Heat pumps, while electrically operated, are distinctly different from resistance electrical heaters. Heat pumps are devices that transfer heat from a lower temperature reservoir, usually the ambient environment, to a higher temperature sink. The low temperature reservoir is usually the air, the ground, or a body of water, and it is essentially an unlimited source of heat. This heat, while unlimited, does not come without cost; work in the form of electricity is required to pump it to the high-temperature sink.
Heat pumps work on the same principle as refrigerators and air conditioners. All remove heat from a cold temperature source and pump it to a higher temperature sink. The difference is simply in the desired effect – cooling vs. heating.
How Does a Heat Pump Work?
A simplified heat pump contains four main parts: a cold source heat exchanger called the evaporator, a compressor, a high-temperature heat exchanger called the condenser, and an expansion valve. This system is filled with a working fluid, such as the refrigerant R‑410A. A diagram of this system is shown here.
In the evaporator, heat is absorbed, vaporizing the refrigerant. This vapor is then compressed, raising its temperature and pressure. The now hot refrigerant vapor is piped to the condenser, where its heat is removed and used for heating. During this heat transfer process, the refrigerant condenses back to a liquid. This liquid then passes through the expansion valve to the low-pressure side of the system, and the cycle repeats.
The effectiveness of a heat pump is called its Coefficient of Performance (COP). The COP is the ratio of heat output to work input. For example, a heat pump operating with a COP of 5 will produce 5 kWh of heat for every 1 kWh of electricity supplied. For this reason, the ‘efficiency’ of a heat pump is often listed as greater than 100%.
The COP is largely dependent on the temperature difference between the source and sink, and the greater the difference, the lower the COP. For example, a heat pump operating between a ground or air temperature of 45°F and an inside temperature of 70°F has a much higher COP than the same system operating between 0°F and an inside temperature of 70°F. Typical COPs for heat pumps tend to be in the range of 1.5 to 6. This is sometimes described as an efficiency of 150% – 600%.
Heat pumps are classified as either air-source heat pumps or ground-source heat pumps. Air-source heat pumps, as their name implies, extract heat from the ambient air. They are the easier and less expensive type to install. Because the COP is a function of the outdoor air temperature, it will vary widely. This variable COP shows one of the air source heat pump’s main disadvantages in cold climates – peak heating demands coincide with the unit’s lowest COP. Heat is therefore most expensive when it is most needed. Nonetheless, there are a number of recent innovations that have led to the development of air-source heat pumps suitable for use down to 0°F.
Ground-source heat pumps have been in use since the late 1940s and use the relatively constant temperature of the earth instead of the outside air for the heat source. This allows the systems to operate with a higher COP in colder weather, making them more appropriate for use in much of Alaska. As with air-source heat pumps, the COP is highest when the difference between the ground temperature and indoor temperature is lowest. Therefore, the colder the ground, the less efficient the system.
There are many configurations for ground-source heat pumps, including an open loop heat pump that pumps water directly from a well or body of water and extracts heat before returning the water back to the same water body or to another one. A dual source heat pump combines an air-source heat pump with a ground-source heat pump.
Links and Resources
- ACEP Publications: Search Keywords: Ground Source Heat Pumps and download the article by authors Jason Meyer, Dominique Pride, and Jonathan O’Toole. A very comprehensive article covering the potential for ground source heat pumps in Alaska's cold climate.