Energy Sources for IoT

The electrical and electronic devices in IoT infrastructure require electrical energy to operate. The energy requirements of the device depend on its size, computing or processing requirements, traffic load, and other mechanical and electrical loads that need to be handled, especially in IoT applications where the feedback commands from fog/cloud computing platforms are used to control a physical process or system through actuators. The main power sources for IoT devices are (figure 1):

• main power,
• energy storage systems,
• energy harvesting systems.

In IoT applications where the hardware devices do not need to be mobile and are energy-hungry (consume significant energy), they can be reliably powered using main power sources. The main power from the grid is AC power, which should be converted to DC power and scaled down to meet the power requirements of sensing, actuating, computing, and networking nodes. The hardware devices at the networking or transport layer and those at the application layer (fog/cloud computing nodes) are often power-hungry and supplied using grid energy.

A drawback of using the main power to supply an IoT infrastructure with many IoT devices that depend on the main power source is the complexity of connecting the devices to the power source using cables. In the case of hundreds or thousands of devices, supplying them using the main power is impractical. If the energy from the main source is generated using fossil fuels, then the carbon footprint from the IoT infrastructure increases as its energy demands increase.

Energy storage systems are systems that are used to store energy so that it can be consumed later. In IoT infrastructures, some sensors, actuators, computing and networking nodes, and other electrical systems are powered by energy storage systems. The energy is stored in forms that can readily be converted into electrical energy required to power the IoT devices, computing and networking nodes and other electrical systems in the IoT infrastructure. In some scenarios, electrical energy from a main power supply or local renewable energy plants (or energy harvesting systems) is converted to storable energy forms and stored in energy storage systems to be used when the source is not able to generate energy to meet the needs of the electrical systems in the IoT infrastructure. Energy storage systems can be categorised depending on the form of the energy (mechanical, electrical, chemical, and thermal energy) that is stored and then subsequently converted into electrical energy.

1. Electrostatic energy storage systems: They use capacitors to store energy as an electric field. They are suitable for high-speed energy release but limited in storage capacity.
2. Magnetic energy storage system: This includes superconducting magnetic energy storage (SMES) systems, which store energy as a magnetic field in superconducting materials. These systems provide high efficiency and rapid discharge but require advanced cooling systems to maintain superconductivity.

Electrochemical energy storage systems Store energy through reversible chemical reactions in batteries. Common types include lithium-ion, lead-acid, alkaline, solid-state thin-film, and 3D-printed zinc batteries. These batteries are suitable for a wide range of applications, from tiny IoT sensors to more extensive infrastructures like data centres.

1. Chemical energy storage systems: The electrical energy generated is converted to chemical energy and stored in chemical fuels that can be easily converted into electrical energy. The energy generated can be stored in chemical forms such as hydrogen for a long time and used when necessary. In this case, energy is harvested from renewable energy sources such as solar or wind when conditions are good, like spring or summer and used during winter when conditions are not favourable for renewable energy generation.
2. Mechanical energy storage systems: The electrical energy produced is converted into mechanical energy (e.g., potential and kinetic energy) and stored in a mechanical energy storage system. The mechanical energy is stored to be easily converted back to electrical energy for consumption. Examples of mechanical energy storage systems include pumped hydro energy storage systems, gravity energy storage systems, compressed air energy storage systems, and flywheel energy storage systems. Mechanical energy storage systems are vast and complex. They may be used as an energy storage option for fixed IoT infrastructures like base station sites or data centres, provided there is space for it and the area's geography is suitable. It may not be an energy storage option for small IoT systems constrained by size and weight.
3. Electrothermal energy storage system: The electrical energy generated is converted to thermal energy, which is stored and used for heating, cooling, or conversion purposes for large-scale infrastructure (e.g., base stations, core network infrastructure, or fog/cloud data centres). The thermal energy can be stored and converted into electrical energy for consumption.
4. Hybrid energy storage system: This system combines multiple storage technologies (e.g., batteries with supercapacitors) to balance capacity, discharge rate, and longevity. It offers flexibility and performance optimisation for diverse IoT applications.

Most IoT devices are powered using a small energy storage system (e.g., battery or supercapacitor) with minimal energy capacity. The energy storage system, in the form of a battery or supercapacitor, is charged to its full capacity when the device is being deployed. The device is shut down when all the energy stored in the energy storage system is completely consumed or drained. The device's lifetime is the time from when the device is deployed to when all the energy stored in its energy storage system is consumed. The capacity of the energy storage is often chosen in such a way as to satisfy the energy consumption demand of the device and ensure a longer lifetime for the device. In a massive deployment of thousands or hundreds of thousands of IoT devices, frequent replacement or recharging of batteries or supercapacitors can be tedious and costly and may also degrade the quality of service.

An energy storage system is recommended mainly for IoT devices that require a tiny amount of power (in the order of micro- or milliwatts) to operate and spend most of their time in sleep mode to save energy. The lifetime of a low-power IoT device powered by a small battery is desired to be at least a decade. The energy storage systems' energy capacity is contained by its size and weight. That is, increasing the capacity of an energy storage system increases its size or weight. Still, it is desired to keep the size and weight of IoT devices as small as possible, especially in IoT applications where mobility is critical.

The computing and networking nodes at the edge/fog/cloud layer of the IoT architecture are energy-hungry devices not often powered solely by energy storage systems. They are often powered by a main power source, such as an electricity grid or renewable energy sources (e.g., wind, solar, pumped hydro-power). A backup energy storage system is often installed to store energy so that when the main power source fails (especially in the case where energy is generated from renewable energy sources as they are intermittent in nature), the energy storage system will supply the computing or networking node until the main source is restored.

Small IoT Devices

Most small IoT devices rely on compact energy storage systems such as batteries or supercapacitors. These devices are typically constrained by:

• Size and Weight: Energy storage capacity must be balanced with the need for compact designs.
• Energy Demand: Devices are optimised for low power consumption (in the range of micro or milliWatts) and often operate in sleep mode to conserve energy.
• Lifetime: The energy storage system's capacity determines the device's operational lifetime, which is designed to minimise frequent replacements or recharging.

The most common energy storage systems used in small IoT devices include:

• Batteries: Lithium-ion and solid-state thin-film batteries are standard in IoT devices due to their energy density and compact size.
• Supercapacitors: Provide rapid charging and discharging capabilities suitable for devices requiring quick energy bursts.

Large IoT Infrastructure

IoT infrastructure at the edge, fog, and cloud layers (e.g., base stations, access points, fog nodes, and data centres) require more robust and large-scale energy storage solutions. These include:

• Battery Energy Storage Systems: Provide reliable backup power.
• Hydrogen Energy Storage Systems: Store renewable energy in chemical form for long-term use.
• Thermal Energy Storage Systems: Store energy as heat, often used for cooling or reconverted to electricity.
• Mechanical Storage Solutions: Pumped hydro or flywheel systems can store vast amounts of energy for large-scale operations.
• Hybrid energy storage: A combination of two or more energy storage systems, e.g., supercapacitor and battery.

Such systems often serve as backup power sources to ensure uninterrupted operation during grid outages or renewable energy intermittency.

Electrical Energy Storage Systems

• Supercapacitors: For high-speed energy release in sensors or actuators.
• Superconducting Magnetic Energy Storage: Suitable for critical applications requiring rapid energy discharge.

Mechanical Energy Storage Systems

• Pumped Hydro: For large-scale energy backup in base stations or data centres.
• Flywheel Storage: Ideal for facilities needing rapid energy delivery.

Chemical Storage

• Flow Batteries: Provide scalability for varying energy demands.
• Hydrogen Storage: Stores renewable energy over long durations.

Thermal Storage

• Cryogenic Energy Storage: Stores energy in liquefied air, suitable for cooling-intensive applications.
• Phase-Change Materials: Efficiently store and release thermal energy.

• Energy Efficiency vs. Size: Increasing energy capacity often results in larger, heavier systems, which may conflict with the need for compact designs.
• Cost: Advanced energy storage systems, such as hydrogen or SMES, can be costly.
• Environmental Impact: Sustainable energy storage solutions are critical to minimising the ecological footprint of IoT systems.
• Reliability: Ensuring consistent performance over long periods, especially in critical IoT applications.

Energy storage systems are pivotal in enabling reliable, efficient, and sustainable IoT operations. These technologies, from small-scale batteries in sensors to large-scale mechanical systems in data centres, ensure that IoT infrastructures can function even without a direct power supply. IoT designers can meet the growing demands of connected ecosystems while addressing environmental and operational challenges by leveraging diverse storage options and optimising for specific use cases.

To deal with limitations of energy storage systems such as the limited lifetime (the time from when an IoT device is deployed to when all the energy stored in its energy storage system is depleted or consumed), maintenance complexity, and scalability, energy harvesting systems are incorporated into IoT systems to harvest energy from the environment. The energy can be harvested from the ambient environment (energy sources naturally present in the immediate environment of the device, e.g., solar, wind, thermal, radiofrequency energy sources) or from external sources (the source of energy is from external systems, e.g., mechanical or human body) and then converted into electrical energy to power IoT devices or storage in an energy storage system for later usage.

Energy harvesting is capturing energy from the ambient environment or external energy sources and then converting it to electrical energy, which is used to supply the IoT systems or stored for later usage. An energy harvesting system converts energy from an unusable form to useful electrical energy, which is then used to power the IoT devices or stored for later usage.

The energy can be harvested from ambient sources (environmental energy sources) such as solar and photovoltaic, Radio Frequency (RF), flow (wind and hydro energy sources), and thermal energy sources. Ambient energy harvesting is the process of capturing energy from the immediate environment of the device (ambient energy sources) and then converting it into electrical energy to power IoT devices. Each energy source has unique characteristics that make it suitable for specific IoT applications, providing tailored solutions to power devices based on their requirements. The ambient energy harvesting systems that can be used to harvest energy to power IoT devices, access points, fog nodes or cloud data centres include:

1. Solar and Photovoltaic Energy Harvesting

Source: Solar energy is derived from natural sunlight, while artificial light sources can be harnessed indoors. Solar panels or photovoltaic cells are the primary tools for capturing this energy.

Process: Photovoltaic (PV) cells, composed of semiconductor materials, absorb photons from light. This absorption excites electrons, generating an electric current that powers IoT devices or charges energy storage systems.

Applications:

• Outdoor IoT devices: Environmental sensors, agricultural IoT systems, and smart city deployments (e.g., solar-powered streetlights or traffic systems).
• Indoor IoT systems: Energy-efficient smart home devices like automated blinds or temperature controllers.

Advantages:

• Solar energy is abundant, renewable, and widely available.
• Photovoltaic cells can be scaled to suit various device sizes and energy needs.

Challenges:

• Performance depends on light availability, weather conditions, and shading.
• Energy storage systems (e.g., batteries) are required for use during periods of darkness or cloudy weather.

2. Radio Frequency (RF) Energy Harvesting

Source: RF energy is emitted by various wireless communication systems such as Wi-Fi routers, mobile networks, and television transmitters.

Process: RF energy is captured using specialised antennas and rectified to produce usable electrical power. Depending on the application, these systems can operate over various frequencies.

Applications: Low-power IoT devices: Wearable sensors, asset trackers, and remote controllers in urban and indoor environments where RF signals are prevalent.

Advantages:

• Utilises an omnipresent energy source in populated areas.
• Offers a continuous power supply in environments with dense RF activity.

Challenges:

• Energy output is relatively low and insufficient for high-power devices.
• Proximity to RF sources and signal strength significantly impact efficiency.

3. Flow Energy Harvesting

Source: Energy from the movement of air (wind) or water (hydro) is captured and converted into electrical energy.

Process:

• Wind energy: Micro wind turbines or harvesters capture the kinetic energy of moving air.
• Hydro energy: Small-scale hydroelectric systems capture water flow in rivers, streams, or pipelines.

Applications: Remote IoT devices in areas with consistent air or water flow, such as wind-powered weather stations or hydro-powered sensors in smart water management systems.

Advantages:

• Renewable and highly scalable for large and small IoT deployments.
• Provides a sustainable energy source in specific geographic locations.

Challenges:

• Requires consistent flow availability and favourable conditions for effective energy generation.
• Infrastructure needs can be costly and space-intensive.

4. Thermal Energy Harvesting

Source: Temperature differences or heat dissipation from industrial processes, human bodies, or natural sources.

Process: Thermoelectric generators (TEGs) use the Seebeck effect, where a voltage is generated due to a temperature gradient across a material, to convert heat into electrical energy.

Applications:

• Industrial IoT systems: Waste heat recovery from factories or power plants.
• Smart home devices: Heat-based systems for energy-efficient homes.
• Wearables: Powering smartwatches or fitness trackers using body heat.

Advantages:

• Utilises existing waste energy, improving overall energy efficiency.
• Ideal for applications with constant heat sources.

Challenges:

• Limited conversion efficiency.
• Reliance on stable and sufficient temperature gradients.

5. Acoustic Noise Energy Harvesting

Source: Pressure waves from sound or vibrations caused by machines, vehicles, or environmental noise.

Process: Piezoelectric or acoustic materials capture sound vibrations and convert them into electrical energy.

Applications:

• Urban IoT devices in noisy environments.
• Sensors in factories or other high-decibel areas.

Advantages:

• Exploits previously untapped sound energy.
• Can be deployed in areas with persistent noise.

Challenges:

• Low energy output.
• Efficiency depends on sound frequency and intensity.

Mechanical energy sources, such as vibrations and pressure changes, are prevalent in dynamic environments like transportation and industrial settings.

1. Vibration Energy Harvesting

Source: Vibrations generated by machinery, vehicles, or natural phenomena.

Process: Devices with piezoelectric or electromagnetic materials capture vibrational energy and convert it to electrical energy.

Applications:

• Monitoring industrial machinery health.
• Powering IoT sensors on vehicles or railways.

Advantages:

• Utilises existing mechanical energy.
• Ideal for environments with continuous movement.

Challenges: Dependent on vibration consistency and intensity.

2. Pressure and Stress-Strain Energy Harvesting

Source: Pressure variations or mechanical stress on materials.

Process: Piezoelectric materials produce electrical charges when subjected to stress or strain.

Applications:

• Medical sensors in wearable devices.
• IoT devices in hydraulic or pneumatic systems.

Advantages: Effective for compact devices.

Challenges: Limited applications outside specific industries.

The human body is a valuable energy source, especially for wearable and implantable IoT devices.

1. Human Activity Energy Harvesting

Source: Biomechanical movements like walking, running, or cycling.

Process: Kinetic systems convert movement into electrical energy, which can power wearables or charge onboard batteries.

Applications:

• Smart fitness trackers.
• IoT-enabled medical monitoring devices.

Advantages: Eliminates external charging needs.

Challenges: Energy generation depends on user activity levels.

2. Human Physiological Energy Harvesting

Source: Body heat, biochemical reactions, or other physiological processes.

Process:

• Thermal: Converts body heat into power using thermoelectric generators.
• Chemical: Biofuel cells harness energy from biochemical reactions.

Applications:

• Implantable medical devices like pacemakers.
• Continuous health monitoring systems.

Advantages:

• Supports self-sustaining devices.
• Minimizes maintenance for medical applications.

Challenges: Requires advanced materials for efficient energy conversion.

Hybrid systems combine multiple energy sources to ensure reliability and maximise efficiency. They are instrumental in scenarios where environmental conditions vary unpredictably.

Advantages:

• Reliable energy supply from complementary sources.
• Improved energy generation and storage flexibility.

Challenges:

• Complex integration of different energy harvesting mechanisms.
• Higher costs and design challenges for seamless operation.

Energy harvesting from ambient sources is a transformative approach to powering IoT devices sustainably. These systems provide self-sufficient, low-maintenance energy solutions by leveraging solar, RF, thermal, acoustic, and mechanical sources. Innovations in hybrid energy systems and advanced materials are expected to enhance the efficiency and applicability of energy harvesting technologies, paving the way for widespread adoption in IoT infrastructures across industries.