Vol 2 April 2002       

summer/winter system

by Carol Hiltner


If you have read our February issue on Free & Alternative Energy, you will know that the heat pump is actually an "overunity" device (it puts out more energy than it takes to run it) and that the common refrigerator has one (although it's not being used very intelligently).

Today, advances in heat-pump technology have brought this amazing device within reach of the individual homeowner. Here, you will learn how they work, and where and how you can have one of your own.

Heat pumps, especially ground-source, are currently the method of choice for heating and cooling living-space. Assuming that the specific system is suited to the climate and usage patterns, and that the system is properly installed and maintained, the higher installation costs are quickly offset by the savings in energy costs.[1]

Heat pumps are a highly efficient means of heating and cooling space, and heating water. Similar to refrigerators, heat pumps use electricity, gas, or oil to move heat from a cool space into a warm space. In the winter, a heat pump will move heat from the cold outdoors into a warm house, and in the summer, it will move heat from a cool house to the warm outdoors by reversing the flow.[2]

Recent improvements in the technology have made heat pumps arguably the most efficient of all currently available space-heating/cooling systems. While one unit of energy into a furnace will net somewhat less than one back out as heat, one unit of energy into a heat pump will more than triple the return.[3]

The technology is now well established, and products and installation are readily available. And there is a huge amount of information available on the Internet. Heat pumps can either replace or supplement furnace and air conditioning machinery. Excess heat, or heat resulting from air conditioning, also can be used to heat water.[4]

However, because heat pump function is sensitive to climate, different configurations are appropriate for each climatic region (see Resource Links at the end of this article).

Initial investment for heat pumps can be almost double that for conventional systems. But that investment can be recouped within a few years.[5] Machinery can be expected to last for upwards of fifteen years.[6]

How do they work?[7]

A heat pump is by definition any device that accepts heat at one temperature and rejects heat at a higher temperature.

Two different technologies are used: vapor compression, and absorption. For cooling, the vapor compression technology is more efficient because of the physical principles involved, whereas the absorption technology is better for heating.

In a vapor compression system, a refrigerant is vaporized in an evaporator, drawing heat from whatever is to be cooled. Then the vapor is compressed until saturation temperature is higher than the heat sink temperature, whereupon the vapor condenses and gives off heat into the heat sink.

The absorption system is essentially the same as the vapor compression system, except that the compressor is replaced with a solution circuit that absorbs the vapor at the loweer pressure and desorbs it at the higher pressure.

What kind will work best for my location?

In selecting a heat pump, the technology used is less important than the source of the heat/cold, which can be provided by air, ground, or water. The efficiency of a given heat pump depends on both climate and usage patterns. Heat pump organizations in all parts of the country offer advice based on local conditions.

Air source heat pumps are recommended for mild and moderate climate regions where winter temperatures usually remain above 30 degrees Fahrenheit.[8] Air can be vented either inside the living space or outside. Externally vented systems can double as ventilation systems in air-tight buildings.[9]

Ground source (geothermal) heat pumps utilize the relatively constant temperature of the ground (50-55 degrees in the American midwest),[10] and are more efficient and economical to operate than air source heat pumps, especially in climates with similar heating and cooling loads.[11] The same technology used for geothermal heat pumps can be used with water sources, such as ponds or wells.[12]

The most recent technical advancement in geothermal systems is the use of a closed loop of buried pipe, made viable by improvements in heat pump design and pipe materials.[13] For heating, fluid is circulated through a pipe loop to absorb heat from outside the house. Loops are buried, and can be installed either vertically or horizontally. The absorbed heat is converted to hot air by circulating it through water-to-refrigerant and refrigerant-to-air heat exchangers (similar to a car radiator). The flow is reversed for cooling.[14]

Besides efficiency, other advantages to geothermal heat pumps include low maintenance (due to few mechanical components), cleanliness and therefore low environmental impact, and low noise.[15]

Can I install one myself?

Heat pumps are not a Saturday afternoon project for the homeowner. Proper sizing and installation are key to efficient function,[16] so a certified installer is recommended.[17] A couple hours of annual preventive maintenance also is highly recommended.[18]

Heat pumps systems use interior ducts to distribute the heat/cold, and they generally require larger ducts than other central heating systems.[19]

Because of their power savings, many local utility companies will finance or offer rebates on heat pumps, offsetting the higher installation cost.[20]

The World's Oceans: Earth's Largest Heat Sink

Heat pumps are being used in wells and ponds, so what about using them in the oceans?

This is a good news/bad news story.

The good news is that the oceans are a vast renewable resource, with the potential to help us produce billions of watts of electric power. On an average day, tropical seas absorb solar radiation equivalent in heat content to about 250 billion barrels of oil. Less than one-tenth of one percent of this stored solar energy could supply more than 20 times the total amount of electricity consumed in the United States on any given day.[i]

The bad news is that we haven't managed to do it yet in a cost-effective manner.

And the good news, again, is that we are working on it, with a technology called OTEC, or "ocean thermal energy conversion."

Most of the United States's OTEC experiments in recent years have taken place in the Natural Energy Laboratory of Hawaii Authority (NELHA), which is recognized as the world's foremost laboratory for OTEC research. The facility has been funded by the State of Hawaii with significant USDOE and private sector participation.[ii]

What is OTEC?[iii]

OTEC converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient — the different temperatures of different layers of ocean water — to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 36 degrees F., an OTEC system can produce a significant amount of power.

The economics of energy production have delayed the financing of a permanent, continuously operating OTEC plant. However, OTEC is very promising for tropical island communities that rely heavily on imported fuel. OTEC plants in these markets could provide islanders with much-needed power, as well as desalinated water and a variety of mariculture products.

The Basic Process[iv]

There are three types of OTEC processes: closed-cycle, open-cycle, and hybrid-cycle.

In the closed-cycle system, heat transferred from the warm surface seawater causes a working fluid (such as ammonia, which boils at a temperature of about 78 degrees F. at atmospheric pressure), to turn to vapor.

The expanding vapor drives a turbine attached to a generator which produces electricity. Cold seawater passing through a condenser containing the vaporized working fluid turns the vapor back into a liquid which is then recycled through the system.

Open-cycle OTEC uses the warm surface water itself as the working fluid. The water vaporizes in a near vacuum at surface water temperatures. The expanding vapor drives a low-pressure turbine attached to a generator which produces electricity.

The vapor, which has lost its salt and is almost pure fresh water, is condensed back into a liquid by exposure to cold temperatures from deep ocean water. If the condenser keeps the vapor from direct contact with seawater, the condensed water can be used for drinking water, irrigation or aquaculture.

A "direct contact" condenser produces more electricity, but the vapor is mixed with cold seawater and the discharge water is salty. That mixture is returned to the ocean. The process is repeated with a continuous supply of warm surface seawater.

Hybrid systems use parts of both open- and closed-cycle systems to optimize production of electricity and fresh water.

Advantages of OTEC[v]

  • OTEC uses clean, abundant, renewable, natural resources.

  • Suitably designed OTEC plants will produce little or no carbon dioxide or other polluting chemicals which contribute to acid rain or global warming. Extensive research indicates little or no adverse environmental effects from discharging the used OTEC water back into the ocean at prescribed depths.

  • OTEC systems can produce fresh water as well as electricity. This is a significant advantage in island areas where fresh water is limited.

  • There is enough solar energy received and stored in the warm tropical ocean surface layer to provide most, if not all, of present human energy needs.

  • The use of OTEC as a source of electricity will help reduce our dependence on imported fossil fuels.

  • The cold seawater from the OTEC process has many additional uses, including air-conditioning buildings, assisting agriculture, and growing fish, shellfish, kelp and other sea plants which thrive in the cold, nutrient-rich, pathogen-free water.

Disadvantages and Challenges[vi]
  • OTEC-produced electricity at present would cost more than electricity generated from fossil fuels at their current costs. The electricity cost could be reduced significantly if the plant operated without major overhaul for 30 years or more, but there are no data on possible plant life cycles.

  • OTEC plants must be located where a difference of about 40 Fahrenheit (F) occurs year 'round. Ocean depths must be available fairly close to shore-based facilities for economic operation. Floating plant ships could provide more flexibility.

  • Although extensive and successful testing of OTEC has occurred in experiments on component parts or small scale plants, a pilot or demonstration plant of commercial size needs to be built to further document economic feasibility.

  • Construction of OTEC plants and laying of pipes in coastal waters may cause localized damage to reefs and near-shore marine ecosystems.

  • Some additional development of key components is essential to the success of future OTEC plants (e.g., less-costly large diameter, deep seawater pipelines; low-pressure turbines and condensers for open-cycle systems; etc.).


  1. The U.S. Department of Energy's Energy Efficiency and Renewable Energy Network, a good starting place for links to other sites on renewable energy.
  2. The National Database of State Incentives for Renewable Energy (DSIRE), a constantly-updated database of information from the 50 states on financial and regulatory incentives for all renewable energy systems.
  3. OTEC, history, facts, and diagrams of open- and and closed-cycle OTEC systems.
  4. Sea Solar Power, a vendor of OTEC technology, with projects in several locations; diagrams, discussions of "land-based or sea-based?", and a comprehensive set of links.
  5. U.S. Department of Energy (DOE) and Electric Power Research Institute (EPRI) Renewable Energy Technology Characterizations.
  6. best estimates of USDOE and EPRI regarding technical and economic status and future performance and cost of renewable energy technologies through the year 2030.


  1. Ocean Thermal Energy Conversion http://www.nrel.gov/otec/what.html
  2. Ocean Thermal Energy Conversion (OTEC) Fact Sheet.
  3. See reference 1.
  4. See reference 2.
  5. Ibid.
  6. Ibid.
  7. Ibid.

Resource Links:

This list was provided by Margaret Thomas, librarian at Washington State University. Email: info@energy.wsu.edu.

Geothermal Heat Pumps Make Sense for Homeowners eren.doe.gov/erec/factsheets/ghp_homeowners.html.

Environmental and Energy Benefits of Geothermal Heat Pumps eren.doe.gov/geothermal/ghp_enviro.html.

Air-Source Heat Pumps eren.doe.gov/erec/factsheets/airheatpump.html.

U.S. Environmental Protection Agency report on "Geothermal Heat Pumps" epa.gov/globalwarming/publications/outreach/technology/geothermalheatpumps.pdf.

The Energy Efficiency and Renewable Energy Clearinghouse (EREC) eren.doe.gov/consumerinfo, P.O. Box 3048, Merrifield, VA 22116, 800-DOE-EREC (363-3732), Fax: 703-893-0400, Email: doe.erec@nciinc.com.

Geo-Heat Center, Oregon Institute of Technology oit.osshe.edu/geoheat, 3201 Campus Drive, Klamath Falls, OR 97601-8801, 503-885-1750.

Geothermal Heat Pump Consortium, Inc. (GHPC) geoexchange.org, 701 Pennsylvania Avenue, NW., Washington, DC 20004-2696, 888-ALL-4-GEO (255-4436).

International Ground Source Heat Pump Association (IGSHPA) igshpa.okstate.edu, 490 Cordell South, Stillwater, OK 74078-8018, 405-744-5175, 800-626-4747.

Air Conditioning Contractors of America (ACCA) acca.org, 2800 Shirlington Rd., Suite 300, Arlington, VA 22206, Phone: 703-575-4477, Fax: 703-575-4449, Email: info@ms.acca.org. A national trade association of heating, ventilation, air-conditioning, and refrigeration contractors.

Air-Conditioning and Refrigeration Institute (ARI) ari.org, 4301 N. Fairfax Drive, Suite 425, Arlington, VA 22203, Phone: 703-524-8800, Fax: 703-528-3816, Email: ari@ari.org. A national trade association representing manufacturers of U.S. produced central air-conditioning and commercial refrigeration equipment.

Consortium for Energy Efficiency CEE-ceeformt.org, One State Street, Suite 1400, Boston, MA 02109-3507, Phone: 617-589-3949, Fax: 617-589-3948. A national, nonprofit, benefits corporation that promotes the manufacture and purchase of energy-efficient products and services.

Eastern Heating & Cooling Council EH-CC-eh-cc.org, 20,000 Horizon Way, Suite 260, Mt. Laurel, NJ 08054, Email: info@eh-cc.org. Educates consumers and contractors on properly designed and installed high-efficiency heating and cooling systems.

Energy Star (R) energystar.gov, DOE and EPA, Phone: 888-STAR-YES (782-7937), Email: info@energystar.gov. Provides lists of energy-efficient, Energy Star-qualified products, including heat pumps.

Home Energy Magazine homeenergy.org, 2124 Kittredge Street, #95, Berkeley, CA 94704, Phone: 510-524-5405, Email: contact@homeenergy.org. A source of information on reducing energy consumption in the home.

The Energy Outlet "Heat Pumps" energyoutlet.com.

DOE Office of Building Technology, State and Community Programs "Heat Pumps for Heating and Cooling" eren.doe.gov/buildings/ee_heatpump.html.

International Energy Agency Heat Pump Centre heatpumpcentre.org.
You can find more information about heat pumps and many other issues at the Energy Ideas site sponsored by the Northwest Energy Efficiency Alliance energyideas.org/energy_solutions.