07. generators for portable power applications

8/12/2019 07. Generators for Portable Power Applications

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158 7 Generators for portable power applications

(a) (b)

Fig. 7.1. Microgenerator pack for soldiers rated at 80 W: (a) micro engine driven PMgenerator; (b) generator pack [52]. Photo courtesy D-star Engineering Corporation ,Shelton, CT, U.S.A.

V/28 V d.c. energy for charging 12/24 V batteries. The basic dimensions of pack are 127×127×127 mm including a wrap-around fuel tank. The weight is

Fig. 7.2. Micro gas turbine layout. 1 — stator of generator, 2 — compressor, 3 —air bearings, 4 — turbine, 5 — combustion chamber, 6 — regenerator, 7 — rotor of generator. Courtesy of Power MEMS , Leuven, Belgium, [120].




(a) (b)

Fig. 7.4. Construction of generator/motor for Power MEMS micro gas turbine: (a)3D layout; (b) magnetic flux paths. 1 — stator, 2 — PM, 3 — phase 1, 4 — phase2, 5 — phase n, 6 — rotor, 7 — magnetic flux [184].

The PM is ring shaped and axially magnetized (Fig. 7.4a). Magnetic fluxpaths are shown in Fig. 7.4b. The PM flux crosses the air gap and enters therotor. Then it goes towards the outer diameter where again it crosses the airgap towards the stator teeth. The stator back iron creates a return path forthe magnetic flux.

(a) (b)

Fig. 7.5. 1 kW, 452 000 rpm mini generator: (a) ironless stator; (b) PM rotor.Photo courtesy of Calnetix , CA, U.S.A.

Calnetix , CA, U.S.A. has developed a 1 kW, 452 000 rpm PM brushlessminigenerator (Fig. 7.5) for U.S. Air Forces (USAF). This is an ironless designwhere the stator is flooded with oil to intensify cooling and increase power



7.1 Miniature rotary generators 161

density. The machine was originally designed to operate on ball bearings. Theefficiency is about 94 %.

(a) (b)

Fig. 7.6. Portable mini gas turbine system developed at Tohoku University, Japan:(a) rotor of turbine integrated with rotor of electric generator; (b) gas turbine engine.Photo courtesy of University of Tohoku, Japan.

Researchers at Tohoku University, Nano-Precision Mechanical FabricationLab, Japan have developed a palm-sized gas turbine engine to power au-tonomous robots and serve as a portable engine for personal transportation

devices for elderly. The tiny engine measures 100 mm in diameter and 150mm in length. With air bearings, 16 mm compressor rotor diameter, 17 mmturbine rotor diameter and combustion chamber, the engine can develop arotational speed of 500 000 to 600 000 rpm (Fig. 7.6). It has been not decidedyet which type off electric generator will be integrated with mini gas turbineengine.

(a) (b)

Fig. 7.7. Micro gas turbine engines for reluctance micro starter/generators: (a)turbine engines (b) generator. Photo courtesy of University of Leicester, U.K.




Fig. 7.10. Stator winding patterns for 8-pole microgenerator: (a) 2 turns per pole;(b) 3-turns pole [16]. Photo courtesy of Georgia Institute of Technology, Atlanta,GA, U.S.A.

Table 7.1. Specifications of a disc type microgenerator with stator electroplatedwinding fabricated at the School of Electrical and Computer Engineering, GeorgiaInstitute of Technology, Atlanta, GA, U.S.A. [16, 17].

Number of poles 2 to 12Number of turns per pole 1 to 6Outer end turn extension 0 to 2.5 mmInner end turn extension 0 to 2.5 mmMagnet outer radius, mm 5.0

Magnet inner radius, mm 2.5Stator radial conductor outer radius, mm 4.75Stator radial conductor inner radius, mm 2.75Hiperco ring thickness, µm 500Magnet thickness, µm 500Radial conductor thickness, µm 200End turn thickness, µm 80Substrate thickness, µm 1000Air gap, µm 100Power electronics equivalent resistance, mΩ 100

A 16 W of mechanical–to–electrical power conversion and delivery of 8 Wof d.c. power to a resistive load at a rotational speed of 305 krpm has beendemonstrated [16]. The power density per volume of the disc type microgen-erator with stator electroplated winding was 59 W/cm3.

7.2 Energy harvesting devices

Self-powered microsystems have recently been considered as a new area of technology development. Interest in self-powered microsystems have been ad-



7.2 Energy harvesting devices 165

Fig. 7.11. Electro mechanism of Seiko kinetic watch.

Fig. 7.12. Electromechanical generator harvesting vibration energy (46 µW, 428mV rms, 52 Hz, 1 cm3 volume). Four PMs are arranged on an etched cantilever beamwith a wound coil located within the moving magnetic field [199]. Photo courtesy of University of Southampton, U.K.

dressed in several papers, e.g., [7, 75, 112, 119, 172, 173]. Main reasons whichstimulate research in this new area are:

• large numbers of distributed sensors;• sensors located in positions where it is difficult to wire or charge batteries;• reduction in cost of power and communication;• Moore’s law (the number of transistors per unit area of an IC doubles

every 18 months).

Microsystems can be powered by energy harvested from a range of sources

present in the environment. Solar cells, thermoelectric generators, kinetic gen-erators, radio power, leakage magnetic or electric fields are just a few examples.In some applications, e.g., container security systems, condition monitoring of machine parts (motors, turbines, pumps, gearboxes), permanent embedding




Fig. 7.13. NightStar R flashlight with moving magnet linear generator. 1 — sta-tionary coil, 2 — moving magnet, 3 — repulsion magnet, 4 — capacitor and chargingcircuit, 5 — LED, 6 — precision acrylic lenses, 7 — sealed magnetic switch. Photo

courtesy of Applied Innovative Technologies , Fort Lupton, CO, U.S.A.

F

1 2 3 4 5

B

Fig. 7.14. Principle of operation of a simple electromechanical energy harvestingdevice. 1 — stationary coil; 2 — NdFeB PMs; 3 — mild steel magnetic circuit;4 — cantilever beam (flat spring); 5 — base [73].

in inaccessible structures (bridges, towers, roads), or animal tracking, the onlysource of electrical energy is the kinetic energy.

The harvesting of kinetic energy is the generation of electrical power fromthe kinetic energy present in the environment. The nature of the kinetic energyharvesting mechanism in a self contained system depends upon the nature of




l M

g

N S

mild steel

c o i l

c o i l

hM

hM

Fig. 7.15. Magnetic field distribution excited by NdFeB PMs and coil current (300turns, 0.07 A) in an electromechanical energy harvesting device with cantilever beam[73].

the application [75, 112, 183]. Kinetic energy harvesting devices can be dividedinto two groups:

• acceleration/vibration and spring mass system devices, e.g., kinetic watchesAsulab (Swatch Group) and Seiko (Fig. 7.11), cantilever beam vibrationgenerators (Fig. 7.12), moving magnet linear generators (Fig. 7.13);

• repeated straining physical deformation devices, e.g., piezoelectric genera-tors or magnetic shape memory (MSM) generators [119, 173, 183].

The Seiko kinetic watch (Fig. 7.11) uses the movement of the wearer’s armto produce the electrical energy to keep the watch running. The movementof wrist rotates the oscillating pendulum (weight) attached to a relativelylarge gear which is engaged with a very small pinion. The pendulum canspin the pinion up to 100 000 rpm. The pinion is coupled to a miniatureelectrical generator which charges a capacitor or a rechargeable battery. Oncethe kinetic watch has been fully charged, the wearer can enjoy a full six monthsof continuous use.

In electromechanical energy harvesting device with cantilever beam (Figs7.12, 7.14) the input mechanical energy coming from external source of vibra-tion (vehicle, marine vessel, engine, road, etc.) is converted into the outputelectrical energy. A spring–mass system with moving PM is mechanically ex-cited by external vibration. The voltage is induced in a stationary coil that isembraced by PM poles. When the coil is loaded with an external impedance,




shaker

PM

coil

cantilever

beam

terminals

yoke

Fig. 7.16. Prototype of energy harvesting electromechanical device placed on asmall variable-frequency shaker [73]. Photo courtesy of United Technologies ResearchCenter, East Hartford, CT, U.S.A.

an electric current proportional to the induced EMF arises in the external cir-cuit. Fundamental equations for performance calculations can been derived on

the basis of elementary beam theory and circuit analysis. The magnetic fielddistribution excited both by PMs and coil, as obtained from the 2D FEM, isshown in Fig. 7.15.

In NightStar R flashlight (Fig. 7.13) the kinetic energy of motion is trans-formed into electrical energy by means of repeatedly passing a PM througha stationary coil. Stationary PMs oriented to repel the moving magnet aremounted at both ends of the flashlight. Repulsion PMs smoothly deceler-ate and accelerate the moving magnet back through the stationary coil. Ki-netic energy is therefore efficiently converted into electrical energy. The gener-ated electrical energy is stored in a capacitor. NightStar R is most effectivelyrecharged when it is turned off and shaken between two and three times persecond over a distance of approximately 5 cm. On a full charge NightStar Rwill provide highly effective illumination for over 20 minutes.




0

2

4

6

8

10

12

14

16

18

0 5 10 15 20 25 30 35

frequency, Hz

o u t p u t p o w e r , m W

output power, 5-mm beam




Fig. 7.17. Output power versus frequency for different lengths of the steel cantileverbeam of an electromechanical energy harvesting device shown in Fig. 7.16 [73]

.

Portable electromechanical energy harvesting devices with the magneticfield excitation system integrated with the cantilever beam (flat spring) anda stationary multiturn coil are the most efficient devices (Figs 7.12, 7.14 and7.16) [73]. The maximum generated energy is when the mechanical resonance occurs, i.e., when the natural frequency of the cantilever beam-based vibratingsystem is the same or close to the input frequency of vibration. The outputpower for different lengths of the steel cantilever beam of an electromechanicalenergy harvesting device shown in Fig. 7.16 is plotted versus frequency in Fig.7.17. Potential applications of energy harvesting devices include:

• condition-based monitoring of machinery and structures;• wireless sensors installed in security systems of containers or trailers,• implanted medical sensors;• wearable computers;• intelligent environments (smart space ), etc.

It is, in general, not difficult to design electromechanical energy harvestingdevices in the range of microwatts, but it is very difficult to design properlyfunctioning devices rated at milliwatt level. Electromechanical energy devicesare down scalable to the microelectromechanical system (MEMS) levels. Inthis case such effects as micro-collisions, air friction dissipation, and cushioningeffects cannot be ignored in their analysis and synthesis [73].

07. generators for portable power applications

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