The word geothermal
comes from the Greek words geo (earth) and therme (heat). So, geothermal energy
is heat from within the earth. We can use the steam and hot water produced
inside the earth to heat buildings or generate electricity. Geothermal energy is
a renewable
energy source because the water is replenished by rainfall and the heat is
continuously produced inside the earth.
Geothermal energy is generated in
the earth's core, about 4,000 miles below the surface. Temperatures hotter than
the sun's surface are continuously produced inside the earth by the slow decay
of radioactive particles, a process that happens in all rocks. The earth has a
number of different layers: The core itself has two layers: a solid iron core
and an outer core made of very hot melted rock, called magma. The mantle which
surrounds the core and is about 1,800 miles thick. It is made up of magma and
rock. The crust is the outermost layer of the earth, the land that forms the
continents and ocean floors. It can be three to five miles thick under the
oceans and 15 to 35 miles thick on the continents. The earth's crust is broken
into pieces called plates. Magma comes close to the earth's surface near the
edges of these plates. This is where volcanoes occur. The lava that erupts from
volcanoes is partly magma. Deep underground, the rocks and water absorb the heat
from this magma. The temperature of the rocks and water get hotter and hotter as
you go deeper underground. People around the world use geothermal energy to heat
their homes and to produce electricity by digging deep wells and pumping the
heated underground water or steam to the surface. Or, we can make use of the
stable temperatures near the surface of the earth to heat and cool buildings.
Most geothermal reservoirs are
deep underground with no visible clues showing above ground. Geothermal energy
can sometimes find its way to the surface in the form of: volcanoes and
fumaroles (holes where volcanic gases are released) hot springs and geysers. The
most active geothermal resources are usually found along major plate boundaries
where earthquakes and volcanoes are concentrated. Most of the geothermal
activity in the world occurs in an area called the Ring of Fire. This area rims
the Pacific Ocean.
When magma comes close to the
surface it heats ground water found trapped in porous rock or water running
along fractured rock surfaces and faults. Such hydrothermal resources have two
common ingredients: water (hydro) and heat (thermal). Naturally occurring large
areas of hydrothermal resources are called geothermal reservoirs. Geologists use
different methods to look for geothermal reservoirs. Drilling a well and testing
the temperature deep underground is the only way to be sure a geothermal
reservoir really exists. Most of the geothermal reservoirs in the United States
are located in the western states, Alaska, and Hawaii. California is the state
that generates the most electricity from geothermal energy. The Geysers dry
steam reservoir in northern California is the largest known dry steam field in
the world. The field has been producing electricity since 1960.
Some applications of geothermal
energy use the earth's temperatures near the surface, while others require
drilling miles into the earth. The three main uses of geothermal energy are:
1) Direct Use and District
Heating Systems which use hot water from springs or reservoirs near the
surface.
2) Electricity generation in a
power plant requires water or steam at very high temperature (300 to 700 degrees
Fahrenheit). Geothermal power plants are generally built where geothermal
reservoirs are located within a mile or two of the surface.
3) Geothermal heat pumps use
stable ground or water temperatures near the earth's surface to control building
temperatures above ground.
The direct use of hot water as an
energy source has been happening since ancient times. The Romans, Chinese, and
Native Americans used hot mineral springs for bathing, cooking and heating.
Hot water near the earth's
surface can be piped directly into buildings and industries for heat. A district
heating system provides heat for 95 percent of the buildings in Reykjavik,
Iceland.
In Iceland, there are five major
geothermal power plants which produce about 26% (2006) of the country's
electricity. In addition, geothermal heating meets the heating and hot water
requirements for around 87% of the nation's housing. In 2006, 26.5% of
electricity generation in Iceland came from geothermal energy, 73.4% from hydro
power, and 0.1% from fossil fuels
Geothermal power plants use
hydrothermal
resources which have two common ingredients: water (hydro) and heat (thermal).
Geothermal plants require high temperature (300 to 700 degrees Fahrenheit)
hydrothermal resources that may come from either dry steam wells or hot water
wells. We can use these resources by drilling wells into the earth and piping
the steam or hot water to the surface. Geothermal wells are one to two miles
deep.
The United States generates more
geothermal electricity than any other country but the amount of electricity it
produces is less than 1 percent of electricity produced in United States. Only
four states have geothermal power plants:
California - has 33
geothermal power plants that produce almost 90 percent of the nation's
geothermal electricity.
Nevada - has 15 geothermal
power plants.
Hawaii and Utah - each have
one geothermal plant
There are three basic types of
geothermal power plants:
Dry steam plants - use steam
piped directly from a geothermal reservoir to turn the generator turbines.
The first geothermal power plant was built in 1904 in Tuscany, Italy at a
place where natural steam was erupting from the earth.
Flash steam plants - take
high-pressure hot water from deep inside the earth and convert it to steam
to drive the generator turbines. When the steam cools, it condenses to water
and is injected back into the ground to be used over and over again. Most
geothermal power plants are flash plants.
Binary power plants -
transfer the heat from geothermal hot water to another liquid. The heat
causes the second liquid to turn to steam which is used to drive a generator
turbine.
GEOTHERMAL HEAT
PUMPS
While temperatures above ground
change a lot from day to day and season to season, temperatures in the upper 10
feet of the Earth's surface hold nearly constant between 50 and 60 degrees
Fahrenheit. For most areas, this means that soil temperatures are usually warmer
than the air in winter and cooler than the air in summer. Geothermal heat pumps
use the Earth's constant temperatures to heat and cool buildings. They transfer
heat from the ground (or water) into buildings in winter and reverse the process
in the summer.
According to the U.S.
Environmental Protection Agency (EPA), geothermal heat pumps are the most
energy-efficient, environmentally clean, and cost-effective systems for
temperature control. Although, most homes still use traditional furnaces and air
conditioners, geothermal heat pumps are becoming more popular. In recent years,
the U.S. Department of Energy along with the EPA have partnered with industry to
promote the use of geothermal heat pumps.
Types of
Geothermal Heat Pump Systems
There are four basic types of
ground loop systems. Three of these—horizontal, vertical, and pond/lake—are
closed-loop systems. The fourth type of system is the open-loop option. Which
one of these is best depends on the climate, soil conditions, available land,
and local installation costs at the site. All of these approaches can be used
for residential and commercial building applications.
Closed-Loop Systems
Horizontal
This type of installation is
generally most cost-effective for residential installations, particularly for
new construction where sufficient land is available. It requires trenches at
least four feet deep. The most common layouts either use two pipes, one buried
at six feet, and the other at four feet, or two pipes placed side-by-side at
five feet in the ground in a two-foot wide trench. The Slinky™ method of looping
pipe allows more pipe in a shorter trench, which cuts down on installation costs
and makes horizontal installation possible in areas it would not be with
conventional horizontal applications.
Vertical
Large commercial buildings and
schools often use vertical systems because the land area required for horizontal
loops would be prohibitive. Vertical loops are also used where the soil is too
shallow for trenching, and they minimize the disturbance to existing
landscaping. For a vertical system, holes (approximately four inches in
diameter) are drilled about 20 feet apart and 100–400 feet deep. Into these
holes go two pipes that are connected at the bottom with a U-bend to form a
loop. The vertical loops are connected with horizontal pipe (i.e., manifold),
placed in trenches, and connected to the heat pump in the building.
Pond/Lake
If the site has an adequate water
body, this may be the lowest cost option. A supply line pipe is run underground
from the building to the water and coiled into circles at least eight feet under
the surface to prevent freezing. The coils should only be placed in a water
source that meets minimum volume, depth, and quality criteria.
Open-Loop System
This type of system uses well or
surface body water as the heat exchange fluid that circulates directly through
the GHP system. Once it has circulated through the system, the water returns to
the ground through the well, a recharge well, or surface discharge. This option
is obviously practical only where there is an adequate supply of relatively
clean water, and all local codes and regulations regarding groundwater discharge
are met.
GEOTHERMAL ENERGY
AND THE ENVIRONMENT
The environmental impact of
geothermal energy depends on how it is being used.
Direct use and heating
applications have almost no negative impact on the environment.
Geothermal power plants do
not burn fuel to generate electricity, so their emission levels are very
low. They release about 1 to 3 percent of the carbon dioxide emissions of a
fossil fuel plant. Geothermal plants use scrubber systems to clean the air
of hydrogen sulfide that is naturally found in the steam and hot water.
Geothermal plants emit 97 percent less acid rain - causing sulfur compounds
than are emitted by fossil fuel plants. After the steam and water from a
geothermal reservoir have been used, they are injected back into the earth.
