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:

• 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 a significant amount of energy), they can be reliably powered using main power sources. The main power from the grid is in the form of AC power and should be converted to DC power and scaled down to meet the power requirement of sensing, actuating, computing, and networking nodes. The hardware devices are the networking or transport layer, and those at the application layer (fog/cloud computing nodes) are often power-hungry and are supplied using energy from the grid.

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 it's 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 using energy storage systems. The energy is stored in forms that 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. The various categories of energy storage systems include:

1. Electrostatic energy storage systems:
2. Magnetic energy storage system
3. Electrochemical energy storage systems
4. Chemical energy storage systems: The electrical energy generated is converted to chemical energy and stored in the form of 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 then used when necessary. In this case, energy is harvested from renewable energy sources such as solar or wind when conditions are good like during the spring or summer and used during winter when conditions are not favourable for renewable energy generation.
5. Mechanical energy storage systems: The electrical energy produced is converted into mechanical energy (e.g., potential and kinetic energy) which is stored by a mechanical energy storage system. The mechanical energy is stored in such a way that it can easily be 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 very large and complex and may be used as an energy storage option for fixed IoT infrastructures like base station sites or data centres provided that there is space for it and that the geography of the area is suitable. It may not be suitable as an energy storage option for small IoT systems that are constrained by size and weight.
6. Electrothermal energy storage system: The electrical energy generated is converted to thermal energy which is stored and used for heating, cooling or converted purposes for large-scale infrastructure (e.g., base stations, core network infrastructure or fog/cloud data centres). The thermal energy can be stored in such a way that it can be converted into electrical energy for consumption.
7. Hybrid energy storage system

Most IoT devices are often powered using a small energy storage system (e.g., battery or supercapacitor) with very limited 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. When all the energy stored in the energy storage system is completely consumed or drained, the device is shut down. The time from when the deployed to when all the energy stored in its energy storage system is consumed is called the lifetime of the device. 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 very tedious and costly and may also degrade the quality of service.

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

The computing and networking nodes at the edge/fog/cloud layer of the IoT architecture are energy-hungry devices that are not often powered solely by energy storage systems. They are often powered by a main power source from an electricity grid or from 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.

1. Batteries
   • Lithium-ion batteries
   • Lead acid batteries
   • Alkaline batteries
   • 3D-printed Zinc batteries
   • Solid-state thin film batteries
2. Supercapacitor
3. Superconducting magnetic energy storage
4. Hybrid energy storage system

Energy storage systems for edge/fog/cloud layer devices (access points, base stations, fog computing nodes, cloud data centers)

1. Battery energy storage systems
2. Hydrogen energy storage systems
3. Thermal energy storage systems
4. Supercapacitors
5. superconducting magnetic energy storage
6. Pumped hydro energy storage
7. Hybrid energy storage systems

electrical energy storage (supercapacitor, superconducting magnetic energy storage) mechanical energy storage (flywheel, pumped hydro storage, CAES) chemical storage (Including cold storage, such as conventional batteries, flow batteries, hydrogen energy storage, gas storage, biomass and cryogenic energy storage (liquid air energy storage)) Thermal energy storage (aquiferous cold energy storage, cryogenic energy storage, high-temperature storage, such as water tanks, phase-change materials, and concrete thermal storage.)

In order 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, Radio frequency 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 the process of 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.

Energy harvesting from ambient energy sources
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. 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:

• Solar and photovoltaic energy harvesting: capturing natural light energy (in the case of light) or artificial light (in indoor deployments) and converting it into electrical energy to power IoT devices.
• Radio frequency (RF) energy harvesting: Capturing RF energy from the environment and converting it into electrical energy to power IoT devices.
• Flow energy harvesting: Converting the energy generated from the flow of air (e.g., wind energy harvesting) or water (e.g., hydro energy harvesting) into electrical energy to power IoT or other IT infrastructures.
• Thermal: Capturing the energy that is generated from temperature differences and converting it into electrical energy to power IoT systems and other IoT infrastructures.
• Acoustic noise: Capturing the energy resulting from the pressure waves produced by a vibrating source and converting it into electrical energy to power IoT devices.

Harvesting energy from external sources

1. Energy harvesting from mechanical sources
   • Vibration energy harvesting: harvesting the energy created by vibrations (e.g., due to car movements, operations of machines etc.) and converting it into useful electrical energy, which can be used to power IoT devices or stored in the battery for later use.
   • Pressure energy harvesting: Harvesting the energy from pressure sources and converting it into useful electrical energy.
   • Stress-strain energy harvesting: Harvesting energy from mechanical vibrations by exploiting the property of some materials (e.g., piezoelectric materials) that, when they are subject to mechanical strain, produce an electrical charge that is proportional to the stress applied to it.
2. Energy harvesting from human body sources

Human body energy harvesting is the process of harvesting energy from the human body and then converting it to electrical energy, which is used to power wearable IoT devices, especially IoT devices designed for smart health applications. The source of energy could be from the vibration or deformations created by human activity (mechanical energy). The source of energy could be from human temperature differences or gradients (thermal energy) or from human physiology (chemical energy).

• Human activity energy harvesting: Capturing the biomechanical energy resulting from human activities (walking, cycling, running, and other forms of exercises) and then converting it into useful electrical energy that can be used to power the IoT devices or stored for later use.
• Human physiological energy harvesting: Capturing the biochemical energy resulting from human physiological processes and then converting it into electrical energy that can be used to power IoT devices, especially medical implantable IoT devices.