Performance of SDHW systems with fully mixed and stratified tank operation under radiative regimes with different degree of stability

In this paper we investigate whether, and to what extent, the radiative regime has an influence on the performance of a solar domestic hot water system. We compared the results of two mathematical models (a more accurate model with stratification in the storage tank and a simple, less accurate, fully mixed model), simulating the dynamic behaviour of a system with two flat-plate solar collectors, under three commonly used tapping cycles. Four summer and four winter days with different radiative regimes have been selected for simulations. For the less stable winter day of 19 January (relative sunshine σ = 0.4–0.7), when the system operates with a constant working fluid flow rate of 0.01 kg/(s m2) and hot water is extracted according to a medium consumption tapping cycle, the mean value predicted by the more accurate, stratification model, for the useful heat gain by the solar collector is 388.87 W. Using this value as a reference, the MBE and RMSE values of the results predicted by the less accurate model are 41.54 W and 293.28 W, respectively. This proves that approximation models may not be used successfully for simulation. For variable working fluid mass flow rate, depending on the level of incoming solar irradiation and collecting surface area, the simplified model accuracy increases with the increase of the radiative regime stability.