You can get around that problem by draining the collector when it is not in use or by adding antifreeze to the water. Other problems with water solar collectors include leaks in the plumbing and metal corrosion. Solar air heaters are the most common choice for livestock housing because of the water collector's disadvantages and because air collectors do not require a heat exchanger for space heating. 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.