Energy from waste water

The heat energy from warm water production in households disappears mostly unused into the canal at present. The project aims at investigating the possibility of recovering heat from waste water and to establish this in Austria. There exist already many such projects in Switzerland and Germany. According to foreign studies, about 5% of the buildings can thus be supplied with heat.  
The heat pump as such is used all over Austria already. A frequent problem being the low temperature during the main heating period at the so-far used heat sources (amongst others air). Waste water has a constant temperature all year round of between 10°C and 15°C. This means that waste water is optimal as a heat source for heat pumps: thus higher annual working figures can be achieved respectively, higher supply temperature can be realised with the same efficiency. By using the waste water heat – the only heat transfer leakage, which is normally not considered even in a low-energy and passive building – is closed.  

Prerequisites for using waste water

In order to make use of waste water heat, sewerage systems and also consumers must fulfil certain prerequisites; these are in parts conditions that different heating systems do not require necessarily.

This type of heating is particularly suitable for areas with a high density of houses, as the transport paths for achieving a certain performance capacity are lower (lower investment costs, less heat loss). Further conditions for realising the use of a waste water heat plant are the vicinity to  a sufficiently large sewerage pipe (800 mm in diameter), a dry weather flow of at least 15 l/s (the more the better as this increases the continuous waste water flow), as well as low-temperature heating systems in the objects to be heated. This type of heating becomes especially economic in larger objects, and the use of a heat and hot water power plant for several individual objects would make sense.  
Particularly favourable economic framework conditions are always given in situations where investment in new sewerage systems, heating for buildings or even new buildings etc. become necessary anyway.

As is the case with using many renewable energy carriers, there are initially comparably high starting up costs and low operating costs. The performance of the heat pump should not go below 100 kW, as otherwise high investment costs are incurred that do not or only partly depend on the size of the plant.

Further possibilities for using Waster Water Energy

Within the scope of the project, the possibility of cooling by means of waste water is also being investigated. A particularly energy-saving variation of cooling is the so-called free cooling. As soon as the waste water temperature falls sufficiently below the desired cooling temperature it is sufficient to let the heating medium circulate without a heat pump and, due to the natural thermal gradient, heat energy is transported from the building.  
Feeding the district heating network is also possible under certain circumstances. 

Project Dates
ClientEnergy and climate fund
Project ManagerFranz Zach
Project PartnerOchsner Wärmepumpen GmbH
Universität für Bodenkultur
EnergieSchweiz in Infrastrukturanlagen
Fernwärme Wien GmbH
Duration11/2009 – 11/2012
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