Energy and exergy analysis of waste-water heat recovery in a multi-family residential complex

This article provides an approach to analysing the energy and exergy efficiency of a waste-water heat recovery system. The system studied is installed in a multi-family residential complex in Stockholm, Sweden. The heat recovery unit in question consists of eight coaxial, counter-flow heat-exchangers connected in parallel, with waste-water flowing through the internal pipe and cold brine (propylene glycol 25%) ducted through the external pipe. The analysis was carried out based on data collected by a building monitoring system (BMS) during five winter months (heating season). The energy analysis (using average hourly values) showed that on average 10.7 kW (Span: 1.1 - 38.7 kW) of waste-water heat was delivered to the cold brine. On the other hand, the exergy analysis (using the same data) showed that on average 12% (Span: 3 - 24%) of the exergy contained in the waste-water was delivered to the cold brine, while on average 76% (Span: 60 - 88%) was consumed during the process. Access to accurate and detailed performance measurement data was found to be essential for analysing the exergy and energy performance of the heat recovery system. In conclusion, this article demonstrates that it is vitally important to consider both the first and second laws of thermodynamics to achieve a wholesome understanding of heat recovery from waste-water systems. (paper)