By taking advantage of the vagaries of the natural world, Duke University engineers have developed a novel approach that they believe can more efficiently harvest electricity from the motions of everyday life.

Energy harvesting is the process of converting one form of energy, such as motion, into another form of energy, in this case electricity. Strategies range from the development of massive wind farms to produce large amounts of electricity to using the vibrations of walking to power small electronic devices.

Although motion is an abundant source of energy, only limited success has been achieved because the devices used only perform well over a narrow band of frequencies. These so-called “linear” devices can work well, for example, if the character of the motion is fairly constant, such as the cadence of a person walking. However, as researchers point out, the pace of someone walking, as with all environmental sources, changes over time and can vary widely.

“The ideal device would be one that could convert a range of vibrations instead of just a narrow band,” said Samuel Stanton, graduate student in Duke’s Pratt School of Engineering, working in the laboratory of Brian Mann, assistant professor of mechanical engineering and materials sciences. The team, which included undergraduate Clark McGehee, published the results of their latest experiments early online in Applied Physics Letters.

“Nature doesn’t work in a single frequency, so we wanted to come up with a device that would work over a broad range of frequencies,” Stanton said. “By using magnets to ‘tune’ the bandwidth of the experimental device, we were able verify in the lab that this new non-linear approach can outperform conventional linear devices.”

Although the device they constructed looks deceptively simple, it was able to prove the team’s theories on a small scale. It is basically a small cantilever, several inches long and a quarter inch wide, with an end magnet that interacts with nearby magnets. The cantilever base itself is made of a piezoelectric material, which has the unique property of releasing electrical voltage when it is strained.

The key to the new approach involved placing moveable magnets of opposing poles on either side of the magnet at the end of the cantilever arm. By changing the distance of the moveable magnets, the researchers were able to “tune” the interactions of the system with its environment, and thus produce electricity over a broader spectrum of frequencies.

“These results suggest to us that this non-linear approach could harvest more of the frequencies from the same ambient vibrations,” Mann said. “More importantly, being able to capture more of the bandwidth makes it more likely that these types of devices could someday rival batteries as a portable power source.”

The range of applications for non-linear energy harvesters varies widely. For example, Mann is working on a project that would use the motion of ocean waves to power an array of sensors that would be carried inside ocean buoys.

“These non-linear systems are self-sustaining, so they are ideal for any electrical device that needs batteries and is in a location that is difficult to access,” Mann said.

For example, the motion of walking could provide enough electricity to power an implanted device, such as a pacemaker or cardiac defibrillator. On a larger scale, sensors in the environment or spacecraft could be powered by the everyday natural vibrations around them, Mann said.

Mann’s research is supported by the Office of Naval Research.

Within the Consolider HOPE project (projects funded by the Ministry of Innovation and Science), a group of scientists at Universidad Pablo de Olavide (UPO), headed by Juan Antonio Anta, are working on the optimisation of a type of photovoltaic cell (Grätzel cell) that artificially mimics photosynthesis.

Grätzel cells are photovoltaic devices that take advantage of the interaction of a structured semiconductor less than nanometre in size and an organic dye that acts as a solar collector.

According to Elena Guillén, member of UPO’s Coloides y Celdas Solares Nanoestructuradas (Nanostructured Colloids and Solar Cells) Group, this dye can be either synthetic or natural and can even enable the use of chlorophyll for this type of cell.

Thus, researchers at UPO have begun a study with which they hope to increase the efficiency of these eosin or mercurochrome -based organic components by incorporating ionic salts, known as green solvents, with a view to preventing evaporation of the liquid compounds and the consequent reduction in efficiency.

Previous studies show that ionic salts are less volatile and it is this characteristic that the group headed by Professor Anta seeks to exploit. “Notwithstanding its liquid state, these types of solvents have high viscosity levels and, therefore, during the coming months we will continue our study, working on different alternatives within ionic liquids, their synthesis, etc.,” comments Elena Guillén.

The pros and cons of the new generation

Although there are already some third generation cells on the market (for example, for recharging mobile phones), according to the researchers their practical use is anecdotal. However, due to their properties of flexibility and variety of colours and shapes, the future of these cells lies in new market niches such as decoration or use in coloured windows that not only allow light through but use this light to generate electricity.

On the other hand, apart from the rapid amortisation of energy production costs -estimated in one year’s use-, there is also the low cost of the materials. “Organic materials are usually cheaper,” affirms the researcher, despite which the search continues for an alternative organic dye to the one currently used, derived from ruthenium.

“The paradox lies in the fact that if one uses these cells because their competitive edge is that they are cheaper and more readily available, and then one uses a dye based on a precious metal, what is the advantage?” points out Elena Guillén.

On the other hand, the researchers are aware that it is a relatively new technology -this type of cell was invented in 1991- that still need to be greatly developed. Furthermore, the maximum efficiency obtained in laboratory is only 11%, which is competitive but it drops when extrapolated to an industrial scale.

The main technological challenge is currently the problem of cell degradation. “If you use an organic dye, it can be degraded by the action of sunlight, with the consequent reduction in useful life compared to silicon cells. On the other hand,” the researcher highlights, “our group is working on one of the key aspect for improving cell stability – elimination of the need to use liquids that can present problems with evaporation, etc. and for which, as already mentioned, our focus is on the use of ionic salts.”