[ExI] gravel batteries

[Damien Broderick]

Giant Gravel Batteries Could Make Wind Power a Reliable Energy Option
CLAY DILLOW - POPSCI

<http://www.popsci.com/technology/article/2010-04/giant-gravel-batteries-could-make-wind-power-consistent-energy-option>

Wind Power Wind power is green as can be, but incredibly inconsistent. A 
new gravel-battery storage scheme could cheaply store excess power when 
the wind is strong to supplement wind turbines when the gusts die down.

Wind and solar are such promising technologies for the hydrocarbon-free 
energy sources of tomorrow, but intermittent, inconsistent output 
renders them unfeasible as anything other than secondary power sources. 
But UK firm Isentropic thinks it may have solved the problem as it 
pertains to wind power; all we need to stabilize the energy flow from 
turbines are giant batteries made out of gravel.

The battery consists of two large silos filled with crushed rock. 
Electricity generated by the turbine heats and pressurizes argon gas and 
feeds it into the first silo. The gravel is heated to more than 900 
degrees as the hot, pressurized argon passes through, though by the time 
the argon leaves the chamber it has cooled to ambient temperature.

The argon is then fed into the second silo where it returns to normal 
atmospheric pressure, initiating a cooling effect that chills the gas 
and rock to -256 degrees. Thus, the electricity is stored as a 
temperature difference between the two chambers. If the wind ceases to 
blow, the process is reversed, feeding the cold gas back into silo 
number one, powering a generator as it makes the transition back to hot 
from cold.

The process isn't a perfect closed energy loop, but Isentropic claims a 
complete trip through the cycle retains up to 80 percent of the original 
electricity. Even better, gravel is cheap; the cost per kilowatt-hour 
falls somewhere between $10 and $55, depending on the costs of other 
materials. Isentropic also claims the batteries are highly durable; 
according to the company's founder, a 164-foot tall silo with an equal 
diameter would retain half its energy even if left untouched for three 
years.

All that sounds pretty good, but Isentropic has yet to fully prove out 
the idea. The vast temperature differences generated by the argon sound 
quite drastic, and the director of the UK Energy Research Centre points 
out, gravel isn't the ideal material to have inside of machine with 
moving parts. As such, Isentropic is designing a pilot plant that could 
store 16 megawatt-hours in two silos just 23 feet tall by 23 feet in 
diameter. That's enough to cover a pretty big neighborhood during a 
long, windless stretch. The company is also in talks with an unnamed 
utility to build a larger demonstration facility.