Life Cycle Assessment of Residential Heating and Cooling Systems in Minnesota A comprehensive analysis on life cycle greenhouse gas (GHG) emissions and cost- effectiveness of ground source heat pump (GSHP) systems compared to the conventional gas furnace and air conditioner system A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Mo Li IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE Timothy M. Smith, Advisor December, 2012 2 Copyright 2012, Mo Li © Mo Li 2012 i Acknowledgements First of all, I would like to acknowledge my thesis committee. I want to thank Dr. Timothy Smith for all his guidance, support, and advice during this research project and during my graduate school career and Dr. Jason Hill and Dr. Elizabeth Wilson for providing valuable help to my graduate studies and feedback on this thesis. I would also like to thank Professor Patrick Huelman and Research Fellow Tom Shirber for their generous help on providing the operational data and their willingness to take time to answer or find an answer to my questions. I also wish to express my gratitude to the sponsors of the research. Office of Energy Security at Minnesota Department of Commerce provided funding for the research towards this thesis, which will also serve as a basis for a report to them. I want to thank the fellow graduate researchers from the NorthStar research team and Room 309 at the Department of Bioproducts and Biosystems Engineering. Thank you to Rylie Olson for taking time to help me and discuss with me on manipulating the GaBi 5 Software and Tom Nickerson for reviewing my thesis and providing advice on the economic analysis. Finally, I would like to thank my family and friends for their understanding and support throughout my graduate studies and thesis writing. ii Abstract Ground Source Heat Pump (GSHP) technologies for residential heating and cooling are often suggested as an effective means to curb energy consumption, reduce greenhouse gas (GHG) emissions and lower homeowners’ heating and cooling costs. As such, numerous federal, state and utility-based incentives, most often in the forms of financial incentives, installation rebates, and loan programs, have been made available for these technologies. While GSHP technology for space heating and cooling is well understood, with widespread implementation across the U.S., research specific to the environmental and economic performance of these systems in cold climates, such as Minnesota, is limited. In this study, a comparative environmental life cycle assessment (LCA) is conducted of typical residential HVAC (Heating, Ventilation, and Air Conditioning) systems in Minnesota to investigate greenhouse gas (GHG) emissions for delivering 20 years of residential heating and cooling – maintaining indoor temperatures of 68ºF (20ºC) and 75ºF (24ºC) in Minnesota-specific heating and cooling seasons, respectively. Eight residential GSHP design scenarios (i.e. horizontal loop field, vertical loop field, high coefficient of performance, low coefficient of performance, hybrid natural gas heat back-up) and one conventional natural gas furnace and air conditioner system are assessed for GHG and life cycle economic costs. Life cycle GHG emissions were found to range between 1.09 ! 105 kg CO2 eq. and 1.86 ! 105 kg CO2 eq. Six of the eight GSHP technology scenarios had fewer carbon impacts than the conventional system. Only in cases of horizontal low-efficiency GSHP and hybrid, do results suggest increased GHGs. Life cycle costs and present value analyses suggest GSHP technologies can be cost competitive over their 20-year life, but that policy incentives may be required iii to reduce the high up-front capital costs of GSHPs and relatively long payback periods of more than 20 years. In addition, results suggest that the regional electricity fuel mix and volatile energy prices significantly influence the benefits of employing GSHP technologies in Minnesota from both environmental and economic perspectives. It is worthy noting that with the historically low natural gas price in 2012, the conventional system’s energy bill reduction would be large enough to bring its life-cycle cost below those of the GSHPs. As a result, the environmentally favorable GSHP technologies would become economically unfavorable, unless they are additionally subsidized. Improved understanding these effects, along with design and performance characteristics of GSGP technologies specific to Minnesota’s cold climate, allows better decision making among homeowners considering these technologies and policy makers providing incentives for alternative energy solutions. iv Table of Contents Acknowledgements!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!#! Abstract!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!##! List of Tables!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!$#! List of Figures!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!$##! Nomenclature!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!$###! 1. Introduction!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!%! 2. Literature review!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&! 2.1. Recognition of GSHP and status of GSHP industry in the US!""""""""""""""""""""""""""""""""""""""""""!&! 2.2. GSHP Technology!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!'! 2.2.1. Outdoor loop system!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!#! 2.2.2. Heat pump unit!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!$$! 2.3. Life Cycle Assessment on GSHP systems!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!%(! 2.4. Policies and economic incentives on residential GSHP systems!"""""""""""""""""""""""""""""""""""""!%)! 2.5. Summary!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!%*! 3. Methodology!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!%+! 3.1. LCA Goal and scope definition!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!%'! 3.1.1. Goal of the study!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!$#! 3.1.2. Scope of the study!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!$%! 3.1.3. LCA functional unit!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&'! 3.2. Life cycle assessment models!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!(%! 3.2.1. Overall system design!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&$! 3.2.2. Loop system and anti-freeze liquid!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&&! 3.2.3. Ground source heat pump and circulating pump!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&(! 3.2.4. Gas furnace, air conditioner and ductwork!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&)! 3.2.5. House model and operational energetic usages!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&*! 4. Results!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!,%! 4.1. Life cycle assessment on benchmark HVAC systems!"""""""""""""""""""""""""""""""""""""""""""""""""""""""!,%! 4.1.1. Life cycle inventory analysis (LCI)!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!($! 4.1.2. Life cycle GHG emissions!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!((! 4.2. Sensitivity analysis (SA)!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!,&! 4.2.1. Life cycle inventory analysis (LCI)!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!(+! 4.2.2. Life cycle GHG emissions!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!(,! 4.3. Comparison among benchmarks and sensitivity analysis results!""""""""""""""""""""""""""""""""""!)-! 4.4. Economic analysis!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!),! 5. Discussions and future work!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!)+! 5.1. Scenario designs!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!)+! 5.2. Electricity fuel mix!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!).! 5.3. Policies and economics!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&-! 5.4. Model applicability in other regions!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&%! 6. Conclusion!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&(! Reference!"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!&)! v Appendices!""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!*-! Appendix A: Applicable incentive programs for residential GSHP systems in Minnesota !""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""!*-! Appendix B: LCA primary data collections and GHG calculations!"""""""""""""""""""""""""""""""""""""!*+! Appendix C: Economic analysis primary data collections and calculations!"""""""""""""""""""""""!+)! ! ! vi List of Tables Table 1. Regional research focused on GHG emissions reductions achieved by GSHP systems ........................................................................................................................ 7! Table 2. Comparison between horizontal slinky and vertical loop systems ..................... 10! Table 3. LCA research focused on carbon impacts from GSHP systems ......................... 13! Table 4. Federal and state financial incentives for residential GSHP consumers ............ 14! Table 5. Composition of benchmark scenarios ................................................................. 22! Table 6. Summary of equipment specifications ................................................................ 26! Table 7. Annual energetic consumption for space heating and cooling in benchmark scenarios .................................................................................................................... 29! Table 8. Life cycle inventory of material and energy inputs in the benchmark systems .. 32! Table 9. Life cycle GHG emissions (CO2 eq. kg) in the four GSHP systems and the one conventional HVAC system from aspects of material/energetic inputs and life stages ................................................................................................................................... 34! Table 10. Life cycle inventory of material and energetic inputs in the hybrid systems ... 36! Table 11. Life cycle GHG emissions in hybrid systems from aspects of material and energetic inputs and life stages ................................................................................. 38! Table 12. Comparison of the life cycle GHG emissions (kg CO2 eq.) from the hybrid systems and their corresponding GSHP systems ...................................................... 39! Table 13. Life cycle costs PV (present value) of each system ......................................... 46! vii List of Figures Figure 1. Heating and cooling system load characteristics in Minneapolis/St. Paul, Minnesota .................................................................................................................... 4! Figure 2. Configurations of typical GSHP horizontal slinky and vertical loop systems in Minnesota: (a) horizontal slinky loop and b) vertical loop system ........................... 11! Figure 3. A schematic of the ground source heat pump system ....................................... 12! Figure 4. Numbers of Minnesota electric and natural gas utilities that offer and do not offer residential GSHP incentive program by utility category (RECs, IOUs, and MUs) ........................................................................................................................ 16! Figure 5. System boundary and life stages of residential HVAC systems . ..................... 20! Figure 6. Design process of the house model ................................................................... 28! Figure 7. MISO (Midwest Independent Transmission System Operator) real-time generation fuel mix in July 2012 .............................................................................. 30! Figure 8. Life cycle GHG emissions (CO2 kg eq.) of all the HVAC systems from material and energetic inputs perspective .............................................................................. 41! Figure 9. Proportions of each material and energetic input in the life cycle GHG emissions by HVAC systems .................................................................................... 42! Figure 10. Life cycle GHG emissions (CO2 kg eq.) of all the HVAC systems from life stages perspective ...................................................................................................... 43! Figure 11. Life cycle costs PV (present value) of each system ...................................... 46! viii Nomenclature AC Air Conditioner AFUE Annual Fuel Utilization Efficiency CO2 Carbon Dioxide COP Coefficient Of Performance DSIRE Database of State Incentives for Renewable Energy GF Gas Furnace GF&AC Gas Furnace and Air Conditioner GHG Greenhouse gas GREET Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation GSHP Ground Source Heat Pump GSHP_HH Horizontal Ground Source Heat Pump with High efficiency rate (5.0 COP/18 SEER) GSHP_HL Horizontal Ground Source Heat Pump with Low efficiency rate (3.0 COP/14 SEER) GSHP_VH Vertical Ground Source Heat Pump with High efficiency rate (5.0 COP/18 SEER) GSHP_VL Vertical Ground Source Heat Pump with Low efficiency rate (3.0 COP/14 SEER) GWP Global Warming Potential HDPE High-density polyethylene HVAC Heating, Ventilation, and Air Conditioning Hybrid _HH Hybrid (GSHP + gas furnace) with Horizontal loop and High efficiency rate (5.0 COP/18 SEER) Hybrid _HL Hybrid (GSHP + gas furnace) with Horizontal loop and Low efficiency rate (3.0 COP/14 SEER) Hybrid _VH Hybrid (GSHP + gas furnace) with Vertical loop and High efficiency rate (5.0 COP/18 SEER) Hybrid _VL Hybrid (GSHP + gas furnace) with Vertical loop and Low efficiency rate (3.0 COP/14 SEER) LCIA Life Cycle Impact Assessment MISO Midwest Independent Transmission System Operator MUs Municipal electric Utilities PACE Property Assessed Clean Energy PV Present Value RECs Rural Electric Cooperatives SA Sensitivity analysis SEER Seasonal Energy Efficiency Ratio 1 1. Introduction Residential buildings comprised 23% of the end-use sector of total energy consumption in the United States in 2010 (EIA, 2011a). Heating and cooling – which combined account for 52.7% of site energy consumption (US-DOE, 2011a), 42.3% of primary energy consumption (US-DOE, 2011a), and 41.2% of CO2 emissions (US-DOE, 2011b) – drove residential buildings energy demand in 2010. The application of the most efficient HVAC (heating, ventilation, and air conditioning) system in residential buildings has potential to contribute to energy savings, emissions reductions, and economic cutbacks. Ground source heat pump (GSHP) systems, one type of HVAC system, have been recognized as an efficient delivery mechanism of the renewable geothermal energy (Curtis, Lund, Sanner, Rybach, & Hellström, 2005). GSHP systems extract geothermal heat energy from the ground to provide indoor heating and cooling. A residential GSHP system consists of a heat pump, a circulating pump, and an outdoor loop system buried underground. The circulating pump propels an anti-freeze liquid (25% glycol and 75% water) to circulate in the loop system and draw geothermal energy from soil back to the heat pump. Then the heat pump achieves the refrigeration cycle, in which heat is moved from a lower-temperature location to another higher-temperature location via the refrigerants’ state change and circulation, and delivers warmth or cold in the house through indoor ductwork. The entire GSHP system transports geothermal energy for heating and cooling by consuming electricity, rather than converting heat from directly burned fossil fuels in conventional natural gas or oil HVAC systems or direct electric heating or cooling. 2 GSHP systems have been employed as an alternative for heating and cooling in residential buildings. GSHP technologies are invested politically and financially from many aspects and are thought to provide significant reductions in greenhouse gas (GHG) emissions and energy consumption in many situations (Lund, 1988) (Curtis, Lund, Sanner, Rybach, & Hellström, 2005) (Lienau, 1997). Researchers have inspected GSHP systems in terms of different scales and various regions, but significant concern exists regarding the effectiveness of these systems in cold climates and in regions where the electricity fuel source mix includes significant amounts of high carbon emitting coal, such as Minnesota. In addition, given the increased importance of natural gas in recent years in the electricity mix, driven by new extraction technologies and historically low prices – as well as the direct substitution of natural gas combustion heat of a residential furnace by GSHP technologies requiring significant electricity inputs, it is unclear that GSHPs provide economic or GHG benefits to Minnesota residents. In this study, inclusive and comparative life cycle assessments (LCA) and economic analysis on residential GSHP, conventional HVAC, and hybrid HVAC systems in Minnesota were accomplished. Life cycle GHG emissions impacts and cost effectiveness of the systems over a time period of 20 years were the targets of this study. Results from this study can provide scientific evidences and suggestions influencing decision-making of stakeholders including consumers, manufacturers and policy makers... The analysis began by investigating five basic residential HVAC systems as benchmark systems: four GSHP system designs (horizontal and vertical systems with low and high efficiency, respectively) and one conventional HVAC system containing a gas 3 furnace and an air conditioner. LCA models for the basic HVAC systems were established in GaBi 5 software. In addition, use phase estimates associated with home energy loads were calculated using REMRate software and Microsoft Excel 2011 to determine life cycle GHG emissions. In Minnesota, however, hybrid systems (GSHP and gas furnace) commonly exist in residential houses for specific reasons: first, heating services by individual GSHPs are sometimes not sufficient enough in extreme cold; second, consumers can receive a discounted electricity rate if enrolled in a “Dual Fuel” program (Minnesota Power, 2012) to interrupt the operations of their electric devices and switch to using natural gas during heating peak hours in winter; and finally, consumers can also receive up to $350 rebate by upgrading their gas furnace to the 95% efficient level (Minnesota Energy Resources, 2012). Therefore, sensitivity analyses were conducted based on the LCA benchmark models to investigate life cycle GHG emissions of the hybrid systems in terms of various heating strategies during peak hours. The strategies were defined to simulate scenarios by which the heat pump is disabled during times when the outside ground temperature becomes too cold to effectively heat the home, and a gas back-up furnace is employed to provide certain heating load proportions (10%, 20% and 30%). Based on a heating and cooling load distribution of a typical Minnesotan residential HVAC system (Figure 1), it was assumed that the consumers usually apply their strategies in the months of November to March in the following year. 4 Figure 1. Heating and cooling system load characteristics in Minneapolis/St. Paul, Minnesota. Monthly characteristics are based on the natural outdoor temperature and typical residential demands in Minneapolis/St. Paul, Minnesota. This load distribution is altered to represent the monthly load hours of the four basic GSHP systems and the one conventional system, bases on RETScreen 4 software (RETScreen, 2012). The integrated LCAs offered distinct evaluations of life cycle GHG emissions of all the HVAC systems and helpful comparisons among them. The comprehensive sensitivity analysis added investigations into the extent to which the hybrids’ performances would be improved or degraded when certain replacements between the GSHP and the gas furnace occur during peak hours in winter. It also revealed interrelations among various strategies of providing residential heating and cooling in Minnesota. Besides the environmental impacts, other influential factors of the applications of residential GSHP systems were also reviewed and analyzed, for example, current policies and tax incentives. A preliminary economic analysis on all the HVAC systems was completed to explore their overall costs over 20 years and how federal tax credit and Minnesota state rebate will impact the costs. In the following sections of this thesis, I explore these issues. Chapter 2 provides a literature review focused on recognition and industry status of residential GSHP in the US, existing LCA research on GSHP, and 5 current policies and economic incentives on residential GSHP in Minnesota. Chapter 3 applies LCA methods for evaluating all the HVAC systems. It introduces components of each system and fundamental definitions of the LCA models: research scope, system boundary and functional unit. In Chapter 4, results of the LCAs as well as a preliminary economic analysis are presented. Life cycle GHG emissions of each system are displayed separately and compared jointly. The economic analysis calculates and compares the net present values of life cycle costs of each system and investigates how financial incentives on residential GSHP can be used to offset its high capital cost. Chapter 5 covers discussions of findings in this study and recommendations for future work. Finally, research conclusions are drawn in Chapter 6. 2. Literature review 2.1. Recognition of GSHP and status of GSHP industry in the US Since the 1970s energy crisis, increasing energy costs and climate changes have been propelling the rapid development of renewable and efficient energy technologies both internationally and nationally. With the aim of reducing energy bills, dependence on scarce resources and environmental impacts, the US government has implemented policies and programs to employ renewable energy and enhance energy efficiency. One path has been found in the area of residential HVAC systems, since energy consumption in residential buildings accounted for nearly a quarter of the country’s total energy consumption in 2010 (EIA, 2011a), and is largely driven by heating and cooling loads. Many studies have demonstrated that the application of ground source heat pump systems, which utilize the renewable geothermal energy from underground to provide heating and cooling, have the potential to contribute to energy savings, emissions 6 reductions, and economic benefits. In the report “Climate Change 2007: Mitigation of Climate Change”, the Nobel-Prize-winning Intergovernmental Panel on Climate Change (IPCC) identified GSHP systems as “economically feasible under certain circumstances” in continental and cold climate (IPCC, 2007). Cases cited in the report also indicated that GSHP systems could reduce energy use for heating by 50% to 60% (Shonder, Martin, McLain, & Hughes, 2000) (Johnson, 2002) and for dehumidification in cooling by 30 to 50% (Fischer, Sand, Elkin, & Mescher, 2002) (Niu, Zhang, & Zuo, 2002). GSHP systems have been cited as a helpful and realistic option to achieve the goal of limiting primary energy use in 2030 to the same level as in 2008, as opposed to a business-as- usual case of a projected 30% increase over the same period (APS, 2008) (Dirks, Belzer, Anderson, Cort, & Hostick, 2008). Another report by National Renewable Energy Laboratory (NREL) categorized GSHP systems as a low risk technology to develop cost- neutral net zero-energy homes (Anderson & Roberts, 2008). Saner et al. (2010) summarized the findings from a diversity of international case studies and found GHG emission savings of residential and commercial GSHP systems varying from 15% to 80%. The reduction range depends on regional hydro and geological conditions, technology and technological designs, (time-dependent) heating and cooling requirements, CO2 intensity of the primary energy for running heat pump, as well as available alternative heating and cooling systems (Saner, Juraske, Kubert, Blum, Hellweg, & Bayer, 2010). In addition to the general case studies, regional research more precisely represents the reduced carbon impacts by GSHP systems, compared to conventional HVAC systems (Table 1). 7 Table 1. Regional research focused on GHG emissions reductions achieved by GSHP systems Findings Scale Region Reference From a regional scale perspective, one installed GSHP unit can achieve annual CO2 savings ranging between 1.8 and 4.0 t, which accounts for between 35% and 72% of CO2 emissions from one unit of conventional heating systems. Commercial Baden- Württemberg, Germany Blum, et al. (2010) GSHP systems in the UK can obtain carbon savings up to 80%. Residential UK Jenkins, et al. (2009) GSHP systems can save 33-50% in CO2 emission compared to fossil fuel fired boilers, even 100% by using hydroelectricity. Commercial and residential Global Fridleifsson, et al. (2008) GSHP systems can reduce GHG emissions by 2.3 ! 108 t CO2 eq./year with an expected annual increase rate of 5.4 million heat pumps in Europe and the average European electrical energy mix, which represents 20% of the European GHG emission savings goal by 2020. Commercial and residential Europe Rybach (2008) GSHP systems at COP 4 can provide GHG emissions savings up to 0.76 kg CO2 eq./kWh and 1.24 kg CO2 eq./kWh compared to 95% ef"cient natural gas furnaces and 85% ef"cient heating oil furnaces, respectively. Commercial and residential US Hanova and Dowlatabadi (2007) GSHP system in a proposed office building on the Winnebago Reservation can achieve annual GHG emissions of 15 t and 30 t CO2 eq. compared to gas heat and air source heat pumps, respectively. Commercial Nebraska, US Chiasson (2006) Horizontal GSHP systems in large buildings can save CO2 emissions by 30% and 40% compared to natural gas furnaces and oil-fired boilers, respectively, and even 100% when utilizing a renewable energy resource for electricity. GSHP systems are estimated to save about 2.4 ! 1012 kWh/year in primary energy use and 6.17 ! 105 t CO2 eq./year in GHG emissions for residential and tertiary sectors Commercial and residential Ireland O’Connell and Cassidy (2003) A planned district GSHP heating system can achieve annual CO2 savings of 1,516 ton and annual fossil fuel savings of 4.7 ! 106 kWh. Commercial and residential Greece Agioutantis and Bekas (2000) In the late 1970s, GSHP industry started in the US. With some ups and downs at the beginning of its first development (Hughes, 2008), the industry has had robust growth in recent years, with a total number of 101,000 units installation in 2008 (EIA, 2008), which was an increase of 40% from 2007 to 2008 (NREL, 2008). The strong developing trend is possibly to be continuing in the next several decades: GSHP systems in the US were estimated to have developable resources of 1.85 ! 104 MWt (thermal megawatt) in 8 2015, 6.64 ! 104 MWt in 2025, and more than 1.0 ! 106 MWt in 2050 (Green & Nix, 2006). Adoption and dissemination of a new energy technology into the mainstream market does not come without major hurdles, and GSHP systems are no different (NREL, 2008). The GSHP industry is comprised of manufacturers of water-source heat pumps, high-density polyethylene (HDPE) pipe and fittings, circulating pumps, and specialty components, as well as a design infrastructure, an installation infrastructure, and various trade allies, most notably electric utilities (Hughes, 2008). Therefore, climatic and geological conditions and more importantly, the availability of trained contractors, drillers, and engineers, who tend to be clustered together mainly in the Midwestern states, California, and Colorado, caused sales of GSHP systems to be regional rather than spread uniformly over the U.S. (NREL, 2008). 2.2. GSHP Technology Compared to conventional HVAC systems, GSHP systems avoid fossil fuels combustions at the residence by consuming electricity to extract and transport underground geothermal energy and provide heating and cooling in ground buildings. For residential GSHP systems specifically, construction can be easily completed in both new and existing houses, basically requiring two sections: an outdoor underground loop system and an indoor heat pump unit with a circulating pump. Individually reviewed outdoor loop systems and heat pumps are described in detail in following. 2.2.1. Outdoor loop system The outdoor loop system consists of a series of small diameter HPDE pipes buried underground, with an anti-freeze liquid circulating through the pipes as a medium to transfer heat between the soil and the heat pump. Various types of outdoor loop systems 9 applied in practice can be classified as closed and open systems, with a third category for those not truly belonging to one or the other (Omer, 2006). To choose the right system for a specific installation, several factors have to be considered: geology and hydrogeology of the underground, area and utilization on the surface, existence of potential heat sources like mines, and heating and cooling characteristics of the building(s) (Omer, 2006). After reviewing all categories of the loop systems, focus of this study narrowed into two typical closed loop systems: the horizontal slinky loop and the vertical loop. They were examined in every reviewed study due to their wide implementations and representative benefits and drawbacks (Table 2). Reasons for excluding all the other choices were: first, the open loop system requires relatively high water usage volumes during peak seasonal operation, and it is regulated that all well water usages should be consulted with the Minnesota Department of Health (DOH) and Department of Natural Resources (DNR) (MNGHPA, 2012); second, the closed loop system associated with pond lake is not realistic in most houses because of the availability of such resource; and third, due to the frost problems in ground surface caused by the cold climate in Minnesota, another type of closed loop system – horizontal trench loop is ruled out (MNGHPA, 2012). 10 Table 2. Comparison between horizontal slinky and vertical loop systems. Horizontal slinky and vertical loop systems are highlighted as they are widely used in Minnesota. Benefits and drawbacks comparison between systems are adapted largely from Omer (2006). GSHP loop types Advantages Disadvantages Horizontal slinky • Lower installation costs • Flexible installation options • Larger ground area required • Longer pipe lengths required • Thermal performance subject to soil, season, and rainfall • Antifreeze solution viscosity increases pumping energy, decreases the heat- transfer rate and overall efficiency Vertical • Shorter pipe lengths required • Smaller ground area required • Highest ability to provide sufficient heat exchange capacity • Thermal performance not subject to seasonal variation • Higher installation costs, mostly drilling • Potential for underground extension In the horizontal slinky loop system, a large and 1-2 m deep pit is dug in the residential yard and a series of parallel HDPE pipes are laid horizontally in loops as shown in Figure 2 (a). After setting the pipes as an entire closed loop, the soil is distributed back over the pipes (Florides & Kalogirou, 2007) (Omer, 2006). In the vertical loop system (Figure 2 (b)), several holes are drilled 30-120 m deep and 6 m apart (US-DOE, EERE, 2011). Series of HDPE pipes with U-bend ends are inserted into the hole to form a closed loop. The remaining space in the hole is grouted with soil-like materials. Additional pipes are horizontally laid in the 1 m deep trenches, with one end linked to every borehole pipes and the other end connected with the indoor heat pump. 11 Figure 2. Configurations of typical GSHP horizontal slinky and vertical loop systems in Minnesota: (a) horizontal slinky loop and b) vertical loop system. Horizontal slinky loop systems are the easiest and most cost-effective to install, but it requires the most land area (Florides & Kalogirou, 2007). Vertical loop systems require less piping to reach the “constant temperature zone” (15-20 m depth) and are favored in instances were land area is constrained (Omer, 2006). Images courtesy of: US-DOE the Office of Energy Efficiency and Renewable Energy (EERE). Accessed on 2/8/2012, from: http://www.energysavers.gov. 2.2.2. Heat pump unit A ground source heat pump moves thermal energy against a thermal gradient at the expense of an external source of electricity. The heat pump usually includes an evaporator (refrigerant-to-water heat exchanger), a compressor, a condenser (refrigerant- to-air heat exchanger), and an expansion valve (Figure 3) (Rafferty, 2001) (Greening & Azapagic, 2012). In the heating season, the anti-freeze fluid flows from the outdoor loop system into the heat pump, carrying heat from underground. As it flows through the evaporator (a double-layer tube), liquid refrigerant inside the evaporator absorbs the heat and changes its state from liquid to gas. In the compressor, the gasified refrigerant is pressurized to raise its temperature to a high enough degree. After compression, the refrigerant loses its heat to the air in the condenser and returns to liquid. The warmed air is blown into indoor distribution system by a fan coil unit, while the liquid refrigerant flows back to the evaporator through the expansion valve and starts the next refrigeration cycle. In cooling season, the entire refrigeration cycle reverses: indoor air lowers indoor temperature by losing its heat to the refrigerant and eventually to the circulating fluid. 12 A circulating pump is a much smaller piece of equipment connected with the heat pump. It operates like a motor and powers the glycol-water solution to continuously circulate in the closed circuit consisting of the outdoor piping system and the heat pump. Figure 3. A schematic of the ground source heat pump system. In the figure: 1. Outdoor loop system – heat collector; 2. Flow center – circulating pump; 3. Evaporator (refrigerant/water heat exchanger); 4. Compressor; 5. Condenser (refrigerant/air heat exchanger); 6. Expansion valve. Refrigerant circulates in it and provides heat cycle via change of state. (Edited from the work by Greening & Azapagic (2007)) 2.3. Life Cycle Assessment on GSHP systems Life Cycle Assessment is a tool to assess the potential environmental impacts and resources used throughout a product’s life cycle, that is, from raw material acquisition, via production and use phases, to waste management, including disposal and recycling (ISO, 2006a). LCA is a comprehensive assessment and considers attributes or aspects of natural environment, human health, and resources (ISO, 2006a). The unique feature of LCA is the focus on products from a life cycle perspective. An LCA study requires a precise structure involving goal and scope definition, life cycle inventory analysis (LCI), life cycle impact assessment (LCIA), and interpretation. The goal and scope definition phase includes the reasons for carrying out the study, the intended application, and the 13 intended audience (ISO, 2006a). It is also the phase where the system boundary of the study is described and the functional unit is defined. The system boundary is the set of criteria specifying which unit processes are part of a product system (ISO, 2006a). The functional unit is a quantitative measure of the functions that the goods (or service) provide (Finnveden, et al., 2009). The result from the LCI is a compilation of the inputs (resources) and the outputs (emissions) from the product over its life cycle in relation to the functional unit. The LCIA is aimed at understanding and evaluating the magnitude and significance of the potential environmental impacts of the studied system. In the interpretation phase, the results from the previous phases are evaluated in relation to the goal and scope in order to reach conclusions and recommendations (ISO, 2006a). Worldwide, LCA research has often targeted life cycle carbon impacts from GSHP systems. Researchers have claimed that GSHP systems are capable to achieve CO2 savings (Table 3). Table 3. LCA research focused on carbon impacts from GSHP systems. The study by Shah et al. (2008) included Minnesota as a region scope to examine residential GSHP systems, but it did not consider different GSHP technologies (horizontal and vertical) or different efficiency levels (high and low). Nor did it investigate the hybrid systems included in this study. Findings Scale Region Reference GSHP has the maximum carbon impacts in regions where fossil fuels accounts for the highest proportion of power generation. Residential US (Minnesota, Oregon, Pennsylvania, Texas) Shah et al. (2008) GSHP systems were not competitive in the overall environmental impacts comparison, but they do have advantages with respect to GHG emissions and Global Warming Potential (GWP) Residential UK Greening & Azapagic (2012) Installed (replacing air source heat pumps) in a 1 km2 of high-rise building, the examined GSHP system can save CO2 emissions of nearly 4.0 ! 104 t (more than 50%) and offer an environmental payback time of 1.7 years. It is also found that 87% of the carbon impacts came from the digging stage. Commercial/industrial Nishi (West)- Shinjuku area of Tokyo, Japan Genchi, et al. (2002) 14 2.4. Policies and economic incentives on residential GSHP systems Many federal and state policies and economic incentives have been employed to support the development of residential GSHP systems, for example, efficiency standards, laws, tax incentives, and utility rebates. In this study the effective policies and incentives in Minnesota were reviewed, including the subsidizing policies and mechanisms from federal and state governments (Table 4), and the loan and rebate programs from local utilities (Figure 4). Table 4. Federal and state financial incentives for residential GSHP consumers According to the Database of State Incentives for Renewable & Efficiency (DSIRE), residential GSHP consumers can receive various economic subsidies from both federal and state governments in the forms of personal tax credit, rebate etc. In addition to the listed financial incentives, Minnesota state government has also authorized certain local governments to establish similar programs (DSIRE, 2012c). In the programs, being qualified for Energy Star criteria is the primary requirement for residential GSHP systems (ENERGY STAR, 2012). Financial incentive Incentive specifications Source Federal personal tax credit 30% of capital costs (expire date: 12/31/2016) DSIRE (2012a) Minnesota state loan program Maximum incentive of $35,000 with requirements including loan terms from 1 to 20 years at a fixed rate of 5.75% and maximum household income of $96,500 DSIRE (2012b) PACE (Property Assessed Clean Energy) financing Property owners are allowed to borrow money from local governments to pay for energy improvements and repay the loaners via a special assessment on the property over a period of years. Loans amounts may not exceed 10% of the assessed value of the property and may include costs related to the required energy audit or feasibility study, equipment and labor costs, and performance verification. Interest rates are locally determined, but must be sufficient to cover program costs, including the issuance of bonds and any financing delinquencies. Under the state law, implementing entities must set loan maturities at the weighted average of the useful life of improvements made to the property, not to exceed 20 years. DSIRE (2012c) Minnesota state law requires Minnesota electric and natural gas utilities to invest a portion of their revenues in conservation improvement programs that promote energy- efficient technologies and practices to their customers (Minnesota Office of Energy Security, 2011). Therefore, Minnesota Energy Conservation Improvement Program (CIP), a utility-administered program, was established with regulatory oversight from the 15 Minnesota Office of Energy Security to implement the plan (Minnesota Office of Energy Security, 2011). Roughly speaking, there are three types of electric utilities in Minnesota: rural electric cooperatives (RECs), which are member-owned utilities serving mostly rural areas; investor-owned utilities (IOUs), which are privately-owned but publicly-traded utilities regulated and authorized by states utilities commissions; and municipal utilities (MUs), which are publicly-owned and non-profit government entities (EIA, 2012a) (Wilson, Plummer, Fischlein, & Smith, 2008). Natural gas utilities in Minnesota are categorized into two types: IOUs and MUs (Minnesota Public Utilities Commision, 2012b). In Minnesota, there are 178 utilities provide electric service, in which 8 utilities also provide natural gas service (EIA, 2010) (Minnesota Public Utilities Commission, 2012a) (Minnesota Public Utilities Commision, 2012b). However, not every utility offers incentive program on residential GSHP system (Figure 4). From a quantitative perspective, MUs have the most utilities but the lowest proportion of incentive program supporting residential GSHP. In contrast, RECs and IOUs have relatively fewer utilities but show stronger support in proportion (summarized in Figure 4). Details of the incentive programs are given in Appendices A. 16 Figure 4. Numbers of Minnesota electric and natural gas utilities that offer and do not offer residential GSHP incentive program by utility category (RECs, IOUs, and MUs). In the figure, GSHP means the numbers of the utilities offering residential GSHP incentive program, and non-GSHP means those do not. RECs offer the most residential GSHP incentive programs in numbers and proportion among all, because they cover rural areas where residential houses are widespread. Availability of residential house leads to large demands and marketing potentials of residential GSHP systems. Contrary to RECs, MUs mostly cover cities and small towns, which leads to fewer resources to implement residential GSHP systems. IOUs are much fewer in number of utilities but quite larger in size compared to the other two categories. In 2010, IOUs account for nearly half of electricity sales in Minnesota. Source: EIA (2010); Minnesota Public Utilities Commission (2012a); Minnesota Public Utilities Commission (2012b); DSIRE (2012a) 2.5. Summary Ground source heat pump appears to be a promising technology for residential HVAC industry. Powered by electricity, GSHP systems are capable of extracting geothermal energy from deep underground and providing space heating and cooling to meet domestic needs. In a standard GSHP system, the outdoor loop system plays the role of heat exchanger, while the indoor heat pump and accessory units perform the duty of command and distribution center. All the equipment comprises a closed ring containing energy flow. Closed and open loop GSHP systems are accepted to be the most typical types, among which the closed owns more acceptance in Minnesota due to local geological and climatic conditions. In the U.S., GSHP has been a developing category in residential HVAC industry since the late 1970s, with existing barriers in market and optimistic trend under certain financial incentives and political regulations. According to the Database of State Incentives for Renewable & Efficiency (DSIRE) and the Minnesota Energy Conservation Improvement Program (CIP), residential GSHP consumers can receive subsidies including personal tax credit, rebates and loan programs supported by federal, state and local governments and local public utilities. Existing studies have demonstrated competitiveness of GSHP systems in improving energy efficiency and utilizing the renewable geothermal energy. Potentials in 17 GSHP encourage its popularization in residential houses, especially in the regions with huge heating and cooling demands and affluent geothermal resources. Although GSHPs are often purported to apply renewable energy (i.e. from the sun’s heat captured in the soil), they still consume electricity to support extraction of the renewable geothermal resource. This additional energetic usage is used in GSHPs’ operations, so its fuel mix essentially influences the overall performances of GSHPs on GHG emissions reductions. Comprehensive life cycle assessments on the technologies offer explicit examinations to demonstrate their benefits and drawbacks in cutting GHG emissions. According to the principals of LCA, numerous studies have carried out cradle-to-grave analysis on GSHP systems and published results based on various cases. Generally, GSHP systems are proven to have fewer life cycle carbon environmental impacts than conventional systems. The carbon gap can be extended specifically under greener electricity grid mix. 3. Methodology Life cycle assessment method was used in this study to evaluate GHG emissions of residential HVAC systems from cradle-to-grave perspective. Four GSHP systems and one conventional HVAC system (gas furnace and air conditioner) were established to inspect individual application of each technology. Four additional hybrid systems (GSHP and gas furnace) were investigated to simulate the actual residential applications of these technologies in Minnesota. Life cycle assessment models of the HVAC systems were created in GaBi 5 software to accomplish the analysis on all the systems described above. GaBi 5 software, developed by PE INTERNATIONAL, provides access to and use of Ecoinvent Integrated and Professional Extensions databases, which contain international data of material and 18 energy inputs and outputs for the basic processes. Other data sources included GREET model 2011 by Argonne National Laboratory (Wang, 2011) and surveys with local residential GSHP system installers, such as Hass Geosystems Inc., Massmann Geothermal and Mechanical, and Summit Heating & Air Conditioning. Based on the LCA models, a sensitivity analysis on the hybrids was accomplished to simulate the actual residential heating and cooling demands in Minnesota. In the analysis, three cases were hypothesized and analyzed in each hybrid regarding the gas furnace’s three load proportions (10%, 20% and 30%). A preliminary economic analysis on all the HVAC systems was also completed to compare the cost effectiveness of each system by calculating the present value (PV) of their life cycle costs. Energy prices were used as an uncertainty to examine their influences of the economic performances of the HVAC systems. 3.1. LCA Goal and scope definition 3.1.1. Goal of the study In defining the goal of a life cycle assessment, items including the intended application, the reasons for carrying out the study, the intended audience, that is, to whom the results of the study are intended to be communicated, and whether the results are intended to be used in comparative assertions intended to be disclosed to the public, are required to be explicitly stated (ISO, 2006b). The intended application in this study was to assess life cycle GHG emissions by each of the HVAC systems. The reasons for carrying out this study comprised the recognition of the research gaps described in the above text and the intention to approach 19 a comprehensive life cycle assessment on carbon impacts of diverse residential HVAC systems in Minnesota. The intended audience will be stakeholders of the promotion of residential GSHP technologies in Minnesota, such as consumers, suppliers, state policy makers, and related researchers. The results will be disclosed to the public to provide appropriate assistance for future development of residential GSHP industry. 3.1.2. Scope of the study The scope of a life cycle assessment is basically defined by product systems, functions, and system boundary. The intended application has explained the product systems – all the HVAC systems designed for this study, whose functions are to provide space heating and cooling for residential houses in Minnesota year round. Life stages of a typical product and details of all the systems helped shape up the system boundary in this study. An integrated illustration of the system boundary and life stages of each system is shown in Figure 5, followed by a legend explaining the unique system boundary and specific components in each system. Overall inputs (materials and energy) and outputs (GHG) are displayed. According to the ultimate objective of this study – to attain aggregate GHG emissions – and specific analysis requirements, the life stages have been properly compacted and integrated to fit the following analysis. 20 Figure 5. System boundary and life stages of residential HVAC systems. Based on the data sources in GaBi 5 software, certain independent processes have been aggregated accordingly. The extraction of raw materials is integrated with the manufacture stage and simplified as inputs. The transportation stage represents the conveyance of products from their manufacturers to local retailers then to consumers. The installation stage includes not only the actual GSHP constructions in consumer’s yard and house, such as drilling, trenching, and piping, but also the correlated vehicle trips that have GHG emissions. In the operation stage, annual electricity and natural gas consumptions appear as inputs. Due to the absence of empirical data on the disposals of GSHP systems, the system boundary is enclosed after the operation stage. 3.1.3. LCA functional unit The functional unit is a key component of any life cycle assessment. It is a measure of the function of the studied systems. The functional unit shall be consistent with the goal and scope of the study and shall provide a reference to which the input and output data are normalized (in a mathematical sense) (ISO, 2006b). In this study, functions of the HVAC systems are providing space heating and cooling for residential houses in Minnesota. Therefore, the functional unit was defined to be 20 years of heating 21 and cooling to maintain indoor temperature at 68ºF (20ºC) and 75ºF (24ºC) in heating and cooling season, respectively. 3.2. Life cycle assessment models 3.2.1. Overall system design Based on practical conditions in Minnesota and existing studies (Greening & Azapagic, 2012) (Rey, et al., 2004) (Shah, Col Debella, & Ries, 2007), the LCA models designed for this study included four GSHP systems, one conventional HVAC system (gas furnace and air conditioner), and four hybrid systems (GSHP and gas furnace). Besides the two configurations of GSHP loop (horizontal and vertical), two levels of heat pump efficiency rate were chosen based on the house model design to characterize the GSHP-involved systems: low efficiency rate of 3.0 COP/14 SEER and 5.0 COP/18 SEER. The first five systems involving individual technology, either GSHP or conventional, were considered as benchmarks, while the other four hybrids were designed to be dual systems containing a GSHP system, either horizontal or vertical, and one piece of the same gas furnace from the conventional HVAC system. A composite description of the benchmarks is displayed (Table 5). Details of the hybrids can be inferred based on the same components (Table 5) and the LCA system boundary illustration (Figure 5). All the residential HVAC systems selected in this study were hypothetically installed in the same residential house model. Therefore, evaluation and comparison could be properly achieved based on a fixed parameter. Residential Energy Analysis and Rating (REM/Rate) software, a product of Architectural Energy Corporation, helped design the house model to reflect the actual characteristics of a typical residential house 22 in Minnesota, which is capable for GSHP system and conventional HVAC systems as well. Table 5. Composition of benchmark scenarios The gas furnace has the annual fuel utilization efficiency (AFUE) of 92%. The air conditioner has the efficiency rate of 13 SEER. A 4-ton GSHP was adopted for this study. Horizontal slinky and vertical represent the two typical types of outdoor loop configuration in GSHP systems. Two typical efficiency rates of ground source heat pump are considered: low efficiency (L) of 3.0 COP/14 SEER and high efficiency (H) of 5.0 COP/ 18 SEER. COP (coefficient of performance) is a measure of the ratio of useful heat output by the heat pump to the amount of energy input for operation (Greening & Azapagic, 2012). SEER (seasonal energy efficiency ratio) is defined to be the total heat (Btu) removed from the conditioned space during the annual cooling season divided by the total electrical energy (Wh) consumed by the air conditioner or heat pump during the same season (ANSI/AHRI 210/240, 2011). COP of a heat pump is usually between 3.0 and 5.0 (Yang, Cui, & Fang, 2010). SEER of a heat pump is usually above 14 (Omer, 2006). Components Gas furnace & air conditioner Horizontal slinky GSHP Vertical GSHP Low High Low High Outdoor Loop Field Horizontal ! ! Vertical ! ! Anti-freeze liquid ! ! ! ! Indoor Equipment Heat pump ! ! ! ! Circulating pump ! ! ! ! Gas furnace ! Air conditioner ! Indoor distribution Ductwork ! ! ! ! ! 3.2.2. Loop system and anti-freeze liquid As described in literature review, horizontal slinky and vertical loop systems were selected for this study. According to the survey conducted with the local installers, the GSHP industry accepts the 3/4” SDR-11 ASTM D3035 HDPE as the most common pipe type used in both the loop systems and 25% as the volume proportion of glycol in the anti-freeze liquid. The lifetimes of the pipes and the anti-freeze liquid were estimated to be 70 years. The survey also showed that to support a 4-ton ground source heat pump, a total length of 3,200 foot and 1,600 foot HDPE pipe were demanded in horizontal and vertical loop systems, respectively. Loop volume calculator by Hydro-Temp Corporation (2012) provided parameters and customized calculations of the exact pipe type. 23 Calculation results of the HDPE pipe weight and the anti-freeze liquid volume required by horizontal and vertical loop systems are displayed in Table 6. It was assumed the HPDE pipes and glycol were manufactured in China and transported to Minneapolis, Minnesota, US. Manufacturer locations and transport methods are given in Table 6 as well. 3.2.3. Ground source heat pump and circulating pump A 4-ton heat pump with desuperheater was selected in this study, because it is the most common ground source heat pump for residential uses in Minnesota. The 4-ton represents output capacity and size of this heat pump. Specifications of the heat pump were adopted from models and findings in existing studies. Roth North America (2011) offered an exact 4-ton ground source heat pump with shipping weight of 240 kg (assumed net weight of 200 kg) and an estimated life of 35 years. Greening and Azapagic (2012) provided a ground source heat pump with a net weight of 132.8 kg and COP of 3.9 but lacking of size information. The total weight of 200 kg and the material specifications of the heat pump in the study by Greening and Azapagic (2012) were integrated to model the targeted heat pump for this study. Considering different efficiency rates (3.0 COP/14 SEER and 5.0 COP/18 SEER) of the heat pump, specifications of the evaporator and the condenser heat exchangers change slightly because of their direct effects on the overall efficiency of the heat pump. Generally, the larger (heavier) the heat exchangers the more efficient the system becomes (National Air Warehouse, 2012), but their weight differences were not distinguished in this study due to lack of empirical references. Impacts of the weight differences might be considered minor and negligible in existing 24 studies. Otherwise, it would be a research gap for neglecting them, and future work was recommended to investigate it. A circulating pump, also called flow center, powers the glycol-water solution to continuously circulate in the closed circuit consisting of the loop system and the heat pump. Compared to the heat pump, the circulating pump is a much smaller piece of equipment, which is commonly made of cast iron (BOSCH, 2012). Table 6 includes material compositions of the flow center capable for this study provided by Roth North America (2011). It contains double pumps, weighs 19.1 kg (assumed net weight of 18 kg) and supports a 3.5 to 6 ton heat pump. Manufacturers information and transport methods of the heat pump and circulating pump are also given in Table 6. 3.2.4. Gas furnace, air conditioner and ductwork In this study, the gas furnace was designed to individually provide heating in the conventional HVAC system and operate part-time as the substitution for the GSHP in the hybrids. The case study by Shah et al. (2007) provided a suitable gas furnace with the model number of 58DLA and manufactured by Carrier Corporation (Carrier Corporation, 2006). Annual fuel utilization efficiency (AFUE) of the furnace was adjusted to 92% to meet the function simulation in this study. The furnace uses an electric spark to ignite a mixture of natural gas and air. The air is warmed up by the natural gas combustion then loses its heat to the air that circulates in the indoor ductwork and delivers heat to the indoor environment. Major constituents given in Table 6 form the bulk of the furnace’s weight. Other smaller fractions of material usages and energy usages during the assembly of the furnace are not available in the study by Shah et al. (2007). 25 Similar to the gas furnace, the air conditioner used in this analysis was adopted from the case study by Shah et al. (2007) as well, which was manufactured by Carrier Corporation (Carrier Corporation, 2006) with the model number 24ACR3. The seasonal energy efficiency ratio (SEER) of this unit is 13. Table 6 provides the major components of the air conditioner. Unlike the heat pump or the gas furnace, the air conditioner has a condenser located outside the house to extract air. Cool refrigerant absorbs heat from the air flowing through indoor ductwork and provide cooling to the house. It was assumed that both the gas furnace and the air conditioner are manufactured in Mexico and transported to Minneapolis, Minnesota. Manufacturer information and transport methods are given in Table 6. The indoor distribution system (ductwork) was adopted from Shah et al. (2007). All the HVAC systems were hypothesized to share the same indoor ductwork, which was designed for both heating and cooling services. Existence of the same ductwork helped simplify the comparison among all the HVAC systems. Material constituent of the ductwork is provided in Table 6, as well as the manufacturer information and transport method. 26 Table 6. Summary of equipment specifications. The heat pump also consumed energy during assembly: medium-voltage electricity 606.6 MJ and natural gas 1,575 MJ. The 3/4” SDR-11 HDPE pipes were used here. Refrigerant R-22 was replaced by R-134a in the heat pump and the air conditioner considering available data source, their compatibility, and the small proportion of refrigerant in the entire equipment unit. All equipment was transported to consumers’ houses in Minneapolis, Minnesota. Equipment Material Weight/kg Estimated life/year Manufacturer locations & transport methods Heat pump Low alloyed steel Reinforcing steel Copper Elastomere Polyvinylchloride Lubricating oil R-134a (refrigerant) 30 112.5 33 15 1.5 2.55 4.64 35 Roth North America, Watertown, New York, US ! Train: 1,800 km ! Truck: 57 km HDPE pipe High-density polyethylene 174 (horizontal) 87 (vertical) 70 Tianjin Junxing Pipe Group Co., Ltd., Tianjin, China ! Ocean freight: 9,914 km ! Train: 2,700 km ! Truck: 93 km Anti-freeze Glycol (25% in volume) 92.4 (horizontal) 46.2 (vertical) 70 Tianjin Jixin Industrial & Trade Co., Ltd., Tianjin, China ! Ocean freight: 9,914 km ! Train: 2,700 km ! Truck: 93 km Circulating pump Cast iron 18 35 Roth North America, Watertown, New York, US ! Train: 1,800 km ! Truck: 57 km Gas furnace Steel Galvanized steel Aluminum Copper 46 18 9 3 20 Carrier Corporation, Monterrey, Nuevo León, Mexico ! Train: 2,300 km ! Truck: 20 km Air conditioner Steel Galvanized steel Aluminum Copper R-134a (refrigerant) 78 35 17 17 6 20 Carrier Corporation, Monterrey, Nuevo León, Mexico ! Train: 2,300 km ! Truck: 20 km Ductwork Galvanized steel 265 35 Minnesota, US ! Truck: 50 km 3.2.5. House model and operational energetic usages In this study, REM/Rate software was used to design a typical Minnesotan single- family residential house model. This software was developed as a user-friendly, yet highly sophisticated, residential energy analysis, code compliance and rating software by Architectural Energy Corporation (Architectural Energy Corporation, 2012). It has been employed for residential building design and home energy usage simulation in many 27 studies (Bourassa, Rainer, Mills, & Glickman, 2012) (Walsh, Bashford, & Anand, 2003) (Sawhney, Mund, & Syal, 2002) (Wong, Lam, & Feriadi, 2000). With the help from REM/Rate software, the house model for this study was designed to be a standard building, which was not as accurate as a given residential building but reasonable and sufficient for this analysis. Characters of the house were determined based on the basic principles of energy flows in buildings and software computations associated with the capacity (load) of the ground source heat pump, 4 ton or 48,000 Btu/hr. To maintain comfort in a residential house, heating and cooling systems supply or remove heat at a rate roughly equaling heat’s flow rate through the building shell (Krigger & Dorsi, 2004). Heating energy demand in the residential house in heating season equals to the heat transmission losses through the floor, exterior walls, and ceiling, added to the air leakage, minus solar radiation gain and internal heat production; cooling energy needs in cooling season are determined by solar radiation, internal heat, air leakage, and heat transmission (Krigger & Dorsi, 2004). In the REM/Rate computations, heat transmission losses of the house were firstly calculated based on the heating and cooling demands (to maintain indoor temperature at 68ºF (20ºC) and 75ºF (24ºC) in heating and cooling season, respectively), the heat pump’s load, and the seasonal outdoor temperature in Minneapolis/St. Paul area. Then, the loss numbers were used as the index to design building parameters. The house was designed to be a universal model, which has square floor, same wall area on each of the four sides, and same window area in each wall. It has two stories above grade, one conditioned basement as foundation, four bedrooms with 3,072 sq. foot area of conditioned space and 27,648 cubic foot of conditioned space. In order to reach a fair 28 comparison, it was assumed that all the HVAC systems in this study were newly placed in the house model. Figure 6. Design process of the house model. First, given the heating and cooling demand (to maintain indoor temperature at 68ºF and 75ºF), the heat pump’s load (4 ton), and the seasonal outdoor temperature in Minneapolis/St. Paul, heat transmission loss in all directions was determined in REM/Rate software. Then a standard framework of residential house was built up in REM/Rate software according to the heat transmission loss number. The house was designed to be a universal model, whose construction materials and all parameters were selected to be standard, such as square floor, same wall area on each of the four sides, and same window area in each wall. After characterizing the house model, REM/Rate software helped output a simulation of space heating and cooling services by the GSHP systems and the conventional system. Since the heating and cooling supply equals to the heat transmission loss of the residential building, heating and cooling delivered by each HVAC system was easily determined. Then, according to the unique efficiency rate of each system, in-use energy consumed by each system was calculated and outputted by REM/Rate software. It is worth noting that the results from REM/Rate software 29 contained natural gas usages for water heating in each HVAC system, but a water boiler was actually not included in the system boundary of this LCA study. However, the water heating service is a key component in residential life and a true benefit of the GSHP system. By utilizing the extra heat from geothermal energy to satisfy domestic hot water demand, the GSHP system avoids additional natural gas consumption compared to the conventional HVAC system. Carbon savings are therefore attained as well. In order to effectively reflect this benefit of the GSHP system and fairly compare all the systems, an offset among natural gas usages for the water heating service in each system was calculated. Since the conventional system demanded the most natural gas for water heating (20,995 MJ), the differences between this number and that in each GSHP system were subtracted from the electricity usages in each GSHP system in the unit form of kWh. The adjusted energy use numbers are displayed in Table 7. Table 7. Annual energetic consumption for space heating and cooling in benchmark scenarios. REM/Rate software helped simulate annual electricity and natural gas usages in a typical Minnesotan single-family residential house model. All the numbers are based on the output from the REM/Rate software with some adjustments regarding the offset of natural gas usages for water heating. Gap values of natural gas usages between each GSHP system and the conventional HVAC system were deducted from total electricity usage in each of the GSHP systems. Horizontal GSHP Vertical GSHP Gas furnace & air conditioner (AFUE 92% & 13 SEER) 3.0 COP/ 14 SEER 5.0 COP/ 18 SEER 3.0 COP/ 14 SEER 5.0 COP/ 18 SEER Heating Electricity/kWh 9,893 5,548 8,758 6,086 526 Natural gas/MJ N/A N/A N/A N/A 96,216 Cooling Electricity/kWh 1,015 875 850 552 1,224 Natural gas/MJ N/A N/A N/A N/A N/A Direct electricity usages included in the system boundary of this study located in the assembly of GSHP units in the life stages of manufacture and operation. The US medium voltage electricity mix (Ecoinvent, 2004) was used for manufacturing the ground source heat pump unit in Watertown, New York, and the MISO (Midwest Independent 30 Transmission System Operator) real-time generation fuel mix in July 2012 (MISO, 2012a) was used in the operation stage to specifically and accurately reflect the local situations (Figure 7). Figure 7. MISO (Midwest Independent Transmission System Operator) real-time generation fuel mix in July 2012. Nonrenewable sources (coal, natural gas and oil) accounted for 85% of the mix. Vast carbon impacts from them made electricity an influential factor in determining life cycle GHG emissions of the HVAC systems. Sources: (MISO, 2012a) Similar to electricity, direct natural gas usages included in the system boundary also located in the assembly of GSHP units in life stages of manufacture and operation. The US burned-in-power-plant natural gas (Ecoinvent, 2004) was selected for manufacturing the heat pump unit in Watertown, New York, and the USA at-consumer natural gas (PE INTERNATIONAL, 2012) was used in the operation stage to reflect the local situations specifically and accurately. 31 4. Results 4.1. Life cycle assessment on benchmark HVAC systems Life cycle assessments on the benchmark HVAC systems, that is, GSHP_HL (horizontal GSHP system with low efficiency rate), GSHP_HH (horizontal GSHP system with high efficiency rate), GSHP_VL (vertical GSHP system with low efficiency rate), GSHP_VH (vertical GSHP system with high efficiency rate), and GF&AC (gas furnace and air conditioner) are firstly accomplished. 4.1.1. Life cycle inventory analysis (LCI) Life cycle inventory analysis consists of data collection, calculation, and allocation. In order to achieve an explicit inventory, information and data of every component are sorted into the four life stages: manufacture, transportation, installation, and operation. A clear checklist of the involved elements fills the foundation of the analysis. Based on the specific information and characteristics of each component included in the system boundary (Table 5 and 6), amounts of each material and energetic input are collected and organized accordingly in Table 8. 32 Table 8. Life cycle inventory of material and energy inputs in the benchmark systems. Material/energetic inputs in the manufacture stage are aggregated numbers based on the specifications in Table 6. In the transportation stage, fuel consumptions in each system were calculated based on the selected ocean freight/train/truck transport processes in GaBi 5 software and according to different cargo weights and travel distances. Diesel usage in the installation stage refers to on-site works: digging, drilling etc., while gasoline is used in necessary trips associated with the installation procedure. All numbers in the stages of manufacture, transportation and installation represent true material and energetic inputs during one-time activities. Numbers in the operation stage are 20-year values. Data sources: a) Shah et al. (2007); b) Greening and Azapagic (2012); c) Roth North America (2011); d) surveys with local residential GSHP system installers, such as Hass Geosystems Inc., Massmann Geothermal and Mechanical, and Summit Heating & Air Conditioning; e) house model and operational numbers outputted from REMRate software; f) Tianjin Jixin Industrial & Trade Co., Ltd. (2012); and g) Tianjin Junxing Pipe Group Co., Ltd. (2012). Gas furnace + air conditioner Horizontal slinky GSHP Vertical GSHP Low High Low High Manufacture Aluminum/kg a 26 0 0 0 0 Cast iron/kg c 0 18 18 18 18 Copper/kg a,b,c 20 33 33 33 33 Elastomere/kg b,c 0 15 15 15 15 Glycol/kg 0 92 92 46 46 HDPE/kg 0 174 174 87 87 Lubricating oil/kg b,c 0 3 3 3 3 Polyvinylchloride/kg b, c 0 2 2 2 2 R-134a/kg a, b, c 6 5 5 5 5 Steel/kg a, b, c 442 408 408 408 408 Electricity/kWh b, c 0 506 506 506 506 Natural gas/MJ b, c 0 1,575 1,575 1,575 1,575 Transportation (ocean freight, train and truck) Diesel/kg a, b, c, f, g 1 18 18 10 10 Installation Diesel/kg d 0 225 225 911 911 Gasoline/kg d 0 9 9 9 9 Operation (20 years) Electricity/kWh e 35,000 197,860 110,960 163,520 112,620 Natural gas/MJ e 1,924,320 0 0 0 0 33 4.1.2. Life cycle GHG emissions Life cycle GHG emissions of the four GSHP systems and the one conventional HVAC system were calculated based on various data sources. Ecoinvent Integrated and Professional Extensions databases provided applicable data in most processes, including extractions of raw materials, refineries of primary products, and productions of secondary products. GREET model 2011 contributed to the transportation stage by providing data on certain GHG emissions from fuel combustion. Results in all the systems are presented in Table 9 and Figure 8. Overall, total GHG emissions in the five systems range from 105,900 kg CO2 eq. to 186,100 kg CO2 eq. in the time period of 20 years, which equals to annual emissions ranging from 5,295 kg CO2 eq. to 9,305 kg CO2 eq. 34 Table 9. Life cycle GHG emissions (CO2 eq. kg) in the four GSHP systems and the one conventional HVAC system from aspects of material/energetic inputs and life stages. The GSHP_HL system emitted more GHG than any other systems, up to 80,200 kg CO2 eq. (76%) more than the smallest emitter GSHP_HH. The conventional system ranked right after the worst case. Systems with high efficiency rate (High) saved energetic usages and reduced CO2 emissions compared to those with low efficiency rate (Low). The vertical GSHP system performed better than the horizontal at low efficiency rate but worse at high efficiency rate. From the perspective of inputs, energetic inputs (electricity and natural gas) were responsible for the majority (more than 95% and 99% in the GSHP systems and the conventional system, respectively) of the total GHG emissions due to the huge amounts of their usages during the 20 years. This also resulted in the dominant status (93% to 99%) of the operation stage in total GHG emissions. The life stage of transportation was proven to have the least carbon impact even though cargo weights and travel distances seem quite big. Each system’s emissions numbers were close in the manufacture stage but quite various in the installation stage: the vertical systems emit the most due to the heavier on-site works compared to the horizontal. GHG emissions/ CO2 eq. kg Gas furnace + air conditioner Horizontal slinky GSHP Vertical GSHP Low High Low High Material and energetic inputs Aluminum 82 0 0 0 0 Cast iron 0 57 57 57 57 Copper 63 104 104 104 104 Elastomere 0 47 47 47 47 Glycol 0 292 292 146 146 HDPE 0 549 549 275 275 Lubricating oil 0 8 8 8 8 Polyvinylchloride 0 5 5 5 5 R-134a 19 15 15 15 15 Steel 1,395 1,286 1,286 1,286 1,286 Diesel 35 857 857 3,167 3,167 Gasoline 0 36 36 36 36 Electricity 32,315 182,792 102,559 151,088 104,087 Natural gas 131,345 86 86 86 86 Life stages Manufacture 1,559 2,558 2,558 2,138 2,138 Transportation 35 74 74 44 44 Installation 0 819 819 3,159 3,159 Operation 163,659 182,682 102,450 150,979 103,977 TOTAL 165,300 186,100 105,900 156,300 109,300 35 4.2. Sensitivity analysis (SA) In addition to the life cycle assessments on the basic HVAC systems, a sensitivity analysis based on the same LCA models was approached to specifically simulate practical situations in Minnesota. The task was accomplished through the four additional hybrid systems: Hybrid_HL (horizontal slinky GSHP with efficiency rate of 3.0 COP/14 SEER and gas furnace), Hybrid _HH (horizontal slinky GSHP with efficiency rate of 5.0 COP/18 SEER and gas furnace), Hybrid _VL (vertical GSHP with efficiency rate of 3.0 COP/14 SEER and gas furnace), and Hybrid _VH (vertical GSHP with efficiency rate of 5.0 COP/18 SEER and gas furnace). All the hybrids were composed of the basic GSHP systems and the gas furnace from the conventional HVAC system. 4.2.1. Life cycle inventory analysis (LCI) The sensitivity analysis started with a newly targeted life cycle inventory of all material and energetic inputs for the four hybrid systems. Similar to those in the inventory of the basic HVAC systems (Table 8), inputs numbers here were gathered from the elementary Table 5 and 6 and displayed in categories of life stages. In the first three stages, all the numbers represent one-time activities of manufacture, transportation, and installation. In the operation stage, different amounts of energetic usages were applied to analyze each hybrid’s sensitivity towards the changes of heating load. 36 Table 10. Life cycle inventory of material and energetic inputs in GSHP-GF hybrid systems. Numbers in the categories of manufacture, transportation, and installation represented one-time activities and slightly change compared to those in Table 8, because the gas furnace was the only added piece. In the operation stage, energetic usages cover the 20-year lifetime, and the percentages characterize the situation when the gas furnace accounts for 10%, 20% and 30% of the hybrid’s heating load. Horizontal slinky GSHP + gas furnace Vertical GSHP + gas furnace Low High Low High Manufacture Aluminum/kg 9 9 9 9 Cast iron/kg 18 18 18 18 Copper/kg 36 36 36 36 Elastomere/kg 15 15 15 15 Glycol/kg 92 92 46 46 HDPE/kg 174 174 87 87 Lubricating oil/kg 3 3 3 3 Polyvinylchloride/kg 2 2 2 2 R-134a/kg 5 5 5 5 Steel/kg 472 472 472 472 Electricity/kWh 506 506 506 506 Natural gas/MJ 1,313 1,313 1,313 1,313 Transportation Diesel/kg 18 18 10 10 Installation Diesel/kg 225 225 911 911 Gasoline/kg 9 9 9 9 Operation (20 years) Electricity/ kWh 10% 184,300 104,200 152,600 105,300 20% 170,600 97,400 141,600 97,900 30% 157,000 90,700 130,600 90,600 Natural gas/MJ 10% 156,800 156,800 156,800 156,800 20% 313,600 313,600 313,600 313,600 30% 470,400 470,400 470,400 470,400 37 4.2.2. Life cycle GHG emissions In the sensitivity analysis, life cycle GHG emissions of the hybrid HVAC systems were calculated based on the same data sources as those founding the LCAs on the benchmark HVAC systems. This sensitivity analysis focused on the hybrids helped recognize the practical situations in Minnesota. The life cycle GHG emissions of each hybrid were calculated and shown in Table 11 and also illustrated in Figure 8 along with the benchmark HVAC systems. Overall, total GHG emissions in the five systems range from 110,600 kg CO2 eq. to 184,500 kg CO2 eq. in the time period of 20 years, which equals to annual emissions ranging from 5,530 kg CO2 eq. to 9,225 kg CO2 eq. Similar to the situations of the basic GSHP systems, the vertical hybrid performed better than the horizontal at low efficiency rate but worse at high efficiency rate. In addition, the emissions numbers and influences of each input and life stage barely change – energetic usages and the operation stage are still the dominant. 38 Table 11. Life cycle GHG emissions in hybrid systems from aspects of material and energetic inputs and life stages. Hybrid _HH system provided the least life cycle GHG emissions of 110,614 kg CO2 eq., while the Hybrid _HL had the most life cycle carbon impacts of 184,500 kg CO2 eq. Majority (69% to 92%) of the carbon impacts came from electricity usages and 6% to 27% from natural gas usages in all the hybrids. Each hybrid’s sensitivity towards the changes of heating load can be revealed by looking at the emissions numbers according to the back-up load percentages (10%, 20% and 30%): the longer the gas furnace stays on load, the fewer GHG emitted by the Hybrid _HL hybrid system, however, the more by the other three hybrids. GHG emissions/ CO2 eq. kg Horizontal slinky GSHP + gas furnace Vertical GSHP + gas furnace Low High Low High Material and energetic inputs Aluminum 28 28 28 28 Cast iron 57 57 57 57 Copper 114 114 114 114 Elastomere 47 47 47 47 Glycol 292 292 146 146 HDPE 549 549 275 275 Lubricating oil 8 8 8 8 Polyvinylchloride 5 5 5 5 R-134a 15 15 15 15 Steel 1,488 1,488 1,488 1,488 Diesel 868 868 3,178 3,178 Gasoline 36 36 36 36 Electricity 10% 170,225 96,319 140,968 97,309 20% 157,659 90,079 130,848 90,531 30% 145,092 83,840 120,727 83,752 Natural gas 10% 10,788 10,788 10,788 10,788 20% 21,490 21,490 21,490 21,490 30% 32,192 32,192 32,192 32,192 Life stages Manufacture 2,798 2,798 2,377 2,377 Transportation 85 85 55 55 Installation 819 819 3,159 3,159 Operation 10% 180,818 106,912 151,560 107,901 20% 178,953 111,374 152,142 111,825 30% 177,089 115,837 152,724 115,749 TOTAL 10% 184,500 110,600 157,200 113,500 20% 182,700 115,100 157,700 117,400 30% 180,800 119,500 158,300 121,300 39 Results from the sensitivity analysis were summarized with those from the LCAs of the basic GSHP systems to compare the hybrids with their corresponding GSHP systems and to reveal each hybrid’s sensitivity towards certain changes in the percentages of back-up load (Table 12). The life cycle GHG emissions of the basic GSHP systems were introduced as baselines, to which the results of each hybrid were compared correspondingly. According to longitudinal comparisons, the hybrids, except the Hybrid _HL system, emitted more GHG emissions than their corresponding GSHP systems, ranging from 900 kg CO2 eq. to 13,600 kg CO2 eq. or from 0.58% to 12.84%, while the Hybrid _HL system reduces life cycle GHG emissions by 1,600 kg CO2 eq. to 5,300 kg CO2 eq. or 0.86% to 2.85%, compared to its corresponding GSHP_HL system. Besides, it was found that according to the same change in the percentage of back-up load (10% to 30%), the Hybrid _VL system emitted the fewest amounts (1,100 kg CO2 eq.) of additional GHG, followed by the Hybrid _HL system with 3,700 kg CO2 eq., the Hybrid _VH system with 7,800 kg CO2 eq., and the Hybrid _HH system with 8,900 kg CO2 eq. Table 12. Comparison of the life cycle GHG emissions (kg CO2 eq.) from the hybrid systems and their corresponding GSHP systems. In the table, 0% represents the corresponding GSHP system of each hybrid, and the numbers in the 0% row are set to be baselines. The numbers and percentages in the following rows (10%, 20% and 30%) are the cuts (-) and increments (+) based on the baselines. Clearly, The Hybrid _HL system was the only one to achieve GHGs cuts. Some universally applied trends were also revealed considering each hybrid’s sensitivity towards changes in the percentage of back-up load. With the same loop configuration (horizontal or vertical), the high efficiency hybrids were more sensitive to the percentage changes than the low efficiency ones. However, with the same efficiency rate (low or high), the horizontal hybrids were more sensitive than the vertical ones. In the categories of the shares, the longer the GF stayed on load, the fewer share electricity accounted for, while the more share natural gas took. Percent back- up load Hybrid _HL Hybrid _HH Hybrid _VL Hybrid _VH 0% 186,100 105,900 156,300 109,300 10% -1,600 (-0.86%) +4,700 (4.44%) +900 (0.58%) +4,200 (3.84%) 20% -3,400 (-1.83%) +9,200 (8.69%) +1,400 (0.90%) +8,100 (7.41%) 30% -5,300 (-2.85%) +13,600 (12.84%) +2,000 (1.28%) +12,000 (10.98%) 40 4.3. Comparison among benchmarks and sensitivity analysis results Based on the LCAs of all the HVAC systems, the life cycle GHG emissions results are displayed to compare each of the systems to another accordingly and reveal essential relations among them (Figure 8). Overall, the GSHP_HL system and the GSHP_HH system maintained to be the biggest and the smallest GHG emitters, respectively. They outlined a scope of total emissions for all. Within the scope, each of the hybrids specifically ranked between its corresponding basic GSHP system and the conventional system, for example, emissions number of the Hybrid _HL system ranked between those of the GSHP_HL and the GF&AC. At the level of low efficiency rate, the horizontal GSHP system emitted more GHG than the vertical GSHP system, but the situation reversed at the level of high efficiency rate. The hybrids followed this law as well. In addition to the total GHG emissions display, emissions proportions of all inputs are also illustrated with an adjusted y-axis is created (Figure 9), in which the ratios of every material and energetic inputs can be clearly identified, especially the minor shares of the material inputs. The total GHG emissions from life stage perspective are exhibited in a similar way – the y-axis is modified to explicitly illustrate the emissions numbers from all life stages (Figure 10). 41 Figure 8. Life cycle GHG emissions (CO2 kg eq.) of all the HVAC systems from material and energetic inputs perspective. Clearly, electricity and natural gas were the dominant emitters. Emissions from other inputs were inconspicuous. Findings from the sensitivity analysis can be more thoroughly observed and explained here: in each hybrid, as the gas furnace operates for more hours, carbon impact from electricity gradually declines and that from natural gas dramatically increases; however, the rates of the decline and the increase are not synchronized and therefore cause the inconsistent trends of total GHG emissions among the hybrid. More specifically saying, total GHG emissions reduce when the declining rate is higher than the growth rate (Hybrid_HL), and vice versa (the other three hybrids). 42 Figure 9. Proportions of each material and energetic input in the life cycle GHG emissions by HVAC systems. In this figure, the y-axis is designed to be non-linear with the lower part (0% to 5%) being enlarged to display the certain small proportions of the GHG emissions by the material inputs. More attention is drawn to the material inputs: diesel and steel were the biggest emitters among all material inputs, though they merely accounted for less than 3% of the total. The rest material inputs were existing but negligible. There were no huge variances in the emissions by steel among all the systems due to steel’s wide use in manufacture, while obvious differences existed in the emissions by diesel especially between the horizontal and vertical systems, which reflected the fact that the vertical loop system requireed more diesel usages in installation. 43 Figure 10. Life cycle GHG emissions (CO2 kg eq.) of all the HVAC systems from life stages perspective. In this figure, the y-axis is designed to be non-linear with the lower part (0 to 6,000) being enlarged to display the GHG emissions by the life stages of manufacture, transportation and installation. It is clearly shown in the figure that the operation stage is where the bulk of the total emissions occur, while the other life stages together account for a quite small share of the total emissions. Overall, the carbon impacts from manufacture were not changing much among all the systems, so were those from transportation. Condition was different with the carbon impacts from installation: the vertical GSHP-involved systems had much more GHG emissions than the horizontal ones, just as the observations on diesel in Figure 9. 4.4. Economic analysis Preliminary economic analyses are performed to examine the economic feasibility of all the HVAC systems. The task is accomplished through a present value (PV) analysis by calculating the capital cost (the installation cost included), the maintenance cost, the operational cost, and the available rebates of each system over 20 years (from 2012 to 2031). A discount rate of 3% and an annual fuel price escalation rate of 5.4% are used to in the PV calculations (Kleine, 2009) (Rushing, Kneifel, & Lippiat, 2011) (Meyer, Pride, O’Toole, Craven, & Spencer, 2011). 44 Estimates of the capital cost and the annual maintenance cost were obtained from the existing studies and manufacturer pricings (Kavanaugh, Gilbreah, & Kilpatrick, 1995) (Geothermal Systems, LLC, 2009) (Kleine, 2009) (Meyer, Pride, O’Toole, Craven, & Spencer, 2011) (EERE, US-DOE, 2011) (QualitySmith, 2012). Capital costs of individual components are from different years but all normalized to the year 2012. Price variance between the high-efficiency and the low-efficiency heat pumps was considered. It was assumed that the annual maintenance costs of the four GSHP systems are the same, so are all the hybrids. The annual energy cost was estimated based on the annual energy usages from the house model described earlier in this study and the average Minnesota energy prices in the past 10 years (2002-2011): electricity $0.1162/kWh (EIA, 2011b) and natural gas $13.14/mcf (EIA, 2011c). Each energy price range in the past 10 years was added to and deducted from the average price to estimate a high level ($0.1319/kWh and $18.71/mcf) and a low level ($0.1004/kWh and $7.57/mcf) of energy price. The three levels of energy prices were used to accomplish a sensitivity analysis on the fluctuations of energy prices. For the financial incentives, the conventional HVAC system received a $150 rebate for the 92% AFUE gas furnace (CenterPoint Energy, 2012) but no rebate for the air conditioner due to its low 13 SEER. All the GSHP- involved systems obtained capital deduction from the federal personal tax credit of 30% and the average rebate of $1,600 ($400/ton) from Minnesota local utilities (DSIRE, 2012a) (EIA, 2010) (Minnesota Public Utilities Commission, 2012a) (Minnesota Public Utilities Commision, 2012b). Overall, the life cycle costs of the systems were found to range between $33,900 and $49,200 (Table 13). Seen from each cost and rebate (Figure 11), the average capital 45 cost of the GSHP systems is more than three times of that of the conventional system, ($24,000 compared to $7,600). The hybrids are even more expensive due to their natures of combining the GSHP and the gas furnace. For the maintenance cost, the GSHP systems show their benefits over the conventional system, $1,800 compared to $4,200. For the operational cost, the conventional system is the most expensive among all by costing $35,200 over 20 years. Besides, the GSHP-involved systems, whose operational expenses are largely influenced by heat pump efficiency rates, cost from $16,400 to $29,800. The higher the efficiency rate, the greater realized savings are. Basic GSHPs can save small amounts of operational costs compared to their corresponding hybrids. For the rebates, the $150 rebate for gas furnace is too negligible compared to the 30% and $1,600 subsidies for the GSHP-involved systems, which greatly benefit them. Targeting at the long-term projection (20 years) in this study, the energy rate’s 10- year averages ($0.1162/kWh and $13.14/mcf) and spectrums ($0.1004/kWh to $0.1319/kWh and $7.57/mcf to $18.71/mcf) successfully indicated that the GSHP technologies were more cost-effective than the conventional system in most cases with average and higher energy rates. However, no GSHPs, but the GSHP_HH, were preferred than the conventional when natural gas price reaches the low level of $7.57/mcf. In fact, the unsteady and historically low natural gas price ($3.00 - $5.00/mcf (Center Point Energy, 2012)) in recent short term has made the GSHP technologies unfavorable. In this case, as the gas price drops to $3.00/mcf in Minnesota (Center Point Energy, 2012), the conventional system’s 20-year expense is propelled to $23,600, which is even less than the tag price of the high-efficient GSHP units. Compared to the fluctuating gas price, the relatively stable electricity rate nails the life-cycle cost of GSHP 46 technologies at a relatively high level, unless the GSHPs could receive subsidies from perspectives of capital investment, in-use electricity rate, and carbon tax. Table 13. Life cycle costs PV (present value) of each system GSHP systems and high-efficient hybrids can save money over the conventional system. For the low- efficient hybrids, only the vertical 10% and 20% ones achieve life cycle net savings. Each hybrid costs more than its corresponding GSHP. The longer the gas furnace operates, the more the hybrid costs. GF&AC GSHP_HL GSHP_HH GSHP_VL GSHP_VH $46,800 $42,700 $33,900 $40,200 $36,600 Hybrid_HL Hybrid_HH Hybrid_VL Hybrid_VH 10% $47,800 $40,000 $45,700 $42,600 20% $48,300 $41,500 $46,500 $44,000 30% $48,800 $42,900 $47,400 $45,400 Figure 11. Life cycle costs PV (present value) of each system. Capital cost of the GSHP technologies is a significant weak point, but it can benefit the operational cost: the higher priced the system is, the more energy bills it saves. The variance bars represent the life cycle costs range influenced by energy price changes, which were suggested to be influential drivers of the total costs of each system. The energy rate’s averages ($0.1162/kWh and $13.14/mcf) and spectrums ($0.1004/kWh to $0.1319/kWh and $7.57/mcf to $18.71/mcf) were based on the 10-year (2002-2011) rate range in Minnesota (EIA, 2011b) (EIA, 2011c). These 10-year numbers were selected to suit the long-term projection (20 years) of this study. In the long- term lifetime, the conventional system costs more than most GSHP technologies with the average and high level of the energy rates, but it starts to save money over the GSHPs, except the GSHP_HH, when natural gas price reaches the low level $7.57/mcf. In fact, as the gas price drops to its historically lowest level of $3.00/mcf (Center Point Energy, 2012) in Minnesota, the conventional system’s 20-year expense is reduced to $23,600, which makes the GSHP technologies unfavorable in any case. 47 5. Discussions and future work 5.1. Scenarios design Scenarios design is the cornerstone of this study. The scenarios, including the conventional system (92% AFUE natural gas furnace and 13 SEER air conditioner) and the GSHP scenarios (horizontal and vertical loop configurations, 3.0 COP/14 SEER and 5.0 COP/18 SEER heat pump efficiency rates), were designed based on this study’s motivations, which were driven by the fact of cold climates in Minnesota and more importantly the policies and incentives associated with residential GSHP technologies. In the processes of designing the house model and simulating the annual energy consumption of each scenario in REM/Rate software, some parameters were theoretically assumed being constant, like the efficiency rate of the heat pump, or excluded, like the energy loss by the circulating pump. In practice, however, the COP/SEER of the heat pump dynamically changes along with the changes of the external temperature or the internal operating indexes, for example, the compressor speed (Madani, Ahmadi, Claesson, & Lundqvist, 2010); energy usage by the circulating pump is also a factor that cannot be ignored. Therefore, future field study is necessary for improving and extending the research dimensions. Various house sizes, monitored energy usages, and variable efficiency rates are recommended to help amend the assumptions and accomplish more accurate examinations on the scenarios. Results from the life cycle GHGs and costs analyses suggested that the high- efficient GSHP scenarios played a better role in reducing carbon emissions and economic expenditures, compared to the conventional system and their corresponding low-efficient scenarios, because they were able to provide outstanding carbon and economic performances in operation even with intensive capital costs. Promotions of the high- 48 efficient units, however, may run into market barriers because of consumers’ unwillingness to invest in energy efficiency without receiving substantially higher returns on their investment (Golove & Eto). High capital costs straightforwardly put obstacles in the consumers’ way of purchasing high-efficient units in the first place, even though the units are capable of reducing energy bills and emissions later in use phase. Overall, with their deserving low-carbon and low-cost statuses, the high-efficient scenarios appeared worth being incentivized by additional subsidies in the future. That is to say, the subsidies attempting to incentivize low-carbon designs should be better focused on high- efficient GSHP units, especially if future studies corroborate the necessity of back-up heating systems. The results also indicated that the horizontal GSHP designs have better carbon and economic performances than the vertical when coupled with high-efficient units but worse when with low-efficient units. Specific research models and available data sources in this study led to the certain results and revealed the reversed outcomes from the high and low efficiency units. Further examinations on GSHP system designs are recommended, in terms of loop configurations, hybrid operational strategies, and geological and hydrogeological conditions. In this study, the life cycle system boundary closed up after the operation stage and excluded the disposal of all HVAC units due to the lack of empirical data. Environmental impacts of replacing and disposing each component, especially the refrigerants in heat pump and air conditioner, may not be huge because of their few amounts of usage, but the refrigerants truly have significant GHG effects per unit. 49 Therefore, it is recommended that future studies could explore the end-of-life stage of each unit with appropriate hypothesis and empirical data. 5.2. Electricity fuel mix In the assumed lifetime of 20 years, life cycle GHG emissions of all the systems have been investigated and compared. It was found that although the GSHP equipment emits zero GHG during operations, electricity with huge GHG impacts from generation still drew back the systems’ overall carbon emissions performances. Sourced from the MISO grid, the electricity consumed in GSHP operations had a severe non-renewable fuel mix: 68% by hard coal and 15% by natural gas (Figure 7). This non-green fuel mix directly led to the fact that vast carbon impacted come from electricity: more than 95% in the basic GSHP systems and more than 70% in the hybrids. Improvement in the fuel mix, for example, replacing the fossil fuels by clean energy like wind, obviously can impact the consequences of GHG emissions and achieve carbon savings. It is unrealistic for the GSHP systems in Minnesota to achieve zero GHG impacts like those in Oregon due to the absolute hydropower (Shah, Col Debella, & Ries, 2007), but progress towards cleaner energy can be made starting from the 68% coal and 15% natural gas to improve GHG performances of GSHPs. Exponential wind capacity growth from 2006 in the MISO (MISO, 2012b) can help realize some GHG emissions reductions. However, improvement in greening the fuel mix could raise electricity prices, especially in the regions that are lack of renewable resources for power generation. Appropriate discounts on residential electricity rates could be one solution to promote the cleaner power. Therefore, rational policies on leveraging the fuel mix to meet both 50 environmental and economic demands would be a challenging task and essential drivers for development of residential GSHP technologies. 5.3. Policies and economics The preliminary economic analysis was performed to examine the economic feasibility of all the systems by calculating and comparing the present value of their life cycle costs. Subsidized by certain financial incentives, GSHP technologies were suggested to be able to compete over the conventional system in residential HVAC market. It is worth noting that the subsidies were all applied to the capital cost, which benefited the implementations of GSHP technologies from the purchasing perspective. However, with increasing concerns of environmental impacts of the technologies, economic incentives should also influence the operation phase of GSHP systems. For example, certain subsidies should be incentivized to the consumers who choose to power their GSHP with the electricity from greener grid. Economic incentives are driven by policies. Therefore, how to leverage the incentives to promote the GSHP development should be a next-step task for policy makers. The policy makers are recommended to comprehensively promulgate and implement policies and incentives from perspectives of residential HVAC industry and related energy market. Environmental and economic benefits of GSHP technologies could be optimized by future policies such as progressively subsidizing residential GSHP applications, especially the high-efficient units, improving the electricity mix and providing discounts to the consequent high electricity rate. Low electricity rates will benefit not only the GSHP technologies but also other household electric appliances, such as fridges and TVs. However, too low prices would lead to more consumption by homeowners and heavier burden on the entire grid. In addition to electricity, attentions 51 should also be paid to the increased importance of natural gas in recent years in energy market. Driven by new extraction technologies and historically low prices, natural gas could become preferred by both end-use consumers and electricity fuel mix. With the historically low short-term gas price of $3.00 - $5.00/mcf in Minnesota (Center Point Energy, 2012) and possibly lower rates in near future, promotions of GSHP technologies would hit barriers if homeowners’ swing towards cheap natural gas made GSHPs’ direct substitution of residential natural gas furnace hardly possible and carbon benefits from the GSHPs were offset by heavy carbon impacts from increasing natural gas’ share in the grid mix. The situation could get better for GSHPs if carbon market was well regulated with accurately priced carbon tax. This economic benefit gained from the GSHPs’ fewer GHG emissions could help reduce their life cycle costs and promote their popularizations. 5.4. Model applicability in other regions In this study, the scenarios of HVAC systems were all designed to simulate the practical situations in Minnesota. However, the same GSHP technologies and hybrid selections may be ineffective in other regions across the country due to local climatic and geological conditions. Vertical configuration may be challenged in regions with hard rocks in shallow underground or weak ground foundation. Geothermal resource availability may also affect the heating and cooling performance of the GSHP technologies. The geothermal resource can be treated as the “electric energy” stored in underground soils (rechargeable battery), whose charge and storage availabilities are influenced by weather and soil moisture. Weather impacts the soil’s absorption of solar radiation, which is the “recharging” process. Soil moisture impacts the availability of storing the energy. Overall, the wetter the soil is, the more available the geothermal 52 resources are. The furnace-backup hybrid is valuable in cold climate regions, such as Minnesota, but may be unnecessary in mild climate places, and an AC-backup hybrid may be needed in the hot climate regions. Future work is recommended to cover these topics and assess the model applicability in other regions. 6. Conclusion The primary goal of this study was to determine the life cycle (20 years) GHG emissions and economic costs of the residential HVAC systems in Minnesota. Four basic GSHP systems and one conventional HVAC system were firstly evaluated as benchmarks. Then four hybrid systems were investigated to simulate the actual residential heating and cooling situations in Minnesota. In the overall GHG emissions range of 1.09 ! 105 to 1.86 ! 105 kg CO2 eq., the environmental LCA suggest that six of the eight GSHP technology scenarios had fewer carbon impacts than the conventional. The increased GHGs only occurred in the horizontal low-efficient GSHP and hybrid. The economic analysis found that GSHP technologies could be cost competitive with effective financial incentives to help reduce their high up-front capital costs. The hybrid scenarios, specifically designed for Minnesota, were proven able to reduce life cycle GHG emissions and lower homeowners’ total costs compared to the conventional system. Results also suggest that high-efficient GSHP and hybrid scenarios cost more at capital phase than the low- efficient scenarios, but they can achieve life cycle GHG reductions and economic cutbacks. Regarding the loop configurations, the horizontal GSHPs and hybrids performed better than the vertical at high-efficient level but worse at low. Therefore, 53 horizontal high-efficient scenarios have the top environmental and economic performances among all. The regional electricity fuel mix and fluctuating energy prices were found to be influential drivers of the benefits of employing GSHP technologies in Minnesota from both environmental and economic perspectives. To what extent the GSHP technologies will be leveraged in the future depends on the management of these two key drivers. It is worthy noting that with the historically low natural gas price in 2012, the conventional system’s energy bill cutback could be large enough to propel its life-cycle cost fewer than the GSHPs. As a result, the environmentally favorable GSHP technologies would become economically unfavorable, unless they are additionally subsidized. 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Applied Energy (87), 16-27. 60 Appendices Appendix A: Applicable incentive programs for residential GSHP systems in Minnesota Program Incentive amount & cap Requirements Supplementary Federal – Personal Tax Credit Residential Renewable Energy Tax Credit • 30% - unlimited of cost • Installation cost included • $2,000 incentive cap for GSHP systems placed in service before 12/31/2008 • Existing and new houses qualified • Rentals not qualified • Closed loop: EER ! 14.1; COP ! 3.3 • Open loop: EER ! 16.2; COP ! 3.5 • Validation: 1/1/2006 – 12/31/2016 Minnesota – PACE Financing Local Option – Energy Improvement Financing Programs • Funding source: Implementing entities authorized to issue revenue bonds Minnesota – State Loan Program Neighborhood Energy Connection (NEC) Minnesota Energy Loan Program • Varies by project • Up to $35,000 • Loan terms from 1-20 years at a fixed rate of 5.75% • Maximum household income of $96,500 • Funding source: Minnesota Housing and Finance Agency (MHFA) Minnesota – Utility Loan Program Minnesota Valley Electric Cooperative – Residential Energy Resource Conservation Loan Program • Up to $5,000 • Rate: 5% • Repayment: up to 5 years • Available for replacement items only • Minimum monthly payments $25 • Minimum of 6 months membership of MVEC before qualified for ERC loan • Available for Energy Wise program(s) members • Funding source: Otter Tail Power Company – DollarSmart Energy Efficiency Loan Program • $150 - $20,000 • Competitive Plan Rate: 8.5% • Energy Efficiency Plan Rate: 1.9% • Loans fewer than 80% of the project's total cost • Maximum payment terms: 60 months for loans of $5,000 or more • Funding source: Minnesota – Utility Rebate Program Great River Energy (28 Member Cooperatives) – Commercial and Industrial Efficiency Rebates Varies • Funding source: Great River Energy Alliant Energy Interstate Power • $300/ton + $50/EER/ton • 240,000 Btu or 20 tons or less in • Funding source: 61 and Light (Electric) – Residential Energy Efficiency Rebate Program a system size • Closed loop: EER !14.1 • Open loop: EER !16.2 • Alliant Energy must supply primary heating energy • Home built before 1/1/2001 • Renters need owner’s prior approval (Alliant Energy provides the service to contact owner) Alliant Energy Interstate Power and Light (Gas) - Residential Energy Efficiency Program • Closed loop: ($300 " tons) + [(EER - 14.1) " $50 " tons] • Open loop: ($300 " tons) + [(EER - 16.2) " $50 " tons] • Desuperheater $100 • Maximum 180,000 Btu or 15 tons in system size • Closed loop: EER ! 14.1; COP ! 3.3 • Open loop: EER ! 16.2; COP ! 3.5 Austin Utilities (Gas and Electric) – Residential Conserve and Save Rebate Program • $200/ton plus an additional rebate of $25/ton for each 1 EER above the minimum qualifying efficiency • Maximum $100,000 per customer location per technology per year • Closed-Loop Water-to-Air: EER ! 17.1; COP ! 3.6 • Closed-Loop Water-to-Water: EER ! 16.1; COP ! 3.1 • Open-Loop Water-to-Air: EER ! 21.1; COP ! 4.1 • Open-Loop Water-to-Water: EER ! 20.1; COP ! 3.5 • Direct GeoExchange (DGX): EER ! 16.0; COP ! 3.6 • Effective from 1/1 to 12/1 in respective calendar year • Rebates will be awarded on a first-come, first- served basis • Renewed from the program listed in DSIRE database Connexus Energy – Residential Efficient HVAC Rebate Program • $400/ton • New GSHPs purchased and installed between 1/1/2012 and 12/31/2012, or while funds last, within Connexus Energy electrical service area • Limit one rebate per member account • Renewed from the program listed in DSIRE database Crow Wing Power – Residential Energy Efficiency Rebate Program • $100/ton • Dual fuel heating systems: GSHP system backed up by an automatic fossil fuel source (natural gas, propane, or fuel oil) • Dual Fuel meter and load control • Closed loop: EER ! 14.1 • Open loop: EER ! 16.2 • Funding source: • Rates: $0.053/kwh (includes PCA effective 1/1/2012) Dakota Electric Association – Residential Energy Efficiency Rebate Program • $400/ton • Funding source: 62 East Central Energy (ECE) – Residential Energy Efficiency Rebate Program • $400/ton • Limit one rebate per member account • GSHP must be installed in 2012 within ECE territory • Funding source: • Rebates will be awarded on a first-come, first- served basis Elk River Municipal Utilities – Residential Energy Efficiency Rebate Program • $400/ton • Limit one rebate per member account • Must be closed loop GSHP • System must be bought in 2012 and installed within Elk River Municipal Utilities service territory • Funding source: Fairmont Public Utilities - Residential Energy Efficiency Rebate Program (Renewed in 2012) • $200/ton plus an additional rebate of $25/ton for each 1 EER above the minimum qualifying efficiency • Closed-Loop Water-to-Air: EER ! 17.1; COP ! 3.6 • Closed-Loop Water-to-Water: EER ! 16.1; COP ! 3.1 • Open-Loop Water-to-Air: EER ! 21.1; COP ! 4.1 • Open-Loop Water-to-Water: EER ! 20.1; COP ! 3.5 • Direct GeoExchange (DGX): EER ! 16.0; COP ! 3.6 • Funding source: • Effective from 1/1/2012 • Renewed from the program listed in DSIRE database Lake Country Power – Residential Energy Efficiency Rebate Program • $400/ton • Limit one rebate per member account • GSHP must be installed in 2012 within Lake Country Power service territory • Funding source: • Effective from 1/1/2012 • Renewed from the program listed in DSIRE database Lake Region Electric Cooperative – Residential Energy Efficiency Rebate Program • Closed loop: $200 - $400/ton • Open loop: $100/ton • Maximum 20 ton in system size • Limit one rebate per member account • Rebate application must be received by 12/15/2012 • Funding source: • Effective in 2012 • Renewed from the program listed in DSIRE database Marshall Municipal Utilities – Residential Energy Efficiency Rebate Program • $200/ton • Funding source: Minnesota Power – Residential Energy Efficiency Rebate Program • Closed loop: $150/ton • Open loop: $250/ton • $200 Electronically Commutated Motor (ECM) rebate only applies in new GSHP installation not replacement of existing units • Up to $1,400 rebate plus tax credit • Closed loop: COP ! 3.3 • Open loop: COP ! 3.6 • Validation: 1/1/2006 – 6/30/2016 • Funding source: • Renewed from the program listed in DSIRE database 63 (some restrictions may apply) Minnesota Valley Electric Cooperative (MVEC) - Residential Energy Efficiency Rebate Program • $200/ton • Maximum 5 ton in system size • Limit one rebate per member account • GSHP must be installed in 2012 within MVEC's service area • Must be new GSHP system purchased no earlier than 1/1/2009 • Funding source: • Renewed from the program listed in DSIRE database Northern Municipal Power Agency – Residential Energy Efficiency Rebate Program • New installation: ! Closed loop: $400/ton, maximum $5,000/home ! Open loop: $200/ton, maximum $2,500/home • Replacement: ! Closed loop: $200/ton, maximum $2,500/home ! Open loop: $100/ton, maximum $1,250/home • New installation and replacement: ! Closed loop: 16.2 EER/3.6 COP, system size < 135,000 Btuh @ 59ºF ! Open loop: 14.1 EER/3.3 COP, system size < 135,000 Btuh @ 77ºF • Funding source: • Renewed from the program listed in DSIRE database Otter Tail Power Company – Residential Energy Efficiency Rebate Program • $600/ton • Closed loop: Water-to-Air COP ! 3.6; Water-to-Water COP ! 3.1 • Open loop Water-to-Air COP ! 4.1; Water-to-Water COP ! 3.5 • Direct GeoExchange (DGX): COP ! 3.6 • Owatonna Public Utilities – Residential Conserve and Save Rebate Program • $250/ton + $150/EER over the minimum efficiency requirement • Desuperheater $250 • Water-to-Air: EER ! 16.1; COP ! 3.5 (2 stages) • Water-to-Water: EER ! 15.1; COP ! 3.0 • Funding source: • Renewed from the program listed in DSIRE database Rochester Public Utilities – Residential Conserve and Save Rebate • $200/ton + $150/EER over the minimum efficiency requirement • Desuperheater $250 • Water-to-Air: EER ! 17.1; COP ! 3.6 (2 stages) • Water-to-Water: EER ! 16.1; COP ! 3.1 • Renewed from the program listed in DSIRE database Stearns Electric Association – Residential Energy Efficiency Rebate Program • $200/ton • Maximum 5 ton in system size • Must have a dual fuel program (a full size backup heating system) • Limit one rebate per member account • GSHP must be installed within Stearns Electric Association service territory • Rebate application must be received • Funding source: • Renewed from the program listed in DSIRE database • Rates: $0.042/kwh 64 by 12/17/2012 Wright-Hennepin Cooperative Electric Association – Residential Energy Efficiency Rebate Program b Xcel Energy (Electric) – Residential Energy Efficiency Rebate Programs • $200/ton • Closed loop with EER ! 14.1 • Maximum 5 ton in system size • Must include an air cooling application • Newly purchased and installed GSHP system between 1/1/2012 and 12/31/2012 • Limit one rebate per member account • GSHP must be installed within Xcel Energy service territory • Rebate application must be received by 7/21/2013 • Funding source: • Renewed from the program listed in DSIRE database Note: a Joint with the next program; b Expired program. 65 Sources: Federal Residential Renewable Energy Tax Credit: http://dsireusa.org/incentives/incentive.cfm?Incentive_Code=US37F&re=1&ee=1 http://www.energystar.gov/index.cfm?c=tax_credits.tx_index Minnesota Local Option – Energy Improvement Financing Programs: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN113F&re=1&ee=1 http://mn.gov/commerce/energy/topics/financial/PACE.jsp Minnesota Neighborhood Energy Connection (NEC) Minnesota Energy Loan Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN113F&re=1&ee=1 http://thenec.org/financing/minnesota-energy-loan Minnesota Valley Electric Cooperative – Residential Energy Resource Conservation Loan Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN65F&re=1&ee=1 http://www.mvec.net/residential/financing/ Minnesota Otter Tail Power Company – DollarSmart Energy Efficiency Loan Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN33F&re=1&ee=1 https://www.otpco.com/ProductsServices/Pages/Financing.aspx Great River Energy (28 Member Cooperatives) – Commercial and Industrial Efficiency Rebates: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN119F&re=1&ee=1 http://www.greatriverenergy.com/savingelectricity/energyefficiency/ Alliant Energy Interstate Power and Light (Gas) – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN160F&re=1&ee=1 http://www.alliantenergy.com/SaveEnergyAndMoney/Rebates/HomeMN/030052 Austin Utilities (Gas and Electric) – Residential Conserve and Save Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN46F&re=1&ee=1 http://www.austinutilities.com/files/pdf/CI%20Heat%20Pumps%2012_www.pdf Connexus Energy – Residential Efficient HVAC Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN125F&re=1&ee=1 http://www.connexusenergy.com/pdfs/GSHP_Rebate.pdf Crow Wing Power - Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN75F&re=1&ee=1 http://www.cwpower.com/dualfuel.shtml Dakota Electric Association – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN70F&re=1&ee=1 https://www.dakotaelectric.com/residential/programs/rebates/residential_heating_and_cooling_rebates East Central Energy (ECE) – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN69F&re=1&ee=1 http://www.eastcentralenergy.com/PDFs/resgshprebate.pdf Elk River Municipal Utilities – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN50F&re=1&ee=1 http://www.elkriverutilities.com/pdfs/rebates/2012_res_geo_thermal_grnd_source_heat_pump.pdf Fairmont Public Utilities - Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN82F&re=1&ee=1 http://www.smmpa.org/upload/2012RESrebatesummaryFM.pdf 66 Lake Country Power – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN68F&re=1&ee=1 http://www.lakecountrypower.coop/userfiles/2012%20Rebate%20Form%20GSHP%20Rebate.pdf Lake Region Electric Cooperative – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN133F&re=1&ee=1 http://www.lrec.coop/Documents/OnlineForms/2012GSHPrebateApplication.pdf Marshall Municipal Utilities – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN51F&re=1&ee=1 http://www.marshallutilities.com/residential/heatingandcooling.php Minnesota Power – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN25F&re=1&ee=1 http://www.mnpower.com/powerofone/one_home/energystar/special_offers/index.php Minnesota Valley Electric Cooperative (MVEC) - Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN64F&re=1&ee=1 http://www.mvec.net/wp-content/uploads/2011/09/rebates-gshp.pdf Northern Municipal Power Agency – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN163F&re=1&ee=1 http://www.minnkota.com/Conservation/Documents/Incentive%20Forms/Residential%20Prescriptive.pdf Otter Tail Power Company – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN151F&re=1&ee=1 https://www.otpco.com/SaveEnergyMoney/Pages/Rebates.aspx Owatonna Public Utilities – Residential Conserve and Save Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN55F&re=1&ee=1 http://www.owatonnautilities.com/residential-customers/residential-rebates/heat-pumps-geothermal Rochester Public Utilities – Residential Conserve and Save Rebate: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN112F&re=1&ee=1 http://www.rpu.org/documents/2012%20Electric%20Efficiency%20Rebate%20Application.pdf Stearns Electric Association – Residential Energy Efficiency Rebate Program: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN129F&re=1&ee=1 https://www.stearnselectric.org/energywiserebatesres.htm https://www.stearnselectric.org/Forms/2012GroundSourcerebateformfillable.pdf Xcel Energy (Electric) – Residential Energy Efficiency Rebate Programs: http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=MN39F&re=1&ee=1 http://www.xcelenergy.com/Save_Money_&_Energy/For_Your_Home/Heating_&_Cooling/Ground_Sourc e_Heat_Pump_-_MN http://www.xcelenergy.com/staticfiles/xe/Marketing/Files/MN-Res-GSHP-Rebate-Application.pdf 67 Appendix B: LCA primary data collections and GHG calculations Table B-1. Manufacture of the components (air conditioner, gas furnace, ductwork, and circulating pump) 68 Table B-2. Manufacture of the components (heat pump, HDPE pipes, glycol) 69 Table B-3. Diesel usage 70 Table B-4. Gasoline usage 71 Table B-5. Natural gas usage 72 Table B-6. Electricity generation according to the MISO fuel mix in July 2012 73 Table B-7. Electricity usage 74 Appendix C: Economic analysis primary data collections and calculations Table C-1. Primary data for basic (default) analysis Table C-2. Primary data (energy prices) for sensitivity analysis 75 Table C-3. Basic (default) analysis calculation. Electricity: $0.1162/kWh. Natural gas: $13.14/mcf 76 Table C-4. Sensitivity analysis calculation (high energy prices). Electricity: $0.1319/kWh. Natural gas: $18.71/mcf 77 Table C-4. Sensitivity analysis calculation (high energy prices). Electricity: $0.1004/kWh. Natural gas: $7.57/mcf