Analysis of Grey-water Heat Recovery System in Residential Buildings

Abstract

Annual operating costs for buildings are a substantial cost in a lifetime. It is therefore of interest to try and reduce these costs. A large fraction of this cost today as the buildings become more and more energy efficient is the cost of hot tap water. The study in this report looks into the potential for energy savings from grey wastewater. It is here looked at the amount of energy which can be recovered from hot water leaving the building and reused for pre heating of hot tap water and heating of building. The unit which would recover this energy is referred to as the grey-water heat recovery unit in this report. A residential building with three floors where each floor has one washing machine, one shower and one dishwasher has been as the case building for the report. The total living area of the building is 450 m2. In the case building used in this report as much as 17.1 % of the total used energy goes to heating of hot tap water. By installing a heat recovery system which can recover some of the energy stored in the used hot water which leaves the building, this this could be reduced to 10.9 % of the total used energy according to simulations done in SIMIEN. There are also possibilities of using this energy for heating of the building as well as pre heating of hot tap water. There are a few different solutions for implementing a grey-water heat recovery unit which could give different energy recovery between 2 716 kWh/year to 3 759 kWh/year. The best solution would be to connect the grey-water heat recovery unit to pre-heating of hot tap water, heating of the building as well as installing an accumulation tank to store recovered energy in. The most simple solution which would give the lowest amount of recovered energy would be to just connect the grey-water heat recovery unit to pre heating of hot tap water. In this report two different simulation programs have been used, EnergyPlus and SIMIEN, to find what impact the energy reduction would have on the building and to see if the simulations would correspond to the theoretical estimates done in this report. The theoretical estimates based on equations for heat recovery and measured data for energy use in the case building gave a little bit better results than the simulated results for the same case building. Although there is a difference both gave a positive indication that a heat recovery unit would not only reduce the energy consumption but also reduce the annual operating cost of a building. The investment cost for a heat recovery system could be a bit large for small buildings compared to the annual savings but for larger buildings the investment cost could be substantially higher. Regarding the energy as much as 87.7 % of the energy stored in the grey-water could be recovered for a system with an accumulation tank and a connection to the buildings heating system. For a system without the accumulation tank and district heating as the energy source it would have a theoretical efficiency of 76.7 % and a simulated efficiency of 63.3 % when simulated in EnergyPlus.