Geothermal Basics and Ground Heat Exchangers
HOW DO GEOTHERMAL HEAT PUMPS WORK?
Someday, nearly every new residence and building will utilize the geothermal resources of the shallow earth. We’re not talking about magma-influenced hot rocks and the steam in isolated geysers like Yellowstone’s Old Faithful. We’re talking about a thermal resource resting under every square foot of the earth’s surface, from a few feet underground to hundreds of feet—and it’s been around awhile. The sun deposits nearly half its energy underground and it’s all renewable, sustainable, and free for the taking. Why wouldn’t we go after it to save energy, money, and to protect our climate from combustion that generates more greenhouse gases?
THE SHALLOW EARTH IS WHERE IT’S AT
If we’re not talking about steam and hot water from underground and instead just the dirt under our feet at temperatures of 40 to 70 degrees, how can someone get winter heating from that? The refrigerant/compression cycle so common in our refrigerators and auto air conditioners is what comes to our rescue; this time in the form of a heat pump. This equipment doesn’t make heat (as combustion-based furnaces do) it concentrates and moves it where we want. GHPs (geothermal heat pumps) are transfer agents, but they are also part of a solar collection system. That’s because the sun’s energy striking the surface of the ground enters and stays there. At less than 25 foot depth, temperatures cycle warmer and cooler with the seasons, but less than air temperatures do; they lag the seasonal calendar’s highs and lows. Below 25 feet, the temperature is stable and unchanging. The simplified diagram to the right shows that GHPs actually drive three loops:
- Is the circulation of a fluid loop from underground to the heat pump and back underground.
- Is the refrigerant circuit inside the heat pump with compressor, condenser, and evaporator (which sends/removes heat to/from the distribution loop).
- Is the distribution loop, using ducted air or piped water to deliver heat at the condenser (winter) or remove it (summer) at the evaporator.
WORKING BOTH WAYS
All heat pumps have the capability to perform both heating and cooling functions because of a reversing valve in the refrigerant loop that changes the direction of the refrigerant gas’ flow. This changeover is performed by the thermostat, either manually or automatically—there are no (necessary) seasonal adjustments to the heat pump itself.
The HDPE (high density polyethylene) pipe we place underground carries water or a water and antifreeze solution in (most cases) a closed loop. We either take thermal energy from the ground that is carried by this water to the heat pump to generate heat in our building or we take heat from the building into the liquid loop to be transferred into the earth. The function of this piping carries the name of “ground heat exchanger”, or GHEX because it serves to either pull from our heat source or reject to our heat sink. There are many loop variations.