1618  Energy Harvesting Concepts and Design

Theoretical Foundations and Design Workflow for Self-Powered Systems

1618.1 Learning Objectives

TipWhat You’ll Learn

After completing this chapter, you will be able to:

  • Explain the energy balance equation for autonomous systems
  • Identify key power management considerations
  • Compare harvester efficiency across different technologies
  • Apply a systematic design workflow for energy harvesting systems

1618.2 Understanding Energy Harvesting

NoteFor Beginners: What is Energy Harvesting?

Energy harvesting (also called energy scavenging) is the process of capturing small amounts of energy from the environment and converting it into electrical power for low-energy devices like IoT sensors.

Why does it matter?

  • Eliminates battery replacement in remote or hard-to-access locations
  • Enables truly autonomous IoT deployments
  • Reduces environmental impact from battery disposal
  • Can reduce total cost of ownership over device lifetime

Common sources:

  • Solar - Light energy from sun or indoor lighting
  • Vibration - Mechanical motion from machinery or human movement
  • Thermal - Temperature differences (hot pipes, body heat, HVAC)
  • RF - Radio frequency energy from Wi-Fi, cellular, or dedicated transmitters

1618.3 Key Concepts

1618.3.1 Energy Balance Equation

The fundamental equation for energy-autonomous systems:

\[E_{harvested} \geq E_{consumed}\]

Where:

  • \(E_{harvested}\) = Energy captured from environment over time period
  • \(E_{consumed}\) = Energy used by device over same time period

For daily self-sufficiency:

\[P_{harvest} \times t_{available} \geq P_{avg} \times 24h\]

Think of energy harvesting like filling a water tank:

  • Harvest = Water flowing into the tank (from rain, a hose, etc.)
  • Consumption = Water flowing out (for drinking, washing, etc.)
  • Storage = The tank itself (battery or supercapacitor)

If more water flows in than flows out, your tank stays full. If more flows out than in, eventually your tank empties and you run out of water!

The same applies to energy: harvest more than you consume, and your IoT device runs forever.

1618.3.2 Power Management Considerations

ImportantCritical Design Factors
  1. Impedance Matching - Maximum Power Point Tracking (MPPT) extracts 15-30% more power
  2. Storage Sizing - Must handle peak loads and autonomy requirements
  3. Voltage Regulation - Harvested voltage varies; devices need stable supply
  4. Startup Energy - Cold-start requires minimum stored energy threshold
  5. Efficiency Chain - Each conversion stage has losses (harvester → regulator → storage → load)

1618.3.3 Harvester Efficiency

Different harvesting technologies have varying efficiencies:

Technology Typical Efficiency Key Limitation
Solar (outdoor) 15-25% Weather, orientation
Solar (indoor) 5-15% Low light levels
Piezoelectric 10-25% Frequency matching
Electromagnetic 20-40% Size constraints
Thermoelectric 3-8% Small ΔT
RF Rectenna 30-60% Low power density
NoteEfficiency vs. Power Density

High efficiency doesn’t always mean more power. RF harvesting at 60% efficiency might only produce 0.1 mW, while solar at 18% efficiency produces 50 mW. Match your source to your power requirements.

1618.4 Design Workflow

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flowchart TD
    A[Define Power Requirements] --> B[Survey Environment]
    B --> C[Select Harvesting Source]
    C --> D[Size Harvester]
    D --> E[Design Power Management]
    E --> F[Size Storage]
    F --> G[Prototype & Test]
    G --> H{Self-Sufficient?}
    H -->|No| I[Optimize Design]
    I --> D
    H -->|Yes| J[Deploy]

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    style B fill:#16A085,color:#fff
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    style D fill:#16A085,color:#fff
    style E fill:#E67E22,color:#fff
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    style H fill:#7F8C8D,color:#fff
    style I fill:#E74C3C,color:#fff
    style J fill:#27AE60,color:#fff

1618.4.1 Step-by-Step Design Process

1618.4.1.1 Step 1: Define Power Requirements

Document your device’s power profile:

  • Sleep current (µA) - Typically 0.1-100 µA for modern MCUs
  • Active current (mA) - Processing, sensing, communication
  • Peak current (mA) - Radio TX bursts, motor activation
  • Operating voltage (V) - Usually 1.8-5V for IoT devices
  • Duty cycle (%) - Ratio of active to total time

1618.4.1.2 Step 2: Survey Environment

Assess available energy sources at your deployment location:

  • Light levels (lux) - Outdoor sun (100,000), office (300-500), warehouse (50-100)
  • Vibration (Hz, g) - Machinery frequency and amplitude
  • Temperature differential (°C) - Between hot and cold surfaces
  • RF availability - Distance to Wi-Fi APs, cellular towers

1618.4.1.3 Step 3: Select Harvesting Source

Match source to environment and power needs:

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flowchart TD
    A[Power Need?] --> B{> 10 mW?}
    B -->|Yes| C{Outdoor?}
    C -->|Yes| D[Solar Panel]
    C -->|No| E{Vibration Available?}
    E -->|Yes| F[Piezo/EM Harvester]
    E -->|No| G{Heat Source?}
    G -->|Yes| H[TEG Module]
    G -->|No| I[Consider Battery]

    B -->|No| J{< 1 mW?}
    J -->|Yes| K[RF Harvesting]
    J -->|No| L[Small Solar/Thermal]

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    style F fill:#9B59B6,color:#fff
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    style K fill:#16A085,color:#fff
    style L fill:#27AE60,color:#fff

1618.4.1.4 Step 4: Size the Harvester

Calculate required harvester capacity:

\[P_{harvester} = \frac{E_{daily} + E_{storage\_charging}}{t_{available} \times \eta_{system}}\]

Where:

  • \(E_{daily}\) = Daily energy consumption (mWh)
  • \(E_{storage\_charging}\) = Energy to charge storage (mWh)
  • \(t_{available}\) = Hours of harvesting per day
  • \(\eta_{system}\) = System efficiency (typically 0.6-0.8)

1618.4.1.5 Step 5: Design Power Management

Select appropriate Power Management IC (PMIC):

  • MPPT for solar (e.g., BQ25504, LTC3105)
  • Low-voltage boost for thermal/piezo (e.g., LTC3108)
  • Rectifier + boost for RF (e.g., custom rectenna designs)

1618.4.1.6 Step 6: Size Storage

Balance storage capacity with form factor and cost:

\[E_{storage} = P_{avg} \times t_{autonomy} \times \frac{1}{\eta_{discharge}}\]

Storage Selection Guide:

Storage Type Best For Considerations
Supercapacitor Peak loads, rapid charge High leakage, limited energy
LiPo Battery Long autonomy, compact Cycle life, temperature limits
LiFePO4 High cycles, safety Lower energy density
Hybrid Best of both More complex PMIC

1618.5 System Architecture

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flowchart LR
    subgraph Sources
        S1[Solar Panel]
        S2[TEG Module]
        S3[RF Antenna]
    end

    subgraph PMIC["Power Management IC"]
        M1[MPPT 1]
        M2[MPPT 2]
        M3[Rectifier]
        OR[Power ORing]
    end

    subgraph Storage
        SC[Supercap]
        BAT[Battery]
    end

    S1 --> M1
    S2 --> M2
    S3 --> M3
    M1 --> OR
    M2 --> OR
    M3 --> OR
    OR --> SC
    SC --> BAT
    BAT --> LOAD[IoT Device]

    style S1 fill:#F1C40F,color:#000
    style S2 fill:#E74C3C,color:#fff
    style S3 fill:#16A085,color:#fff
    style LOAD fill:#2C3E50,color:#fff

1618.5.1 Hybrid System Benefits

Combining multiple energy sources provides:

  1. Redundancy - If one source fails, others continue
  2. Higher total power - Sum of all harvested energy
  3. Better availability - Different sources active at different times
  4. Improved reliability - Less dependence on single environmental factor

1618.6 Energy Source Deep Dive

1618.6.1 Solar Photovoltaic Harvesting

Solar cells convert light energy directly into electricity through the photovoltaic effect. When photons strike the semiconductor material, they excite electrons to create an electrical current.

Key Parameters:

  • Irradiance: Solar power per unit area (W/m²)
    • Full sun: 1000 W/m² (100 mW/cm²)
    • Cloudy day: 100-300 W/m²
    • Indoor office: 0.3-5 W/m² (300-5000 lux)
  • Panel Efficiency: Percentage of light converted to electricity
    • Monocrystalline: 18-25%
    • Polycrystalline: 15-20%
    • Amorphous/thin-film: 7-13% (but better in low light)
  • Fill Factor: Ratio of maximum power to theoretical maximum
    • Good panels: 0.7-0.85

Solar Power Calculation:

\[P_{solar} = I_{rradiance} \times A_{panel} \times \eta_{panel} \times FF\]

Where:

  • \(I_{rradiance}\) = Light intensity (W/m²)
  • \(A_{panel}\) = Panel area (m²)
  • \(\eta_{panel}\) = Panel efficiency
  • \(FF\) = Fill factor

Hey there, young engineers! Think of a solar panel like a garden:

  • Sunlight is like rain - the more you get, the more your garden grows
  • Panel size is like your garden plot - bigger gardens grow more plants
  • Efficiency is like using good soil - better soil means more plants from the same rain
  • Storage is like a water tank - you save extra water for cloudy days

So a good solar-powered device is like a garden with a water tank - it collects energy when the sun shines and saves it for nighttime!

1618.6.2 Vibration/Piezoelectric Harvesting

Piezoelectric materials generate voltage when mechanically stressed. This makes them ideal for harvesting energy from vibrating machinery or human motion.

Types of Vibration Harvesters:

Type Mechanism Voltage Output Best Frequency
Piezoelectric Crystal deformation High (10-100V AC) 10-1000 Hz
Electromagnetic Coil + magnet Low (0.1-5V AC) 1-100 Hz
Electrostatic Capacitance change Medium (1-10V) 10-1000 Hz

Vibration Power Equation (Roundy Model):

\[P_{vib} = \frac{m \cdot a^2}{4 \cdot \omega \cdot \zeta_e}\]

Where:

  • \(m\) = Proof mass (kg)
  • \(a\) = Acceleration amplitude (m/s²)
  • \(\omega\) = Angular frequency (rad/s)
  • \(\zeta_e\) = Electrical damping ratio

Critical Design Consideration: The harvester must be tuned to the dominant vibration frequency for maximum power extraction.

1618.6.3 Thermoelectric Harvesting

Thermoelectric generators (TEGs) use the Seebeck effect to convert temperature differences into electrical voltage. The voltage generated is proportional to the temperature differential.

Seebeck Effect:

\[V_{oc} = S \times \Delta T\]

Where:

  • \(V_{oc}\) = Open-circuit voltage (V)
  • \(S\) = Seebeck coefficient (V/K), typically 20-50 mV/K for Bi2Te3
  • \(\Delta T\) = Temperature difference (K)

TEG Power:

\[P_{TEG} = \frac{S^2 \times \Delta T^2}{4 \times R_{internal}}\]

Practical Temperature Sources:

Source Typical ΔT Expected Power
Body heat 5-15°C 10-100 µW/cm²
Hot water pipe 20-40°C 1-5 mW/cm²
Industrial machine 50-100°C 5-20 mW/cm²
Exhaust system 100-300°C 20-100 mW/cm²

1618.6.4 RF Energy Harvesting

RF harvesting captures electromagnetic radiation and converts it to DC power using rectifying antennas (rectennas).

RF Harvesting Chain:

Antenna → Matching Network → Rectifier → DC-DC Converter → Load

Friis Transmission Equation (received power):

\[P_r = P_t \times G_t \times G_r \times \left(\frac{\lambda}{4\pi d}\right)^2\]

Where:

  • \(P_r\) = Received power (W)
  • \(P_t\) = Transmitted power (W)
  • \(G_t, G_r\) = Transmitter and receiver antenna gains
  • \(\lambda\) = Wavelength (m)
  • \(d\) = Distance (m)

Practical RF Power Levels:

Source Distance Typical Power Available
Wi-Fi AP (100 mW) 1 m 10-100 µW
Wi-Fi AP (100 mW) 10 m 0.1-1 µW
Cellular tower 100 m 1-10 µW
Dedicated TX (1W) 1 m 1-10 mW
WarningRF Harvesting Limitations

RF harvesting produces very low power (typically < 100 µW) from ambient sources. It’s best suited for:

  • Supplementary power alongside other sources
  • Dedicated transmitter applications where TX is nearby
  • Very low power devices with long sleep periods

1618.7 Efficiency Analysis

1618.7.1 End-to-End System Efficiency

Real energy harvesting systems have losses at every stage:

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flowchart LR
    A[Environment<br>100%] --> B[Harvester<br>15-25%]
    B --> C[MPPT/PMIC<br>85-95%]
    C --> D[Storage<br>80-95%]
    D --> E[Regulator<br>85-95%]
    E --> F[Load<br>60-80%<br>total]

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    style C fill:#E67E22,color:#fff
    style D fill:#F1C40F,color:#000
    style E fill:#2C3E50,color:#fff
    style F fill:#27AE60,color:#fff

Example Efficiency Calculation:

For a solar harvesting system:

  • Panel efficiency: 18%
  • MPPT efficiency: 90%
  • Battery charge efficiency: 90%
  • DC-DC regulator: 90%

Total system efficiency: 0.18 × 0.90 × 0.90 × 0.90 = 13.1%

This means only 13.1% of incident solar energy reaches the load!

1618.7.2 Improving System Efficiency

  1. Use MPPT - Can improve harvester output by 15-30%
  2. Minimize conversion stages - Each stage adds losses
  3. Match voltages - Direct use of harvested voltage when possible
  4. Efficient regulators - Choose high-efficiency DC-DC converters
  5. Low-leakage storage - Battery > supercapacitor for long storage

1618.8 Knowledge Check

CautionTest Your Understanding

Question 1: A solar-powered sensor needs 2 mWh/day and has 6 hours of sunlight. If system efficiency is 70%, what minimum harvested power is needed?

Show Answer

\[P_{harvester} = \frac{2 \text{ mWh}}{6 \text{ h} \times 0.7} = 0.48 \text{ mW}\]

The harvester needs to produce at least 0.48 mW during the 6-hour window.

Question 2: Which storage type would you choose for a sensor that experiences 100 mA TX bursts but only needs 1-day autonomy?

Show Answer

Hybrid (supercap + small battery) - The supercapacitor handles the peak TX bursts without voltage droop, while the small battery provides the 1-day autonomy. This is more efficient than using a battery alone for peak loads.

1618.9 Summary

TipKey Takeaways
  1. Energy balance is fundamental - Harvest must exceed consumption for autonomy
  2. Efficiency chain matters - Account for losses at each conversion stage
  3. Match source to application - Solar for outdoor, vibration for industrial, thermal for HVAC
  4. Size storage appropriately - Consider both autonomy and peak load requirements
  5. Use MPPT - 15-30% more power extraction justifies the complexity
  6. Iterate the design - Prototype, test, and optimize before deployment

1618.10 What’s Next

Continue to the Energy Harvesting Practical Guide chapter to see real-world examples, common pitfalls, and advanced topics like Maximum Power Point Tracking and hybrid system design.