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.
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.
Growers are also improving thermal efficiencies of their structures by opting for modern insulation materials in the walls. Synthetic insulation blankets, which are better suited for moist environments, are also in use because the straw mats become heavier and have a lower insulating capacity when wet. Some of the most recently built greenhouses have more sophisticated ventilation and insulation systems, including insulation blankets that roll up and down automatically. Some greenhouses even have a double roof or reflective insulation. The plastic film used for the greenhouses — obviously the least sustainable component of the system — is also continually being improved, resulting in a longer life span.
The performance of a Chinese greenhouse depends on its design, the latitude, and the local climate. A recent study observed three types of greenhouses in Shenyang, the capital of Liaoning Province, China. The city lies just south of the 42nd parallel north (almost exactly as far north as Chicago, Illinois) and is one of the most northern areas where Chinese-style greenhouses are built (between latitudes 32 degrees and 43 degrees north, wherein much of the continental United States lies).
In this study, three greenhouses were examined, all with the same shape and dimensions (197 by 41 by 18 feet). The simplest construction has walls of rammed earth with bricks lining the interior to increase the structure’s stability. Its covering is a thin plastic film that is insulated at night with a straw blanket. The other two greenhouses have a northern wall of brick with extruded polystyrene foam acting as an insulating material — the thickness of the wall is half that of the rammed earth model. These more “modern” greenhouses are also covered with a thicker polyvinyl chloride (PVC) plastic film, and one of them also has a reflective coating on the insulation blanket, further reducing heat loss at night.
The study found that, at 42 degrees north, only the most sophisticated greenhouse — with its reflective insulation layer — succeeded in keeping the inside temperature above freezing at all times using only solar energy. What’s more, the interior temperature stayed above 50 degrees most of the time, which is the minimum temperature for the cultivation of warm-season plants, such as tomatoes and cucumbers. Of course, passive solar greenhouses in more southern locations would require less-sophisticated insulation techniques to be operated without additional heating.
If we go farther north, similar passive solar greenhouses would require extra heating during the coldest months of the year, no matter how well they’re insulated. The farther north the greenhouse is located, the greater its roof slope should be, angled to be perpendicular to the sun’s rays when it’s lowest on the horizon. (For more on calculating the roof angle of a greenhouse, read Small-Greenhouse Plans for Winter Growing.) In 2005, a 23-by-98-foot Chinese-style greenhouse was tested and observed in Manitoba, Canada, at a latitude of 50 degrees north. The greenhouse had a well-insulated northern wall that utilized R-20 fiberglass insulation and a cotton insulation blanket with an R-value of 7. During February, the coldest month, the outside temperature varied between 40 and minus 20 degrees. While the interior temperature was on average 32.4 degrees higher than the exterior, it turned out to be impossible to cultivate plants without added heat during winter.
Nevertheless, energy savings can be huge in comparison with a glass greenhouse. To keep the temperature above 50 degrees at all times, the heating system of the Canadian structure only has to deliver a maximum of 5.4 British thermal units (Btu) per hour per square foot, which is 12,172 Btu per hour, or 3.6 kilowatts (kW) for the entire building. In comparison, a glass greenhouse of equal proportions at the same interior and exterior temperatures would require a maximum capacity of 125 to 155 kW — that means the Manitoba solar greenhouse is up to 43 times more energy-efficient than an all-glass greenhouse!
These results can’t be applied to all locations at the 50th parallel north because solar output has a greater impact on the inside temperature of the structure than does the outside temperature. The correlation between inside temperature and sunlight is almost four times greater than the correlation between inside temperature and outside temperature. So, while Brussels lies at the same latitude as Manitoba, a solar greenhouse in Brussels wouldn’t be nearly as effective because Manitoba receives on average 1.5 times more sun.
Thermal capacity can be further improved by placing black-painted water-storage tanks against the north wall inside the structure. These capture extra solar energy during the day and release it at night. A different method to improve the heat retention of a greenhouse is by earth berming the north, east, and west walls.
A passive greenhouse has the potential to save a lot of energy, but there’s a catch: Currently, the profits generated by each Chinese greenhouse are two to three times lower per square foot than the profits of its fully glazed energy-hog counterpart. The more-efficient Chinese greenhouses can grow an average of 6 pounds of tomatoes and 6 pounds of cucumbers per square foot, while the average production in a glass greenhouse is about 12 pounds of tomatoes and 20 pounds of cucumbers per square foot. A passive greenhouse industry would have to take up two to three times more space to produce the same amount of food.
Another issue with a solar-powered greenhouse is the lack of a carbon dioxide (CO2 ) source. To increase crop yield in modern greenhouses, operators aim to have a CO2 level at least three times the levels generally found outdoors. The CO2 used is a byproduct of the fossil-fuel-based heating systems inside the greenhouses. If growers don’t use fossil fuels, they must find another source of CO2. This isn’t only an issue for solar greenhouses; it’s also one of the main reasons why geothermal energy and electric heat pumps aren’t advancing in the modern greenhouse industry.
In Chinese solar greenhouses, this issue is sometimes solved by raising produce and animals together. Pigs, chickens, and fish all produce CO2 that the plants can absorb, while the plants produce oxygen (and green waste) that the animals can use. The animals and their manure also contribute to the heating of the structure. Research of such integrated greenhouse systems has shown that the combined production of vegetables, meat, milk, and eggs raises yields quite substantially.
Justin Walker, an American now living in Siberia, is building an integrated system in a Siberian monastery using horses, goats, and sheep. Considering the intensely cold climate, the structure is built partly below ground, while its above-ground parts are earth bermed. The animals provide CO2 and generate heat that benefits the plants. Above the barn area is a hayloft that provides added winter insulation as well as ventilation in summer when it’s empty. Walker has also integrated a compost heat-recovery system that produces hot water that’s piped through radiant heating zones in the floor of the greenhouse.
Heating and CO2 production can also be done without housing animals in a greenhouse. Their manure will suffice. The use of horse manure for heating small-scale greenhouses dates back several centuries in Europe, and in China it was practiced as far back as 2,000 years ago. Since the 1980s, several compost-heated greenhouses have been built in the United States. These have shown that a greenhouse can be entirely heated by compost if it’s well-insulated, and that this method drastically enriches the CO2 levels in the soil and in the greenhouse air. As a bonus, the compost also serves to increase soil fertility.
Kris De Decker is the founder of, and main writer for, Low-Tech Magazine, a blog that promotes traditional knowledge and low-tech solutions to modern problems. This article is excerpted from “Reinventing the Greenhouse.”
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