21.1 Vibration-based energy harvesting in structures
3 min read•august 9, 2024
Vibration-based energy harvesting in structures taps into to generate power. This method uses piezoelectric materials to convert mechanical strain into electricity, offering a sustainable power source for sensors and small devices in buildings and infrastructure.
Designing effective harvesters involves understanding , optimizing geometry, and matching frequencies. Key considerations include , , and developing techniques to harvest energy across a wide range of vibration frequencies.
Vibration Characteristics
Ambient Vibrations and Structural Resonance
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Ambient vibrations occur naturally in structures due to environmental factors (wind, traffic, machinery)
happens when external forces match of a structure
Natural frequencies depend on mass, stiffness, and geometry of the structure
, increasing potential for energy harvesting
Structures exhibit multiple resonant frequencies, each corresponding to a different mode of vibration
Vibration Modes and Structural Dynamics
represent distinct patterns of motion in a structure
involves the entire structure moving in phase
display more complex patterns with nodes and antinodes
describe the relative displacement of different parts of the structure
Structural dynamics studies how structures respond to dynamic loads over time
affects vibration amplitude and duration in structures
model basic structural dynamic behavior
Equation of motion for a single degree of freedom system: mx¨+cx˙+kx=F(t)
m: mass, c: damping coefficient, k: stiffness, F(t): external force
Harvester Design
Cantilever Beam Harvesters
consist of a fixed at one end
Piezoelectric material attached to the beam converts strain energy to electrical energy
often added to lower resonant frequency and increase strain
Beam dimensions and material properties affect resonant frequency and power output
Single beam harvesters typically operate in a narrow frequency band
Multi-beam arrays can harvest energy across a wider frequency range
Cantilever beams can be designed for unimorph or bimorph configurations
Unimorph: single piezoelectric layer
Bimorph: two piezoelectric layers, can be connected in series or parallel
Tuned Mass Dampers and Frequency Matching
(TMDs) absorb vibration energy in structures
TMDs can be modified to harvest energy while damping vibrations
Frequency matching involves tuning harvester resonance to match ambient vibration frequencies
adjust harvester properties to maintain optimal performance
Methods for include:
Adjustable tip mass
Variable beam stiffness
Magnetic force tuning
extend operational frequency range
Array of harvesters with different resonant frequencies
(bistable or tristable systems)
Modal Analysis for Harvester Design
identifies vibration characteristics of structures
(FEA) used to simulate structural behavior
employs sensors to measure actual structural response
Mode shapes and frequencies guide optimal placement of harvesters
indicate which modes contribute most to overall response
Modal analysis helps in:
Selecting appropriate harvester designs
Determining optimal harvester locations
Predicting energy harvesting potential
Performance Metrics
Power Density and Energy Conversion Efficiency
Power density measures harvested power per unit volume or mass of the device
Typical power densities range from microwatts to milliwatts per cubic centimeter
Energy conversion efficiency quantifies how much mechanical energy is converted to electrical energy
Factors affecting power density and efficiency:
Piezoelectric material properties (, )
Harvester geometry and design
(resistive load, synchronized switching)
: k2Q
k: coefficient
Q: quality factor
Maximum theoretical efficiency for linear piezoelectric harvesters: ηmax=4+2k2k2
Measurement and Optimization Techniques
optimizes power transfer to the load
enhance energy extraction
SSHI (Synchronized Switch Harvesting on Inductor)
SECE (Synchronized Electrical Charge Extraction)
:
Shaker table tests for controlled excitation
Accelerometer measurements for ambient vibration analysis
condition harvested energy for storage or use
converts AC to DC
DC-DC converters optimize voltage levels
Performance comparison metrics:
Normalized power density (NPD) accounts for input acceleration
Effectiveness compares harvester performance to an ideal harvester