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Energy Currents
A Blog by Enerdynamics

How Heat Pumps Work: One of Electrification's Biggest Mysteries

by Bob Shively, Enerdynamics President and Lead Facilitator

Central to many decarbonization plans in Europe and the United States is the displacement of fossil fuel space heating with electric heat pumps. This raises the question: What is a heat pump? The simple answer is, it’s a device that draws heat or cooling from the air or ground and transfers it into the house. But when it’s 90 degrees outside how can a heat pump cool my house by drawing in cool from outside air? Or, alternatively, how does it get heat from 20-degree air? Maybe a more complete explanation is needed!

Unlike gas or oil furnaces that burn fuel to create heat, or electric resistance heat which 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. While many of us have never thought much about how our refrigerators or air conditioners work, the same principles are applied to heat pumps.

At this point, I’m guessing what you’ve read so far still doesn't make much sense. To figure it out, we must know a few things about a refrigerant:

1. The refrigerant can be changed from a liquid to a gas, or vice versa from a gas to a liquid.

2. The refrigerant can be changed from a liquid to a gas by absorption of heat, which causes the surrounding air to get colder.

3.    The refrigerant can be changed from a gas to a liquid by rejection of heat, which causes the surrounding air to get warmer.

4. When the pressure of a gas is raised, the temperature of the gas goes up.

5. When the pressure of a gas is reduced, the temperature of the gas goes down.

6. 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.  

Now that we understand these principles, we are ready to explain how a heat pump works. There are two general categories of heat pumps, ground source and air source. They differ by whether the outside coils are in the air or are buried underground. Otherwise, the principles are basically the same. We’ll focus on air-source heat pumps since they are more common.

First, we’ll look at how a heat pump can heat your home in the winter:

  1. 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. 
  2. The cold refrigerant gas flows into the compressor, which raises its pressure. Since increasing pressure increases temperature, the gas becomes a hot gas. 
  3. 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. 
  4. 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:

 

  1. In cooling mode, the flow of the heat pump is reversed from heating mode. Now, 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.
  2. The gas flows into the compressor, which raises its pressure resulting in a hot gas.
  3. The hot gas flows through the outdoor coils at the outdoor temperature, and at the high pressure the gas becomes a liquid.
  4. 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.

Sound like magic? We must admit it does. But it is proven to work. And in recent years air-source heat pump technology has advanced rapidly. These heat pumps have proven to be effective in heating buildings even in cold places like Minnesota and Canada. Many regions of the world now expect heat pumps to replace gas or oil furnaces over time, resulting in a low-carbon heating solution so long as the electricity used to power the heat pump is green. If you currently have a gas or oil furnace, expect that when it has lived out its useful life you will be replacing it with a heat pump. And if you are buying a new home, expect that it may already be equipped with one.  

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