Numerical simulation of a new biomass/solar micro-cogeneration ORC system

This paper presents numerical simulation results for a new prototype of a 6 kWe solar/biomass system. The system uses a micro-cogeneration Organic Rankine Cycle (ORC), driven by a combination of solar thermal and biomass sources. Both sources may be used separately or combined. The solar thermal energy is obtained through medium-temperature concentrating compound parabolic solar collectors (CPC) with evacuated tubes. This system is able to generate electrical energy and useful heat at two different temperature levels. The computer model of the overall system was developed by combining EES (Engineering Equation Solver) with TRNSYS software. EES was used for the power cycle calculation and TRNSYS for simulating the solar circuit and overall system. The ORC/power cycle model consists of a set of algebraic equations representing the thermodynamic behaviour of each ORC component. The ORC simulated by EES includes the following components: evaporator, expander, generator, regenerator, condenser and pump. For an electrical output equal to 6 kW, a fluid inlet temperature in the ORC evaporator of 180ºC was assumed, achieved with the solar collectors and biomass boiler. The ORC is expected to operate at nearly steady-state conditions, as the cycle driving temperature is kept approximately constant. However, TRNSYS is needed to take into account the transient behaviour of solar energy. The TRNSYS model includes the following components: solar collector, boiler, thermal storage, economizer, pumps and control systems. The system is under development and a prototype will be installed and tested in Benguerir (Morocco), in the framework of the REELCOOP project, funded by the EU. For solar resource assessment purposes, Meteonorm software was used to generate climatic data of Benguerir. Results of system simulation under different operating conditions are presented. Annual solar fraction and annual global electrical efficiency for different collector areas and storage capacities were calculated.