2.4 Power sources and energy harvesting techniques

Power sources are crucial for wireless sensor networks. Batteries are common, but energy harvesting techniques like solar cells and piezoelectric generators offer sustainable alternatives. These power sources determine how long sensors can operate and how often they need maintenance.

Efficient power management is key to extending sensor node lifespans. Techniques like duty cycling, where nodes alternate between active and sleep modes, help conserve energy. Power Management ICs optimize energy use, while low-power design strategies minimize consumption across all components.

Power Sources

Battery Technologies for Wireless Sensor Nodes

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• Batteries are the most common power source for wireless sensor nodes due to their high energy density and relatively low cost
• Lithium-ion (Li-ion) batteries offer high energy density, low self-discharge rate, and long cycle life, making them suitable for long-term deployments
• Lithium-polymer (LiPo) batteries provide a thin, lightweight, and flexible form factor, which is advantageous for wearable and implantable sensor applications
• Alkaline batteries, such as AA or AAA cells, are inexpensive and widely available but have lower energy density and shorter lifespans compared to Li-ion and LiPo batteries

Solar Cells for Energy Harvesting

• Solar cells convert light energy into electrical energy through the photovoltaic effect, providing a renewable power source for wireless sensor nodes
• Monocrystalline solar cells have the highest efficiency (15-20%) but are more expensive to manufacture compared to polycrystalline and thin-film solar cells
• Polycrystalline solar cells offer a balance between efficiency (13-16%) and cost, making them a popular choice for outdoor sensor deployments
• Thin-film solar cells, such as amorphous silicon (a-Si) and copper indium gallium selenide (CIGS), are flexible and lightweight but have lower efficiencies (5-13%) compared to crystalline solar cells

Supercapacitors for Energy Storage and Power Delivery

• Supercapacitors, also known as ultracapacitors or electrochemical double-layer capacitors (EDLCs), store energy in an electric field between two electrodes
• Supercapacitors have a high power density, allowing them to deliver large amounts of current quickly, which is useful for powering energy-intensive tasks like data transmission
• Compared to batteries, supercapacitors have a longer cycle life (>100,000 cycles) and can operate in a wider temperature range (-40°C to +65°C)
• Supercapacitors can be used in conjunction with batteries or energy harvesting sources to provide a hybrid power solution that combines high energy density and high power density

Energy Harvesting Techniques

Piezoelectric Energy Harvesting from Mechanical Vibrations

• Piezoelectric materials, such as lead zirconate titanate (PZT) and polyvinylidene fluoride (PVDF), generate an electric charge when subjected to mechanical stress or strain
• Piezoelectric energy harvesters convert ambient vibrations, such as those from machinery, human motion, or infrastructure, into electrical energy
• Cantilever-based piezoelectric energy harvesters consist of a piezoelectric material attached to a flexible beam that oscillates when exposed to vibrations, generating an alternating current (AC)
• Cymbal-type piezoelectric energy harvesters use a piezoelectric material sandwiched between two metal endcaps, which amplify the stress on the piezoelectric material and increase power output

Thermoelectric Generators for Waste Heat Recovery

• Thermoelectric generators (TEGs) convert temperature differences directly into electrical energy through the Seebeck effect
• TEGs consist of p-type and n-type semiconductor materials connected electrically in series and thermally in parallel, creating a voltage when a temperature gradient is applied
• Bismuth telluride (Bi2Te3) is the most common thermoelectric material for near-room-temperature applications, such as harvesting body heat or waste heat from industrial processes
• TEGs can be used to power wireless sensor nodes in environments with consistent temperature gradients, such as pipelines, engines, or human skin

RF Energy Harvesting from Ambient Electromagnetic Radiation

• RF energy harvesting captures ambient electromagnetic radiation from sources like TV and radio broadcasts, cellular networks, and Wi-Fi routers
• RF energy harvesters consist of an antenna to receive the electromagnetic waves, an impedance matching network to maximize power transfer, and a rectifier to convert the AC signal to DC
• The amount of energy available from ambient RF sources is relatively low (in the range of nanowatts to microwatts), requiring efficient antenna design and power management techniques
• Multi-band RF energy harvesters can capture energy from multiple frequency bands simultaneously, increasing the total power output

Power Management

Power Management ICs for Efficient Energy Utilization

• Power management integrated circuits (PMICs) are designed to optimize the power consumption and distribution within a wireless sensor node
• PMICs typically include voltage regulators, battery chargers, and power switches to control the flow of energy between the power sources, energy storage devices, and loads
• Maximum power point tracking (MPPT) is a technique used by PMICs to extract the maximum available power from solar cells or other energy harvesting sources under varying environmental conditions
• PMICs with integrated energy harvesting capabilities, such as the Texas Instruments BQ25570, can efficiently manage the power from multiple energy sources and store it in a rechargeable battery or supercapacitor

Low-Power Design Techniques for Extended Battery Life

• Low-power design techniques aim to minimize the power consumption of wireless sensor nodes, extending their battery life and reducing the need for frequent battery replacements
• Microcontrollers with low-power sleep modes, such as the Atmel ATmega328P or the Texas Instruments MSP430, can significantly reduce the power consumption when the sensor node is idle
• Selecting low-power sensors, such as MEMS accelerometers or temperature sensors, can minimize the power required for data acquisition
• Dynamic voltage and frequency scaling (DVFS) adjusts the operating voltage and clock frequency of the microcontroller based on the workload, reducing power consumption during periods of low activity

Duty Cycling for Energy Conservation

• Duty cycling is a power management technique that periodically switches the sensor node between active and sleep modes to conserve energy
• In the active mode, the sensor node performs tasks such as sensing, data processing, and communication, consuming relatively high power
• In the sleep mode, the sensor node disables most of its components and maintains only essential functions, such as a real-time clock or a low-power timer, resulting in significantly lower power consumption
• The duty cycle, defined as the ratio of active time to the total time, can be adjusted based on the application requirements, such as the desired sampling rate or the expected event frequency
• Synchronous duty cycling protocols, such as S-MAC and T-MAC, coordinate the sleep/wake schedules of neighboring nodes to minimize idle listening and reduce overall network power consumption