Green Energy Sources in IoT

Powering IoT devices using energy storage systems (e.g., batteries or capacitors/supercapacity/ultracapacitor) faces some challenges, 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. In an IoT infrastructure with massive numbers of IoT devices (e.g., massive IoT), frequent change of IoT batteries results in maintenance complexities, high cost, and sustainability challenges. One of the solutions to these challenges is using energy harvesters to harvest energy from the ambient environment or external sources (e.g., vibrations or human body sources) to power the IoT devices. Energy harvesting is capturing energy from the ambient environment or external sources, which is then converted into electrical energy that can be used to power IoT devices or stored for later use.

IoT end node devices (Edge class) are usually powered with low current and voltage. This raises new capabilities to use green energy sources, which is essential in particular in distant and remote locations (e.g. earthquake sensors).

When selecting a renewable energy source, it is essential to consider:

• An energy budget - is the renewable energy source able to deliver enough energy for the duty cycle of the IoT device?
• Is there a need to provide a backup energy source to ensure continuous operation of the IoT device?
• How do ageing and time (daytime, season) affect the energy received from the green energy source?
• What is the cost of the green energy source compared to other powering opportunities?
• Is there an AC needed?

Answers to those questions drive a selection of the green energy source, which always regards a specific duty cycle and working conditions of the IoT device.
A short characteristic of selected green energy sources can help during powering design.

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 airflow (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 generated from temperature differences and converting it into electrical energy to power IoT systems and other IoT infrastructures.
• Acoustic noise: Capturing the energy from the pressure waves produced by a vibrating source and converting it into electrical energy to power IoT devices.

Solar and photovoltaic energy harvesting
So far, solar energy is the easiest and most widespread option to power remote IoT devices. It is available virtually worldwide, simple to implement and integrated with other energy resources.

Solar energy is grabbed using solar panels. Solar panels deliver DC. Solar panels work best in mid-temperature (overheating decreases efficiency) and when located perpendicular to the solar rays. As the sun changes its position during the day and during the season, it is important to ensure the correct angle to maximise the solar exposition possible when using a fixed mounting of the solar panels. Some active trackers can follow the sun's location in the sky and change the angle of the solar panel accordingly, but that requires an additional control system and extra energy. Tracking is usually unsuitable for small, low-powered IoT devices such as sensors.

Depending on the region, the weather (primarily clouds, snow and rainfalls) seriously impacts panels' efficiency and, thus, the amount of energy available. For this reason, it is common for solar panels to be oversized and equipped with backup energy storage, such as a battery pack. Moreover, in subpolar and polar regions, daylight is very short during the winter or even unavailable for latitudes beyond the polar circle.

Radio frequency (RF) energy Radiofrequency (RF) energy harvesting is among the most popular energy harvesting technologies developed to power self-powered IoT devices like IoT RFID tags and smart cards. The RF electromagnetic is captured and converted into electrical energy, which is then used to power the IoT devices or stored in a battery or capacitor/supercapacitor/ultracapacitor to be utilised later. Specialised antennas (including RF input filter and impedance matching network) are used to capture RF signals rectified by passing them through a rectifier, which rectifies the RF signals, converting the RF power into DC power. The DC power can then be used to power IoT devices or stored for later use.

The sources of RF signals could be from mobile cellular networks, radio and television wireless transmitters, and other Wireless access points (e.g., WiFi). The RF energy harvesting is influenced by the frequency of the signal, antenna gain, and the distance of the device from the source of the RF signal (e.g., the distance of the IoT device from a cellar base station, especially in situations where a cellular base station generates the RF signal). Although the amount of energy harvested from RF sources is relatively tiny, RF energy harvesting systems can easily be implemented. RF energy is readily available, making RF energy harvesting a cheaper and more convenient energy harvesting solution for power tiny and low-power (less energy-hungry) IoT devices. A significant possible drawback of RF energy harvesting is that when millions or tens of billions of RF-powered IoT devices are deployed in a given environment, RF energy harvesting may pose a health risk.

Flow energy

1. Wind energy
Wind energy is grabbed using a wind turbine, which converts rotation into a magnetic field that generates electric energy. Raw turbine delivers AC, which must be converted into the DC suitable for IoT devices. This conversion drops efficiency.

Wind energy is weather-dependent, so it is usually not a single energy source but works in parallel with other sources and frequently requires backup energy storage. Winds too strong for a turbine may cause damage; thus, the turbine has to be switched off in such cases. When the wind is too low, it cannot push the propellers, so energy is not generated.

Wind turbines tend to be big, and as they contain complex mechanics (blades, rotor, gear, generator), they require inspection and maintenance. Thus, they are not suitable for the “set and forget” IoT applications.

2. Hydro energy
Water energy is considered a stable energy source, eventually depending on the season. Its advantage is the ability to generate energy for a whole day, regardless of the day and night. Water energy is complex in use, however, because it uses additional infrastructure (e.g. pipes that deliver water).

Water turbines work with similar principles to wind turbines but use water to push the propellers instead of wind. Water turbines generate AC, so that needs to be converted to DC. Because of their size and the need for maintenance, they share a similar development area as wind turbines.

Water can also be considered as a backup energy battery regarding gravity: during the energy overhead, it can be pumped with that energy up, and then, thanks to the gravity and use of water turbines, this energy can be re-used when there is a lack of other energy resources. This process is used on a large scale and is known as a “pumped storage power plant”. However, principles remain scalable; they involve complex infrastructure and other (usually green) energy sources such as wind or solar.

Due to complexity, water turbines are not the first choice to power small IoT devices but rather to set up a local medium-scale energy source or support the grid.

Water has recently been considered a medium to generate hydrogen using external energy sources (such as solar panels or wind). When energy is available, hydrogen is generated and stored in tanks; later, it is used for energy generation using fuel cells and converted back from hydrogen and oxygen into water. Similar to the aforementioned pumped storage power plant, this solution delivers clean energy storage but also requires complex and extensive infrastructure. Hydrogen is also an explosive gas.

Thermal energy harvesting

Thermal energy harvesting is the capture of thermal energy and conversion into electrical energy to power IoT devices or store it for later use. Thermal energy is readily available in the environment (at home, in factories, and in regions with high temperatures). Some heat sources include car engines, geothermal heat from the ground, and heart from industrial operations. With the use of thermoelectric generators, thermal energy is captured and converted into electrical energy to power IoT systems or to store for future use.

Geothermal energy is considered to be very constant but of low availability. Its application is based on steam and hot water conversion to electrical energy, usually via high and low-pressure turbines. Due to the complex processing involving dealing with high temperatures (e.g. overheated steam of >200C)^[1], it is suitable for mass-scale energy production for a grid rather than as a small energy source to power a single IoT device.

Below is a short list of energy harvesting characteristics from non-ambient, in general, external sources.

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 valuable 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 subject to mechanical strain, produce an electrical charge proportional to the stress applied to it.

Energy harvesting from human body sources

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

• Human activity energy harvesting - capturing the biomechanical energy resulting from human activities (walking, cycling, running, and other 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.