Challenges
This chapter focuses on micro energy harvesters for indoor applications other than photovoltaics. Some generators are already available commercially. A current challenge for researchers, product designers and customers is their lack of characterization and test standards. The reproducibility and comparability of test results and data sheets remain difficult, if not impossible, without standardized and representative test conditions.
Toxicity, recycling and lifetime are important topics for future energy harvesting systems, especially for IoT applications. The devices might remain in the field for decades, and some might even be forgotten or left installed after their functional life has ended. This needs to be considered during design. The long lifetime also poses a challenge to the security of IoT edge nodes. Such a system needs to protect and secure its data now and in the future, which might be 15 years ahead. So far, security has been neglected in many IoT and comparable systems. In addition, the visible and invisible interconnection of many edge nodes quickly results in giant networks of an accordingly high attack surface [43].
Finally, a common motivation for the installation of IoT devices is the classical idea of Big Data, that is, gathering as much data as possible in order to realize data analytics applications. However, from a statistical point of view, high quality data from a well-planned sample often yield a better result for these applications than an unlimited amount of data of unknown reliability [43, 44]. Thus, the real value gained from the data obtained by an IoT edge node needs to outweigh its cost to security, privacy, operational effort, toxicity, ecological considerations and so forth.
2.8 Conclusions and Outlook
The use of ambient energy to power low-power electronic devices seems like a natural choice today. This chapter presented the most common types of generators other than photovoltaic converters, their sources of energy, and typical output power levels for indoor applications. From these, kinetic converters including micro turbines and thermoelectric generators are the most mature generators. Commercial products are available on the market, and the output power range is the highest discussed in this chapter. While some of the converters and applications presented in the sections above might seem exotic, and are elaborate in their fabrication or too unreliable for use in commercial products, micro energy harvesting is a valid approach to power IoT edge nodes and applications and a standard in space applications. The appropriate type of generator, its detailed design and the overall design of the IoT edge node depends on the application. Does the system only need to send data at certain events? Does it have to be constantly available or are delays or even outage times allowed? Is the ambient energy easy to predict in its frequency of occurrence and level or is a battery part of the system? A reliable micro energy harvesting system requires a design specific to its application and as a whole system, including its data storage, processing and communication units. An example are kinetic converters, which are tuned to a certain frequency. The need for this application-specific design limits economic mass production effects. Especially when the systems aim for miniaturization, standardized circuit boards that are produced on a mass scale for different product lines cannot be used in micro energy harvesting.
An exception to this, with power levels as high or above the ones presented so far, is indoor photovoltaics, which is introduced in detail in the following chapter.
Acknowledgment
Parts of this work have been translated and revised from the PhD thesis of the author at the Albert Ludwig University of Freiburg, Graduate School of Micro Energy Harvesting DFG GRK 1322-1.
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