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Heat Pump Technology in the New Century

Although physicists discovered the basic principles 150 years ago, and successful commercial installations have existed since the 1930s, more recent emphasis on energy conservation has brought renewed interest in heat pump technology.

The heat pump is a device for moving heat from one location to another, functioning to either heat or cool a building interior without direct combustion of fuel. During cooling, it operates like an air conditioner. transferring thermal energy from inside air to outside air via interior and exterior heat exchangers and fans. During the heating cycle, it moves heat in the reverse direction. Even at extremely low temperatures, air contains enough heat energy to be useful.

That's known as an "air-source" heat pump. Also widely used is the "water-source" design, in which heat is either supplied by (heating cycle) or rejected to (cooling cycle) some body of water. That can be a lake or stream (open-loop system) or a self-contained loop (sometimes consisting of a set of intake and rejection wells). For a "ground-source" heat pump, the earth itself is the energy source, transferring heat to a buried fluid circulation network. Heating may also be carried out using recovered thermal energy (from building occupants and appliances, or an industrial process).

Because atmospheric air temperature varies widely with the season, whereas underground water or earth temperature does not, an air-source heat pump is best suited to a warm climate. In colder regions where a water-source unit is more appropriate, supplemental heating is usually needed during the coldest weather. A "dual-fuel" heat pump can then be the best choice, coupling the heat pump with a gas-fired furnace.

Heat pumps generally have much higher efficiency and lower operating costs than conventional climate control equipment. Installed equipment costs, however, tend to be much higher. Water supplies, for example, can require large land areas and excavations; local water usage restrictions can present problems. Operating life and maintenance requirements are highly variable. Thus, the homeowner or building designer can face complex choices.

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