On Mon, 06 Aug 2007 17:57:47 GMT, Orval Fairbairn
wrote in
:
In article ,
Orval Fairbairn wrote:
Look at the problem this way: In an all-electric machine, you carry ALL
of your energy supply with you: fuel and oxidizer -- to make electricity.
With any IC engine, you carry the fuel only -- the air is free (20%
oxygen), so, at 15:1 air/fuel ratio, you would need 90 lb of air for
each gallon of fuel.
Therefore, for a nominal 50 gallon fuel capacity (300 lb), you would
have to carry an additional 7500 lb of air.
That is a lot of weight for a 3000 lb aircraft!
DUH! I meant 4500 lb of air! That is still a lot of weight penalty.
I hadn't thought of that. I wonder if a zinc-air batter might be
lighter than a lithium-ion polymer battery. Lithium, being number
three in the periodic table of elements, is pretty light; zinc is
number 30, so it's ten times heaver. But there are other concerns
like packaging requirements that come into play.
Here's some information about zinc-air batteries:
http://en.wikipedia.org/wiki/Zinc-air_battery
Zinc-air battery
Zinc-air batteries, also called “zinc-air fuel cells,“ are
non-rechargeable electro-chemical batteries powered by the
oxidation of zinc with oxygen from the air. These batteries have
very high energy densities and are relatively inexpensive to
produce. They are used in hearing aids and in experimental
electric vehicles. They may be an important part of a future zinc
economy.
Particles of zinc are mixed with an electrolyte (usually potassium
hydroxide solution); water and oxygen from the air react at the
cathode and form hydroxyls which migrate into the zinc paste and
form zincate (Zn(OH)42-), at which point electrons are released
and travel to the cathode. The zincate decays into zinc oxide and
water is released back into the system. The water and hydroxyls
from the anode are recycled at the cathode, so the water serves
only as a catalyst. The reactions produce a maximum voltage level
of 1.65 volts, but this is reduced to 1.4–1.35 V by reducing air
flow into the cell; this is usually done for hearing aid batteries
to reduce the rate of water drying out.
The term zinc-air fuel cell usually refers to a zinc-air battery
in which zinc fuel is replenished and zinc oxide waste is removed
continuously. This is accomplished by pushing zinc electrolyte
paste or pellets into an anode chamber. Waste zinc oxide is pumped
into a waste tank or bladder inside the fuel tank, and fresh zinc
paste or pellets are taken from the fuel tank. The zinc oxide
waste is pumped out at a refueling station and sent to a recycling
plant. Alternatively, this term may refer to an electro-chemical
system in which zinc is used as a co-reactant to assist the
reformation of hydrocarbon fuels on an anode of a fuel cell.
Zinc-air batteries have properties of fuel cells as well as
batteries: the zinc is the fuel, the rate of the reaction can be
controlled by controlling the air flow, and used zinc/electrolyte
paste can be removed from the cell and replaced with fresh paste.
Research is being conducted in powering electric vehicles with
zinc-air batteries.
http://micro.magnet.fsu.edu/electrom...s/zincair.html
Zinc-air batteries produce electrochemical energy by using oxygen
straight from the air. Oxygen becomes the cathode reactant, and is
diffused directly into the battery. The air cathode uses an
aqueous alkaline electrolyte to catalytically promote the reaction
of oxygen, but is not depleted or transformed at discharge. The
cathode is compact, yet at the same time has an almost unlimited
capacity, and achieves high energy densities due to the additional
volume available for the zinc anode.
The advantages of a zinc-air battery include flat discharge
voltage, safety and environmental benefits, good shelf life, and
low cost. In addition, zinc-air batteries have high volumetric
energy density compared to most primary batteries. The
disadvantages of such batteries are that they rely on ambient
conditions, they dry out once exposed to outside air, they have
flooding potential, they have limited output, and their active
life is short. It is important to note that when zinc turns it
into zinc oxide it expands, and there must be adequate space
within the battery for this expansion. The main form of gas
transfer degradation is water vapor transfer.