Experimental evaluation study of collection efficiency of cavity receivers

摘要

Purpose - This paper aims to present experimental work examining the effect of opening size on the collection efficiency of cavity-type receiver geometries, e.g. modified cavity and spherical cavity with single- as well as dual-stage water heating. The correlations, obtained using the experimentally obtained data, are helpful in designing of cavity receivers (modified and spherical geometry type) to be used in solar-power harnessing assignments/projects, for yielding better system performance. Design/methodology/approach - The parameters of study encompass receiver opening or aperture ratios (dID, ratio of diameter of opening to the maximum diameter of spherical cavity) of 0.4, 0.47,0.533 and 0.6; flow Reynolds numbers of 938,1,175,1,525 and 1,880 with water as a coolant; and receiver inclination angles of 90, 60, 45 and 30° (with 90° as receiver-opening facing downward and 30° as receiver-aperture facing closer to sideway). A modified cavity receiver was examined for opening ratios of 0.46, 0.6, 0.7 and 0.93. The glass covers, with thickness 2, 4 and 6 mm, were positioned at the opening of cavity to mitigate the energy losses. Findings - The experiments have been conducted at a lesser incoming radiative heat flux, for receiver cavity wall surface temperatures ranging from 90°C to 180°C. The collection efficiency values of both the receivers, modified cavity and spherical cavity types, are seen increasing with coolant flow rate and receiver tilt (inclination) angles, i.e. 30° -> 90°. The collection efficiency exhibits maxima at an opening ratio of 0.533 in case of both single- and double-stage spherical cavity receiver. This value was observed as 0.6 for modified cavity receiver. The mathematical correlations developed for obtaining the collection efficiency values of modified cavity-type receiver, spherical cavity receiver with single stage and spherical cavity receiver with dual-stage water heating are given as η=0.4667 Re~(0.0798) (d/D)~(0.1651) (δ)~(0.0281) (θ)~(-0.011), η=0.2317 Re~(0.124) (d/D)~(1.265)(δ)~(0.0192)(θ)~(0.2914) and η = 0.1137 Re~(0.1715)(d/D)~(0.8702)(θ)~(0.2751), respectively. Social implications - The findings of the paper may be helpful in erecting concentrating solar collector systems for household water heating, concentrating solar-based power generation as well as for various agricultural applications. Originality/value - The experimental investigations are fewer in the literature examining the combined geometrical influence on the efficiency of cavity receivers with single- and double-stage water heating provisions.