January 30, 2018
Energy harvesting promises to sweep aside a key impediment to ubiquitous sensing and, in the process, facilitate the realization of the internet of things (IoT). No longer dependent on powerline energy supplies and batteries, engineers can now use this technology to open the door to a broader field upon which to deploy all types of sensors.
In some cases, harvesting precludes the use of batteries all together. In others, it allows devices to operate for much longer periods of time without requiring engineers to replace batteries. This enables sensors and other electronic devices to function virtually autonomously in a broad range of applications, including remote and difficult-to-access locations.
The rise of energy harvesting is driven by advances that make the technology more practical and desirable for mobile and IoT applications. Energy conversion systems can now capture even trace amounts of energy from the environment and transform them into electrical energy, making the systems relevant in more use cases.
Another factor in play here is the fact that electronic systems have become more energy efficient, bringing the demand for power down into a range that harvesters can support. In particular, processor technology has achieved greater power efficiency, which reduces overall system power consumption. Growth of the number of low-power electronic devices—driven by demand for mobile and wearable products—also increases the number of use cases where the services of harvesting systems can be brought to bear.
Before designers can take advantage of energy harvesting, however, they have to understand how power conversion works. The process begins with a transducer converting ambient energy—such as mechanical, thermal, light, electromagnetic energy, and chemical and biological energy sources—into harvested power. An example of this would be a piezoelectric generator transforming mechanical vibrations, strain or stress into electrical voltage or current that the system and the energy storage devices can use. This means matching the input impedance to provide maximum harvested energy, charging intermediate energy storage, routing power from a primary cell battery and generating the correct output voltage for the system.
In the final step, the energy conversion system buffers the processed energy in a storage device (such as a super capacitor or battery) to meet the device’s ongoing power requirements. These include providing support for sensing, data processing and communications functions.
Designers have to consider a number of technical issues and variables when developing a harvesting system for a device. But perhaps the most fundamental lies in the nature of the energy source.
When designing a self-powered system, the engineer should start by estimating just how much power can be harvested from the available energy. For example, if the engineer is developing a thermoelectric harvester, the first step is to measure the temperature gradient that can be developed. If the designer plans to use vibration as the energy source, then the engineer must evaluate the acceleration level and vibration frequency. Once the available energy has been established, the designer can select the best harvester type for the application.
The next issue to consider is the energy-balance equation, where the engineer balances the harvester-enabled device’s power demand with available energy. At this point, the development team may have to apply ultra low-power system design techniques to reduce the power requirements of the device, bringing power demand in line with available energy.
Designs may also require optional components to meet the application’s requirements. For example, an application may call for an electronic interface module to condition the energy captured from a low-voltage source. This may take the form of a low-voltage step-up booster module.
The engineers may also have to include a supplementary energy storage device, such as a thin-film battery or a super capacitor.
Adding more components can often improve performance, but there’s a catch: When considering the inclusion of optional components, development teams should remember that adding these new elements would likely increase the system’s energy consumption. The best way to handle the cost–benefit trade-offs is by adopting a holistic view.
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