T. P. Williamson, B. Bauer, P. T. McEntee, P. M. McKernan, Thermomax Ltd, Balloo Crescent, Bangor, BT19 7UP, Northern Ireland, tel: +44 (0) 2891 270411, tommy_williamson@thermomax. co. uk

Current test methods for solar collectors require that the irradiance distribution over the collector aperture should be >800 Wm-2 during data acquisition. However, evacuated tube collectors have been designed to operate at northerly latitudes where irradiance distributions are commonly lower than this value during a large part of the year. Reported here are the results of an investigation into how the optical efficiency of a direct-flow evacuated tube collector varies with respect to incident irradiance in the range 200 — 1400 Wm-2 using a simulated solar irradiance source.

A number of domestic solar thermal control systems have utilized a variable flow­rate method to manage the contribution of useful energy gained by the system. However to date little experimental data relating to how varies with flow-rate for evacuated collectors was available in the literature1. Stated here are the results for optical efficiency variation of an evacuated tube collector with mass flow from 0.02 — 0.15 kgs-1 and the corresponding variation in AT for two irradiance levels.

Many evacuated solar collectors are installed with slopes p which do not conform to those recommended by standard test methods (i. e. 45°), the consequences are that the relative efficiencies of these installations are unknown. Evaluation of for slopes in the range 0° to 60° with respect to the horizontal plane were investigated.

1 Introduction

Standardised testing2,3 for thermal solar collectors has proven to be an effective method for direct system comparison. By applying a strictly controlled input parameter technique, different collector performances can be quantitatively evaluated. Valuable additional information can be gained in relation to the collector characteristics by shifting these input parameters away from their recommended magnitudes. The variation of optical efficiency По for a direct-flow evacuated collector with relation to the incident irradiance G, mass flow m and slope p with respect to the horizontal plane are reported. Global heat losses UL from the collector were calculated under these experimental conditions where the collector temperature Tcwas held at ~3 K above the ambient temperature; which is the upper limit for optical efficiency measurements according to EN12975-2. All optical efficiencies stated are in relation to the collector absorber area.