## 3 Experimental Setup
### Employed Testing Protocol
For each recirculating pumps' constant rotational speed value, more than 12 different runner rotational speed values of the PAT have been acquired (see Table 3), which makes a total of more than 120 operating points necessary to draw the full-3D efficiency characteristic of the machine. Compared to the microturbine, the number of operating points is less since its only degree of freedom is the rotational speed of the runner. Focusing on the dynamic method, first the optimal acceleration/deceleration ramps of the electrical drives have been identified in order to ensure a quasi- steady-state operation of the testing model. The main objective was to reduce at maximum the measurement time, while avoiding measurement errors and hysteresis of the acquired characteristics. Then, the hydraulic efficiency measurements have been performed at respectively 9 and 19 different test rig recirculating pumps' constant rotational speed values (see Tables 4 and 5) while increasing and decreasing the speed of the microturbine/PAT runner(s) from minimum to maximum, and vice versa. In the case of the microturbine, for each recirculating pumps' constant rotational speed value 11 different constant runners absolute rotational speed ratios have been tested. Finally, the results obtained by the dynamic method have been validated with the ones obtained by the classical static point-by-point measurements method. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**H = 0.5/0.75/1/1.25/1.5/1.75/2/2.25/2.5/2.75/3 [bar]**} \\ \hline \multicolumn{2}{c}{\(\mathbf{N_{A}}\)**[\%]**} & \multicolumn{8}{c}{\(\mathbf{N_{B}}\) [\%]} \\ \hline
0 & 0 & – & – & – & – & – & – & – & – & – \\
8.33 & 0 & 2.08 & 4.17 & 6.25 & 8.33 & 11.08 & 16.67 & 33.33 & 8.33 \\
16.67 & 0 & 4.17 & 8.33 & 12.5 & 16.67 & 22.17 & 33.33 & 66.67 & 16.67 \\
25 & 0 & 6.25 & 12.5 & 18.75 & 25 & 33.33 & 50 & 100 & 25 \\
33.33 & 0 & 8.33 & 16.67 & 25 & 33.33 & 44.47 & 66.67 & – & 33.33 \\
41.67 & 0 & 10.42 & 20.83 & 31.25 & 41.67 & 55.42 & 83.33 & – & 41.67 \\
50 & 0 & 12.5 & 25 & 37.5 & 50 & 66.67 & 100 & – & 50 \\
58.33 & 0 & 14.58 & 29.17 & 43.75 & 58.33 & 77.58 & – & – & 58.33 \\
66.67 & 0 & 16.67 & 33.33 & 50 & 66.67 & 88.9 & – & – & 66.67 \\
75 & 0 & 18.75 & 37.5 & 56.25 & 75 & 100 & – & – & 75 \\
83.33 & 0 & 20.83 & 41.67 & 62.5 & 83.33 & – & – & – & 83.33 \\
91.67 & 0 & 22.92 & 45.83 & 68.75 & 91.67 & – & – & – & 91.67 \\
100 & 0 & 25 & 50 & 75 & 100 & – & – & – & 100 \\ \(\alpha\) = \(\mathbf{N_{B}}\)/\(\mathbf{N_{A}}\) [\(-\)1] & **0** & **0.25** & **0.5** & **0.75** & **1** & **1.33** & **2** & **4** & \(\infty\) (\(\mathbf{N_{A}}\) = 0)** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Combinations of rotational speeds of the microturbine runners for point-by-point performance measurements. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**Np1,2,3 [\%]**} \\ \cline{2-11}  & **24** & **33.33** & **41.67** & **50** & **58.33** & **66.67** & **75** & **83.33** & **91.67** & **100** \\ \hline  & 8.33 & 8.33 & 8.33 & 8.33 & 8.33 & 8.33 & 8.33 & 8.33 & 50 & 78 \\  & 16.67 & 16.67 & 16.67 & 16.67 & 16.67 & 16.67 & 16.67 & 16.67 & 58.33 & 83.33 \\  & 25 & 25 & 25 & 25 & 25 & 25 & 25 & 25 & 66.67 & – \\  & 33.33 & 33.33 & 33.33 & 33.33 & 33.33 & 33.33 & 33.33 & 33.33 & 33.33 & 75 & – \\  & 41.67 & 41.67 & 41.67 & 41.67 & 41.67 & 41.67 & 41.67 & 83.33 & – \\  & 50 & – & 50 & 50 & 50 & 50 & 50 & 50 & 91.67 & – \\  & 58.33 & – & 58.33 & 58.33 & 58.33 & 58.33 & 58.33 & 58.33 & 100 & – \\  & – & – & – & 66.67 & 66.67 & 66.67 & 66.67 & 66.67 & – & – \\  & – & – & – & 75 & 75 & 75 & 75 & – & – \\  & – & – & – & – & 83.33 & 83.33 & 83.33 & 83.33 & – & – \\  & – & – & – & 91.67 & 91.67 & 91.67 & 91.67 & – & – \\  & – & – & – & – & - & 100 & 100 & 100 & – & – \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Combinations of rotational speeds of the recirculating pumps and of the PAT for performance measurements using the static method.