Originally Posted by MI3GTO
A still can be built for £10 to produce ethanol
I'd be surprised if a £10 still can produce enough ethanol (relatively dry) to be of use as a fuel. A £10 still also will be relatively unsafe device and potentially a good way to kill yourself or at the very least destroy your garage!
I don't know if you can distill fuel without appropriate licenses but I assume the Irish tax man is sensitive about stills that could be used for making drinkable spirit without paying tax.
Assuming you can convert all the energy in raw potatoes into energy stored in ethanol (probably unlikely - but lets assume you could) then 1 kg of raw potatoes has about 3350 kJ or energy. This is roughly about the amount of energy in 140 mL of Ethanol. i.e. it will take at least 7 kg of tatties to make a litre of "pure" ethanol. So unless you are going to grow them yourself (and have a very big field and enough time to do that) or have access to some ridiculously cheap source of tatties I think you will not be able to do it cheaper than petrol.
Worse than that (even if you could find a farmer with half a tonne of tatties to get rid of) is that first you normally boil the tatties. That takes energy. Then you let it cool down (that wastes that energy) so the yeast can do its work, Then you heat it all back up again to boil off (and condense) the ethanol. Now if your giant vat of tatties was perfectly insulated and efficiently heated (and for simplicity assuming the heat capacity of tatties is similar to water) then for each kg of water/tatties it is going to take about 0.1 kWhr to bring it up to boiling. Assuming you have equal quantities (by mass) of water to tatties that would be 1.4 kWh just to do the first stage of bringing the tatties to the boil. I'd guess you need at least the same amount of energy to keep them boiling until cooked. Then after the yeast has done its magic (you will have about 10% ethanol) you'll need roughly the same energy again to bring it to the boil again, plus another 2.4 kWh to vaporize the ethanol. So the energy requirements will be at the very least 6.6 kWh per litre of ethanol (and even an "efficient" system is probably 2x that - and a hombuilt system probably 4x it). Domestic electricity costs something around 15p per kWh. So its going to cost £1/L in energy absolute minimum to distill your ethanol... and potentially way more.