Geothermal:
How it Works
(Thank you to the Geothermal
Heat Pump Consortium for the information contained on this page and the vapor compression
cycle diagram. © Copyright 1995, 1996, 1997 by Geothermal
Heat Pump Consortium, Inc. Used with permission.)
Overview
Introduction
Geothermal Heating and Cooling
Systems provide space conditioning -- heating, cooling, and humidity
control. They may also provide water heating -- either to supplement
or replace conventional water heaters. Geothermal Heating and
Cooling Systems work by moving heat, rather than by converting
chemical energy to heat like in a furnace. Every Geothermal Heating
and Cooling Systems has three major subsystems or parts: a geothermal
heat pump to move heat between the building and the fluid in the
earth connection, an earth connection for transferring heat between
its fluid and the earth, and a distribution subsystem for delivering
heating or cooling to the building. Each system may also have
a desuperheater to supplement the building's water heater, or
a full-demand water heater to meet all of the building's hot water
needs.
See Ground Loop's Geothermal
Simulator for a basic look
at summer and winter geothermal operation.
Geothermal Heat Pump
The geothermal heat pump is packaged
in a single cabinet, and includes the compressor, loop-to-refrigerant
heat exchanger, and controls. Systems that distribute heat using
ducted air also contain the air handler, duct fan, filter, refrigerant-to-air
heat exchanger, and condensate removal system for air conditioning.
For home installations, the geothermal heat pump cabinet is usually
located in a basement, attic, or closet. In commercial installations,
it may be hung above a suspended ceiling or installed as a self-contained
console. (Please see Details
and Diagrams below)
Distribution Subsystem
Most residential geothermal systems
use conventional ductwork to distribute hot or cold air and to
provide humidity control. (A few systems use water-to-water heat
pumps with one or more fan-coil units, baseboard radiators, or
under-floor circulating pipes.) Properly sized, constructed, and
sealed ducts are essential to maintain system efficiency. Ducts
must be well insulated and, whenever possible, located inside
of the building's thermal envelope (conditioned space).
Geothermal heating and cooling systems for large commercial buildings,
such as schools and offices, often use a different arrangement.
Multiple heat pumps (perhaps one for each classroom or office)
are attached to the same earth connection by a loop inside the
building. This way, each area of the building can be individually
controlled. The heat pumps on the sunny side of the building may
provide cooling while those on the shady side are providing heat.
This arrangement is very economical, as heat is merely being transferred
from one area of the building to another, with the earth connection
serving as the heat source or heat sink only for the difference
between the building's heating and cooling needs.
Water Heating
Many residential-sized systems
installed today are equipped with desuperheaters to provide domestic
hot water when the system is providing heat or air conditioning.
The desuperheater is a small auxiliary heat exchanger at the compressor
outlet. It transfers excess heat from the compressed gas to a
water line that circulates water to the house's hot water tank.
In summer, when the air conditioning runs frequently, a desuperheater
may provide all the hot water needed by a household. It can provide
four to eight gallons of hot water per ton of cooling capacity
each hour it operates. A desuperheater provides less hot water
during the winter, and none during the spring and fall when the
system is not operating.
Because the heat pump is so much more efficient than other means
of water heating, manufacturers are beginning to offer "triple
function," "full condensing," or "full demand"
systems that use a separate heat exchanger to meet all of a household
hot water needs. These units cost-effectively provide hot water
as quickly as any competing system.
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Types of Systems
Geothermal systems use the earth
as a heat source and heat sink. A series of pipes, commonly called
a "loop," carry a fluid used to connect the geothermal
system's heat pump to the earth.
Closed
and Open Loops
There are two basic types of loops: closed and
open.
Open loop systems are the simplest. Used successfully for decades,
ground water is drawn from an aquifer through one well, passes
through the heat pump's heat exchanger, and is discharged to the
same aquifer through a second well at a distance from the first.
Generally, two to three gallons per minute per ton of capacity
are necessary for effective heat exchange. Since the temperature
of ground water is nearly constant throughout the year, open loops
are a popular option in areas where they are permitted. Open loop
systems do have some associated challenges:
-
Some local ground water
chemical conditions can lead to fouling the heat pump's heat
exchanger. Such situations may require precautions to keep
carbon dioxide and other gases in solution in the water. Other
options include the use of cupronickel heat exchangers and
heat exchangers that can be cleaned without introducing chemicals
into the groundwater.
-
Increasing environmental
concerns mean that local officials must be consulted to assure
compliance with regulations concerning water use and acceptable
water discharge methods. For example, discharge to a sanitary
sewer system is rarely acceptable.
Closed loop systems are becoming the most
common. When properly installed, they are economical, efficient,
and reliable. Water (or a water and antifreeze solution) is circulated
through a continuous buried pipe. The length of loop piping varies
depending on ground temperature, thermal conductivity of the ground,
soil moisture, and system design. (Some heat pumps work well with
larger inlet temperature variations, which allows marginally smaller
loops).
Horizontal
Loops
Horizontal closed loop installations are generally most cost-effective
for small installations, particularly for new construction where
sufficient land area is available. These installations involve
burying pipe in trenches dug with back-hoes or chain trenchers.
Up to six pipes, usually in parallel connections, are buried in
each trench, with minimum separations of a foot between pipes
and ten to fifteen feet between trenches.
Vertical
Loops
Vertical closed loops are preferred in many situations. For example,
most large commercial buildings and schools use vertical loops
because the land area required for horizontal loops would be prohibitive.
Vertical loops are also used where the soil is too shallow for
trenching. Vertical loops also minimize the disturbance to existing
landscaping.
For vertical closed loop systems, a U-tube (more rarely, two U-tubes)
is installed in a well drilled 100 to 400 feet deep. Because conditions
in the ground may vary greatly, loop lengths can range from 130
to 300 feet per ton of heat exchange. Multiple drill holes are
required for most installations, where the pipes are generally
joined in parallel or series-parallel configurations.
Slinky Loops
Increasingly, "Slinky" coils -- overlapping coils of
polyethylene pipe -- are used to increase the heat exchange per
foot of trench, but require more pipe per ton of capacity. Two-pipe
systems may require 200 to 300 feet of trench per ton of nominal
heat exchange capacity. The trench length decreases as the number
of pipes in the trench increases -- or as Slinky coil overlap
increases. (Illustration below shows a slinky coil in a pond)
Pond
Loops
Pond closed loops are a special kind of closed loop system. Where
there is a pond or stream that is deep enough and with enough
flow, closed loop coils can be placed on the pond bottom. Fluid
is pumped just as for a conventional closed loop ground system
where conditions are suitable, the economics are very attractive,
and no aquatic system impacts have been shown.
Conclusion
Geothermal heating and cooling systems can be connected to the
earth in a variety of ways -- all thoroughly field proven. However,
high performance requires the use of experienced professionals
who understand local conditions. Contact
Ground Loop today to discuss the best system for you.
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Details and Diagrams
Introduction
Refrigerators and air conditioners both contain heat pumps. In
a refrigerator, heat is moved from the food storage sections and
discharged to the kitchen air. Air conditioners work the same
way -- they move heat from the inside of a building and discharge
it to the outside air. Conventional or air source heat pumps differ
from those in a refrigerator or air conditioner because they are
reversible -- they can concentrate heat from the outside air and
move it inside to provide warmth, as well as move heat out of
the building to provide cooling. To do this, air-source heat pumps
(and central air conditioners) need a large outside unit to exchange
heat with the outdoor air.
Geothermal
Heating and Cooling Systems (see our Geothermal
Simulator)
In a geothermal heating and cooling systems, the heat pump is
connected to the building by a distribution system -- most commonly
air ducts. And the heat pump is connected to the earth through
a series of pipes called a "loop" (see Types
of Systems above). The system exchanges
heat with the earth, meaning that no noisy or unsightly outdoor
unit is needed.
Vapor Compression Cycle
(click here
for illustrative diagram)
All heat pumps use a vapor compression cycle to transport heat
from one location to another. In heating mode, the cycle starts
as the cold liquid refrigerant within the heat pump passes through
a heat exchanger (evaporator) and absorbs heat from the low-temperature
source (fluid circulated through the earth connection). The refrigerant
evaporates into a gas as heat is absorbed. The gaseous refrigerant
then passes through a compressor where it is pressurized, raising
its temperature to over 180 degrees F. The hot gas then circulates
through a refrigerant-to-air heat exchanger where the heat is
removed and sent through the air ducts. When the refrigerant loses
the heat, it changes back to a liquid. The liquid refrigerant
cools as it passes through an expansion valve, and the process
begins again.
Although heat pumps are complex internally, they are marvels of
compact design for reliability. Some include features such as
additional heat exchangers for water heating, and microprocessor-based
automatic controls and protection devices.
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Geothermal Advantages
Efficiency
Geothermal heat pumps are much more efficient than air source
heat pumps because earth temperatures are much more uniform through
the year than air temperatures. Not only are earth temperatures
more constant, but also the range of temperatures in ground water
is rather small in the United States, varying from upper 40s to
upper 70s nationwide.
To further improve efficiency, many manufacturers use variable-speed,
electronically-controlled motors on the duct system fans. Depending
on unit size, manufacturers may opt for reciprocating, inertia,
rotary, or scroll compressors -- all of which are hermetically
sealed and mounted in the indoor cabinet. Some advanced heat pumps
feature two-speed or variable speed operation while others feature
dual compressors to vary output capacity and match loads.
The Benefits Speak for
Themselves
Tens of thousands of homes are being built or retrofitted with
geothermal heating and cooling systems every year, because of
the advantages these systems offer: economical operation, noise
reduction, and product quality. Initially reserved for the most
expensive homes, now these systems have become affordable options
for thousands of low and moderate income housing units because
of the system's low life cycle costs compared to all other alternatives
in almost every region of the country. According to the Environmental Protection Agency, geoexchange systems save homeowners more than 30-70% in heating costs 20-50% in cooling compared to conventional systems.
Geothermal Heating and
Cooling Makes Sense
Geothermal systems are efficient, environmentally-sensitive, comfortable,
and economical. Operating savings often provide paybacks of considerably
less than five years -- sometimes less than two years.
The key is that geothermal heat pumps use electricity to move
heat, not to generate it by the burning fuel or using electric
resistance elements. Indeed, the U.S. EPA has found that no other
technology with more favorable operating efficiencies and economics
than emerging geothermal heat and cooling systems.
An Important Renewable
Energy Technology
The U.S. Environmental Protection Agency has concluded that well-designed
and properly installed high efficiency geothermal heat pump systems
produce less environmental harm than any other alternative space
conditioning technology currently available. On a full fuel cycle
basis, emerging geothermal systems are the most efficient technology
available, with the lowest CO2 emissions for minimum greenhouse
warming impact. Overall, the EPA found emerging geothermal heating
and cooling systems to have the lowest environmental cost of all
technologies analyzed -- including air-source heat pumps and natural
gas furnaces.
Geothermal Heat Pump
Systems Work!
No existing space conditioning technology offers greater comfort,
economy, or environmental benefits than the geothermal heat and
cooling systems now available for residential and commercial installations.
Over 250,000 installations are in place in the United States today,
and the number is rapidly increasing. More than 95 percent of
all geothermal heat and cooling customers are completely satisfied
with their systems.
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Ground Loop Heating and Air Conditioning
installs and services many loop
types and different
manufacturers for geothermal heat
pump systems. We can recommend the best system for your particular
needs.
Type of loop
Three factors that affect your
choice for type of loop: characteristics of your lot, size of
your house, and your budget. Ground Loop can custom design loops to accommodate all factors.
Horizontal loops are usually the most economical. They also require the most amount
of property area.
Vertical loops use the least amount of space and are thus the most practical,
and sometimes the only choice for small lots. Drilling the vertical
holes can increase the time to realize a return on your investment.
Slinky (coiled) loops can save time and money. They require less area than the standard
horizontal loops and are generally more economical than vertical
loops. They are usually placed flat in a horizontal position,
but can sometimes be placed vertically (upright), using even less
space.
Other possible types are open
well and pond closed loop. Both types
require specific conditions at the building site to be installed.
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Other Considerations
Piping
Special thick but flexible piping is used in geothermal
installations. The most commonly used type of pipe is high quality,
high density polyethylene. All below-grade connections must be
made by heat fusing, which yields connections stronger than the
pipe itself and without any threads. All installations must be
purged to remove construction debris, flushed to remove air, and
pressure tested (to 100 PSI) before backfilling or grouting. Ground
Loop guarantees their loops for twenty years.
Sizing
Numerous studies have shown that residential heat pumps are often
sized too large. Heat pumps that are too large waste energy and
do not provide proper humidity control. Where heating needs are
much greater than cooling needs, advanced heat pumps (see above)
can improve Summer comfort. Ground Loop uses detailed information
about your home as well as their experience to recommend the right
type of unit and loops for your home.
Purchase and Installation
As in the case of all major decisions, choosing a geothermal heating
and cooling system requires some careful consideration to insure
that the consumer receives the highest quality system and installation.
The Geothermal Heat Pump Consortium (GHPC) offers the following
guidelines:
-
Ratings and certification: The Air-Conditioning and Refrigeration Institute (ARI) is
a non-profit organization that certifies performance of residential
and small commercial geothermal equipment. For equipment classes
rated by ARI, the GHPC suggests that consumers look for the
ARI seal. (ARI does not rate large, commercial geothermal
equipment greater than 135,000 Btu/hr).
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Warrantees: Manufacturers' terms of warranty vary. To assure a high quality
installation, consumers should seek a performance guarantee
on the installed system, as opposed to coverage limited to
the heat pump itself.
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Sizing: Numerous studies have shown that heat pumps are often sized
too large. For maximum efficiency, heating and cooling loads
should be carefully determined. The actual unit size should
be within 15 percent of the calculated load. Heat pumps that
are too large waste energy and do not provide proper humidity
control. For an excellent article on sizing, see Bigger
is Not Better: Sizing Air Conditioners Properly in the May/June issue of Home Energy Magazine. While the article
focuses on air conditioning, the same principles apply to
heat pumps. [Use the "Back" button on your browser
to return to this page.]
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Design: Residential system design is straight-forward, but the most
satisfactory results are obtained with experienced contractors.
Aspects to consider include careful duct design and installation
in which ducts are kept in conditioned spaces and are permanently
sealed; high quality insulation, glazing, and other envelope
features to minimize loads and reduce air infiltration; and
careful matching of heat pump and earth connection capacity
to building load. It may be advantageous to specify two-speed
or variable speed systems where heating requirements are much
greater than cooling, to assure good humidity control. Finally,
the contractor should examine ways to use the geothermal system
to provide hot water.
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Installation: Proper installation is a key to success. The GHPC recommends
that consumers employ experienced contractors who will provide
the names of customers with comparable systems.
Dealing with experienced professionals
who size and install ground loops is essential. It is critical
that the installation prescriptions of the International
Ground Source Heat Pump Association (IGSHPA) are followed.
Ground Loop is IGSHPA certified as well as a member of the Geothermal
Heat Pump Consortium (GHPC).
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