## 1 Introduction  
Growing environmental awareness and the increasing shortage of fossil fuels lead rethinking in politics and society. Today, a lot of effort and money are invested in environmentally friendly energy production. In addition to completely replacing fossil fuels, which is currently still not possible, a further focus is placed on the increase in efficiency of existing processes, which is a very important point and essential for a successful energy transition. As demonstrated in [1], nearly 35% of the inserted primary energy is already lost in the energy sector as waste heat or as transport losses. So, only 65% of the inserted energy reaches the end-user, whereas in this paper, high energy losses, mainly in the form of waste heat, are documented, e.g., in bulbs or in combustion engines of automobiles. Up to this point, only 34% of the primary energy has been used reasonably. In addition to this, the effective energy is not used meaningfully in all cases, e.g., the illumination of an empty room. In total, Ref. [1] comes to the conclusion that only 20% of the originally inserted primary energy is used wisely. In all of these three areas, namely in energy production, the energy conversion and the wise energy usage, big potential for energy saving exists. Regarding the first two areas, thermoelectric energy harvesting systems (EHSs) can be used, amongst others, to slightly increase the energy yield. In general, energy harvesting is the transformation of ambient energy sources into small amounts of electrical energy, which are mostly buffered in an accumulator or a supercapacitor [2; 3]. The energy sources include light, vibration, radio frequency, water flow or heat and a great advantage is the fact that the energy sources used are normally free of charge and freely available. That is why EHSs are often added to the sustainable energy sector depending on the used energy source, or at least as a system which enhances the energy efficiency of existing processes. If the energy comes from a non-exhaustible source, the EHS actually produces sustainable energy. However, if it is a manmade, exhaustible energy source, the waste energy would normally be staying unused and consequently be lost. So, it is definitely beneficial to use the waste energy with an EHS. A very common understanding of EHS is that the complete system from the transformation of the ambient energy source to the supply of an electrical device is taken into account [4]. Consequently, the EHS consists of an energy converter, electrical energy storage and very often an energy management system as well as the electronics which produce a regulated output voltage for the power supply of electrical devices. A special case of EHSs are thermoelectric EHSs. They use a temperature difference to generate electrical energy with thermoelectric generators (TEGs). The underlying physical effects are the thermoelectric effects, whereby the Seebeck Effect is the key factor for the functionality of TEGs. So far considerable efforts have been made toward enhancing the thermoelectric efficiency of various material classes, including tellurides [5; 6; 7], half-Heuslers [8], and silicides [9]. The advantages of the TEGs are that they are quiet, need neither working fluid and they do not have moving parts, are maintenance free and do not pollute the environment. One application is to use the temperature of the exhaust gas line in automobiles with TEG-based EHSs, see [10; 11]. Another application is the more efficient use of solar energy, in addition to using only the visible light; the authors in [12] use the invisible, infrared light of the sun. As more than half of the energy coming from the sun is thermal energy, this seems reasonable. Other researchers consider thermoelectric EHSs in combination with phase change materials; see [13; 14]. In [15] an experimental study of a thermoelectric generation system for application in exhaust heat of kilns is developed.