Did you know that your house probably has at least two heat pumps in it? One is your refrigerator and the other is your air conditioner. Both work on the same principle as the familiar air source heat pump and the geothermal heat pump. All use a refrigerant, a compressor, and have a hot end and a cold end.

But how do they work?

In order to truly understand how the heat pump works, you have to understand some basic physics. This page walks you through this information.

Solid - Liquid - Gas - The Basic States of Matter

Think about water. When it's cold and freezes. Most of the time, it's a liquid. And if you heat it up enough, it becomes steam. This is very familiar. But did you know that the heat pump relies on this process - converting a substance, called a refrigerant, from liquid to gas and back.

Now look a bit closer. When you put a pot of water on the stove, it quickly heats up, but then, it seems to take forever to start boiling. This is because it takes much more energy to change from liquid to gas than it does to raise the temperature of a liquid or a gas. This is called the "heat of vaporization" (also referred to as "latent heat"). A very good description of this phase change process, and the energy required, is here. Understanding this process is necessary if you want to understand how a heat pump works.

High Pressure Increases Boiling Point and Temperature

If you took high school chemistry, you may remember an equation relating temperature and pressure. Basically, under otherwise constant conditions, the temperature of a gas is directly related to the pressure. This principle also tells us that the boiling point of a liquid can be changed by increasing or decreasing the pressure.

Scroll about half way down the page here to "The Steam Tables". You see that at low pressure, the boiling point of water is much lower. For example, at 1 pound of pressure, the boiling temperature is ~102F. Conversely, at 26 pounds of pressure, the boiling temperature rises to 242F. Standard atmospheric pressure is ~15 pounds of pressure.

Remember this - high pressure = high temperature; low pressure = low temperature.

Temperature vs. Pressure Curve for R-22 refrigerant

What is a Refrigerant

You've heard of Freon and perhaps you've heard of the new refrigerant, Puron. These are just trade names for substances that have favorable phase changing temperatures. In the case of Freon (typically R-22). These refrigerants share a common property - they have a very low boiling point. At normal temperature and pressure, they are a gas. Compress them enough and you raise the boiling point to room temperature and convert it to a liquid. Release that pressure, and the liquid reverts to a gas, absorbing energy from its surroundings.

So, How DOES a Heat Pump Work?

Heat pump animation - An simplified animation of heat pump operation.

Heat pump animation 2 - A more complete animation of heat pump operation.

Heat pump animation 3 - An even more complete animation

So far, we've reviewed that matter has various states, and to convert from one state to another takes considerable energy. We've also seen that the temperature at which the state change occurs varies depending on the pressure - the higher the pressure, the higher the temperature. Finally, we also know that applying pressure to a gas, while keeping other things constant, will raise the temperature of the gas. These are all critical bits necessary to understand so that you can put it all together in a heat pump.

In a heat pump (or a refrigerator or air conditioner), you have just a few key components that make it work - the refrigerant, a condenser, an evaporator, a compressor and an orifice. Note: the following is a simplified explanation of how the components work)

  • Condenser - where the refrigerant condenses from a gas to a liquid. As the refrigerant condenses, it releases the latent heat of evaporation.
  • Evaporator - where the refrigerant changes from a liquid to a gas. This requires energy equal to the latent heat of evaporation.
  • Compressor - takes the cool, low pressure gas (refrigerant) and compresses it to a warm, high pressure gas.
  • Orifice - a small opening that allows pressure to build up behind it and release after it. The orifice at the output end of the condenser keeps the pressure high in the condenser so the compressor can do it's job. After the liquid passes through the orifice, it travels to another orifice in the evaporator, where the liquid experiences the lower pressure caused by the suction from the compressor, and it turns into a gas, drawing energy to do so.

Recall your air conditioner. Outside, you have a big fan that blows outdoor air across a coil with fins. What is it doing? This is the condenser. The heat picked up in the house is released outside in the condenser. Inside, the evaporator is very similar. It blows the warm inside air across the cold evaporator coil, where the refrigerant picks up the energy and transfers it back outside.

How about a conventional heat pump? It's exactly the same as the air conditioner, except the evaporator and condenser positions are swapped, so the hot air is blown into the house and the cool air is released to the outside.

Remember, the refrigerant boils at far below room temperature and, when pressurized, gets much hotter than room temperature. The outside air blown over the coil just allows the coil to rise in temperature from, for example 20F up to the outside air temperature, 35F for example. This is why heat pumps lose their effectiveness when it gets cold outside and air conditioners don't work well when it's really hot out.

The Geothermal Heat Pump

Finally, we come to the geothermal heat pump!

With a geothermal system, rather than blowing air across the cold evaporator to warm it up, as you do in a conventional heat pump, you use the ground temperature, typically around 50F, to warm the refrigerant. Remember! The heat pump does not use the 50F ground temperature directly to warm your house! It only adds energy to the refrigerant. The compressor adds more energy to the refrigerant, heating it up to well above 100F, and this hot refrigerant is what heats your home.

Review:

  • The compressor pumps a low pressure gas, adds energy to it, and turns it into a high pressure gas. The high pressure gas is hotter than the original, low pressure gas.
  • The hot (~120F), high pressure gas travels to the condenser, where it is used to heat the air in your home.
  • The cool (~20F) liquid refrigerant travels to the evaporator, where it picks up energy from the geothermal earth loops and turns back into a gas, at low temperatures. The energy required to evaporate the refrigerant comes from the ground, cooling the ground in the process.
  • The low temperature and pressure gas travels back to the compressor and the cycle repeats.

More Geothermal Details

Closed Loop Geothermal Caveats

As noted, the geothermal heating system takes energy from the ground in order to evaporate the refrigerant. In a closed loop geothermal system, an anti-freeze solution is pumped through the loops, and through a heat exchanger with the refrigerant. It adds heat from the anti-freeze to the refrigerant, cooling the anti-freeze, which then goes back to the ground loop to be brought back to ground temperature.

You may be asking, "why doesn't the ground freeze"? That's an excellent question. In fact, if you draw out more energy from the ground than the ground can replenish (from surrounding areas of ground), then the ground temperature will drop. As this happens, the energy available in the ground drops as does the ability of the geothermal system to generate heat. This is why it is vitally important for the ground loops to cover enough ground. The ground must be able to supply energy at least equal to that extracted by the heat pump, or it will get increasingly cold and the heat pump will not provide as much heat as it was designed for.

Open Loop Geothermal Benefits

An open loop geothermal system doesn't have the problem increasing cold. In an open loop system, fresh well water provides the ~50F temperature that is used to give energy to the refrigerant. Since the cooled water is then injected into another well, it does not affect the supply well water. For this reason, an open loop system can work at optimal efficiency for as long as there is a supply of groundwater.

Note too that the water is not polluted by this process. All that happens is that it gets cooled by a few degrees before it goes back into the ground. The ground will then warm the water back up as it enters the water table.

The one potential issue when using an open loop system is that water deposits can clog the heat exchanger. It is therefore important to have a means to keep the heat exchanger cleaned out, as deposits will reduce the effectiveness of the heat exchanger and can lead to system malfunction.