Geothermal heat pumps – a primer:

Thermodynamics 101:

First of all, there is no such thing as ‘cold’, as far as something that is the opposite of heat; there is just more or less heat energy. It’s the same as dark and light, the dark is merely the lack of light and ‘cold’ is the lack of heat.

Second, “Well, everyone knows heat rises, right?” No. Hot air rises because it is lighter than cold air. Heat goes one direction only, from a higher temperature toward a lower temperature. A hot-air balloon rises because the heated air in it is lighter than the cold air surrounding it, but the heat in the balloon is moving from the inside (hotter) toward the outside (colder) and in all directions.

Third, if there is no temperature difference (equilibrium) then no heat moves. The lower the difference and the more resistance (insulation) the slower the heat moves. The greater the difference and the less resistance, the faster it moves.

Basic refrigeration:

Most residential refrigeration uses the vapor-compression cycle. This mechanically harnesses the phenomenon that if you compress a gas it will get hotter. If you cool the gas enough it will condense into a liquid and then if you allow the liquid to evaporate and expand, it will absorb heat. The coils in which you cool the gas get hot (giving off heat) and the coils in which you evaporate the gas get colder (absorbing heat). The mechanism controls the compression and expansion to move the heat energy from one side to the other. In an air conditioner, this only moves heat from the inside to the outside. With a heat pump, you can reverse the direction of the process to bring heat inside.

Air-source heat pumps:

Using the refrigeration process described above, an air-source heat pump uses the atmosphere as a heat sink and moves heat from the outside air to the inside air for heating and from the inside air to the outside for air conditioning. When it is 50 F outside and your thermostat is set to 70 F inside, the heat pump has to “lift” the temperature of the heat energy it is harvesting from the atmosphere from 50F to above the 70 F inside. The compressor consumes electricity to perform this work. When it is 30 F outside and 70 F inside, it has to raise that heat energy farther to get it into the house. This requires the compressor to do more work and it consumes more electricity. In air conditioning, it is reversed, if the inside is 70 F and the outside is 90 F, it has to lift the heat energy from inside above the 90F outside. If it is 105F outside, it requires more work and more power.

 Geothermal heat pumps:

Geothermal heat pumps, (also called “ground-source”,  “water-source” or “geo-exchange” heat pumps) work the same way that an air-source does. The difference is that instead of using the atmosphere as their heat sink, (which has higher or lower temperatures depending on the conditions), they use the temperature of the earth which is virtually unaffected by weather. The average temperature of the ground at 5 feet down is about 54 F. Since the difference in temperature between the inside air and the earth mass is less, the energy required to move heat between them is less too. By taking advantage of the temperature of the earth, a ground source heat pump can provide higher operating efficiencies for heating and cooling. The earth temperatures are harnessed using a ground loop “geo-exchange” most commonly consisting of High-Density-Poly-Ethylene (HDPE) pipe. The pipe is buried in the ground in loops contacting enough earth mass to provide the thermal energy required to meet the conditioning needs of the structure. An anti-freeze solution is circulated through the pipe to move the heat between the loop field and the heat pump equipment. The ground loop can be installed horizontally in trenches or vertically in boreholes. The design of the ground loop is dictated by the heating/cooling load of the structure, the site details, and the thermal characteristics of the soil. The capacity of the loop field is affected by both the amount of pipe in the ground, the amount of earth mass that is contacts and the characteristics of the soil.


There will be a quiz.