Environmental activists have an aboveground and a below-ground view of the world. Energy sources harnessed on or very close to the surface, like wind, wave, tidal, solar and hydro power, are good. They are renewable and do not emit carbon dioxide, the main greenhouse gas that is widely thought to be responsible for global warming.
However, energy sources found under the ground, such as coal, oil and natural gas, and uranium used for nuclear power, are bad. Fossil fuels are major greenhouse- gas emitters while nuclear power, although it produces almost no global warming emissions, is still regarded by many environmentalists as too much of a safety and proliferation risk.
Yet there is another form of underground energy gaining increasing prominence as a future source of power that gets an environmental seal of approval: geothermal heat. What comes out of the ground with this form of energy are hot water and steam, and almost no pollution.
Moreover, advocates point out that geothermal is currently the only form of renewable energy that provides a near-constant supply of baseload electricity to commercial grids in the same way that plants powered by coal, oil, gas and nuclear fuel do.
Other types of renewable energy generate electricity intermittently, depending on the strength of the sun, wind, waves and tides. They can make a useful contribution to a cleaner energy future. But they do not appear to have the capacity of geothermal power to cut greenhouse-gas emissions and change the way electricity is generated. Subterranean energy also has great potential for providing direct heat to homes and offices through district heating systems.
Geothermal energy is working in dozens of countries. Of some 10,000 megawatts of geothermal power installed around the world, nearly one-third is in the Philippines and Indonesia — the two largest generators of electricity using underground heat after the United States. Japan, with 535 megawatts installed, is the sixth-biggest producer of subterranean power.
Geothermal energy provides only a tiny fraction of global electricity supply, most of which comes from burning fossil fuels. But installed geothermal capacity is expected to reach 13,500 megawatts next year, with the number of countries producing power from underground heat rising to 46, from 21 a decade ago.
This power system has a long track record of successful development and use. But it is currently limited to areas where volcanic activity produces very hot underground water in reservoirs, which may stretch for as much as 50 square kilometers and can be tapped to drive steam turbines installed in power plants on the surface.
Among countries with the richest volcanic resources are those on the so-called Ring of Fire that circles the Pacific. They include New Zealand, Indonesia, the Philippines, Japan, Russia’s Kamchatka Peninsula, the west coasts of the U.S. and Canada, Central America, and the west coast of South America.
The Ring of Fire is a zone where tectonic plates collide to create the earthquakes and tsunami so much in the news in recent days in Indonesia and islands of the South Pacific. But these same forces also create subterranean heat reservoirs that can easily be reached with current oil and gas drilling technology. Volcanic geothermal power exploits less than 5 percent of the very hot underground water resources that could be exploited worldwide, according to the International Energy Agency.
However, advanced drilling technology in geologically stable parts of the world is opening up a much bigger underground heat source for future power generation. This has unleashed a wave of exploration and development activity in Australia, Europe, the U.S., China and India.
Known as Enhanced Geothermal System (EGS), or “hot rock,” technology, it focuses on high heat-producing granite typically found 3 to 5 km below the surface. In this zone, which draws heat from the molten core of Earth and the decay of radioactive uranium, thorium and potassium in the crust, the temperature can reach 300 C.
By some calculations, the heat energy content in the upper 10 km of Earth’s crust is 50,000 times greater than the energy content of all known oil and gas resources. Exploiting this power — by drilling down, fracturing the rock with water pumped in under high pressure and then drawing very hot water from the resultant reservoir up a separate well — poses major technical and financial challenges.
Since 2006, two EGS projects in Europe, both near urban centers, have been halted amid concerns that the underground rock fracturing had caused tremors. However, this appears to be a manageable problem. The Massachusetts Institute of Technology concluded in a 2006 report that hot rock geothermal energy in the U.S. could support a generating capacity of 100,000 megawatts by 2050.
Geoscience Australia, a government agency, calculates that extracting just 1 percent of the energy from rocks hotter than 150 C (the minimum for generating electricity) and less than 5 km below the surface would yield about 26,000 times Australia’s primary power usage in 2005.
A report last year commissioned by the Australian Geothermal Energy Association, which represents companies involved in the burgeoning industry, concluded that geothermal power could provide 2,200 megawatts of baseload capacity by 2020, about one-third of the new generating capacity likely to be needed by then.
This will require investment of at least $10 billion. Geothermal electricity costs also will have to fall. The industry reckons this will happen as commercial-scale generation results in improved efficiency and climate change concerns impose added costs on fossil fuel power.
Michael Richardson is a visiting senior research fellow at the Institute of South East Asian Studies in Singapore.