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:
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.
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.
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.
[source: US Department of Energy - Energy Efficiency and Renewable Energy]