Production of agricultural crops depends on two main factors: solar radiation and climate. Plants need solar radiation for photosynthesis, and the interior environment (temperature and humidity) must match the plants requirements. Generally greenhouses can sustain such conditions, but need fuel based heating for sustaining through the cold winters.
A passive solar greenhouse is a greenhouse heated entirely by sunlight, with no additional fuel based heating.
Passive solar greenhouse absorbs as much as possible solar energy (sunlight during the day) in the winter season for the following gradual emission of accumulated solar energy for heating purposes.
The main operation principle of a solar greenhouse lies in accumulation and storage of solar energy (sun radiation) during daytime and gradual release of the energy (heat) during the night.
- Collects the maximum amount of solar radiation during the day;
- Efficiently stores the heat collected from sun radiation during the day;
- Releases this heat to the interior of the building during the night;
- Heat losses are reduced by insulation of the whole greenhouse;
- Can be ventilated to avoid overheating.
- Enough solar radiation is accumulated for the photosynthesis process, needed for the plants;
- Interior climatic conditions required for growing vegetables all year round is shaped and sustained;
- Heat losses are reduced due to thick 3-layer walls on the east, west, and north sides;
- Design of the efficient passive solar greenhouse along an east-west axis, with the length of the south face increased and angled allows the largest possible surface area to the sun;
- Basic vegetables subsistence needs in remote areas can be fulfilled;
- Rural population gets additional income generation possibility;
- Materials for constructing of solar greenhouse are locally available wood, straw, stone), except for the transparent cover sheet or glass for covering the southern wall;
- Can be constructed by local builders;
- The investment costs can be recouped in less than three years if the production is well-managed and the products sold;
- Comparatively moderate costs: for Uzbekistan 1 sq.m. of interior space of a greenhouse with a southern wall covered with glass costs around 160 thousand UZS, of a greenhouse with southern wall covered with plastic costs 130 thousand UZS.
Passive solar greenhouses are widely disseminated in China since 1970s; where currently over 700 thousand ha are occupied by solar greenhouses.
Greenhouses of this type are suitable for use in other areas of the Hindu Kush, Himalaya, and Pamir ranges with a similar climate and socioeconomic situation, such as the high valleys of India; Nepal; the Tibetan Plateau in China; Bhutan; Pakistan; Afghanistan; Tadjikistan, Kirghizstan and Uzbekistan in Central Asia.
Passive solar greenhouse is designed along an east-west axis, with the length of the south face increased and angled to present the largest possible surface area to the sun. The size of the east and west facing walls are reduced to minimize heat loss and shade inside the greenhouse, and the north wall is heavily insulated.
The design of a greenhouse for a specific location is influenced by the site characteristics, the climate, and the expected amount of snowfall.
The structure of a passive solar greenhouse consists of the 3 main elements:
(1) walls on the east, west, and north sides;
(2) Wooden carcass with a polythene sheet on the south side, which picks up the largest amount of solar energy. The polythene sheet is set at an angle and supported by a wooden frame. Depending on financial capacities, weather conditions and the will of agricultural producers the carcass can be made of metal and covered with glass;
(3) A (solid) roof on the north side to limit heat loss. The roof is tilted to avoid shading in winter and reduce the interior volume.
The walls of a solar greenhouse need to have heat insulating properties for preventing heat loss through the walls.
For this purpose, the walls are composed of three layers:
(1) an inner wall used to store heat during the day and release it a night, also built with mud brick, rammed earth, or stone and painted in black for better heat absorbing;
(2) an insulating layer of materials like straw, sawdust, wood shavings, dry leaves, dry grass, or wild bush cuttings pressed between the two.
(3) an outer load-bearing wall built with mud brick, rammed earth, or stone.
Very important is also to install a good ventilation system with doors on both western and eastern walls and windows in the roof.
Moveable insulation (parachute, cloth) is used as a curtain below the polythene after sunset to reduce heat loss; it is removed after sunrise. Alternatively, a so called thermoblanket can be installed above the polythene cover (or glass cover) to block the heat inside the greenhouse and establishing a buffer layer between the cold air and the transparent (southern) wall of the solar greenhouse.
The actual construction can start after the appropriate location site is selected and all the materials are prepared. The construction itself can be divided into the following basic steps:
(1) Constructing the foundation. Foundations are the basis of every structure. The orientation and outline of the wall positions must be exactly as given in the design to ensure maximum efficiency of the greenhouse. The outline of the walls is first drawn on the ground and the foundations are then dug and filled.
(2) Building the walls. The walls are built on the foundations. The walls must be shaped in the precise way shown in the designs so that the roof angle is correct. The back wall is a simple vertical wall whose top is parallel to the bottom. Low density materials (light materials like straw, sawdust, wood shavings, dry leaves, and dry grass) are poor conductors and storers of heat and are thus good insulators: they help retain the heat inside the greenhouse. These materials are filled into the cavity between the loadbearing wall and the thermal mass wall.
(3) Making and installing the door. A door is built into the wall to provide access and act as a ventilator for cooling. It is constructed in the wall opposite to the prevailing wind to reduce unwanted drafts.
(4) Making and installing the wall ventilator. In order to cool the greenhouse by natural ventilation, a side shutter is installed across from the main door in the opposite wall. The ventilator is composed of a fixed frame and an articulated shutter.
(5) Constructing the roof. The north and side-walls of the greenhouse are constructed in a shape that supports the roof.
(6) Making and installing the roof ventilator(s). One or two ventilators are installed in the roof of the greenhouse, and the door and wall shutter provide the lower openings. The roof ventilator frames are installed at the same time as the roof structure is constructed.
(7) Finishing the walls.
(8) Installing the polythene sheet. It is very important point to ensure that the polythene sheet is stretched tight and attached firmly to prevent it flapping in the wind. If this is done properly, the sheet can last as long as seven years. If not, damage resulting from flapping and rubbing can destroy the sheet in one season. Polythene expands with heat, so it should be fixed during the warmest hour of a sunny day when it is well-expanded so that it becomes taught as it cools. If it is fixed when cold, it will later expand and become loose, and be more susceptible to wind damage.
(9) Installing night insulation. The insulation is only effective if a space is left between the polythene and the cloth and the air inside this space is trapped; the cloth must be air-tight and the join between the cloth and the polythene sealed at the top, bottom, and sides. The cloth may be wet in the morning and should dry during the day.
Schemes, technical sheets for constructing passive solar greenhouses can be found via the following links.
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