The efficiency of a solar collector is defined as the ratio of the amount of useful heat collected to the total amount of solar radiation striking the collector surface during any time period. One of the problems with flat plate systems designed to date has been the relatively low efficiency compared to other heating systems. Efficiencies are low because of the high losses in collection and transport of solar energy. Heat is lost through the front, sides and back of the collector, by reflection from the cover, and by direct radiation from the heated flat plate. Typical day-long efficiencies for different types of flat plate collectors are presented in Table 1.
Collection efficiencies for flat plate collectors.
Remember this when evaluating manufacturer's or designer's claims for a particular type of collector. Several modifications of the basic collectors will improve efficiency. If the collector will operate at more than 50 degrees difference between the outside air temperature and the circulating medium's temperature, you can increase efficiency by adding an additional cover. Each additional cover reduces convection and radiation loss. But by adding a cover, you increase cost and reflection losses from the glass or other material.