Experimental and theoretical analysis of a micro-cogenerative solar ORC-based unit equipped with a variable speed sliding rotary vane expander

A promising solution for the Combined Heat and Power (CHP) micro production is certainly represented by Organic Rankine Cycle (ORC)-based power units. In the domestic appliances with electrical power range of the units below 1 kW, the reduced dimensions of the components represent a critical aspect as well as the need to guarantee a high reliability. When the hot source is represented by solar energy, the optimization of the electricity production keeping insured the thermal energy availability represents an aspect which invites to a proper management of the unit. Solar-based ORC-recovery units frequently work in off-design conditions due to the variability of the hot source and to the Domestic Hot Water (DHW) requirements. For this reason, the design and the selection of the components should be carefully performed. The expander is commonly retained the key component of the unit being the one that mainly affects the behaviour. For the mentioned power ranges, the volumetric expander is the best technological option and, among those available, Sliding Rotary Vane Expander (SVRE) are gaining a sensible interest. At off design conditions, according to permeability theory, the expander intake pressure linearly varies with mass flow rate of the Working Fluid (WF) which is the most suitable and easiest parameter to be changed. This modifies the performances of the unit, both from a thermodynamic and technological point of view. In this paper, the speed variation of the expander is considered as control parameter to restore design expander intake pressure. In order to assess a strategy for the speed variation of the expander, in this paper a comprehensive model of the SVRE is presented when it operates in a solar-driven ORC-based unit. The model is physically based and recovers and widens the permeability theory developed by the authors in previous works. An experimental ORC-based unit was fully instrumented and operated, coupled with a reservoir, usually present when flat plate solar collectors are used, which store the thermal energy which fulfils thermal energy requests and feeds the generating unit. The model was widely validated with the experimental data properly conceived for the purpose. In the unit the expander speed was varied and, thanks to the permeability theory, the relationships between WF flowrate variations, inlet expander pressure and expander speed variation were investigated. The potentiality of a control strategy of the expander revolution speed of the expander was fixed as well as a deeper understanding of the SVRE behaviour and relationships between operating variables. In particular, it was observed that varying the speed from 1000 RPM up to 2000 RPM, the expander behaviour was optimized ensuring proper working condition matching with a (30-100 g/s) flowrate range.