Performance of Flat-Plate Solar Collector in a Closed-Loop Thermosiphon Using Refrigerant-11.

The performance of a refrigerant charged flat-plate solar collector in a closed-thermosiphon loop was analyzed. The analysis resulted in a generalized form of the Hottel-Whillier-Bliss equation, which allows for the consideration for full (Z* = 0) or partial (0.0 (LESSTHEQ) Z* (LESSTHEQ) 1.0) boiling in the collector tube and for both (or either) thermosiphon flow or forced flow. In the limit of no-boiling (Z* = 1.0) the developed equation was identical to the Hottel-Whillier-Bliss equation. The refrigerant charged systems showed superiority in performance over identical hydronic flat-plate solar collector systems under the same weather conditions because they are utilizing latent heat transfer (phase change) rather than sensible heat. Those systems operate at the highest possible efficiency if the collector tubes are full of refrigerant (100% submergence) and the inlet fluid is saturated (T(,in) = T(,sat)) with normal incidence of solar beam radiation. Also, there are several advantages to using refrigerant as the working fluid in a closed thermosiphon loop. The most obvious is the elimination of the circulation pump. This increases the reliability of the system, reduces the operating cost, and makes possible its operation in remote areas where electricity is not available. The problem of freezing is completely eliminated by using refrigerants since most refrigerants freeze only at exceptionally low temperature. The circulation rate was found to depend primarily, in a complex manner, on solar irradiance, system geometry, weather conditions, physical properties of the working fluid, and operating conditions. A relationship was developed and tested for predicting the flow rate at any operating conditions. It showed good agreement with experimental measurements. The collector performance was analyzed for transient operation. The analysis indicated that those systems have a spontaneous response to perturbations in weather conditions. The system is self-regulating after any changes in solar irradiance.PhDChemical engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159128/1/8304434.pd