The Solar Greenhouse
Ladakh's climate is too harsh that the growing season is limited to a maximum of five months: June to October. While some vegetable can be stored underground or dried for use in winter, they are necessarily extremely scarce at the end of the cold season. In an attempt to extend the growing season, a few farmers have experimented with 'trench sowing'. A trench is dug to a level below the frost line, and covered with a sheet of translucent polythene. Plants are gown at the bottom of the trench, where they are somewhat protected from the freezing night-time temperatures. This technique has had considerable success on a small scale.
Solar Greenhouses make it possible to extend the season still further, and thereby greatly increase the availability of fresh produce the year round. The design which we have introduced is very simple, and requires only the most standard building materials, which are available locally. We have already installed more than 20 greenhouses in villages along the Indus valley.
The principles involved are simple. The objective is to collect as much of the sun's energy as possible during the day, and store it for release during the night, when ambient temperatures fall. With proper insulation, a sufficient temperature can be maintained inside the greenhouse until the following morning.
The design which we have developed consists of three insulated mud-brick walls, and a south-facing area of glass set at a 45 degree angle. (Asteeper angle would be more efficient, but creates construction problems), The thermal mass of the north wall is augmented by two or more 200-litre barrels, which are painted black and filled with water. The interior is excavated to increase space, and all four walls are lined with planting beds. A rolled cover is provided as insulation for the glass. The upper panes of glass can be removed in summer to provide a means of venting excess heat.
Foundations for the greenhouse are constructed of stone. To prevent moisture from seeping into the walls. North, east and west walls are constructed of two layers of mud-brick with a straw filled cavity. The insides of the walls are plastered and painted black. A single tight-fitting door is provided on either the east or west side, and is constructed of plywood with coconut fibre insulation. The insulating cover may be wool blankets or heavy cotton quilting.
The green house can be either 'free-standing' or attached to the house. If the house has a south-facing wall, this can serve as the back wall of the greenhouse. At night, heat stored in the thermal mass will be radiated into the living space as well as the greenhouse. If vents are added at the top and bottom of the thermal mass wall, a convection cell can be created. Which can help to keep the room warm during the day.
Performance of the greenhouse varies with the aspect and orientation of the site and with the quality of construction, but is always sufficient to extend the growing season by at least four months. In many cases, vegetable can be grown all the year round. Temperatures in properly built greenhouses never fall below freezing, even during those winter nights when ambient temperatures fall to as low as -30C.
The greenhouses which we have installed have been used to grow a wide variety of vegetables, including turnips, spinach, radishes and Chinese cabbage. Up to three crops can be obtained in a winter season. In March and April, cabbage, cauliflower and other vegetables can be stored in the greenhouse, and transplanted outside when temperatures rise.
Copyright 1998-2005, Tibet Environmental Watch (TEW)