Numerical analysis of heat recovery options in old Finnish apartment buildings

Abstract

Ventilation heat recovery in residential buildings is well established. However, waste heat recovery from sewage is focused on industrial applications and is not commonly done in Finland at the residential building level. This study examines the CO2 emission reduction potential of heat recovery from sewage and ventilation exhaust air in various configurations in an apartment building built before ventilation heat recovery systems were mandatory. The study was done by dynamic simulation using TRNSYS and the IDA-ICE building simulation software. The lowest costs and emissions were obtained by combining both the exhaust air and sewage heat recovery using heat pumps. Thus, heat recovery can reduce emissions even while lowering life cycle costs. CO2 emissions were reduced by 12 to 50% using a series connection between district heating and waste heat sources and 21 to 37% using a parallel connection. Utilities enforce the use of parallel connection, which reduces heat recovery potential. With heat exchanger based ventilation heat recovery, the emissions were reduced by 23 to 29%. The key performance indicator for energy efficiency is the primary energy consumption. However, systems with similar primary energy consumption can have very different emissions. The mismatch between emissions and primary energy use suggests a need for a policy update.