Reaching competitiveness in a small ORC unit: dynamic simulation, experiments and economic evaluation

Organic Rankine Cycle is one of the most efficient, profitable and feasible technology for low-to-medium temperature heat recovery. The boost to the development of small-scale systems that could easily enter the market poses challenges in layout design and selection of components. This work deals with a micro-scale ORC unit conceived to be cheap, efficient and compact. The transient response of the system has been experimentally investigated and simulated with large variations of the heat source. To this end, a dynamic model is built and validated against a large set of experimental data. The final goal is to show the real potential of the system in terms of market applications, for which best design choices, operational limits and response time to variable heat sources must be identified. Results proved the fast time response of the system that promptly reacts to variations of the heat source temperature. The model is able to capture well the system dynamics, showing maximum relative errors of about 5% in the prediction of both output power and high pressure. The choice of scroll expander, volumetric pump and plate heat exchangers is made according to the lowest cost-to-efficiency ratio to keep the investment cost below 2500 euro/kW already at the premarket phase. The additional heat production (around 15 liters/min at 40°C) amplifies the market potential in isolated domestic applications and paves the way for further cost reductions.