Using A Heat Pump To Heat And Cool Your Home
(Some of the sections in this Heat Pumps chapter may repeat information from other Heat Pumps sections. This will assist you with understanding the topics more clearly, especially geothermal heat pumps.)
For climates with moderate heating and cooling needs, heat pumps offer an energy efficient alternative to furnaces and air-conditioners. Like your refrigerator, heat pumps use electricity to move heat from a cool space into a warm place (heat pumps powered by natural gas are also commercially available now). During the heating season, heat pumps move heat from the cool outdoors into your warm house; during the cooling season, heat pumps move heat from your cool house into the warm outdoors.
Because they move heat rather than generating heat, heat pumps can provide up to 4 times the amount of energy they consume. If you heat with electricity, a heat pump can trim the amount of electricity you use for heating by as much as 30% to 40%. High-efficiency heat pumps also dehumidify better than standard central air-conditioners, resulting in less energy usage and more cooling comfort in summer months.
If your home lacks a ductwork system, you would need to add one to convert to a heat pump system. Even if your house has ducts, you may need to modify them, as heat pump systems generally require larger duct sizes than other central heating systems. For proper heat pump operation, air flow should be 50 to 60 liters per second per kilowatt-hour or 400 to 500 cubic foot per minute per ton of cooling capacity. Consult a local heating and cooling contractor to assure that your ductwork is sized properly.
The "conventional" model of heat pump is the air-source heat pump, which transfers heat between your house and the outside air. Although air-source heat pumps can be used in nearly all parts of the United States, they do not perform well over extended periods of sub-freezing temperatures. In regions with sub-freezing winter temperatures, it may not be cost effective to meet all your heating needs with an air-source heat pump.
Higher efficiencies are achieved with ground-source (or geothermal) heat pumps, which transfer heat between your house and the ground. Although they cost more to install, geothermal heat pumps have low operating costs because they take advantage of relatively constant ground temperatures. However, the installation depends on the size of your lot, the subsoil and landscape. If your house is located near a body of water, such as a pond or lake, water-source heat pumps offer similar advantages. Ground-source or water-source heat pumps can be used in more extreme climatic conditions than air-source heat pumps, and customer satisfaction with the systems is very high.
Heat pumps function by moving (or pumping) heat from one place to another. Like a standard air-conditioner, a heat pump takes heat from inside a building and dumps it outside. The difference is that a heat pump can be reversed to take heat from a heat source outside and pump it inside. Heat pumps use electricity to operate pumps that alternately evaporate and condense a refrigerant fluid to move that heat. In the heating mode, heat pumps are far more "efficient" at converting electricity into usable heat because the electricity is used to move heat, not to generate it.
The most common type of heat pump-an air-source heat pump-uses outside air as the heat source during the heating season and the heat sink during the air-conditioning season. Ground-source and water-source heat pumps work the same way, except that the heat source/sink is the ground, groundwater, or a body of surface water, such as a lake. (For simplicity, water-source heat pumps are often lumped with ground-source heat pumps, as is the case here.)
The efficiency or coefficient of performance (COP) of ground-source heat pumps is significantly higher than that of air-source heat pumps because the heat source is warmer during the heating season and the heat sink is cooler during the cooling season. Ground-source heat pumps are also known as geothermal heat pumps.
Ground-source heat pumps are environmentally attractive because they deliver so much heat or cooling energy per unit of electricity consumed. The COP is usually 3 or higher. The best ground-source heat pumps are more efficient than high-efficiency gas combustion, even when the source efficiency of the electricity is taken into account.
Ground source heat pumps are generally most appropriate for residential and small commercial buildings, such as small-town post offices. In residential and small (skin-dominated) commercial buildings, ground-source heat pumps make the most sense in mixed climates with significant heating and cooling loads because the high-cost heat pump replaces both the heating and air-conditioning system.
Because ground-source heat pumps are expensive to install in residential and small commercial buildings, it sometimes makes better economic sense to invest in energy efficiency measures that significantly reduce heating and cooling loads, then install less expensive heating and cooling equipment. The savings in equipment may be able to pay for most of the envelope improvements (see Integrated Building Design).
Figure 32: This vertical, closed-loop ground source heat pump serves the Chesapeake Bay Foundation headquarters. Forty-eight 300-feet-deep wells provide heating in winter and cooling in summer.
If a ground-source heat pump is to be used, plan the site work and project scheduling carefully so that the ground loop can be installed with minimum site disturbance or in an area that will be covered by a parking lot or driveway.
Ground-source heat pumps are generally classified according to the type of loop used to exchange heat with the heat source/sink. Most common are closed-loop horizontal and closed-loop vertical systems (see illustration). Using a body of water as the heat source/sink is very effective, but seldom available as an option. Open-loop systems are less common than closed-loop systems due to performance problems (if detritus gets into the heat pump) and risk of contaminating the water source or, in the case of well water, inadequately recharging the aquifer.
Ground-source heat pumps are complex. Basically, water or a nontoxic antifreeze-water mix is circulated through buried polyethylene or polybutylene piping. This water is then pumped through one of two heat exchangers in the heat pump. When used in the heating mode, this circulating water is pumped through the cold heat exchanger, where its heat is absorbed by evaporation of the refrigerant. The refrigerant is then pumped to the warm heat exchanger, where the refrigerant is condensed, releasing heat in the process. This sequence is reversed for operation in the cooling mode.
Direct-exchange ground-source heat pumps use copper ground-loop coils that are charged with refrigerant. This ground loop thus serves as one of the two heat exchangers in the heat pump. The overall efficiency is higher because one of the two separate heat exchangers is eliminated, but the risk of releasing the ozone-depleting refrigerant into the environment is greater. Direct-exchange systems have a small market share.