## 2 System Description  
### System Analysis  
As described previously, a thermoelectric EHS for the exhaust pipe of a heating system is here developed. That is why a normal heating system has to be analyzed. However, the exhaust pipe of the heating system can be representative for other possible exhaust pipes, e.g., in industry or in automobiles. Related test benches on heating systems are already described in [21; 22]. In general, the main electrical consumers in gas- or oil-fired heating systems for space heating and domestic hot water preparation are the blower of the boiler, the electronics and the different pumps. In these systems, there are generally two main pumps. One to supply the heat emission system (in this case radiators), which is now called'space heating pump', as well as the pump for charging a domestic hot water storage, called 'domestic hot water pump'. Additionally, for comfort and to ensure water hygiene and legionella prophylaxis, in many systems an additional 'hot water circulation pump' is installed, which circulates the heated tapping water in the circulation circuit permanently or with defined control and e.g., night shut-off. Moreover, data has to be collected from heating systems to calculate previously, if such an EHS is promising or not. That is why a measurement system was installed in one building at our university, where an oil-fired heating system is available. To avoid disturbing the real heating system, only temperature sensors are attached and the following temperatures are measured: room and ambient temperature, outside pipe temperature at the beginning and in the middle of the exhaust gas pipe as well as inside pipe temperature (equal to medium temperature) at the beginning and in the middle. The data are stored in a database and can be visualized via a web interface. An exemplary typical curve progression is shown for the 15 th of November 2016 between 08:42 and 10:22 o'clock in Figure 1.  
Visible are the aforementioned temperatures and an approx. 20/80 on/off behavior of the heating. The outside temperatures of the pipe have their maximum at about 74 \({}^{\circ}\)C, whereas the inside pipe temperatures reach a temperature of 163 \({}^{\circ}\)C. For the time being, the temperature levels seem to be appropriate to install a TEG-based EHS. A second measurement system was installed in a private house. This building represents a normal one-family house and besides different temperatures, the energy consumptions of the pumps as well as the temperatures of a cockle stove in the living room are measured. Figure 2 shows a typical behavior for the 23 rd of January 2016 between 00:00 and 23:59 o'clock. The measured data are again stored in a database and can be visualized by the web interface. Figure 1: Measured temperatures at an oil-fired heating on the campus, visualized by the web interface. In the upper diagram of Figure 2, the temperatures are depicted and in the lower diagram the power consumptions are depicted. The central gas-fired condensing boiler recovers the latent heat of vaporization, which is reflected in the low maximum exhaust gas temperatures of about 75 \({}^{\circ}\)C. Therefore, a TEG-based EHS is not suitable for new condensing boilers as the exhaust temperatures are too low. Nevertheless, the on/off behavior of the boiler is also recognizable for this heating system. The power consumptions of the pumps are, on average, 45 W for the domestic hot water pump (runs only a few times a day), 6 W for the space heating pump (runs all day) and 2 W for the hot water circulation pump (runs almost continuously between 5:30 and 23:30 o'clock). The consumption of the other components (like blower and the heating electronics) is about 23 W, which results from the overall consumption minus the pump consumptions.