The layout of the hybrid system  (Source: Nico Hotz)
New system utilizes a water/methanol combination along with catalytic nanoparticles flowing through copper tubes to create hydrogen, which produces electricity through a fuel cell

A Duke University researcher has created a new hybrid system for converting sunlight into electricity, which can be placed in residential rooftops as a form of alternative energy.

Nico Hotz, study leader and assistant professor of mechanical engineering and materials science at Duke University's Pratt School of Engineering, has developed a hybrid system based on traditional solar panels, but instead utilizes sunlight to heat water and methanol in copper tubes on a house/building's rooftop.

This hybrid system consists of a maze of copper tubes coated with aluminum and aluminum oxide, and these tubes are partially filled with catalytic nanoparticles. A combination of water and methanol, which is sealed in a vacuum, flow through the tubes while sunlight is collected.

"This set-up allows 95 percent of the sunlight to be absorbed with very little being lost as heat to the surroundings," said Hotz. "This is crucial because it permits us to achieve temperatures of well over 200 degrees Celsius within the tubes. By comparison, a standard solar collector can only heat water between 60 and 70 degrees Celsius."

The evaporated combination reaches these high temperatures, and small amounts of the catalyst are added to produce hydrogen. This hydrogen is then guided to a fuel cell to create electricity for the building, or to be stored in a tank for later on.

Hotz compared the system to three separate technologies according to their exergetic performance, which is a way of describing the amount of a given quantity of energy that can be converted to "useful work." The three different technologies were the standard photovoltaic cell, which converts sunlight into electricity to split water electrolytically into oxygen and hydrogen; a system where photovoltaic cells turn sunlight into electricity, which is then stored in batteries such as lithium ion, and a photocatalytic system that creates hydrogen much like Hotz's system, but is not quite "mature" yet. Hotz made sure to conduct these comparisons in February and July in order to observe system performance in winter and summer.

"The hybrid system achieved exergetic efficiencies of 28.5 percent in the summer and 18.5 percent in the winter, compared to 5 to 15 percent for the conventional systems in the summer and 2.5 to 5 percent in winter," said Hotz.

Hotz also performed a cost analysis and discovered that his hybrid solution is the least expensive compared to traditional systems, with the hybrid's installation costs totaling $7,900 for summer requirements. He did admit that a fossil fuel-fed generator would be the only cheaper option.

"The installation costs per year including the fuel costs and the price per amount of electricity produced, however, showed that the (hybrid) solar scenarios can compete with the fossil fuel-based system to some degree," said Hotz. "In summer, the first and third scenarios, as well as the hybrid system, are cheaper than a propane or diesel-combusting generator."

Other benefits associated with the hybrid system include the efficient production of hydrogen without "significant" impurities, and the option to shut down portions of the rooftop structure or even sell excess energy back to the grid if the system produces more energy than needed in the summer. A hybrid rooftop system would need to be large enough to produce electricity needed in the winter, and if it can accomplish this, then that would mean it would produce excess electricity in the summer, according to Science Daily. Those located in rural areas could especially benefit from such a system, since traditional forms of energy can be too expensive or difficult to receive.

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