The current hype about energy harvesting generates lots of attention in and beyond the scientific world. But it is not uncommon for research results to be extrapolated to claims which set the expectations of the public unrealistically high.
The reality is that for an energy harvester to be successful, it should somehow outperform batteries. For many applications, the combination of a small solar cell and a rechargeable battery will always be the best solution.
Energy harvesters will add value for applications where batteries or solar cells are undesirable because they cannot last long enough, cannot be applied easily, or have weak performance. The list includes applications where a battery replacement would be costly or impossible and applications that don’t have access to enough light.
Two application areas have been identified as specifically suitable for vibration energy harvesting: predictive maintenance in the industrial machine environment and tire-pressure monitoring systems (TPMS) for car tires. For both areas battery replacement is difficult and costly because batteries are needed in hard-to-reach locations. In addition, light is weak or non-existent, but vibrations are present, and there is a market demand for wireless sensing.
Many energy harvesters are still not ready for prime time.
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The table above shows an estimate of power levels for centimeter-sized harvesters. It illustrates the excellent position of solar cells for outdoor applications and shows there is still some way to go for other techniques.
The technology for downsizing thermal harvesters is still under development, and the approach has not yet reached high-enough power levels. The table also shows that, where industrial vibrations are available, small-sized harvesters can generate a power level of about 100 µW, which is reasonable given the specifications for autonomous systems.
Next to generating energy, the harvesters should also rectify the energy, convert it, and temporarily store it. Therefore, they also need an efficient power management circuit with a storage capacitor or rechargeable battery.
The other side of generation is energy consumption. Harvesters can only be effective in true low-power-consuming circuits. Therefore it is key to have a real low-power design for the sensor electronics and to make use of smart duty cycling. Taking these factors into account, a harvester that generates around 10 µW is sufficient for an autonomous wireless sensor system.
When looking at what is already available on the market today, we see only a few cm-sized vibration energy harvesters. For example, Omron has developed a low-frequency fine-machined harvester, and Imec has a high-frequency MEMS harvester. Both are currently undergoing field tests for machine monitoring and TPMS apps respectively. Depending on the results of these tests, these harvesters may soon find their way into the first applications of small vibration energy harvesters.
— René Elfrink is a senior researcher who works on sensors and energy harvesters at the Imec research institute in Belgium.