# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 3 Energy Supply for Wireless Sensor Nodes
### Hybrid Converters  
A strategy for increasing energy income and more energy reliability for a WSN is the development of hybrid converters harvesting energy from multiple sources. In hybrid solutions, more converters from the same source or combinations of harvesters from different energy sources can be considered. A good synergy between harvesters can be realized, if the sources are relatively independent and can be available in the same environment. In the case of hybrid converters, the amount of harvested energy compared to the single source is of great interest. Hybrid solutions therefore aim to have a compromise between improving the generated energy and the improvement of the reliability of the harvesting system. That is why the hybrid solution is used more often where several ambient sources coexist, which can be complementary and ensure a better reliability. If they have a different behavior under operation conditions, they may get more complementarity, so one source could take over in phases, where the other lacks energy from ambient. In addition, the dependence on environmental conditions and aging may differ, bringing a better total system availability over the lifetime. In general, it is not necessary that the harvesters contribute with the same amount of energy. The challenge is more to increase the total energy income and the availability of the system under the different operation and environmental conditions as well as independently of aging. Often reported hybridizations consist of the development of multi-converters harvesting from solar source combined with thermal or kinetic source, the thermal source in combination with the kinetic source and the kinetic source combined with the wind source. Figure 3 provides an overview of interesting combinations of ambient sources reported in literature. For the realization of converters combining thermal and solar sources (see Figure 4a), a laminated structure is proposed in [36; 37], where solar and thermoelectric cells are used. This hybridization enabled in this case an improvement of the maximum power output by 47% and 36% in comparison to the single solar cell and thermoelectric cell, respectively. The resulting system structure is complex and contains several layers to be able to ensure the hybridization. In [37; 38], a solution combining thermoelectric and solar cells is developed to power wearable medical devices. In this case, the main purpose of combining both sources is to ensure the continuous reliable energy availability as photovoltaic cells have a good performance limited to outdoors. Thermal source is added, in this case, especially to compensate the gap in cold environment and at night, where solar cells have a low performance or even cannot generate energy. In [39], an interesting solution, where one single cell combining intrinsically thermoelectric and photovoltaic effects is proposed and showed an efficiency improvement by 23.3%. Figure 3: Overview of selected hybrid converters from multi-sources. In [40; 41], the combination of solar energy with kinetic energy is investigated. In this case, the kinetic source consists of a raindrop harvester aiming to improve the performance of solar cells during rain by using the triboelectric effect. The hybridization is in this case limited to the presence of rain. Other intermediate conditions such as cloudy or foggy days are not considered. Another solution, combining triboelectric principle, thermoelectric principle and solar cells together under a single structure (see Figure 4b) is proposed in [23]. It can harvest simultaneously or individually mechanical, thermal and solar energies. It is a promising solution in the future especially for applications with variable operating conditions. Aiming to harvest from vibration and wind sources simultaneously, the authors of [43] developed converters based on electromagnetic and piezoelectric principles (see Figure 4c,d) able to harvest energy from ambient vibration of bridges.