GEOTHERMAL energy, long a poor relation among the more glamorous renewable technologies of wind and solar power, is poised to smarten its dowdy image. Piping-hot underground water and steam, percolating up through fissures in rocks fractured by seismic activity, have been a welcome feature of the European landscape since the Romans popularised bathing. On the other side of the globe, the Japanese have luxuriated since the Heian era in hot-spring onsen that dot their volcanic and quake-strewn archipelago. Even so, as a source of renewable energy, geothermal electricity has gone largely ignored as fortunes have been heaped on its rivals.
Today, a handful of countries that sit astride seismic belts or have active volcanoes in their midst, such as Iceland, the Philippines, Costa Rica and New Zealand, get a significant proportion of their heat and electricity from geothermal sources. America actually has more geothermal generating capacity (3.4 gigawatts) than any other country. But because of its huge resources of coal and natural gas, along with heavy investments in hydroelectricity and nuclear power, geothermal juice contributes a mere 0.4% of its electrical output.
There is, however, much to like about geothermal energy. It is reasonably clean; leaves behind little in the way of waste; does not suffer the vagaries of the weather or the inevitability of sunset; makes the tiniest of footprints on the land; and is pretty well inexhaustible. Above all, it is more or less free for the taking. Yet, lacking the political clout of wind and solar power, geothermal electricity has never received the attention it deserves.
Over the past five years, for example, the American government has spent close on $150 billion on clean energy, through a combination of grants and tax credits. Half went on handouts for electrical vehicles, advanced batteries, high-speed rail, electricity distribution, nuclear power and new fossil-fuel technologies. A further sixth was spent on subsidising biofuels. Of the remaining third, devoted to various forms of renewable electricity, wind and solar took the lion’s share. The Department of Energy’s 2014 budget for solar research, for instance, amounts to $257m, while geothermal’s is a modest $45m. Overall, geothermal has received around a thirtieth of the federal support—in terms of research grants, matching funds and tax credits—that has been handed out to wind and solar.
Whether such parsimony has truly hindered innovation in geothermal engineering is hard to say, for in a sense it is two different industries. In volcanic areas the heat comes to you. It is just a question of corralling it and using it—not a matter that needs state subsidy. Elsewhere, though, you have to dig deep to get at useful amounts of heat, and it is certainly true that exploration and drilling costs have remained stubbornly high for the deeper wells needed outside hot-spring regions, and that developers have been slow to devise better ways of extracting heat from such rocks, even if wells are sunk. Here, a little financial lubrication for research might pay dividends.
One important advance has been made—or, rather, borrowed from the oil and gas industry. This is the use of hydraulic fracturing ("fracking"), in which, in the case of oil or gas, water is injected into rocks whose hydrocarbons are too tightly bound to the rocky matrix to gush to the surface of their own accord. The high-pressure water shatters the matrix, releasing the bound payload.
Fracking is the technique behind the new “enhanced geothermal systems” that extract energy from rocks which are hot enough, but too dry, to produce steam. In such cases, developers bore two wells several kilometres down to the basement rocks and fracture the matrix between them with either high-pressure water or explosives. Water is then pumped down one of the boreholes and rises, heated, up the other. The pressure drops as the boiling water approaches the surface, causing it to flash into steam. This steam is used to drive turbines for generating electricity.
A report prepared several years ago by scientists at Massachusetts Institute of Technology (MIT), which examined the potential for enhanced geothermal systems, reckoned $1 billion spent over 15 years on research and development could lead to 100 gigawatts of geothermal generating capacity being established by 2050 in the United States alone. Worldwide, the amount of geothermal energy that might be extracted this way could exceed 200 zettajoules (ie, over 50 million-billion kilowatt-hours). With further refinement, the MIT researchers estimated that ten times more geothermal energy could be made available—enough to meet the world’s current needs for several thousand years.
The reason enhanced geothermal wells have to be deep is that the thermal efficiency (and thus the profitability) of geothermal generation is particularly sensitive to the temperature of the water brought to the surface. That temperature needs to be 150ºC or more to produce steam powerful enough to drive electrical turbines. Away from places where tectonic plates abut, the temperature of the underlying rocks increases by roughly 25º-30ºC per kilometre (23º-26ºF per 1,000 feet) of depth. This means that to get water hot enough to raise steam, you have to drill down several kilometres.
Short of sinking wells to unprecedented depths (the deepest so far is 12.3km), the water temperature is unlikely to be high enough to produce the quality of steam found in, say, a boiler heated by fossil fuel. At best, the thermal efficiency of geothermal power generation is around 23%—about half that of a coal-fired power station.
This does not mean geothermal electricity is uncompetitive. The capital costs of geothermal plants are high—as much as $2m-7m per megawatt of capacity. But with the “fuel” being essentially free and maintenance and environmental problems minimal, operating costs are particularly low. Typically, geothermal generating plants produce a kilowatt-hour of electricity for around five cents (the same as coal), compared with eight cents for wind and 13 cents for solar.
And unlike wind or solar, geothermal generating stations can run day and night, year in and year out. Their average capacity factor (a measure of the amount of electricity produced compared with the capacity installed) is 73%, though some operate as high as 96%. The average capacity factor for solar-generating arrays is no more than 12%, while wind farms manage around 23%. In many ways, geothermal plants are similar to nuclear-power stations (with a capacity factor of 90%), albeit on a smaller scale and without the radiation or waste-disposal problems.
Unfortunately, these apparent advantages have turned into an incubus. What the geothermal industry needs, more than any subsidy, is to change the message it gives out. Until recently, it has boasted that, unlike other renewables, such as wind or solar, it is a base-load resource similar to coal, gas or even nuclear, but without greenhouse gases or radiation fears. Such a claim, far from being a virtue, has become something of a curse. The problem, as Dave Olsen of the California Independent System Operator Corporation sees it, is that utilities are hobbled by the inflexibility of their base-load generating stations.
Ironically, when Southern California Edison was forced last year to retire its San Onofre nuclear power station that served the greater San Diego area, the local grid became more stable rather than less so, despite losing its biggest chunk of steady, base-load capacity (see “Too hot to handle” June 17th 2013). Meanwhile, the amount of renewable power delivered to households increased.
What this incident revealed was that, as more and more wind and solar power are added to the grid, many utilities are facing a severe over-supply of electricity during the middle of the day. In 2013, California had to dump over 19 gigawatt-hours of pre-purchased renewable energy, because it could not throttle its inflexible base-load supplies sufficiently. As more renewables are mandated into existence (California plans to get a third of its electricity from renewable sources by 2020), the base-load problem can only get worse.
Recently, Mr Olsen told a meeting of the Geothermal Resources Council that the last thing they should be promoting their product as is a carbon-free alternative for base-load power. What the utilities are crying out for is more flexible power that can be ramped up quickly. Rather than be seen as part of the problem, geothermal needs to present itself as a cheaper, cleaner, more reliable and efficient form of auxiliary power that can provide utilities with the flexibility they urgently seek.
It is premature to declare, as some have, that base-load power is dead. It still provides the cheapest electricity—and will continue to do so for decades to come. But the virtue of geothermal electricity is that it can provide base-load power, flexible power or anything in between. In other words, it can be a base-load producer that runs all the time, a "load-follower" that operates during the day and into the early evening, or even a "peaking power" plant that ramps up quickly to meet sudden spikes in demand. If the geothermal industry manages to get that message across, the days of wind and solar could well be numbered.