Tubular Solar Panels Create Electricity, Hot Water

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Look closely at these solar panels. They aren't the flat panels commonly seen on rooftops, but arrays of tubular components.

These tubes, developed by Naked Energy, in Guildford, England, are a kind of hybrid solar-energy contraption that make more efficient use of the sun's energy to produce both electricity and hot water.

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They're hybrid because they do two jobs. Most solar panels either making electricity or heat water, not both. The former is done by a photovoltaic silicon solar panel, familiar in small devices such as calculators and big rooftop installations. Solar hot water heaters are basically arrays of black plastic pipes that get heated up during the day.

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The problem with either system by itself is that a lot of energy gets wasted. Photovoltaics operate at efficiencies of 20 percent, which means that most of the potential sunlight is not captured for energy and is simply lost. On top of that, when panels heat up (as they will, in the sunlight) they become less efficient — above about 77 degrees they start to generate less power. Meanwhile, a solar hot water heater can save on gas (or electric) heating, but they miss out on the chance to generate electricity.

Naked Energy's array of tubes contains solar panels that capture sunlight to convert to electricity, but it's also able to support water, which is pumped through the tubes, absorbing excess heat. Cooling the panels keep them running at peak efficiency. The panels thus double as hot water heaters, in a process called cogeneration.

Naked Energy isn't the only player in the field. In California, Cogenra Solar has installed a combination photovoltaic and hot water heating system at the Kendall-Jackson winery near Santa Rosa, Calif., providing electricity and hot water for a bottling and blending facility.

The idea isn't new. Solar thermal plants use the sun's light to heat water for turbines, but that tends to be useful for big installations, covering hundreds of acres.

Cogeneration systems like these can make solar energy available to those who don't need megawatts. Cogenra's system, for instance, can produce about 241 kilowatts. That's enough, according to the company, to save Kendall-Jackson some $30,000 per year in electricity costs. CEO Gilad Almogy told Discovery News that the basic model of the system provides about 30 kilowatts of heat, and is a good fit for apartment buildings.

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There are differences in the way they work. Cogenra's system tracks the sun, and consists of solar panels that are curved to focus the light onto the solar panels, with the pipes containing the water to be heated running behind them. Naked Energy's panels are in vacuum tubes that can be mounted flat, with the water being pumped in pipes that draw the heat from the panel directly. (A similar innovation was unveiled last year by researchers at MIT and Boston College).

That also reflects a difference in purpose. Cogenra is for big buildings such as small factories and apartment houses, whereas Naked Energy is geared to homes. That's one reason Naked Energy went with a design that doesn't need motors to track the sun across the sky — such systems tend to be too expensive (and complicated) for the average homeowner to install. 

Cogenera says it has systems installed in several states, and Alomogy said the payback on the system is five years, even given the low price of natural gas (the usual way water gets heated in many areas). Naked Energy is still a start-up,seeking enough investment capital to mass-produce the solar panels.

Photo: Naked Energy's panels on a rooftop

Credit: Naked Energy