Thermal energy storage for solar-powered organic Rankine cycle engines

• New thermal storage concepts specific to solar ORC power plants.
• Contains a review of thermal storage concepts for Rankine cycle power systems.
• New thermodynamic cycle configuration for high-temperature ORC systems.
• Simulation of test case: 100 kW modular system.

The feasibility of energy storage is of paramount importance for solar power systems, to the point that it can be the technology enabler. Regarding concentrated solar power (CSP) systems, the implementation of thermal energy storage (TES) is arguably a key advantage over systems based on photovoltaic (PV) technologies. The interest for highly efficient and modular CSP plants of small to medium capacity (5 kWE–5 MWE) is growing: organic Rankine cycle (ORC) power systems stand out in terms of efficiency, reliability and cost-effectiveness in such power-range. In this paper a thorough investigation on thermal storage systems tailored to high-temperature (≈300 °C) ORC power plants is addressed first, stemming from the observation that the direct storage of the ORC working fluid is effective thanks to its favourable thermodynamic properties. The concept of complete flashing cycle (CFC) is then introduced as a mean of achieving an unmatched system layout simplification, while preserving conversion efficiency. This is a new variant of the Rankine cycle, whereby the vapour is produced by throttling the organic working fluid from liquid to saturated vapour conditions. The presentation and discussion of a case study follows: a 100 kWE CFC system with direct thermal energy storage, coupled with state-of-the-art parabolic trough collectors. The proposed turbogenerator achieves an estimated 25% efficiency, which corresponds to a value of 18% in design conditions for the complete system. Considering siloxanes as working fluids, the estimated values of storage density are around 10kWeh/mstorage3.