THE POTENTIAL ROLE OF HIGHER ENTHALPY GEOTHERMAL SYSTEMS IN THE USA

After more than 50 years of development only ~3,400 MWe of electric power is currently being produced from geothermal resources in the USA, that is only about 0.33% of the country's total installed electrical capacity. While some 2,500 MWe of new geothermal electrical capacity is under development, the growth rate of this environmentally benign energy resource is overshadowed by the rapid increase in the installed capacity of wind and solar energy, in spite of the large demonstrated potential of geothermal resources.

Most of the new geothermal developments in the USA involve relatively small, moderate-temperature, geothermal systems. In contrast, development of higher enthalpy geothermal systems for power production has obvious advantages; specifically higher temperatures yield higher power outputs per well so that fewer wells are needed, leading to smaller environmental footprints for a given size of power plant. Disadvantages include that the fact that locations of suitable geothermal systems are restricted to young volcanic terrains, production of very high enthalpy fluids usually requires drilling deeper wells and may require enhanced geothermal (EGS) technology, and drilling deep into hot hostile environments is technologically challenging. However the potential for very favorable economic returns suggests that the USA should begin developing such a program.

One approach to mitigating the cost issue is to form a consortium of industry, government and academia to share the costs and broaden the scope an investigation. An excellent example of such a collaboration is the Iceland Deep Drilling Project (IDDP) which is investigating the economic feasibility of producing electricity from supercritical geothermal reservoirs. This industry-government consortium planned to drill a deep well in the volcanic caldera of Krafla in NE Iceland. However drilling had to be terminated at 2.1 km depth when 900°C rhyolite magma flowed into the well. The resultant well was highly productive capable of generating >35 MWe from superheated steam at a well-head temperature of ~450°C. In the USA it should be feasible to start a similar program aimed at developing high-enthalpy systems.