Bladeless wind power harvester and aeroelastic harvester

Chapter 4

Bladeless wind power harvester and aeroelastic harvester Chapter outline 4.1 Bladeless electromagnetic energy harvester driven by airand water-flow 4.1.1 Measurement configurations and design parameters 4.1.2 Experimental results 4.1.3 Summary

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4.2 Aero-elastic-piezo-electric energy harvester 4.2.1 Measurement of configurations and design parameters 4.2.2 Experimental results 4.2.3 Concluding remarks References

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358 361 367 368

Recently, there are strong industrial and academic interests on developing miniature energy harvesters so that ambient energy associated with wind or rainwater may be harnessed and converted into electrical power. It is aimed to meet the increased power demand resulting from the wide application and rapid development of electronic devices, such as LED lights and mobile phones. A number of energy harnessing technologies have been successfully developed and implemented to achieve this milestone. Generally, the energy harvesting techniques are developed and designed on the effects and working principles of thermosacoustic [1–9], magneto-electric [10], piezo-electric [11,12], thermos-electric [13,14] or opto-electric [15]. There are many renewable energy resources. Two of the resources include rainwater or air flow [16,17], and their mechanical energy may be harnessed. The power generation technology from wind or flowing water [16,17] is well-developed and commercially available by implementing large-size wind- or hydro-turbines. A classical hydro-turbines is an impulse one, which extracts the kinetic energy of moving water via rotating large-diameter blades. The blade diameter is typically in the unit of meter. Hydro- and wind-power harvesting systems/devices work as reliable energy source in the past 3 decades. They are playing more and more important roles and become major industries for electric power generation. The total contribution from those hydro- and wind-power systems to the global power consumption has dramatically increased. This is due to the fact that wind- and hydro-power help on abating

Wind Turbines and Aerodynamics Energy Harvesters. https://doi.org/10.1016/B978-0-12-817135-6.00004-1 © 2019 Elsevier Inc. All rights reserved.

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global warming by reducing CO2 emission resulting from power generation. However, the effects (direct or indirect) of implementing hydro- and windturbines on aquatic and other wildlife are typically neglected and not well addressed. Neither does the local concerns of visual intrusion of such large or huge turbines. Finally, another real challenge for implementing large hydro- and wind-turbines to be addressed and solved is the by-product of hydro- and aero-dynamic noise. These factors contribute to the social opposition to implementing large air- or water-driven energy harvesters. Therefore, it is becoming more and more popular to develop ‘small’ or ‘micro’ energy harvesters [18,19]. Compared with the conventional large energy harvesting systems, these small harvesters are more attractive due to (1) the reduced visual and ecological impacts and (2) the high power to weight ratio. The power production performances of hydro- and wind-turbines are related strongly to the hydro- or aerodynamic characteristics of turbine runner blades [16]. Thus it is critical to determine the optimum design of the blade profiles and angle of attack (AOA). The performances of different blade profiles at different wind speeds and angles of attack can be investigated by solving Reynoldsaveraged Navier–Stokes (RANS) equations via FV (finite volume) method [18]. Conventional energy harvesting systems driven by air- or water-flow involve implementing large-diameter blades (>1 m) [20]. However, bladeless energy harvesting systems/devices [21–28] could be as efficient, low-cost and reliable as those bladed ones. The bladeless system has many advantages [29,30]. The first and most important one is that it is simple to build. The components are easy to fabricate and manufacture, as the rotor assembly comprises on flat smooth discs instead of the complex geometries involved with curved blades. This will result in lower manufacturing costs as well as a better quality control of the discs. The simplicity of the design allows for parts being easily interchanged and adjusted. An example would be the modification of the discs and the spacing between them. As a result of such a flexible design, ease and subsequently lower costs of maintenance is possible. The faulty component may be easily removed and replaced [31,32]. Hence, the bladeless turbine offers the advantage of cheap construction and maintenance, whilst allowing for flexibility in altering the turbine parameters. The second advantage of the bladeless turbine is safety. Compared to its conventional bladed turbine counterparts, the bladeless turbine is superior in terms of safety. In the case of a bladed turbine, when the blades fracture, the shrapnel from the fractured fragments have been documented to slice through hydraulic lines and control surfaces of engine systems. For the bladeless turbine, previous studies have shown that at high speeds of 85,000 rpm, a critical component failure would result in the implosion into numerous tiny fragments, which are subsequently ejected via the exhaust [8]. In addition, the stator assembly could be made of a strong material, which will contain the failed fragments within its case.

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The third advantage of the Bladeless Turbine is flexibility in operations [9]. The bladeless turbine can be used in conjunction with a wide variety of working fluids, not limited to water, steam, conventional air, corrosive and/or abrasive working fluids [33]. In addition, the bladeless turbine has a large operating envelope with regard to temperatures. The removal of such limitations opens up numerous possibilities of operations, in which a conventional turbine may not be suitable for (e.g. abrasive working fluids damaging conventional turbine blades). The fact that the bladeless turbine can run on steam alone, giving rise to more practical applications and thus reducing the carbon footprint of energy generation. The final advantage of the bladeless turbine is that it does not suffer from cavitation of the working fluids, this result in longer disc operating life. The turbine is also simple to use due to its start–stop operating nature, quiet operation and experiences a low mechanical stress due to its small size and good ability to function at low speeds. All these features indicate that the bladeless system can be used for developing a miniature energy harvesting system. Abundant rainfall is received in many tropical countries throughout the year. For example, the annual rainfall in Singapore is about 2400 mm on average. Such rainfall is a renewable source of energy harvesting. It has great potential to be utilized for both rainwater collection and power generation purposes. In this chapter, miniature energy harvesters driven by air or water are designed and tested. Experimental studies on air-driven harvesters are performed first to shed light on its working principles and performances, as the flow rate is set to four different values. For this, three harvesters with different diameters are designed and tested. This is described in Section 4.1. The working principles and numerical simulations are also discussed. The harnessed electrical power could be measured in either open- or closed-loop electrical circuit configuration, as discussed in Section 4.1.1. In addition, four critical design parameters are identified. In Section 4.1.2, experimental results are shown and discussed. Finally, in Section 4.1.3, the potential of applying such miniature bladeless system for harvesting energy from rainwater is experimentally investigated. Comparison is then made between the rainwater-driven harvester and the same-size airdriven one.

4.1 Bladeless electromagnetic energy harvester driven by air- and water-flow A miniature bladeless energy harvester is experimentally tested. It is driven by turbulent air flow and designed for exploring an alternative device/system to harness mechanical energy of air or rainwater flow. The energy harnessing device is shown in Fig. 4.1A. It includes a number of co-rotating CDs (compact discs) (see Fig. 4.1B). These discs are attached to a central shaft and they are evenly and axially spaced. To produce electricity, a magnet is attached to the shaft. The inter-disc gap (axial distance) could be changed from 0.2 to

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Bladeless harvester

Magnet south pole

Air flow inlet

Magnet north pole

Blu-tack Wound coil

Loading resistance/LEDs

G-clamp

Acrylic cylinder

Exhaust orifice

(A)

(B)

FIG. 4.1 (A) Schematic of a bladeless electromagnetic energy harvester, (B) photo of compact discs [34]. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

3.5 mm. In addition, three harvesters with disc diameters of 120, 80 and 40 mm are built. The minim disc is chosen to be 40 mm. This enables the disc diameters of these three harvesters being varied with a constant step size of 40 mm. The main working principle of the bladeless systems with rotating discs is that air or water is tangentially (or nearly) injected. The injected air or water flows through narrow gaps between the discs and moves spirally towards the exhaust orifices, which are located at the centre of each disc [27]. The relative motion between the disc and the working fluid gives rise to a viscous drag force. It leads to the disc rotating. The rotating discs are typically enclosed in a cavity with a small clearance in both radial and axial directions. Let’s assume that the air or water is injected at a velocity U through a stationary disc as shown in Fig. 4.2A. Then the relative velocity of the disc is
U. The viscous drag force

Ζ ω Δd R0 Ri

(A)

(B)

FIG. 4.2 (A) Schematic diagram of two neighbouring discs, (B) photo of a 120 mm disc with exhaust orifices [34]. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

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on the discs will act in the direction of the air/water flow, opposing the motion of the working fluid. Because there is a relative velocity between the working fluid and the disc surface, a velocity gradient near the disc surface is present. This leads to the generation of a shear stress s τw: τ w ðr Þ ¼ μ f

∂U θ ðr, zÞ ∂z

(4.1)

The shear stress τw contributes to the generation of a torque Π on the disc. It is determined by integrating the elemental torque over the entire surface of the disc as ZR0

Z2π Π¼2

τw ðrÞr2 dr

dθ 0

(4.2)

Ri

If the produced torque Π is larger than the frictional one, then the discs start rotating. As the disc rotation speed is increased, the velocity of the working fluid relative to the disc is decreased. Finally, a steady state at which the frictional toque is equivalent to Π is reached, and the discs rotate at a constant speed. When the working air is replaced with water, the viscosity effect on the energy harvesting performance can be evaluated. This will be discussed later. To shed lights on the aerodynamic torque driving the rotating discs via injecting the air to the turbine, numerical studies are performed by using ANSYS CFX [28–30]. Here, ideal gas is assumed to be the working medium. Thus the thermodynamic state equation holds: p c2 ¼ RT ¼ γ ρ

(4.3)

Navier–Stokes equations are solved numerically by using the CFX-mesh as shown in Fig. 4.3 [28,30]. The three dimensional flow is turbulent.

FIG. 4.3 Mesh generated for CFD simulations [34]. (A) 3D mesh and (B) cross-sectional view of the disk mesh. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

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The state-of-the-art CFD code employs a control volume based method to solve the unsteady conservation equations as given as: (1) mass conservation: ∂ρ + r∙ðρ∙UÞ ¼ 0 ∂t

(4.4)

(2) Momentum conservation: ∂ρU + r∙ðρU  UÞ ¼
rp + r∙τ ∂t

(4.5)

(3) Energy conservation: ∂ρHtot ∂p
+ r∙ðρUHtot Þ ¼ r∙ðλrTÞ + r∙ðU∙τÞ ∂t ∂t

(4.6)

where Htot is the total enthalpy. It is related to the static enthalpy H(T,ρ) by Htot ¼ H + U2/2. T is temperature, λ denotes the thermal conductivity and τ is the stress tensor and it is related to the strain rate by   2 (4.7) τ ¼ μ rU + ðrUÞT
δr∙U 3 where δ is the identity matrix. Superscript T denotes the transpose. The boundary conditions are set to be velocity inlet and pressure outlet respectively. The turbulence model is k
ε and the number of meshes is 3,841,356. Note that in order to capture the boundary layer superposition and accurate solution, a finer structured mesh is applied at the vicinity of the disc solid surface (boundary layer), as shown in Fig. 4.3. Eleven nodes are generated between two neighbouring discs in the axial direction. The disc edges with the desired intervals and ratio of increasing or decreasing the cell width are meshed first. The boundary layer thickness for turbulent flow [28] is approximated by using δb ¼ 0.036φ(μ/ρUφ)1/5 at zero angle of incidence. Here φ is the length downstream from the start of the disc, and U is the working fluid’s bulk velocity. In the axial direction
δb < z < δb , the region near the disc surface is not movable. As the system governing equations are iteratively solved, the pressure distribution along the surfaces of discs placed at two different locations are determined, as shown in Fig. 4.4. When the inlet volume flow rate is chosen to two values, the pressure distributions reveals that the maximum pressure is different but the patterns are similar (see Fig. 4.4A and C or Fig. 4.4B and D). Closer observation shows that there is a great pressure gradient. In addition, with the working fluid flowing towards the disc centre, its radial pressure is decreased, since the flow area is gradually decreased and the radial velocity

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FIG. 4.4 Calculated pressure distribution along the side- (A) and (C) and centering-disc (B) and (D): (A and B) the volume flow rate is set to 0.00502 m3/s, (C and D) the volume flow rate is set to 0.00328 m3/s [34]. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

is increased. The pressure gradient gives rise to the shear stress. Thus the aerodynamic torque is produced to drive the discs to rotate. Although the present numerical simulations shed light on the working principles and physics of the disc-involved energy harvester, there is no direct comparison between the numerical and experimental results. This is due to the fact that the experimental tests are performed in terms of the electrical power output only and there are no detailed flow field measurements. With the compact discs and the shaft rotating resulting from the aerodynamic torque Π, the magnet attached at the end of the shaft rotates too. The magnet rotation in a wound coil of copper leads to the electrical power being produced, since the magnetic flux is changed periodically (sinusoidally) in the coil [31]. If the generated emf (electromotive force) as measured in real-time is sinusoidally varied, then the current I and the electrical output power E of the closed-loop electrical circuit are alternating ones as I¼

uN u2 ，and E ¼ N Rtot Rtot

(4.8)

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where Rtot denotes the total resistance of the closed-loop electrical circuit. The emf uN is generated due to the magnetic flux varied in the cooper wire loop. Here the cooper wire loop is stationary and has N turns in our experiment. In theory, uN is proportional to the rate of the magnetic flux change through the circuit surface S. According to Maxwell-Faraday’s law [31]
r  E ¼ ∂ B/∂t, uN could be determined in integral form by applying Stokes’ theorem as I Z Z Z Z ∂ ðr  EÞ∙ndA ¼
N B∙ndA (4.9) uN ¼ N E∙ds ¼ N ∂t T s

s

where T is a closed curve, S is the surface bounded by it, E is the electric field and B is the magnetic field. Here it is assumed that emf produced from each turn of the cooper coil is the same. Substituting Eq. (4.9) into Eq. (4.8) gives 2 0 13 2 2 0 13 2 Z Z Z Z 4∂ @ 4∂ @ B∙ndAA5 B∙ndAA5 ∂t ∂t s s max max ¼N (4.10) E ¼ N2 2R0 2Ri It can be seen that and the generated electrical power E is N2 times larger than the wound cooper coil with 1 turn, as R0 ¼ NRi remains constant. Eq. (4.10) reveals that more electricity could be produced, if the number of the copper wire turn N is increased. Furthermore, if N remains unchanged, the power output E and voltage uN are related to the rate of the magnetic flux change as,   ∂ð∙Þ ∂ð∙Þ 2 (4.11) ，and E∝ uN ∝ ∂t ∂t Eq. (4.11) is validated and confirmed by our experimental measurements as discussed later.

4.1.1 Measurement configurations and design parameters In practice, there are two typical configurations to measure the electrical power generated by an energy harvester. One configuration is open-loop, as shown in Fig. 4.5A. A multi-meter is implemented directly to measure the voltage output and electrical power. Here R0 denotes the coil wire resistance. The other configuration is closed-loop. A known resistor with an resistance of RL is applied to build a closed-loop electrical circuit. As the voltage across the resistor is measured, the electrical current and power generated by the energy harvester are then determined. Experimental measurements are conducted in both configurations, since some researchers/readers could be interested in the electric power or current, which might be measured in open- or closed-loop configuration.

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Harvester

V

RL

Multimeter

(A)

V

Resistor

(B)

FIG. 4.5 Open- and closed-loop electrical circuits built by the energy harvester and the loading resistor [34]. (A) Open-loop configuration and (B) close-loop configuration. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

TABLE 4.1 Design parameters to be studied [34]. Parameters

Names

Unit

D

Disc diameter

mm

Δd

Inter-disc distance

mm

N

The number of discs

J

The number of orifices

Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.

Four design parameters are identified and evaluated, as summarized in Table 4.1. These parameters play important roles on affecting the miniature harvester performance. The impact of each given parameter is examined later by performing a parametric measurement in terms of the electrical output power.

4.1.2

Experimental results

The power output Eo as revealed in Eq. (4.10) could be increased by increasing the number N of the copper wire turns. This finding is validated by our experimental results, as illustrated in Fig. 4.6. It shows that the output RMS (root mean square) voltage and power are varied, as N ¼ 280 or N ¼ 560 and the inlet volume flow rate is set to four different values. It is clear that the 560 turns produces approximately 100% more output voltage and power in comparison with

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5

N = 280 N = 560

4.5

0.3 Output Power E0 (Watt)

Output RMS Voltage (Volts)

4 3.5 3 2.5 2 1.5

0.25 0.2 0.15 0.1

1 0.05

0.5 0

(A)

1

4 5 2 3 Volume flow rate (m3/s) × 10−3

0

6

1

(B)

2 3 4 5 Volume flow rate (m3/s) × 10−3

6

FIG. 4.6 Comparison of the measured voltage and power output from the 80 mm air-driven harvester, as N is set to 280 and 560 respectively [34]. (A) Output RMS voltage and (B) output power E0. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

that of 280 turns. The reason is that because the electrical resistance of the cooper coil is proportional to its length i.e. R0/N  Ri. And this explains why E0 ∝N. Note that the cross-sectional area of the inlet nozzle is measured. The average velocity of the air flow is measured by using a constant temperature HWA (hot wire anemometry), which is placed near a sidewall exhaust orifice. The hot wire probe is calibrated in a wind tunnel at the Fluid Mechanics Laboratory in Nanyang Technological University. Singapore, before it is implemented to measure the air jet velocity. The hot wire probe is connected to a mini CTA 54T30 system from DANTEC dynamics. It is then linked up with NI USB6008 card from National Instruments for data acquisition. If air leakage is assumed to be negligible and the volume flow rate from the exhaust orifice is assumed to be the same, then the discharge of the air flow could be determined. Eq. (4.11) reveals that the output voltage and power from the harvester are strongly related to the rate of the magnetic flux change. The variation of the electrical output from the 80 mm harvester with RPM (revolutions per minute) is shown in Fig. 4.7. The electrical power E0 is measured by applying Fluke 2 digital multi-meter. Here the following relationship E0 ¼ V2rms/R p0ffiffiffi¼ V0/2R0 is applied. The output voltage is denoted by V0 and Vrms ¼ V0 = 2 denotes its RMS (root mean square). It can be seen from Fig. 4.7 that the output RMS voltage is increased linearly with RPM. And the output power E0 is quadratically increased. The maximum output electrical power is approximately 0.3 W, as RPM  6300. For measurements, AI3030 Tachometer is applied.

4

0.4

3.5

0.35

3

0.3

Output Power E0 (Watt)

Output RMS Voltage (Volts)

Bladeless wind power harvester and aeroelastic harvester Chapter

2.5 2 1.5 1 0.5 0 0

(A)

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experiment prediction

0.25 0.2 0.15 0.1 0.05

2000 4000 6000 RPM (rev/min)

0 0

8000

(B)

0 2000 4000 6000 RPM (rev/min)

8000

FIG. 4.7 Variation of output RMS voltage and power from the 80 mm harvester with revolutions per minute (RPM) of the discs [34]. (A) Output RMS voltage and (B) output power. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

Time evolution of the instantaneous measurement of the AC voltage is measured by using Tektronix TDS1012B digital oscilloscope (100 MHz). This is shown in Fig. 4.8. It can be seen that two different regions are present in the oscilloscope trace of the acquired signal. One is the transient stage due to the inertia effect at t < 30 s. The other is ‘the stable stage’ at t > 30 s. The power output is determined when ‘saturated’ emf is produced. Parametric measurements are conducted to shed light on the effects of the design parameters on the energy harvesting performance and to find out the optimum design. There are four parameters as summarized in Table 4.1. The key parameters involve (1) the disc diameter D, (2) the number of discs N, (3) the distance between two neighbouring discs △ d, and (4) the number of orifices J characterizing the exhaust flow rate. Fig. 4.9 summarizes the output electrical power varied with these four main parameters. It can be seen that as the inlet volume flow rate is increased, the output electrical power is increased. Fig. 4.9A and B reveal that when the diameter D or number N of the discs is increased, the output electrical power is increased. However, the maximum power output is generated, as the inter-disc distance and the sidewall orifices’ number is ‘optimized’. The optimum Δ d and J are found to be approximately 0.5 mm and 8 respectively, as shown in Fig. 4.9C and D, as the inlet volume flow rate is small. These determined optimum values are applied later to design rainwater-driven miniature energy harvesters.

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15

emf (Volts)

10 5 0 –5

–10 –15 0

Electrical Voltage (Volts)

(A)

5

10

15

20

25

30

35

40

4 Measured numerical

2 0 –2

(B)

–4 0.3

0.4

0.5

0.6 0.7 Time (s)

0.8

0.9

1

FIG. 4.8 Variation of measured emf from the 80 mm aerodynamic energy harvester [34]. (A) Time evolution of the emf and (B) zoom-in graph during 0.3 < t < 1.0 s. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

As shown in Fig. 4.5B, the generated power could also be measured in closed-loop configuration via connecting a resistor. If the resistor’s resistance RL is mathematically equal to that of the copper wires, then electrical power output is maximized in theory. The variations of measured output emf and electrical power are shown in Fig. 4.10. It is apparent that the emf output is increased with the loading resistance RL. However, maximum output power is obtained as RL ¼ 146 Ohms. And the corresponding maximum current is about 40 mA. This loading resistance RL is approximately equal to the resistance of the copper wire. The practical application of such bladeless energy harvesting system is evaluated by using rainwater. It is collected and stored in a water tank with a volume of 20 L. This is schematically shown in Fig. 4.11A. A soft PVC pipe with a diameter D of 6.25 mm is applied to connect the bladeless harvester and water tank, as illustrated in Fig. 4.11B. Three different-diameter bladeless harvesters are experimentally tested. The minimum diameter of the disc is 40 mm, as shown in Fig. 4.11C. When the water tank is placed at about 2 m above the ground, a water flow with approximately 3.6 m/s is generated. Its average velocity is measured by using the conventional volumetric method. Since the inlet nozzle’s diameter is 6.0 mm and the cross-sectional area is known, the time (about 9.7 s) taken

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Output power Eo (Watt)

0.3

(A)

0.2

0.2

0.1

0.1

0 20

0 60 100 Disc diameter (mm)

140

Output power Eo (Watt)

0.3

0

4

(B)

8

12

16

Disc number

0.2

0.2 3

0.00238 m /s

0.1

3

0.00328 m /s

0.1

3

0.00426 m /s 3

0.00502 m /s 0 –1 10

(C)

0

10 Inter-disc distance (mm)

10

1

0 0

(D)

8 16 24 Sidewall Orifice number

32

FIG. 4.9 Variation of the output electrical power with the system parameters (A) disc diameter D, (B) the number of discs N, (C) inter-disc distance △d and (D) the number of exhaust orifices J, as the inlet volume flow rate is set to four-different values [34]. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

to discharge 1000 mL water is then determined and so the mean flow velocity. The measured velocity could also be determined in theory by applying Bernoulli equation [32] as Δp Δv2 X Δv2 +ΔH¼ + εi 2g 2g ρw g

(4.12)

where ρw is the water density, g is the gravitational acceleration, Δ p  0, since the water discharges into the atmosphere and the water in the tank is open to the atmosphere. Δ H ¼ 1.96 m is the elevation difference between the water level in the water tank and the bladeless turbine inlet, Δ v2 is the kinetic energy difference between the free surface of the water tank and the outlet of the pipe. However, since the tank is large relative to the pipe outlet, the ‘free surface’ P velocity is zero (assumed stationary in the tank), Δ v is the outlet velocity. εi is the total head loss coefficient. And it consists of the entrance loss ε1 ¼ 0.8, entrance elbow loss ε2 ¼ 0.3 and friction loss ε3.

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7 10

6

–1

Output power Ec (Watt)

Output emf (Volts)

5

4

3

2

10

–2

1

0 0 10

(A)

10 2

10 RL (Ohms)

10

4

–3

10

0

(B)

2

10 RL (Ohms)

10

4

FIG. 4.10 Measured electrical circuit voltage and power from the 80 mm air-driven harvester with varying resistance RL, as N ¼ 280 [34]. (A) Output emf and (B) output power Ec. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

εi is empirically chosen [32]. The friction loss ε3 ¼ f(L/D) depends on the pipe length L and diameter, D according to Darcye-Weisbach energy balance equation. The pipe friction factor is denoted by f, which is related to Reynolds number and the surface roughness ξ for turbulent flow as   ξ (4.13) f ¼ F Re D According to the Moody chart, the friction factor f is determined as 7.54  10
3. With these values determined, the flow velocity estimated from Eq. (4.12) is approximately 3.4 m/s. And the corresponding Reynolds number is Re ¼

ρw ΔvD ¼ 1:9  104 μw

(4.14)

Thus it is confirmed that the inlet water flow is turbulent. As the water flows in the bladeless harvester, electrical power is generated. The density of the rainwater is 991.65 kg/m3 at 23.8°C as shown in Table 4.2 and Fig. 4.12. The electricity produced from the rainwater-driven harvester could be used for many purposes. However, for demonstration, the bladeless harvester is coupled with a red LED (light-emitting diode) for convenience, as schematically illustrated in Fig. 4.11D. The demonstration application confirms that the miniature bladeless

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Water Tank

harvester

(A)

(B)

(C)

(D)

FIG. 4.11 (A) schematic of the experimental setup, (B) experimental setup of the 40 mm rainwater-driven harvester, (C) comparison of a 40 mm rotating disc with a 50 cent Singapore coin and (D) a red LED is powered by the 40 mm harvester (please refer to the video attached) [34]. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

TABLE 4.2 Measured density ratio of rainwater ρw with respect to tap water ρt by using volumetric method [34]. Measured volume (mL)

Net weight (g)

Density ratio

Rainwater

Tap water

Rainwater

Tap water

ρw/ρt

400

400

392.47

391.26

1.0031

700

700

696.70

692.96

1.0068

1000

1000

997.07

995.97

1.0012

Source: D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.

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Temperature (deg) (120.0) 31.1 29.6 28.1 26.6 25.1 23.6 22.1 20.6 19.1 (–40.0)

FIG. 4.12 Measured rainwater temperature contour by using an infrared thermal imaging camera [34]. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

harvester provides a simple, but robust, low-cost platform to harness the kinetic energy of rainfall to produce electricity, as shown in Fig. 4.8B. The harvester performance could be characterized by using the overall energy conversion efficiency η0, which could be determined in open-loop configuration as η0 ¼

E0 V2 =2R0 ¼ 0 2 Ei m _ w ΔV =2

(4.15)

Eq. (4.15) describes how efficiently of the total input energy being converted into electricity. η0 varied with the disc diameter D is shown in Fig. 4.13C. It can be seen that as the disc diameter D is increased, η0 is decreased. The variation of the measured output voltage and power are shown in Fig. 4.13A and B respectively. Similar trends are observed. The overall efficiency is also known as Brayton cycle thermal efficiency characterizing a conventional miniature gas turbine. It is typically about 2–3% [28], which is slightly less than that of the present harvester with a diameter of 40 mm. A closed-loop electrical circuit is created, as a known resistor is applied connected to the bladeless harvester. The voltage and current through the resistor and so the electrical power can be measured directly. The measured output voltage and power from the 40 mm rainwater-driven harvester in the closed-loop configuration are illustrated in Fig. 4.14A and B respectively. It is observed that the output voltage Uc is increased with increased loading resistance RL. However, when RL is matching the internal resistance of the harvester, i.e. RL  146 Ohms, the power output Ec is maximized. Furthermore, the output Uc and Ec from the 8-disc harvester is much greater than that from those obtained from

(A)

1.5 1 0.5 0 20

0.03

Output power E0 (Watt)

Output emf (Volts)

2

60 100 Disc diameter (mm)

140

60 100 Disc diameter (mm)

140

0.02

0.01 0 20

(B)

60 100 Disc diameter (mm)

140

Efficiency η

0

0.04 0.03 0.02 0.01

(C)

0 20

FIG. 4.13 Measured open-loop output voltage, power and efficiency η0, as the disc diameter in the rainwater-driven harvester is set to 40, 80 and 120 mm respectively [34]. (A) output emf, (B) output power Ec, and (C) efficiency. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

2.5

0.01

(A)

2

Output Power Ec (Watt)

Output RMS U0 (Volts)

water: 8 discs water: 4 discs

1.5

1

0.5

0 0

water: 8 discs water: 4 discs 500

1000

1500

0.008

0.006

0.004

0.002

(B)

0 0

0.01

c

0.01

Efficiency η

c

Efficiency η

1500

water: 8 discs air: 8 discs

water: 8 discs water: 4 discs

(C)

1000

0.015

0.015

0.005

0 0

500

500 1000 RL (Ohms)

0.005

0 0

1500

(D)

500 1000 RL (Ohms)

1500

FIG. 4.14 Measured electrical circuit voltage and power from the 40 mm rainwater-driven harvester with varying resistance RL, as R0 ¼ 146 Ohm [34]. (A) Output RMS, (B) output power, (C) efficiency, and (D) efficiency. (Adapted from D. Zhao, C. Ji, C. Teo, S. Li, Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air, Energy 74 (2014) 99–108.)

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the 4-disc harvester. As far as the measured overall efficiency ηc ¼ Ec/E is concerned, same trend can be seen in Fig. 4.14C. Finally, the efficiency ηc of the rainwater-driven harvester is compared with that driven by air flow, as shown in Fig. 4.14D. The efficiency ηc of the rainwater-driven harvester is found to be much larger due to the increased viscosity, i.e. μw > μa. As revealed by Eqs (4.1) and (4.2), increased shear stress (viscous drag) gives rise to increased torque. This confirms the important role of the viscosity being played in determining the output performance of the bladeless harvester. Note that these miniature bladeless harvesters are applied to rainfall collection systems or rivers. However, the harvesters are designed to harness more renewable energy in Singapore with abundant rainfall, with the overall aim of maximizing the abstraction of rainwater collection from Changi airport or residential buildings.

4.1.3 Summary In summary, the feasibility study of harnessing the kinetic energy of turbulent air and rainwater flow is conducted via evaluating the performance of three different diameter bladeless systems. The miniature bladeless system provides a simple but reliable platform to generate electrical power by using a number of co-rotating discs. The working principle of the harvester is to take advantage of a shear stress force and torque, which is generated due to the presence of the velocity gradient between the working fluid and the compact discs. Numerical studies are performed first to gain insights on the aerodynamics of the rotating discs driven by injecting turbulent air flow. The designed harvesters are bladeless and electromagnetic-coupled. The harvesters are demonstrated to be applicable by using either air or water flow. Compared with the conventional blade-involved harvester, this is one of the attractive features. Experimental measurements on the system performance and responses reveals that approximately 0.3 W electrical power can be produced. To shed light on the effects of (1) the disc number, (2) diameter, (3) the exhaust flow rate and (4) the inter-disc distance, parametric measurement studies of the system performance is then performed. Finally, a 40 mm bladeless harvester is designed, manufactured and experimentally tested in order to harness energy from rainwater flow. The rainwater-driven harvester are found to works more efficiently due to the dramatically increased viscosity in comparison with the same-size air-driven one. Moreover, the overall efficiency of the harvester is shown to be approximately 2–3% in terms of energy conversion. The efficiency is found to be slightly higher than a conventional miniature gas turbine. However, the overall efficiency is decreased with increased disc diameter D. The maximum electric current is approximately 4.5 mA. To demonstrate its practical application, the electricity generated from the 40 mm water-driven harvester is used to power a red LED. The present research reveals

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that there is a great potential of applying such small-scale bladeless systems to harness rainwater energy in countries with abundant rainfall, like Singapore.

4.2

Aero-elastic-piezo-electric energy harvester

Ambient environment contains great amount of energy in different forms to be potentially harnessed. The available energy could be harvested via different energy transform mechanisms. By now, many devices and technologies are developed and tested to extract useful energy from the ambient environment. The energy is harnessed by applying magneto-electric [10,12], piezoelectric [35–38], thermos-acoustic [38,39] and thermos-electric generators [18–20]. As one of renewable and low-cost (even free) energy sources, wind flow is typically available. Electrical power is produced via horizontal or vertical axis wind turbines [40–43]. Wind turbines technologies are well-developed and commercially available. The diameter of the conventional wind turbine blade is in meters. Such large blades is associated with a high tip speed and thus produces loud aero/hydrodynamic noises [12,44,45]. The performance of these turbines could be deteriorated with laminar separation occurred at a very low Reynolds number [18]. However, as the demand for small or portable electronic devices is increased, there is a strong industrial need to harness kinetic energies from wind to power remote sensors or LED lights [46,47]. An interesting but classical energy harvesting approach is to apply low-frequency piezoelectric generators [36,40–42]. There are a number of previous studies on taking advantage of piezoelectric generators to harvest ambient energy from natural (wind) or forced air flow [48– 50]. Li et al. [41] conducted experimental measurements on the performances of a wind-driven piezoelectric generator, as it undergoes either parallel- or crossflow flutter. They found that cross-flow flutter can produce more electrical power from the piezo generator. Flutter is characterized by nonlinear limit cycle oscillations. Such oscillations were experimentally observed by Erturk et al. [42], as an uniform air flow speed is set ‘properly’ and passing a piezo generator [42]. Further studies were conducted by Kwon et al. [43] on a cantilevered piezoelectric beam shaped like ‘T’, as a forcing air flow was present. The critical air flow triggering limit cycle oscillations were determined. Theoretical studies on predicting the power output from a piezoelectric generator were conducted by Kwuimy et al. [35]. It is found that more energy can be harvested, if the harvester is close to the condition of stochastic resonance. Typically, vibration-based piezoelectric generation devices are operating more efficiently and effectively over a limited frequency bandwidth [51–54]. Maximum power output is achieved, when the excitation frequency is reaching the fundamental frequency of the piezo harvesters [55–58]. However, if the excitation frequency is deviated from the resonant frequency of the harvester, then it will give rise to a decreased energy output. This will make the energy harvester working inefficiently [43]. Thus it will limit the practical application of such vibration-based

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FIG. 4.15 Experimental setup of an aeroelastic-piezo energy harvesting platform in a wind tunnel.

energy harvesting systems [44,45]. Ambient wind flow acts as a continuous kinetic energy resource, which has great potential to be harnessed. Recent application is demonstrated by using a coherence resonance of a bi-stable piezoelectric harvester [45,46]. However, there are few reported designs and practical demonstration, especially in the field of aero-elastic-piezo-electric energy harnessing [59,60]. In this chapter, aero-elastic-piezo-electric energy harnessing systems are experimentally tested by designing three rectangular large aspect ratio wings, to which a piezoelectric generator is attached [61–64]. The energy harvesters were placed in the closed-loop wind tunnel in Singapore Nanyang Technological University, as schematically shown in Fig. 4.15. The energy harvesting system uses a rectangular wing with a large aspect ratio, which is implemented on a shaft and anchored in a test section in the wind tunnel [65–67]. The dimensions of the test section are 2000 (L)  780 (W)  720(H) in millimetre. The air flow average speed in the wind tunnel is variable from 0 to 35 m/s. To decrease the mechanical frictions, an air bearing (NEWWAY) is applied. The air bearing system is designed to use compressed air with a pressure of approximately 689 KPa to create a thin ‘air film’ for ‘lubrication’ purpose.

4.2.1 Measurement of configurations and design parameters The dimensions and geometry of the tested rectangular wings with aspect ratio β are summarized in Table 4.3. For comparison, the surface areas of the wings are chosen to be invariant. However, the ratio β of the wing span to the chord length is variable.

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TABLE 4.3 Dimensions and geometry of the large aspect ratio wings tested in the experiments. Name

Chord (mm)

Span (mm)

Aspect ratio β

Wing area (mm2)

Wing A

75.0

337.5

4.5

25312.5

Wing B

85.0

297.6

3.5

25312.5

Wing C

100.6

251.6

2.5

25312.5

Harvesting the kinetic energy of oncoming air flow is achievable by producing self-sustained limit cycle rolling oscillations of the designed wings, as shown in Fig. 4.16A. Such oscillations can be used to excite a piezoelectric generator attached to the rectangular-shaped wings [68–70]. Detailed information on the piezoelectric generator such as the specifications and dimensions are summarized in Table 4.4. Attaching the rectangular-shaped piezoelectric generator to the wings could be achieved with two different configurations, as shown schematically in Fig. 4.16. One implementation is placing the piezo generator so that it is perpendicular to the air flow direction (streamline) as depicted schematically in Fig. 4.16B. The other implementation is to place it to be

FIG. 4.16 (A) Without piezo generator, (B) and (C) two different implementation configurations of the piezo-electric generator on the rectangular wing, (D) LED demonstration of the electrical power harnessed from the wing rolling motion.

360

Piezo material

Resonance frequency (Hz)

Free deflection (μm)

Capacitance (nF)

Weight (g)

Stiffness (N/m)

Length (mm)

Height (mm)

Width (mm)

5A4E

68

1260

260

9.5

245

66.7

2.5

31.8

Wind turbines and aerodynamics energy harvesters

TABLE 4.4 Dimensions and specifications of the piezo-electric generator (piezo Q260-A4-503YB).

Bladeless wind power harvester and aeroelastic harvester Chapter

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FIG. 4.17 (A) Time trace of the rolling angle θ of wing B in the presence and absence of the piezoelectric generator, as the air flow speed is set to 20 m/s. (B) Phase diagram of the rolling angle θ.

parallel, as illustrated schematically in Fig. 4.16C. However, in order to optimize the output electric power generated by the piezo generator, a tip mass could be introduced and added. Thus there are four different implementation configurations in the wind tunnel tests. Implementation configurations PC1 and PC3 involved with the piezo-generator being placed at the wing trailing edge, as the tip mass is attached. Implementation configurations PC2 and PC4 are associated with no additional tip mass being attached. Time trace of the measured flow-sustained rolling angle θ is obtained as shown in Fig. 4.17A. It is obvious that limit cycle rolling oscillations are successfully generated at t > 15 s. Insights on the critical flow conditions are then gained by conducting sensitivity measurements to determine the critical conditions to trigger limit cycles. These measurements are illustrated in Fig. 4.17B. It can be seen that limit cycle oscillations are achieved with the minimum air flow speed umin about 15 m/s and an AOA (angle of attack) αmin 16°. In the following experimental tests, the air flow speed and the AOA do need to meet the minimum requirement conditions to generate periodic aeroelastic limit cycle oscillations.

4.2.2

Experimental results

When the piezoelectric generator is not implemented, the amplitude of the periodic rolling motions is slightly increased [71–73], as described by the blue curve in Fig. 4.17A. This is mainly due to the decreased mass of the rectangular ‘wing.’ The phase diagrams of the rolling angle θ with and without the piezo generator implemented are illustrated in Fig. 4.17B. The noncircle-like shape reveals that there are harmonic resonances at high frequencies. This confirms that the energy harvesting system is a nonlinear one. All these measurements

362

Wind turbines and aerodynamics energy harvesters

confirms the successful production of limit cycle periodic rolling motions [74,75]. The produced electricity is used to power a LED light as shown in Fig. 4.16D. Comparison with the curve fitting of a sinusoid waveform is then made. The blue dash line denotes the curve-fitting data via applying the sinusoid function as θ ¼ 56 sin ð2π  0:909  ðt
60ÞÞ

(4.16)

Comparing the measured periodic rolling of wing-C and the curve-fitting one reveals that good agreement is obtained. This curve-fitting analysis confirms that the wing is undergoing periodic oscillations with a dominant frequency of 0.909 Hz. Fig. 4.17B illustrates the phase diagram of θ and its gradient θ_ of the rolling angle. The experimental results of the three wings with different aspect ratios and a piezoelectric generator implemented are summarized in Table 4.5. It is obvious

TABLE 4.5 The summarized measurement results of the period rolling motions of the wings involving piezoelectric generator.

Wing type

Piezo config

Airflow speed

Reynolds number

Peak angle

Rotation period (s)

Rolling freq (Hz)

A

1

16

7.39E+ 04

22.1

1.28

0.78

A

1

20

9.24E+ 04

34.9

1.13

0.88

A

1

24

1.11E+ 05

49.6

1.01

0.99

A

2

16

7.39E+ 04

50

1.53

0.65

A

2

20

9.24E+ 04

54

1.32

0.76

A

2

24

1.11E+ 05

69.6

1

1.00

A

3

16

7.39E+ 04

33.1

1.2

0.83

A

3

20

9.24E+ 04

44.4

0.97

1.03

A

3

24

1.11E+ 05

60.5

0.77

1.30

A

4

16

7.39E+ 04

37.5

1.09

0.92

A

4

20

9.24E+ 04

42.7

0.93

1.08

A

4

24

1.11E+ 05

67.8

0.74

1.35

B

1

16

8.38E+ 04

35.1

1.98

0.51

B

1

20

1.05E+ 05

51.2

1.76

0.57

B

1

24

1.26E+ 05

74.2

1.58

0.63

B

2

16

8.38E+ 04

63

1.84

0.54

B

2

20

1.05E+ 05

69.2

1.6

0.63

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TABLE 4.5 The summarized measurement results of the period rolling motions of the wings involving piezoelectric generator.—cont’d

Wing type

Piezo config

Airflow speed

Reynolds number

Peak angle

Rotation period (s)

Rolling freq (Hz)

B

2

24

1.26E+ 05

75.8

1.1

0.91

B

3

16

8.38E+ 04

52

1.18

0.85

B

3

20

1.05E+ 05

53.6

0.93

1.08

B

3

24

1.26E+ 05

69.2

0.82

1.22

B

4

16

8.38E+ 04

43.8

1.1

0.91

B

4

20

1.05E+ 05

47

0.89

1.12

B

4

24

1.26E+ 05

64.4

0.68

1.47

C

1

16

9.91E+ 04

52.6

1.61

0.62

C

1

18

1.12E+ 05

53.1

1.59

0.63

C

1

20

1.24E+ 05

53.4

1.56

0.64

C

1

22

1.36E+ 05

63.6

1.19

0.84

C

1

24

1.49E+ 05

73.1

1.14

0.88

C

2

16

9.91E+ 04

63.2

1.4

0.71

C

2

18

1.12E+ 05

63.8

1.32

0.76

C

2

20

1.24E+ 05

71.1

1.22

0.82

C

2

22

1.36E+ 05

72.5

1.21

0.83

C

2

24

1.49E+ 05

85.4

0.97

1.03

C

3

16

9.91E+ 04

47

1.1

0.91

C

3

18

1.12E+ 05

50

1.03

0.97

C

3

20

1.24E+ 05

61.4

0.99

1.01

C

3

22

1.36E+ 05

65.5

0.76

1.32

C

3

24

1.49E+ 05

86

0.7

1.43

C

4

16

9.91E+ 04

57.7

1.02

0.98

C

4

18

1.12E+ 05

60.4

0.92

1.09

C

4

20

1.24E+ 05

61.8

0.84

1.19

C

4

22

1.36E+ 05

72.6

0.78

1.28

C

4

24

1.49E+ 05

87.5

0.68

1.47

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Wind turbines and aerodynamics energy harvesters

that the rolling frequency is typically around 1.0 Hz. However, the maximum peak angle is varied from 22.1 to approximately 87.5 degrees, depending on the wing aspect ratio and the Reynold number and the piezo implementation configuration. Now the flow-excited aeroelastic rolling are used to generate electricity by applying a piezo-electric generator [76–78]. The working principle is based on the mechanical-electrical coupling effect. The piezo generator is operated at the resonant frequency of the vibrational structure. If the deformations of the piezoelectric generator is maximized, then the output electrical power is optimized. This is achieved by shifting its resonance frequency via attaching a tip mass to the piezo generator [79–81]. Thus the rolling frequency of the wing can be as close as possible to the resonant frequency. So does the deformation of the piezo materials. For simplicity, the bending behaviours of the piezo-electric beam could be modelled into a 1DOF (one degree of freedom) system near the resonance frequency. Thus the simplified modal vibration of the piezo structure is applied to evaluate the energy harvesting performance of the device [82]. For a 1DOF piezo generator undergoing bending motion, the governing equations characterizing mechanical-electrical coupling are given as: m

d2 z dz + c
kme v ¼ ΔpAp dt2 dt 1 q
kme z
v ¼ 0 cp

(4.17) (4.18)

where z is the modal displacement. c, m, and k are modal damping coefficient, mechanical mass and stiffness, respectively. cp is the material capacitance. kme and q are the modal piezoelectric coupling stiffness and the electric charge on the electrodes. v and Δ p are the difference in electric potential between the electrodes and the modal external mechanical pressure applied respectively. Taking Laplace transform to Eqs(4.17) and (4.18) and conducting further simplification give rise to the transfer function of v^ðsÞ=Δ^ pðsÞ as ^ vð s Þ Ap    ¼  CðsÞ 1 Δ^ pðsÞ 2 CðsÞ m+s C
kme s

kme cp kme kme cp kme

(4.19)

where CðsÞ ¼ q^ðsÞ=^ vðsÞ. It can be seen that the exciting force resulting from the mechanical-electrical coupling effect leads to the displacement in the piezo. It then induces the electrical charge on the electrodes. This explains well how electricity is produced from piezoelectric generator. When the denominator of Eq. (4.19) is zero, i.e. the vibration is near the resonance frequency of the generator, the output power is maximized. The performance of the wing-C is evaluated and shown in Fig. 4.18, as the angular velocity is varied. It is clear that the electric power output is strongly related to the angular velocity and the implementation configuration of the

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FIG. 4.18 Produced electric power varied with the angular velocity for wing-C, as α ¼ 16 degrees.

piezo generator. In summary, the maximum electric power of approximately 55 mW is produced from the implementation PC4. Such implementation configuration is an optimum one in terms of the power production. Further insights on the power production performance of the harvester are obtained, as the dominant frequency of the aeroelastic oscillations and electric power output in terms of voltage rms values are measured [83–85], as depicted in Fig. 4.19. It is observed that as the oncoming air flow velocity u0 is increased, the produced voltage is increased dramatically, no matter what implementation configuration is used. However, the implementation configuration PC4 is involved with the maximum voltage rms compared with other implementation configurations. Correspondingly, the rolling frequency is maximized, as illustrated in Fig. 4.19B. Autocorrelation study of the measured electric voltage is conducted and illustrated in Fig. 4.20. It is observed that there are two dominant frequencies of approximately 35 and 1.5 Hz, respectively. The periodic motion at 1.5 Hz is due to the rolling motion, which is sustained by the oncoming air flow. It is characterized by V(t) ¼ 4.41 sin(2π1.47(t
20.4)), which is denoted by the blue curve with cross marker. In order to shed lights on the deformation and vibration of the piezo materials in the presence or absence of a tip mass, modal analysis is performed by using ANSYS 15.0 or COMSOL [86–88]. For the given geometric and dimensions of the piezo plate, the 1st bending mode vibration is found to contribute more to the energy harvesting. As a tip mass is applied, the resonant frequency of the piezo generator is quite closed to the dominant vibration

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Wind turbines and aerodynamics energy harvesters

FIG. 4.19 Variation of the measured voltage (rms) and rolling frequency with oncoming air flow speed u0 and the maximum rolling angle θmax, from wing-C, as the angle of attack is set to α ¼16°. (A) Variation of Vrms with θmax and u0 and (B) variation of rolling frequency with θmax and u0.

FIG. 4.20 Autocorrelation coefficient of the measured voltage normalized with its maximum value, as the oncoming air flow speed u0 ¼ 16 m/s and AOA of α ¼16°.

frequency (around 35 Hz). Thus the electric power output is maximized [89]. However, introducing the tip mass gives rise to the resonant frequencies of torsional motions being reduced. In addition, the torsional vibration are associated with large losses such they occur at much higher frequencies. There are many application of the electric power generated from the aeroelastic

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pieoelectric system [90,91]. For demonstration, these energy harvesters are connected with a red LED (light-emitting diode), as illustrated in Fig. 4.16D. The demonstration tests confirm that the aeroelastic-piezo harvester serves as a simple and robust system to harness the kinetic energy of air flow for power harnessing and generation [92–96].

4.2.3

Concluding remarks

In general, energy harvesting from three large aspect ratio rectangular wings is achieved by implementing a piezoelectric generator. For this, flow-excited aeroelastic limit cycle oscillations are successfully produced. The performances of the energy harvester are evaluated as AOA (angle of attack) α, aspect ratio β, the air flow speed u0, or the implementation configurations of the piezo-electric generator is varied one at a time. The experimental measurements show that the aeroelastic rolling frequency is somehow increased but the rolling amplitude is decreased, when the piezoelectric generator is attached to the wing. In addition, the limit cycle oscillations are found to be easier to be triggered, as the aspect ratio of the wing is minimized. Maximum power output is achieved with the piezoelectric generator attached in parallel with the oncoming air flow streamline. Approximately 55 mW electricity is successfully generated via the piezoelectric generator (with the internal resistance 100 kΩ) from a rectangular wing; its surface area is about 0.025 m2. Conventional wind energy harvesting technology takes advantages of blades rotation. However, the present work proposed and tested another feasible way to harness air flow energy via producing nonlinear limit cycle aeroelastic oscillations. The aerodynamic-driven energy harvesters have been demonstrated practically on small scale prototypes. However, the harvesters may need to be scaled up in industries for a large-size host structures. For example, an UAV (unmanned aerial vehicle) with a wind span of a few meters for long-range mission. The scaling impact on both energy harvesting performances and the aerodynamics of UAV should be analyzed completely. In addition, the introduction of energy harvester will added additional weight and complexity due to its implementation on the airframe structure. A good design is obtained, when (1) the added weight is minimized, (2) the energy harnessed is maximized, (3) the implementation and integration of the energy harvester should be seamless. No further or extra redesign to the host structures, (4) the implementation of the energy harvester did not degrade the aerodynamic performances of the hosting platform, (5) the energy harvesting system should be continuously operating. In practice, the energy outputs of energy harvesters need to be enhanced. However, there is a tradeoff between a higher energy output and better energy conversion efficiency. To achieve a higher power output and a better efficiency, systematic and parametric design and analysis of the energy harvesting system such as the bladeless and the aeroelastic-piezo harvester is needed. This include

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the studies on the development of better functional piezoelectric materials, and a higher efficient structural configuration et al.

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