Michigan
State University Extension
Home Maintenance And Repair
06/24/03
Energy Tips - Geothermal Heating and Cooling
An
under-appreciated/utilized source of thermal energy is the earth itself
– the source of geothermal power, the world’s largest power source in
common use for centuries. The word “geothermal” means earth plus heat
and it is an environmentally friendly, renewable resource in contrast
to fossil fuels mined from the earth that are most commonly used as
power generation fuels.
Geothermal energy contributes both to energy supply, with electrical
power generation and direct-heat uses, and to reduced energy demand,
with savings in electricity and natural gas usage by using geothermal
heat pumps to heat and cool buildings. Geothermal power plants operate
worldwide and show that the earth’s thermal energy can be readily
converted to electricity in geologically active areas. Facilities can
be heated directly with hot water from underground reservoirs and the
availability of geothermal heat pumps allows people to tap this
abundant source of thermal energy for use in home heating and
cooling systems.
How It Works
The earth’s core is about 4000 miles deep and scientists
estimate its temperature is 7200º F or higher and heat is
constantly flowing from the interior to the surface. Wells tap
underground reservoirs of steam or hot water and the steam rotates
turbines that generate electricity. Water returns to the ground to
recharge the reservoir and complete the renewable energy cycle. Hot
water can be used directly for greenhouses, spas, fish farms, swimming
pools, and home hot water needs. The U.S. Department of Energy reports
that the world capacity for using geothermal heat is growing at about
9% per year. In addition, the earth itself at depths of 5 to 500 feet
maintains a constant temperature from geothermal heating. We can use
the earth as a heat source or a heat sink by means of a geothermal heat
pump. Heat naturally flows from a higher to a lower temperature and
heat pumps can force heat flow in the other direction for cooling
purposes. In the winter, heat pumps draw from “earth heat” to warm a
house and in summer, they transfer heat from the house to the earth as
a “heat sink.” The subsoil temperature ranges from 50 to 70 degrees
depending on latitude.
Advantages of Geothermal Heating
Geothermal heat is much less expensive in general and causes
almost no environmental pollution in sharp contrast to traditional
power generation plants.Another advantage is that geothermal power is a
domestic energy source so its use avoids the political influences of
importation and foreign policy difficulties. For instance, a greenhouse
owner decides to use geothermal heat to cut exorbitant energy bills so
he can stay in business. He could reduce heating costs by up to 80% and
save about 5% to 8% in total operating costs. Or a homeowner installs a
geothermal heat pump and for her investment she gets low operating and
maintenance costs and usually the lowest life-cycle costs. The heat
pump may cost $15 per month more in mortgage payments, but it may save
$30 per month on the electric bill. And you help reduce the
environmental pollution that comes with traditional power generation.
Types of Geothermal Heat
Hydrothermal resources: reserves of steam or hot water, tapped
by drilling wells for direct use or electricity generation
Geopressured resources: deeply buried water that contains
dissolved methane
Hot dry rock resources: cold water is injected down one well,
circulated through hot fracture rock, and drawn off as hot water from
another well
Magma or molten rock: extremely high temperature geothermal
resources currently not available for heat recovery, but a subject of
intense research
Earth energy: use of the heat contained in soil and rocks at
shallow depths and accessed by geothermal heat pumps.
Environmental Concerns
Traditional power production costs range from 4 to 8
cents/kilowatt hour. Geothermal production promises a life-cycle energy
cost of 3 cents/kilowatt hour. The cost savings is apparent, so let’s
look at the environmental impacts. The production of geothermal energy
involves no combustion so this is a big advantage over fossil fuels
that produce greenhouse gasses when burned. Very low levels of air
emissions are produced in the use of thermal energy. Geothermal energy
production is reliable and efficient and systems need very little
maintenance.
Home Heat Pump Installation and Operation
A geothermal heat pump does not create electricity: it greatly reduces
the consumption of it. Systems don’t convert electricity to heat;
rather, they use electricity to move thermal energy from the ground to
the building. Electricity use is reduced 30 – 60% compared to
traditional heating and cooling systems. This allows a payback on
system installation in two to ten years. And surveys show that the
customer satisfaction rate with geothermal heat pump systems stands at
95% or higher. U.S. DOE estimates that there are more than 400,000
systems operating in the United States today.
Heat pumps transfer heat from natural heat sources in the ground
or water for home heating. For summer cooling, heat is transferred in
the oppposite direction from your home to the surrounding soil and air.
And sometimes the excesss heat from cooling can be used to heat a
home’s hot water supply, saving even more money.
Heat pumps work on the principle of vapor compression and heat
exchangers. The components are connected to form a closed loop and a
fluid circulates in the loop. By pumping a refrigeration fluid through
loops of pipe buried underground around the permineter of your home,
this system uses the relatively constant temperature (45 - 55°F) of
the Earth to transfer heat into buildings in winter and out into the
ground in the summer.
The evaporator keeps the temperature of the fluid lower than the
temperature of the heat source, and heat flows from the heat source to
the liquid. Vapor from the evaporator is compressed to a higher
pressure and temperature; hot vapor enters the condenser where it
condenses and gives off useful heat.The compressor is driven by an
electric motor.
The underground loops can be an open loop system that uses a well and
pumps ground water, or a looppond system when a pond and a discharge
area are available; closed underground loops use the earth's thermal
energy as a resource. Loops can be installed either horizontally or
vertically, or in a pond or lake.This choice is made based on space,
soil and rock types, and ground water/pond/lake availability.
A Renewable and Environmentally-friendly Resource
The U.S. EPA says geothermal heat pumps are one of the most
efficient and least-polluting heating, cooling, and water-heating
systems available. They are an important technology to reduce gaseous
emissions that harm the environment such as carbon dioxide, sulfur
dioxide, and nitrogen oxide. And they can save the consumer money on
fuel costs. Geothermal energy can be a win-win choice for you and the
environment.
References
Energy Tips - Geothermal Heating and Cooling, Michigan State University
Extension E2796
Geothermal
Resources Council, P.O. Box 1350,2001 Second Street, Suite 5,Davis, CA
95617-1350,
http://www.geothermal.org
U.S Department of Energy (DOE), Energy Efficiency and Renewable Energy
Network (EREN),
http://www1.eere.energy.gov/geothermal/
U.S. Environmental Protection Agency,
http://www.epa.gov/
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Thomas G. Coon, Director, MSU Extension, East Lansing,MI 48824. This
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