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The Organic Rankine Cycle (ORC) is a thermal driven power cycle operated with an organic working fluid. It is especially suitable to exploit low to medium temperature heat sources at small to medium size capacity ranges. For these conditions, it outperforms the standard Clausius Rankine Cycles. Suitable heat sources can be either renewable in nature (i.e. biomass, solar, geothermal) or waste heat (i.e. mobile, stationary). The size of these power units ranges from very few kW to several dozen MW. It is the intention of this report to present an up-to-date overview on the recent developments in the ORC industry. This publication provides insights on the number and capacity of most of the installed ORC plants worldwide as well as the current and future market expectations from ORC manufacturer’s perspective. Information from 30 manufacturers, representing a total of more than 2600 projects have been collected and analyzed providing information on trends in the past and the market evolution during the last four years in terms of installed capacity and installed plants divided by application, macro region and manufacturer. At the end of 2020, the cumulated ORC installed capacity was 4.07 GW. Since 2016, the overall ORC market increased by 40 % (+1.18 GW) in terms of installed capacity and by 46 % (+851) in terms of installed plants. Regarding the capacity increase between 2016 and 2020, the largest increase is due to geothermal application (+970 MW, +45 %), while waste heat systems have the largest growth concerning the number of installed systems (628 plants, + 207 %). For the market survey, ten established ORC manufactures contributed their current and future perspective on the ORC market. The results highlight that, despite the impact of the COVID-19 situation, the current market situation is in good shape and that the majority of the manufacturers have positive expectations concerning the short-term development of both the ORC market and number of employment. Central and Eastern Europe as well as the ASEAN region are rated as the regions with the highest growth potential. Geothermal energy and small waste heat sources are seen as the most relevant heat sources for the coming years.

Currently, turbomachinery design optimization methodologies are mainly restricted to steady state approaches, due to the high computational cost associated with time-accurate shape optimization algorithms. However, the possibility to include unsteady effects in turbomachinery optimization can significantly increase the level of accuracy of the design predictions, leading to a more realistic representation of the actual performance and ultimately to a substantial increase in operating efficiency. Unsteady effects are particularly relevant in Organic Rankine Cycle turbines. A trade-off between high-fidelity time-accurate unsteady simulations of the flow solution and computational cost is therefore needed at design level. In this paper, a first application of the harmonic balance method to non-ideal compressible flows is presented. The methodology allows to solve the unsteady flow equations for a set of specified frequencies only, with significant computational time savings. An algorithm is proposed for non uniform time sampling in order to resolve frequencies that do not need to be integral multiple of one fundamental harmonic. This enables the solution of quasi-periodically forced non-linear flow problems, in combination with complex fluid models based on accurate equations of state. The method is applied to the unsteady analysis of a supersonic Organic Rankine Cycle stator with quasi-periodic inlet operating conditions, showing about one order magnitude lower computational cost compared to time-accurate simulations.

High temperature Organic Rankine Cycles power systems of low power capacity, i.e. 3-50 kWe, are receiving recognition for distributed and mobile energy generation applications. For this type of power plants, it is customary to adopt a radial-turbine as prime mover, essentially for their ability to cope with very large volumetric flow ratio with limited fluid-dynamic penalty. To date, the design of such turbines is based on design guidelines and loss models developed mainly for turbo-chargers, subsequently adapted by means of non-validated computational fluid-dynamic calculations. In the attempt to provide data sets for CFD validation and calibration of loss models, a mini-ORC radial inflow turbine delivering 10kW of mechanical power will be realized and tested in the Propulsion and Power Laboratory of TU-Delft. The fluid dynamic design and characterization of the machine is detailed in this paper. According to available models, the results indicate that the optimal layout of mini-ORC turbines can substantially differ from that of radial-inflow turbines utilized in more traditional applications, strengthening the need of experimental campaigns to support the conception of new design practices.