Green Wednesday: Geothermal Power, Reinventing the Steam Engine

From on December 03, 2008 in Green Remodeling

Renewable energy is getting a lot of overdue attention, most of which goes to popular resources such as wind and solar. There is, however, an underdog that you may not have heard of: geothermal power. Geothermal literally means heat from the earth and that is exactly what geothermal power plants harness. While there is not much said about geothermal power as of yet, interest is growing and the technology is advancing, so be prepared to hear a lot more about it, starting now.

Geothermal energy has three big benefits: it is clean, reliable, and domestically abundant.

Below the earth’s surface, rock is insulated and heated to high temperatures by the molten rock below it. Driving water down into this warm rock creates steam, which is then pushed to the surface where it drives a turbine, creating electricity. As the Department of Energy itself puts it, “Geothermal energy is an enormous, underused heat and power resource.”

Geothermal heat pumpsOn a small, single-home scale, geothermal energy has already gained some notoriety as an efficient heating and cooling system. Geothermal heat pumps utilize the nearly constant temperature of the upper 10 feet of the earth’s surface. In winter, the warm air from the shallow ground around the house is pumped via a network of pipes through a heat exchanger and into the house. In summer, the hot air that collects in the house is drawn into the geothermal system and the cool ground. The heat removed from the house is often recycled to heat water for the home.

On the utility scale, geothermal energy is harnessed in three ways: dry steam plants, flash steam plants, and binary-cycle plants.

  1. The first dry steam plant, which uses steam directly to spin turbines, dates back to 1904.
  2. Flash steam plants draw high-pressure hot water up into lower-pressure tanks. When the higher-pressure water hits the tank, much of that water immediately evaporates (in a “flash,” so to speak), creating steam to spin the turbines.
  3. Binary-cycle plants use water and a fluid other than water, which has a lower boiling point. Water is heated in the deep rocks and pumped to the surface and through a heat exchanger. At the same time the second fluid is running through that heat exchanger. The hot water causes the second fluid to evaporate into steam, which is then sent to the turbine to create electricity.

The binary-cycle method is growing in popularity because the water need not be heated to as high a temperature. This allows for functioning geothermal wells at shallower depths – saving time and money.

In addition to such low-temperature resources, there is another innovation that is driving new interest in geothermal power: Enhanced (or Engineered) Geothermal Systems (EGS). These systems involve drilling deep holes into the earth. Relatively cool water is then pumped down the hole and circulated through the heated rock below. This water is heated and produces steam which drives a turbine at the surface. EGS system greatly expand the accessibility of geothermal power around the world.

These innovations, plus generous estimates for geothermal potential, are inspiring the federal government and some corporations to increase funding for geothermal research and development. Google alone has promised over $10 million to geothermal projects.