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Fig. 1 Comparison of the temperatures in °C and the electrical power (Pel ) between turbines and steam expansion engines.
© DeVeTe
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More electricity with fewer components

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.”

Organic Rankine Cycle (ORC)

The ORC system is named after Scottish physicist William Rankine (1820 - 1872), who is considered one of the founding fathers of thermodynamics. The ORC system is similar to the steam cycle used in coal-fired power plants, except in this case water is replaced as the working fluid with an organic working fluid that evaporates at lower temperatures and thus possesses more efficient thermodynamic properties than water. In order to achieve the greatest possible efficiency, the temperature of the heat source determines whether alcohol, (silicon) oil or a refrigerant is used. The working fluid also makes it possible to apply an effective steam pressure on the expansion engine or turbine – whose rotating shaft is used to generate electricity – with waste heat at temperatures below 600 °C. ORC technology can use waste heat from both industrial processes and renewable energy sources. Potential users include large-scale bakeries, glass and paper factories, steel and cement works, CHP plants that use biomass or biogas, coalmine gas or landfill gas, geothermal energy and solar energy (solar thermal).

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Addresses

Steam expansion engine
DeVeTec GmbH

Working fluids
Evonik Industries AG

Heat exchangers
SCHOTT AG