Evaluation of a Large Solar Thermal Drainback System for Hay Bales Drying
Abstract
In this paper, over one year of measurement data of a 127 m drainback solar thermal system for hay drying is analysed. The study shows that high temperatures in the solar collector loop lead to partial boiling when the system starts, slowing down the filling process. More severe, due to siphon establishment, the flow rate drops and the efficiency of the field is impacted anytime the collector outlet temperature exceeds 83 °C. The existing system is then modelled with the simulation software TRNSYS 16, based on measured data. Four other hydraulic designs are as well modelled and simulated for a comparative analysis. Three use the drainback principle and one represents a “conventional” pressurized system. As expected the results demonstrate that the drainback with load-side heat exchanger has the best thermal performance and the “conventional” system the worst one. The energy yield of the drainback design with collector-side heat exchanger is comparable to the one of the “conventional” system. The parasitic electricity consumption of the solar collector loop pump is also calculated. It varies significantly depending on the system, the most demanding one being the drainback without siphon formation, with a total consumption more than doubled compared to the same system with siphon formation.
Keywords
Drainback system, Solar process heat