The experimental thermal performance of a flat-plate solar collector containing a boiling fluid is given for steady and unsteady thermosiphon operation. The steady-state thermal efficiency is shown to compare favorably with a new generalized formulation for collector efficiency (Al-Tamimi and Clark 1983). The result is a generalized heat-removal factor that may be applied to any flat-plate solar collector whether it is cooled by a single-phase or a boiling fluid. The experimental results include both full (saturated inlet) and partial (subcooled inlet) boiling in the coolant channel. In the case of a nonboiling collector, this formulation becomes identical to the Hottel-Whillier-Bliss (HWB) equation. Accordingly, the HWB equation is found to be a limiting form of a generalized result.

Collectors containing a boiling fluid have a thermal efficiency that is inherently greater than a nonboiling collector having the same value of the thermal parameter a (see nomenclature). Further, a boiling collector represents a class of solar collectors whose thermal efficiency approaches the theoretical maximum more closely than any other class of collector. The experimental data presented in this paper are taken on an R-11 charged boiling collector in natural sunlight. Experimental time response data are included.

The results of this research were used to help develop a new ASHRAE test standard for boiling collectors.