Geothermal Energy

geothermal energy


Many technologies have been developed to take advantage of geothermal energy—the heat from the earth. This heat can be drawn from several sources: hot water or steam reservoirs deep in the earth that are accessed by drilling; geothermal reservoirs located near the earth’s surface, mostly located in the western U.S., Alaska, and Hawaii; and the shallow ground near the Earth’s surface that maintains a relatively constant temperature of 50°–60°F.

So how might geothermal energy fit into your life? Would you like to be able to sit back, relax, and let Mother Nature provide you with free utilities? Unfortunately, it’s not that easy. But even if you don’t intend to buy a hot spring or drill your own hot water well, you may be interested in knowing how geothermal energy is used for heating purposes.

Major Factors to Consider

Four major factors must be considered before you undertake a geothermal heating project:

  • Finding a geothermal resource and establishing ownership
  • Piping geothermal fluid to your home or business
  • Using the proper heating equipment
  • Disposing properly of the spent geothermal fluid

Several thousand miles beneath the surface, the earth’s core is surrounded by molten rock (called magma) with temperatures approaching 6,000°F. We are protected from these extreme temperatures by the insulating soil and rock between the earth’s crust and the magma. There are deep cracks, or fault lines, in the earth. Groundwater that finds its way into these fault lines circulates much deeper than most water, and comes into contact with much warmer areas of the earth’s interior. If this warmed groundwater encounters another fault line, it may move upwards to the earth’s surface, where it emerges as a hot spring.

Well drillers occasionally hit hot water by accident. More often, geologists assist in determining where geothermal wells should be drilled. Many of the same techniques used in discovering oil and gas are used to find geothermal deposits: seismic soundings, gravity measurements, resistivity, and test drilling have all been used.

Before you start drilling a geothermal well or using a hot spring for heating, you must obtain a water rights permit from the Department of Natural Resources and Conservation, Water Resources Division.

Before you can use your geothermal resource for heating, you must pipe the hot water from the well or spring to your home or business. More than a century ago, wooden pipelines were used to transport geothermal water. Remains of these pipelines can be found near some of Montana’s early-day hot springs resorts. Today, wood is rarely the preferred material for geothermal piping. Instead, various types of plastic, adapted to specific temperature ranges, are used. Among the materials used are polyethylene, polybutylene, PVC and CPVC. For extremely hot water (over 200°F), carbon steel piping is used.

Water quality can also affect the type of piping used for a geothermal system. Contaminants can cause corrosion or sedimentation in geothermal piping. To prevent this from happening, a device called a “heat exchanger” is sometimes used. This consists of a series of stainless steel plates, with the dirty geothermal water on one side of the plate and clean cold water on the other side of the plate. The geothermal water exchanges its heat through the plate to the clean water, which is in turn warmed. The heated clean water can then be used in the piping and heating system.

Geothermal piping is often insulated to prevent heat loss from the pipe before the water reaches its end use. The simplest insulating method is burying the bare pipe in the ground. Sometimes insulating foam is blown around the pipe before it’s buried, or rigid insulation is installed around the pipe during manufacturing. A well-insulated geothermal pipeline can transport geothermal water for several miles without losing more than one or two degrees F per mile. The cost of the piping may be one of your biggest expenses in developing a geothermal resource. Be sure that the piping material selected and the insulation method used are the best suited for your particular application.

Space Heating

One potential that geothermal energy holds is space heating. Geothermal resources as cool as 85°F are being used to heat homes, greenhouses, and commercial buildings. However, this isn’t the lower temperature limit of geothermal’s potential. Devices called ground-source heat pumps or ground-source heat pumps can pull energy out of water as cold as that from your cold water tap (around 45°F).

If geothermal water is available in the 85°F range upwards, it can be used directly for heating without using a heat pump. Four direct heating methods can be used, depending on the water temperature. When geothermal water over 85°F is available, radiant slab heating works well. Plastic or metal piping is laid in a concrete floor as it is poured, a few inches beneath the floor surface. Geothermal water is then pumped through the piping, warming the floor, and heating the space above.

Geothermal water above 105°F can be used in a forced-air system similar to a natural gas furnace. In this heating system, the water is piped through a series of finned tubes resembling a car radiator placed inside the plenum of a gas or electric furnace. The furnace fan forces cold air through the heated finned tubes, warming the air, which then blows through the furnace ducts into the house.

If the water is warmer than 140°F, finned tubes may be placed along the walls of a room. Geothermal water then flows through the tubes, heating the fins that radiate heat into the room. Cooler water temperatures also work in this system, but the length of finned tubes required to comfortably heat a space increases dramatically if cooler geothermal temperatures are used.

Finally, geothermal water warmer than 160°F can be used in the old-style cast-iron radiators. You’re probably familiar with this type of heating system in many older steam-heated buildings. Although this is the least efficient method of using geothermal heat, it has been used for more than one hundred years.

The flow rate required for heating a building varies with the temperature of the water. In general, the warmer the geothermal water, the less flow is needed. For example, a 1,000?square-foot home in western Montana might require 14 gallons per minute of 85°F water in a radiant slab heating system, but would only need 2 gallons of 140°F water to heat the same building with a finned tube heating system.

No matter what type of heating system you are considering, conservation is almost always a good first step. Weatherstripping, caulking and insulating a structure will make your home feel more comfortable and will require less geothermal energy than a poorly insulated structure. Do a careful analysis of exactly how much energy you need before you invest in geothermal energy equipment? It may turn out that a combination of conservation and solar energy may be a better investment.

Waste Disposal
After you’ve heated your home or business with geothermal energy, you’re faced with one final problem: What do you do with the spent geothermal water? This water often contains a high percentage of minerals, and may have an elevated temperature that could thermally pollute lakes, streams, and groundwater supplies. Proper disposal of the spent geothermal fluid is needed to help maintain high water quality.

You have three options when considering geothermal waste disposal: surface discharge, discharge to an artificial impoundment, or re-injection into an aquifer. The method you select for disposing of the used geothermal water will depend both on environmental considerations and on economics. The Department of Natural Resources and Conservation requires permits for any surface discharge or re-injection of geothermal water; contact them before you decide on your disposal method.

Geothermal Resources

Geothermal Heat Pumps

If you’re planning to build a new house, office building, or school, or replace your heating and cooling system, you may want to consider a geothermal heat pump (GHP) system. GHP systems are also known as ground-source, or water-source heat pumps (as opposed to air-source heat pumps). Regardless of what you call them, energy-efficient geothermal heat pumps are available today for both residential and commercial building applications.

A GHP system can be installed in virtually any area of the country and will save energy and money.

The biggest benefit of GHPs is that they use 25%–50% less electricity than conventional heating or cooling systems. This translates into a GHP using one unit of electricity to move three units of heat from the earth. According to EPA, geothermal heat pumps can reduce energy consumption—and corresponding emissions—up to 44% compared to air-source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment. GHPs also improve humidity control by maintaining about 50% relative indoor humidity, making GHPs very effective in humid areas.

Geothermal heat pump systems allow for design flexibility and can be installed in both new and retrofit situations. Because the hardware requires less space than that needed by conventional HVAC systems, the equipment rooms can be greatly scaled down in size, freeing space for productive use. GHP systems also provide excellent “zone” space conditioning, allowing different parts of your home to be heated or cooled to different temperatures.

Because GHP systems have relatively few moving parts, and because those parts are sheltered inside a building, they are durable and highly reliable. The underground piping often carries warranties of 25–50 years, and the heat pumps often last 20 years or more. Since they usually have no outdoor compressors, GHPs are not susceptible to vandalism. On the other hand, the components in the living space are easily accessible, which increases the convenience factor and helps ensure that the upkeep is done on a timely basis.

Because they have no outside condensing units like air conditioners, there’s no concern about noise outside the home. A two-speed GHP system is so quiet inside a house that users do not know it is operating: there are no tell-tale blasts of cold or hot air.

Many technologies have been developed to take advantage of geothermal energy – the heat from the earth. NREL performs research to develop and advance technologies for the following geothermal applications:

Geothermal Energy Technologies:



Even though a number of successful geothermal heating projects are operating, we’ve only started to tap the enormous energy potential of the earth’s natural heat.

Perhaps you’ve got your own ideas on how to use geothermal energy and would like to know more about adapting it to your own project. If so, you may want to visit the Geo-Heat Center website, which features library databases, publications, maps and a comprehensive list of geothermal links. But even if your only contact with geothermal is soaking in a hot springs on a frosty winter evening with snow falling on your ears, geothermal resources will have proved their value.