of the human lower leg has been measured to be 10 m/s2 [18], where the frequency f of a human walking is around 2 Hz. For a mass m = 0,2 kg, and ζm=ζe= 0.015 an electric power of 6.64W could be harvested. Decreasing the mass to a more comfortable m = 0.01 kg yields 331 mW. This would already be enough to power small transceivers with low frequencies of use, such as for paging applications in hospitals. As outlined in the introduction of this book by Joe Paradiso, those applications are more of research interest due to their low comfort for the user.
Exemplary demonstrated and commercial systems are outlined in the following sections.
2.2.1.1 Human Motion
EnOcean uses kinetic converters in order to power their batteryless radio switches [19]. Kinetron offers weight, rotational and linear based converters with a broad application range including watches, water industry, sanitary applications and similar applications [15].
The PowerWalk system by Bionic Power aims at avoiding soldiers having to carry batteries on their mission by using kinetic converters, with a targeted harvest of 10–12 W [20, 21]. Future applications are aimed at forestry, mining and consumers.
Fu et al. demonstrated the use of airflows in micro turbine implemented in a shoe with a power of 6 mW [22]. Many researchers demonstrated the use of piezoelectric converters. A review has been provided by Sun et al. [23].
2.2.1.2 Vibrations
The commercially available converters for higher frequencies obtain their maximum power at their resonance frequencies, usually 50–60 Hz, and 100–120 Hz, respectively. The bandwidth typically ranges between 1–2 Hz. It follows that the stability of the excitation frequency is their critical parameter. An approach is to order generators that are tuned to application frequencies.
ReVibe Energy sells converters for frequencies from 20–200 Hz with power output from 500 mW to a few W [16]. Figure 2.2 depicts an exemplary generator with equipment from ReVibe Energy.
Figure 2.2 ReVibe Energy kinetic converter. (Photo reproduced by permission of ReVibe Energy, 2019).
Non-resonant vibrational harvesters have been a research topic since the beginning of micro energy harvesting. One approach is to tune the electronics [24], the other is to adjust the springs and the length of the resonating mass [25]. Nanoscale generators for implementation in clothes, for example, have been developed by the Georgia Institute of Technology with a corresponding power density of 10 mW/cm2 [26].
Piezo generators and engineering services for customized generators are offered by different companies, such as Piezo.com [27]. Piezoelectric generators mounted on the wings of beetles yielded 11.5 and 7.5 µW during flight in prototypes from the University of Michigan [28]. The recent state-of-the-art in piezoelectric harvesting has been published by Safaei and coworkers [29].
As in most energy harvesting generators, characterization standards with reproducible test conditions are missing, including test temperature and test frequencies. However, regarding their industrial stage, kinetic converters have achieved a mature state.
2.2.1.3 Flow of Gas and Fluids
The theoretical limit of usable power Pmax of a gas flow from an electric turbine is defined by the conversion coefficient cP following the Betz limit [30]
(2.3)
where A denotes the flow area, ρ the density of theflow medium and v its speed. cp is defined by the ratio of the flow velocity v1 before the rotor of the turbine to the flow velocity v2 after the rotor
(2.4)
with cp(x) defined as
(2.5)
and the rotor speed v as
(2.6)
The function has a maximum with x = 1/3 with power coefficient von 16/27. This equals 59.3% of the incoming kinetic power of the theoretical limit of the electrically usable power. The Betz limit covers flow losses due to the design of the rotor blades. Additional mechanical losses are caused by fraction, the inertia of the bearing, and the converter behind the rotor, typically electromagnetic. So far, large wind turbines demonstrated a conversion efficiency of about 40%. New types of design might exceed those limits. However, microturbines are expected to reach a range of 10% due to nonlinear effects and the small ratio of converter to loss surface [13, 31].
The flow speed in HVAC plants can reach up to 6 m/s. For an air density ρ = 1.2 kgcm-3, an efficiency of 10% and an area of 1 cm², an electric power of 1.3 mW could be achieved. The current European low-pressure systems reach up to 3 m/s, with a resulting power output of approximately 0.2 mW.
Although their maximum power limit is significantly below the limits of large gas flow turbines, indoor air flow of all sources can be a very valuable power source with achievable power in the milliwatt scale. This is sufficient to power most IoT applications.
2.2.1.4 Acoustic Vibrations
Acoustic vibrations in indoor environments are available from humans, machines and outdoor vibrations, including traffic. Their harvest is one of the youngest disciplines in Micro Energy Harvesting with applications ranging from typical IoT systems, noise reduction to espionage and hacking. Typical designs are based on, but not limited to, the use of electromagnetic and piezoelectric converters or a combination of both. Yuan et al. provided a recent review of the field [32].
Most converters achieve a power in the range of a few microwatts with converters reaching the lower milliwatt scale as the exception.
2.2.1.5 Elastic Energy
Elastic energy has had a long tradition in human application, long before electricity was discovered. Watches and clocks have been built based on winding springs that stored energy in its elastic form, which then actuated the pendula and the mechanics. The first turntables and many other devices used the same principles. A common example of an elastic micro energy harvester is the Seiko Kinetic watch.
2.3 Thermoelectric Conversion
Thermal energy can be converted to electric energy with thermoelectric generators (TEGs). In their typical design, TEGs consist of semiconductor materials. Thermocouples, that is pairs of p- and n-type legs, are connected in parallel thermally and in series electrically. The main physical effects in TEGs are the Seebeck, Thompson and Peltier effect, respectively, and
