In order to grow plants in greenhouses, a proper combination of light and heat is required. At times, the requisite light and heat are provided naturally from the sun. However, in most northern climates, it is often too cold during the winter to grow plants without some source of heat. Typically, heat is provided using forced air heaters. This does not prove to be entirely satisfactory, however, because much of the hot air is lost through the roof and walls of the greenhouse. As a result, during the winter days, the temperature in the greenhouse can be as low as 50.degree. F. even with the hot air heaters at full blast.
One solution to this problem has been to use insulating materials on the roof and walls of the greenhouse. Systems of this type are disclosed in U.S. Pat. Nos. 4,064,648 to Cary, 4,313,650 to Ward et al. and 4,375,232 to Heescher et al.
Cary discloses use of a flexible sheet of insulating material, wound in a roll, that can be installed near the roof of the greenhouse. The material can then be deployed at night to cover the roof and prevent heat loss. During the daytime, the insulation is retracted into the roll so that light may enter the greenhouse.
A similar system is disclosed in the Heescher patent. There, an insulative foil, made of two layers of plastic separated by air pockets, is selectively deployed over the roof and walls of the greenhouse to prevent heat loss. The foil is folded when stored and opens to form an insulative barrier. Both the Heescher and Cary references suggest that the foil might be coated with aluminum to increase the insulation effect.
In Ward et al., reflective and insulative slats are arranged venitian blind style below the greenhouse roof. This construction permits selective capture or reflection of heat or light.
Aluminized insulating materials, such as those disclosed in the Heescher et al. and Cary references, are also useful in summer months to prevent overheating from direct sunlight. By deploying the insulating material over the southern exposure of the greenhouse (in the Northern hemisphere), direct sunlight will be reflected out of the greenhouse. Diffuse light still enters the greenhouse through the northern exposure providing light for the plants.
While hot air systems, used in conjunction with insulating materials, can be effective to promoting plant growth during cold winter months, such systems are still relatively energy inefficient. One primary drawback to the use of forced air is that the hot air naturally rises to the roof. Thus, the plants do not receive the full benefit of the forced air output. Although the insulating layers prevent much of the heat from escaping, the air near the roof will be warmer than the air near the floor, an indication that the system is not energy efficient.
Greenhouses also require large amounts of water. The water is often taken straight from the outside and sprayed on the plants. This water is very cold, especially in winter and, when sprayed on the plants, it cools the plants, soil and surrounding air. Thus, the environment must be reheated to maintain proper growing conditions. In order to maintain a steady temperature in the greenhouse, the water is sometimes heated before spraying. This is typically accomplished using a large capacity water heater. Thus, separate air and water heating systems must be purchased and installed. The water heating systems in the prior art are not able to take advantage of the excess heat produced by the air heating systems and so efficiency is not maximized.
Further problems arise due to the build-up of snow and ice on the roof above the insulating material. The prior art has attempted to solve this problem using additional devices which either scrape off the snow or heat the roof. None of the additional devices make use of the excess heat in the heating system, however, so, again, maximum efficiency is not achieved.