Geothermal Heat Pumps
Reprinted
from US Department of Energy Web Site
Geothermal
heat pumps (sometimes referred to as GeoExchange,
earth-coupled, ground-source, or water-source heat
pumps) have been in use since the late 1940s.
Geothermal heat pumps (GHPs) use the constant
temperature of the earth as the exchange medium instead
of the outside air temperature. This allows the system
to reach fairly high efficiencies (300%-600%) on the
coldest of winter nights, compared to 175%-250% for
air-source heat pumps on cool days.
While many parts of the country experience seasonal
temperature extremes—from scorching heat in the summer
to sub-zero cold in the winter—a few feet below the
earth's surface the ground remains at a relatively
constant temperature. Depending on latitude, ground
temperatures range from 45°F (7°C) to 75°F (21°C). Like
a cave, this ground temperature is warmer than the air
above it during the winter and cooler than the air in
the summer. The GHP takes advantage of this by
exchanging heat with the earth through a ground heat
exchanger.
As with any heat pump, geothermal and water-source heat
pumps are able to heat, cool, and, if so equipped,
supply the house with hot water. Some models of
geothermal systems are available with two-speed
compressors and variable fans for more comfort and
energy savings. Relative to air-source heat pumps, they
are quieter, last longer, need little maintenance, and
do not depend on the temperature of the outside air.
A dual-source heat pump combines an air-source heat
pump with a geothermal heat pump. These appliances
combine the best of both systems. Dual-source heat
pumps have higher efficiency ratings than air-source
units, but are not as efficient as geothermal units.
The main advantage of dual-source systems is that they
cost much less to install than a single geothermal
unit, and work almost as well.
Even though the installation price of a geothermal
system can be several times that of an air-source
system of the same heating and cooling capacity, the
additional costs are returned to you in energy savings
in 5–10 years. System life is estimated at 25 years for
the inside components and 50+ years for the ground
loop. There are approximately 50,000 geothermal heat
pumps installed in the United States each year.
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.
Benefits
of Geothermal Heat Pump Systems
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 the 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.
Selecting and Installing a Geothermal Heat Pump System
Heating and Cooling Efficiency of Geothermal Heat Pumps
The heating efficiency of ground-source and water-source heat pumps is indicated by their coefficient of performance (COP), which is the ratio of heat provided in Btu per Btu of energy input. Their cooling efficiency is indicated by the Energy Efficiency Ratio (EER), which is the ratio of the heat removed (in Btu per hour) to the electricity required (in watts) to run the unit. Look for the ENERGY STAR® label, which indicates a heating COP of 2.8 or greater and an EER of 13 or greater.
Manufacturers of high-efficiency geothermal heat pumps voluntarily use the EPA ENERGY STAR label on qualifying equipment and related product literature. If you are purchasing a geothermal heat pump and uncertain whether it meets ENERGY STAR qualifications, ask for an efficiency rating of at least 2.8 COP or 13 EER.
Many geothermal heat pump systems carry the U.S. Department of Energy (DOE) and EPA ENERGY STAR label. Ask your contractor about special financing or incentives for purchasing energy efficient products, including ENERGY STAR qualified products.
Economics of Geothermal Heat Pumps
Geothermal heat pumps save money in operating and maintenance costs. While the initial purchase price of a residential GHP system is often higher than that of a comparable gas-fired furnace and central air-conditioning system, it is more efficient, thereby saving money every month. For further savings, GHPs equipped with a device called a "desuperheater" can heat the household water. In the summer cooling period, the heat that is taken from the house is used to heat the water for free. In the winter, water heating costs are reduced by about half.
On average, a geothermal heat pump system costs about $2,500 per ton of capacity, or roughly $7,500 for a 3-ton unit (a typical residential size). ). A system using horizontal ground loops will generally cost less than a system with vertical loops. In comparison, other systems would cost about $4,000 with air conditioning.
Although initially more expensive to install than conventional systems, properly sized and installed GHPs deliver more energy per unit consumed than conventional systems.
And since geothermal heat pumps are generally more efficient, they are less expensive to operate and maintain — typical annual energy savings range from 30% to 60%. Depending on factors such as climate, soil conditions, the system features you choose, and available financing and incentives, you may even recoup your initial investment in two to ten years through lower utility bills.
But when included in a mortgage, your GHP will have a positive cash flow from the beginning. For example, say that the extra $3,500 will add $30 per month to each mortgage payment. The energy cost savings will easily exceed that added mortgage amount over the course of each year.
On a retrofit, the GHP's high efficiency typically means much lower utility bills, allowing the investment to be recouped in two to ten years. It may also be possible to include the purchase of a GHP system in an "energy-efficient mortgage" that would cover this and other energy-saving improvements to the home. Banks and mortgage companies can provide more information on these loans.
There may be a number of special financing options and incentives available to help offset the cost of adding a geothermal heat pump (GHP) to your home. These provisions are available from federal, state, and local governments; power providers; and banks or mortgage companies that offer energy-efficient mortgage loans for energy-saving home improvements. Be sure the system you're interested in qualifies for available incentives before you make your final purchase.
To find out more about financing and incentives that are available to you, visit the Database of State Incentives for Renewable Energy (DSIRE) Web site. The site is frequently updated with the latest incentives. You should also check with your electric utility and ask if they offer any rebates, financing, or special electric rate programs.
Evaluating Your Site for a Geothermal Heat Pump
Because shallow ground temperatures are relatively constant throughout the United States, geothermal heat pumps (GHPs) can be effectively used almost anywhere. However, the specific geological, hydrological, and spatial characteristics of your land will help your local system supplier/installer determine the best type of ground loop for your site:
Geology
Factors such as the composition and properties of your soil and rock (which can affect heat transfer rates) require consideration when designing a ground loop. For example, soil with good heat transfer properties requires less piping to gather a certain amount of heat than soil with poor heat transfer properties. The amount of soil available contributes to system design as well — system suppliers in areas with extensive hard rock or soil too shallow to trench may install vertical ground loops instead of horizontal loops.
Hydrology
Ground or surface water availability also plays a part in deciding what type of ground loop to use. Depending on factors such as depth, volume, and water quality, bodies of surface water can be used as a source of water for an open-loop system, or as a repository for coils of piping in a closed-loop system. Ground water can also be used as a source for open-loop systems, provided the water quality is suitable and all ground water discharge regulations are met.
Before you purchase an open-loop system, you will want to be sure your system supplier/installer has fully investigated your site's hydrology, so you can avoid potential problems such as aquifer depletion and groundwater contamination. Antifreeze fluids circulated through closed-loop systems generally pose little to no environmental hazard.
Land Availability
The amount and layout of your land, your landscaping, and the location of underground utilities or sprinkler systems also contribute to your system design. Horizontal ground loops (generally the most economical) are typically used for newly constructed buildings with sufficient land. Vertical installations or more compact horizontal "Slinky™" installations are often used for existing buildings because they minimize the disturbance to the landscape.
Installing Geothermal Heat Pumps
Because of the technical knowledge and equipment needed to properly install the piping, a GHP system installation is not a do-it-yourself project. To find a qualified installer, call your local utility company, the International Ground Source Heat Pump Association or the Geothermal Heat Pump Consortium for their listing of qualified installers in your area. Installers should be certified and experienced. Ask for references, especially for owners of systems that are several years old, and check them.
The ground heat exchanger in a GHP system is made up of a closed or open loop pipe system. Most common is the closed loop, in which high density polyethylene pipe is buried horizontally at 4-6 feet deep or vertically at 100 to 400 feet deep. These pipes are filled with an environmentally friendly antifreeze/water solution that acts as a heat exchanger. In the winter, the fluid in the pipes extracts heat from the earth and carries it into the building. In the summer, the system reverses and takes heat from the building and deposits it to the cooler ground.
The air delivery ductwork distributes the heated or cooled air through the house's duct work, just like conventional systems. The box that contains the indoor coil and fan is sometimes called the air handler because it moves house air through the heat pump for heating or cooling. The air handler contains a large blower and a filter just like conventional air conditioners.
Most geothermal heat pumps are automatically covered under your homeowner's insurance policy. Contact your insurance provider to find out what its policy is. Even if your provider will cover your system, it is best to inform them in writing that you own a new system.