Dynamic thermal analysis of a coupled solar water heaters-thermal storage tank for solar powered district heating networks

Abstract

The integration of solar thermal and energy storage systems into district heating networks (DHNs) has been considered as one of the most advantageous solutions to reduce the use of fossil fuels and carbon emissions. In this context, this work deals with the numerical investigation of a coupled solar water heaters-thermal storage unit for urban heating networks. The studied system contains flat plate solar collectors (FPCs) for hot water production, a shell-and-tube thermal storage unit with phase change material (PCM) for latent storage or water for sensible storage, hydraulic pumps and a mass flow rate regulation system. The functioning of the coupled system is simulated using a detailed dynamic model based on transient energy and mass balances. The reliability of the developed model is verified through comparing numerical results with measurement data from the literature and a good agreement is obtained with a maximum relative error of about 4.5 %. The validated model is used to investigate the system thermal performance under the weather conditions of the city of Pau, France, and the effects of multiple parameters such as the FPC area, the DHN target thermal power, and the storage medium type (latent or sensible) during both charging and discharging processes are presented and analyzed. Results show that the use of FPCs is beneficial for DHNs during summer as they allow the production of thermal energy for both the DHN use and the storage process. It was also shown that increasing the DHN target thermal power from 30 kW to 50 kW leads to a reduction in the thermal storage process duration by up to 40 %. For both charging and discharging operations, the use of PCM (latent storage) as a storage medium is more suitable than water (sensible storage) as it leads to an increase in the storage density and extends the duration of hot water production with constant thermal power by about 65 %.