Electrical/thermal energy storage in geothermal powerplants

Adding energy storage capability to geothermal resources enables the power produced/offset to be dispatched as necessary based on changing grid conditions, expanding the usage and utility of geothermal energy, while reducing the unsubsidized levelized cost of geothermal energy. Geothermal power-plant emissions can still be substantial, like non-condensable gases (NCGs) such as CO2 and H2S. These effluent NCGs can vary from 1% to 20% of the inlet stream to the turbine. These NCGs can be an additional source of energy, augmenting the economics of geothermal plant operations and decarbonization of geothermal plants. Special polymers developed in Phase I and II (US Patent 9,782,719) proved capable of H2S/CO2 absorption (250 g CO2 absorbed per ml of solvent). In addition to the steam-to-hydrogen process developed in Phase II, further R&D is proposed to use these captured NCGs for energy storage: hydrogen production by electrolysis of H2S at, and use of captured CO2 for energy storage as compressed CO2 in power generation in ORC generators. The Phase II project successfully demonstrated flexible power generation with thermal and electrical energy storage. Thermal energy was stored in concentrated osmotic solutions, and converted to electric energy, as needed, in hydro-osmotic turbines, coupled with Organic Rankine Cycle (ORC) turbines. Electrical energy was stored in hydrogen, produced by a Low Temperature Steam Electrolysis (LTSE) process from waste steam, for power generation on-demand. Nrgtek has conceptualized and tested a pathway enabling absorption of the NCGs in polyetheramines, wherein both H2S and CO2 are absorbed. The resultant solution is electrolyzed in a low-voltage membrane-less cell, wherein elemental sulfur and hydrogen are produced from H2S electrolysis. The process produces hydrogen at extremely low voltage (= 0.55 V, as demonstrated in a small cell in our laboratory), in comparison to water electrolysis (energy reduction from 50 kWh/kg H2 to = 20 kWh/kg H2). The CO2 in the remnant solvent is desorbed at low temperatures (= 85oC) and the solvent recycled back. When compressed to liquid CO2, carbon capture and energy storage (CCES) is possible. The CCES process will capture and store the CO2 gas under high pressure when electricity is plentiful; when needed, the stored gas can be expanded through a turbine to generate electricity, at an average round-trip efficiency estimated at 70-80%. We will develop a ORC system (similar to our Phase II system) for the proposed CCES system. The systems and technologies developed can be used for flexible power generation in geothermal applications, as well as low-temperature waste heat and solar-thermal applications. Additional benefits will be decarbonization and production of “green H2”.