“Simply, a climate battery is a system that pushes warm, humid air around the greenhouse, using hot air that would otherwise be exhausted and using that to heat the soil underneath,” explained Jim Schultz.

His Red Shirt Farm is a small mixed vegetable operation located in Lanesborough, MA. Schultz and other vegetable growers discussed climate battery systems as part of the Hudson Valley Farming Webinar and Field Day Series: Farming in a Changing Climate.

Climate batteries are also known as subterranean heating and cooling systems, ground-to-air heat transfer systems, geo-air exchangers and poor man’s geothermal (although Schultz said the last description is a misnomer because a climate battery is not a true geothermal system).

While climate batteries can also be installed in polycarbonate greenhouses, the scope of this discussion was limited to using them beneath the high tunnel structures used on most vegetable operations.

How does a climate battery work?

Climate battery systems can be used for heating and cooling. During hot days, high tunnels accumulate a lot of heat, which is typically removed passively via roll-up sides and endwall doors or mechanically via fans. Essentially, this heat is wasted. In a climate battery system, the excess heat is forced underground with a fan through pipes. As the hot air circulates underground, the soil absorbs thermal energy from the air. Cooler, drier air is then exhausted back into the high tunnel.

Conversely, the system can heat the high tunnel. When the tunnel needs heating at night or during winter, the fan can draw cool air through the warm soil and the warmer air is exhausted back into the high tunnel.

Kyle Nisonger from Maple View Farm in Poughquag, NY, operates a one-acre no-till market garden. He built a 30-by-96-foot climate battery in 2021. So far, he’s used his new system for winter greens.

He said, “Have realistic expectations for growing temperatures. You aren’t going to be growing everything lush and warm in the middle of the winter. It’s just taking the edges off, for the most part, in the peak of the summer and the coldest depths of the winter, providing more ideal conditions throughout the shoulder seasons.”

What does a climate battery build entail?

Schultz’s climate battery lies beneath a 30-by-72-foot high tunnel. Because a climate battery involves digging a large, deep hole, he suggested probing the site prior to excavation to help assess if there might be massive rocks in the way.

He cautioned that wet, clay soils are not suitable for a climate battery. “Think about what clay turns into when it’s heated. You’d get pottery around your pipes,” he said.

Excavation is the first step of the build. Schultz hired friends who brought an excavator and a bulldozer. They removed the topsoil and then the subsoil, being careful to keep the piles separate. The hole was four feet deep – it looked like they were installing an in-ground pool.

Next, Schultz laid three 30-foot manifolds parallel to the width – one on each end and one in the middle. The manifolds are 12-inch culvert pipes. Using a hole saw, he drilled three rows of holes into each manifold. Each hole is about four feet apart along the length of the manifold. At the above ground end of each manifold, there is a five-foot pipe with a fan attached.

Installing a climate battery system involves digging down into the subsoil. Photo courtesy of Jim Schultz

This spacing was specific to Schultz’s site. “Depending on the type of soil you have, you want to space your pipe accordingly,” he said. “If it’s a heavier soil, you can have the pipe a little bit farther apart because the heat’s going to transfer better. If you have a lighter soil, you want to have it closer together.”

Then he began installing the heat exchange tubing which he had pre-cut into 30-foot sections (the width of the tunnel). The heat exchange tubes are made of four-inch perforated ADS plastic drainage pipe with a sock. The sock prevents silt from falling into the pipes and eventually clogging them.

The climate battery has three layers of heat exchange tubes. For the first layer, he attached the 30-foot sections of heat exchange tubing into the manifolds. Once the first layer was inserted, he backfilled, mostly by hand because heavy equipment could potentially crush the heat exchange tubing. He then ran a compactor over the top. This process was repeated two more times.

Using a skid steer equipped with tracks for weight distribution, the final step was to spread 14 inches of topsoil on the surface. Schultz needs this depth because he uses a broadfork to loosen the soil in his high tunnels and wanted to be sure he wouldn’t puncture the pipes he took such great care to install.

Finally, Schultz installed two-inch rigid foam boards around the perimeter of the climate battery. His strategy was to cut each four-by-eight-foot sheet in half, with two horizontal feet and two vertical feet. “If you go down two feet and out two feet, it’s the equivalent of going down almost five feet straight down. So you save yourself excavation time and a little material,” Schultz said.

Nisonger’s climate battery has some distinct differences. He opted for two layers of heat exchange tubing rather than three. He made this decision because he was nervous about having tubing close to the surface of his beds. For insulation, Nisonger also used two-inch rigid foam boards but buried the entire four feet vertically rather than cut the larger sheets in half.

How are the climate batteries performing?

Schultz finds that his climate battery is as much an asset in summer as in winter. During summer, he grows tomatoes, cucumbers and some ginger. In the past, he struggled with growing high tunnel tomatoes through the heat of summer. With the climate battery, he doesn’t need to roll up the sides of the high tunnel until 10 a.m.

He said his employees even seek out the climate battery high tunnel in summer for some cool air. “They go and stick their head over the output tunnel because it literally feels like air conditioning. Even in summer, at four feet below grade, it is around 45º.” The air is also dry, and Schultz feels like it helps prevent fungal diseases.

Nisonger has had one full growing season with his climate battery. He finished his tunnel late in 2021 and then filled it with winter greens. After they were established, the plants began to suffer. Nisonger believes that the soil was compromised during construction with not enough emphasis placed on keeping the topsoil separate from the subsoil.

Covering and uncovering beds is one of the more labor-intensive parts of growing winter high tunnel greens. Nisonger hopes that the climate battery will help moderate temperatures enough during winter to eliminate some of the covering and uncovering.

Both growers remain optimistic about their climate battery systems. Schultz said, “We have three identical houses. Only one is a climate battery and, hands down, it’s the one that is the most productive for us in the summer months and in the winter without any extra heat. It’s also the nicest to work in. I would say, if you can have your payback be four years or less, it’s definitely worth it.”

by Sonja Heyck-Merlin