A Standard-Based Method to Simulate the Behavior of Thermal Solar Systems with a Stratified Storage Tank

Thermal solar systems are interesting solutions to reduce CO2 emissions and gradually promote the use of renewable sources. However, sizing such systems and analysing their behavior are still challenging issues, especially for the trade-off between useful solar energy maximization and stagnation risk minimization. The new EPB (Energy Performance of Buildings) standard EN 15316-4-3:2017 offers several methods to evaluate the performance of a forced circulation solar system. One of them is a dynamic hourly method that must be used together with EN 15316-5:2017 for the simulation of the stratified storage tank connected with the solar loop. In this work, such dynamic hourly method is extended to provide more realistic predictions. In particular, modeling of the pump operation due to solar fluid temperature exceeding a set threshold, or due to low temperature differential between solar field and storage tank, is introduced as an on–off control. The implemented code is applied to a case study of solar system for the preparation of domestic hot water and the impact of different design parameters is evaluated. The model predicts a higher risk of overtemperature lock-out or stagnation when the solar field surface is increased, the storage volume is reduced and water consumption is set to zero to simulate summer vacation periods. Finally, a simple modulating control with a time step of a few seconds to a few minutes is introduced, quantitatively showing the resulting benefits in terms of useful solar energy increase, back-up operation savings and reduced auxiliary energy use.