How Does Geothermal Heating Work in a Greenhouse?

Geothermal equipment is based around the stable temperature of buried earth or water, but it’s a little more complicated than that. When designed for heating alone, these systems can vary greatly in their heat source and method for distributing the warmth. It’s important to understand the variations in geothermal design to make sure you’re getting the right heating system for your agricultural pursuits.

Low or Medium Heat Source

First, geothermal systems are based on the temperatures available. If you have a source of hot water like a natural geothermal spring, it’s possible to circulate this water through pipes and radiators to release the heat into the air. To qualify as a medium heat source, the water must be 212 to 300 degrees Fahrenheit. If you have warm water that’s under 212 degrees, it’s still considered a low energy heat source like ground to air tunnels that don’t involve any water. Low energy systems heat the majority of geothermal greenhouses due to the limited numbers of hot springs available around the world.

A Steady Temperature Underground

It’s possible to use water or air from deep underground, year-round, for reliable results because changes in surface and air temperatures don’t penetrate very far beneath the surface. By tapping into a source of constant 55-degree temperatures, you’re creating a reliable system that isn’t affected by cold snaps or blizzards. Solar panels, that power electric heating systems quickly, can become covered with snow. Battery systems stop operating efficiently if the temperature drops enough, but none of that will affect the geothermal system.

Moving the Heat with Radiators and Blowers

Unless you choose a completely passive design, in which the warm air rises from wells drilled next to or under the greenhouses, you’ll need to either move heated air or water through the space to transfer the heat. Forced air systems rely on fans and blowers just like any other furnace, while water-based geothermal systems circulate hot water through pipes and radiators that release the heat slowly into the air. Both systems require electricity, but they allow you to centralize your heat production like you would with a traditional boiler or furnace.

Using a Heat Pump to Maximize Warmth

Except for greenhouse owners lucky enough to live near a naturally hot source of water, most systems involve heat pumps that extract heat from the air. Heat pumps can extract heat from practically any air source that’s above freezing. These pumps still capture heat at lower temperatures, but not efficiently enough to make it worth using. Since you can tap into an air supply that’s always around the 50-degree F mark by drilling underground, the heat pump functions all winter long regardless of air temperatures above ground.

This differs from home heat pumps that draw only from outdoor air and therefore often fail to heat during the coldest parts of the winter. The main drawback to using a heat pump in a geothermal greenhouse rather than a passive system is the need for electricity. However, heat pumps use electricity far more efficiently than direct space heaters or furnaces because they’re only extracting existing heat from an air supply rather than generating heat from resistance. When heat pumps are used with radiant loops and radiators, they typically use antifreeze to carry the heat rather than plain water, but otherwise use the same circulation system as naturally heated water.

Heat pumps function similarly to refrigeration units. Rather than generating any heat itself, the heat pump simply uses a refrigerant liquid or gas to pull all the heat out of the source air. This is how your home refrigerator cools air to keep your food from spoiling. If you hold your hand over its vents, you’ll feel a steady stream of warm air. Heat pumps simply direct that warm air to where it’s needed and release the cooled air in a reverse of the refrigeration process you’re familiar with. This is why it’s possible to extract plenty of heat from air that’s only around 50 degrees F, which may feel chilly to bare skin and not very warm at all. As long as there’s some amount of heat, the heat pump can capture most of what’s available with relatively low electrical demand. This makes a heat pump much more efficient than most furnaces, regardless of fuel type.

Thermostatic Controls and Automatic Vents

The same equipment used to keep greenhouses cooler in the summer can help spread the heat of a geothermal system. Thermostats that allow for precise control of each structure ensure that the furthest greenhouses stay just as warm as those nearer to the heat source. If you’re using heating in the spring and fall when warm days can suddenly send indoor temperatures rocketing, automatic vents work just as well as in the summer when they’re attached to the thermostats.

Trapping Heat with Insulated Materials

Geothermal heating works best in a highly insulated and tightly sealed greenhouse environment. Many greenhouses are built with a little air leakage on purpose to release fumes from natural gas or propane heaters. This isn’t necessary for any kind of geothermal heat, whether it’s distributed as warm air, hot water, or direct air flow from underground wells. You want to trap every BTU of heat generated by your heat pump or released from a hot water source rather than letting it escape through gaps and thin layers of film. Double wall film installation is essential for the insulation buffer of the air gap. When using a heavy-duty yet highly emissive film like BTL Liner’s ArmorClear for a double-wall design, the insulation value is often just as good or higher than what’s offered by polycarbonate panels or even glass.

Humidity Levels

Most greenhouses are naturally humid from the constant evaporation of irrigation water and transpiration from the leaves of healthy plants. In most areas, any minimal amount of humidity added from direct air circulation from underground wells is barely noticeable. If it’s a problem, it should be easy to counter with a large enough dehumidifier. This isn’t a problem with water loop systems, which are sealed, and heat pumps that dry the air as heat is extracted.

Geothermal Does More Than Just Heating

Even passive earth tubes, designed to exchange air directly with the underground wells, provide cooling in the summer in addition to the warming in the winter. With a cascaded geothermal system, it’s possible to maximize the return on your investment in geothermal wells or water sources.

Power Generation

Power generation from excess hot water can provide a low-cost power source for running fans, lights, and greenhouse automation equipment. Unless there’s a natural source of high-temperature water, it’s unlikely you’ll be able to produce enough electricity to power a heat pump from geothermal electricity generation. Many greenhouses are located near large public geothermal plants to use the low-emissions energy for electrical heating instead of having to drill their own wells.

Cooling the Greenhouse

Geothermal systems are easily designed to provide cooling in the summer in addition to heating for the cool seasons. This can take the form of direct air exchange with underground pipes and wells, or by generating power for air conditioning units. If you use a heat pump in the winter to generate heat with a ground-to-air system, you may be able to directly circulate the unheated air in the summer with a few system modifications. Geothermal power generation is often tricky to handle on a small scale, but new emerging technologies are making it possible for large greenhouse operations to supply both lighting and cooling in the summer from the same source they tap for winter heating.

Secondary Heat Distribution

If a geothermal heat pump is efficient enough to produce far more heat than the greenhouses need, it’s often redirected to nearby homes and businesses. Providing a secondary heat distribution circuit could cut costs for the farm’s other structures or even produce income if you can sell the heat to neighboring properties. These customers will want a steady source of heat rather than occasional supply in most cases, so size the geothermal system to handle the total demand even during the coldest periods of the year.

The exact method of exchanging heat from the ground with geothermal equipment varies depending on whether you use a ground-to-air, heat pump, or water-to-air system. Regardless of the design, your heating bill will cost the least when you’re working with an insulated and tightly sealed greenhouse structure. Whether you’re designing a new greenhouse or repairing an older one, choose a reliable product from us here at BTL Liners to make the process as easy as possible.

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