How would be the chemistry/electrolyte if one were to use either Al or Fe in a galvanic cell, for generating electricity ?
==> The scrap-Al-price consists maybe 80-90 % of the energy-price ...
==> ... so if 50 % efficiency could be achived there would be a cheap way to get maybe 0.7 V or somethibng like that ...
I searched several times in the past for some AL-cell, but nothing to be found ... also I want to use standard-scrap, tot Al/Mg-alloy (that is used
for some newer sort of cells ...)
Anty Ideas ? Into what cheap electrolyte could I hang some Al-Plates to recover the chemical energy as electricity ?12AX7 - 25-3-2010 at 06:28
Aluminum in NaOH doesn't have a terrible self-discharge rate, and generates 0.8V or so. The byproduct is hydrogen, which could be piped to a fuel
cell, or oxidized by MnO2 as in an alkaline cell, roughly doubling the voltage output.
Downside: Al(OH)3 clogs *everything*. In some areas, it builds up on the metal, passivating it; other areas remain clear, but the cell container and
cathode still get plastered.
As for iron, you get maybe 0.4V if you're lucky, so you need an awful lot of cells. Iron is cheap, but you need something to dissolve it in, or a
more reactive metal to displace (like copper), either of which raises the cost noticably.
Tim
[Edited on 3-25-2010 by 12AX7]not_important - 25-3-2010 at 07:02
Al-air batteries/fuel-cells have been seriously proposed, with designs that supposedly handle the oxide problems. For at least one design the target
chemistry was to generate OAlOH so as to reduce the energy going into reclaiming the Al - less dehydrating to do.
chief - 25-3-2010 at 09:08
I have now checked numbers, and suddenly it seems expensive ... ; but the numbers are right I think:
Faraday-constant 96485.339 C/mol alias As/mol
Aluminum: 37.06449 mol/kg (from molar mass of 26.98 g/mol)
==> oxidation to Al3+ gives 111.193474 mol of charge,
==> times the Faraday-constant gives 1.072854e7 As
==> times 0.8 V gives 8582832.0 Ws
==> eq. 2.38 kWh per kg of AL ....
But Al-scrap costs 1000 EUR/ton, so the kWh (at 100 % efficiency) would cost 41.9442 ct ...
==> I'm quite sure that the kWh at the factores, where the Al is initially electrolyzed from kryolithe, comes at maybe 1 or 2 ct. per kWh ...
... wonder wonder ...
But the calculation is right, isn't it ?
================
Or is it the hydrogen where most of the energy goes ?
[Edited on 25-3-2010 by chief]Ozone - 25-3-2010 at 12:23
See:
Power and Hydrogen Generation System
Abstract:
A galvanic cell system was discovered that is based on two dissimilar electrodes in an electrolyte solution of hypochlorite and peroxide. The oxidant
electrolyte solution contains preferably sodium hypochlorite and hydrogen peroxide in a 10:1 ratio. The cathode (e.g, a copper electrode) was not
appreciably consumed. The anode preferably was composed of an aluminum/manganese alloy. This galvanic cell system produced significant current density
(e.g., 23 mA/cm.sup.2) at a useful voltage (e.g., 1.6-1.7 V/cell). It also produced hydrogen gas, with the maximum production being approximately 1.5
moles of hydrogen per mole of expended anode material. The by-products of this fuel system were environmentally friendly products, including sodium
chloride, aluminum hydroxide, and a trace of permanganate ion.
The energy production cost of Al seems to range fairly widely. The following seem to be good numbers
Quote:
Smelting a ton of aluminum requires ideally 4610 kwh.
14500 kWh/ton
However, to produce 1 ton of electrolyzed aluminum there is only about 13000–13300 kWh of direct electricity consumption in foreign advanced prebake
cel
With the first being the base reduction power, the other two including energy to melt the bath and Al produced, and otherwise lost in the smelting
process.
Using that ideal figure your result is roughly half of the theory, but yu used a rather low voltage.
The high energy cost of producing metallic Al makes the scrap of high value, energy cost of Al from recycled scrap is around 1/6 that of production
from ore. This makes it unlikely that you can save money by using the scrap as a power source unless you have a good supply for free.
JohnWW - 25-3-2010 at 13:27
The most efficient producers of Al metal from alumina, using the cryolite catalyzed electrolysis of a mixture of refined bauxite (Al3O3) and cryolite
(Na3AlF6), are in New Zealand and Norway, and after that Canada, where giant hydro-electric power stations provide the electricity. There should be
comparative figures for the cost of electricity to the Al smelters somewhere.bquirky - 26-3-2010 at 09:01
hi, i have made al cells, Including Zn ions in the Naoh electrolyte helps slow the spontanious reactions of al but only by about 12 hours you need a
counter electrode that can act as a depoleriser like a copper oxide to keep a constant currant flowchief - 26-3-2010 at 12:50
How much does the current go down ? Would it work to just compensate the current-drop by making a bigger cell ?
How good does a _simple_ cell work ? Just the KOH-solution (or NaOH ?), some Al and a Kathode of maybe stainless steel or even plain steel ...
==> The KOH would self-recycle itself, right ? Just feeding Al intio the cell ... ?
==> NaOH might form the Na/Al-mix-hydroxide, but KOH wouldn't and ist _therefore_ preferred ?
Also: The mentioned Hydro-Electricity comes at maybe 1 or 2 ct/kWh ... which is quite a big contrast to the maybe 50ct/kWh when calculating the
electricity-content of the aluminum ...
==> This can't be all explained by efficienciencies, can it ?
