Theoretical evaluation of falling film evaporation for organic rankine cycle processes

  • Theoretische Untersuchung der Rieselfilmverdampfung in Organic Rankine Cycle Prozessen

Höfler, Matthias; Kneer, Reinhold (Thesis advisor); Wirsum, Manfred Christian (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019


The exploitation of heat sources at low and medium temperature levels by means of Organic Rankine Cycle (ORC) power plants offers a significant potential for resource saving energy conversion. However, economical and technical constraints reduce the feasibility of this technology. In particular, evaporators are critical key components and driving cost factors. To achieve an increased exploitation of the unused ORC potential, a general improvement of the evaporator properties and performance is compulsory. In this respect, the application of a horizontal falling film evaporator could result in both technical and economical benefits compared to actual evaporator technologies. As main advantages, horizontal falling film evaporators would allow for a minimisation of the installation size, increased heat transfer efficiencies and a significant reduction of the required working fluid charge. Although falling film evaporators are successfully used in different fields of industrial application, there is no implementation at operating conditions of an ORC yet. In this dissertation, detailed numerical studies were conducted to demonstrate the potential of the falling film evaporator in the context of an optimisation of multi-Megawatt ORC plants. For this purpose, detailed models of heat exchangers and power plant components were used for both analysis of the heat transfer process within a falling film evaporator and the overall performance of a representative model power plant equipped with different evaporator technologies. First, respective influences of the tube bundle configuration as well as the recirculation ratio of a falling film evaporator were assessed. Based on the findings, a reference specification for the ORC falling film evaporator was developed. Performance analyses of power plant operation subsequently revealed a comparable stationary operation regardless of the applied evaporator technology. However, the application of the falling film evaporator allows for a significant reduction of the working fluid charge in the evaporator. Finally, the transient power plant operation was analysed with regard to the impact of the evaporator technology, revealing that the falling film evaporator allows for a reduction of local fluctuations of the working fluid charge in the heat exchangers.