| Abstract |
Although base-load electricity generated from geothermal resources is cost competitive compared to electricity generated from solar and wind resources, it is less attractive to utilities compared to natural gas because gas-fired generation can be ramped up or down quickly to respond to the problem of balancing the grid due to daily and seasonal variation in the ratio of wind and solar generated electricity to demand. One approach to adding value to geothermally generated electricity is to address the problem of flexibility by creating electrical storage capacity. We suggest, however, that load following could, under appropriate conditions, be achieved more economically and effectively by using geothermal resources both (A) to generate electricity both for sale and for electrolysis to produce hydrogen as a non-polluting transportation fuel, and (B) to preheat the feedwater for such electrolysis. As both power generation and electrolysis of water are more efficient at high temperature conditions, the best environment to test this concept is in very high temperature, preferably supercritical, geothermal reservoirs. In Iceland, the Iceland Deep Drilling Project (IDDP) aims to investigate such supercritical geothermal conditions. In 2009 Phase 1 of the IDDP created the hottest geothermal well in the world (450oC) but it was too shallow to reach supercritical pressures. Phase 2, beginning in May 2016, will drill to greater depth to reach the appropriate pressure and temperature conditions. In California the Salton Sea and the Geysers geothermal fields both offer high temperatures at drillable depths. In fact, in some parts of the Salton Sea geothermal field temperatures exceeding the critical temperature are likely at ~3.5 km depth. This creates the possibility of large-scale production of hydrogen to alleviate the serious levels of greenhouse gas and air pollution produced by combustion of fossil fuels in Southern California. However the greatest potential world-wide for such systems is along oceanic spreading centers, which produce supercritical geothermal resources for 65,000 kilometers around the world. High temperature geothermal wells could produce at a constant flow rate, generating electricity for the grid when needed for balancing, and otherwise generating electricity and heating feedwater to supercritical temperatures for electrolysis to produce hydrogen as a fuel for transportation and oxygen for industrial purposes. Electrolysis can be ramped down in seconds so that balancing power can be provided to the grid immediately, more responsively than gas-fired generation. By providing standby capacity to utilities, we can avoid wasting capital on stand-by gas fired plants (which create greenhouse gases) or batteries or other forms of storage that do not create any additional clean power. That money can, instead, be invested in a flexible form of generation that runs in baseload mode, cleanly, and can supply both balancing capacity and fuels, as needed. Implementation of such systems will require deep drilling of the high temperature geothermal fields, development of technology for water treatment and supercritical electrolysis, and the negotiation of flexible forms of capacity and pricing for electricity sales that respond to supply and demand at any given time. |