Optimization of the flow resistance characteristics of the direct return flat plate solar collector field

The uneven (flow) resistance of branched pipes commonly causes local overheating and freezing in solar collector fields. In this paper, the resistance characteristics of the direct return flat plate solar collector field are studied using a numerical model. The accuracy of the model is verified with experimental data, and the maximum relative error is below 10%.

A new evaluation index, the ratio of resistance imbalance rate to efficiency (RRIRE), is introduced. In this way, it is possible to comprehensively evaluate the effect of different factors (configuration, flow rate per unit heat collection area, inlet temperature, working medium) on both the resistance characteristics and efficiency of the solar collector field.

The results show that, when the number of rows (Nrow) exceeds the number of collectors in each row (Ncol), the inlet temperature is below 40 °C. In addition, the flow rate per unit heat collection area is below 0.06 m3/(h m2). Hence, the RRIRE is smaller and the system has better resistance efficiency.

Considering the viscosity change due to the temperature change in the fluid of the solar collector field, a new temperature difference correction method is proposed. This makes it possible to calculate the frictional drag for a fluid in the collector field, which allows a more realistic calculation of flow resistance.

This study lays the foundation for improved layout design and parameter optimization in solar collector fields.