Solar collectors, evacuated tube, solar concentration, model
Solar collectors can provide a useful response to the heat demand in buildings, such as heating of domestic water and spaces. Among the different types of solar collectors, the evacuated tube ones can better display their features when the temperature difference between operating fluid and ambient air is high, which are the typical operating conditions during space heating of buildings. Beside the heating application, there is also need for addressing the increasing energy consumption due to the summer air conditioning. Solar cooling can be an interesting opportunity, because the cooling demand matches the period with the higher solar radiation availability. Also in this application the evacuated collectors are an adequate type, due to the temperature level of the heat required as an input to the absorption machines. Therefore, accurate modeling and simulation of this type of collectors is highly recommended to fully assess the performance of the collectors already available in the market and propose improved designs, for example with higher concentration ratio, to get more benefits particularly at higher levels of temperature. This paper reports a new model for the performance analysis of evacuated tube solar collectors. The analyzed collectors are truncated CPC (compound parabolic collectors). An original software is developed under MATLAB environment for the simulation purposes. A novel numerical procedure is implemented to obtain the solution for the nonlinear set of equations representing the mathematical model. The length of the tube is divided into a specified number of segments. The analysis is performed for each segment along the tube length in order to obtain the variation of the different parameters. The model analyzes separately the optics and the heat transfer in the evacuated tubes and this approach allows to extend the analysis to new configurations. The model can simulate the efficiency curve under steady state conditions, according to the standard EN 12975-2 (EN 12975-2. Thermal solar systems and components - solar collectors - part 2: test methods. Brussels: CEN; 2006), but it is also able to describe the performance during the day with a quasi-dynamic approach. A comparison with experimental data shows the accuracy of the model. The results will provide a comprehensive characterization of the performance of evacuated tube collectors, discussing modeling and experimental data of efficiency and heat production at different levels of temperature. Particular attention will be paid to the applications of heating and cooling of buildings, industrial process heat generation and district heating.