Design of a System for the Production of Domestic Hot Water from Wastewater Heat Recovery Based on a Heat Pump Optimized to Work at High Water Temperature Lift

Although many efforts towards more energy-efficient houses have been done, the domestic hot water (DHW) production represents still 18% of the energy consumption in the US domestic sector and will remain roughly similar in the future. Water from the sewage in the domestic sector or from condensing loops in tertiary sector is a potential heat source usually wasted to the ambient. The main potential of this source -apart from energy recuperation- is its temperature, which is slightly higher than the ambient (20-40ºC) and that it does not have significant variability neither through the year nor through the day and its availability. The main objective of the present work is to study the best system configuration in order to use this heat source to satisfy the DHW demand. The solution founded is composed of an initial heat exchanger, a water-to-water heat pump (WtWHP) and a hot water storage tank. The wastewater flow passes through a heat exchanger in order to preheat the DHW. Later on, it is used as a heat source in the evaporator of the WtWHP. The WtWHP increases the temperature of the preheated DHW up to 60 ºC. Finally, the water is stored in a tank at this temperature. The WtWHP has being optimized from the exergetic point of view in order to be able to work with high water temperature lift in the condenser in subcritical conditions with a high COP. This is possible by the integration of a control system that optimizes the subcooling in the condenser based on external conditions and a liquid to suction heat exchanger. The WtWHP configured in this way is able to work at COPs up to 6. The produced water is stored in a hot water storage tank at uniform temperature. The size of the binomial WtWHP-tank has been optimized in order to minimize the energy consumption of the system. Finally, annual energy consumption and CO2 emissions of the final configuration has been calculated and compared with the obtained using a gas boiler with the same profile demand