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A Device That Converts Sunlight Into Hydrogen Fuel

Posted September 16, 2013 11:26 AM

From Popular Science - New Technology, Science News, The Future Now:

"The idea is to take the energy in light and store it as a fuel we can use later. So we made the GRAFSTRR (Gravity-Fed Solar-Thermochemical Receiver/Reactor)-a 1,000-pound cylinder of insulated steel, about 3 feet wide and 2.5 feet tall. In the lab, 10 lamps simulate only 10 to 20 kilowatts of sunlight. (In the real world, though, tens of thousands of small mirrors across a field would reflect sunlight into the reactor.) The light enters the top of the reactor and passes through a circular quartz window that keeps out air, which can contaminate the chemical reaction inside. At the light's most concentrated and hottest point-3,000°F-it enters the reaction cavity. Fifteen hoppers drop zinc oxide powder into the cavity. When the radiation there hits the zinc oxide, it breaks the bond between the zinc and the oxygen, making free zinc. In the future, a second reactor would use the zinc to strip the oxygen from water, making hydrogen gas.Theoretically, we could capture about 40 percent of the energy, but in lab experiments to demonstrate the design, we get less than 3 percent. Our reactor is mostly a proof-of-concept, but I think it could be scaled up in my kids' lifetime."

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#1

Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/16/2013 10:46 PM

Why involve zinc, mirrors, focussing , (which requires tracking of sun) etc.? Simply use a solar photovoltaic panel to produce 12V DC current, use it to electrolyse brine into hydrogen and oxygen, collect both, burn them and get energy and water back.

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#2
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Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/17/2013 9:20 AM

It may have to do with overall economy based on solar efficiency. The Japanese Technical Institute has demonstrated Magnesium oxide conversion using a Cr:Nd:YAG laser (1.06 mirometer wavelength), to heat the oxide much hotter than can be done with sunlight, however the solar efficiency is not as high as the zinc experiment. On the other hand magnesium trumps zinc in planetary abundance, and relatively low toxicity. The magnesium metal produced would be used with water to generate steam and hydrogen (on demand), thereby producing even more steam by combustion of the hydrogen. Steam would be utilized in a variety of engines designed to cope with some magnesium oxide contamination of the steam. Magnesium would be the primary storage medium of energy at that point.

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#4
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Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/17/2013 10:29 AM

Because the idea of this device is to store the energy for later use. It is much easier to store zinc than it is to store hydrogen and oxygen.

Now there are a lot of details to be worked out for this system to be useful. For example, What happens to the zinc produced? Ideally it would fall down as zinc powder, but unless it is rapidly cooled, it will fall into a molten pool and later cool to form a solid block, which would be difficult to remove, and would only slowly react with the water.

Also, they do not mention storing the oxygen released, so there goes a significant part of the energy. At least oxygen is easier to store than hydrogen...

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Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/17/2013 2:55 PM

I am pretty sure there is a way to draw off zinc as liquid, due to the high temperature, unless it stays in the vapor and is somehow separated from oxygen prior to condensing, once in liquid there are simple means of making small pellets, shot, prills, etc.

As to the magnesium production scheme of the Japanese scientists, it is not clear how the separation takes place, but is going to involve separation of vapor and oxygen at such high temperatures. The magnesium stores the energy recovered from the thermolytic reduction of magnesium (just as with zinc). In the parlance of thermochemistry, the fuels contain the heat of combustion/oxidation, not the oxidizer.

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Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/17/2013 3:11 PM

When steam engine was invented and steam powered automobiles were on the road, there was considerable trepidation about the safety. So also for gasoline, which proved to be one of the best store houses of energy for its weight. Electricity itself is so dangerous that utmost care is needed to handle high voltages.

Energy is energy, whether in the form of steam, gasoline, electricity, or hydrogen. Methods for safe storage, utilisation etc., will be evolved over a time.

What I was referring about, is the convenience of handling gaseous hydrogen, rather than solid zinc, its repeated handling wtc.

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Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/17/2013 4:04 PM

Obviously if this is wanted for transportation energy, then BTU's/lb fuel, and the ability to store significant weight of active fuel onboard becomes the issue. There are always trade offs. Check out cyclone power's web site for the new radial steam engines. These have no large boiler vessel, just tubing that is heated, and that delivers ample, high quality steam for such an engine to be quite efficient.

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#3

Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/17/2013 9:38 AM

Or you can plant a tree, cornfield, . . . . or even a shrubbery.

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#5

Re: A Device That Converts Sunlight Into Hydrogen Fuel

09/17/2013 1:59 PM

When the efficiency of the process reaches more than the photovoltaic cell, then we can talk about a good idea! Otherwise, Nature does a better job and as #3 comments, we should plant more trees and corn ... (more ecnomical and cleaner)

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