What Is A Geothermal Heat Pump System?
Geothermal heat pumps use the natural properties of the earth to provide heating and cooling to a building. Heat addition and rejection take place below the ground (or in a body of water), inside hundreds of feet of high-density polyethylene pipe, known as a loop. Fluid is circulated through the loop and into the geothermal units, each of which serves a specific zone within the building. The units then amplify and direct conditioned air into their zone based on its cooling or heating requirement.
Heat is continually supplied to the ground in the form of solar energy. Approximately 46% of the sun’s energy is absorbed by the earth. The remaining 54% is either reflected back into space or absorbed by the earth’s atmosphere. At a depth of approximately 15 feet the ground temperature remains fairly constant, with an average temperature between 42-77°F year-round, depending on the local climate, terrain and soil type.
The size of the loop is dependent upon the cooling and heating load for the building, the type of loop installed (see Geothermal System Types) in addition to the local climate, terrain and soil type. Computer design software is best used to determine the loop size.
In the cooling mode, the earth acts as a heat sink enabling the circulating fluid to transfer the excess heat, absorbed by the unit, from the building zones to the earth where it is absorbed and stored for future heating requirements.
The earth has a built in time delay. During those times of the year when the cooling requirements are highest, the Earth’s temperature is still relatively cool, allowing easy absorption of excess heat into the ground. The ground temperature actually lags the outside temperature by several months, so that by the time its temperature has increased, the building’s total cooling requirement has been reduced.
In the heating mode the Earth acts as a heat source, allowing the circulating fluid to extract natural heat from the Earth and transfer it to the space where it can be used for heating.
During those times of the year when the heating requirements are highest, the earth’s temperature is still relatively warm which makes heat extraction from the ground easy. In the heating mode, the unit function is reversed from that of the cooling mode. The geothermal unit absorbs heat from the ground and transfers it to the building zones that require heat. The ground temperature again lags the outside temperature so that by the time its temperature has decreased, the building’s total heating requirement has diminished.
For most of a year the building requires both cooling and heating, allowing the water temperature in the ground loop to remain relatively constant throughout the day. This is known as a balanced system.
Supplemental Hot Water
Geothermal systems can also provide supplementary hot water to the building’s domestic hot water system, which amounts to "free" hot water at no penalty to the units. A desuperheater is essentially a water to refrigerant heat exchanger placed either inside or outside the geothermal unit and tied into the unit’s refrigeration circuit. During cooling operation, the desuperheater takes the heat that is extracted from the return air and rejects it into the building’s hot water system before it reaches the ground loop.
The benefit of this system is twofold. First, "free" hot water is being provided to the building’s hot water system. Second, by rejecting some of the heat to the hot water system, less heat is rejected to the earth, making the loop more efficient. During heating operation, hot water is generated at the expense of space heating, but is often cheaper than heating water with fossil fuels such as gas or electricity.