Power/Performance Bits: Dec. 9

Solar capture and storage
Researchers at the University of Houston developed a device capable of both capturing and storing solar energy. Unlike solar panels and solar cells, which rely on photovoltaic technology for the direct generation of electricity, the hybrid device captures heat from the sun and stores it as thermal energy.

The device combines molecular energy storage and latent heat storage to produce an integrated harvesting and storage device for potential 24/7 operation. The researchers report a harvesting efficiency of 73% at small-scale operation and as high as 90% at large-scale operation.

Up to 80% of stored energy was recovered at night, and the researchers said daytime recovery was even higher.

Hadi Ghasemi, Associate Professor of Mechanical Engineering at UH, said the high efficiency harvest is due, in part, to the ability of the device to capture the full spectrum of sunlight, harvesting it for immediate use and converting the excess into molecular energy storage.

The device was synthesized using norbornadiene-quadricyclane as the molecular storage material, an organic compound that the researchers said demonstrates high specific energy and exceptional heat release while remaining stable over extended storage times. Ghasemi said the same concept could be applied using different materials, allowing performance – including operating temperatures and efficiency – to be optimized.

T. Randall Lee, University Chair professor of chemistry at UH, said the device offers improved efficiency in several ways: The solar energy is stored in molecular form rather than as heat, which dissipates over time, and the integrated system also reduces thermal losses because there is no need to transport the stored energy through piping lines.

“During the day, the solar thermal energy can be harvested at temperatures as high as 120 degrees centigrade (about 248 Fahrenheit),” said Lee. “At night, when there is low or no solar irradiation, the stored energy is harvested by the molecular storage material, which can convert it from a lower energy molecule to a higher energy molecule.”

That allows the stored energy to produce thermal energy at a higher temperature at night than during the day, boosting the amount of energy available even when the sun is not shining, he said.

Knee power
Researchers at the Chinese University of Hong Kong developed a lightweight energy harvester for scavenging energy from the movement of the human knee during regular walking. The device could power wearable electronics such as pedometers, health monitors, and GPS.

The device makes use of piezoelectric macro fiber composites, lightweight materials that can produce electricity under deformation. The proposed energy harvester employs a bending beam and a slider-crank mechanism to capture the motion of the human knee when walking. Then, the captured motion is used to deform piezoelectric macro fiber composites pieces bonded to the bending beam so that electricity is produced when the human knee flexes or extends.

Wei-Hsin Liao, a professor at CUHK, said, “The human knee joint has a larger range of motion than other lower limb joints such as the ankle and hip, which enables energy harvesters to capture the motion more easily and generate more electricity.”

The prototype harvester can generate an average power of 1.6 mW, when the wearer walks at about 2-6.5 km/h. The generated electricity is efficient to power common wearable electronic devices such as smart bands.

The prototype weighs only 307 grams, making the wearer’s metabolic cost when walking with the device almost the same as that when walking without it.

“This apparatus will attract much attention from mountaineers and hikers,” Liao predicts. “If they get lost in remote mountains or a wilderness where the power grid is unavailable, the device can derive energy from their motion and convert it to electricity, enabling wearers to continuously monitor their vital signs, know their position, or even send out an SOS signal at any time when they need help. At present, we are focusing on improvement in the performance of the harvester by reducing the weight of the device and increasing energy harvesting efficiency. We plan to commercialize the harvester and market it through cooperating with garment manufacturers to embed the device in sportswear.”