This woman is able to grow vegetables long into the winter in her passive solar greenhouse located in Ladakh, India, even when the outside temperature falls to negative 13 degrees Fahrenheit. Photo by Martin Wright/Ashden

Typical glass greenhouses require massive inputs of energy to grow crops out of season. That’s because glass, even if it’s triple-glazed, loses much more heat than an insulated wall. But with some thoughtful design, growing fruits and vegetables out of season can be accomplished by using and storing the energy from the sun.

Contrary to its fully glazed counterpart, a passive solar greenhouse is designed to retain as much warmth as possible using thermal mass and insulation. These features make it possible to grow crops year-round with solar energy alone, even when it’s freezing outside, in regions where doing so would otherwise be impossible without large energy inputs. Solar greenhouses are especially successful in China, where many thousands of such structures have been built in recent decades.

The quest to grow warmth-loving crops in temperate regions initially didn’t involve any glass at all. In northwestern Europe, gardeners planted Mediterranean crops close to specially built walls with high thermal mass. These walls created a microclimate that could be 14 to 22 degrees Fahrenheit higher than the surrounding outside temperatures (learn more at Low-Tech Magazine). Later, greenhouses built against these walls further improved yields from solar energy alone. It was only at the very end of the 19th century that the greenhouse turned into a fully glazed and artificially heated structure where heat is rapidly lost — a far cry from the technology it evolved from.

The oil crisis in the 1970s prompted a renewed interest in the passive solar greenhouse. However, the attention quickly faded when energy prices came down again, and the all-glass greenhouse remained the horticultural workhorse in many parts of the world. The Chinese, on the other hand, have built nearly 2 million acres of passive solar greenhouses during the past three decades.

-Advertisement-

The Chinese Solar Greenhouse

The Chinese passive solar greenhouse generally has three walls of brick or clay that make up the north, east, and west sides of the structure. Only the south side of the building consists of transparent material (usually plastic film) through which the sun can shine. During the day, the greenhouse captures energy from the sun in the thermal mass of the walls, which is then released as heat at night. The walls also help block the cold, north winds, which would otherwise speed up heat loss. At sunset, an insulating sheet — usually made of straw, pressed grass, or canvas — can be rolled out over the plastic to further slow heat loss. These features keep the indoor temperature of a Chinese passive solar greenhouse up to 45 degrees higher than the outdoor temperature.

The Chinese government’s incentive program has made the solar greenhouse a cornerstone of food production in central and northern China. Solar greenhouses now make up a fifth of the total area covered by greenhouses in China, and they’re expected to cover at least 3.7 million acres by 2020. The first Chinese-style greenhouse was built in 1978. However, the technology really took off during the 1980s, following the arrival of transparent plastic film. Not only is plastic film cheaper than glass, it’s also lighter and doesn’t require a strong weight-bearing frame like glass does, which makes the construction of the structure much less expensive. Since then, the design has continually been improved. The structure has become deeper and taller, allowing sunlight to be better distributed and thus reducing temperature fluctuations.