# Operational Optimisation of a Non-Recuperative 1-kWe Organic Rankine Cycle (ORC) Engine Prototype
## 4 Steady-State ORC Performance
### Part-Load Performance Maps  
The working fluid mass flowrate \(\dot{m}_{\mathrm{wf}}\), and the expander spindle speed \(\omega_{\mathrm{exp}}\), are intelligible variables that can be readily used in comprehensive or heuristic models, together with the system parameters described in Table 1.  
Figure 4 presents the part-load performance maps for three investigated temperatures, using both the net power output and the thermal efficiency as performance indicators. The contour plots are made of isometric curves ranging from the lowest to the highest value measured for a set heat-source temperature, which allows to visually ascertain the optimal working conditions. With the heat-source temperature set at \(120\,\mathrm{\SIUnitSymbolCelsius}\), a maximum thermal efficiency of \(4.2\pm 0.1\%\) (that corresponds to a power output of \(399\pm 2\,\mathrm{W}\) and \(16.2\pm 0.3\%\) exergy efficiency) is reached for a mass flowrate of \(35.3\pm 0.2\,\mathrm{g/s}\), according to Figure 4d, while a maximum net power of \(470\,\pm\,3\,\mathrm{W}\) (that corresponds to \(3.9\pm 0.1\%\) thermal efficiency) is generated with a working fluid mass flowrate of \(45.1\pm 0.3\,\mathrm{g/s}\) and a resistive load of \(36\,\mathrm{\SIUnitSymbolOelsius}\) according to Figure 4c. Unlike the latter, the conditions found to maximise the thermal efficiency constitute a set of global optimum variables, as changing further the pump RPM or expander load would not lead to higher \(\eta_{\mathrm{th}}\) values. Higher power production could however be obtained with the current apparatus in similar conditions, if higher working fluid flowrates were investigated--which unfortunately was not possible with the current equipment. Moreoever, the operating conditions corresponding to the maximum net power differ from those that correspond to the maximal thermal efficiency, as a higher mass flowrate also increases the heat input. Table 2 summarises the results obtained for the three heat-source temperatures investigated. Figure 3: ORC gross power output part-load performance map as function of the pump speed, \(\omega_{\mathrm{p}}\) and the expander load, \(R_{\mathrm{exp}}\) for a heat-source temperature set at \(140\,\mathrm{\SIUnitSymbolCelsius}\). Two objective functions are considered to determine the optimum results listed in Table 2, namely the net power output (max \(\dot{W}_{\text{net}}\)) and the thermal efficiency (max \(\eta_{\text{th}}\)). Maximising the latter is equivalent to maximising the exergy efficiency, \(\eta_{\text{eeg}}\), as the Carnot efficiency is fixed for given heat-source and heat-sink temperatures, which is why only two optimisation scenarios are listed. For a single heat-source temperature, the mass flowrate found to maximise the efficiency is lower than that maximising the power output, as a higher flow leads to a higher heat input. For \(T_{\text{hs}}=100\,^{\circ}\)C, however, the same operating parameters are found to maximise both the power and efficiency of the  
Figure 4: ORC part-load performance maps: contour plots of the net power output, \(\dot{W}_{\text{net}}\)**(a,c,e)** and of the thermal efficiency, \(\eta_{\text{th}}\)**(b,d,f)**, as functions of the expander spindle speed, \(\omega_{\text{eep}}\) and of the working fluid mass flowrate, \(\dot{m}_{\text{wf}}\), for heat-source temperatures of 100 **(a,b)**, 120 **(c,d)** and 140 \({}^{\circ}\)C **(e,f)**. Performance map obtained from cubic-spline interpolation of experimental data (indicated as grey dots). The optimum values highlighted in red are further described in Table 2.  
system, with a steady cycle that exhibits a relatively low superheat at the exhaust of the evaporator, \(\Delta T_{\rm sup}\ =\)7.2 \({}^{\circ}\)C. Increasing the working fluid flowrate further actually led to a negative superheat, that is, a two-phase mixture at the expander intake and a drop in thermal efficiency. This limit could however be exceeded with a higher evaporating capacity, either with an increased heat-source mass flowrate or increased evaporator area.