## 3 Development of the Energy Harvesting System  
To develop the EHS for the exhaust pipe of the heating, a development process, which has already been presented in [23] is used. The modified scheme is shown in Figure 4 and the EHS is developed according to this guideline. The steps in the red frame can also be realized within a multiphysics simulation. Figure 4: Model-based development process for thermoelectric energy harvesting systems [23]. Figure 3: Real system versus mockup: (**a**) Real oil-fired heating on the campus; (**b**) Heating mockup in the laboratory. Regarding the development process, the selection of the heat source and the settings of the objectives have already been done in the previous section. The other steps can be summarized to five main steps, which are the 'Thermal Design', the 'Selection of Thermoelectric Device', the 'Mechanical Design', the 'Electrical Design' and 'Control Concepts'. Concerning the thermal design, a segment cooling aggregate, [24], which is originally designed to cool down a flowing medium, is installed in the exhaust gas pipe. It is built up of cooling elements, which penetrate into the medium, collect the heat and transport it to the outer side of the pipe, see Figure 5a. There, the TEGs and the corresponding cooling units can be easily attached. This component is used to collect as much heat as possible from the exhaust pipe (the heat of the medium). This is, of course, an intervention in the real process and that is why a heating mockup was planned and finally built-up in our laboratory. Consequently, all following results are valid for a hot flowing medium in a pipe, but cannot be transferred directly to a real system, e.g., a real heating, before doing more studies on the influence for the real process. First measurements on the mockup with the installed clean segment cooling aggregate and a real heating behavior are shown in Figure 5b. The temperatures for each section of the segment cooling aggregate--in total there are four sections--are measured. At each section, four thermoelectric devices can be attached, which are consequently under the same temperature conditions. In [25], it was shown that in some cases expensive TEGs can be replaced by cheap thermoelectric coolers (TEC, Peltier element). According to [25], for the first two sections, TEGs are used and TECs are used for the third and fourth sections. The next step of the development process, the heat transfer of the waste heat, will be done with controlled air forced cooling elements. These are more suitable for real heating with an on/off scenario of 20/80 than liquid cooling elements, as described in [26]. Due to the lack of space, special small central processing unit (CPU) cooling elements are attached and the fans of the cooling elements are only mounted at the outside on the first and the fourth section. The fans on the first section blow air through the heat sinks and the fans on the last section pull the air from the heat sinks. So, there is airflow from the first section to the fourth. The mechanical setup of the heating mockup is shown in Figure 6. There, the four sections and the temperature sensors are visible for these sections; the inside pipe temperature sensor at the beginning, the cooling elements and the fans for sections 1 and 4 are also visible. The blue arrow in Figure 6b shows the air flow direction through the cooling elements. Regarding the last hardware point of the development process, the electrical design, the prospective was to be very flexible. That is why a switch cabinet with a front panel was built up (see Figure 7a). As the thermoelectric devices of one section have the same temperature differences, they are connected in parallel. The individual blocks can then be individually interconnected (see front panel in Figure 7b).