The Renewable Electron Economy Part V.4: Power from Hot Rocks (Geothermal) August 2, 2007Posted by Michael Hoexter in Renewable Energy.
Overlooked in most discussions of renewable sources of energy is geothermal energy. The word “geothermal” is applied two different energy sources: 1) generating electricity from the heat from deep in the earth and 2) using the constant temperature of the upper layers of soil to heat and cool facilities using a heat pump. The first will only work in special locations on the earth’s crust or using extremely deep drilling. The second is possible in almost every location on the globe. Here I will be discussing the first geothermal energy while later in the series I will discuss what are properly called “ground-source heat pumps” which are a way-cool technology in their own right.
Around cracks and fissures in the earth’s crust, the heat of earth’s mantle and core seeps through in the form of hot steam, gases or exposed hot rocks and the intrusions of liquid rock, i.e. magma. Geothermal energy in the form of steam and hot gases can be harnessed to turn conventional steam turbines that turn dynamos. As hydrogen sulfide and some other gases may be mixed in with the water vapor, some geothermal power plants need to scrub their emissions to release just the water vapor into the air. Some geothermal plants can produce hydrogen and sulfur as saleable byproducts of scrubbing their emissions. Geothermal steam or hot water can also be used to directly heat buildings, greenhouses, or industrial processes (this is not the same as a geothermal or ground source heat pump).
Iceland meets much of its electric and all of its space-heating needs through geothermal energy. In the United States, the Mountain States and the Pacific coast have a vast majority of the high geothermal potential areas. Currently there are around 3 gigawatts (GW) of geothermal power in the US, which covers the energy needs of around 3 million households. There are around 9 GW geothermal electric installed for the entire world combined.
Potential of Geothermal
Estimates of the potential for geothermal energy vary widely for the US and worldwide depending upon assumptions about technology and the depth to which project leaders and financers would be willing to go to tap into geothermal energy. If we are talking about depths of 20 or 30 kilometers, most areas of the world have the potential for geothermal energy but drilling this deep is very expensive. Using this type of future technology, all the world’s energy needs many times over could be met through geothermal energy but energy costs would have to warrant the building of facilities needed.
Using current technology and energy pricing, the areas of the earth that have the potential for generating electricity from geothermal are usually located at the edges of tectonic plates with notable geological activity. Current estimates in the US indicate that there are 13 GW of geothermal potential or over 4 times the current installed base. With consistent production incentives, at least this capacity can be built in the next few years.
Because of a high capacity factor (near 100%) geothermal energy functions very well as always-on baseline power.
If so-called “Enhanced Geothermal Systems” (EGS) are used which involve drilling to depths of at least 10km, the potential of the US would go up to 100 GW, a substantial portion of US electrical demand. EGS is an attainable future technology that uses conventional oil exploration techniques with the exception that larger boreholes would be needed. Water would be injected down one borehole and steam would be harvested from a paired borehole to drive turbines.
There are three main ways to harvest the heat of the earth depending on the temperature of the water or steam as it emerges at the surface. The earliest geothermal technology is called “dry steam” which uses steam at temperatures above 230 degrees Celsius (455 degrees Fahrenheit) directly from the earth but requires active steam vents from the earth. “Flash steam” uses lower temperature pressurized hot water at over 186 degrees Celsius that vaporizes into steam at lower pressures and drives the turbine. The most common, cleanest, and widely applicable technology is called a binary cycle power plant that uses more common lower temperature hot water pushed through a heat exchanger to vaporize fluids that have a lower boiling temperature which drive a turbine. Binary cycle plants are zero emission as none of the gases are emitted to the atmosphere, while dry and flash steam may release gases other than water vapor in small amounts including carbon dioxide, hydrogen sulfide and nitric oxide.
Some resources have adequate heat but not enough water, so wastewater is injected into the geothermal hot spot to be harvested by the geothermal plant when it reaches the surface.
Criticisms of Geothermal
Geothermal’s limited geographic range under current technology and price constraints is one of its primary drawbacks. Some areas can benefit from geothermal while others would need either deeper drilling or much better transmission infrastructure to get the benefit of geothermal power.
The two older technologies, dry and flash steam, do emit small amounts of gas, including carbon dioxide, though in far lower quantities than fossil fuel plants.
A more fundamental criticism that looks at geothermal on a time scale of decades, centuries and millennia, is whether it is actually technically a renewable resource. There are suggestions that some hot water and steam resources can cool down over time, necessitating more drilling or exploration. Currently, the oldest geothermal field in Italy is still generating power over 100 years later.
Exploiting geothermal resources in an era of carbon constraint is a no-brainer, even if those resources may cool down at an undetermined time in the future. With modern binary-cycle power plants, geothermal has no emissions. Geothermal can function as clean baseline power which supplements well other renewables and replaces coal-fired or even nuclear plants with their attendant environmental and security problems. An investment in research and development of Enhanced Geothermal Systems, estimated at $1 billion would unlock 100 GW of clean energy for the US alone.
An additional benefit of investing in geothermal is that it can leverage the human and financial capital of oil exploration and drilling companies to develop clean energy rather than continue to mine fossil reserves that pollute the air and endanger future generations.