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.

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

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!

Hydrogen as a Secondary Fuel:

While hydrogen is the most abundant element in the Universe on the Earth its mostly found as water.

Hydrogen can be easily separated from Oxygen in water via Electrolysis. This process is about 67% efficient

Burning hydrogen combines with oxygen to form water --> no other combustion products (except for small amounts of nitrogen oxides formed around high temperature combustion zone)

For use as a secondary fuel, Hydrogen needs to be stored as a liquid. (20 K; -253 C).

As a liquid its energy density per unit volume is 1000 times higher.

For a given stored energy requirment, a cryogenic hydrogen facility is much less expensive than a pumped hydro facility

But overall efficiency is 25% cryogenic storage is energy intensive

But, one can make a hydgrogen-oxygen fuel cell Using a catalyst, hydrogen combines with oxygen to make water plus electricity. In the lab, such cells can acheive 85% efficiency but large scale value is unknown and untested although there have been some recent breakthroughs:

• All about Hydrogen Fuel Cells

Hydrogen is already produced mainly to form ammonia to be used in fertilizer. Hydrogen is extracted from methane and steam to make Carbon Dioxide.

• Ammonia = NH3
• Methane = CH4
• Carbon Dioxide = CO2
• Water = H20

Problems with the use of Hydrogen:

For 10% of our national energy budget, 400 1000 Megawatt power plants operating at 24 hours per day would be required to produce hydrogen via electrolysis. This is twice the current national demand.

Hydrogen is incredibly explosive (Hindenburg disaster). More explosive than natural gas. Can explode when mixed with air at concentrations of 4-75%. The ignition energy for this mixture is also very small and easily generated from a spark of static electricty.

Transport of Hydrogen Gas:

• use existing natural gas pipelines
• for a given energy requirement, 3 times as much hydrogen is needed as natural gas
• but hydrogen has lower density and can be pumped at 3 times the flow rate of natural gas.
• pipeline systems are very efficient compared to transmission losses over long electrical grids

Costs:

Because of the inefficiency in producing it, hydrogen will always be more expensive than the electricity that produced it, if you do the price comparison at the production site

But, for situations where customers are 1000 miles away from the production site - it is cheaper to deliver hydrogen through a pipeline system than electricity through the power grid.

A possible strategy is to build large, sturdy windmills in the Aleutian Island Chain (one of the windiest places on the Earth), for the purposes of producing electricity to make hydrogen from Sea Water. The hydrogen would then be shipped over the pipeline network to customers thousands of miles away.

The use of liquid hydrogen as a fuel source has potential (particularly on jet airplanes) but technical problems associated with storage and delivery have not yet been overcome Add your questions or comments about this particular lecture

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