Energy harvesters for small devices create energy from the environment, such as where there is a temperature gradient, movement, radio frequency (RF) field, light and more. For most developers of energy harvesters, wireless sensors is a high priority market, where 90% of trials fail to move to adoption because the conventional battery in the wireless sensor makes the system unaffordable - finding and replacing batteries at frequent intervals is a challenge. Negating the conventional battery and replacing it with an energy harvester sees the life time extend to decades. While wireless sensors are commonly discussed, energy harvesting will also - and has already - had prolific use in many consumer electronics applications. From the humble piezoelectric lighter today sold in hundreds of millions to new consumer devices such as cellphones with assisted power from light and heat.
For grid connected systems, photovoltaics and wind power are commonly used in large energy harvesting farms. However, there are now other forms of energy harvesters developed initially at the micro level which are now being scaled to produce enough energy to replace or supplement grid power. For example, take the humble bicycle dynamo - based on an electrodynamic energy harvester. The same technology is also used in large scale wind power, but now it has been redesigned to work beyond a rotary means. EnOcean, Germany, offer more than 500 products based on this technology from light switches powered by pressing the switch to wirelessly monitored mouse traps powered by the mouse entering the trap. Re designing the decades old technology is now making other, larger scale applications possible. Many retailers across Europe are using the technology to power electronic signage on entry to the supermarket - the vehicle drives over a plate which moves and the energy generated by the harvester is captured and used. Energy harvesters can also be applied to stairs - as one walks up the stairs they depress slightly and the energy captured can be used for lighting or other applications - reducing reliance on the grid.
Others are scaling piezoelectric energy harvesters to become an alternative to grid power in some cases. Piezoelectric materials generate power when strained in a particular way. The devices are small but over a large area can amount to significant power generation. In Italy and Israel trials are underway with piezoelectric energy harvesting roads - the vibrations due to vehicle motion are turned into electricity. In a trial by the Israeli government, 2,000 watt hours of electricity was generated on a 10 meter stretch of highway. Highway energy Systems, UK, are installing a device that monitors the occupancy of parking spaces. An installation will provide about 36 KW in continual traffic. However, these technologies would be best placed where a vehicle is slowing down, otherwise the systems can lower the fuel efficiency of the vehicle. Approaches to crossings, ramps, parking lots etc are ideal locations.
In addition, companies such as Pavegen, New Energy Technologies, Powerleap, The Facility and Innowattech are among the leading developers, but supporting them with appropriate technologies are many large materials, energy storage and power electronics companies.
Localising Grid Energy Production
Another attraction to such systems is the means to locally generate grid power. Even with large scale renewables now such as photovoltaics and wind power, while very successful, they suffer from being required to be used in optimal locations, such as in high light levels, tracking the sun, or in areas with strong wind. Often where the energy is optimally generated it is not used and needs to be transported, conditioned and stored for use in the grid. Other energy harvesters such as those described above meet unmet needs. For example, powering signage or lighting on roads or in pedestrian areas can be done by harvesting energy from the traffic or footfall near it - no expensive infrastructure is needed.
