The behavior of a solar collector with a boiling fluid is analyzed to provide a simple algebraic model for future systems simulations and to provide guidance for testing. The efficiency equation is developed in a form linear in the difference between saturation (boiling) and ambient temperatures, whereas the expression upon which ASHRAE Standard 109P based utilizes the difference between inlet and ambient temperatures. The coefficient of the revised linear term is a weak function of collector parameters, weather, and subcooling of the working fluid. For a glazed flat-plate collector with metal absorber, the coefficient is effectively constant. Therefore, testing at multiple values of insulation and subcooling, as specified by ASHRAE 109P, should not be necessary for most collectors. The influences of collector properties and operating conditions on efficiency are examined.

Units: Dual