Oak Ridge National Laboratories has developed a unique pyroelectric generator that can cool electronic devices, photocells, computers, and even large waste-heat producing systems while generating electricity. The device is based on a MEMS pyroelectric capacitor at the end of a bimetal cantilever that oscillates between hot and cold surfaces. The tip of the hot cantilever comes into contact with a cold surface, the heat sink, where it rapidly loses its heat and causes the cantilever to move back and make contact with the hot surface. The oscillation continues as long as long as there is a sufficient temperature differential – anywhere from a few degrees to several hundred degrees – between the two surfaces.
The cantilever structures are only about 1 mm² and generate 1 to 10mW per device; however 1,000 of them can be attached to a one square inch substrate, creating a relatively high output power source. Due to the fast cycle time of the cantilever, the developer projects 10 to 30% efficiency – far better than current thermoelectric and piezoelectric energy harvesting devices.
Photovoltaic cells are the most widely used energy harvesting source, but they are not very efficient. The best monocrystalline PV cells – with a theoretical maximum efficiency of 30% – do well to top 20% efficiency. Now scientists at the University of Missouri and the Idaho National Laboratory have developed a flexible solar film that can theoretically achieve 90% efficiency.
In contrast to conventional photovoltaic cells, the film is essentially an array of nanoantennas (or “nantennas”), each tuned to a specific frequency of light. Rather than generating single electron-hole pairs, as in the case of PVs, the incoming electromagnetic field from the sun induces a current in the antenna that is then collected at the feed point, rectified, and stored. Nanoelectronic electromagnetic collectors (NECs) can be configured as frequency selective surfaces to efficiently absorb the entire solar spectrum. Or NECs can be configured as a reflective bandpass filter centered at a wavelength of 6.5µm; this would enable them to absorb infrared rays, thus recycling waste heat from engines, furnaces, and other high-temperature power sources.
NEC devices have been successfully prototyped on both silicon and polyethylene substrates, however developing economical mass production processes will require further funding and time. The researchers foresee a product that complements conventional PV solar panels by capturing currently unused infrared energy. As a film, it could be incorporated into building materials and infrastructure. NECs can be integrated into polymer materials so they might also be incorporated into the skin of consumer electronic devices to continuously charge batteries.
The development of ultra-low-power MCUs has created a huge and rapidly expanding energy harvesting market on which they are becoming increasingly dependent. The first wave of energy harvesting has given rise to low-power wireless sensors, which are turning up seemingly everywhere. But the ripple effect will continue throughout consumer, industrial, and medical markets, creating new applications that we can only begin to imagine. Whether planning portable battery-powered devices or the desire to improve the energy efficiency of larger ones, all design engineers should consider incorporating energy harvesting techniques into their products. Investigate a wide range of energy harvesting devices at Mouser Electronics’ Energy Harvesting site to find out more.