How is a geothermal heat pump like a refrigerator?

Like a refrigerator, a geothermal heat pump simply transfers heat from one place to another. When a refrigerator is operating, heat is being carried away from the inside food storage area to the outside, your kitchen. Therefore, cooling is not being added to the inside; heat is being taken out.

To understand the operation of a geothermal heat pump, it helps to understand how a refrigerator works. A refrigerator uses a refrigeration circuit with four main components, a compressor (1), a condenser (2), an expansion device (3), and an evaporator (4). Refrigerant (sometimes referred to by the brand name Freon) is pumped through the circuit to transfer heat from the inside of the refrigerator to the outside.

The compressor (1) is the pump. It also pressurizes the refrigerant gas. Since temperature and pressure are directly related, as the pressure increases, the temperature increases. The high temperature/high pressure gas flows from the compressor to the condenser (2).The cooler air in the kitchen (relative to the temperature of the refrigerant, 150 to 180°F [65 to 85°C]) causes the refrigerant to condense into a liquid. When two surfaces at different temperatures touch (or are very near – separated only by tubing), the hotter surface cools and the cooler surface warms. This is a law of physics called the second law of thermodynamics. The condenser therefore releases heat to the kitchen.

The next step in the process involves the expansion device (3).The expansion device is a small orifice that the refrigerant is forced through. The small hole creates a pressure differential between the two sides of the device. Think of an expansion device like a dam on a river with a hole in the dam. The water leaking through the hole is at a low pressure on the downstream side; the water on the other side (being held back by the dam) is at a high pressure. Once again, the pressure/temperature relationship (lower pressure/ lower temperature) creates a cold, low pressure liquid refrigerant that gets fed to the evaporator (4).

As warm air inside the refrigerator (relative to the very cold temperature of the refrigerant) passes through the evaporator coil (4), the hotter surface (air inside the refrigerator) gets cooler and the cooler surface (refrigerant in the evaporator (4) tubing) gets warmer. The liquid refrigerant evaporates back into gas form, and the cycle starts over again as the refrigerant enters the compressor (1). The evaporator therefore absorbs heat from the inside of the refrigerator, which keeps the food cold.

An air conditioner or refrigerator transfers heat in only one direction. A heat pump can transfer heat in two directions, thereby heating or cooling the space. Most heat pumps heat or cool the air. Some heat pumps heat or chill water. An additional component, a reversing valve, is added to a heat pump, which allows the refrigerant to change direction, allowing the space that was being cooled to be heated.
A geothermal heat pump has a compressor, a condenser, an expansion device, and an evaporator like a refrigerator, but also includes a reversing valve to allow both heating and cooling. The big difference between a refrigerator or traditional air conditioner and a geothermal heat pump is the way heat is transferred. A geothermal heat pump transfers heat between the refrigerant circuit and the ground instead of between the refrigerant circuit and the air. The ground is a much milder heat source, since the temperature changes very little over the course of the year. The outside air temperature, however, varies significantly over the year, making a geothermal heat pump much more energy efficient than a traditional air conditioner or heat pump. A geothermal heat pump compressor also operates at lower pressures because of the milder heat source/heat sink (the ground), helping provide longer life expectancies.

A geothermal heat pump is a like a refrigerator in many ways. Simple refrigerator technology coupled with the stable temperature of the Earth provides quiet, reliable, and energy efficient heating and cooling systems for today’s discerning homeowners.