It’s a little-known fact: Most year-round greenhouses are energy guzzlers. To grow a variety of crops through the off-season, in most climates, a standard greenhouse needs large amounts of heating – usually propane or natural gas. This makes greenhouses costly to operate year-round for many growers, and not all that green. One study found growing tomatoes locally in New York year-round created more CO2 emissions than shipping tomatoes from far-away states like Florida.
Fortunately, there are easy and affordable solutions to this challenge, allowing growers to create a lush, abundant year-round garden with a variety of crops. Using passive solar design, greenhouses can dramatically reduce energy costs by maximizing the use of free solar energy.
Instead of a completely plastic or glass structure, passive solar greenhouses balance the area of glazing (glass or plastic) and insulation. In the Northern hemisphere, the North wall is insulated much like a standard wall of a home. This reduces heat loss at night, and the need for fossil-fuel heating.
To compensate for the smaller area glazing area, passive solar greenhouses use glazing strategically: They are oriented so the majority of glazing faces the sun, at the right angle. Further principles include insulating underground to couple the structure to the stable temperatures of the soil, and incorporating sufficient natural ventilation for passive cooling. (For more on the principles of passive solar greenhouse design, see the book The Year-Round Solar Greenhouse, or our summary blog How to Design a Year-Round Solar Greenhouse.)
Though passive solar greenhouse design goes a long way to reducing energy costs and enabling year-round growing, some climate control is usually still needed. (The amount and cost depends on what you are trying to grow and your climate.) Fortunately, there are many sustainable options to heat and cool a greenhouse year-round. Most of them can be described with the not-so-sexy name of “thermal storage.”
Thermal storage rely on a simple fact: Greenhouses normally collect excessive amounts of heat during the day, due to the large area of glazing (glass or plastics). For example, on a sunny winter day, a greenhouse can easily reach temperatures over 100 F if it is not ventilated. As a result, most growers continually ventilate the structure on winter days, flushing that heat outside with exhaust fans.
The problem is that after the sun goes down, the greenhouse almost immediately overcools. Glazing materials are extremely poor insulators; they conduct heat very easily, and thus the greenhouse cools down immediately. The challenge with regulating a greenhouse’s temperature, therefore, is not so much the total amount of heat, but simply timing.
Instead of venting all the greenhouse’s heat out during the day, a smart self-heating greenhouse stores this heat for when it is needed. It takes advantage of the natural greenhouse effect during the day, using this free heat to warm the greenhouse at night.
There are several ways to store thermal energy. Incorporating thermal mass materials, like water, is the most common. Stacking several large barrels of water in a greenhouse creates a water wall, which will passively absorb heat during the day and re-radiate this at night. While simple, water walls come with some practical challenges. First, they are bulky. You need a lot of the material to get a significant effect, and this takes up room in the greenhouse. Secondly, thermal mass relies on direct sunshine in order to have a major effect, which is not always available in locations with cloudy winters. (More tips on using water as thermal mass here.)
The perfect thermal storage medium is low-cost, space-efficient, and has a large heat capacity (i.e. there is lots of it). Fortunately, every greenhouse already has one of these — the soil underground. The soil can be used to regulate a greenhouse’s temperature, just like water. But in contrast to water, there is an immense amount of it already underneath the greenhouse, conveniently out of the way.
A ground to air heat exchanger, often called climate battery, allows the greenhouse to tap into this natural reserve of thermal mass. It uses the soil to heat, cool and dehumidify the greenhouse – three critical functions all in one. Here’s how it works: when the greenhouse heats up during the day, fans automatically turn on and move hot air from the greenhouse through a network of pipes buried in the soil underground. As the hot air is circulated underground, heat is slowly transferred to the soil. After moving through the pipes, the air is then exhausted back into the greenhouse, cooler and drier. In this way, the system cools the greenhouse by transferring heat to the soil.
When the greenhouse gets too cold (at night or on cold days), the system provides heating as well. When the air drops below a threshold temperature, the fans automatically turn on and circulate air underground. This time, the soil is warmer than the air, and heat is transferred back to the air. (For further explanation, a video shows how a climate battery works.)
In addition to a heater / cooler, a climate battery is also a de-humidifier. During the day, the air in the greenhouse is usually hot and humid. As it moves underground it cools, reaches the dew point and condenses. Water droplets then percolate into the soil through small perforations in the pipes. In effect, the system takes water out of the air and drops it into the soil, where it’s available to plants roots. (The phase change from water vapor to liquid is also a major driver of the cooling power during the day, making it a vital part of the system functioning.)
A climate battery partly works by storing excess heat from the greenhouse in the soil. It also relies on the stable temperatures of the soil deep underground. Throughout most of the US, the soil remains roughly 40-50F a few feet below ground, despite much more extreme air temperatures). Though 50F is not hot, during the coldest parts of the year, this moderate temperature air is usually much warmer than the outside, helping prevent freezing in the greenhouse.
Though climate batteries use the stable temperature of the earth, they are different than geothermal heat pumps, often simply called ‘geothermal systems’. Heat pumps involve a refrigeration cycle (circulating a fluid underground), making them much more complicated and expensive. Climate batteries, in contrast, circulate air. The only required components are fans, thermostats and drain pipe buried underground. Pipes of a climate battery are also buried at a comparatively shallow depth – usually 2 to 5 ft. underground. Thus, these systems are vastly cheaper and simpler to install compared to geothermal heat pumps – the reason some people call them ‘geothermal lite’ or ‘the poor man’s geothermal.’
Ground to air heat exchangers have been given several other names as well, due to a long history of development with many actors. John Cruickshank coined the term ‘climate battery’ when installing one at the Central Rocky Mountain Permaculture Institute (CRMPI) in Colorado, where Jerome Osentowski now grows in permaculture greenhouses. (You can read more about their greenhouse and climate batteries in The Forest Garden Greenhouse.)
Ceres Greenhouse Solutions altered the system slightly to increase and called the design a Ground to Air Heat Transfer (GAHT) system. (You can read more about GAHT systems in The Year-Round Solar Greenhouse.) ‘Earth Tubes’ are another name that gets thrown in the mix. They are significantly different due to the fact that they are one-way airflow systems, sucking air from the outside and exhausting it inside, usually in conjunction with an earth ship home. Climate batteries and GAHT systems, in contrast, return air to the greenhouse as closed-loop systems.
The concept of using the stable temperatures and thermal mass of the soil pre-dates all of the above monikers. The first installation of a similar ground to air heat exchanger was in the 1940s. Currently, they are still rare in greenhouse applications. That is changing, though, as people discover the incredible benefits of greenhouses’ immense thermal energy potential, and the soil’s capacity to store heat over long periods. Some growers may need to incorporate back-up heating for the coldest parts of the winter, but largely a climate battery allows the greenhouse to regulate its temperature using its own naturally generated heat. The result is a naturally abundant and self-reliant greenhouse, able to grow much more year-round without fossil fuels.
For more on how to install a climate battery system, resources include:
Lindsey Schiller is a greenhouse designer and co-founder of Ceres Greenhouse Solutions, which researches, designs and builds energy-efficient year-round greenhouses. She is also co-author, along with Marc Plinke, of The Year-Round Solar Greenhouse: How to Design and Build a Net-Zero Energy Greenhouse. Read all of Lindsey’s MOTHER EARTH NEWS posts here.
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