No Hot Springs, No Volcanoes
To most people in the world, "geothermal energy" means steaming hot water in very
isolated areas that power hot springs for relaxing tourists, health resorts, commercial complexes or maybe even some greenhouses. One thing we know for certain, "geothermal energy" is very rare. ...or is it? It’s true that high grade and medium grade geothermal sources are quite rare in most of the world. Yet, low grade geothermal energy, while considerably less dramatic, is common in virtually every country and climate of the world. In fact, low grade geothermal energy is plentiful, stable, renewable and available everywhere where there is dirt, sand, rocks or vegetation. It's that simple: if you have earth under your home, you also have unlimited geothermal energy.
To illustrate, we know that home basements remain warmer in the winter and cooler in the summer than the rest of the home. We also know that traditional root cellars, sunk just below the ground, have been employed for centuries to store fruits and vegetables and protect them from freezing, even when outside temperatures are bitterly cold.
The fact is, in much of the world, at a depth of 6' to 8' the earth remains at a constant 50°F - 65°F year ‘round, whether you live in sub-zero weather or in hot deserts. (Yes, exceptions do exist.) This consistent temperature is called "low grade geothermal" energy.
Power Bills are Bad
Greenhouses are often considered too expensive to be practical during cold seasons, but it’s possible to build a passive geothermal system that could keep your greenhouse operating year-round at zero cost. If you’re using a system that includes an inexpensive, low energy, circulating blower, your cost will still be very low. In fact, you can estimate the cost by checking the power draw of a typical blower sized to your greenhouse, multiplying that by the number of hours you’ll use it per day, then multiplying it again by 30, and again by the price of electricity from your local utility.
Are All Geothermal Systems the Same?
When you’re considering geothermal heating for your greenhouse, it’s important to understand exactly which method you’re researching and don’t mix or match between them. One small example: geothermal battery systems depend on condensation to transfer heat into the soil, so tubes for those systems must be perforated to allow water to pass through and they won’t perform well in dense clay soil. However, geo-air systems need solid tubes so that water does not enter the system, and clay soil is actually a good heat-transfer medium in this case. Keep in mind that both systems can be used for both heating and cooling, but you’ll need to do some research to decide which system will do a better job for you based on the soil and climate in your area.
Passive geo-air systems are simple and have the lowest power requirements. Geo-air systems with a supplemental heat pump are efficient and use relatively simple, easily available equipment.
Geo-liquid systems are more complex to set up and use more expensive equipment, but they are good solutions where the soil is not ideal or there’s not a suitable area to bury air tubes. If a plentiful supply of clean, warm groundwater is available, or even a sizeable natural aquifer, this could be a good solution for low-cost greenhouse heat. The major downside is the initial outlay for relatively expensive specialized equipment, the cost of drilling, and continuing maintenance on said equipment.
If you plan to grow warm weather plants that are sensitive to cooler night temperatures, like tomatoes and cucumbers, you may find that using a thermal battery system is better suited for your needs since it keeps nighttime temperatures higher in the winter.
How Deep is Geothermal Heat?
The ideal depth of your subterranean tubes is also determined by the type of system you’re using and what level of energy efficiency you’re aiming for. A true Geo-air system is much more energy efficient at 8', but a geo-air with a supporting heat pump may be able to function well at 4'. The presence of heat exchangers in the latter example makes it difficult to calculate how much difference the depth affects efficiency. Just keep in mind that if you’re not planning to use supplemental equipment, 4' is not efficient enough in most climates to provide 100% of your heat requirements.
Our Winters are Cold and Dark. Can Geothermal Still Work?
Geothermal systems are not reliant on variable weather conditions like wind or sun. Geothermal energy is present and constant no matter the season or the weather. In fact, the temperature below ground where air tubes or refrigerant loops are buried remains far above freezing all the time, no matter how cold the weather is. This makes geothermal the most dependable source of steady, year-round warmth for your greenhouse.
Details, Details
If you’re planning on using a pond or underground aquifer for an air-liquid (geothermal) heat pump, you’ll need to make sure your water source is large enough to store the thermal energy you’ll draw on each day. For a surface pond, you’ll need at least 1 acre in surface area for each 50,000 Btu per hour of heat pump capacity. Keep in mind that you’re withdrawing heat from the pond to direct it to your greenhouse - make sure that the lowered temperatures in your pond won’t negatively affect any fish or other wildlife that rely on it.
If you’re using geo-air to heat your greenhouse, you can choose between horizontal and vertical loops when you’re planning your tubes. If you’re short on land, vertical loops may work best for you, but that requires deeper (and more expensive) excavation. If you have enough available land close to your greenhouse, horizontal loops are cheaper & easier to install.
Consider the soil in your area when planning your geothermal system. For geo-air systems, the soil needs to be able to transfer heat to and from the air in the tubes. Dry, sandy soils aren’t very efficient, but wet sandy soils work very well. Dense clay soils are also great at storing and transferring heat.
