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Invited talk on Energy harvesting devices - a basis of energy supply of sensors and sensor systems
Prof. Dr. Norbert Schwesinger
Associate Professorship of Micro-Mechatronical Systems
Technische Universität München


Energy harvesting devices - a basis of energy supply of sensors and sensor systems Energy Harvesting (EH) is still a hype area in engineering. Many governments in industrialized countries support projects of energy harvesting very benevolent. On the other hand, only a few companies operate in this field. Their products show a comparable low output of electrical energy. There is a necessity to analyze what EH can afford at all. EH is understood as a technique that turns waste energy into electrical energy. Losses in technical processes are the main resources for energy conversion. Although these losses are inevitable, engineers are constantly striving to reduce them. However, EH targets to a circumstance opposite to the trend of technical developments. Loss of energy in artificial processes is heat, vibration and radiation, scattering as well flow losses. Over 80% of publications in EH focus on the conversion of vibration into electrical energy using the piezoelectric effect. Despite of the use of the high performance ceramic PZT ("Lead-Zirconate-Titanate"), the power density of piezoelectric transducers is only in the range of μWs /cm3. This low value is the reason to search for better structural and/or electrical solutions. Numerous designs and circuits developed show a minimal rise of effectiveness but an increase in complexity and fabrication costs. Note, this kind of EH is only useful when no external electrical supply and no rotational movements are present. Even with the necessary repeated maintenance, batteries are often much cheaper than harvesters are. This and the low power density make EHs and the research doubtful. Rethinking is imperative. EH needs to be considered from view of applications. Fields, possessing of a large number of small consumers of energy, can no longer be maintained manually. Large sensor nodes and systems appear in certain circumstances to be a useful application for EHs. Another area of application could be sensors at nearby inaccessible locations. Some examples show ways in which EH in combination with regarding sensors can be sensibly and economically justifiable used.