The geothermal heat resource is not limited the way fuel or iron or other materials are. But the low efficiency of geothermal power systems is unsatisfying and contributes to often-poor economics. To improve efficiency, engineers could look for a different thermodynamic cycle in the power plant. And, the economics can be improved by developing a facility whereby geothermal heat and electricity is the by-product of a process to produce lithium.
Engineers pride themselves on finding efficient solutions to problems. The resources we have to work with are invariably limited, so there is always a push to do the most with the least. For instance, engineers have worked to wring the most work from a unit of fuel-designing systems that can work at higher temperature or higher pressures, or devising cogeneration or combined-cycle systems that can produce two products from the same source of heat. This pursuit of efficiency continues until the cost of improvement exceeds the benefit, what economists call diminishing returns.
Geothermal resources present limits to efficiency. They inherently have lower temperatures and pressures than fossil-fueled power plants where the heat source are the high temperature combustion products. The heat source for a geothermal plant is the hot brine that comes from a deep well. Since the maximum amount of heat that can be extracted is the product of flow rate, heat capacity, and the difference between the initial and ambient temperature, geothermal power plants feature inherently much lower efficiencies than combustion systems. Existing geothermal power plants often operate with single-digit efficiency, thus throwing away more than 90 percent of the heat delivered to the surface.
So why bother? In spite of the efficiency limitation, geothermal energy can be an attractive alternative to fossil fuels, and it also has advantages relative to other renewable options. A geothermal power plant can operate full time at installed capacity, whereas solar and wind have a relatively low availability due to weather and seasonal variations. And unlike hydropower, which can see output decline because of decreased river flows and reservoirs from drought, geothermal is not affected by climate change.
Importantly, the geothermal heat resource is not limited the way fuel or iron or other materials are. Geothermal heat is created within the Earth and rises to the surface whether we tap it or not.
Still, that low efficiency is unsatisfying and contributes to the often-poor economics of geothermal systems. After all, the wells and turbines are capital-intensive and require selling a product to offset those costs. To improve efficiency, engineers could look for a different thermodynamic cycle in the power plant, one that is better optimized for wringing work from low heat. And, the economics can be improved by developing a facility whereby geothermal heat and electricity is the by-product of another industrial process. It would be like cogen, except the two products are electricity and lithium.
TEMPERATURE AND EFFICIENCY
Ancient civilizations built geothermal baths for comfort and healing, but they didn’t know why hot water sprung from the ground. Only relatively recently did scientists understand that most geothermal heat comes from the decay of radioactive uranium, thorium, and potassium. A lesser amount is from chemical reactions, hysteresis, friction, and a gentle squeezing of the planet due to the varying gravitational forces between the Earth, moon, and Sun.
The geothermal heat flux by conduction through the upper crust is about 0.1 W/m2, about 10,000 times less than the heat flux from the sun. This heat flux causes an average temperature gradient of about 100 °F per mile of depth, though geothermal heat is not evenly distributed across the Earth. Places with volcanic activity—and thus molten lava close to the surface—have more heat to tap.
About 26 countries have installed 15,400 MW of geothermal electric capacity, with geologically active Iceland, Kenya, and the Philippines each using geothermal power stations to contribute more than one-quarter of their electricity. There are numerous geothermal sites in the United States—the first to be harnessed was the Geysers in northern California in 1960—and their aggregate installed capacity is 3,600 MW.
While geothermal power has been around a long time, the sector has not experienced the growth or investment seen in the large-scale wind and solar power sectors. More investment could enable a better optimization of geothermal power plants, including finding ways to extract more heat from the brines.
Efficiency is the ratio of the work or power produced to the amount of heat extracted from the brine. While attaining the maximum efficiency is not realistic, it can provide guidance for improving the efficiency of practical cycles. It also allows for comparing the efficiency of an actual cycle with the ideal.
The Wicks cycle is described in engineering literature as the ideal for producing power from hot streams such as combustion products or from the hot brine of a geothermal well. The cycle requires absolute temperatures, like the Carnot efficiency that is valid for constant temperature heat sources and heat sinks.
Wicks Ideal Efficiency = 1 - Tcold* ln (Thotmax/Tcold)/(Thotmax-Tcold)
A well that produces a brine at 340 °F or 444 K in a location with an ambient temperature is 40 °F or 278 K would have a maximum attainable efficiency of 21.66 percent.
Vapor cycles in which the working fluid evaporates and then condenses are the most practical working fluids for geothermal conditions. The brine from the well will be a pressurized liquid. For instance, the saturation steam and water property tables show a minimal absolute pressure of 115 psia is required to prevent boiling at 340 °F. The flash system is the simplest system and was used on early geothermal plants. The brine is throttled to reduce pressure, resulting in a lower-temperature mixture of saturated steam and water in a separator. The saturated steam flows out the top to a turbine and the liquid flows out the bottom. It combines with the condensate from the condenser and is pumped back into the ground via a separate injection well.
A more efficient cycle is called a binary cycle. The brine stream remains separate from the working fluid of the cycle. It provides the heat to a four-process Rankine cycle. Heat is transferred to the cycle via a counter flow heat exchanger. The cooling brine is on the hot side and the working fluid is on the cold receiving side. The resulting steam flows to a turbine and condenser, and a pump returns the water to the steam-producing heat exchanger or steam generator. The process is similar to the steam generator on a pressur-ized-water reactor nuclear power plant.
Due to the relatively low temperature of many geothermal brines, some companies have replaced the working fluid in their Ran-kine-cycle turbine from water to a variety of refrigerant type fluids called organics that feature boiling points below 212 °F. The refrigerants have the advantage of higher density and thus a more compact turbine and condenser. However, they have no fundamental thermodynamic efficiency advantage. Also, they can be toxic and have environmental risk relative to water.
Instead of one working fluid, another possibility for optimizing a power system to low-temperature brine is to use a mixture of two working fluids, such as water and ammonia. By adjusting the mixture of fluids, a designer can improve the efficiency of the system by lowering the average temperature difference between the brine and the boiling mixture.
This two-fluid mixture was patented in 1985 by Alex Kalina, a Russian engineer who immigrated to the U.S., and he further developed it over the next three decades. Geothermal plants utilizing the Kalina cycle have been built in Iceland, Germany, and Japan, but the technology has had a limited uptake. Kalina cycle plants are more complicated and expensive than ones using a conventional Rankine cycle and have the additional concern of toxic ammonia.
It should be possible, however, to combine the simplicity of a Rankine cycle with the lower average stream temperature difference of the Kalina cycle. Such a system would be comprised of multiple Rankine cycles receiving heat from the brine over different temperature ranges. The turbine for each cycle could then be designed to best match the pressure and temperature of the steam supply. Design engineers might find a satisfying challenge in determining the optimal hot side temperature range for each Rankine cycle turbine in the train.
PRODUCT AND BY-PRODUCT
The other direction in improving the prospects of geothermal power is economic, making the resource easier to tap or the plants less reliant on energy sales to balance the books. For instance, the petroleum industry deploys some of the most advanced technology found anywhere. Fortunately, the drilling equipment, technologies, and engineering methods are similar to the requirements for harnessing deep geothermal resources. Rather than being all lost with the transition from fossil fuels in the coming decades, much can be transferred to the development of geothermal energy.
A recent proposal looks to alter the economic fortunes of geothermal energy by getting more than one product from the brines.
As one might suspect from the name, the brines raised from geothermal heat sources are rich in alkalis, such as sodium and calcium, and halides, chiefly chlorine. But other minerals are dissolved in the water as well. A 2020 report from the University of California Riverside looked at the composition of the brines used in power plants in the Salton Sea Geothermal Field, one of California’s largest, and found concentrations of zinc and manganese that could be economically important. But, the quantities of lithium in the water were eyeopening.
Lithium, of course, is the major component of lithium-ion batteries, the high power-density electrical source for most handheld electronics and electric vehicles. The global production for the metal is 77,000 tons per year, with Australia, Chile, and China accounting for 86 percent of the total. Over the past decade, there have been periodic concerns over the long-term availability of lithium, either due to depleted resources or to potential geopolitical barriers to the global lithium trade.
Based on the concentration of lithium found in the brine pulled up from wells in the Salton Sea and elsewhere in the Imperial Valley, UC Riverside researchers Michael A. McKibben, Wilfred A. Elders, and Arun S.K. Raju estimated that the nine geothermal power plants currently in operation in the Salton Sea area could produce 17,000 tons per year, a large fraction of the present-day global production. Doubling the power production of those plants, to 700 MW, would enable the extraction of 40,000 tons per year. Utilizing this resource would turn the U.S. into a major lithium producer. The technology for extracting the metal is already developed and ready for deployment.
There are additional benefits to using brines to mine lithium. For one, the brines are already being brought to the surface as part of geothermal power production, so this would have minimal environmental impact compared to hard-rock mining. What’s more, it’s expected that the cost of extracting the metal from the brine could be up to 40 percent less than the cost of rock mining.
Mining lithium as a by-product of geothermal power production radically alters the economics of the plants there. “Geothermal lithium production could enable the geothermal power companies to become a major net exporter and dominant supplier to the expanding global market,” McKibben, Elders, and Raju wrote. Having two products to sell helps support both.
There are other geothermal brines that are rich in lithium, including ones in Cornwall in southwest England, the site of historic copper and tin mines, and along the Rhein Valley in Germany. The presence of lithium could enable the development of geothermal resources in those places, because a site can go from commercially marginal or even unviable to profitable if the production of lithium can be combined with electricity.
This isn’t the way engineers usually think about efficiency or go about achieving it, but combining lithium mining with geothermal power could meet multiple energy challenges.