Creating Season Extension with Climate Batteries

March 1, 2024

By Sonja Heyck-Merlin

A quick glance at most seed catalogs reveals a plethora of season-extending tools, from sprouting trays to cold frames. From there, season extension gets progressively more complicated: bendable hoops for do-it-yourself caterpillar-style tunnels, small backyard greenhouses with polycarbonate walls and large high tunnels most commonly found on commercial vegetable farms.

The use of these products is driven by a singular goal — having something fresh to eat during the long dark months of winter when the crunch of fresh butterhead or the zing of a red radish becomes a faint memory. Some farms in the Northeast, whose income depends on providing their customers with a consistent supply and diversity of greens throughout the winter, have turned to a novel method of season extension: the climate battery greenhouse.

Climate batteries are also known as subterranean heating and cooling systems, ground-to-air heat transfer systems or geo-air exchangers. The concept is very simple. Take heat from the high tunnel during the warm hours of the day and store it deep underground to be released during the dark, cold hours of the night.

This is accomplished with fans and a heat exchanger buried 4 feet below the high tunnel. The heat exchanger is typically made with a series of perforated plastic pipes through which air is forced by the fans. The heat is conducted to the soil through the pipes, and moisture is condensed from the air as it is cooled underground. Dry 45-degree air is then exhausted back into the high tunnel. Conversely, during the summer months, the climate battery can be used to help cool the air in the high tunnel using the same forced air circulation that heats the tunnel in the winter.

Constructing a Climate Battery at Red Shirt Farm

Jim Schultz of Red Shirt Farm in Lanesborough, Massachusetts, installed a 30-by-72-foot climate battery seven years ago. Schultz grows about 10 acres of mixed vegetables, most of which are sold through a community supported agriculture (CSA) program, and also raises heritage pork and poultry. The climate battery construction was in part funded by a Berkshire Agricultural Ventures grant.

heat exchange tubing Red Shirt Farm
Fourteen inches of topsoil cover the heat exchange tubing in the climate battery system at Red Shirt Farm. This depth allows for broadforking in the high tunnel beds. Courtesy of Jim Schultz

To build the climate battery, Schultz first excavated a 4-foot-deep hole, which he equated to prepping for the construction of an in-ground swimming pool. He encouraged anyone interested in constructing a climate battery to do a thorough site assessment beforehand. “See what you have for a soil profile,” he said. “If you have bedrock, if you have a high water table, these are things that may be showstoppers for you.” Sandy soils are ideal for a climate battery; soil with too much clay (more than 20-25%) can be problematic because, according to Schultz, water condensing underground may cause a clay shell to form around the underground heat exchange tubing, potentially clogging the system.

Next, Schultz laid three 30-foot manifolds made of standard drainage pipe along the width (short length) of the tunnel — one on each end and one in the middle. Using a hole saw, he drilled three layers of holes into each manifold. Each hole is about 4 feet apart. Attached to the end of each manifold is a 30-inch riser pipe. Two of the riser pipes have fans forcing air underground, and the riser pipe in the middle manifold exhausts air back into the high tunnel.

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 further apart because the heat’s going to transfer better. If you have a lighter soil, you want to have it closer together.”

Next, he installed the heat exchange tubing pre-cut into 30-foot sections, the width of the tunnel. The heat exchange tubes are made of 4-inch perforated ADS plastic drainage pipe with a sock that prevents silt from falling into the pipes and clogging them.

climate battery insulation Red Shirt Farm
Schultz installed rigid insulation around the base of the climate battery to increase the soil temperature near the greenhouse sidewall. Courtesy of Jim Schultz

Shultz’s climate battery has three layers of heat exchange tubes totaling about two-thirds of a mile. For the first layer, he attached the 30-foot sections of heat exchange tubing into the manifolds. Each joint is caulked to prevent soil from entering the climate battery. Once the first layer was inserted into the manifold, he backfilled by hand to prevent crushing the heat exchange tubing. Then, he 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 was careful to keep the topsoil and subsoil separate during excavation). He needs this depth because he uses a broadfork to loosen the soil and wanted to prevent puncturing the pipes.

Finally, Schultz installed 2-inch rigid foam boards around the base of the climate battery. His strategy was to cut each 4-by-8-foot sheet in half, with 2 horizontal feet and 2 vertical feet. “If you go down 2 feet and out 4 feet, it’s the equivalent of going down almost 5 feet straight down. So, you save yourself excavation time and a little material,” Schultz said.

High tunnels used for commercial production are typically covered with two layers of plastic inflated by a blower. Schultz’s climate battery, however, is covered with a product called SolaWrap, a bubble wrap-like material composed of three layers of transparent polyethylene film with 1,000 bubbles per square meter. It is installed in sections unlike standard high tunnels that employ a single sheet covering the entire structure. It has an R-value of 1.72 and is guaranteed against ultraviolet degradation for 10 years.

Schultz harvests greens throughout the winter from the climate battery system and uses it for tomatoes, cucumbers and ginger in the summer. He finds it as much of an asset in the summer as the winter. Employees actually seek the climate battery during the summer months to cool off. “They go and stick their head over the output tunnel because it literally feels like air conditioning. Even in summer, at 4 feet below grade, it is around 45 degrees,” Schultz said. The exhausted air is also dry, and Schultz feels like it helps prevent fungal diseases in the summer months.

Growing in a Climate Battery at Maple View Farm

Kyle Nisonger of Maple View Farm in Poughquag, New York, grows three-quarters of an acre of certified organic mixed vegetables in an intensive no-till system. Although he has one small tractor, it is used only for moving materials, such as compost, around the farm. About one-third of Nisonger’s revenue comes from the year-round sale of baby greens, the majority of which he sells at farmers’ markets.

Winter greens have always been a mainstay for Nisonger, but they were harvested from two 15-by-96-foot homemade caterpillar structures. As he harvested from the caterpillars, his busy brain and tired back dreamed of a more efficient system. Specifically, Nisonger was seeking to reduce time spent on the constant setting and removing of floating row covers in the caterpillars. “In our climate we have to cover and uncover generally every morning. That can get kind of old. It’s hard to get away sometimes,” he said.

climate battery Maple View Farm
In 2021, Maple View Farm in Poughquag, New York, constructed a 30-by-96-foot high tunnel with a climate battery system consisting of two layers of heat exchange tubing. Kyle Nisonger photo

After reading Jerome Osentowski’s “The Forest Garden Greenhouse” (Osentowski is the director and founder of the Central Rocky Mountain Permaculture Institute) and studying some climate battery systems, Nisonger decided to make the investment. He constructed a 30-by-96-foot high tunnel equipped with a climate battery at the end of 2021. Nisonger’s climate battery has some distinct differences from Schultz’s. He opted for two layers of heat exchange tubing rather than three, because he was nervous about having tubing too close to the surface of his beds. For insulation, Nisonger also used 2-inch rigid foam boards, but he buried the entire 4 feet vertically around the entire perimeter of the high tunnel rather than cutting the 4-by-8-foot sheets in half.

By early January of 2024, Nisonger’s first full winter using the climate battery, he had achieved his goal of eliminating the row cover chores. Despite many nights dipping into the low 20s and multiple nights in the teens, his propane back-up heater — a 200,000 BTU Modine — had kicked on for less than an hour, which happens automatically if the thermostat in the high tunnel registers below-freezing temperatures.

Even though Nisonger is spending less time with row covers, his farming system remains labor-intensive since everything is done by hand. There are sixteen 30-inch-wide by 50-foot-long beds with 12-inch pathways. Each bed is planted with multiple rows per bed to maximize space. As soon as a crop is harvested, it is removed and the bed is replanted. Because his markets demand variety over large quantities of a few crops, the climate battery greenhouse, in mid-December 2023, was full of mustard greens, bok choy, tatsoi, baby kale, bunching kale, collards, arugula, scallions, radishes, cilantro, dill and parsley. Lettuces, especially Johnny’s Selected Seeds’ Salanova mini heads, are a mainstay in the climate battery since lettuce doesn’t perform as well in the unheated caterpillars.

climate battery greenhouse Maple View Farm
Winter greens are a mainstay of Maple View Farm’s intensive crop plan. The climate battery greenhouse allows for more efficient production of a diversity of successions, including mustard greens, lettuce, arugula, scallions and tatsoi. Kyle Nisonger photo

Maximizing production on his small acreage is essential to Nisonger, and he made sure that the two large risers in the climate battery (one for pulling air in and one for exhausting air) were located on diagonally opposite corners. He said, “I’ve seen so many designs with massive manifold tubes coming up in the middle of the greenhouse, and it’s just wasting growing space and making it so you can’t standardize anything. Mine are within the space that’s essentially a pathway. If you’re going to spend all the money, you need to be able to maximize that space.”

Another important component of the system is the automation of the ventilation in the high tunnel structure, including automated roll-up curtain sides, two automated louvers in one end wall and a large exhaust fan on the opposite end wall. The high tunnel ventilation system is monitored by a single thermostat and humidity sensor combined with an iGrow environmental controller that controls all the different equipment in the high tunnel.

A hypothetical example of the high tunnel structure and climate battery working together takes place on a cold spring morning. The fans in the climate battery are circulating air to release heat from underground into the high tunnel in the pre-dawn hours. Nisonger’s system is designed for seven and half air exchanges per hour. As the sun comes up and the high tunnel begins to passively warm up, the fans in the climate battery switch off. Later in the morning, when the high tunnel begins to warm beyond a certain point, the climate battery fans come on again and the system begins storing heat underground. By noon, the high tunnel is still getting hotter even as the climate battery is storing heat underground, so the ventilation system is activated. The sides roll up and, if it is hotter yet, the louvers will open and possibly the ventilation fan may turn on.

Automation does, however, come with a certain amount of risk. In the summer of 2023, when Nisonger was on a family vacation, a temperature sensor failed on a hot sunny day and the inside of the climate battery high tunnel hit 130 degrees. “It fried my tomato crop and the beginnings of my winter plantings. I should have had some backup alarms to notify someone that things were out of whack,” he said.

Despite the temporary setback, Nisonger is pleased with his investment in the climate battery. 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.”

About the author: Sonja Heyck-Merlin is a regular feature writer for The MOF&G. She and her family own and operate an organic farm in Charleston, Maine.

This article was originally published in the spring 2024 issue of The Maine Organic Farmer & Gardener.

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