THERMODYNAMIC ANALYSIS OF THE OFF-DESIGN PERFORMANCE OF A MICRO SOLAR ORGANIC RANKINE CYCLE TRIGENERATION SYSTEM

Organic Rankine Cycle (ORC) systems are considered as one of the most suitable technologies to produce electricity from low-temperature sources. ORC units can efficiently convert low-temperature solar energy into electric and thermal power. Independently from the solar technology used, the hourly and the seasonal fluctuations of solar energy entail challenging dynamic effects and bring these systems to operate in off-design conditions. Such effects are even more influential at micro-to-small scales granting paramount importance to the comprehensive understanding of their behavior. In this study, the annual performances of a 4 kWe/50 kWth solar ORC trigenerative system for residential applications are numerically investigated. Four different modeling approaches commonly used in annual system-level simulations of ORC systems are compared. These models differ in the system-level modeling approach and the components modeling method. The analysis has shown that the simplest ORC model results in the lowest discrepancy compared to the model with the least assumption, in which the components are modeled empirically, and the high and low pressures of the system are found iteratively. The difference between the produced electric energy using the four models is significantly higher in hot months, in which the average temperature of the water tank is high due to the requirements of the vapor generator of the absorption chiller. In this case, the expander pressure ratio drops drastically depending on the system model algorithm, which affects the produced electric power depending on the adopted expander model. On the contrary, the discrepancy between the models for the produced thermal energy is negligible.