The emerging field of optofluidics is offering new techniques for directing light and concentrating with it microscopic precision where it can be most efficiently used. This has the potential for greatly increasing the efficiency of existing solar energy systems such as PV cells, as well as possibly creating entirely new forms of energy production, according to Demetri Psaltis, Dean of the Ecole Polytechnique Fédérale de Lausanne (EPFL) School of Engineering in Switzerland and a pioneer in the optofluidicfield. EPFL is considered the world leader in optofluidics.
A new study: Optofluidics for Energy Applications, published online September 11, 2011, in the journal Nature Photonics, highlights the work by Professor Psaltis in collaboration with David Erickson of the Sibley School of Mechanical and Aerospace Engineering, Cornell University, and David Sinton, at the Department of Mechanical and Industrial Engineering at the University of Toronto. The study focuses in particular on optofluidic opportunities in sunlight-based fuel production in “photobioreactors” and “photocatalytic” systems, as well as optofluidically- enabled solar energy collection and control. The researchers believe that optofluidics technology can be used in solar energy systems and other sunlight-based fuel production systems.
Optofluidics
Optofluidics, the study of microfluidics, is a relatively new interdisciplinary field that combines the technology of microscopic delivery of fluids combined through optics. This science of light and fluids has become increasingly intertwined since 1862 when Léon Foucault discovered the speed of light. What Foucault also noticed was that the speed of light actually slows when it passes through a liquid such as water. Using this knowledge, researchers are finding ways to use optics and fluids in harmony to collect and focus light to where it can be used most efficiently.
Psaltis’ research team found that when you combine “microfluidics” - the delivery of fluids through extremely small channels or nanotubes - with optics, you can deliver both the fluid and light simultaneously and direct the concentrated light with microscopic precision. The sunlight is first captured by a light-concentrating system that follows the sun’s path by changing the angle of the water’s refraction. Doing this at the nanoscale is highly efficient because you create more surface area for interactions to occur, resulting in greater output and reduced cost.
Optofluidic Technologies
Over the last five years, the researchers have developed many new technologies to precisely deliver light and fluids to the same place at the same time, and are now applying it to the area of energy. Their past work had focused on photobioreactors, a technology that uses microorganisms such as algae or cyanobacteria to convert light and carbon dioxide into hydrocarbon fuels using photosynthesis for energy conversion. The study also describes how optofluidic technology could be used with “photocatalytic” systems, in which light energy is used to split water into its hydrogen and oxygen components and convert water into hydrocarbon fuels.
Now the EPFL team is focusing on using optofluidic technology, such as plasmonic nanoparticles or photonic waveguides, to optimize how light can more directly target a solar collector.
Prisms and mirrors are commonly employed to direct and concentrate sunlight to heat water on the roofs of homes or fluids at CSP plants. These techniques already employ the same principles of controlling and manipulating light found in optofluidics, but without the precision offered by nano technology. By combining optical elements into microfluidic devices, called “optofluidic chips,” the technique has promise in portable devices for applications such focusing light energy to PV panels, according to the researchers.
Technical Challenges
The EPFL team does acknowledge the need to sort out a few technical challenges of such optofluidic systems, including up-scaling the optofluidics technology for practical, utility-scale energy applications. The main challenge for the use of optofluidic technology is how to maintain the precision of nano and micro light and fluid manipulation while creating industrial-sized installations large enough to produce usable amounts of power. According to the researchers, this would require following a modular approach much like the way a super computer is built from the integration of many small components, and up-scaling the number of optofluidic chips in order to achieve the desired output. By scaling down the size of the optofluidic channels to the nano level allows for thousands more channels in the same available space, greatly increasing the overall surface area and leading to a huge reduction in the size and cost of the project.
Optofluidic Solar Applications
The EPFL team puts forward several possibilities for up-scaling optofluidics, such as using optical fibers to transport sunlight into large indoor biofuel reactors or PV systems with mass-produced optofluidic nanotubes. The researchers believe that that actually using these smaller spaces could increase the power density and reduce overall operating costs because offer a flexibility when concentrating and directing sunlight for solar collection and photovoltaic panels.
An optofluidic solar lighting could also exploit the sunlight that hits the outside or roof of a building and actually channel it into the building. This could be done by capturing the sunlight with a light concentrating system that follows the sun's path by changing the angle of the water's refraction and then distribute the sunlight throughout the building through light pipes or fiber optic cables to indoor PV panels, allowing the PV panels to be totally protected from weather and other environmental elements allowing them to experience a longer useful lifespan, according to the researchers. The light itself could even be channeled and used for ceiling lighting within office or industrial spaces.
One concern is maintaining a constant light source for powering PV panels or ceiling lighting since the changing of the intensity of the light due to clouds passing over would cut the efficiency. In order to maintain a constant light source during such unforeseen changes in intensity, researches have suggested using a system that utilizes “electrowetting” in order to deviate light from one nano channel into another quickly and inexpensively. Such a system involves allowing a droplet of water to sit on the outer surface of light tubes. A small electrical current would then be applied to excite the ions in the water, which would push them to the edge of the water droplet and expand it just enough for it to touch the surface of the next nano tube. The expanded droplet would then create a “light bridge” between these two light tubes and effectively smooth-out the amount of light traveling through the tubes.
