Geothermal Power: Opportunities for Research
Kenneth H. Williamson
Unocal Corporation,
1300 N. Dutton Avenue,
Santa Rosa, CA 95401
ABSTRACT
Geothermal power utilizes heat stored in the earth as an energy source. The total thermal energy stored within the earth is estimated at 1031 Joules, which is more than 1010 times the current annual global energy usage. The thermal energy stored beneath the continents to a depth of 1 km is estimated to be 105 times current annual usage. Worldwide installed geothermal power capacity is 8GW, and 50 TWh were generated in 1999, mostly from the vicinity of tectonic plate boundaries where heat transfer from the earth's interior is greatly enhanced by volcanism. In the past 5 years, global installed capacity has increased by 17% whereas the U.S. capacity of 2.4 GW has decreased by 20% over the same period, reflecting declines at The Geysers Field.
Current geothermal fluids are normally produced from the depth range 500 - 3,000 m and the temperature range 200 - 350 ºC, from naturally occurring fractures in volcanic rock. Research into the use of artificially stimulated fractures for creating geothermal systems in impermeable rock is currently focussed in Europe and Japan. This Hot Dry Rock initiative, if successful, greatly expands the geothermal resource base and in principle allows geothermal power generation anywhere on earth.
A typical geothermal system produces from a liquid brine reservoir. The brine enters a borehole ~0.2 m in diameter, partially flashes to steam as it rises to the surface, and then the steam is separated in a pressure vessel on the surface before being scrubbed and sent to the turbine inlet. Spent brine from the separator is returned to the reservoir through injection wells. This injected brine helps to maintain reservoir pressure, but has the potentially negative effect of cooling the reservoir and reducing the steam output of production wells.
A priority for research at this time is therefore directed at maximizing the pressure support, and minimizing the cooling effect of injection. Optimizing the location and rates of injection wells in a way that maximizes heat recovery has the potential to greatly extend the useful reservoir life, sustaining production at some value below the peak level for 100 years or more. In the Geysers Field in California, which has been operating for 40 years, injection into the reservoir is being supplemented by treated wastewater, and an increase in power output has been demonstrated.
Another priority for research at this time is reducing the development cost of geothermal projects. Recent reductions in the cost of natural gas combined cycle power units has made geothermal power unable to compete in the markets where relatively inexpensive gas is available. Significant progress has been made in reducing the cost of geothermal, and the goal of being competitive is within reach, but will require additional research and development of drilling and power conversion technologies.
To harness vast geothermal resources currently inaccessible due to technological limitations, it is essential to continue research in exploring for hidden systems, and in stimulating permeability in the margins of existing natural reservoirs. The ultimate goal should be the development of "Hot Dry Rock" reservoirs, where permeability of the entire reservoir is created through artificial stimulation.
The following strategy is therefore proposed for geothermal research:
Immediate
Optimize exploited geothermal systems It has been estimated that typically a small fraction (~20%) of available heat is extracted during a 30 year life. In an existing operation, where plant and field facilities have been already constructed, it is economically effective to make changes to the injection system to extract the remaining available heat. However the state-of-the-art in our understanding of the heat transfer properties of naturally fractured systems is inadequate. It is proposed that continuing research into the development and interpretation of liquid tracers, which have the potential to register maximum path temperature, average path temperature, and surface area of contact along the flow path, should be made a priority.
Reduce development cost of high Temperature systems
The priority here is to reduce the cost of drilling and completing wells, and in increasing the success rate of wells in finding adequate permeability. It is also critical to reduce the cost of energy conversion, i.e. power plants, but the principal manufacturers of geothermal turbines are not currently based in the U.S.
Within 5 years
Locally enhance permeability in the tight margins of existing systems (EGS)
Most commercial systems have impermeable hot margins, and have an adequate supply of fluid for injection, and existing power plants and production wells. Once the existing reservoir goes into decline, makeup steam could be supplied from artificially stimulated regions on the hot margins of the field. Research into artificial stimulation of fractures will be required to make this economically feasible.
Explore for and develop "hidden" high enthalpy systems, with no surface features
Almost all existing geothermal developments were found because of surface seeps, in an analogous way to the early oil industry. It is expected that many more geothermal reservoirs without associated hot springs or fumaroles exist, and technology improvements will be required to find them economically.
Within 20 years
Develop impermeable systems with artificial fracturing (HDR)
Once it becomes economically viable to drill to 5-10 km, and to create a reservoir in naturally impermeable rock through artificial stimulation, the size of the geothermal resource available for exploitation worldwide will increase by several orders of magnitude. Efforts to demonstrate the technical feasibility of HDR are currently underway in Japan and in Europe. It is recommended that U.S.-based research focus on developing the necessary technologies and testing them in existing high-enthalpy geothermal systems, and that high-cost demonstration projects be delayed until their risk of failure has been significantly reduced. In this way the cost of conventional systems can be lowered as progress to the ultimate HDR goal is made incrementally, and the maximum value can be extracted from the research.