Study of pump control in residential grid-tied solar domestic hot water photovoltaic-thermal (PV-T) systems

Abstract

A study of pump control focusing on active residential grid-connected solar domestic hot water (SDHW) photovoltaic-thermal (PV-T) systems was conducted. The main goal was to determine how the two main pump controls for this segment compare, namely the differential temperature static two-level hysteretic control (DTSTLHC) and the differential temperature static saturated hysteretic-proportional control (DTSSHPC), given the dual outputs of PV-T technology: heat and electricity. In order to do so, a dynamic PV-T collector model was developed for use in transient simulations and incorporated into a SDHW PV-T system model. A substantial number of annual simulations for each of the various locations selected were conducted to encompass the best performances using each control, with emphasis on multiple combinations of controller setpoints and mass flow rates. The results show PV-T systems using DTSSHPC and optimised for maximum auxiliary energy savings consistently outperforming those using DTSTLHC and optimised using the same criterion, though the opposite was true when seeking to optimise the electrical efficiency, with those using DTSTLHC performing best. However, the advantages at best correspond to single-digit percentages of the annual thermal energy demand, and less than 0.1% of the annual electrical efficiency. Similarly low performance advantages were reached from the standpoint of primary energy efficiency and load provision cost-effectiveness by using DTSSHPC, though not consistently due to the inability to reconcile electrical, thermal and parasitic performance advantages over DTSTLHC. Moreover, the advantages presented by DTSSHPC are low enough to be offset by one additional maintenance operation, which systems using this control are likelier to require first due to its complexity and higher switching frequencies. Finally, a study on setpoint selection for differential temperature controllers, namely DTSSHPC and DTSTLHC, for use in PV-T systems was also conducted using steady-state methods, which revealed marginal differences between setpoint selection for hybrid and non-hybrid systems.