Carbon-Free Compressed Air Energy Storage

The inherent intermittency of the two fastest growing renewable energy sources, wind and solar, presents a significant barrier to widespread penetration and replacement of fossil-fuel sourced baseload generation. These intermittencies range from short term ramp events experienced by wind farms to the diurnal fluctuation of solar installations. Traditionally, grid operators have had very little control over demand and full control over supply. With the increase in wind and solar based generation onto an electric grid comes a decrease in control of supply. Grid operators are required to have a certain amount of spinning reserve ready to respond when wind or solar resources suddenly decrease. One solution to this problem of grid integration is the use of energy storage. While traditionally used as merchant facilities to buy and sell energy on the spot market, storage is actively being investigated as a means of enabling renewable energy sources to achieve widespread penetration onto national grids. Pumped hydro storage has been used extensively as a means of large scale energy storage. Standard compressed air energy storage (CAES), while only two installations currently exist, shows potential to store large amounts of energy. Both traditional technologies have drawbacks; they are site specific, and in CAES, require natural gas combustion. In this thesis, a short-to-medium term energy storage system is presented. While similar to the CAES technique in that compressed air is still used for energy storage, it differs as an incompressible liquid is the working fluid in the turbine, thus eliminating the need for supplementary combustion when the energy is recovered. Energy is stored above ground in pressure tanks until power is needed; at this time, it exerts a force on the incompressible fluid, pushing it through a hydroturbine. This family of approaches combines the best concepts attributed with pumped hydro storage and CAES in a system that is not site-specific, does not require natural gas and has the potential for being very efficient. Thermodynamic analyses were performed to determine energy flows into and out of the system. The results from that analysis were used to verify a system model created from individual component models. This system model was then used to simulate the application of this energy storage technology in various applications and accurately assess its performance. This research found that this approach to energy storage is feasible with existing technology. The upper bound on the round-trip efficiency of energy stored in this manner is 65%