Toward Small-Scale Wind Energy Harvesting: Design, Enhancement, Performance Comparison, and Applicability

Table 5

Summary of various flutter energy harvester devices.

Author	Transduction	Flutter instability	Flap at the tip	Cut-in wind speed (flutter speed) (m/s)	Cut-out wind speed (m/s)	Maximum power (mW)	Wind speed at max power (m/s)	Dimensions	Power density per volume (mW/cm^3)	Advantages/disadvantages and other information
Bryant and Garcia [63]; Bryant and Garcia [26]	Piezoelectric	Modal convergence flutter	Airfoil, profile NACA0012	1.86	—	2.2	7.9	Airfoil: semichord 2.97 cm, span 13.6 cm.; Cantilever: 25.4 × 2.54 × 0.0381 cm^3	7.17 × 10^−3	(i) Semiempirical model of the nonlinear electromechanical and aerodynamic system accurately predicted electrical and mechanical response.; (ii) Successfully predicts the flutter boundary with one of the real parts of the first two eigenvalues turning positive and the two imaginary parts coalescing.; (iii) Wide operational wind speed range.; (iv) Subcritical Hopf bifurcation, a large initial disturbance is fundamental for system startup.
Bryant et al. [64]	Piezoelectric	Modal convergence flutter	Flat plate	Stiff host structure				Flat plate tip: chord 3 cm, span 6 cm, thickness 0.79 mm.; Cantilever: 7.6 × 2.5 × 0.0381 cm^3	Stiff host structure	(i) Compared to a stiff host structure, a compliant host structure reduces the cut-in wind speed, cut-in frequency and oscillation frequency.; (ii) The peak power is shifted toward the lower wind speeds with the compliant host structure.
				17.3	29	43	26		20.0
				Compliant host structure					Compliant host structure
				15.2	29	35	25		16.3
Bryant et al. [65]	Piezoelectric	Modal convergence flutter	Flat plate	17.3	29	43	26	Flat plate tip: chord 3 cm, span 6 cm, thickness 0.79 mm.; Cantilever: 7.6 × 2.5 × 0.0381 cm^3	20.0	(i) Confirms the feasibility of using ambient flow energy harvesting to power aerodynamic control surfaces.
Erturk et al. [66]	Piezoelectric	Modal convergence flutter	Airfoil	9.30	—	10.7	9.30	Airfoil: semichord 12.5 cm, span 50 cm. Cantilever: —	2.27 × 10^−3	(i) Effect of electromechanical coupling on flutter energy harvesting is analyzed.; (ii) Found that the optimal load gave the maximum flutter speed due to the associated maximum shunt damping effect during power extraction.
Sousa et al. [67]	Piezoelectric	Modal convergence flutter	Airfoil	Linear configuration				Airfoil: semichord 12.5 cm, span 50 cm.; Cantilever: —	Linear configuration; 2.55 × 10^−3	(i) The free play nonlinearity reduces the cut-in wind speed and increased the output power.; (ii) Theoretically determining that the hardening stiffness brings the response amplitude to acceptable levels and broadens the operational wind speed range.
				12.1	—	12	12.1
				With free play nonlinearity					With free play nonlinearity
				10.0	—	27	10.0		5.73 × 10^−3
Bibo and Daqaq [68]	Piezoelectric	Modal convergence flutter	Airfoil,; profile NACA0012	2.3	—	0.138	3; (With base acceleration 0.15 m/s^2)	Airfoil: semichord 4.2 cm, span 5.2 cm.; Cantilever: —	8.38 × 10^−4	(i) Concurrent flow and base excitations enhances power generation performance.; (ii) Concurrent excitations increases output power by 2.5 times below the flutter speed, and over 3 times above the flutter speed.; (iii) Above the flutter speed, requiring careful adjustment because power is sensitive to base acceleration frequency.
Kwon [69]	Piezoelectric	Modal convergence flutter	Flat plate tip; Whole device T-shape	4	—	4.0	15	Flat plate tip: 6.0 × 3.0 cm^2.; Cantilever: 10.0 × 6.0 × 0.02 cm^3	2.56	(i) Simple T-shape structure, easy to fabricate.; (ii) No rotating components.; (iii) Wide operational wind speed range.
Park et al. [70]	Electromagnetic	Modal convergence flutter	Flat plate tip; Whole device T-shape	4	—	1.1	8	Flat plate tip: 3.0 × 2.0 cm^2.; Cantilever: 4.2 × 3.0 × 0.01016 cm^3	5.82	(i) Determining that the onset of the harvester requires the load resistance to surpass the flutter onset resistance.
Li et al. [71]	Piezoelectric	cross-flow flutter	—	4	—	0.615	8	adhered double-layer, stalk: 7.2 × 1.6 × 0.041 cm^3	1.30	(i) Cross-flow configuration generated one order of magnitude more power than the parallel configuration.; (ii) Having high power density per weight and per volume.; (iii) Being robust, simple, and miniature sized.; (iv) Being easy to blend in urban and natural environments due to its “leaf” appearance.
Deivasigamani et al. [72]	Piezoelectric	cross-flow flutter	Triangle	—	—	0.0883	8	Isosceles triangle tip: 8 cm in base, 8 cm in height, 0.35 mm in thickness.; Stalk: 7.2 × 1.6 × 0.0205 cm^3	0.0651	(i) Determining that vertical stalk configuration is superior to the horizontal stalk with five times more output power.
Humdinger	Electromagnetic	cross-flow flutter	—	—	—	≈9	10	Membrane: 12 × 0.7 cm^2.; Casing: 13 × 3 × 2.5 cm^3	0.923	(i) Successfully powers wireless sensor nodes.; (ii) Being compact and robust.; (iii) Low cut-in wind speed and wide operational wind speed range.
Hobeck et al. [73]	Piezoelectric	Dual cantilever flutter	—	≈3	—	0.796	≈13	Two identical cantilevers: 14.6 × 2.54 × 0.0254 cm^3	0.422	(i) Very wide operational wind speed range with efficient power generation.; (ii) Generating a significant amount of power from 3 m/s to 15 m/s when gap is small.

Calculated by 18 mw/g × (0.15 m/s²/9.8 m/s²)/2 from the information given by the authors of the reference.
Obtained from the figure in the datasheet of μicroWindbelt (http://www.humdingerwind.com/pdf/microBelt_brief.pdf).