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)

So potentially, Hydrogen wins big but the main problem is that there are no naturally occurring sources of hydrogen so it must be made and that has an energy cost associated with it.

Note: the actual fire on the Hindenburg was the result of a spark that ignited the outer skin and not the spontaneous explosion of hydrogen

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:

Hydrogen Fuel Cells

This animation shows the process that goes on inside an individual fuel cell. The red Hs represent hydrogen molecules (H2) from a hydrogen storage tank. The orange H+ represents a hydrogen ion after it's electron is removed. The yellow e- represents an electron moving through a circut to do work (like lighting a light bulb or powering a car). The green Os represent an oxygen molecule (O2) from the air, and the blue drops at the end are for pure water--the only byproduct of hydrogen power.

Notes:

• To produce power in large amounts, many of these cells are combined into a fuel cell stack.

• there will be some electron loss depending on the work done so the process is not 100% efficient as the animation shows.

• Until recently, a fuel cell stack was difficult and expensive to build.

Advantages of Hydrogen technology and fuel cells

Good overview of different kinds of Fuel Cells

Basic Chemisty of a Fuel Cell:

Anode: 2H₂ --> 4H⁺ + 4e^-
Cathode: 4e^- + 4H⁺ + O₂ --> 2H₂O

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Overall: 2H₂ + O₂ --> 2H₂O

What about fuel cell yield? This information is hard to find, but GM announced in October 2001 the following specifications:

• Rated Output: 1.75 KWH per Liter of Hydrogen (not this would cost about 25 cents at current KWH prices)

• Fuel Cell Stack weighs 180 pounds and gives continuous output of 100 KW to scale things, your home requires about 2KW of power or therefore 4.5 pounds of Hydrogen full cell stack.

  
  
  
  

  Production of Hydrogen

  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 very explosive when mixed with oxygen. In its pure state, it poses no explosive threat so is perfectly safe to ship in pipelines. 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 (Hindenburg Disaster).

  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/Problems:

  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

  Best hope Hydrogen fuel cell technology will continue to improve and soon (3-5 years) individual vehicles will use this as a main fuel source.

  However, it is quite unlikely that we will ever see electric production from hydrogen as the economics of the efficiency cycle is against this. Previous Lecture Next Lecture Course Page