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The yellow dwarf star at the center of our solar system that reliably illuminates and warms this planet was considered a deity in ancient cultures. The science behind the sun’s energy only gradually occurred to the curious and the ingenious. Today, the ability to harvest solar energy is viewed as one of the foremost solutions to this country’s energy challenges, and rightly so, as the advent of new technologies has initiated novel and exciting possibilities…

Patrick Pinhero, an associate professor of chemical engineering at the University of Missouri, along with colleagues at Idaho National Laboratories (INL), MicroContinuum Inc. in Cambridge, Mass., and the University of Colorado (CU), is developing a solar nanoantenna (nantenna) device that could potentially revolutionize our approach to solar power and the harvest of industrial waste heat.

Solar energy 101

French physicist Edmond Becquerel discovered the photoelectric effect — that sunlight could be converted to electricity — and 92 years later, an explanation of the process earned Albert Einstein a Nobel Prize.

Solar cells are made of a semi-conductor material, such as silicon, specially treated to form an electric field, positive on one side and negative on the other. A conductor is attached to both sides, forming an electric circuit. When photons hit the silicon and electrons are released, the energy is captured in the form of direct-current (DC) energy that is then converted into alternating current (AC) to power electrical devices.

It is the wavelength, or frequency, of light and not its intensity that determines the amount of energy released: the shorter its wavelength, the greater its frequency. Visible light has a shorter wavelength than infrared light. But because solar cells can’t cover the entire light spectrum, they are relatively inefficient, converting only eight to 25 percent of available light to electricity.

In addition, solar panels, made up of many cells, are expensive to manufacture and operate at extremely high temperatures, up to few hundred degrees Celsius.

Pinhero and his team have developed an alternative direct collection process to collect solar energy and convert it into power that addresses the limitations in prevailing solar technology.

At left, Patrick Pinhero, an associate professor of chemical engineering, gestures to a poster detailing the solar nanoantennas that he and fellow researchers have developed that can harvest up to 90 percent of direct and indirect light energy.

Solar power of the future

In partnership with Dale Kotter and Steven Novack of INL, and Dennis Slafer of MicroContinuum, Pinhero helped conceive and fabricate nanoantenna electromagnetic collectors of various geometries — including square spirals that are 1/25 the width of a human hair — that can collect energy from the entire light spectrum in the same way a radio antenna collects electromagnetic waves — by resonance.

Professor Garett Moddel at CU is working to fabricate diode devices that can convert the very high frequency of these resonators into an electrical direct current. It is the research team’s plan to integrate the diodes, which work as one-way valves for the oscillating electrons, directly into the nantannas.

An array of these nantennas can be printed using conductive metals like gold onto a flexible sheet of polymer or a thin metal foil. One early prototype contained 1.4 billion nantennas on a six-foot square sheet. The device is predicted to have the ability to potentially collect on the order of 90 percent of light energy, direct and indirect.

Slafer fabricates the arrays using a roll-to-roll technology to keep manufacturing costs low and thus commercially viable.

“It’s inexpensive, non-toxic, lightweight, and operates at room temperature,” said Pinhero. “And it [the array] also has a wide angle of acceptance, so you don’t have to change its angle with respect to the emitter to maintain its efficiency.”

The team is seeking funds from the Department of Energy, and in January formed a consortium with the help of Pat Brady of RedWave Energy, Inc. in Chicago, Ill., to raise capital from private investors. They believe that within five years the nantenna technology will be able to harvest direct sunlight with an efficiency that Pinhero describes as orders of magnitude better than current solar energy technologies.

An array of nanoantennas printed in gold create a flexible panel of interconnected nantennas could eventually replace solar panels.

The nanoantennas are imaged with a scanning electron microscope.

In the meantime

The researchers are still fine-tuning the device, getting the nantennas and the diodes to “talk” to each other.

Because the nantennas have the ability to collect energy from the infrared spectrum, the next step will be to tweak the technology to harvest industrial waste heat. As such, nantennas will be complementary to photovoltaics, offering increased energy efficiencies through thermal harvesting when coupled to existing solar PV collection devices.

“Efficiencies are proportional to the change in temperature. You could potentially harvest waste heat from an aluminum smelting operation with greater than 60 percent efficiency,” said Pinhero. “The hotter the better for harvesting infrared energy from waste heat.”

Pinhero speculated that the nantennas might be integrated into building materials or electronics. “They potentially could be used in electric vehicles, and there may be no need for a battery in a car,” Pinhero said.

“It’s what nations need to proceed: renewable resources with well-thought out manufacturing and scale-up. That’s our goal: a quality product that is inexpensive enough to be accessible to all people,” said Pinhero.


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