.

Fig. 2 Layout of the demonstration plant.
© DeVeTe

Fig. 3 At the biogas plant in Kirchwalsede, a crane lifts the roof structure comprising the heat exchanger and chimney pipe onto the standard container housing the ORC module.
© DeVeTec
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Demonstration plant as development hub

The standard technology for utilising waste heat is provided by ORC turbines. Their intermediate thermal oil circuit means that they often use complicated system technology. Fraunhofer researchers in Oberhausen have now developed turbines that dispense with thermal oil. However, turbines have a narrow optimal operating point, which with lower loads in partial load operation considerably reduces their efficiency. With the ORC process using a steam expansion engine, on the other hand, the high temperature difference in the heat exchanger (HE) leads to a greater overall efficiency, which enables the electricity production (Pel) to be doubled for the same heat provision (Fig. 1). The high efficiency is due to the high enthalpy gradient in the engine. That means that a considerable amount of the energy available in the waste heat is transformed into usable energy. Interesting in energy terms is the further utilisation of the waste heat in the two ORC modules: the cooling water from the condenser is also available as usable heat for drying and heating. This has enabled the demonstration plant for the steam expansion engine in Fenne in Saarland to utilise more than 90% of the heat energy used. In contrast to turbines, here it has been shown that the same engine can be used for various output levels between 100 and 200 kW. This enables variable balancing of the different pressure and temperature levels. Schmidt refers to the experience gained in Fenne: “When the cooling water temperature increases in summer, we can adjust the expansion ratio and thus the output of the gas piston engine. A turbine would have to be idle.”

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Addresses

Steam expansion engine
DeVeTec GmbH

Working fluids
Evonik Industries AG

Heat exchangers
SCHOTT AG