Researchers harness sun's energy during day for use at night

Jan 14, 2014
Tom Meyer's new system generates hydrogen fuel by using the sun's energy to split water into its component parts. After the split, hydrogen is sequestered and stored, while the byproduct, oxygen, is released into the air. Credit: Tom Meyer

Solar energy has long been used as a clean alternative to fossil fuels such as coal and oil, but it could only be harnessed during the day when the sun's rays were strongest. Now researchers led by Tom Meyer at the Energy Frontier Research Center at the University of North Carolina at Chapel Hill have built a system that converts the sun's energy not into electricity but hydrogen fuel and stores it for later use, allowing us to power our devices long after the sun goes down.

"So called 'solar fuels' like hydrogen offer a solution to how to store energy for nighttime use by taking a cue from natural photosynthesis," said Meyer, Arey Distinguished Professor of Chemistry at UNC's College of Arts and Sciences. "Our new findings may provide a last major piece of a puzzle for a new way to store the sun's energy – it could be a tipping point for a future."

In one hour, the sun puts out enough energy to power every vehicle, factory and device on the planet for an entire year. Solar panels can harness that energy to generate electricity during the day. But the problem with the sun is that it goes down at night—and with it the ability to power our homes and cars. If solar energy is going to have a shot at being a clean source for powering the planet, scientists had to figure out how to store it for night-time use.

The new system designed by Meyer and colleagues at UNC and with Greg Parsons' group at North Carolina State University does exactly that. It is known as a dye-sensitized photoelectrosynthesis cell, or DSPEC, and it generates by using the sun's energy to split water into its component parts. After the split, hydrogen is sequestered and stored, while the byproduct, oxygen, is released into the air.

"But splitting water is extremely difficult to do," said Meyer. "You need to take four electrons away from two water molecules, transfer them somewhere else, and make hydrogen, and, once you have done that, keep the hydrogen and oxygen separated. How to design molecules capable of doing that is a really big challenge that we've begun to overcome."

Meyer had been investigating DSPECs for years at the Energy Frontier Research Center at UNC and before. His design has two basic components: a molecule and a nanoparticle. The molecule, called a chromophore-catalyst assembly, absorbs sunlight and then kick starts the catalyst to rip electrons away from water. The nanoparticle, to which thousands of chromophore-catalyst assemblies are tethered, is part of a film of nanoparticles that shuttles the electrons away to make the hydrogen fuel.

However, even with the best of attempts, the system always crashed because either the chromophore-catalyst assembly kept breaking away from the nanoparticles or because the electrons couldn't be shuttled away quickly enough to make hydrogen.

To solve both of these problems, Meyer turned to the Parsons group to use a technique that coated the nanoparticle, atom by atom, with a thin layer of a material called titanium dioxide. By using ultra-thin layers, the researchers found that the nanoparticle could carry away electrons far more rapidly than before, with the freed electrons available to make . They also figured out how to build a protective coating that keeps the chromophore-catalyst assembly tethered firmly to the nanoparticle, ensuring that the assembly stayed on the surface.

With flowing freely through the nanoparticle and the tether stabilized, Meyer's new system can turn the sun's energy into fuel while needing almost no external power to operate and releasing no greenhouse gases. What's more, the infrastructure to install these sunlight-to-fuel converters is in sight based on existing technology. A next target is to use the same approach to reduce carbon dioxide, a greenhouse gas, to a carbon-based fuel such as formate or methanol.

"When you talk about powering a planet with energy stored in batteries, it's just not practical," said Meyer. "It turns out that the most energy dense way to store is in the chemical bonds of molecules. And that's what we did – we found an answer through chemistry."

Explore further: An improved, cost-effective catalyst for water-splitting devices

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wealthychef
4 / 5 (3) Jan 14, 2014
Makes sense to use chemical bonds to store energy. The fact that they skip the "generate electriciy" step and go straight to storage sounds promising. The old questions are always the killers though: cost to build, cost to maintain, cost to distribute, i.e., what will consumers pay per kilowatt-hour and how does this compare to coal?
tylab
3.8 / 5 (4) Jan 14, 2014
"In one hour, the sun puts out enough energy to power every vehicle, factory and device on the planet for an entire year." - Did you mean this is how much energy the sun puts out that REACHES earth? How much energy the sun puts out, and how much energy the sun puts out that reaches earth are two very different numbers and only the former is relevant! Not to be the nitpick nazi, but I think its an important distinction. Otherwise, this is well written article about a very relevant technology!

Maggnus
4 / 5 (4) Jan 14, 2014
Without looking it up tylab, I would guess the former. Having said that, it should be easy enough to look up and/or figure out...
tylab
5 / 5 (2) Jan 14, 2014
aghh I'm an idiot. I meant that only the amount of energy that the sun puts out that reaches earth is relevant.
Returners
4.2 / 5 (5) Jan 14, 2014
Solar Constant for Earth's cross-section equals 174 petawatts.

Multiply by 3600 seconds gives:

626,400,000,000,000,000,000 Joules

or 626 exajoules, which is slightly more, I think, than humans use per year.

We only need to harvest a fraction of a percent of the solar constant throughout a year in order to power all human needs, at least where practical.

===

Article makes no mention of the efficiency of capture and conversion of the energy, obviously there may be refinements and further improvements to the technology, but it would be nice to have a baseline amount.

How much hydrogen could we safely store in tanks practical to an automobile, say a coupe?

Compressed or liquified hydrogen would probably be a hell of a lot more dangerous than gasoline. What safety measures are in development for these issues?

Will there be training for refueling? Presently it's pretty easy to fill up with gasoline, since it's not an air-tight system.

Hydrogen stations won't be as "idiot proof".
Maggnus
3.3 / 5 (3) Jan 14, 2014
How much hydrogen could we safely store in tanks practical to an automobile, say a coupe?
I don't think that's what they are proposing. I read it to be that they would stiore it on the site of the solar energy plant for use in generating electricity when the sun goes down.

Returners
3 / 5 (1) Jan 14, 2014
Oh yeah, I can see battles over water rights in some locations, such as Colorado, where it's literally illegal to collect rain water on your own property in some locaitons, becoming even more intense.

Making 10 kilograms of hydrogen requires 5000 mol of water, or 90 liters.

Unlike a dam, the water doesn't go back to the basin it was naturally draining to; it ultimately ends up as water vapor after the burning of the hydrogen. You can see that a facility creating tens of thousands, perhaps a million kilograms of hydrogen per day would suck up enormous amounts of water from the basins downstream. Additionally, most of the "good" locations for solar, such as the Sahara and the U.S. desert southwest, don't have an excess of water nearby.
Returners
not rated yet Jan 14, 2014
i don't think that's what they are proposing. I read it to be that they would stiore it on the site of the solar energy plant for use in generating electricity when the sun goes down.


I realize that, but the usual claim of the "hydrogen fuel economy" crowd is that hydrogen burning autos (or fuel cell equipped,) would replace gasoline-burning autos.

Obviously, if you made hydrogen gas you could easily store it on site as the back-up or night time power, the same as is done with Methane right now.

You could also pipe hydrogen to other locations, most likely a lot more efficient than long-distance electric lines, which have huge losses over distance.
MR166
5 / 5 (1) Jan 14, 2014
"You can see that a facility creating tens of thousands, perhaps a million kilograms of hydrogen per day would suck up enormous amounts of water from the basins downstream. Additionally, most of the "good" locations for solar, such as the Sahara and the U.S. desert southwest, don't have an excess of water nearby."

Good Point! I wonder if sea water could be used and the brine returned to the sea.

Or a fuel cell could generate electricity and as a byproduct water would be produced making the cycle almost self sustaining.
mytwocts
5 / 5 (1) Jan 15, 2014

Making 10 kilograms of hydrogen requires 5000 mol of water, or 90 liters.

Unlike a dam, the water doesn't go back to the basin it was naturally draining to; it ultimately ends up as water vapor after the burning of the hydrogen. You can see that a facility creating tens of thousands, perhaps a million kilograms of hydrogen per day would suck up enormous amounts of water from the basins downstream. Additionally, most of the "good" locations for solar, such as the Sahara and the U.S. desert southwest, don't have an excess of water nearby.


The water can be recovered by condensation, which also increases the energy efficiency.
antialias_physorg
not rated yet Jan 15, 2014
it ultimately ends up as water vapor after the burning of the hydrogen

That's why you don't burn it but use it in a fuel cell - which renders liquid water as the product (i.e. you can have a closed cycle for the water if you want to. And you probably want to because you'll want to use distilled water if possible).

How much hydrogen could we safely store in tanks practical to an automobile, say a coupe?

As much as you would store gas (by energy content, because it's the energy content that counts.) If you release all the energy you need to drive 300km all at once it doesn't matter whether that's in hydrogen, gas, or bellyrubs - you'll get killed in any case)
Eikka
4 / 5 (1) Jan 15, 2014
How much hydrogen could we safely store in tanks practical to an automobile, say a coupe?


How far do you want to drive?

A small sub-compact car with minimal energy use can do about 12 US-MPG on liquid hydrogen, hence if you want 300 km you need a 15 gallon tank of cryogenic hydrogen. 30 gallons if you want to drive a typical five door sedan.

That's about the upper limit of how tightly you can pack it without combining it into other chemicals.
betterexists
not rated yet Jan 16, 2014
¾ of this Planet is Ocean covered & Directly Exposed to the Sun with NO shade whatsoever. All of the Water is at 1 Place.
It is just a matter of scaling up the technology & running up pipes from ocean underground towards the beaches!
Once Fuel becomes plentiful....Larger ships also may be used to transport condensed/liquefied stuff!
dedereu
not rated yet Jan 17, 2014
The global efficiency to make again electricity remains low, below a few percent, similar to the efficiency of natural photosynthesis giving wood to burn.
A quite better efficiency is possible, without any new technology, storing the solar thermal energy underground to use it later, even, interseason, from summer to winter. It works gratis, to heat houses, like used in ww w.dlsc.ca since 2007, perpetually, gratis, consuming nothing.
Concentrated solar heat energy at 150°C up to 300°C stored underground, in simple usual geothermal bores, can be used to generate electricity continuously 365 days in the year, day and night, with a quite better global efficiency and without completely new technological complex research.
It can even use unused exhausted shale gas boreholes to store this solar heat in ecological solar plants so that shale gas lost boreholes give solar energy, without any more CO2 or pollution, perpetually gratis, consuming nothing, even if there is no sun in winter.
Stiltpro
not rated yet Jan 19, 2014
Site this plant next to cattle feed lots, pig farms, etc and use the urine as a feed stock. That would : reduce use of scarce water, treat waste water and increase efficiency.

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