Modelling and testing of a hybrid solar-biomass ORC-based micro-CHP system

Expanders employed recently in organic Rankine cycle (ORC)-based systems suffer from key problems including excessive working fluid leakage, thermal losses, low isentropic efficiency and high cost. The majority of the units available in the market are for medium and large-scale applications (>100 kW) with no commercial micro-scale expanders available and applicable for ORC units for residential and building applications. Moreover, the majority of the studies conducted on ORC expanders employed HFC and HCFC working fluids which have high global warming potential leading to negative environmental impacts. In this study, a micro-scale CHP system based on the ORC technology is theoretically and experimentally investigated to provide the thermal needs and part of the electrical demands for residential applications. An innovative design for a hybrid ORC-based micro-CHP system is proposed using a biomass boiler and a solar concentrator to run the CHP system providing more reliable and clean operation compared to conventional natural gas-driven units. The micro-CHP system employs a new type small-scale scroll expander with a compact design, integrating the generator and the turbine in a single unit. A numerical model was developed using the Engineering Equation Solver (EES) software to simulate the thermodynamic behaviour of the ORC unit predicting the thermal and electrical performance of the overall CHP system. In addition, an experimental setup was built to test the whole ORC-CHP system performance under different conditions, and the effect of various operational parameters on the system performance has been presented using an environmentally friendly HFE7100 working fluid. The maximum electric power generated by the expander was in the range of 500 W at a pressure differential of about 4.5 bars. The attained expander isentropic efficiency was over 80% at its peak operating conditions with no fluid leakage observed. Being mass-produced with low cost in the automotive industry along with the high isentropic efficiency and the leakage-free performance, the proposed compact scroll expander represents a potential candidate to be used in the development of micro-scale ORC-CHP units for building applications. Copyright (c) 2013 John Wiley & Sons, Ltd.