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The Hottel-Whillier-Bliss (HWB) equation has been the standard tool for the evaluation of collector thermal performance for four decades. This paper presents a technique that applies the criteria of ASHRAE Standard 93-77 to the determination of the HWB equation coefficients from field-derived data. Results of the analysis of a sample collector array illustrate the technique. Actual dynamic performances of various collector arrays in the field are compared to those predicted by the steady-state efficiency models for the individual panels. In certain cases, the HWB model produces deviations of over 100% from measured hourly performances and 35% from measured monthly performances when compared with the single-panel laboratory-derived model. However, when the field-derived HWB model is used as the basis of comparison the performance deviations were typically less than 5%.
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