Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable, and have long lifetimes. The general principle of compressed-gas or compressed-air energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If a body of gas is at the same temperature as its environment, and expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas will remain at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of high-pressure gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, expansion where no heat is exchanged between the gas and its environment—e.g., because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during expansion and compression, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. Pat. No. 7,832,207, filed Apr. 9, 2009 (the '207 patent) and U.S. Pat. No. 7,874,155, filed Feb. 25, 2010 (the '155 patent), the disclosures of which are hereby incorporated herein by reference in their entireties. The '207 and '155 patents disclose systems and techniques for expanding gas isothermally in staged cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas may be used to drive a hydraulic pump/motor subsystem that produces electricity. Systems and techniques for hydraulic-pneumatic pressure intensification that may be employed in systems and methods such as those disclosed in the '207 and '155 patents are shown and described in U.S. Pat. No. 8,037,678, filed Sep. 10, 2010 (the '678 patent), the disclosure of which is hereby incorporated herein by reference in its entirety.
In the systems disclosed in the '207 and '155 patents, reciprocal mechanical motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of techniques, for example as disclosed in the '678 patent as well as in U.S. Pat. No. 8,117,842, filed Feb. 14, 2011 (the '842 patent), the disclosure of which is hereby incorporated herein by reference in its entirety. The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
The power density (volumetric or mass-based) of an energy-storage system that approximates isothermal expansion and compression of a gas by mingling a heat-exchange liquid with the gas may be defined as the maximum sustained power (kilowatts, kW) that the system can either convert to a stored form or extract from storage, divided by either the volume (m3) or mass (kg) of the system. The power density (either volumetric or mass-based) of an energy-storage system therefore may have units of kW/m3 or of kW/kg. An energy-storage system having higher power density will in general be capable of more economic storage and retrieval of energy than an otherwise comparable system with lower power density, i.e., averaged over the lifetime of the system its use will require fewer cents per kilowatt-hour stored and retrieved (¢/kWh).
Power density may be increased by a number of techniques; one such technique is to increase the rate at which thermal energy is exchanged by the heat-exchange liquid and the gas. One technique for achieving rapid heat exchange between the heat-exchange liquid and the gas is to spray the liquid through the gas as a mist or rain of droplets, which tends to increase the surface area of a given volume of liquid compared to the surface area of the same volume of liquid in a compact shape, e.g., a single cylinder or sphere. However, in many applications even more rapid heat exchange is desirable, and increasingly small heat-exchange droplet size (i.e., for increased heat-exchange surface area) may be difficult or impractical to attain. Thus, there is a need for systems and techniques for more-rapid heat exchange between a heat-exchange fluid and a gas to be or being compressed and/or expanded in compressed-gas energy storage and recovery systems.