Objective
Utilization of the heat potential of cleaned waste water and sewage gases from a sewage treatment plant by means of an absorption heat pump for the heating of several large buildings of the town of Waiblingen.
The Waiblingen plant has operated satisfactorily.
Only the development of micro-organisms in the treated waste water on a few days in 1984. These micro-organisms brought about severe fouling of the automatic backwashing filter, which could only be removed by manual cleaning.
It is, however, possible to avoid such upset conditions by careful monitoring and by applying adequate cleaning methods.
As far as the energetic aspects are concerned, plant operation in practice shows positive and negative deviations from design and engineering data.
Performance measurements revealed an annual heat pump plant coefficient of performance of 1.37 whereas the contractual coefficient was 1.3.
The district heat losses were calculated to be 5.5%, whereas in practice the values proved to be more favourable.
Whereas the original plant concept, proceeding from a sewage deficiency at night, provided for intermittent absorption heat pump operation, it was meanwhile possible to ensure a continuous wastewater flow, by the respective intervention and control of the sewage treatment process. The utilization of the heat contained in the waste water is thus improved.
At the same time, the plant switching cycles are reduced, which has a positive effect on a stable and economic heat pump operation which finally also influences the service life of the plant.
These positive aspects are faced by low heating load requirements and/or the low energy consumption of the consumers.
Whereas a maximum heating load on Winter days was designed to be 6.500 kW, the maximum heating load during operation proved to be approximately 70% of this value.
The same applies to useful heat consumption. The design value was 15,800 MWh, whereas provisional extrapolations made so far bring about approximately 12,000 MWh.
Calculation of the heat requirements according to DIN 4701 and the respective safety factors, unexpectedly low coincidence factors within the buildings and the total system, as well as energy savings as a result of the modification and retrofit of high-temperature consumers into low-temperature systems suitable for heat pumps finally brought about that it was possible to keep below the designed performance and consumption data.
As far as the consumer side is concerned, the Waiblingen absorption heat pump system has not yet reached its full extension. There is still capacity for another 3,500 kW.
Engineering was done with a look into the future. Many investments are made with a view to future requirements and also under economic efficiency aspects (e.g. district heat transportation systems).
The advantages of using treated waste water as a heat source were particularly evident in 1985, when in January and February we had extremely low outdoor temperatures, down to - 20 deg. C.
The heat pump could be operated even with these very low outdoor temperatures, which is not all possible in heat pump systems with air or surface water being used as heat source.
The planned district heating of the town of Waiblingen consists of an absorption heat pump and 2 gas boilers, operating in a bivalent parallel connection. It has a total heating capacity of 9500 kW (2500 kW heat pump, 2 x 3500 kW boilers). For the supply of the users, a heating capacity of 6450 kW is necessary, the surplus capacity serves as reserve. The plant utilizes the heat potential of cleaned waste water and sewage gases from the town's sewage treatment plant to produce heating waste which is fed in to the network for the heating of 6 public buildings : town hall, covered market, indoor swimming pool, civic center, sewage plant, hospital.
The absorption heat pump operates with NH3 a heat carrier and a NH2 - water solution as solvent. The heat source is the waste water from the sewage treatment plant which is cooled down from 9 deg.C to 5 deg.C in the heat pump evaporator. An automatic brush cleaning system keeps the evaporator free of dirt. The ejection boiler is fired withsewage gas and natural gas. Apart of the ejection boiler's exhaust gas, heat is recovered in a heat exchanger for the heating of the district heating water. In the whole heat pump system, the district heating water is heated from its return temperature of 40 deg.C to a supply temperature of 65 deg.C.
The heat pump covers the base load of the district heating network, it supplies about 77% of the total annual output of the district heating plant.
In the case of consumption peaks at low outside temperatures, the boilers, using natural gas and sewage gas as fuel, are switched on. When using the boilers, the temperature of the supply water of the heating network can be raised to 110 deg.C.
A surplus of hot water produced by the heat pump is fed into an 80 m3 storage tank and can again be taken out in case of an increasing heat demand in the district heating circuit.
The calculated energy saving of this heat pump - boiler plant amounts to 880 TOE/y, compared with a monovalent decentralgas boiler concept.
The cost of the project amounts to DM 11,434,246.
The construction phase of the project has started in 1983. The completion of the demonstration is expected for the end of 1984.
Programme(s)
Topic(s)
Call for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
7050 WAIBLINGEN
Germany