Liquid batteries that can store excess energy generated by sources such as wind turbines could accelerate adoption of the green technology.

Banks of scorching hot batteries filled with molten metals may be the long-sought silver bullet to make large-scale adoption of wind and solar energy a practical, purely green reality.

Such a storage solution is needed because, as we know, the wind doesn't always blow and the sun doesn't always shine where and when it's needed.

"Right now, if you run a solar farm or a wind farm and you want to deliver electricity when the wind isn't blowing or the sun isn’t shining, the cheapest way is to get a gas-fired peaking unit," Donald Sadoway, a materials chemist at the Massachusetts Institute of Technology, told me Monday.

Gas-fired peaking units are mini power plants that can be turned on and off quickly to meet demand for electricity when other sources are unavailable or maxed out.

"Those things are cheap to buy, they are cheap to run, and the price of natural gas has been falling recently in the United States. So the way people have been looking at (shoring up) renewables is to turn to natural gas," Sadoway explained.

"And that's fine. It's not illegal. There's nothing immoral about it," he added, "but it is not 100 percent green at this point."

For the industry to adopt the greener battery technology, the cost of the battery has to be as cheap, efficient, and reliable as state-of-the art natural gas-fired peaking plants. 

Sadoway and his colleagues are hard at work on a liquid battery they believe will meet these criteria. On Monday, they described their progress in the Journal of the American Chemical Society.

Metal cocktail
The battery is a cocktail of metals that naturally settle into distinct layers because of their different densities, similar to a "black and tan" pint served at British pubs, where dark stout rests on top of denser pale ale.

Batteries need three layers — positive and negative poles and a membrane between the charges. In the case of the liquid battery, molten metals on the top and bottom serve as the positive and negative poles and a layer of molten salt serves as the membrane.

"The principle of the battery is an alloying reaction," Sadoway explained. 

Alloys are metals made via the combination of two or more metallic elements. In the liquid battery, the top layer is magnesium and the bottom layer is antimony.

"The driving force for current is the desire of magnesium to enter the antimony and form an alloy," Sadoway said. "In order to alloy, the magnesium has to first get across the molten salt and in order to do so, the magnesium has to lose two electrons and become a magnesium ion."

Those two electrons are what escape to the wires to power our gadgets and appliances. 

"When the magnesium ions get to the interface with the antimony, they acquire two electrons which have been pulled out of the external circuit and then that makes neutral magnesium which then alloys with the antimony," said Sadoway.

To charge the battery, the process is reversed. 

Sadoway and his colleagues have tweaked the recipe of this liquid cocktail for several years and gradually scaled up the size of the batteries. 

The initial tests consisted of a battery about the size of a shot glass; then they went to a battery the size of a hockey puck and, now, the team reports a six-inch-wide version that has 200 times the storage capacity of the original.

Keeping them hot
To keep the metals in a liquid state requires a battery operating temperature of 700 degrees Celsius (1,292 degrees Fahrenheit).

This heat comes at an energy cost — "we have to lose some of the energy we are storing in order to keep the battery at temperature," Sadoway explained.

Tests show about 75 percent efficiency — that is, for every 100 units of electricity put in the battery, 75 units come out. The rest is spent keeping the battery hot and lost due to inefficiencies in power electronics and converting back and forth between AC and DC.

A loss of 25 percent is actually quite reasonable, according to Sadoway. As long as more than a 25 percent spread between the price of electricity when the battery is charged and discharged, a utility can recoup its investment cost and make a buck.

For example, a utility could charge up its battery in the middle of the night when the wind is blowing and rates are low and then sell it back to the grid in the afternoon when rates are high. 

"In certain markets like California, there can be day-night price swings that can be not so many percent, but so many X," Sadoway noted. "In a market like that, this thing would do just fine."

Battery vs battery
According to Sadoway, who has started a company, Liquid Metal Battery Corp. to scale up and sell these batteries, the liquid approach is potentially better than competing technologies such as lithium ion batteries, which require the expansion and contraction of solid parts in order to work.

All this swelling and contracting amounts to wear and tear, which is often why the lithium ion batteries in laptops, for example, go kaput after a few years.

"Those kinds of failure mode are absent in this battery because it is all liquid and liquid can accommodate volume changes," Sadoway noted. 

Lab tests, he said, show that the lifetime of the battery isn't limited by its capacity to hold a charge so much as by the lifetime of materials used to encase and insulate it.

Current materials, he said, may begin to corrode after 10 to 15 years sufficiently to change the chemistry of the battery or permit the battery "to eat its way out of the case."

Another advantage to the liquid technology, he added, is the abundance of the raw materials used to build it. Magnesium and antimony are abundant in the United States and low cost.

As well, assembly of the battery is straightforward. Due to density differences, for example, the layers self assemble. "No clean rooms, no fancy nano-tech, nothing like that," Sadoway said.

Daniel Kammen directs the Renewable and Appropriate Energy Laboratory at the University of California at Berkeley. He said the biggest challenge for the liquid battery is the high operating temperature.

"Even if the waste heat can be harvested for an added benefit, systems operating at over 1,000 degrees are going to be a challenge for long-term maintenance," he told me in an email exchange on Tuesday. 

Shipping-container-sized battery
Sadoway's startup up is focused on scaling up the battery technology with the best chemistry that comes out of his lab at MIT. While the lab has reached a six-inch diameter cell, the company has cells that are 16-inches in diameter, he said.

The idea is to take these cells, stack them about 20 high, and link the stacks together in rows about 20 deep that that fit inside a 40-foot shipping container. This shipping-container-sized battery would provide about 2 megawatt hours of juice.

By the end of 2014, the goal is "to have something that can be readily shipped to a potential customer for testing," Sadoway said.

While the utility companies may be interested in the batteries as an alternative to gas-fired peaker plants to make their solar and wind farms viable, the batteries also could ease transmission line congestion.

This could be particularly useful in tech-heavy regions such as the Bay Area, where the energy demands of server farms are steadily climbing, noted Sadoway.

On certain days of the year, for example during a heat wave in the middle of the summer, "you can't get enough electricity through the lines. The transmission lines are running at full capacity," he said.

Instead of building additional transmission capacity — bringing more wires into the city, which is usually controversial and requires a drawn-out permitting process — companies could plop a battery in the basement of their buildings.

"From midnight to 5 a.m., when the lines aren't congested [and rates are low] you could be shipping electricity into the center of the city and storing it in the basement of these buildings," Sadoway explained. 

"Then, in the middle of the day, you just take it right out of the basement into the servers."

Kammen, the University of California energy professor, said this is "exciting stuff and a welcome area of long-overdue innovation."

Updated at 1:40 pm PT to reflect comments from Daniel Kammen.

More on battery and storage technology:

• Energy storage breakthroughs on the horizon
• Pourable batteries could store green power
• Building a better battery
• Battery tech improving as demand soars
• Electric battery gets you gooing, gooing, gone!
• Can EVs solve wind power puzzle

John Roach is a contributing writer for msnbc.com. To learn more about him, check out his website and follow him on Twitter. For more of our Future of Technology series, watch the featured video below.

Grass clippings could be turned into solar cells using inexpensive chemicals and materials, according to new research.

Within a few years, a special powder sold in little plastic baggies could turn your grass clippings into an electricity-generating solar cell, scientists reported Thursday.

"That's the dream," Andreas Mershin, a researcher at the Massachusetts Institute of Technology and co-author of a paper describing the process, told me.

The powder in the bag is an inexpensive chemical cocktail that stabilizes the molecules in green plants that carry out photosynthesis known as photosystem-I so that they can be used to generate electricity.

Instructions on how to build the rest of the so-called biophotovoltaic would be printed on a cartoon included with the baggie.

One step is to extract and concentrate photosystem-I from yard waste, for example, with a membrane such as cheesecloth and spinach. "It is not that hard," Mershin promised. "The green stuff is easy."

In addition, these do-it-yourselfers will need to roughen up a piece of glass or metal, which increases the surface area, to stick the stabilized green goo onto.

Wires connected to this plate would deliver the trickle of electricity to a battery, cell phone or a light.

Mershin and his colleagues explain their process for building one of these biophotovoltaics in the open access journal Scientific Reports. 

The research improves on previous work by Mershin's MIT colleague Shuguang Zhang, who coated photosystem-I on a flat glass surface. 

This produced an electric current, but such a small amount that it was practically useless. In addition, the stabilizing chemicals used were expensive and assembling it all involved expensive lab equipment.

Mershin looked to nature for inspiration and found a potentially better design in forests of pine trees that allow "for more light to be absorbed," he said.

He mimicked this forest effect with zinc oxide nanowires and a sponge-like titanium-dioxide nanostructure. 

When this chip is coated with the light-harvesting material extracted from plants, it creates a solar cell with 0.1 percent efficiency.

"At 0.1 percent, you can only do this as a proof-of-principle," Mershin said. "Nobody is going to be doing this in real life until we get to about 1 or 2 percent efficiency and about 12 months of lifetime."

The hope is that researchers around the world will replicate the results — which can be done with inexpensive materials and equipment — and improve on the design to reach that milestone.

If so, this technology could be a way to bring electricity to the 1.2 billion people in the world who live without it today.

Ideally, he said, not even the plastic baggie with the powder will be required. "We'll just send out fliers that have the information."

More on solar energy technology:

• Tree power could save forests from fires
• Quantum dots: A big boost to solar tech?
• Sunflowers inspire improved solar power plant
• Ant frying tech could make solar cheap

Susan Montoya Bryan / AP file

Solar panels at a 2-megawatt photovoltaic array in Albuquerque, N.M. are shown. Charged quantum dots could increase the efficiency of solar cells by 45 percent, according to researchers.

Itsy bitsy particles with a built-in charge could provide a big boost to the efficiency of solar cells, according to researchers aiming to take their innovation to market.

The particles, called charged quantum dots, are embedded into conventional solar cells, and increase their efficiency by up to 45 percent, the team from the University at Buffalo reports.

The boost comes because the dots permit harvesting of infrared light, which is otherwise lost, and the charge on the dots prevent them from absorbing free-flowing electrons in the cell.

"These two special effects we can use to increase solar cell efficiency," Andrei Sergeev, an electrical engineer at the university, told me Monday. 

He and colleagues published their findings in May 2011 in Nano Letters and recently created a company, OPtoElctronic Nanodevices, to commercialize the technology.

The company aims to develop solar cells with the tiny particles and then license them to manufacturers.

"These cells will be at least 50 percent and up to 100 percent more efficient than current solar cells," according to a presentation given at an energy conference in October.

Such improved cells could be a boost to the U.S. military, which is on the lookout for light and powerful energy technologies for use on the battlefield. 

In fact, researchers with the U.S. Air Force and Army collaborated on the project.

Key to the team's success is doping their quantum dot, which is made of semiconductor materials, so that it has a charge. 

"This built-in charge is beneficial because it repels electrons, forcing them to travel around the quantum dots," the University of Buffalo explains in a news release.

"Otherwise, the quantum dots create a channel of recombination for electrons, in essence 'capturing' moving electrons and preventing them from contributing to electric current."

The team calls their quantum dot with a built-in charge Q-BICs. 

Working in the lab, the team has demonstrated a "substantial increase in photovoltaic efficiency," Sergeev said. They now hope to scale it up and make it a viable technology. 

"This is only the beginning," he added.

In other words, whether this solar breakthrough will be the one that succeeds in the marketplace remains unknown. To check out more ideas in the solar technology landscape, see the stories below.

More on solar technology:

• Sunflowers inspire improved solar power plant
• Himalayas: The future of solar?
• Ant frying tech could make solar cheap
• 'Greenhouse effect' used to generate electricity
• Artificial leaf makes real fuel

California's investments in renewable energy help make San Diego one of the hottest markets for green jobs in the U.S.

In 2013, the cost of solar power in San Diego will be cheaper than electricity from the local utility grid, according the predictions of an energy policy analyst who created a handy graphic to illustrate when so-called grid parity will be achieved.

Sam Mircovich / Reuters

A prototype sun tracking solar panel made by Concentrix Solar collects energy from its location at the University of California San Diego in this file photo.

The interactive graphic posted on the Energy Self Reliant States website shows when this moment will be reached in major U.S. cities between now and 2027. 

Parity is a "tipping point, when democratization of the electricity system not only makes political and economic sense, but becomes more competitive than using utility-delivered electricity," writes analyst John Farrell.

His calculations assume that the cost of solar will continue to fall by 7 percent a year and grid electricity will rise at 2 percent a year. 

If true, then San Diego will be the first to reach the parity milestone, followed by New York in 2015. From there, parity is progressively reached across the southern tier of the U.S. with my cloudy, rainy, northern hometown of Seattle not reaching parity until 2027.

More on solar power:

• Solar power is beginning to go mainstream
• Google pulls plug on solar power plan
• U.S. trade panel to probe solar dispute with China
• Ten hot green energy trends to watch
• Himalayas: The future of solar?
• PG&E makes deal for space solar power