As mobile phones morphed from wireless analog telephones to handheld computers, users kept demanding more and more energy-consuming features, such as web browsing, videos, gaming, and email, while still requiring extended battery life. Since battery makers were not much help, semiconductor manufacturers devised numerous energy-saving techniques to make it all possible. They have been wildly successful.
Low power has been the most important electronic design criterion for at least the last ten years. Thanks to Moore’s Law and a lot of smart engineers, semiconductor power levels have dropped dramatically, often consuming milliwatts in run mode and nanowatts in standby mode. As a direct result, ultra-low-power wireless sensorless networks finally became possible and their adoption has been dramatic. Now, sensors stand alone in remote or hard-to-reach areas to warn of building and bridge stresses, air pollution, forest fires, pending landslides, worn bearings, and wing vibration. Low-power wireless sensor networks are at the heart of numerous industrial, medical, and commercial applications.
However, off-grid, as well as portable sensor nodes, rely on batteries for power and face the same problem as cell phones. In such cases, it is advisable to prolong battery life by harvesting environmental energy sources – most often available as light, heat, vibration, motion, or ambient RF. If a device’s energy requirement is low enough and battery replacement would be difficult or expensive, it may be possible to dispense with the battery altogether and rely exclusively on harvesting ambient energy sources for power. The combination of ultra-low-power MCUs and energy harvesting have given rise to a wealth of applications that previously were not possible.
The energy harvesting market is large and growing rapidly. According to analysts at IDTechEx, energy harvesting was a $0.7 billion market in 2012 and is expected to exceed $5 billion by 2022; by then 250 million sensors will be powered by energy harvesting sources. The market for thermoelectric energy harvesting alone will reach $865 million by 2023.
There are several energy harvesting technologies in common use, with some innovative techniques just over the horizon. The most common energy sources are light, heat, vibration, and RF. Short of rooftop solar panels none of them generate a great deal of energy (see Figure 1), but one or more of them may be more than adequate to power low-power devices in a particular environment.
|Source||Source Power||Harvested Power|
|Indoor||0.1 mW/cm²||10 µW/cm²|
|Outdoor||100 mW/cm²||10 mW/cm²|
|Human||0.5m at 1 Hz|
|1m/s² at 50 Hz||4 µW/cm²|
|Machine||1m at 5 Hz|
|10m/s² at 1 kHz||100 µW/cm²|
|Human||20 mW/cm²||30 μW/cm²|
|Machine||100 mW/cm²||1-10 mW/cm²|
|GSM BSS||0.3 µW/cm²||0.1 µW/cm²|
There is hardly a home or office that does not have at least one solar-powered calculator – actually, a calculator with a coin-cell battery and a small front panel photovoltaic (PV) cell to top it up. These polycrystalline silicon or thin-film cells convert photons to electrons with a typical efficiency of about 15 to 20% for polycrystalline and 6 to 12% for thin film cells. Since the power available from indoor lighting is typically only about 10 µW/cm², their usefulness depends on the size of the module plus the spectral composition of the light.
Small solar cells are frequently used in consumer and industrial applications, including toys, watches, calculators, street lighting controls, portable power supplies, and satellites. Since light sources tend to be intermittent, solar cells are used to charge batteries and/or supercapacitors to provide a stable energy source.
Thermoelectric harvesters exploit the Seebeck effect, where a voltage is created when a temperature differential exists at the junction of two dissimilar metals. Thermoelectric generators (TEGs) consist of an array of these thermocouples connected together in series to a common heat source such as an engine, water heater, or even the back of a solar panel. Output depends on the size of the TEG and the temperature differential that can be maintained. TEGs are typically used to power wireless sensor nodes in high-temperature environments such as industrial heating systems. A TEG mounted between a power transistor and its heatsink can recycle some of the energy that would otherwise be lost as heat.
Micropelt’s TE-CORE7 Thermal Energy Harvesting Modules convert locally available waste heat to provide long-life operation for low-power devices. The TE-CORE TEG converts heat to an electrical charge which is then boosted, stored in a 100µF capacitor, and regulated to supply up to 5.5V. Running at 50°C the TE-CORE7 can deliver 6.424mAh annually, the equivalent of three to four AA batteries – at that rate the batteries would need to be changed every few months.
Forcing a current to flow through the junction of dissimilar metals will cause heat to transfer from the hot to the cool junction – the Peltier effect, essentially the opposite of the Seebeck effect. The Peltier effect is the basis for thermoelectric heat pumps.