A preliminary study for localized waste heat recovery system at hot water shower drains in highrise residential buildings of Hong Kong

This paper investigates the potential of thermal energy recovered from hot water shower drains in bathrooms equipped with instantaneous type water heaters in high-rise residential buildings of Hong Kong. In particular, a simple single-pass counter-flow heat exchanger installed at a horizontal shower drainage pipe was used as a preliminary heat recovery measure by heating up the cold water supply to a water heater. The thermal energy exchange at the heat exchanger was evaluated with the effectiveness number-of-transfer-units (ε-NTU) approach. User shower patterns including shower operating time and water flow rate were sampled from an interview survey with the Monte-Carlo sampling technique. Yearly water temperatures at the shower heads, shower drains and cold water supply taps were recorded in sample operations of hot water showers in bathrooms and used for sample inputs in this study. The results showed that 4-15% thermal energy would be recovered from a shower drain with a 1.5 m long single-pass counter-flow heat exchanger for a drain pipe of 50 mm diameter.