There are several ways that greenhouses can take advantage of geothermal heating, and each type has its own set of advantages. Sometimes one approach is obviously a good match for a set of needs and conditions, occasionally there are several to choose from. Let’s discuss a few options - keep in mind that most of these can be combined in different ways - don’t shy away from being creative if a single approach doesn’t quite fit.
Most people think of geothermal heat in the context of geysers, hot springs, and active volcanic regions. Those are definitely dramatic examples, but they’re found in very few places around the world, and much like trying to drink from a firehose, they’re probably too powerful to safely harness for your greenhouse. Fortunately, low-temperature geothermal heat is available virtually everywhere on the planet. It’s safe and it’s free. All our discussions will be focused on the use of this global resource for greenhouse heating and cooling.
Sunken Greenhouses: Pits, Walipini, and Chinese Style
There are several variations of sunken greenhouses. They typically have some elements of passive solar construction and may also access certain elements of geothermal heat, whether or not that’s an intentional focus. Sunken greenhouses are popular because they are often less expensive to build than traditional above-ground models. They also may have particular appeal to DIY types who have no hesitation about spending a weekend digging a giant hole. Although all these variations tend to be lumped together as one, there are some notable differences.
Traditional sun-pit, or earth-sheltered, greenhouses originated in New England around the 18th century. Pit greenhouses are basically large versions of time-honored cold frames used to protect tender plants from winter cold. The frames collect and retain warmth from the daytime sun and store it in the ground. As air temperatures drop at night, the ground releases its stored heat into the enclosure and keeps the plants from freezing. Full size pit greenhouses are designed to take advantage of the fact that, several feet below ground, the earth maintains a constant temperature that’s well above freezing. Generally, a pit greenhouse is sunk approximately 4 feet below ground with clear half walls and roof emerging above.
Originating In South America, walipini are typically sunk up to 8 feet into the ground, with only a translucent roof, angled so that it receives maximum sunlight on the winter solstice. This type of design was popularized in the early 2000s after US missionaries traveled to Bolivia to help the indigenous population improve their access to fresh food by constructing affordable, accessible and effective greenhouses. Reports of their success inspired a lot of gardening enthusiasts who realized that affordable greenhouses were suddenly within their reach.
Walipini take advantage of some elements of both passive geothermal and passive solar heating. At 8’ below ground, the ground maintains a steady temperature of about 55 degrees, even when winter nights fall below freezing. That’s a great head start, but the earthen walls also absorb heat from the sun during the day and release it during the colder nights, much like the cold frames we discussed earlier. From these two simple, passive and free energy sources, the Bolivian walipini can maintain a comfortable interior environment for growing food year-round.
It’s true that walipini are very effective in equatorial regions, where the sun tracks high in the sky at all times of the year. In that situation, the sun shines pretty directly straight down and provides close to 12 hours of light for growing all year-round, even at the bottom of an 8’ hole. The low-profile roof protects the plants and the structure itself from strong winds on the Altiplano, while the arid climate minimizes any risk of flooding. While many enthusiasts in the higher latitudes have embraced the concept of walipini greenhouses, this traditional design needs a lot of adjustments to apply effectively in regions where winter days are short and the sun tracks low in the sky.
Chinese Style Greenhouses
It’s probably not fair to exclusively credit Chinese gardeners with the development of this style of greenhouse. Still, in recent decades they have definitely embraced and popularized the style, building 2 million acres or more of passive solar greenhouses.
The Chinese style greenhouse generally has 3 walls of brick or clay enclosing the growing area, leaving the southern exposure enclosed with some type of transparent skin, typically plastic film. During the daytime, warmth from the sun is absorbed and stored by the thermal mass of bricks that make up the walls. The bricks perform double duty, releasing warmth during the night and protecting the tender plants from punishing north winds, which would otherwise accelerate heat loss. At sunset, growers can also spread an additional insulating sheet of straw or woven grasses to slow heat loss even more. With these features, the interior temperature of this style of passive solar greenhouse can be sustained as much as 45 degrees warmer than outside.
Even though Chinese style greenhouses are not actually sunken, they rely exclusively on heat from the sun and thermal mass to store and then radiate warmth as needed, very much like pit and walipini styles.
Disadvantages of Sunken Greenhouses
When greenhouses are sunk into the ground, exposure to sun is limited and the lower half of the greenhouse may spend a significant amount of time in shade with inadequate sun, while the upper half may get too much. If supplemental UV light is used, it’s hard to balance the exposure.
Sunken greenhouses are subject to flooding and effective preventive measures can be difficult to establish. Additionally, the greenhouse may need significant insulation on the floor and lower walls lest the stored warmth from the sun be conducted away from the greenhouse interior.
This kind of basic, easily constructed project relies entirely on the stability of the earth to prevent collapse and other safety concerns. Nowadays, most states require that sunken greenhouses have fully engineered walls on all four sides and top to bottom on the external perimeter. These restrictions effectively eliminate most of the advantages of sunken greenhouses as an easy, inexpensive project for DIYers.