SOLUTIONS
Storing Renewable Energy
by Jon Luoma
Renewables, like the solar-thermal system above, use clean but intermittent natural energy souces. So robust storage is the "holy grail" factor that can transform them into base-load systems that produce "power-on-demand" for the electricity grid.
The new "holy grail" of clean energy
The great, carbon-free power sources of wind and solar have, so far, been are limited by intermittency. Baseload "power on demand” has been thought to belong only to fossil coal, gas and oil, or to nuclear power. Now practical affordable electricity storage is beginning to break the limiting belief that links renewables to intermittency.
Grid parity has been the “Holy Grail” of clean energy for decades. Critics claimed that wind and solar could not compete, dollar-for-dollar, with coal and nuclear. In the last few years however, rapid technological advances and growing economies of manufacturing scale have put wind power almost at grid parity—ie. the cost of generating electricity from wind or coal are now much the same. Solar power is likely to follow suit within five years.
As grid parity looms, the new "holy grail" is the discovery of ways to store millions of watts of excess green electricity for times when the wind doesn’t blow and the sun doesn’t shine. The day when such large-scale energy storage becomes practical and cost-effective now too seems within reach. Some technologies that can store sizeable amounts of intermittent power are already deployed. Some that hold great promise lie just over the horizon.
Large-scale electricity storage will unshackle alternative energy
New storage approaches include improvements to existing lithium-ion batteries and storage of energy as huge volumes of compressed air in geologic vaults. Another feasible plan is to create a network of small, energy-dense batteries in tens of millions of homes—a “distributed storage” scheme whereby utility computers coordinate electricity flows over a “smart grid” that continually adjusts the flow of power to and from local batteries. The “V2G” (vehicle-to-grid, see also below) concept would include batteries in stationary plug-in hybrid or all-electric vehicles.
A123 Systems Hymotion Plug-in Conversion Module turns a standard
Toyota Prius into a plug-in hybrid getting 100+ mpg (60% less fuel
consumption/emissions). It charges from standard household outlets.
In a world run on fossil fuels, finding ways to store electricity was not an issue. Power plants across the grid simply burn more fuel when demand is high. But large-scale electricity storage will be an energy game-changer, unshackling alternative energy from the constraints of intermittency. If a wind or solar farm is the cheapest, cleanest way to generate power, it will no longer matter when the sun shines or the wind blows.
One obvious storage approach is improving battery technologies. Efficient, enormous batteries could store tens of millions of watt-hours of power. Today, the vast majority of rooftop solar photovoltaic panels are connected to the grid, using it as a giant battery, pushing excess power onto the grid when ther panels provide excess power. The building then draws power from the grid when the sun is not shining—the meter spins backward and forward with the ebb and flow of power. With relatively few solar roofs yet in play, utilities manage this by drawing down and ramping up generation at conventional power plants that are designed to balance fluctuating supply and demand.
Robust solar and wind power could be better served by giant batteries—more likely many widely-distributed batteries. This concept is a proven one: the world’s largest nickel-cadmium battery (as in todays laptop computers) has been storing energy for Fairbanks, Alaska for six years. This isolated 100,000-resident city needs an electricity backstop since its sub-zero winters freeze pipes in a couple of hours. Housed in a giant warehouse, a 1,300-tonne battery, larger than a football field, cranks out 40 million watts of power, enough for 12,000 residents for 7 minutes. That was sufficient to prevent 80+ blackouts its first 2 years.
Cost will determine which storage technologies prevail
In Japan, flow batteries have been used for years to store backup power at industrial plants. Conventional batteries store energy in chemical form. In a flow battery, charged chemicals are pumped into storage tanks, allowing yet more chemical to be charged and pumped away, then pumped back into the active portion of the battery, and drawn down as needed. Flow battery capacity is expanded simply by adding more chemical storage tanks. One Australian utility has used a large flow battery to soak up and release excess power from a wind farm for the last 7 years.
Storage technologies must considera factor termed round-trip efficiency—some energy is lost as it goes into storage, and some more is lost as it comes out. That is why there are high hopes for lithium ion batteries, since they have impressive round-trip efficiencies, can pack in high densities of energy, and can charge and discharge thousands of times before becoming degraded. This is the technology that dominates already in laptop computers and cell phones. On a larger scale, we see it in new vehicles like the plug-in hybrid Chevrolet Volt.
Lithium ion batteries also have applications on the grid. A123 Systems has installed a 2MW lithium ion storage unit atone California power plant. Although lithium ion is still 10 times more costly than lead-acid batteries, technological improvements and manufacturing scale should bring costs down with time. Then again, someone may find a way to build an even better battery. IBM has a major research program into the promising lithium metal-air battery, delivering 10 times the energy density of today’s best lithium ion technology. Pound for pound, it offers the energy density of gasoline, because it uses oxygen drawn from the air, replacing certain chemical reactants in the lithium ion system. However air isn’t just oxygen and also contains humidity. Lithium is capable of acting like ignited gasoline when exposed to moisture. So it may take 5 years or more to develop a membrane that will let oxygen into the battery but keep moisture out.
V2G
“Vehicle to Grid” storage (V2G) relies on idled storage in the batteries of the millions of plug-in hybrid or all-electric automobiles. More than 90% of the time cars sit idled, and aside from days they’re used for long trips, most of their full energy storage capacity goes unused.
A single idle, electric-powered car could generate as much as 10 kilowatts of power, enough to meet the average demand of 10 houses. With V2G technology controlled by an array of smart meters, car owners plugged in at home or work could allow the grid to draw off unused chunks of power at times when short-term demand is high. Conversely, cars will be recharged when demand is low.
Electrolysis of Water: Hydrogen Storage
Using excess electricity to make hydrogen that can be stored could be a big breakthrough for energy storage. Hydrogen can be produced through simple electrolysis, but the technical and cost hurdles have far made that impractical. Industrial-scale hydrogen is currently produced from natural gas as (fossil fuel) feedstock. At the Massachusetts Institute of Technology, however, Daniel Nocera and colleagues have recently made a major discovery—a new cobalt/phosphate catalyst (i.e. abundant,non-toxic materials) that kick-starts electrolysis. This photosynthesis-like process could mean solar or wind-generated electricity being stored by splitting (and later recombining) simple water. It has been hailed by James Barber of Imperial College London as having “enormous implications for the future prosperity of humankind.” Nocera has formed a start-up company Sun Catalytix to develop it further.
On an entirely different front, excess electricity can be used to pump compressed air into caverns, salt domes, and old natural gas wells. Then this air can be released to help state-of-the-art natural gas power plants spin their turbines, lowering the amount of fuel consumed by as much as 70%. A consortium of utilities in Iowa, Minnesota, and the Dakotas is working with the Sandia National Laboratory to develop a giant 268MW compressed-air system, the Iowa Stored Energy Park. It will store excess energy from that region’s burgeoning wind power industry.
Jon Luoma has written 3 books on environmental issues & is a contributing editor for Audubon. He writes lucidly on science & environment topics forThe New York Times, National Geographic and Discover. The above was edited, with new illustrations, from his 2009 piece for the excellent website Yale Environment 360.