Electronics Guide

Energy Sources and Power Levels

Ambient energy exists in many forms, each with characteristic power densities and availability patterns. Solar energy provides the highest power density in outdoor applications, with photovoltaic cells delivering 10-100 milliwatts per square centimeter under direct sunlight. Indoor lighting yields far less, typically 10-100 microwatts per square centimeter, but remains useful for low-power applications.

Thermal gradients between surfaces at different temperatures can be converted to electricity using thermoelectric generators. Industrial equipment, human body heat, and solar thermal effects provide temperature differences that yield power densities of 10-100 microwatts per square centimeter per degree Celsius of temperature differential.

Mechanical vibrations in machinery, vehicles, and infrastructure contain significant energy. Piezoelectric and electromagnetic transducers convert vibration energy to electricity, with power outputs ranging from microwatts for low-amplitude vibrations to milliwatts for industrial machinery.

Radio frequency energy from WiFi, cellular, and broadcast transmissions can be captured using rectifying antennas. While power levels are typically low, ranging from nanowatts to microwatts, RF harvesting can power very low duty-cycle sensors in areas with adequate RF coverage.

Energy Harvesting Transducers

Each energy source requires an appropriate transducer to convert ambient energy into electrical form:

Photovoltaic cells generate current when photons excite electrons in semiconductor junctions. Cell technologies range from crystalline silicon offering high efficiency to thin-film materials providing flexibility and low cost. Cell selection involves tradeoffs between efficiency, cost, size, and spectral response matching the intended light source.

Thermoelectric generators (TEGs) use the Seebeck effect to produce voltage from temperature differences across semiconductor junctions. TEG modules contain many junction pairs connected in series for useful voltage output. Their solid-state construction provides high reliability but relatively low conversion efficiency.

Piezoelectric transducers generate voltage when mechanically stressed. Ceramic and polymer piezoelectric materials can be configured as cantilevers, stacks, or patches to harvest energy from vibrations, impacts, or bending. Resonant designs maximize energy capture at specific frequencies.

Electromagnetic transducers use coils and magnets to convert mechanical motion to electrical energy. Linear generators harvest from oscillating motion while rotational designs capture from spinning sources. Electromagnetic harvesting scales better than piezoelectric approaches for larger displacements.

RF rectifying antennas (rectennas) combine antenna elements with rectifier circuits to convert RF energy to DC power. Design requires matching antenna characteristics to available RF frequencies and optimizing rectifier efficiency at very low input power levels.

Harvesting Challenges

Energy harvesting presents unique challenges that distinguish it from conventional power supply design:

Variable availability: Ambient energy sources are inherently variable and often unpredictable. Solar power depends on time of day, weather, and seasonal variations. Vibration sources may operate intermittently. System design must accommodate these variations while maintaining required functionality.

Low power levels: Harvested power is typically microwatts to milliwatts, far less than conventional power sources. Every microwatt counts, demanding extremely efficient power conversion and aggressive load power management.

Impedance matching: Transducers have internal impedances that vary with operating conditions. Extracting maximum power requires maintaining optimal impedance matching as source conditions change.

Cold start: Systems must be able to start from zero stored energy, bootstrapping power conversion circuits before adequate harvested energy is available.

Photovoltaic Cell Characteristics

Photovoltaic cells exhibit characteristic current-voltage relationships that determine harvesting circuit design. The cell produces maximum current at short circuit and maximum voltage at open circuit. Maximum power occurs at an intermediate operating point where the product of current and voltage is greatest.

The maximum power point (MPP) varies with illumination intensity, spectrum, temperature, and cell aging. Under reduced illumination, both the MPP voltage and current decrease. Cell temperature increases reduce voltage while slightly increasing current. These variations require adaptive power management to maintain optimal energy extraction.

Cell technologies offer different characteristics for various applications. Monocrystalline silicon provides highest efficiency (15-22%) but at higher cost. Polycrystalline silicon offers good efficiency (13-17%) at moderate cost. Thin-film technologies including amorphous silicon, CIGS, and CdTe provide lower efficiency (8-13%) but enable flexible form factors and lower manufacturing costs. Organic and dye-sensitized cells, while less efficient, offer unique advantages for indoor applications and building integration.

Maximum Power Point Tracking

Maximum power point tracking (MPPT) algorithms continuously adjust the operating point of the photovoltaic cell to extract maximum available power. Several algorithms are commonly employed:

Perturb and observe (P&O) periodically adjusts the operating point in one direction, measures the change in output power, and reverses direction if power decreased. Simple to implement, P&O oscillates around the MPP and may lose tracking during rapid irradiance changes.

Incremental conductance compares the incremental conductance of the cell to its instantaneous conductance to determine MPP location. This method can track the MPP more precisely than P&O and handles changing conditions better, but requires more computation.

Fractional open-circuit voltage exploits the approximately linear relationship between MPP voltage and open-circuit voltage. Periodically measuring open-circuit voltage and setting operating voltage to a fixed fraction (typically 0.76-0.78) provides simple tracking with modest accuracy.

Fractional short-circuit current similarly uses the relationship between MPP current and short-circuit current. This approach works well but requires briefly short-circuiting the cell for measurement, losing harvested energy during measurement intervals.

Advanced MPPT algorithms combine multiple techniques, using faster but less accurate methods during stable conditions and more sophisticated tracking during transients.

Indoor Solar Harvesting

Indoor solar harvesting operates under fundamentally different conditions than outdoor applications. Artificial lighting provides 100-1000 times less power than direct sunlight, and the spectrum differs significantly from natural light. Incandescent lighting produces a continuous spectrum while LED and fluorescent sources have discrete spectral peaks.

Cell selection for indoor applications must consider spectral response matching. Amorphous silicon cells perform well under fluorescent lighting, while some organic cells are optimized for LED light spectra. Crystalline silicon cells designed for outdoor use may perform poorly under artificial lighting.

Indoor harvesting power management must handle microwatt to milliwatt power levels with ultra-low quiescent current. Specialized power management ICs designed for indoor solar harvesting feature very low start-up voltages and self-powered operation from harvested energy alone.

Thermoelectric Generator Fundamentals

TEG modules contain arrays of semiconductor couples, alternating n-type and p-type elements, connected electrically in series and thermally in parallel. This arrangement provides useful output voltage while maintaining efficient heat flow through the module.

The Seebeck coefficient determines voltage output per degree of temperature difference, typically 20-60 millivolts per Kelvin for commercial TEG modules. Larger temperature differences produce higher voltage and power output. However, maintaining temperature differential requires attention to thermal management on both hot and cold sides.

TEG efficiency depends on the thermoelectric figure of merit (ZT) of the semiconductor materials and the temperature differential. Commercial bismuth telluride modules achieve ZT values near unity at room temperature, yielding conversion efficiencies of 2-5% for typical temperature differentials. Research materials promise higher ZT values, potentially improving efficiency significantly.

Thermal Considerations

Effective thermal energy harvesting requires maintaining temperature difference across the TEG while providing thermal pathways for heat flow:

Hot side interface: Efficient heat transfer from the source to the TEG hot side maximizes available temperature differential. Thermal interface materials, spring loading for good contact, and adequate contact area minimize thermal resistance.

Cold side heat rejection: The cold side must reject absorbed heat to maintain temperature differential. Heat sinks, forced convection, or coupling to thermally massive structures provide heat rejection paths. In body-worn devices, the human body serves as the heat sink.

Thermal shunting: Heat flowing around rather than through the TEG wastes available energy. Thermal insulation between hot and cold sides minimizes shunt paths and maintains useful temperature differential.

The thermal design challenge often exceeds the electrical design complexity. A well-designed thermal interface can double or triple the power output from a given TEG module.

Low-Voltage Boost Conversion

TEG modules produce low open-circuit voltages, often below 100 millivolts for modest temperature differentials. Starting boost converters at such low voltages requires specialized techniques.

Mechanical oscillators using electromagnetic or piezoelectric elements can bootstrap converter operation without stored energy. Once running, the converter can self-power from harvested energy. Alternatively, specialized boost converter ICs with ultra-low startup voltages (some operating from as little as 20 millivolts) enable direct startup from TEG output.

Charge pumps provide another approach for low-voltage boosting, using switched capacitors rather than inductors. While less efficient than inductive converters at higher power levels, charge pumps can operate from very low input voltages and scale well to microwatt power levels.

Vibration Source Characteristics

Effective vibration harvesting requires understanding the amplitude, frequency, and temporal characteristics of available vibrations. Industrial machinery typically produces vibrations at specific frequencies related to rotation speeds and gear meshing, with accelerations from 0.1g to several g. Vehicle vibrations span a broader frequency range with variable amplitude. Human motion produces low-frequency (1-5 Hz), high-amplitude vibrations during walking or running.

Characterizing vibration sources involves measuring acceleration spectra at the intended mounting location. This data guides harvester design, particularly resonant frequency selection for maximum energy capture.

Piezoelectric Harvesters

Piezoelectric materials generate voltage when mechanically stressed. Ceramic materials like PZT (lead zirconate titanate) provide high piezoelectric coefficients but are brittle. Polymer PVDF offers flexibility at lower output. Composite structures combine benefits of different materials.

Cantilever beam configurations are most common, with piezoelectric elements bonded to a resonant beam that amplifies input vibrations. A tip mass tunes the resonant frequency to match the dominant vibration frequency. Operating at resonance can increase power output by factors of 10-100 compared to off-resonance operation.

Piezoelectric harvesters produce AC output that must be rectified for use. High output impedance requires careful rectifier design; synchronous rectifiers reduce losses compared to diode bridges. The optimal electrical load impedance for maximum power transfer is frequency-dependent and related to the mechanical characteristics of the harvester.

Electromagnetic Harvesters

Electromagnetic harvesters use the relative motion between coils and magnets to generate voltage. Linear oscillating designs suit reciprocating vibrations while rotational designs capture energy from spinning sources.

Electromagnetic transducers generally provide lower output voltage but higher current than piezoelectric equivalents. This characteristic suits lower impedance loads and may simplify power conditioning. Electromagnetic designs scale favorably for larger harvesters where higher output power is required.

Voice coil configurations with axially magnetized moving magnets provide good performance for linear vibration harvesting. Spring elements provide mechanical resonance, with damping designed to balance peak power output against bandwidth.

Resonance and Bandwidth

Resonant harvesters achieve maximum output at their designed resonant frequency but provide significantly less power at other frequencies. This narrow bandwidth limits effectiveness when vibration frequency varies or contains multiple frequency components.

Widening bandwidth involves tradeoffs: increasing mechanical damping broadens the response but reduces peak output. Arrays of harvesters tuned to different frequencies can capture energy across a wider band. Nonlinear resonators with bistable or monostable dynamics can respond to broadband excitation, though at reduced peak efficiency.

Adaptive resonance tuning adjusts the harvester mechanical properties to track changing vibration frequencies. Techniques include variable stiffness springs, adjustable tip masses, and active tuning using harvested energy. The complexity and power consumption of tuning mechanisms must be justified by increased energy capture.

RF Energy Sources

Ambient RF energy varies dramatically with location, frequency, and time. Urban environments with dense wireless infrastructure provide more harvesting opportunities than rural areas. Common RF sources include:

Cellular signals: Base stations transmit continuously at relatively high power, providing consistent RF energy in populated areas. Multiple frequency bands (700 MHz to 3.5 GHz) offer diverse harvesting opportunities.

WiFi: 2.4 GHz and 5 GHz WiFi signals are ubiquitous in indoor environments. Signal strength varies with distance from access points and network activity patterns.

Broadcast: FM radio, digital television, and other broadcast signals provide consistent power levels but at lower frequencies requiring larger antennas.

Dedicated sources: Purpose-built RF transmitters can deliver significantly more power to harvesters than ambient sources, enabling higher power applications within regulatory limits.

Rectenna Design

The rectifying antenna (rectenna) combines antenna and rectifier functions to convert RF energy to DC power. Design optimization requires matching antenna characteristics to available RF frequencies and optimizing rectifier efficiency at very low input power levels.

Antenna selection involves tradeoffs between gain, bandwidth, size, and orientation sensitivity. High-gain antennas capture more power from specific directions but require pointing. Omnidirectional antennas work with any source direction but with lower gain. Wideband or multiband designs can harvest from multiple frequency bands simultaneously.

Rectifier circuits for RF harvesting must efficiently convert very low RF power levels. Schottky diode rectifiers with low forward voltage and fast switching suit high-frequency operation. Voltage multiplier configurations (Dickson charge pumps) boost output voltage at the expense of efficiency. Matching networks optimize power transfer from antenna to rectifier.

RF Power Management

RF harvesting presents extreme challenges for power management due to very low and variable power levels. Typical ambient RF harvesting yields nanowatts to low microwatts, requiring exceptional power management efficiency.

Dedicated RF power transmission systems can deliver milliwatts at moderate distances, enabling more conventional power management approaches. Regulatory limits on transmitted power and specific absorption rate (SAR) for human exposure constrain system design.

Hybrid harvesting systems combine RF with other energy sources, using RF to bootstrap operation or supplement primary harvesting during periods of low availability from other sources.

Energy Harvesting PMIC Architecture

Energy harvesting PMICs typically integrate several functional blocks:

Input stage: Accepts energy from one or more harvesting transducers. May include impedance matching networks, MPPT controllers, and input protection.

DC-DC converter: Converts harvested energy to appropriate voltage for storage or direct use. Buck, boost, or buck-boost topologies accommodate different source and output voltage relationships. Ultra-low quiescent current is essential to avoid consuming more power than harvested during low-energy periods.

Storage management: Controls charging of energy storage elements (batteries, supercapacitors) while protecting against overcharge, overdischarge, and overcurrent. May include fuel gauging for monitoring stored energy levels.

Output regulation: Provides regulated supply voltages to system loads. May include multiple rails, load switching, and power good signaling.

System control: Manages overall operation including cold start, energy prioritization between storage and loads, and low-energy shutdown sequences.

Cold Start Operation

Cold start capability enables system operation from zero stored energy, a critical requirement for energy harvesting applications. Without cold start capability, systems require pre-charged storage or manual intervention to begin operation.

Cold start circuits use minimal, self-powered structures that can operate from very low input voltages. Common approaches include:

Mechanical bootstrapping: Electromagnetic or piezoelectric oscillators provide initial power to start the main converter.

Ultra-low-voltage charge pumps: Specialized charge pump circuits can operate from millivolt inputs, accumulating charge until sufficient voltage exists to start the main converter.

Depletion-mode transistors: Transistors that conduct without gate bias enable circuits that operate from essentially zero input voltage, providing self-starting capability.

Cold start power capability is typically lower than steady-state operation. Harvesting sources must provide minimum power for cold start, which may exceed the minimum for continued operation.

Multi-Source Harvesting

Combining multiple energy sources improves reliability and average power availability. Multi-source PMICs accept inputs from different harvesting transducers and intelligently combine their contributions.

Series combining adds voltages from multiple sources, useful when individual sources provide insufficient voltage. Parallel combining adds currents, useful when sources provide adequate voltage but limited current. Intelligent combining selects or weights sources based on instantaneous availability, maximizing total harvested energy.

Challenges in multi-source systems include preventing reverse current flow between sources, managing different source impedances, and coordinating MPPT for multiple inputs. Dedicated multi-input harvesting PMICs address these challenges with integrated source management.

Energy-Aware Load Management

Energy harvesting PMICs often include features to manage system loads based on available energy:

Power good signals: Indicate when sufficient energy is available for load operation, enabling microcontrollers to determine when to wake and perform work.

Programmable voltage thresholds: Configure when loads are enabled or disabled based on storage voltage, implementing hysteresis to prevent oscillation at threshold boundaries.

Load prioritization: In multi-output systems, enable critical loads before less essential ones as energy becomes available.

Energy budgeting: Integrate current over time to track energy consumption and remaining capacity, enabling intelligent duty-cycling decisions.

Storage Technologies

Several storage technologies suit energy harvesting applications:

Supercapacitors: Offer high cycle life (millions of cycles), wide operating temperature range, and rapid charge/discharge capability. Energy density is lower than batteries, limiting total storage capacity. Supercapacitors excel for buffering transient loads and bridging brief harvesting gaps.

Lithium batteries: Provide high energy density for extended operation when harvesting is unavailable. Lithium-ion and lithium-polymer batteries require protection circuits and careful charge management. Cycle life of hundreds to thousands of cycles may limit useful life in frequently cycled applications.

Thin-film batteries: Solid-state construction provides safety advantages and unusual form factors. Lower capacity than conventional lithium batteries but suitable for space-constrained applications.

Hybrid storage: Combining supercapacitors for transient buffering with batteries for bulk storage optimizes both power and energy requirements. The supercapacitor handles high-current transients while the battery provides long-term storage.

Storage Sizing

Proper storage sizing balances several factors:

Energy requirements: Total energy needed during the longest expected harvesting gap determines minimum storage capacity. For outdoor solar, this may be overnight operation; for indoor applications, weekend building shutdown.

Power requirements: Peak load power determines storage power rating. Supercapacitors handle high peak currents better than batteries.

Harvesting capacity: Storage must be sized to absorb available harvested energy without excessive overcharge. Oversized storage takes longer to charge, delaying system availability.

Physical constraints: Size, weight, cost, and environmental requirements may limit storage options.

Energy budget analysis, accounting for harvesting profiles and load patterns, enables accurate storage sizing for specific applications.

Charge Management

Proper charge management extends storage life and ensures safe operation:

Voltage limiting: Prevents overcharge damage by limiting maximum storage voltage. Supercapacitors tolerate minor overcharge better than batteries.

Current limiting: Controls charge rate to prevent damage and manage heat generation. Energy harvesting typically provides low charge currents, reducing this concern.

Temperature compensation: Adjusts charge parameters based on storage temperature. Lithium batteries require reduced charge voltage at elevated temperatures.

Balancing: Series-connected storage elements require balancing to prevent individual cell overcharge or underutilization.

Energy Budget Analysis

Energy budget analysis compares available harvested energy against load requirements over relevant time periods:

Harvesting profile: Characterize energy availability over daily, seasonal, and annual cycles. Include worst-case periods when designing for reliability.

Load profile: Determine energy consumption patterns including active operation, sleep modes, and duty cycles. Include all system components in the analysis.

Conversion efficiency: Account for losses in power conversion, storage charge/discharge, and voltage regulation. End-to-end efficiency may be 50-80% depending on system design.

Margin: Include safety margin for component aging, environmental variations, and unexpected loads.

If harvested energy exceeds load requirements with adequate margin, the system can achieve energy autonomy. If not, design changes, additional harvesting capacity, or supplemental energy sources are needed.

Duty-Cycling and Load Management

When harvested energy is limited, duty-cycling reduces average power consumption to match available supply:

Periodic wake-up: The system wakes at intervals to perform measurements, process data, or communicate, then returns to sleep. Wake interval depends on application requirements and available energy.

Event-triggered operation: Low-power detection circuits monitor for events of interest, waking the main system only when events occur. This can be more energy-efficient than periodic polling for rare events.

Graceful degradation: When energy is scarce, reduce functionality progressively: reduce sample rates, defer non-critical tasks, or enter minimal operation modes.

Energy-aware scheduling: Schedule energy-intensive tasks when harvesting is abundant and energy storage is adequate. Defer tasks when energy is scarce.

Reliability Considerations

Energy harvesting systems face reliability challenges unique to their autonomous operation:

Degradation: Harvesting transducers, storage elements, and power management circuits degrade over time. Design for expected lifetime with adequate margin.

Environmental exposure: Outdoor harvesting systems face temperature extremes, moisture, UV exposure, and mechanical stress. Select components rated for environmental conditions and provide appropriate protection.

Recovery from energy depletion: Systems must recover gracefully from complete energy depletion, including cold start, state recovery, and time synchronization.

Fault tolerance: Autonomous systems cannot rely on human intervention for fault recovery. Design for self-diagnosis and autonomous recovery where possible.

Testing and Validation

Validating energy harvesting systems requires testing under realistic conditions:

Source simulation: Simulate harvesting sources with programmable power supplies to test across the range of expected conditions, including worst-case scenarios.

Long-duration testing: Energy harvesting issues may only manifest over days or weeks of operation. Extended testing reveals subtle energy budget problems and aging effects.

Environmental testing: Verify operation across the full environmental range, including temperature, humidity, and vibration.

Power measurement: Accurate measurement of harvested power, storage efficiency, and load consumption enables energy budget validation and optimization.

Wireless Sensor Networks

Battery replacement in large sensor networks is costly and impractical. Energy harvesting enables sensors that operate indefinitely without maintenance. Environmental sensors use solar harvesting; industrial monitoring sensors harvest vibration from machinery; building sensors harvest indoor lighting or thermal gradients from HVAC systems.

Wearable Electronics

Body heat, motion, and indoor lighting provide energy for wearable sensors and devices. Thermoelectric generators harvest from skin temperature, while kinetic harvesters capture energy from walking or arm motion. The challenge is achieving adequate power in compact, comfortable form factors.

Structural Health Monitoring

Sensors embedded in bridges, buildings, and infrastructure monitor structural health over decades. Battery replacement in embedded locations is often impossible. Solar panels, vibration harvesters, and long-range wireless power enable truly permanent monitoring installations.

Industrial IoT

Industrial environments offer abundant energy sources: machinery vibration, process heat, and ambient lighting. Harvesting-powered sensors eliminate the need for power wiring or battery maintenance in harsh industrial locations, enabling more pervasive monitoring and predictive maintenance.