Could optofluidics solve the energy challenge?

Marie Freebody, Contributing Editor, marie.freebody@photonics.com

Optofluidics could be poised to revolutionize energy production, according to Demetri Psaltis, dean of the engineering school at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland.

Optofluidics is the combination of optics and microfluidics, the delivery of fluids through extremely small channels or tubes. Optofluidic principles for energy production can already be found at work where prisms and mirrors on the roofs of homes and buildings are commonly used to direct and concentrate sunlight to heat water.

Psaltis believes that by taking this a step further – with precision design plus nano- and microtechnology – optofluidics can offer more efficient solutions for “solar fuel” generation.

The term solar fuel incorporates not only photovoltaics but also the conversion of energy from the sun into fuel to power other systems that otherwise would require electricity. For example, it is used to convert water and carbon dioxide into methane in large industrial biofuel plants.

“The mechanisms used to generate solar fuels include photocatalysis and biofuels,” Psaltis said. “In photocatalysis, the interaction of light with nanoparticles or materials deposited on the surface of a reactor encourages the chemical reaction that generates fuel. In biofuels, algae or similar organisms are used to convert energy from the sun into a usable fuel.”

Directing light and concentrating it where it can be most efficiently used could greatly increase the efficiency of existing energy-producing systems as well as innovate entirely new forms of energy production, Psaltis and co-authors wrote in a review article published Sept. 11 in Nature Photonics.

He and his EPFL colleagues are working on systems that use solar radiation for water purification and for indoor lighting during the daytime.

For the latter application, a Fresnel lens-type solar collector array can be used to collect and focus sunlight directly into optical fibers. Coupling sunlight to a guiding element allows the light to be channeled to otherwise inaccessible areas – for indoor illumination, for example. Once inside, light could be directed to the ceilings of office spaces, indoor solar panels or even microfluidic air filters. Using sunlight to drive an indoor solar panel means that the panel would be protected from the elements and last longer.

The group is now working on a tunable optofluidic solar concentrator and optofluidic switch, which are the core parts of an optofluidic solar lighting system. As shown in Figure 1, sunlight is concentrated and coupled into the fibers by the optofluidic solar concentrator panel installed on the roof of the building and adaptable to the position of the sun.

Figure 1. A newly proposed solar lighting system is composed of an optofluidic solar concentrator, optical fibers (polymer core or liquid core), an optofluidic switch and the optical lighting terminals. These components constitute a reconfigurable optofluidic illumination network. Images courtesy of EPFL.

The infrared portion of light is separated and directed into the infrared photovoltaic solar cell, while the ultraviolet portion is extracted and can be used instead of the UV lamp for air purification during the daytime. Finally, the residual visible part of the sunlight is directed into each room for interior illumination.

The light flow is dynamically controlled by the tunable optofluidic switch, and the excess visible light can be further used to generate electricity via photovoltaic solar cells.

In a comparison between two different solar energy systems for indoor lighting – photovoltaic and solar indoor lighting – Psaltis and colleagues estimate that the peak energy density of sunlight on the Earth’s surface is about 1000 W/m² at noon. At this power, the team calculates that 1 m² of solar cell can generate just 200 W of electrical power to fluorescent lamps. On the other hand, for an indoor solar lighting system, the luminous power from a 1-m² area of sunlight collector generates 3000 W of electrical power to fluorescent lamps at the same optical flux output. Therefore, directly transporting the sunlight for indoor lighting can be an excellent way to conserve energy and could be much more effective than photovoltaic technology.

Figure 2. Researchers have found that transporting sunlight directly for indoor lighting can be more effective than photovoltaic technology.

Although optofluidics involves precise control over fluids and optics at the small scale, it must be scaled up for successful application to energy problems – a challenge that has yet to be addressed.

As highlighted in the review article, most optofluidic implementations so far have been essentially planar, using traditional microfluidic chips. But using the third dimension would be one way of scaling optofluidic concepts.

“The use of optofluidics in the energy field is just now starting,” Psaltis said. “There are several research activities, but for such systems to have a practical impact in the field of fuel or power generation, a major reduction in the cost of the components is required to allow scaling up to large plants.”