A heat pump is a device that draws heat or cooling from the air or ground and transfers it into a building. Heat pumps are efficient ways to heat or cool indoor spaces and are central to decarbonization plans in many regions of the world as they displace the use of fossil fuels for space heating.
Unlike gas or oil furnaces that burn fuel to create heat, or electric resistance heat that uses electric current to heat up a conductor, heat pumps use electricity to power a compressor that transfers heat from one place to another by moving a refrigerant and changing its pressure. Oddly, a heat pump can cool a building when it is hot outside by drawing in cooling from outside air and can get heat from cold air during the winter. Here’s how they work.
It's first important to understand a few things about the refrigerant used in a heat pump:
- The refrigerant can be changed from a liquid to a gas, or from a gas to a liquid.
- The refrigerant is changed from a liquid to a gas by absorption of heat, which causes the surrounding air to get colder..
- The refrigerant is changed from a gas to a liquid by rejection of heat, which causes the surrounding air to get warmer.
- Raising the pressure of a gas causes the temperature of the gas to go up.
- Reducing the pressure of a gas causes the temperature of the gas to go down.
- The temperature above which a liquid becomes a gas and below which a gas becomes a liquid (called the boiling point) is affected by pressure. At higher pressures a substance transitions between liquid and gas at a higher boiling point, while at lower pressures the boiling point is a lower temperature.
There are two general categories of heat pumps, ground source and air source. They differ by the location of the outside coils (either in the air or buried underground). In a ground source heat pump (also called a geothermal heat pump), refrigerant is circulated through pipes in the ground rather than coils in the outside air. Otherwise, the principles are basically the same. The description below focuses on air source heat pumps since they are more common.
First, here's how a heat pump can heat a home in the winter:
- Cool low-pressure liquid refrigerant flows through the outdoor coil. Even at cold temperatures, given the low pressure of the refrigerant, the temperature of the outside air is sufficient to change the refrigerant from a liquid to a gas.
- The cold refrigerant gas flows into the compressor, which raises its pressure. Since increasing pressure increases temperature, the gas becomes a hot gas.
- The hot gas is passed through the indoor coils, warming the indoor air. At this high pressure, the refrigerant becomes a liquid again as it cools to room temperature. This change in state further warms the indoor air as it releases heat energy.
- The warm refrigerant liquid is passed through the expansion valve, which lowers its pressure and temperature, turning it to a cool low-pressure liquid. Then the cycle can begin again.
Now let’s look at cooling mode:
- In cooling mode, the flow of the heat pump is reversed from heating mode. Here, cool low-pressure liquid refrigerant flows through the indoor coil, cooling the indoor air. As the refrigerant is warmed by the indoor air, it becomes a gas. This change in state further cools the indoor air as it draws heat energy from the air.
- The gas flows into the compressor, which raises its pressure resulting in a hot gas.
- The hot gas flows through the outdoor coils at the outdoor temperature, and at the high pressure the gas becomes a liquid.
- The hot liquid flows through the expansion valve, which reduces the pressure and lowers the temperature of the liquid, resulting in cool liquid that is now ready to again flow through the indoor coils.
In recent years air source heat pump technology has advanced rapidly. These heat pumps have proven to be effective in heating buildings even in very cold places. Many regions of the world now expect heat pumps to replace gas or oil furnaces over time, resulting in a low-carbon heating solution as long as the electricity used to power the heat pump is green.
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