Analyzing the Transient Heat Transfer Characteristics of a Drain Water Heat Recovery Device

This paper investigates the transient behavior of a drain water heat recovery (DWHR) device, which recovers heat from warm grey water in buildings. Experimental and numerical investigations were conducted to study the thermal performance of the device under transient conditions. Thermal performance measurements were carried out, and a mathematical model was developed that considered the thermal inertia of the hot and cold-water streams as well as the device’s material. The experimental data were used to validate the model, and good agreement was observed between the two. Under transient operating conditions, the device’s performance was measured in terms of its effectiveness, and an actual effectiveness model was used to capture the transient effect on the system’s overall performance. The experimental results show that during short hot water usage, the actual device effectiveness is significantly reduced as it does not reach a steady state condition. An economic analysis indicates that considering the device’s transient performance leads to a 27.2% reduction in annual energy savings in a typical domestic installation with regular daily usage of hot water appliances. The presented model and analysis offer valuable insights for the development of improved DWHR devices with the potential to contribute to sustainable engineering and building practices.