An investigation into sky temperature estimation, its variation, and significance in heat transfer calculations of solar cookers

Abstract

The determination of sky temperature assumes great importance in engineering applications such as radiative cooling of buildings. Many studies that involve a radiative exchange with the sky employ different reported models of sky temperature interchangeably.However, until now, hardly any systematic study has been done to quantify the errors/variations that might be encountered in calculating this radiative exchange employing these different correlations. In the current paper, first, a thorough analysis has been presented on the sky temperature correlations and a possible range of variation in sky temperature based on the estimation of sky emissivity is computed. Both diurnal-nocturnal variation in sky temperature and seasonal disparities in sky temperature have been reported. Next, the case of a box-type solar cooker has been taken up for Heat Transfer-Asian Res. 2019;48:1830-1856. wileyonlinelibrary.com/journal/htj 1830 | Nomenclature: ε, sky temperature emissivity; T sky , sky temperature; T a , ambient temperature; F 1 , first-figure-of-merit for solar cookers; ƞ o , optical efficiency; U L , overall coefficient of heat loss; T p , plate temperature; θ p , nondimensional plate temperature; U t , top heat transfer factor; U b , bottom heat transfer factor; U s , side heat transfer factor; Q″ t , rate of heat loss per unit area from absorber plate to inner glass cover; h cpg1 , convective heat transfer coefficient from plate to first glass cover; h cg1g2 , convective heat transfer coefficient between two glass covers; h w , convective heat transfer coefficient on the top of glass cover; ε g , emissivity of glass; T g1 , outward temperature of first glass cover; P v , vapor pressure of water; T dp , dew-point temperature; RH, relative humidity (in percentage); ∅, relative humidity (in fraction); H s , solar insolation; RMSE, root-mean-square error; MAPE, mean-absolute-percentage error; η ob , optical efficiency for the beam component of irradiance; η od , optical efficiency for the diffuse component of irradiance; Nu, Nusselt number; Ra, Raleigh number; h rpg1 , radiative heat transfer coefficient from plate to first glass cover; h rg1g2 , radiative heat transfer coefficient between two glass covers; h rg2s , radiative heat transfer coefficient from glass cover to surrounding; ε p , emissivity of plate; T g2 , outward temperature of second glass cover. investigation with respect to the possible influence of the sky temperature estimation in predicting its performance parameter, first-figure-of-merit on a daily, seasonal, and climatic basis. Our observations show an enormous difference in sky temperature depending upon the expressions of emissivity from which it is derived. The variability of sky temperature has a nominal influence on the prediction of first-figure-ofmerit, although a marked discrepancy is observed across the seasons at the same location.

Production of Fresh Water by a Solar Still: An Experimental Case Study in Australia

“…The value of z is to be determined by the best fit of the HC1 with productivity [17]. This can vary between 0.2 and 0.5.…”

Modeling the Temperature Field in the Ground with an Installed Slinky-Coil Heat Exchanger

“…On the basi coefficient of this equation, the value of Tm in Formula (10) was determined: Tm = and the value of the constant CLW was found CLW = 4.72 W/(m 2 K). The temperature of the sky Tsky is related to the air temperature by the relat resulting from the Stefan-Boltzmann equation [14,15]: The temperature of the sky T sky is related to the air temperature by the relationship resulting from the Stefan-Boltzmann equation [14,15]:…”

“…Figure 2 shows the approximations of the Budyko curve according to the research of various authors. The relationship (15) was approximated by a straight line for the range of Φ from 0.5 to 2, and then transformed to the form:…”