====== IoT Energy Sources ======
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A reliable energy source is required to keep an IoT device alive. An interruption is when the energy source shuts down the IoT device, increasing downtime and reducing the quality of service or the quality of experience the users feel. Therefore, choosing the energy source is very important when designing IoT systems. The following factors should be considered when selecting an energy source for an IoT device:

  *Size: In some IoT applications, it is required that the size of the IoT device should be as small as possible. The chosen power source should be one whose size can be scaled down as much as possible. 
  *Mobility: In some IoT applications, mobility is an essential requirement (e.g., in wearable IoT devices), imposing both size and weight constraints on the IoT devices. The energy source chosen should not be location-dependent sho, should be able to provide energy to the IoT devices during motion, and should not obstruct the device's mobility. 
  *Reliability: The energy source chosen should be reliable. That one will supply energy to the IoT devices when necessary with minimal failures. 
  *Scalability: The energy source chosen should be easily accessible at affordable prices to ensure a continuous supply of energy to the growing number of IoT devices and infrastructure. 
  *Cost: The energy source should be cheap, and the cost of energy should also be reasonable. 
  *minimum maintenance requirement: It is recommended to choose energy sources to ensure that the devices' lifetime is reasonably long. That is a minimal energy-related maintenance requirement, especially in large IoT networks. 
  *Ease of deployment: The energy source should be simple to integrate into the IoT system.  
  *IoT application context: The choice of the energy source depends mainly on the application context. 
  *Power requirement: The choice of the energy source also depends on the energy requirement of the device. The energy sources required for IoT devices in agriculture differ from those in smart factories or smart health applications.
  *Sustainability: The energy sources chosen should be green and sustainable. The energy source should produce minimal environmental pollution and be easily recyclable.

The choice of the energy source is critical in the IoT design process as it will influence the selection of the computing power, communication protocols and technologies, and the security mechanism and other subsystems of the IoT system. The three primary energy sources for IoT devices are: 
  *Main energy 
  *Energy storage systems
  *Energy harvesting systems. 


== Main power ==

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 mains power sources. The grid's main power is 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 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 ==

Energy storage systems are systems that are used to store energy so that it can be consumed later. It is preferable to power IoT devices using energy storage systems. One scenario is to charge the energy storage system (e.g., battery or supercapacitor) to its full capacity and then deploy the IoT device with the energy storage system as its only energy source (figure {{ref>Fig1}}). In this case, when all the energy stored in the system is depleted, the device is shut down, resulting in an undesirable downtime. 

<figure Fig1>
{{ :en:iot-open:hardware2:powering:iot_edge_battery.png?800 | The architecture of an IoT device powered by a battery energy storage system}}
<caption>The architecture of an IoT device powered by a battery energy storage system (( Kuaban, G. Suila, E. Gelenbe, T. Czachórski, P. Czekalski, and J. Kewir Tangka, "Modelling of the Energy Depletion Process and Battery Depletion Attacks for Battery-Powered Internet of Things (IoT) Devices", sensors, vol. 23, issue 6183, 2023 )) </caption>
</figure>

The time from when the IoT device is deployed to the instant when all the energy stored in the energy storage system is depleted is called the device's lifetime. Among other factors such as mobility, scalability, and size, lifetime of the device and the energy density, energy capacity, and cycle life of the energy storage system are critical design parameters that should be considered when choosing an energy storage system to use as an energy source for an IoT device. To increase the lifetime of an IoT device and reduce the downtime resulting from the depletion of all the energy stored in energy storage systems of IoT devices, energy harvesting systems are sometimes incorporated into IoT devices. 


== Energy harvesting systems ==

Energy harvesting systems are also an alternative energy source for IoT devices. They capture energy from the environment and convert it to electrical energy to supply IoT devices. Suppose the energy captured is more than the power demand of the IoT device. In that case, the surplus can be stored in energy storage systems when the energy harvesting system cannot produce enough energy to supply the IoT device. A significant drawback of energy harvesting is that the amount of energy that can be harvested at any given time depends mainly on environmental conditions or the presence of external energy sources, resulting in a fluctuation in the amount of energy harvested over time. Hence, it is vital to carefully size the energy harvesting unit and the energy storage system in such a way as to maximise the lifetime of the IoT device. Sample architecture of a self-powered Green IoT device powered by a battery energy storage system and an energy harvesting system is present in figure {{ref>Fig2}}.


<figure Fig2>
{{ :en:iot-open:hardware2:powering:iot_edge_device1.png?800 | The architecture of a self-powered Green IoT device powered by a battery energy storage system and an energy harvesting system }}
<caption>The architecture of a self-powered Green IoT device powered by a battery energy storage system and an energy harvesting system ((  Kuaban, G. Suila, T. Czachórski, E. Gelenbe, P. Czekalski, "A Markov model for a Self-Powered Green IoT Device with State-Dependent Energy Consumption", 2023 4th International Conference on Communications, Information, Electronic and Energy Systems (CIEES) 23 25 November, 2023, Plovdiv, Bulgaria, IEEE, 2023 (in press).)) </caption>
</figure>

The kind of energy harvesting system to be deployed depends on the available energy sources (e.g., light, radio frequency, heat, vibration, wind, etc.). The amount of energy produced by most IoT energy harvesting systems is minimal compared to the energy needs of the devices. Some of the factors that influence the choice of the energy harvesting system are the availability of the energy sources, the size of the device, the energy needs of the device, and the energy density of the energy source. In a scenario where one energy source can produce sufficient energy, more than one energy harvesting system can be deployed (hybrid energy harvesting sources).