Water and soil below ground contain a vast reservoir of thermal energy, thanks to the natural heat emanating from the magma layers stretching thousands of miles below the earth’s crust. Geothermal greenhouse systems can take advantage of this stored thermal energy and use it to heat (and cool) greenhouses anywhere in the world using the “thermal flywheel” effect.
How do you use Heat from the Soil?
The key to using ground geothermal energy is to access ground that’s deep enough to escape the effects of surface temperatures. In many parts of the world, that’s as little as 4’ deep. Even in very cold climates, soil that maintains a steady temperature of 50°F through the seasonal cycle is typically found around 10’-12’ deep.
Apart from industrial uses, there are two common mediums to draw heat from geothermal sources. The most widely available is simply using the steady ambient temperature of soil. For this kind of low-temperature geothermal heating, air is typically the medium used to transfer heat from the ground itself. One popular mechanism for accessing geothermal heat from soil is to bury a long loop of plastic tube 6-12 feet below the soil surface. During winter, since the ground maintains a constant temperature, air is warmed by the surrounding soil as it passes through the tube. As warmed air returns to the greenhouse, it flows into the growing space, raising the ambient air temperature. At the opposite end of the greenhouse, air is taken back into the tube and recirculated. Depending on the structure, recirculation can be a passive process or may be aided by fans.
If a higher temperature is needed to protect tropical plants or seedlings, a heat pump can be added to extract additional heat from the pre-warmed air. Since the heat pump would be working with air that already contains plenty of heat energy from geothermal, it can efficiently raise temperatures into the 80s and 90s with very little power. The benefit of ground source heat pumps is that they concentrate naturally existing heat, rather than by producing heat through the combustion of fossil fuels. Another significant advantage is that they can be used almost anywhere, especially if power can be supplied through solar energy.
Sometimes, liquid is used as the transferring medium from the ground, perhaps water or a heat-conveying fluid like refrigerant or antifreeze. Since these liquids can absorb and transfer heat more efficiently, they can be more effective than air. Pipes carrying antifreeze don’t need to spend as much time underground in order to take up heat, so pipes can be both smaller and shorter. Once the warmed liquid is returned to the greenhouse, it may be connected to a heat exchanger that moves the heat into the airspace. Like the air system, the cooled fluid travels back through the ground to be heated again.
How do you use Heat from Water?
Water absorbs heat from geothermal energy belowground. Underground lakes, springs, and aquifers at various depths can be used to supply heat in several configurations. Since water is denser than soil, it stores more heat, and may be accessible at much higher temperatures than soil, potentially in the range of 68°F to above 200°F. Geothermally heated water is widely (but not universally) available in the US. When you’re evaluating your options, keep in mind that drilling wells or multiple boreholes to the appropriate depth may be quite expensive.
If a reliable source of heated underground water is reasonably accessible, a fairly simple system can be used to pump heated water from underground sources, circulate it through a series of pipes and move the heat where it’s needed before returning it underground. In some cases, hot water or steam may even rise up through the well without needing an active pump.
Heat pumps can also be used in this scenario, for space heating and cooling as well as for heating water. Geothermally heated water is a concentrated source of energy that can easily keep your greenhouse at an appropriate temperature and may even be used to cool it down in the summer. If it’s available, it’s a great option to consider, but this technology is only practical in regions with natural sources of hot groundwater at or near the surface.
Are Heat Pumps Useful with Geothermal Energy?
Heat pumps actually operate as a kind of reverse refrigerator. Rather than generating heat on their own, heat pumps use a refrigerant liquid or gas to pull heat out of the source air. In a refrigerator, the cold air is directed into the cold compartment and the warmth is exhausted outside the unit. Conversely, in a heat pump, the air is just distributed in reverse - the colder air is sent outside while the warm air is distributed into the greenhouse space.
Anyone who’s had a home heat pump is aware that they are certainly capable of extracting heat from practically any air source that’s above freezing, but at lower temperatures, when less heat is available for extraction, the process becomes inefficient and may be inadequate to keep your home comfortable. The huge advantage of a geothermal system is that you can tap into an air supply that’s warmed through geothermal pipes and is always around 50° F. This allows a heat pump to function efficiently all winter long regardless of air temperatures above ground. While 50° F may not feel warm, there’s plenty of heat that can be extracted by the heat pump using relatively little electricity. This makes a heat pump much more efficient than other heating options.
