Context and socio-economic impact of solar greenhouse
South facing greenhouse with polythene facing towards South
Three side walls facing North, East & West
Double wall with insulation of straw
Wall color along East (White), & North (Black) and roof inclination
Roof ventilator to avoid overheating during the summer
A passive solar greenhouse is designed to trap enough solar radiation for the photosynthesis process and to provide the interior climatic conditions required for growing vegetables all year round. When the outside conditions are very cold, heat is stored during the day in the ground and walls of the greenhouse and released during the night to keep the greenhouse air warm.
During winter, the greenhouse traps enough energy during the day to ensure that the vegetables do not freeze at night. The temperature variation between day and night should be minimized to reduce thermal stress to the plants. Overheating during the day can be prevented using natural ventilation for cooling, regulated by manually operated shutters. Ventilation also regulates the humidity and thus helps to limit diseases and pests.
There are four main factors that work together to make a solar greenhouse an efficient user of the available energy (See Figure)
- Collection of the maximum amount of solar radiation during the day
- Efficient storage of the heat collected from the sun radiation during the day
- Release of this heat to the interior of the building during the night
- Reduction of heat losses by insulation of the whole greenhouse
- Ventilation due to overheating to prevent from diseases and pest
The passive solar greenhouse for cold areas described has several components, which together ensure that these requirements are met:
A polythene sheet: The South side picks up the largest amount of solar energy. The polythene transmits the majority of incident solar radiation into the greenhouse. This warms the interior space and is absorbed by the vegetables, the ground, and the walls. The sheet can be covered with a moveable layer of insulation like a curtain, cloth, or mat after sunset to reduce night time heat loss. The polythene sheet is set at an angle and supported by a wooden frame.
Double wall: Walls on the east, west, and north sides are used where the amount of incident solar energy is limited. These walls are either buried into a hillside or insulated to limit heat loss and increase thermal storage. The walls are composed of three layers: an outer load-bearing wall built with mud brick, rammed earth, or stone; an inner wall used to store heat during the day and release it at night, also built with mud brick; and an insulating layer of materials like straw, sawdust, wood shavings, dry leaves, dry grass, or wild bush cuttings pressed between the two.
Wall Color: The inner side of the west wall is painted white (whitewash) to reflect the morning solar radiation after the coldness of the night; the inner side of the east wall and north wall are painted black to absorb and store the afternoon solar radiation, which is then released at night to heat the interior space.
Roof: The fixed roof is sloped (to the north) at an angle of 35°. In winter, when the sun has a low elevation angle, this angle optimizes the solar radiation absorption on the inside surface. During summer, when the sun is high in the sky, the roof partly shades the greenhouse and reduces the risk of overheating. The roof is covered with a layer of insulation (straw, or similar); a piece of white cloth or parachute material can be added below it to improve the insulation and reflect solar radiation onto the vegetables. The shape of the roof reduces the interior volume compared to traditional greenhouses, thus increasing the interior temperature.
Door & Ventilation: The door is located on the wall opposite to the side from which the prevailing wind blows (the lee side) to reduce infiltration of cold air. Roof on the north side is tilted to avoid shading in winter and reduce the interior volume to limit heat loss.
On sunny days, the air in the greenhouse can become very warm. Overheating (over 30°C) can damage the vegetables and encourage diseases and pests. Manually operated openings (ventilators) are provided in the lower part of both sides (door, wall shutter) and in the roof. The warm air rises and leaves the greenhouse through the roof ventilator, drawing in the cooler ambient outer air through the lower ventilators.
