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Session: Poster session & Sponsor Exhibition
Session starts: Monday 07 October, 14:00

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Melissa Ireland (MIT)
Adriano Desideri (University of Liege)
Matthew Orosz (MIT)
Sylvain Quoilin (University of Liege)
J.G. Brisson (MIT)

Abstract:
Organic Rankine cycle (ORC) systems are gaining ground as a means of effectively providing sustainable energy. Coupling small-scale ORCs powered by scroll expander-generators with solar thermal collectors and storage can provide combined heat and power to underserved rural communities. Simulation of such systems is instrumental in optimizing the control strategy. Several authors have simulated solar thermal ORC systems in steady-state [1,2], or focused on either ORC or thermal storage dynamics in isolation [1,2], or simulated system dynamics assuming a fixed electrical load and a solar loop modeled as a single lumped component [3]. In this work, a model for the dynamics of the solar ORC system is developed to evaluate the impact of variable heat sources and sinks, thermal storage, and the variable loads associated with distributed generation. This model can then be used to assess control schemes that adjust operating conditions for diurnal to annual environmental variation. The Modelica programming language is used to capture the important dynamics of the system, mainly in the storage tank, solar collectors, plate heat exchangers, and air-cooled condenser. Detailed steady-state component models are first developed in Engineering Equation Solver and serve as guides for the dynamic models. In particular, a detailed simulation of the fin-tube condenser with hexagonal tube array provides a better understanding of the influence of the moving liquid boundary with various working conditions. Measurements on a pilot system at Eckerd College are currently underway to validate the steady-state models to ensure an appropriate baseline for the prospective dynamic optimization. Ultimately, the goal of this work is to identify “optimal” control schemes for a small-scale solar ORC. Operating conditions will be controlled through the variation of the heat transfer fluid mass flow rate through the solar array, the speed of the ORC expanders, and the speed and number of operating ORC condenser fans. The control strategy will focus on maintaining the pressure ratio across the fixed volume ratio scroll expanders necessary to avoid both over- and under-expansion of the working fluid under variable ambient conditions.