Power/Performance Bits: Aug. 29

Colored solar panels; magnetic chirality memory; energy from sweat.


Colored solar panels
Researchers from AMOLF, the University of Amsterdam (UvA) and the Energy Research Centre of the Netherlands (ECN) developed a technology to create efficient bright green solar panels in the hopes that a greater array of colors will prompt greater adoption among architects and builders who might see the traditional blue or black panels as an eyesore.

The panels have a green appearance from most angles and show about a 10% power reduction due to the loss of absorbed green light.

“Some people say ‘why would you make solar cells less efficient?’ But we can make solar cells beautiful without losing too much efficiency,” said Verena Neder, a researcher at AMOLF. “The new method to change the color of the panels is not only easy to apply but also attractive as an architectural design element and has the potential to widen their use.”

Most research on solar cells has focused on increasing efficiency and reducing cost. Currently, the solar panels sold to consumers can ideally turn up to 22% of the sun’s light into usable energy. Colored solar panels are already on the market, but the dyes and reflective coatings that give them their color greatly reduce efficiency.

Left: The nanopatterned module appears green, independent of the angle. Right: Schematic of silicon nanoscatterer arrays on top of a sapphire cover slide, integrated into a commonly used solar panel design. (Source: Neder et al.)

The new process uses soft-imprint lithography to imprint a dense array of silicon nanocylinders onto the cell surfaces. Each nanocylinder is about 100 nanometers wide and exhibits an electromagnetic resonance that scatters a particular wavelength of light. The geometry of the nanocylinder determines which wavelength it scatters and can be fine-tuned to change the color of the solar cell. The imprint reduces the solar panel’s efficiency by about 2%.

“In principle, this technique is easily scalable for fabrication technology,” said Albert Polman, a scientific group leader at AMOLF. “You can use a rubber stamp the size of a solar panel that in one step, can print the whole panel full of these little, exactly defined nanoparticles.”

Next, the researchers are designing imprints to create red and blue solar cells, with the end goal of combining the different nanoparticles to create white.

Magnetic chirality memory
Physicists at the University of Nottingham and York University discovered a magnetic phenomenon they think could provide a route to creating a new class of highly efficient, non-volatile information processing and storage technology.

In the course of research into the use of magnetic domain walls (local regions of magnetic charge usually driven by magnetic fields) to increase capacity for information storage and logical processing, the team was able to manipulate the structure of a magnetic domain wall by controlling the chirality of the vortex domain wall using an electric field.

The main benefit of using magnetism is its non-volatile storage of information. Magnetic random access memory (MRAM) is one example, where information is written using electrical current which generates heat and stray magnetic fields.

A magnetic domain wall forms in a magnetic wire and separates regions where the magnetization points in opposite directions. Under certain conditions it consists of a region in which the magnetization rotates around a central vortex core, which points into or out of the wire.

An analogy would be the way in which water rotates around a vortex core as it drains down a plug hole. The sense of rotation of the magnetization in the vortex wall — its chirality — can be clockwise or anticlockwise. There have been proposals to use the chirality to both store and process information. The problem is finding a way to manipulate the vortex domain wall.

Previously, it has been shown that the chirality can be manipulated by applying magnetic fields to complicated nanowire geometries, but the use of magnetic fields is wasteful of energy and limits the ability to address individual domain walls selectively.

The team used the strain induced by an electric field applied to a piezoelectric material (which deforms mechanically in response to an electric field) to manipulate the chirality of the domain wall.

“We didn’t set out to switch the chirality of the domain walls,” said Andrew Rushforth, from the School of Physics and Astronomy at Nottingham. “We were actually trying to see if we could make them move. When we noticed that the chirality was switching, we were rather surprised, but we realized that it was an interesting and novel effect that could potentially have important applications. We then had to go back to the office and perform micromagnetic calculations to understand why and how the phenomenon occurs.”

The research is at an early stage. Until now, it hasn’t been obvious how one could control magnetic domain walls reversibly and predictably using electric fields. The next stage will be to investigate how the chirality switching depends upon the material properties and the geometry and dimensions of the magnetic wire. The University of Nottingham has filed a patent application for a memory device based on the effect.

Energy from sweat
Engineers at the University of California San Diego developed stretchable fuel cells that extract energy from sweat and are capable of powering electronics such as LEDs and Bluetooth radios, generating 10 times more power per surface area than existing wearable biofuel cells.

The biofuel cells are equipped with an enzyme that oxidizes the lactic acid present in human sweat to generate current.

To be compatible with wearable devices, the biofuel cell needs to be flexible and stretchable. Using a “bridge and island” structure developed by the group, the cell is made up of rows of dots that are each connected by spring-shaped structures. Half of the dots make up the cell’s anode; the other half are the cathode. The spring-like structures can stretch and bend, making the cell flexible without deforming the anode and cathode.

The basis for the islands and bridges structure was manufactured via lithography and made of gold. As a second step, researchers used screen printing to deposit layers of biofuel materials on top of the anode and cathode dots.

The biofuel cell can stretch and flex, conforming to the human body. (Source: UC San Diego)

The biggest challenge was increasing the cell’s energy density. To increase power density, engineers screen printed a 3D carbon nanotube structure on top the anodes and cathodes. The structure allows engineers to load each anodic dot with more of the enzyme that reacts to lactic acid and silver oxide at the cathode dots. In addition, the tubes allow easier electron transfer, which improves biofuel cell performance.

For testing, the biofuel cell was connected to a custom DC/DC converter circuit board that evens out the power generated by the fuel cells, which fluctuates with the amount of sweat produced by a user, and turns it into constant power with a constant voltage.

Researchers equipped four subjects with the biofuel cell-board combination and had them exercise on a stationary bike. The subjects were able to power a blue LED for about four minutes.

The team plans future work in two areas. First, the silver oxide used at the cathode is light sensitive and degrades over time. In the long run, researchers will need to find a more stable material.

Also, the concentration of lactic acid in a person’s sweat gets diluted over time. That is why subjects were able to light up an LED for only four minutes while biking. The team is exploring a way to store the energy produced while the concentration of lactate is high enough and then release it gradually.

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