Energy Storage II

Energy Density of Some Materials (KHW/kg)

 Gasoline  --------------> 14
 Lead Acid Batteries ----> 0.04
 Hydrostorage -----------> 0.3 (per meter3)
 Flywheel, Steel --------> 0.05
 Flywheel, Carbon Fiber -> 0.2
 Flywheel, Fused Silica -> 0.9
 Hydrogen ---------------> 38
 Compress Air ------------> 2 (per meter3)

Energy density storage drives the choices that can be made. Technology helps to drive this. As discussed previously, energy storage in batteries is not sufficiently high to solve the basic problem.

Technical Overview of Energy Storage

Hence The Advanced Battery Consortium

Last time we talked about

• Pumped Hydro Storage Systems Pump water up hill for later recovery at times of peak demand.
  

• Fused Silica Flywheels High tensile strength material allows it to be rotated very fast without flying apart
  

  The model with the small yellow disc tends to stop when the crank and connecting rod are in a straight line ('dead' spots) - because sliding the brass knob exerts no turning force on the shaft. In the model with the big yellow flywheel, it is easy to keep the disc turning, once it has started, due to the effect of the flywheel. The mass and the size of the big flywheel helps resist the slowing down of the model as it is turning.

Other Forms of Energy Storage

Compressed Air:

Has high energy storage capacity compared to the alternatives. About 10 times higher per cubic meter than water.

One example (in Germany) to date:

• Storage reservoir is underground cavity in a natural salt deposit
• The storage volume is 300,000 cubic meters
• Sheer weight of the salt deposit is able to pressure confine the air reservoir
• Air is compressed to 70 atm (1000 lbs per square inch)
• Compression is done by electrically driven air compressors

• System delivers 300 Megawatts for 2 hours by using the compressed air to drive a turbine
• Difficult to measure the efficiency of this system. Two major contribution to the inefficiency:
  • Energy required to cool the air as it is being put into storage this is a critical requirement (see below)
  • Energy required (usually from fuel) to expand the cool air taken from storage as it entires the turbine.

• Desireable design feature would be recycle the waste heat from the compression stage and use it to reheat the air during expansion stage

For gases, Pressure is directly related to Temperature (Ideal Gas Law)

Go to the pressure chamber JAVA applet

Example calculation:

If the temperature of the air at 1 atm is 20 C, how much will the temperature raise if we increase the pressure to 100 atm.

In general, pressure and temperature between gases is related as

T₂ = T₁(P₂/P₁)^(n-1)/n

For an ideal gas, n = 1 in above. Air is not an ideal gas and it has n = 1.4 Temperature is measured in Kelvins .

so you get

T₂ = 293(100)^(1.4-1)/1.4 = 293 x 100 ^.286

100^.286 = 3.73

T₂ = 293*3.73 = 1093K = 720 C

which would melt the salt reservoir!

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