Electricity Generation From Ambient Energy Sources

Technology, 12 Oct - 2017 ,

Electricity Generation From Ambient Energy Sources
Credit: pixabay.com

In the era of civilization with electricity as a life line, only dependence on grid electricity can be threatened and disrupted by various natural and manmade calamities like floods, storms, earthquakes and attacks etc.

In the era of civilization with electricity as a life line, only dependence on grid electricity can be threatened and disrupted by various natural and manmade calamities like floods, storms, earthquakes and attacks etc. leaving people with most uncomfortable situations of living.  Need is to develop technology in order to exploit and utilize ambient sources of energy viz., sun, wind, wave, biomass, body energy etc. by converting this energy directly to its useful form as electricity and design devices making use of this low level energy. In the modern world, we are increasingly surrounded by digital sensors, cameras and communications devices 

sending data cloud-based analysis services. Those devices need power, and designers are finding new ways to draw it from ambient sources rather than rely on batteries or hard-wired grid connections. Hence, harnessing ambient energy sources is critical to the rollout of the internet of things in industry. This article examines the development is various ambient energy harvesters and their role in the growing internet of things. Though most of the digital sensors can easily be wired to existing electricity outlet from electrical grids, but this is not always practical for certain locations like the insides of air ducts or the undersides of trains. Similarly, batteries which must be periodically replaced or recharged, may also be unsuitable for hard-to-reach places and most of the time they are fully discharged in no time or at odd times due to human complacent nature to keep them (batteries) always charged.

The rapidly growing number of connected devices that form the backbone of the internet of things must become self-powered. The US research and advisory company Gartner, Inc. forecasts that 8,4 billion connected things will be in use worldwide in 2017, up 31% from 2016, and will reach 20,4 billion by 2020. Powering these with batteries or by connecting them to power networks would be totally impractical, even impossible. For consumer IoT devices and systems such as “smart” phones, wearables, appliances and multimedia equipment as well as some gear used in smart homes, this is not such a problem. These devices don’t come in large numbers in a personal environment and can be connected to the grid, or powered by disposable or rechargeable batteries, which can be easily accessed, replaced or recharged. The problem is totally different in an industrial IoT (IIoT) environment where countless connected devices and wireless power networks are meant to operate independently, over long periods, in places difficult to access or in harsh environments. Connecting these devices to a power network is often not feasible, impractical and costly. Replacing batteries every few weeks or months can also be impractical and costly, or not feasible when, for instance these devices are deployed in very high temperature environments unsuitable for batteries. The IIoT today still relies to a great extent on batteries. The best solution for these devices is to be, as far as possible, self-powered, by relying and drawing on ambient energy sources, a process known as energy harvesting (EH). Tomorrow’s IIoT will be powered by its environment, by EH. EH makes self-powered connected devices possible and brings connectivity for IIoT to previously unreachable places. It can also be combined with storage systems such as disposable and rechargeable batteries, capacitors or super capacitors, each with its own characteristics, making self-powered IIoT environments possible and flexible. 

Under all these circumstances of power failure from electrical grid and batteries, there is always a need for more versatile power sources and device makers are turning to energy harvesters which extract trickles of free power from ambient light, heat or motion. Energy harvesters are needed, for example, in wireless self-powered sensors and medical implants, where they could ultimately replace batteries. In the future, energy harvesters can open up new opportunities in many application areas such as wearable electronics. Energy harvesters and new sensing solutions are among the projected megatrends of the near future. Energy harvesters can replace batteries and other energy sources in applications where maintenance is difficult or impossible. Designs heading to the market include everything from comparatively simple solar cells to a wall switch that, rather than being hard-wired to a light fixture, uses the energy from pressing the switch to send a wireless signal to turn a light on or off. Batteries and wired connections are unlikely to go extinct any time soon, however. Since the ambient energy that harvesters rely on may not always be present, they cannot meet the demand for 100 percent reliability that some applications require. Instead, energy harvesters can be used alongside batteries to extend charge life and provide passive recharging. Still, those in the energy-harvesting business remain optimistic. They say the technology is improving, and that it will soon take off. This article highlights various types of ambient energy harvesting technologies generating electricity directly.

Energy harvesting (EH) sources

Mature EH technologies that can be used currently are:

·    Electrodynamic: like that produced in dynamos (bicycle dynamos, for instance), crank-up devices and connected or not to a storage system

·    Light EH : with energy captured from the sun, from infrared (IR) or ultraviolet (UV) lights via photovoltaic (PV) systems (used in calculators, watches, IoT beacons, wearables)

·      Thermoelectric: which relies on the Seebeck effect, which is the generation of a voltage across a material as a result of a temperature difference. Electricity is produced using thermoelectric generators (TEGs)

·      Piezoelectric: where energy from vibration or movement is captured by piezoelectric effect (most widespread use so far has been in lighters, but is seen as offering interesting prospects)

·    Electrostatic (capacitive):  in which energy is harvested by changing capacitor dimensions by force, e.g. by applying torsion, stretching using elastomer dielectric and stretchable electrodes

·     Magnetostrictive: with movement generating electricity when magnetic properties of a material are changed following applied stress

·   Triboelectric principle, where energy is generated through friction and based on contact electrification and electrostatic induction using materials with opposite charge affinity

·     Pyroelectric: using temperature changes

·    Ambient radio frequency (RF) radiation: with energy harvested from radio waves used for radio and TV transmissions, mobile and WiFi networks. RF EH is used to power RFID devices, such as transceivers (transmitters/receivers).

Vibration energy 

Kinetic energy can be converted into electrical energy by means of the piezoelectric effect; mechanically deforming a piezo crystal (or Micro Electro Mechanical System piezo-MEMS) with tension or pressure to generate electrical charges that can be measured as voltage on the electrodes of the piezo element. Piezoelectric vibration energy harvesters convert this mechanical vibrational energy into alternating electrical energy (AC). This AC is then electronically converted to DC, which can be used to drive wireless applications or recharge a battery. Piezoelectric energy transducers deliver maximum energy when operating at the resonant frequency of the vibrational source, and when operating into a load designed to match the piezoelectric output impedance. Research into small energy harvesters that turn mechanical vibration into electricity has focused on piezoelectric and electrostatic devices.  Research scientists have demonstrated a new technique for generating electrical energy which can be used in harvesting energy from mechanical vibrations of the environment and converting it into electricity. This technique does not require an integrated battery, electrets or piezo materials. Research scientists havesuccessfully generated energy by utilizing the charging phenomenon that occurs naturally between two bodies with different work functions. Work function is the amount of energy needed to remove an electron from a solid and it determines, for example, the well-known photoelectric effect. When two conducting bodies with different work functions are connected to each other electrically, they accumulate opposite charges. Moving of these bodies with respect to each other generates energy because of the attractive electrostatic force between the opposite charges. The energy generated by this motion can be converted into useful electrical power by connecting the bodies to an external circuit. This new energy conversion technique also works with semiconductors.  Scientists estimate that the new electricity generation technology could be introduced on an industrial scale within three to six years. 

Light energy

One of the most well-established energy harvesting technologies is solar energy, which has been used for many years to power small devices such as wristwatches and more recently to provide back-up emergency power for cellphones. Solar power collects sunlight and converts it into electricity, but light-to-electricity conversion efficiency is not very high; typical solar panels are rated at 15% or 20% efficiency and that’s under optimum conditions; solar panels can endure rain, cloudy skies (reducing output to about 20 to 30% of the current generated in bright sunshine) time of year differences (solar intensity is reduced in winter), changes in the number of hours of available daylight and other factors that negatively impact output. Semiconductor companies have developed controllers to optimize energy harvesting from solar panels. These controllers utilize Maximum Power Point Tracking (MPPT). This technique optimizes the match between the solar array (PV panels), and the battery bank or utility grid. Most MPPT's are around 93-97% efficient. Solar is better known for its panels, an alternative energy source that, like wind, feeds into existing power grids. But on a much smaller scale, photovoltaic (PV) energy is harvested and stored by small autonomous (off-the-grid) devices, using ambient indoor light as well as sunlight.  Wibicom Inc. in Montreal, Canada, has productized a combination PV harvester and antenna, which it offers with a range of sensors. Organic photovoltaic materials -- polymers combined with carbon-chain fullerenes, are a replacement for inorganic, typically silicon-based PV materials, which are less environmentally friendly both in manufacture and disposal.

Thermoelectric energy

Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors is converted into an electrical potential, or voltage. Using energy extracted from thermal gradients, low-power circuits can operate for years without the need for battery replacement. Thermal energy at low level can be tapped by taking advantage of available transducers and converter ICs.  For example, an energy-autonomous sensor for measuring airflow temperature might use copper to conduct warm air inside the tube where air is flowing, while a heat sink on the outside conducts the cooler ambient air, creating a difference in temperature between the sides of a Peltier element, and generating power for the sensor. Right now waste heat is being converted to renewable energy from factory equipment such as pumps and motors for different applications. Similarly, usable waste heat could be captured from the human body to power some of the wearable health and medical implant sensors being developed (for typical indoor air temperatures, a harvester attached to a person’s skin will be able to use a ∆T of up to 10°C). Energy harvesting technologies will work for really low-power wearables, not power-hungry smart watches.  Low-power wearables may soon bid adieu to batteries and start drawing energy generated by body heat and movement, and ambient energy from the environment. 

Energy from RF waves

Because of the widespread growth of wireless communications and a concomitant increase in the number of radio transmitters, especially for mobile base stations and handsets, there is a lot of essentially “free” RF energy around. Along with ubiquitous Wi-Fi sources, engineers can find RF energy sources ranging from short-range wireless technologies such as Bluetooth and ZigBee, to long-range cellular services. For RF energy harvesting, a receiving coil serves as the energy source, generating a voltage in response to electromagnetic coupling with the RF transmitter The challenge in RF energy harvesting lies in maximizing the output from the transducer at a given ambient energy level. Efficient RF energy-harvesting design has become simpler thanks to available components and ICs from a variety of manufacturers.

Energy harvested from body heat, motion and ambient light could be used in medical implants, monitoring sensors and disposable medical patches.  The technologies are still emerging, but the chip performance and energy efficiency of some wearables are reaching a point where it has started becoming convenient to replace the battery and replace it with ambient energy. In summary, harvesting ambient energy requires only a few basic components, including an energy source transducer--delivering energy that may come at random times, and in random, usually very small amounts a rectifier, an energy storage device, and an output regulator. Here, we have looked at the four major energy harvesting sources: light, heat, vibration and RF. To successfully design an energy harvesting system for a wireless node engineers have to consider the source, type of transducer available for that renewable energy source, where the node is located and the conditions under which it will operate, the required power levels of the node (including, usually, an MCU and radio) as well as the estimated system efficiency and some knowledge of the power-management electronics available for that type of energy harvesting system.

Ambient energy harvesting is fascinating, free and is everywhere and applications of ambient energy sources are limited only by imagination. Solar, wind, thermal, RF are some of the well-established energy harvesting techniques. It can not only reduce the maintenance cost by a huge amount, but also needed for sustainable future. Although promising, energy harvesting cannot power nodes in all kind of applications. It is not always feasible to obtain energy from the ambient sources.  Solar energy is vastly used in many household and industrial applications around the globe and we know that solar cells can harvest energy only in the presence of the Sun. No energy is harvested during the nights. Clouds can appear or disappear which can affect the harvesting process. Not just solar, but all energy sources are unpredictable. We can never predict the amount of energy we will harvest at some time of the day.  Energy prediction models can help us to predict the incoming energy based on the past energy levels but more than often, we don’t trust them so well. Energy arrival unpredictability can make things complex for network designers.


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