Potenzialstudie zum Einsatz eines erweiterten latenten Wärmespeicherkonzeptes in einem Organic-Rankine-Cycle

Kurzfassung

In order to couple thermal energy storage based on phase change, with an Organic-Rankine-Cycle (ORC), the concept of extended latent heat storage is presented. The extended latent heat storage uses phase change material (PCM) for its latent heat of solid-liquid phase change as well as sensible heat. The potential study is focusing on possible combinations between PCMs and ORC-Fluids. Therefor the ORC is used to unload the heat storage whereby the ORC-Fluid is preheated and evaporated. The design of the entire storage system is reserved for subsequent work. Material combinations emerged due to an extensive literature study in a temperature range between 130 °C and 300 °C. The selection of working fluids is based on previously set criteria like for example environmental impact, work safety and thermodynamic performance. The selected fluids are n-Pentane, Toluene, Ethanol and R245fa. As with the organic fluids, a material research has been carried out for the phase change materials and criteria for a suitable selection have been made. Due to the appropriate melting temperatures in a temperature range between 130 °C and 300 °C the following eutectic salt mixtures were selected: KNO3-LiNO3, KNO3-NaNO3, NaNO3-LiNO3 and NaNO3-LiNO3-CaNO3. In order to better compare the various combinations, an evaluation criterion has been devel-oped which represents the correct ratio of the amount of PCM which is needed in preheating and evaporating of the ORC fluid in the concept of the extended latent heat storage. In addi-tion, the course of the specific heat capacity of the solidified PCM is considered, which does not have a linear course in each PCM and thus does not correspond to the course of the spe-cific heat capacity of the liquid ORC fluid. As a further evaluation measure, a comparison of the expanded latent heat storage with a water-pressure-storage was created as well. It shows that the required mass of water, for preheating and evaporation of the ORC fluid, is greater than or equal to the mass PCM. The criteria show that the combination of KNO3-NaNO3 and toluene is promising. The specific heat capacity of the solid state curve is not linear in the desired temperature range for the selected PCM. The effect of the different characteristics of the specific heat capacity needs further investigation. These first results show that the implementation of the concept is quite possible. Especially combinations of PCMs with relatively low phase change enthalpies and ORC fluids with high phase-change enthalpies are promising. Future work will require the study of other combina-tions of ORC-Fluids and PCMs to identify other suitable combinations.