| Keywords |
supercritical,greenhouse gases, load balancing, electrolysis, hydrogen, CO2 sequestration, methanol, lithium, metal extraction |
| Abstract |
The State of California recently enacted legislation requiring a transition to electrical generation using 100% renewable energy generation by 2045, albeit without indicating how this was to be achieved or funded. Geothermal energy can therefore play a new greater role in California, which already is the world leader in production of electricity from geothermal resources, with an installed capacity that is about 19.5% of the global total. However, because of the rapid growth of solar and wind electrical generation in 2017 geothermal generation was only 17% of California’s renewable portfolio. We suggest that greatly enhancing the contribution of geothermal resources to California’s energy mix, while reducing greenhouse gases (GHG) is highly desirable. A recent development in Iceland suggests a way to do so. The successful drilling of a 4.5 km deep geothermal well that penetrated supercritical conditions, (bottom hole temperature of ~600oC, fluid pressure ~34 MPa) opens up new possibilities of improved geothermal economics. Because of the higher enthalpy and more favorable flow characteristics of supercritical water, a supercritical geothermal well should produce up to ten times more power than a conventional geothermal well. Producing much higher temperature working fluids creates other possibilities to improve geothermal economics by making downstream processes more efficient and by taking a fully integrated and flexible approach. Existing geothermal generation provides baseload electricity, but could be even more valuable in the role of balancing the grid. This would be done by selling electricity when demand is high, and at times of lower demand using electricity to produce hydrogen by electrolysis of hot water. Electrolysis is more efficient at high temperatures, but electrolytic cells require clean water, so heat exchangers and/or desalination would be necessary. Advanced electrolytic cells can be switched on and off in seconds, and permitting rapid balancing of the electric grid to complement solar and wind power. Similarly, when the chemistry of geothermal brine is suitable, salable products such as lithium, base metals and other mineral products could be extracted from the brines. However, existing geothermal steam generating plants are not entirely GHG free, although they emit less than a third of the CO2 emitted by a natural gas turbine, which is the least polluting of all hydrocarbon fueled electrical generating plants. The future development of supercritical and superhot geothermal generation would reduce emission of GHG’s by (1) replacing the use of the hydrocarbons that would be used to produce that electricity, (2) using electrolysis to produce hydrogen, as a transportation fuel or as a form of energy storage, (3) producing methanol using the geothermal CO2 and hydrogen, to use as a gasoline additive, (4) sequestration of CO2 in geothermal reservoirs, (5) producing lithium (reducing the price of batteries needed for pollution free transportation), (5) and extracting metals like zinc, silver, lead, manganese, etc., (without the GHG produced by smelting). |