Are geothermal heat pumps a form of renewable energy? Is geothermal energy the same as a geothermal heat pump? Is there a difference between a ground source heat pump and a geothermal heat pump?
To answer these questions it’s necessary to understand what each of these terms refers to. So first, let’s define renewable energy. Renewable energy takes many forms but in general it is energy that can be continuously harvested and replenished. Common forms of renewable energy include wind power, solar power, hydro power, and biofuels. Each of these is an energy source that can be harnessed to do work. Typically renewable energy is used to generate electricity. It can also be used to create heat. Such is the case with solar thermal systems that use solar radiation to heat a circulated fluid. In the case of biofuels the energy source is combusted to create heat. Other forms of renewable energy exist as well but are less common.
This brings us to the second question. Is geothermal energy the same as a geothermal heat pump? No. Geothermal energy is not the same as a geothermal heat pump. This can be confusing as the terms are sometimes incorrectly used interchangeably. Geothermal energy is a method of generating electricity or heat by harnessing the heat from the earth’s core. The heat from the earth’s core comes to the surface in the form of magma and hot springs. Where this occurs, water can be turned into steam by the earth’s natural heat source. The steam is subsequently used to turn a steam turbine and generate electricity. This is similar to a coal fired power plant except that the heat source is the earth rather than coal.
So if geothermal energy is taking heat from the earth to make energy why isn’t a geothermal heat pump renewable? The answer to that question lies in how a geothermal heat pump works. Most homes and commercial buildings use a refrigeration cycle in the form of an air conditioner to provide mechanical cooling in the summer. Air conditioners remove heat from the air inside the building and move it to the outdoors. To move the heat, energy must be input. Airsource heat pumps are similar to air conditioners. They work on the same principal, except that the cycle is reversible. In the summer an airsource heat pump works the same as an air conditioner. However, in the winter the cycle is reversed so the heat pump removes heat from the outdoor air and adds heat to the indoor air. There are limitations to airsource heat pumps however. As the outdoor air temperature becomes significantly higher or lower than the indoor air temperature, the heat pump must work harder to move the same amount of heat. In cooling this is normally tolerable as the outdoor air temperatures are generally not more than 30°F higher than the indoor air temperature. Heating is a different story though. In heating it is common for the outdoor air temperature to be as much as 60°F or more below the indoor air temperature. At these low outdoor air temperatures, the heating ability of an air source heat pump is severely diminished. As a result, energy efficiency is diminished and additional heat is required to maintain adequate heating. This is where geothermal heat pumps add value. Geothermal heat pumps work on the same principal as airsource heat pumps with the exception of the medium for heat rejection. Instead of using outdoor air as a source or sink for heat, geothermal heat pumps use the earth as a heat source or sink. Geothermal heat pumps still require energy to power the refrigeration cycle, but it is less than that required to power an airsource heat pump of the same capacity. This is because the ground temperature is typically more consistent than the outdoor air temperature. This lowers the temperature difference between the indoors and the heat sink/source and reduces the power required to move heat.
So what is a ground source heat pump then? Ground source heat pump is another name for a geothermal heat pump. Although not technically more accurate, the term ground source heat pump helps alleviate some confusion by avoiding the use of word geothermal.
In summary, geothermal energy is renewable energy. Ground source heat pumps are not geothermal energy. And ground source heat pumps are not renewable energy.