Browsing by Subject "Geothermal"

Now showing 1 - 3 of 3
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    An Investigation into the Seasonal Economic and Energy Performance of CO2 Plume Geothermal (CPG) Power Plants
    (2015-08) Peterson, Margaret
    CO2 Plume Geothermal (CPG) energy production is a renewable form of energy that combines geothermal technology with CO2 sequestration, using the CO2 as the working fluid in naturally permeable thermal reservoirs. In this thesis, we compare the energy and economic performance of an electricity production only CPG plant, as well as CPG plants with that of a combined heat and power (CHP) and district heating cooling (DHC). Initially, the monthly economic parameters of electricity-only CPG power plants are modeled for six cities: Williston, ND, Dallas, TX, New Orleans, LA, Houston, TX, Sacramento, CA, and Williamsport, PA. Meteorological data for each city are used to determine energy production and electric power is assumed to be sold in a competitive market. The monthly economic performance of each plant is compiled over 20 years, the assumed lifetime of a CPG plant, and used to determine each plant's potential for profit. It is found that it is crucial to consider location when determining the economic potential of CPG plants. Cool climates tend to result in higher electricity production as a result of a higher thermodynamic plant efficiency; however, it is also necessary to consider the economic environment, as electricity prices can have just as much of an impact, if not more, on a plant's financial performance. CPG power plants are also found to be economically competitive with other renewable energy options at the same capacity level and current CO2 sequestration and tax incentives can make unfavorable CPG power plants profitable. Next, CPG CHP DHC plants are considered, and three cases of heat production are investigated. Case 1 assumes the system meets peak winter heat demand, Case 2 assumes that some form of thermal storage is available and the system meets average monthly heat demand, and Case 3 assumes that all possible heat produced during winter months is sold. Electricity and heat are assumed to be sold in a competitive market. Six cities are considered, Williston, ND, Dallas, TX, New Orleans, LA, Houston, TX, Sacramento, CA, and Williamsport, PA, spanning 4 of the 5 US climate zones (Zones 1, 2, 3, and 5). Meteorological data are used to estimate energy production and heat demand. CPG is found to produce CO2 at high enough temperatures to be used in a district heating system. Case 1 most closely matches actual demand ratios for power vs heat in the various cities. CPG CHP/DHC plants in cities located in Zone 1 and Zone 2 climates have a higher net present value (NPV) than electricity-only plants. Case 2 and Case 3 CPG CHP/DHC plants in Zone 3 and Zone 5 can have a higher NPV than electricity only, but more consideration must be given to heat demand to ensure profit is increased. In all cities considered, tax credits and CO2 sequestration benefits can increase financial performance of CPG CHP/DHC plants.
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    On the Power Performance and Integration of Carbon-dioxide Plume Geothermal (CPG) Electrical Energy Production
    (2015-05) Adams, Benjamin
    CO2 Plume Geothermal (CPG) energy is a method for producing electricity from heat extracted from hot rock layers or reservoirs deep within the earth's crust. CPG is differentiated from other geothermal technologies by several factors: 1) CPG uses CO2 as the primary geologic working fluid instead of brine, 2) CPG utilizes naturally permeable porous reservoirs to extract heat, such as saline aquifers or depleted hydrocarbon reservoirs, 3) CPG is deep--a CPG reservoir must have a depth of 1 km to maintain CO2 in its supercritical state; though depths of 2 to 5 km are more common, and 4) CPG utilizes reservoirs at common geologic temperature gradients, unlike traditional hydrothermal which utilizes shallow reservoirs of unusually high temperature. Thus, CPG is intended to be integrated into an existing CO2 sequestration site affording an economic return on CO2 capture expenses by providing carbon-neutral, dispatchable electricity. Even when CPG is used as a base-load power source, it correlates well with electrical demand, unlike wind and solar (Chapter 5). Typically, CPG configurations consist of one or more injection and production wells. These wells link the surface plant with the porous reservoir to create a fluid circuit. Cooled fluid is injected at the surface, heated within the reservoir, and then returned to the surface at higher temperature and pressure which can then be used to create electricity. The variation in CO2 density between injection and production wells creates a thermosiphon which can drive circulation of CO2 without the use of pumps (Chapter 2). The geologic CO2 can be passed directly through a turbine, called a direct system, or heat can be extracted and used to power an Organic Rankine Cycle, called an indirect system. Either system may be used to generate electricity, although a direct system will nearly always produce more electricity than the indirect system. With reservoirs at moderate depth and temperature, these direct systems will also produce more electricity than comparable brine hydrothermal systems (Chapter 3). The reservoir well spacing and diameter affect the average power and longevity of a CPG system. For every combination of well diameter and reservoir depth, temperature, permeability, and thickness, an optimum spacing between the central injection well and a circumferential collection well will provide the greatest power output over time; placing the collection well too close to the injection well depletes the reservoir too quickly while spacing it too far away increases pressure losses, decreasing the overall power (Chapter 4). Likewise, the selection of too small a well diameter will limit mass flowrate, and thus power, while an oversized well diameter may quickly deplete the reservoir and provide no additional benefit (Chapters 3 & 4). This research has provided a significantly deeper understanding of CPG power systems and their operation. The impact of this work is to establish a basis of CPG research to be used in several ways. It can directly inform industrial developments, such as a green-field implementation of CPG or the long-term planning of a CPG-ready Carbon Capture and Storage site. This work may also be the basis for future economic or policy analyses that can further argue for the development of CPG. Thus, this work will help enable CPG as part of the 21st century energy portfolio.
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    Optimizing borehole Heat exchanger spacing to maximize advective heat transfer
    (2013-09) Meester, Jennifer
    This generalized study provides first order insights into the effect of heat advection on ground source heat pump (GSHP) system operation and the optimization of spacing between borehole heat exchangers (BHE) using groundwater flow and heat transport models. In these systems, there is a threshold Péclet number, the ratio of heat advection rate to heat conduction rate, beyond which the efficiency of heat transfer between the BHEs and the aquifer is significantly increased, thus lowering the temperature drop of the circulating fluid in the BHE and increasing the overall efficiency of the BHE system. This threshold Péclet number depends on the groundwater flow rate and effective thermal diffusivity (among other factors) of the system and, for the given conditions, is approximately 2 with a 1% change in BHE outlet temperature and approximately 11 with a 5% change. In GSHP systems with standardized spacings between BHEs, groundwater heat advection can cause negative thermal interactions between heat exchangers, which can be eliminated and in some situations replaced with positive thermal interactions by optimizing the spacing between BHEs. Above the threshold Péclet number, there is specified spacing between heat exchangers that will allow for the utilization of the previous season's heat injection or extraction, a half year transport distance. For the GSHP system simulated in this study, the BHE spacings for optimization are 6.65 m and 13.8 at groundwater flow velocities of 2.5 x 10-5 m/s and 5 x 10-5 m/s, respectively. It may also be possible to space the heat exchangers at a distance that captures heat after a year and a half of transport (for systems with only slight heat advection dominance), but more simulations are necessary to investigate the results of such a strategy.