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

The status so far, excerpted from Jon Luoma:

“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.

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.”