In an increasingly competitive landscape, cannabis growers are looking for ways to reduce operational costs in novel places, including right under their feet.

Ground-to-air heat exchangers use the stable temperature of the soil and passive solar energy to provide a renewable source of heating and cooling.

Ground-to-air heat exchangers have garnered a proliferation of names in the past decades. The most common is “climate battery,” coined by a pioneering engineer Michael Cruickshank and the Central Rocky Mountain Permaculture Institute (CRMPI).

Colorado-based Ceres Greenhouse Solutions followed by developing a patent-pending version for cannabis growers called a Ground to Air Heat Exchanger (GAHT). Another acronym is Subterranean Heating and Cooling System (SHCS). Then there’s the most colorful title from a recent video reporting on the concept: “poor man’s geothermal.”

Low-cost geothermal is a summary of how a climate battery works. Like traditional geothermal systems, climate batteries use of the stable temperatures of the soil underground.

However, unlike traditional systems, which involve pipes bored hundreds of feet underground and complex refrigeration systems, climate battery systems are much simpler and cheaper.

A network of underground pipes is buried just a few feet (usually two to five feet) underground. Instead of a liquid coolant, they circulate hot air collected in the greenhouse during the day, taking full advantage of free and plentiful solar energy.

Those factors make a climate battery vastly simpler and cheaper than traditional geothermal systems, in the range of few thousand dollars for a backyard greenhouse.

The systems have been extensively used in greenhouses to provide year-round heating and cooling at a fraction of the cost of fossil-fuel systems (applications in indoor growroom are possible but rare). Now, cannabis cultivators are adopting the technology, taking advantage of the many benefits of low-cost geothermal systems.

Using the Greenhouse Effect to Grow Cannabis

Climate batteries rely on a simple fact: Most greenhouses collect far too much heat during the day, even in winter. In other words, it is fairly common for a greenhouse to overheat on a cold and sunny winter day. Typically, growers vent this excess heat outside, essentially wasting it.

Instead of exhausting hot air outside during the day, a climate battery circulates the heat underground. A fan pumps hot air from the greenhouse through a network of pipes buried in the soil. As air travels underground, the soil absorbs heat from the air. The air is then exhausted back into the greenhouse, cooler and drier.

A corollary function of the system is to reduce daytime humidity, a major benefit to help thwart powdery mildew and a host of other pests and diseases. When hot and humid air is cooled underground, it reaches the dew point. The water vapor condenses into liquid and percolates out of the pipes through perforations. Continuous air movement and drainage prevent mold growth in the pipes.

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Besides cooling and dehumidification, a final function of the system is heating. When the greenhouse gets too cold, the fans again circulate air underground. This time, the soil temperature will be warmer than the air due to the fact that the soil stays a moderate and stable temperature year-round. Thus, working in the same direction, the fans draw cooler air underground, where it is warmed by the soil and then exhausted back into the greenhouse.

In many ways, the soil acts like a battery. During warm periods, the greenhouse heats the soil, essentially charging the battery. During cold periods, it then uses the warmer soil to heat the greenhouse, drawing down on this stored thermal energy. The soil is an excellent source of thermal mass, retaining this heat without taking up any room inside the greenhouse.

Read More: How to Reduce Your Grow Room's Operational Costs

Enabling a Closed Greenhouse

Many greenhouse cultivators are adopting climate batteries simply for the fact that they are a closed loop. The system conditions the indoor air—providing temperature control, air circulation, and de-humidification—without exhausting any air outside. That is a major benefit for growers in populated areas where strict regulations prohibit cannabis cultivators from exhausting air due to odors.

Additionally, less ventilation reduces the exposure to environmental contaminants, such as pests and hemp seeds. As a closed loop system, a climate battery allows the greenhouse to be sealed much of the time, generally aiding environmental control.

Climate Batteries are a Proven Technology

Climate batteries and their counterparts have been around for decades and as such, growers enjoy the confidence that the systems will reliably reduce energy costs. Exactly how great these energy savings are, of course, is the golden question.

Returns on investment vary greatly by the size of the operation and the climate. Marc Plinke, chief engineer at Ceres Greenhouse Solutions, says most growers see energy reductions of 30-60 per cent. Growers in harsher climates with a greater heating and cooling demands will experience greater savings.

Though climate batteries provide renewable heating and cooling almost for free, the systems are not perfect. Importantly, backup heating and cooling systems are needed because of the narrow temperature requirements of most cannabis strains.

Particularly with closed greenhouses, an alternate cooling system is required. That requires a hefty upfront investment in climate control equipment. But once again, once the system is installed, it provides virtually free heating and cooling. The power consumption for running fans is very small, and thus the grower cuts their energy bill and adds to the bottom line year after year.

Currently a niche system for cannabis cultivators, climate batteries are making in-roads to the industry. Three automated commercial cannabis greenhouses in Colorado will be operating with the system them this year. Notably, one at 10,200 feet elevation in the Rocky Mountains operates for much of the year using a GAHT system and an energy-efficient greenhouse without backup heating.

With energy savings and added environmental closed-loop air circulation, it’s likely many cannabis growers will follow, integrating poor man’s geothermal into very high-tech greenhouses.