## 2 Experimental Data and Machine Characterization
### Transient State, Start, and Cool-Down  
The measuring campaigns of MTT Enertwin conducted at DLR included not only steady state operations, but also accelerating and decelerating maneuvers. These are induced by increasing (or decreasing) the fuel mass flow to the combustion chamber; shaft rotational speed is monitored with a 2 Hz frequency. A maneuver is considered complete when both the rotational speed and fuel mass flow remain constant at the new SP. In Figure 3, a cold start maneuver is shown, in which the shaft's rotational speed goes from 0 up to 240 krpm in a staged fashion.  
During the first 4-5 min of the test, the system consumes electric power to drive the turbine up to N = 90 krpm, where ignition takes place. The system is then steadily  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**SP** & **N (krpm)** & \(P_{th}\) (kW) & \(P_{el}\) (kW) & \(P_{f}\) (kW) \\ \hline PC20 & 180 & 7.60 & 0.73 & 10.56 \\ PC40 & 190 & 8.40 & 1.02 & 11.96 \\ PC60 & 200 & 10.00 & 1.34 & 13.44 \\ PC70 & 210 & 11.30 & 1.69 & 15.46 \\ PC80 & 220 & 12.50 & 2.04 & 17.54 \\ PC90 & 230 & 14.20 & 2.39 & 19.77 \\ PC100 & 240 & 15.60 & 2.70 & 22.17 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Steady state properties at various setting points. N = rotational speed, \(P_{th}\) = thermal power, \(P_{el}\) = electrical power, and \(P_{f}\) = fuel power.  
Figure 2: Global and net efficiency of the MTT Enertwin as measured by [17]. Adapted from [17]. Copyright 2017, Instituto Superior Técnico Lisbon.  
accelerated up to 180 krpm, the so-called minimum load (SP = 20). After a first peak in the fuel mass flow rate, which is needed to start the cold system, fuel mass flow can be gradually reduced. A second peak allows the rotational speed to ramp-up to full-load, SP = 100, at 240 krpm. Just like in the work of [PERSON] [17], the cold start maneuver is considered complete after about half an hour. However, the time to reach steady state depends on the initial temperature: the warmer the system, the faster it reaches the steady state. In Maj's tests, 1700 s was the minimum time for the system to reach the steady state after a cold start; in warmer starts, the system reached the steady state earlier. The maneuver time was increased from 1700 s to 2000 s to ensure the steady state under every condition. The authors of the present work also believe that a cold start maneuver can be performed under every condition within 1800 s (exactly half an hour) by reducing the time of the staged fuel mass injection: the second fuel peak in Figure 3 could be anticipated at circa 400 s (the experiments conducted at DLR had multiple goals, and therefore, the maneuvers were performed as compromise between different requirements). Therefore, the present analysis assumes 1800 s as the minimum time to start the system.  
Cool-down maneuvers were not investigated in detail during the measuring campaign, and the data from Maj were kept as a reference. Hence, 150 s was considered to be the time needed to reduce the speed of the engine to 0 rpm, for all loads. During cool-down, the heat exchanger is still warm and allows for further energy extraction; in particular, 0.455 kWh thermal and 0.011 kWh electric energy are produced during the 150 s of this maneuver.