Innovative technology for small CHP plants

When the more than twenty-year-old wood-fired heating system of the Fuhrberg waterworks near Hanover was to be modernized and extended, the municipal energy supplier enercity AG decided to generate not only heat but also electricity for the site in the form of combined heat and power (CHP). The aim was to use an ORC process that would make it possible for the first time to use combined heat and power in particularly small wood-fired boilers from around 250 kWth. As part of a pilot application, Fraunhofer UMSICHT developed and implemented a tailor-made ORC process for the waterworks that is unique to date in this form, and provided scientific support for the trial operation.

The organic Rankine cycle (ORC) is a process in which steam turbines are operated with a working fluid other than steam. Organic liquids with low evaporation temperatures are used as the working fluid. The ORC process is used when the available temperature gradient between the heat source and heat sink is too low or the available heat output is too small to operate a steam turbine. This is often the case with electricity production using combined heat and power generation at small sites. ORC plants make it possible here to generate electrical energy from waste heat or associated with small heat outputs.

Wood is used much more efficiently

For the Fuhrberg waterworks, Fraunhofer UMSICHT developed a tailor-made and so far unique ORC process, which uses the waste heat at the ORC condenser for heating purposes on site and improves efficiency when using the renewable energy source wood. The successful pilot application of this ORC process on the small wood chip furnace near Hanover increases the benefit and efficiency of the decentralized energy supply by generating the required heating heat using combined heat and power generation – especially at lower heat outputs.

The electricity generated locally using cogeneration gives people greater independence from rising and fluctuating electricity prices. The pilot application, therefore, allows smaller wood and biomass heating plants to be converted to CHP operation in an economically profitable manner and represents an option for introducing cost- and resource-efficient combined heat and power generation, particularly with regard to retrofitting existing plants.

At the Fuhrberg site, the hot flue gases produced in the wood chip furnace will be cooled down in a large water boiler after modernization. On the secondary side, saturated steam (refers to the boundary between wet and superheated steam) at a temperature of 230°C is generated, which is fed into the connected ORC process for the supply of heat. The water vapor condenses on the primary side in the evaporator of the ORC circuit. The water condensate is returned to the shell boiler. The transferred heat causes the ORC working fluid to evaporate and the ORC steam is led through a small turbine and expanded. A specially developed turbogenerator generates electricity from the steam, which is fed into the grid.
The working fluid steam emerging from the turbine is liquefied in the condenser and the resulting heat is completely coupled into the local heating system of the waterworks. During maintenance work on the ORC system, the heat from the steam boiler can be fed directly into the local heating network via a bypass condenser.

enercity (Stadtwerke Hannover AG)