(Repeats Feb. 11 item; no changes to text)
By John Kemp
LONDON, Feb 11 (Reuters) - Finding a better way to store electrical energy is the single biggest breakthrough needed to tackle climate change. But the goal of developing batteries that are lighter, as well as more compact, powerful and affordable remains frustratingly elusive.
Virtually all usable energy on Earth originates from the sun: wind, solar, wood, even fossil fuels. Oil, gas and coal are simply the fossilised remains of organisms that lived in ancient forests, lakes and seas, which relied on photosynthesis to harness the sun's energy.
The amount of solar energy reaching Earth's surface every year is many orders of magnitude greater than that needed to meet all the needs of the planet's growing population.
Solar radiation reaching the planet's surface is equivalent to more than 150 watts per square metre on average, according to Roger Barry at the University of Colorado ("Atmosphere, weather and climate", 2010).
The problem is how to store that energy in a usable form available when and where needed. Solar panels are not useful at night, or in the winter at high latitudes, or when the sun is temporarily obscured by clouds. Solar energy ultimately drives the wind too. But wind turbines are useless on a still day.
The ideal storage medium has a high energy content but is light, compact and affordable. Liquid fuels refined from petroleum, especially gasoline and diesel, have the best combination of energy content, density and weight, which makes them especially desirable in mobile applications like cars, trucks and ships.
"Compared to gasoline and diesel, other options may have more energy per unit weight, but none have more energy per unit volume," the U.S. Energy Information Administration explained in a recent note ("Few transportation fuels surpass the energy densities of gasoline and diesel", Jan. 3, 2014).
Liquefied natural gas contains more energy per unit of weight but takes up much more space. Ethanol is inferior in weight and volume terms. Batteries are generally heavier and need more space than gasoline or diesel. Most are also much more expensive.
Burning those fossil fuels, however, contributes to global warming. The challenge is to develop more effective energy storage that can compete with gasoline and diesel. Better batteries are likely to prove the only realistic option.
Improved storage is needed at two very different levels: grid scale (for example to store solar electricity for use at night) where it can be stationary, and on a smaller, more mobile scale (to replace gasoline and diesel with electricity as the source of power in cars, trucks and ships).
Various options have been developed for stationary grid-scale storage - including pump hydro, flywheels and compressed air systems. Better batteries are the most promising technology for grid-scale applications, however, and probably the only one for small-scale mobile applications.
Small-scale batteries are already used in electric vehicles and plug-in hybrid electric vehicles. The main problem is the high cost and weight.
For grid applications, some truly enormous batteries have been built. The city of Fairbanks in Alaska has installed a battery system weighing 1,300 tonnes and taking up 2,000 square metres (21,000 square feet) in a warehouse capable of providing 40 megawatts of power for up to seven minutes in the event of a power failure (Daily Telegraph, "World's biggest battery switched on in Alaska" 2003).
Given Fairbanks' isolated location and winter temperatures as much as 50 degrees Celsius below zero (minus 58 Fahrenheit), a secure supply of electricity is vital. The battery is intended to keep the lights on long enough to start up auxiliary diesel generators.
Even bigger batteries have been built. In 2011, China's State Grid Corporation commissioned a giant battery array that can supply 20-36 megawatt-hours of electricity. The Zhangbei National Energy Storage and Transmission Demonstration Project, in Hebei province, is coupled with a wind farm and solar power plant, and claims to be the largest battery array in the world.
Crucially, Zhangbei was intended to prove that wind and solar can provide a reliable source of power on demand. But in most cases, batteries are still too heavy, too large and too expensive to offer an effective solution to grid-scale and vehicle storage needs.
Developing smaller, lighter and more powerful batteries is now one of the top research priorities for the U.S. Department of Energy, the world's biggest funder of science research.
Most of the department's research focuses on more mature technologies.
Its Advanced Research Projects Agency-Energy (ARPA-E), however, is a specialised unit that aims to catalyse the development of less advanced technologies with transformational potential by providing early-stage funding, technical assistance and commercial expertise.
Established in 2007, ARPA-E was modelled on the Pentagon's Defense Advanced Research Projects Agency, credited with helping develop the global positioning system, the stealth fighter and computer networking.
According to the U.S. National Academy of Science, ARPA-E was designed to support "creative out-of-the-box transformational generic energy research that industry by itself cannot or will not support and in which the risk may be high but success would provide dramatic benefits to the nation".
The agency has an annual budget of $275 million. Most of the funding is allocated to 18 focus areas. Five of them concern energy storage - more than any other area of energy technology.
The storage-related programmes are: Advanced Management and Protection Energy Storage Devices (AMPED); Batteries for Electrical Energy Storage in Transportation (BEEST); Grid-Scale Rampable Intermittent Dispatchable Storage (GRIDS); High-Energy Advanced Thermal Storage (HEATS); and Robust Affordable Next Generation Energy Storage Systems (RANGE).
With the exception of HEATS, which focuses on nuclear and solar thermal energy, all these programmes concentrate on developing better batteries. Since 2010, ARPA-E has allocated well over $100 million of funding to 47 battery-related projects, according to the agency's website.
BEEST is funding research that explicitly aims to make batteries comparable to gasoline and diesel. "To offer performance comparable to gasoline-powered cars, electric vehicles must be able to deliver the kind of power that will make electric cars fast, responsive and exciting to drive," ARPA-E emphasises.
"They also must be able to travel at least 300 miles (480 km) on a single charge - the approximate driving range per tank of gas - for a comparable cost. This would require batteries with double the energy density and one-third the cost of today's state-of-the-art lithium-ion battery packs."
ARPA-E is funding research to re-evaluate each component of a battery (positive and negative electrodes as well as the electrolyte) and the materials used to make them.
"BEEST projects are exploring a variety of potential solutions including radical improvement of today's lithium-ion technologies, designs using other metals such as magnesium, sodium and zinc, and new ways of using lithium in lithium-sulfur and lithium-air batteries," the agency explains.
At a much bigger scale, GRIDS is funding research into technology that could be used to back up intermittent wind and solar generation with storage anywhere on the electrical grid at a capital cost of less than $100 per kilowatt-hour.
According to the agency: "The energy storage facilities that exist today use pumped hydropower, which is only available in a handful of locations. New, more flexible, large-scale energy storage technologies would allow energy to be efficiently stored and sent to any location in the country.
"Cost-effective grid-scale energy storage is critical for increasing the use of renewable alternatives and reducing greenhouse gas emissions from the electric energy sector."
A major breakthrough at either scale still appears some way off. But nothing would do more to combat climate change, which justifies ARPA-E's commitment to research on a new generation of battery technology. (Editing by Dale Hudson)