This invention relates to a heat-gain structure. It is illustrated as a high efficiency cold frame, although its usefulness is not limited thereto.
The need for providing a structure which will increase the growing season for plants has been long felt. One structure for prolonging the growing season is the cold frame or hot bed.
A cold frame consists of a frame and a cover glazed with a light-transmitting material. The cold frame provides a heat gain during daylight, even on most cloudy days, and provides some retention of heat during cold and sunless periods. Cold frames, however, have a number of well-known drawbacks.
On sunny days, cold frames tend to overheat, even in very cold weather. To overcome this problem, temperature responsive openers have been used, as shown, for example, in Anderson, U.S. Pat. No. 3,129,530, and in Caldwell, U.S. Pat. No. 4,005,547. This expedient has not been entirely successful, because opening the cold frame sufficiently to let the heat out has sometimes permitted cold wind to freeze plants, especially near the edges of the cold frame.
Wind causes other problems with cold frames. When the glazing is laid back to open the frame, it is subject to being blown away and broken by the March or October wind. If the glazing is hinged to the frame, it may well carry the entire frame with it or deform the hinges or frame. When the glazing is partially opened, either manually or by an automatic opener, it is even more subject to being caught by the wind than when it is laid back on the ground.
Cold frames are also not particularly efficient heat retainers. On cold nights, the plants within them are frequently damaged. To prevent heat loss, a blanket of insulation is sometimes placed over the cold frame, and a water-resistant sheet is placed over the blanket. The blanket and sheet must then be secured against wind, because if even a corner of the blanket blows loose, its insulation value is largely lost.
The time and physical effort required to manipulate cold frames during harsh weather, as well as their limited effectiveness, has limited their use.
Hot beds are essentially cold frames with a supplemental source of heat, either from an outside energy source or from decaying matter. Although they reduce the problem of freezing, they are inefficient users of energy and suffer all of the other problems associated with cold frames.
Greenhouses are much larger, hence much more expensive, than cold frames. An unheated greenhouse loses heat rapidly. A heated greenhouse requires great amounts of energy to provide supplemental heating at night, during cloudy or extremely cold weather. The cost of energy for heating greenhouses has become a serious problem. Various methods for insulating greenhouses have been proposed, but they have generally been so expensive or cumbersome that they have not been widely adopted.