Dynamic modelling and thermo-economic optimization of a small-scale hybrid solar/biomass organic rankine cycle power system

The use of solar thermal energy to drive both large and small scale power generation units is one of the prospective solutions to meet the dramatic increase in the global energy demand and tackle the environmental problems caused by fossil fuels. New energy conversion technologies need to be developed or improved in order to enhance their performance in conversion of renewable energy. The Organic Rankine Cycle (ORC) is considered as one of the most promising technologies in the field of small and medium scale combined heat and power (CHP) systems due to its ability to efficiently recover low-grade heat sources such as solar energy. This technology is especially in demand in isolated areas where connection to the grid is not a viable option. The present research provides thermodynamic performance evaluation and economic assessment for a small-scale (10 kW) hybrid solar/biomass ORC power system to operate in the UK climate conditions. This system consists of two circuits, namely organic fluid circuit and solar heating circuit in which thermal energy is provided by an array of solar evacuated tube collectors (ETCs) with heat pipes. A biomass boiler is also integrated to compensate for solar energy intermittence. A dynamic model for the hybrid ORC power system has been developed to simulate and predict the system behaviour over a day-long period for different annual seasons. In the thermodynamic investigation, an overall thermodynamic mathematical model of the proposed power system has been developed. The calculation model of the ORC plant consists of a number of control volumes and in each volume the mass and energy conservation equations are used to describe energy transfer processes. The set of equations were solved numerically using a toolbox called Thermolib which works in the MATLAB/Simulink® environment. The numerical results obtained on the performance of the ORC plant were validated against the theoretical and experimental data available in the open literature. The predicted results were in very good agreement with the data published in the literature. The comparison demonstrated that the developed simulation model of the ORC plant accurately predicts its performance with a maximum deviation of less than 7%. The developed mathematical model then has been used to carry out the parametric analysis to investigate the effect of different operating conditions on the system performance. The economic analysis has been performed with the use of equipment costing technique to estimate the system’s total capital investment cost. This approach is based on the individual costing correlation of each component in the system, considering all the direct and indirect costs of the proposed components. The system cost calculations have been conducted for a range of operating parameters and different working fluids for a fixed value of net power output. At the final stage of the research, a thermo-economic optimization procedure has been developed using Genetic Algorithm (GA) approach for selection of the rational set of design parameters and operating conditions for optimum system performance