As pointed out in an earlier B-Green Collaborative article titled “What’s Your R-Value”, the average American home expends between 50% and 70% of its energy use for heating and cooling.  So an efficient heating and cooling system is key to reducing the cost of conditioning the air in your home.  Geothermal heat pump systems (GHPs), also known as ground source heat pump systems or Geoexchange systems, can provide heating efficiencies 50% to 70% higher than other heating systems and cooling efficiencies 20% to 40% higher than available air conditioners as well as provide residential hot water (these efficiencies will vary by region).  

According to the Environmental Protection Agency (EPA), GHPs are “the most energy efficient, environmentally clean, and cost-effective space conditioning systems available”.  The EPA also has determined that GHPs have the lowest life-cycle cost of all systems available today.  The United States National Renewable Energy Laboratory (NREL) has concluded that geothermal energy is more efficient and cost-effective compared with conventional residential systems.  It is estimated that for every 100,000 units of typically-sized residential GHPs installed, more than 37.5 trillion BTU’s of energy used for space conditioning and water heating can be saved, corresponding to an emissions reduction of about 2.18 million metric tons of carbon equivalents, and cost savings to consumers of about $750 million over the 20+ year life of the equipment. 

How Does It Work?

GHPs use the earth’s energy storage capability to heat and cool buildings, and to assist in domestic hot water production.  The earth is a huge energy storage device that absorbs 47% of the sun’s energy in the form of clean, renewable energy.  Twenty feet beneath the surface, the earth’s temperature remains fairly constant year-round, generally in the range of 50°F to 60°F (this range will vary by region).  GHPs take advantage of this constant temperature by using three main components: an earth connection subsystem, a heat pump subsystem and a distribution subsystem.  

The earth connection subsystem typically uses a series of high density polyethylene pipes, commonly called a “loop”, buried in the ground near the home.  The loop circulates water or a mixture of water and antifreeze that absorbs heat from, or releases heat to the surrounding soil, depending on whether the ambient air is colder or warmer than the soil.   In the heating mode, the heat pump subsystem pumps the circulating loop fluid through a cold heat exchanger, where its heat is absorbed by evaporation of a refrigerant.  The refrigerant is then pumped to a warm heat exchanger, where the refrigerant is condensed, releasing heat in the process. This sequence is reversed for operation in the cooling mode (see diagrams below).  The distribution subsystem, typically through conventional ductwork, then distributes the heated or cooled air throughout the home (radiant floor heating distribution can also be used, and technology advances may make GPH applicable to baseboard heating as well). 

 GHPs are more than three times as efficient as the most efficient fossil fuel furnaces and have no combustion or indoor air pollutants.  Nearly all GHP systems on the market have the ability to provide low-cost domestic hot water by using a desuperheater to transfer excess heat from the heat pump’s compressor to the home’s hot water tank.  A desuperheater provides no hot water when the GHP system may not be operating, such as in the spring and fall seasons.  However, manufacturers are beginning to offer “full demand” systems that use a separate heat exchanger to meet all of a household’s hot water needs. These units cost-effectively provide hot water as quickly as any competing system. 

The only external energy needed for a GHP is the small amount of electricity needed to operate the ground loop pump, the heat pump and the distribution fan or pump, and by moving heat that already exists in the earth instead of burning a combustible fuel, GHPs deliver up to four units of energy for every one unit used to power the system.  And, because there are no outdoor units (as with air-source heat pumps or central air conditioners used with combustion-based systems), no weather-related maintenance is required. 

Image Courtesy of DOE EnergySavers

There are four basic loop configurations used for GHPs, three of which are closed loop systems.   The horizontal closed loop system uses pipes buried in trenches four to six feet below the surface and then connected to the heat pump in the home.  Depending on geothermal system needs and space available, the trenches may range in length from 100 to 400 feet.  This approach is most suitable for new construction and for retrofits where sufficient and suitable land is available.  Looping the pipe like a Slinky® is sometimes used to allow more pipe in a shorter trench, thereby cutting down on installation costs.  

Image Courtesy of DOE EnergySavers

If land conditions or availability is not conducive to extensive trenching, a vertical closed loop system may be used.  In this approach, holes are drilled about 20 feet apart and from 100 to 500 feet deep.  Two pipes, connected at the bottom with a u-bend are dropped into each hole and are then connected to horizontally buried manifold piping which completes the connection to the heat pump in the building.  Vertical closed loop systems are more common for commercial buildings and schools due to land constraints, but are appropriate for residential applications as well, especially in areas like New England.  

Image Courtesy of DOE EnergySavers

When a large body of water is available for use by the GPH system, a closed loop pond/lake approach may be considered in which a supply line pipe is run underground to coils of pipe placed on the bottom of the water source at a depth of at least eight feet.  This is the most economical closed loop approach from an installation perspective. 

Image Courtesy of DOE EnergySavers

An open loop system uses water from a well or other source as the heat exchange fluid circulating through the GHP system.  Once circulated, the water is then discharged back to the well or other water source.  Though economical to install, this approach requires the homeowner to ensure there is an adequate supply of water and that ground water discharge is in compliance with all local codes and regulations. 

Due to the equipment and knowledge needed to properly install the loop for a GHP system, it is not a do-it-yourself job.  Local utility companies may be able to provide names of qualified installers.  Also, the International Ground Source Heat Pump Association (IGSHPA) and the Geothermal Heat Pump Consortium (GeoExchange) both have directories of installers you can consider by location. 

How Is GHP Efficiency Measured?

The heating efficiency of geothermal heat pumps is indicated by a Coefficient of Performance (COP), which is the ratio of heat provided in BTUs per BTU of energy input.  Cooling efficiency is indicated by the Energy Efficiency Ratio (EER), which is the ratio of the heat removed (in BTUs per hour) to the electricity required (in watts) to run the unit.  Current ENERGY STAR® rated closed loop units carry a heating COP of 3.3 or greater and an EER of 14.1 or greater.  Current ENERGY STAR ® rated open loop units carry a heating COP of 3.6 or greater and an EER of 16.2 or greater.  Qualified products covered under this specification will be over 45 percent more energy efficient than standard options (see ENERGY STAR® qualified GHPs).  Anticipating advances in technology in the coming years, the EPA will set more stringent efficiency requirements to ensure that ENERGY STAR continues to represent top performers in this category.  New requirements for GHP models will take effect on January 1, 2011, and even more stringent levels will go into effect on January 1, 2012. 

How Much Does It Cost?

The initial purchase price of a residential GHP system is often higher than that of a comparable combustion furnace and central air-conditioning system.  On average, a geothermal heat pump costs about $2,500 per ton of capacity, translating to about $7,500 for a 3-ton unit, which is a typical size heat pump for a 1,500 square foot residence.  High end heat pumps will cost more, and a variety of factors such as existing versus required distribution systems, loop installation types (a closed system using horizontal ground loops will generally cost less than one with vertical loops), and specific GHP system features can significantly impact actual costs for each individual application.  In comparison, combustion-based systems with air conditioning would cost about $4,000.  However, due to its significantly more efficient operation, the GHP saves money every month, with typical annual energy savings ranging from 30% to 60%.   

GHP system life is estimated at 25 or more years for the inside components and 50+ years for the ground loop.  And, GHPs equipped with a desuperheater device can use heat taken from the house in the summer to heat household water for free, while reducing water heating costs in the winter by about half.  Depending upon climate, soil conditions, system features and available incentives, homeowners installing retrofit GHP systems may expect to recoup their investments in as little as 2 to 5 years.  For new construction GHP installations, the extra cost added to a mortgage should be easily offset each year by significantly lower utility bills. 

As of December 1, 2009, and through calendar year 2016, homeowners who install geothermal heat pump systems with qualifying ENERGY STAR® ratings are eligible for a 30 percent federal tax credit.  This tax credit has no dollar maximum and applies to the costs of GHP components as well as installation costs.  The credit applies to both existing homes and new construction and includes second homes in addition to principal residences (rental property is not covered).  In addition, a growing number of states offer tax credits or other forms of incentives for GHP systems (see what your state may offer). 

So Where Are We Now?

According to a December 2008 report by the Oak Ridge National Laboratory, today’s domestic GHP industry is better positioned for rapid growth than ever before.  The report states that the technology is proven, with an estimated 60,000 units being installed domestically per year, of which 50 to 60 percent are for residential applications.  The Geothermal Heat Pump Consortium claims there are more than 1,000,000 GHP installations in the United States which result in the annual elimination of more than 5.8 million metric tons of CO2 and more than 1.6 million metric tons of carbon equivalents, plus annual savings of nearly 8 billion kWh, and nearly 40 trillion BTUs of fossil fuels.  The Consortium goes on to state that “No alternative has such great opportunities to maximize savings by combining good design, good construction, and a system customers like.  Surveys by utilities indicate a higher level of consumer satisfaction for GHPs than for conventional systems: more than 95% of all geothermal heat and cooling customers would recommend a GHP system to a family member or friend”. 

Could a GHP system be right for you?