Optimal coupling of shower drain water heat recovery systems with domestic electrical and heat pump storage water heaters

With the overall increase in the energy performance of buildings, domestic hot water (DHW) will become a major source of consumption. In addition, around 80% of shower water is drained away, representing a significant source of waste heat in buildings. This publication explores the potential for coupling common DHW production solutions (electric and heat pump storage water heaters) with drain water heat recovery units (DWHR). The aim is to identify the optimal combination of systems and settings mainly based on heat recovery from drain water and exhaust air (heat pump), in order to maximize the DHW production efficiency. A theoretical model taking into account biofouling, dynamic effects, seasonality, and static losses was developed and validated using in situ measurements over a year ( cv ( RMSE ) < 6%) and TRNSYS simulations ( cv ( RMSE < 7%). These tools have shown that coefficients of performance can reach values ranging from 6.3 (experimentation) to 8 (simulation) while using commercially available systems that are already widely used. This study also shows that the DWHR increases static heat loss and decreases heat pump COP of about 15–20%. The study also showed that the best energy performance of the overall system [tank + heat pump + heat recovery unit] was achieved with an unbalanced DWHR connected only to the shower mixer, which does not offer the best recovery efficiency but limits heat loss and maximizes heat pump performance. The balanced configuration also offers good performance, though slightly lower, while the unbalanced configuration with a DWHR connected to the storage tank offers significantly lower performance.