The Development of an Isothermal Compressed Air Energy Storage System Prototype Utilizing a Liquid Piston Compressor-Expander

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The Development of an Isothermal Compressed Air Energy Storage System Prototype Utilizing a Liquid Piston Compressor-Expander

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2022-12

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Renewable energy sources such as wind and solar energy prove to be the future of sustainable energy as a whole, however there are several limiting factors to these technologies that are currently limiting their potential to replace fossil fuels. The intermittencyof these renewable sources combined with the varying demand of the power grid creates a need to store the collected energy in times of surplus in order to reproduce that energy at peak times. The current solution to storing this large quantity of excess energy is predominantly the use of large battery storage systems. However, as wind and solar collection systems have progressed over recent years, the batteries used to store that energy still remain inefficient, large, expensive, and are filled with harmful chemicals. The limiting factor of renewable energy has moved from the collection method technology to the energy storage itself as the power grid progresses to a more sustainable future. This thesis provides insight into the development of a prototype utilizing a growing energy storage technology in the field of Compressed Air Energy Storage (CAES). The thesis will discuss the development and testing of a prototype CAES system which attempts to maximize efficiency and power density while minimizing cost and environmental impact to achieve a more sustainable method of energy storage and allow for the advancement of renewable energy within the power grid. Compressed air systems function based on the concept of storing energy through compression of air and regenerating that energy through expansion. These systems have their own limiting factors mainly originating from energy lost through heat during compression and regeneration cycles, slow cycle times, and any possible leaks in the system. The prototype and system cycles discussed in this thesis aim to minimize these losses through utilizing a a liquid piston compressor-expander to compress low pressure air at 7 bar to high pressure at 210 bar. The prototype is custom built from a stainless steel construction, in which a liquid water piston is leveraged to compress air and store this air in an accumulator. A hydraulic pump motor system is used to power the system as the energy source by driving a water pump, therefore actuating the liquid water piston used to compress the air. During regeneration, this liquid piston is pushed back down and through the pump motor that acts a generator, simulating the regeneration of the stored energy. The liquid piston is controlled using a circuit of water lines and valves to direct water into and out of the correct chambers to either compress or expand air as well as allow the chambers to refill with low pressure air. The chamber itself is filled with a mixture of ABS plastic and stainless steel heat transfer porous media, that works in conjunction with the liquid piston as a heat sink to produce a nearly isothermal compression and regeneration cycle and minimize heat losses. The compressed air is then either stored and ejected from a high pressure accumulator through two custom designed solenoid valves. The utilization of the heat transfer media eliminates nearly all loss of energy to heat and the liquid piston itself eliminates the possibility of leaks across the piston. The cycles are also optimized to maximize power density and cycle times in order to produce an optimized system. The actuation of the liquid piston utilizes two different phases to reduce cycle times. The first of which is referred to as the flow intensifier phase in which a larger solid stainless steel piston with an advantageous area ratio is used to compress a large volume of air quickly with a relatively small flow of water into the bottom of the piston. In doing so, the air is pre-compressed quickly while it is at low pressure in which the volume changes quickly. After the intensifier piston locks out, the liquid piston actuates through the smaller chamber filled with heat transfer media to reduce volume more slowly while at higher pressure since the compression would generate the most heat during this phase. By utilizing this flow intensifier step, the system cycles are designed to compress air quickly to high pressures, maximizing power density. This prototype demonstrates the integration of custom components necessary to achieve optimal compression and expansion cycles. It uses two custom air valves with designed passive and active functionality as well as a water level sensor. The two valves designed for this system are for high pressure air expulsion and low pressure air regeneration. The low pressure valve is a solenoid valve that with iron backing and a soft iron core to allow for passive refilling of the system with low pressure air as well as an active state to hold the valve open. The high pressure valve is an electromagnetic locking valve that allows the expulsion of high pressure air passively and can be held open to reintroduce high pressure air back into the system. This thesis discusses the development, electromagnetic simulation, and testing of these valves. The water level sensor was developed to utilize the electrical resistance of water in conjunction with a high resistance array mounted on a PCB in order to determine the water level based on the current running through the system. These components were designed and tested in order to reach the necessary timings and reactivity for this system and its necessary cycles. The details of the system, cycle optimization, and testing of the prototype are discussed in more detail through this thesis.

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University of Minnesota M.S.M.E. thesis. December 2022. Major: Mechanical Engineering. Advisor: Perry Li. 1 computer file (PDF); xi, 131 pages.

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Posner, Jacob. (2022). The Development of an Isothermal Compressed Air Energy Storage System Prototype Utilizing a Liquid Piston Compressor-Expander. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/252465.

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