Potential and concepts for waste water heat recovery in combination with solar collectors and heat pumps (WRGpot)
In low energy buildings the share of the domestic hot water demand on the total heat demand of the building is relatively high, and in passive houses it can be even higher than the space heating demand. Additionally the temperature level that is required to fulfil the comfort requirements for domestic hot water is higher than e.g. the temperature that is necessary for heating with a low temperature heating system. Therefore the relevance of the heat demand for domestic hot water is increasing with an improving insulation standard of buildings. For these reasons the heat recovery from waste water in buildings is becoming more interesting with a decreasing space heating demand.
In the project WRGpot the potential of waste water heat recovery in combination with a solar thermal system and a heat pump was investigated. The coupling of a heat pump with the waste water heat recovery offers the advantage that the heat, which is available at a temperature level that cannot be used directly, can be shifted to higher, usable temperatures. The objective of the project was to point out different options for waste water heat recovery in buildings, and to analyse and evaluate different system concepts based on a combination of solar and heat pumps.
Available information and published work about waste water heat recovery in buildings and about combined solar and heat pump systems was collected and documented.
In order to be able to make an analysis of the potential of waste water heat recovery, the amount and temperature of waste water was determined in a single- and a multi-family building by means of field measurements. The results were used to generate time profiles of the waste water flow and temperature, which were later used in the simulation studies that were carried out in the project.
In the second project part different heating system concepts based on combined solar and heat pump systems with waste water heat recovery were evaluated by means of dynamic simulations in the simulation environment TRNSYS. The emphasis was on systems that use solar energy and waste water as the source for the heat pump, without a conventional heat source (ground or ambient air). The results show that with such systems high system efficiencies can be reached for buildings with a low heat demand. The efficiency is strongly dependent on the climate and on the solar radiation in the cold months of the year respectively. Waste water heat recovery is an interesting option in such systems, as the hot water demand has a high fraction on the total heat demand of low energy buildings and the waste water provides an additional heat source for the heat pump in winter.
Future investigations should deal with the use of unglazed solar collectors, which would offer the possibility to additionally use ambient air as a heat source of such systems in winter. Also the fouling of waste water heat exchangers which can cause a loss of efficiency and the necessity of a cleaning of the heat exchanger surfaces should be examined scientifically.
Dipl.-Ing. (FH) Dr.techn. Andreas Heinz, Dipl.-Ing. Werner Lerch
Technische Universität Graz, Institut für Wärmetechnik
Project or cooperation partnerDipl.-Ing. (FH) Johann Breidler, Ing. Christian Fink, Ing. Waldemar Wagner
Institute of Thermal Engineering, Graz University of Technology
Dr. Andreas Heinz
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