By Kris De Decker
Unlike fully glazed greenhouses, a passive solar greenhouse is designed to retain as much heat as possible. Research has shown that it is possible to grow warm-weather crops year-round using only the sun's energy, even if temperatures are very low outside the greenhouse. The solar greenhouse is especially successful in China, where thousands of these structures have been built in recent decades.
The production of crops originating in warm climates in temperate regions does not initially involve the use of glass at all. In northwestern Europe, Mediterranean crops were planted near special constructions, called “fruit walls”, which, with a high thermal mass, create a microclimate that can range from 8 to 12 ° C (14 to 22 ° F ) warmer than undisturbed weather.
Later, greenhouses built from these fruit walls further improved the performance of solar energy. It was only at the end of the 19th century that the greenhouse became a fully glazed and artificially heated building, where heat is lost almost instantly - the complete opposite of the primitive technology from which these modern greenhouses evolved.
During the oil shocks of the 1970s, there was renewed interest in passive solar greenhouses.  However, attention quickly faded as energy prices fell, and the glass greenhouse remained the workhorse of the Northwest of the world. The Chinese, for their part, built 800,000 hectares of passive solar greenhouses during the last three decades, this constitutes 80 times the area of the Netherlands, which is the country with the largest greenhouse industry in the world.
The Chinese Greenhouse
The Chinese passive solar greenhouse has three brick or mud walls. Only the south side of the structure is made up of transparent material (usually a plastic sheet) through which the sun can shine. During the day the greenhouse captures the sun's heat in the thermal mass of the walls, which is released at night.
As the sun sets, an insulating sheet - made of straw, pressed grass, or canvas - is spread over the plastic, increasing the structure's ability to insulate. The walls also block cold north winds, which if not blocked would accelerate heat loss from the greenhouse.
Being the opposite of the energy-intensive glass greenhouse, the Chinese passive solar greenhouse is heated all year round by solar energy only, even when the outside temperature falls below freezing. The internal temperature of the structure can be up to 25 ° C (45 ° F) higher than the outside temperature.
The incentive policy of the Chinese government has made the solar greenhouse the cornerstone of food production in central and northern China. One fifth of the total greenhouse area in China is now made up of solar greenhouses. In 2020 they are expected to occupy at least 1.5 million hectares. [one]
The Chinese Greenhouse Improvement
The first Chinese-style greenhouse was built in 1978. However, the technology only took off during the 1980s, after the advent of transparent plastic sheets. Not only are these sheets cheaper than glass sheet, but they are also lighter and do not require strong load-bearing capacity, which makes the construction of the structure much cheaper. Since then, the design has continually improved.
The structure was made deeper and taller, allowing the sunlight to be better distributed and ensuring a decrease in temperature fluctuations.
A: The original design from the 1980s with a glass cover. B: An improved design from around 1985, with a plastic sheet, a night curtain, and better insulated walls. This design is the most common. C: An improved design from 1995. The walls are thinner because they are insulated with modern materials. The night curtain works automatically. D: The most recent design, from 2007, with a flysheet that provides more space for extra insulation.
In addition, farmers are increasingly turning to modern insulation materials over rammed earth or air cavities in the walls, which save space and / or improve the heat absorbing characteristics of the structure. Synthetic insulation blankets are also widely used, which are better suited to cope with moisture than the old-fashioned straw mats, which become heavier and less insulated when wet.
In some of the newer greenhouses, the insulation blankets are rolled up and down automatically, and more sophisticated ventilation systems are used. Some greenhouses have a double roof or reflective insulation installed. Furthermore, continuous improvement is being sought for the plastic sheet used for greenhouses - which is obviously the least sustainable component of the system - resulting in a longer useful life.
Chinese Greenhouse Performance
The performance of the Chinese greenhouse depends on its design, latitude, and local climate. A recent study looked at three types of greenhouses in Shenyang, the capital of Liaoning Province. The city is located at 41.8 ° N and is one of the northernmost areas where this type of greenhouse is built (between latitudes 32 ° N and 43 ° N).
The research was conducted from the beginning of November to the end of March, the period during which the outside temperature drops below freezing. The average temperature in the coldest month is between -15 ° C and -18 ° C (5 to -0.4 ° F). [one]
The three greenhouses studied have the same shape and dimensions in common (60 x 12.6 x 5.5 m), but differ in the walls, the plastic sheet and the transparent layer. The simplest construction has rammed earth walls and an inner layer of brick to increase the stability of the structure. The cover is a thin plastic film that is covered at night with a straw blanket.
The other two greenhouses have the north-facing wall made of brick with a Styrofoam extrudate that acts as an insulating material, so the width of the wall can be cut in half. They are also covered with a thick PVC plastic sheet. The best greenhouse adds to this a reflective coating over the insulation blanket, further reducing heat loss at night.
In the simplest greenhouses, temperatures drop below freezing from early December to mid-January. Without additional heating, this greenhouse cannot produce any crops at this latitude. Only in the most sophisticated greenhouses - with their reflective insulation layer - it is possible to keep the temperature inside above zero at all times, using only solar energy.
In addition, the temperature was kept above 10 ° C most of the time, which is the minimum temperature for growing warm season plants such as tomatoes and cucumbers. Of course, passive solar greenhouses located to the south, in more southern locations, require less sophisticated isolation techniques.
Solar Greenhouses in Northern Climates
If we go further north, similar passive solar greenhouses would require additional heating during the colder months of the year, no matter how well insulated they are. The further north the greenhouse is, the steeper it will slope. The roof is sloped to be perpendicular to the sun's rays when the sun is at its lowest position on the horizon.
In 2005, a Chinese-style greenhouse was tested in Manitoba, Canada, at latitude 50 ° N. This greenhouse, 30 x 7 meters with a well-insulated north-facing wall (3.6 RSI fiberglass) and a blanket isolation (1.2 RSI cotton), was observed from January to April.
During the coldest month (February) the outdoor temperature ranged between + 4.5 ° C and -29 ° C (40 to 20 ° F), while the indoor temperature averaged 18 ° C (32.4 ° F ) higher than outside, which made it impossible to grow plants without additional heating in winter. 
However, the energy savings can be enormous compared to a glass greenhouse. To keep the temperature above ten degrees at all times, the Canadian structure's heating system must deliver a maximum of 17 W / m2, or 3.6kW for the entire building.  In comparison, a glass greenhouse of equal proportions, and at the same indoor and outdoor temperatures, would require a maximum capacity of 125 to 155 kW.
These results cannot be applied to all places at 50 ° N. Canadian research shows that solar radiation has a greater impact on the interior temperature of the structure than the exterior temperature. The correlation between indoor temperature and sunlight is almost four times greater than the correlation between indoor temperature and outdoor temperature.  For example, while Brussels is at the same latitude as Manitoba, the latter has an average of 1.5 times more sunshine.
Thermal capacity can be further enhanced by placing water-filled, black-painted tanks within the structure, against the north-facing wall. These tanks capture additional solar energy during the day and release it at night. A different method of improving a greenhouse's heat retention is by placing berms or terraces of earth on the north, east and west facing walls. Another solution to improve insulation is the underground or "greenhouse in wells".  However, this greenhouse receives less sunlight and is prone to flooding.
The passive solar greenhouse could save a great deal of energy, but there is a price to pay: the benefits generated by the Chinese greenhouse are two to three times lower per square meter than its fully glazed counterpart. In the most efficient Chinese greenhouses, an average of 30 kg of tomatoes and 30 kg of cucumbers can be grown per square meter (Data from 2005), while the average production in glass greenhouses is about 60 kg of tomatoes and 100 cucumbers kg (Data from the year 2003). [3. 4].
Therefore, a passive greenhouse area would take up to two to three times more space to produce the same amount of food. This could be seen as a problem, but of course what really uses space most intensively in agriculture is meat production. A more diverse and attractive supply of fruits and vegetables could be a more viable alternative to reduce meat consumption, so land use would not be a problem.
Greenhouses Heated Using Compost
Another problem with solar greenhouses is the lack of a CO2 source. In modern greenhouses, the goal is to have a CO2 level at least three times higher than the level in the open air, in order to increase crop yield. This CO2 is produced as a by-product of fossil fuel-based heating systems inside greenhouses.
However, when fossil fuels are not used, another source of CO2 has to be found. This is not only a problem for solar greenhouses, but is also one of the main reasons why geothermal energy and electric heat pumps are not making a breakthrough in the modern greenhouse industry.
In the Chinese solar greenhouse, this problem is sometimes solved by combining crop growing and animal husbandry. Pigs, chickens and fish produce CO2 that can be absorbed by plants, while plants produce oxygen (and green waste) for animals.
Animals and their manure also contribute to the heating of the structure. Research on these types of greenhouses with integrated systems has shown that the combined production of vegetables, meat, milk and eggs increases yields substantially. 
Justin Walker, an American currently in Siberia, is participating in the construction of an integrated system using horses, goats and sheep at a monastery in Siberia. Taking into account the harsh climate, the structure is built partly underground, while its protruding parts are earth berms.
Above the barn area is a haystack that provides additional winter insulation, as well as ventilation during the summer when it is empty. Its heat recovery system produces hot water that is channeled through the floor, heating the greenhouse floor. CO2 is supplied by animals. 
Heating and CO2 production can also be done without keeping the animals in the greenhouse. Using your manure is enough. As we have seen in the previous article, the use of horse manure for heating small-scale greenhouses dates back several centuries in Europe, and has been practiced in China for 2,000 years. Since the 1980s, several compost-heated greenhouses have been built in the US.
These have shown that a greenhouse can be completely heated by compost if well insulated, and that the method dramatically enriches the CO2 levels in the soil and greenhouse air. Along with all this, compost also serves to increase soil fertility. 
 Energy performance optimization of typical Chinese solar greenhouses by means of dynamic simulation (PDF), Alessandro Deiana et al., International conference of agricultural engineering, 2014, Zurich.
 Winter performance of a solar energy greenhouse in southern Manitoba (PDF), Canadian Biosystems Engineering. 2006.
 The solar greenhouse: state of the art in energy saving and sustainable energy supply. G. Bot et al., 2005
 Structure, function, application, and ecological benefit of a single-slope, energy-efficient solar greenhouse in China. HortTechnology, June 2010
 Integrated energy self-served animal and plant complementary ecosystem in China, in "Integrated energy systems in China - the cold northwestern region experience", FAO, 1994
 The Compost-Powered Water Heater: How to heat your greenhouse, pool, or buildings with only compost, Gaelan Brown, 2014
 For example "The Solar Greenhouse Book" (PDF), published by Rodale Press in 1978
 The Earth Sheltered Solar Greenhouse Book, Mike Oehler, 2007
Low Tech Magazine