ORC systems for small scale energy production : Improving the prediction of electricity production of ORC turbines under varying loads

Sammanfattning: In order to have a sustainable, resource-efficient energy system and no net emissions of greenhouse gases by 2050, energy efficiency is one important aspect to reach this goal and develop a long-term sustainable energy system. As part of a solution to increase energy efficiency in society, Organic Rankine Cycle (ORC) systems can have a far reaching effect by utilizing low-grade heat to produce electricity.The market of ORC systems is currently growing and in Sweden and other Nordic countries, the growth can be attributed to extensive use of district heating. Since the available heat at district heating network changes with season, and is limited by the required supply temperature to the district heating network, this project focused on improving the prediction of electricity production of ORC systems under these varying loads. The investigation started with analysis of off -design performance by obtaining compensation factors which can be used to approximate losses due to part load operation. The calculated compensation factors were then used in the continued investigation to seek the optimal interval of output temperature from the ORC condenser. By using the compensation factors to estimate the electricity production during part load, the right balance between part load and using most of available heat was found, determining the design point to maximise the annual electricity production.The project also investigated how large pressure ratios were possible for different generator sizes, modules and stages of the turbine. Based on the calculations, the overall conclusion was that module T434 was good to use for almost all conditions by only changing the number of stages. As a general trend however, larger modules and an increased number of stages resulted in a higher possible pressure ratio over the turbine. Finally, the project investigated how performed calculations agreed with measured data from an installed ORC system at a district heating plant. Analysis of the measured data and theoretical values from Againity's turbine program were found to be in good agreement with each other.