Natural Convection in Enclosed Spaces—A Review of Application to Solar Energy Collection

Abstract

A useful solar-thermal converter requires effective control of heat losses from the hot absorber to the cooler surroundings. Based upon the theory and some experimental measurements it is shown that the spacing between the tilted hot solar absorber and successive glass covers should be in the range 4–8 cm to assure minimum gap conductance. Poor choice of spacing can significantly affect thermal conversion efficiency, particularly when the efficiency is low or when selective black absorbers are used. Recommended data for gap Nusselt number are presented as a function of the Rayleigh number for the high aspect ratios of interest in solar collector designs. It is also shown that a rectangular cell structure placed over a solar absorber is an effective device to suppress natural convection, if designed with the proper cell spacing d, height to spacing ratio L/d and width to spacing ratio W/d needed to give a cell Rayleigh number less than the critical value.

Performance Study of Solar Air Heater Having Absorber Plate with Half-Perforated Baffles

“…To estimate of the convective heat transfer coefficient between the absorber plate and glass cover h pg , the following three-region correlation of Buchberg et al [21] where Ra (= Ra cos β) is Rayleigh number for the inclined air layers between the absorber plate and glass cover (collector slope β = 0 • for horizontal). The Rayleigh number is given by…”

Small-Scale Flat Plate Collectors for Solar Thermal Scavenging in Low Conductivity Environments

“…Buchberg et al [25] have suggested the following correlation (18) for solving the natural convection heat transfer coefficient in the enclosed space between the absorber plate and glass cover.…”

Thermal-performance study of liquid metal fast breeder reactor insulation

“…Studies include thermal stability and heat transfer of inclined honeycomb cells, e.g., reports by Randall et al (1977) and Buchberg et al (1976). Attention is focused primarily in trying to minimize convection in order to prevent heat losses.…”