Thermal energy storage for dispatchable geothermal power

Dispatchable generation refers to sources of electricity that can be turned on or off, or can adjust their power output accordingly to an order. Geothermal plants are usually used for base-load power rather than dispatchable power. The 8 MW Puna Expansion Facility in Hawaii is the first fully dispatchable geothermal power plant (Nordquist et al., GRC Transactions, Vol. 37, 2013). As described by the Ormat team, the adaptation of a base load power plant to full dispatchability was not easy. The plant is required to adjust its power output quickly in response to a requested ramp rate and maintain its frequency within close tolerance to the grid power. This was a challenge for a geothermal plant because the heat source does not naturally respond quickly to changes in demand. To address this challenge, Ormat decided to maintain the geothermal fluid flows at relatively steady rates while providing a bypass around the generation equipment as needed. Under part load, some of the geothermal fluid is pumped to the surface, bypasses the generation equipment, and is re-injected to the ground without extracting any useful enthalpy. This approach is robust but necessarily incurs high parasitic power draws at part load due to the constant, full-flow pumping power requirements. As an improvement to this existing configuration of a dispatchable geothermal plant, we propose to include a phase-change thermal energy storage (TES) unit that will provide some “inertia” against temperature fluctuations. The TES phase change material (PCM) could be in the form of eutectic molten salts or the like based on the required temperature. This configuration would allow the power plant to meet the ramp rate requirements while allowing the geothermal resource to respond more slowly. As a result, the geothermal fluid flow rates could be modulated to reduce pumping power at part load. In Phase I, Mainstream would design, build and test the TES component. Cornell University would perform the techno-economic analysis (TEA). In Phase II, the team would integrate the PCM TES into a test loop that mimics a sub-scale geothermal power plant with realistic, simulated geothermal resource time response. This hardware testing will allow the configuration to be proved out under various dynamic dispatchable use cases (e.g., load matching). Cornell has plans to become a net-zero campus and geothermal heat and power is a critical component of that plan. As such, Cornell would provide an ideal “living laboratory” for a dispatchable, campus power supply.