Numerical analysis of an organic Rankine cycle under steady and variable heat input

This paper develops a strategy to maintain steady operation of an organic Rankine cycle (ORC) by adjusting evaporator flow rates in relation to the available thermal energy. ORC unit under investigation uses R245fa as the working fluid, for which a regression based approach was implemented to evaluate its state properties. Steady and transient models for unit’s subcomponents (pump, evaporator, expander and condenser) were developed. Heat source is considered as solar heated water between 80 °C and 95 °C at mass flow rates between 2 kg/s and 12 kg/s, while the flow rate of R245fa is ranging between 0.5 kg/s and 1.5 kg/s. Due to possible changes in the available thermal energy, unit’s evaporator was identified as the critical component of the ORC. Evaporator’s effectiveness was characterized as a function of inlet temperatures and mass flow rates to map steady operation scenarios for changing conditions. Steady state analysis shows that the selected ORC system is capable of producing 13–39 kW power for heat inputs varying between 125 kW and 367 kW with maximum efficiency in the defined operating range. Subsequently, the developed steady state map is used to construct a control strategy. This strategy aims to adjust evaporator flow rates in order to achieve maximum and steady energy recovery for any given level of heat input. The unit is simulated to study its dynamic response when available thermal energy gradually or abruptly changes with and without the control strategy. It is demonstrated that adjusting flow rates not only improves the thermal efficiency but also helps maintaining the steady state operation.