Nonstationarity of the Efficiency Coefficient and Heating Temperatures of Water in Flat-Plate Solar Collectors

Abstract

A one-dimensional nonstationary three-element distributed thermal model of a flat-plate collector with allowance for time variation of the temperatures through the “thickness” of the elements and along the length of the solar collector (SC) has been developed and software-implemented. The main data in this model consist of the thermophysical and overall parameters of the collector elements, heat carrier inlet velocity, solar radiation density, and ambient and sky temperature. To decrease the calculation variants, the variation ranges of mass flow of the heat carrier and its flow velocity were estimated for the characteristic parameters of the heat-removal channels of flat-plate SCs. It has been found that the specific mass flows of water in flat-plate SCs are 2.5–10 g/(s m²), and the water flow velocity is 0.0005–0.002 m/s. It is shown that the instantaneous efficiency coefficient and temperature of the heat carrier can fall outside stationary values within 1–3 h. However, the daily efficiency coefficients of the SC do not fall outside the stationary values even within a day. The developed flat-plate SC model can be used to estimate the influence of the thermophysical and overall parameters of the elements, as well as the parameters of the environment, on its thermophysical characteristics when developing flat-plate SCs and methods for experimentally determining their characteristics.