Untersuchung eines Niederdruck-Wasser-/Dampfkreislaufs zur Abwärmenutzung von Verbrennungsmotoren auf technische und wirtschaftliche Realisierbarkeit

This thesis deals with the thermodynamic and economic evaluation of a low-pressure steam Rankine cycle (LP-SRC) for the waste heat utilisation of internal combustion engines. A distinctive feature of the LP-SRC is that the coolant and exhaust gas heat are used in a single loop system. To achieve this, the coolant heat is used to evaporate the working fluid. This limits the evaporation pressure of the LP-SRC to < 1 bar absolute, as the coolant heat is present at a relatively low temperature level of approx. 80 °C to a maximum of 120 °C.
The technical and economic feasibility of the LP-SRC in this thesis is investigated for the waste heat utilisation of combined heat and power plants (CHP) in the power range from 50 kWel to 250 kWel. For the thermodynamic evaluation of the plant concept, a stationary thermodynamic simulation model is developed with the software Ebsilon®Professional and validated by means of a demonstrator in laboratory scale (CHP with 38 kW mechanical power). The methods for the design of the heat exchangers and the turbine are also validated.
As the plant concept is not economically feasible in the power range of the demonstra-tor, the plant will be scaled to the upper target power range (CHP with 220 kWel) and the costs will be determined again by means of offers from the supply companies. The economic feasibility is determined exemplarily for the location Germany with consideration of the combined heat and power act 2016.
Based on the costs of the demonstrator and the cost calculation of the scaled plant, a cost model for the construction of a LP-SRC will be developed, with which the costs of the individual components can be calculated using characteristic values (e.g. heat exchanger surface, turbine output, etc.).
In this thesis it is shown that the LP-SRC is competitive to comparable downstream processes regarding the electrical power increase of the CHP engine, despite a comparatively low electrical efficiency. The calculated specific costs of the scaled LP SRC show that an economically profitable operation of the LP-SRC is possible from an electrical power output of the CHP engine of approximately 220 kW.