in 11 sectors with 3000 companies actively working on them [1]. The authors reported a potential market value of $200 billion USD until 2025, and trillions of IoT edge nodes are expected to be installed worldwide in the near future. At the same time, many nations have set ambitious goals in order to reduce the amount of required electric energy.
How do we want to supply edge nodes with energy? A promising approach is the use of ambient energy in order to power electronic systems. This concept is also known as micro energy harvesting. The following chapter introduces micro energy harvesting and its history, the challenges in its design and future work ahead. Different sources of energy and available conversion mechanisms are discussed. The aim of this chapter is to set indoor photovoltaics in the broader frame of Micro Energy Harvesting in general. An introduction to conversion principles, achievable output power, and literature suggestions for further reading are provided for each type of converter. The principles of miniaturized and indoor photovoltaic devices are the core topic of this book with a general discussion of the theoretical foundations in Chapter 3.
2.1.1 Brief History of Electric Generators and Loads
The use of ambient energy in human operated applications is far older than the electric grid. Wind- and watermills have been constructed and operated by humans over thousands of years. Based on the work of Bélidor [2], the first electrodynamic hydro generators were developed and applied for local grids, such as by the US Grand Rapids Electric Light and Power Company in 1880 [3].
The following decade mainly focused on large-scale electrification in both appliances and connections. According to the World Bank, 89% of the world was considered electrified in 2017 [4]. Grids were generally constructed as alternating current (AC) grids. Especially after the introduction of nuclear plants, electric appliances were mostly designed for an abundance of power. With the progress in semiconductor research, transistor elements were invented in 1947 [5]. This enabled the development of electronic products based on semiconductor elements, such as silicon. Silicon with its bandgap of 1.12 eV has an open circuit voltage of around 0.6 V. A serial connection of many silicon elements then yields an operating DC voltage around 3 to 12 V, which can be provided by batteries. Therefore, these devices were and are preferably battery powered.
Both the small scale and the low required power of electronic devices enabled miniaturization of products. In 1997, Chu et al. demonstrated a cubic corner reflector below the centimeter scale [6]. This was the beginning of the Smart Dust project [7], which aimed at fabricating distributed wireless sensor nodes in a microscale. With a similar approach, DARPA launched a program in order to developed miniaturized wireless sensor nodes [8].
2.1.2 Forms of Energies and Energy Converters
In order to do work on an object, exergy needs to be available. If a system is to use the exergy of another system, which might be surrounding energy or a converter, the systems have to be coupled. The form of coupling between energies of the same form or between different types of energy are called weak and strong. There are nine types of energies [9], and in principle, all of them can be used for energy harvesting:
– gravitational
– kinetic
– heat
– elastic
– electric
– chemical
– radiation
– nuclear
– mass.
Besides the use of mass and gravitational energy, each form of energy has been used in micro harvestings research so far. Reviews on the principles, the history and the current state-of-the-art in industry and research have been provided by various authors, such as [10–12]. As this book focuses on indoor applications, this chapter discusses the level and usability of these types of energy for indoor applications. Some of the most important references are provided for further reading, and principles and examples of demonstrated systems or commercially available devices are included where possible. As most generators aim at converting energy from ambient energy to electric energy, the converters discussed in this chapter will focus on those.
2.2 Kinetic Energy
Kinetic energy can be found in many places in indoor environments, often in the form of vibrating objects or motion from humans and machines. Besides which, there is often flow from both natural and technical sources, such as air or liquid flow streams within buildings, and from air conditioning plants. Amongst the most classic electric converters using kinetic energy as incoming energy are turbines, and piezoelectric generators in forms of sheets.
Kinetic converters transfer oscillations to a mass. Its movements relative to the supporting structures are coupled back under use of the piezoelectric, inductive or capacitive effect onto an electric system. There are various geometries and designs of converters. Reviews, modeling and research
work include the work of Mitcheson et al., Roundy et al. or Safaei et al. [11–14]. Kinetic converters are already available on a commercial scale, with examples being Kinetron or ReVibe [15, 16]. These converters mostly cover industrial applications, including rail and helicopter flight assistance.
The following subsections introduce simple general models and available power densities for some of the indoor applications.
2.2.1 Oscillating Solid Objects
Most kinetic converters for vibrations of solid objects are designed for resonance, which means they achieve their maximum electric power at a certain frequency f for an oscillating mass m. The maximum power Pmax can be approximated following Roundy et al. [17]
where ζe denotes the electric damping, ζm the mechanic damping of the system. For a first approximation, the damping can be modeled to be ζm = ζe = 0.015 [17]. In real applications, the modeling of the damping is complex. The total damping ζ follows from ζm = ζe, with the damping coefficient d = ζ 2ωf m, where ωf = 2πf. a is the acceleration of the vibration. The oscillating mass m is damped electrically and mechanically (d). The resonance frequency depends on the feather constant k, where
Figure 2.1 depicts a schematic of the model.
Figure 2.1 Schematic of the basic model of a kinetic energy converter with an oscillating mass m, a feather constant k, and a damping coefficient d.
Following Eq. 2.2, both maximizing the mass m and minimizing the damping d are required for an optimization of the electric power yield. This is the reason why many converters of this type are very heavy. From the models, it also follows that for a constant amplitude, the electric yield increases with decreasing frequencies.
The
