Modeling and thermal parametric analysis of U-pipe evacuated tube solar collector for four different climates in Iran

In this study, thermal performance of the collector with analytic and quasi-dynamic method is evaluated based on energy balance equations for each part of the U-pipe evacuated tube solar collector. Using this approach, effect of different parameters such as tube size, overall heat loss coefficient, absorber tube absorptivity, mass flow rate and air layer thermal resistance on thermal performance of the collector was analyzed. Results showed that collector efficiency is increased with increasing the length of the tube for a fixed tube diameter but this increase is very small if the length of the tube increases more than two meters. Under the same conditions, collector efficiency increases with increasing tube diameter for a fixed tube length. An optimum flow rate is obtained for different climate condition for which collector efficiency is maximum. It shows different values for different climates. A correlation is obtained for optimum flow rate based on average annual total solar radiation.