Off-design simulation and mathematical modeling of small-scale CHP plants at part loads

In this work a mathematical model of combined heat and power (CHP) plants at part loads was developed on the basis of off-design simulations of four existing small-scale (1–20 MWe) CHP plants. The deregulation of the electricity market and the regional availability of biofuels have given new opportunities for small-scale CHP plants. These plants are often operated for long time periods at part load conditions, which makes the knowledge of their part load behaviour very important. The simulation results presented here showed that although the part load power production of a CHP plant can be described quite accurately with a single line, there is a small nonlinear reduction in the power production as the district heat load decreases. Based on the simulation results two and three line regression models describing the electricity production and its nonlinear changes as a function of the district heat load and the outgoing district heating water temperature were developed. According to a comparison with a single and a two line regression model, the three line model describes the simulated CHP processes at part loads most accurately (i.e., has the highest R2 value). The developed models can be used, e.g., for more detailed optimisation of a district heating network with a small-scale CHP plant. When evaluating the annual production of the CHP plant the accuracy of the heat duration curve may have as high influence on the results as the accuracy of the power production model.