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Energy-efficient greenhouses

Subsurface greenhouse-thermos

Description

Greenhouse-thermos is one of the best winter variants of greenhouses. Unique energy saving technologies form the basis of such a greenhouse. Greenhouse-thermos can operate even in frosts. Properly constructed greenhouse-thermos provides an opportunity to grow vegetables, flowers or even southern heat loving agricultural crops all year round.

In general, constructing a greenhouse-thermos requires considerable investments compared to ordinary greenhouse, but eventually all the efforts will be paid off.

Operation principles

Operation principles lie in the fact that at a depth of 2-2,5 meters soil has almost similar and stable temperature all year round. There might be some temperature fluctuations, but not considerable and mostly depending on the surrounding groundwaters.

And is based greenhouse thermos with his hands, almost half a season without much costs and severe equipment. All that is needed are tools and own desire.

Due to its subsoil construction and solid insulation greenhouse-thermos is in fact the safest greenhouse for tender seedlings: greenhouse’s design allows maintaining the difference between day and night temperatures at not more than 5-7°C, which means that the plants do not face temperature stresses. A variety of agricultural crops, including berries, fruits, vegetables, flowers and even mushrooms, can be grown in such a greenhouse-thermos, since the inner temperature will always be above zero despite severe frosts outside. Thanks to the construction materials with light reflective properties greenhouse-thermos will have good lightening, almost 2 times lighter than outside, even on cloudy and nasty days, which will have a positive effect on the expected yields.

Advantages

-     Long service life and high reliability of the greenhouse-thermos. Durable construction materials, proper design, application of contemporary and tested technologies;

-      Saving on lighting due to a high light carrying capacity (91% and over) of the polycarbonate cover and folio insulation materials, which makes the inner space 2 times lighter compared to an ordinary greenhouse covered with plastic. Plants enjoy natural lighting conditions;

-      Durable and solid basis and the carcass; durable to various weather conditions design sustain greenhouse stability and resistance to strong winds, rains, snow, hail, hurricanes, etc.;

-      Saving on heating due to efficient insulation without additional heating equipment and costly energy sources;

-      A wide list of agricultural crops suitable for this type of greenhouses;

-     Greenhouse-thermos can be constructed by own means and hands during 1 season and without sophisticated equipment. All needed is some tools and a strong will;

-     Suitable and favorable microclimate of the greenhouse-thermos allows growing tender plants and have positive effect on the yields;

-     Greenhouse-thermos guarantees stable, reach harvest of any crop all year round.

Coverage

The basic idea and technology of the greenhouse-thermos is not innovative, during the kings times in Russia such greenhouses provided good harvest of pineapples, which were then exported to Europe by the merchants. A big greenhouse built partly in subsoil was constructed in Belorussia in 1930s which still places tropical and subtropical plants.

Design and structure

Construction of subsoil greenhouse-thermos is a bit more costly than construction of an ordinary greenhouse with plastic cover, but the pay-off period is much shorter and the rate of return is much higher.

The proper placement and orientation of the greenhouse-thermos is very important, since it will allow catching more sunlight and heat. Ideally, its longer part should be designed from West to East on an open and well-lighted place, so that its longer and bigger side is always sun-faced, whereas the opposite side may be heat-insulated with mineral wool or aerated plastic.

It will be of benefit if a building or a mountain slope cover the greenhouse from the North or North-western side. The greenhouse can be of any length, but the desirable and recommended width is below 5 m. A larger width will lower the thermos effect of the greenhouse, i.e. heating and lighting properties will be poorer. The greenhouse-thermos can be variously shaped.

Construction steps

Construction of the greenhouse-thermos consists of several phases.

Step I. Preparation of the foundation. The main part of the greenhouse-thermos will be located in the subsurface. So, the starting point is to prepare a 2-meter deep ditch for foundation. At this depth soil is warm and will transmit its heat to the greenhouse. The foundation can be established at a less depth, but the thermos effect will be not so strong. All edges of the future greenhouse must be thoroughly aligned. On the perimeter of the foundation concrete is filled or laid with concrete blocks. The foundation will serve as a solid basis for the walls and the carcass of the greenhouse.

Step II. Construction of walls. As soonas the foundation is ready, walls can be then constructed. The walls are better to be built from materials with good insulation properties. This will be the thermal blocks, which will be set directly on a concrete base, fixed on an iron frame.

Step III. Insulation and heating. Now the inner part of the walls should be covered with special insulation film, which will perfectly retain heat inside the greenhouse. In conditions of colder regions 2 layers of thick folio can be applied to cover the walls. The main task of such reflective insulation is conservation of heat, water and carbon dioxide in the greenhouse, which is very valuable for plants.

The temperature in subsoil greenhouse-thermos is above zero, but colder than air temperature, required for plants. The upper air layer inside the greenhouse is heated by the sunrays and the inside air, but the soil absorbs the heat easily. So, very often warm floor is constructed in the greenhouse consisting of the electric cable for heating, covered by the metal matrix and concrete layer above it to prevent damage to the electric cable. In such a case, the beds for plants are established above the concrete layer or in special flower pots.

It is desirable also to have heat accumulating devices inside the greenhouse – this can be even ordinary water bottles, which are rapidly heated, but slowly cooled due to the greenhouse effect. You can put even the ordinary barrel with water.

Sophisticated greenhouse gardeners recommend heating both the ground and slightly the inner air. As for plants, the most important thing is that the earth’s temperature was about 25°C and 25-35°C provided proper humidity.

Step IV. Roof construction. The carcass of the roof is usually constructed of wood, impregnated with antiseptics. The roof is usually shaped straight with 2 sloping surfaces. Ridgepole is then centrally positioned, and central cradles are installed along the length of the greenhouse.

The ridgepole and the walls are connected by several trussing pieces. The carcass can be alternatively made without high cradles, these can be exchanged for small cradles placed on cross bars connecting opposite walls of the greenhouse. Such a shape makes the inner part of the greenhouse more spacious.

Polycarbonate is a universal coverage for greenhouses-thermos — up to 12 m in length, it may be completely unconnected to eliminate air flow through the entire structure. In addition polycarbonate is resistant to wind blows, and curious birds can’t be hurt. Polycarbonate sheets are fixed onto the carcass by screws. It would be better to place rubber sheet onto the trussing before fixing polycarbonate sheet cover.

To reduce the loss of heat in the greenhouse-thermos through the roof sometimes a double layer of polycarbonate is used. First layer of polycarbonate is fixed to the carcass on the inner side, second layer as usual on top of the trussing. Such double layer polycarbonate will allow to considerably reduce the loss of heating, but can reduce transparency of the roof 10%. In case of double layer polycarbonate cover the snow will not melt so the roof slope must be at least 30 degrees to ensure that the snow will not accumulate on the roof. In addition, a vibrator can be installed to occasionally shake off the snow from the roof.

Step V. Interior design. The next step is to settle the inner part of the greenhouse. It is very important to make the inner parts absolutely sealed, without any draughts. For this all the stitches between the construction blocks in the walls and roof have to be properly plastered.

The beds inside the greenhouse-thermos should be «high». In the end the greenhouse should be provided with electricity and if needed some heating devices as well as a system of automatic irrigation. The lack of lighting may be resolved by installing modern LED lights, which will support good harvest.

Additional reading

Schemes, technical sheets for constructing passive solar greenhouses can be found via the following links.

Project Purpose

Acting as an information repository and knowledge hub, this website helps to increase the use of innovations developed by the well-established CACILM Project in Central Asia. Its synthesis, compilation, and dissemination of current research provide a secure knowledge base that policymakers and other stakeholders can access and utilize to develop sustainable strategies capable of addressing the region’s severe land degradation.

The Project is funded by IFAD and led by ICARDA under framework of CGIAR Research Program on Dryland Systems.

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