ASHRAE 93-77 Instantaneous and All-Day Tests of the Sunpak™ Evacuated-Tube Collector

The results of ASHRAE 93-77 instantaneous and all day tests for two Sunpak™ evacuated-tube collectors are presented and discussed in terms of a simple dynamic response model and a biaxial incidence angle modifier approximation. The dynamic model predicts, and experimental results confirm, a residence time type of response rather than the exponential response that is assumed to characterize most collectors. The anisotropic geometries of the two collectors require that their optical properties be described by an incidence angle modifier that depends on both solar altitude and azimuth in the tilted aperture plane. The test results support the idea that this dependence can be characterized with sufficient accuracy by a biaxial approximation which requires that incidence angle modifiers be measured in each of two mutually perpendicular symmetry planes of the collectors. The all-day measurements are compared with the results to be expected from the ASHRAE 93-77 instantaneous tests, and excellent agreement is obtained when the dynamic model is used to account for the time variations in insolation and inlet temperature. It is concluded that the two Sunpak™ evacuated-tube models can be characterized adequately within the scope of ASHRAE 93-77, but that measurements over and above those actually required by this standard are necessary for a complete characterization. In addition, it is recommended that these collectors be tested on a tracking mount because of their long response times. Such instantaneous test results, in combination with the dynamic model, can be used to predict daily and long-term performance under most conditions.