A novel micro-cogeneration unit for market applications based on a biomass-fired ORC system

In the transition towards smart grid systems, a problem of increasing importance is the distributed generation of thermal and electric power at low cost and low environmental impact. This work proposes a novel cogeneration system based on a biomass boiler and a micro-Organic Rankine Cycle (ORC) unit. The biomass boiler heats up an unpressurised thermal oil circuit, which, in turn, supplies heat to an ORC unit that produces electricity and hot water for the users. The ORC system is based on a single-pressure regenerative cycle that works in the subcritical region. The goal of this study is twofold: i) the analysis of the design choices that were made to achieve a good compromise between efficiency and cheapness of the micro-CHP system, and ii) the performance evaluation of the system for variations of key parameters, such as temperature and flow rate of the thermal oil, mass flow rate of the cooling water and operational assets of the ORC unit. An in-depth experimental campaign has been carried out, where the rotational speeds of the pump and expander of the ORC unit have been varied to choose first the best operating asset, and then investigate the influence of the other key parameters. The best combination of the speeds has been identified as 2250 rpm for the pump and 2300 rpm the expander. In these conditions, maximum values of electrical efficiency (7.4%) and total energy utilization factor (62%) are found. With an oil temperature of about 150°C, the achieved power production is 2530 W, the ORC utilization factor 93% and the expander global efficiency 57%. Simplicity and low specific cost contribute to increase the efficiency-to-cost ratio, making this novel system appealing for the customer.