# Operational Optimisation of a Non-Recuperative 1-kWe Organic Rankine Cycle (ORC) Engine Prototype
## 6 Conclusions  
Driven by the market potential of small- to-medium scale waste-heat recovery (WHR) systems, a significant research effort is being made to develop organic Rankine cycle (ORC) engines in the range 1-kW to 100-kW. The design of technically and, in particular, economically efficient smaller-scale ORC systems is challenging, especially for converting low- to medium-grade thermal energy into useful power. The simultaneous optimisation of the working fluid and of key system components, accounting for part-load and off-design operating conditions, requires accurate physics-based models, able to predict system performance over a wide range of working conditions. High-fidelity experimental measurements are key to inform and validate semi-empirical and comprehensive models. This paper presents an operational exploration and effort to optimise an experimental non-recuperative 1-kWe ORC engine, comprising a rotary-vane pump, brazed-plate evaporator and condenser units and a scroll expander with an adjustable load. Heat is supplied to the prototype at various temperatures (ranging from 100 \({}^{\circ}\)C to 140 \({}^{\circ}\)C in the current experiments) using an 18-kWh heater that circulates thermal oil in the evaporator, while excess heat is rejected to a cooling water stream supplied at 18 \({}^{\circ}\)C. The optimal operating (or control) parameters, i.e., the pump speed and expander load, are determined for different heat-source temperatures, while the part-load performance of the engine is explored with a total of 124 steady-state data points. The net power output, thermal efficiency and exergy efficiency are reported as functions of the working-fluid mass flow rate and expander-scroll spindle speed, thus providing useful part-load performance maps that can be readily used to inform (develop and/or validate) numerical models. Detailed expander performance maps are also provided--wherein the filling factor and electrical isentropic efficiency of the expansion device are reported as functions of the spindle speed and pressure ratio. A maximum net power output, that is, the net electrical power generated through the vapour expansion minus the pump consumption, of \(508\pm 2\) W is reported for a heat-source stream supplied  
Figure 8: (**a**) Dynamic response of the ORC system to a step in pump veloctiy from 1100 to 2000 RPM at fixed heat-source temperature (\(T_{\mathrm{hs}}=120\,^{\circ}\)C) and expander load (\(R_{\mathrm{exp}}=25\,\Omega\)). (**b**) Rising and falling step-response times as functions of the step amplitude in expander power. at 140 \({}^{\circ}\)C, while a maximum \(4.2\pm 0.1\%\) thermal efficiency is reached with a heat-source temperature at 120 \({}^{\circ}\)C. The maximum exergy efficiency, \(18.7\pm 0.3\%\), is reached for the lowest heat-source temperature investigated in this study, 100 \({}^{\circ}\)C. The contribution of the main components to the exergy destruction in the overall heat-to-power conversion process is determined through a steady-state exergy analysis of the three optimum configurations identified, that is, maximising both the thermal and exergy efficiency. It is found that a negligible amount of the thermodynamic losses occur in the pump, which represents 3% of the overall availability loss, while the two major exergy destruction rates are measured in the expansion and evaporation units, which represent 32% to 46% and 36% to 46%, respectively. Finally, the dynamic characteristics of the prototype are investigated by measuring the key time constants, as derived from the analysis of the expander power-output time response to a rising or falling step in pump rotating velocity. It is found that the rise time is 7% to 30% higher than the fall time. Both step-response times are also found to decrease with the amplitude of the command step. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] designed and constructed the testing facility. [PERSON] conducted the experiments. [PERSON] performed the data analysis.