A Rational Procedure for Predicting The Long-Term Average Performance of Flat-Plate Solar-Energy Collectors With Design Data for the U. S., Its Outlying Possessions and Canada* Benjamin Y. H. Liu and Richard C. Jordan Assistant Professor Professor and Head Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota T HE flat-plate collector is the simplest and one of the most effective means of collecting solar energy for use in systems that require thermal energy at comparatively low temperatures. Flat-plate collectors have been used successfully for many years in the southern United States and elsewhere as water heaters, and potentially can be used for space heating, drying, refrigeration, power generation and similar purposes. The majority of these potential applications of solar energy have not been fully evaluated, but experiments ~-7 in solar space heating conducted during the past two decades have shown that space heating with solar energy is entirely feasible. It is now recognized that with the rapid depletion of fossil fuels, solar energy will be increasingly utilized. In comparison with collectors of the concentrating type, such as those used in high-temperature solarfurnace applications, flat-plate collectors offer these advantages: (1) no complicated mechanisms for following the apparent diural motion of the sun are needed for their operation, (2) construction is simple and cost relatively low, and (3) diffuse as well as direct solar radiation is utilized. This last advantage is especially important in view of the fact that, of the total solar radiation received on the surface of the earth (46 percent of the extraterrestrial radiationS), approximately 40 percent (18 percent of the extraterrestrial radiation 9) is diffuse radiation. Manuscript received August 1, 1962. Basic to the design of any solar-energy utilization system in which fiat-plate collectors are used is the long-term average performance of these collectors. The…