Nuclear fusion

[otolith]

Monty Python said:

Should have been a bit more specific - I don't mean replicating what a plant does exactly, it's using sunlight to turn carbon dioxide and water into fuels like hydrogen or ethanol. The sun puts out enough energy in an hour to power the entire planet for a year.

We need more efficient solar cells and energy storage/transport mechanisms. Not sure that hydrocarbon fuels are ideal, if they are used in inefficient IC engines. Maybe in fuel cells. Not sure hydrogen is a great storage/transport mechanism, it's certainly not the most convenient.

[XM5ER]

Monty Python said:

Should have been a bit more specific - I don't mean replicating what a plant does exactly, it's using sunlight to turn carbon dioxide and water into fuels like hydrogen or ethanol. The sun puts out enough energy in an hour to power the entire planet for a year.

It is an interesting idea that I'm sure is being studied as we speak. But let me just test one assumption, you say that the sun put out enough energy in an hour to power the human world for a year, is that energy that falls on the earths surface or total output? (I suspect, earthfall)? Second question, how much land area is needed to gather enough energy to power our world when taking into account all the efficiency reductions that you will experience as you convert light to chemical to motion via various transmission and conversion processes. These are engineering questions based on scientific assumptions.

Another quick question, actually how efficient is photosynthesis? A tree only needs enough energy to grow and produce flowers for bees to fool around in, that's a fair bit less than I need to throw a vehicle round a track on a weekend.

[ATG]

The photosynthesis process is about 30% efficient at capturing the energy of the light that is actually contributing to the process. (This is almost unbelievably efficient.) Plants aren't matt black and achieve efficiencies of something like 3%, but given that something like a wheat field can remove all the CO2 from the air passing through it on a sunny day, that level of light efficiency is more than adequate given plants' environments.

IIRC sunlight has a power density of about 1kw per m^2. That's quite a lot.

[Catweazle]

Nuclear fusion is solar power.

[XM5ER]

ATG said:

The photosynthesis process is about 30% efficient at capturing the energy of the light that is actually contributing to the process. (This is almost unbelievably efficient.) Plants aren't matt black and achieve efficiencies of something like 3%, but given that something like a wheat field can remove all the CO2 from the air passing through it on a sunny day, that level of light efficiency is more than adequate given plants' environments.

IIRC sunlight has a power density of about 1kw per m^2. That's quite a lot.

I think that figure is for the top of the atmosphere, according to that notorious energy budget diagram (which is ridiculously simplified) sunlight accounts for about 400wpsm. Obviously there will be big variations based on location, date, time, weather etc. Wheat grows in summer not winter after all.

[ATG]

A quick Google says 1.4kw/m^2 at the top of the atmosphere and 1kw/m^2 at ground level.

[XM5ER]

A fair chunk of that energy will be in the useless frequency ranges though I guess.

[Simpo Two]

Monty Python said:

Should have been a bit more specific - I don't mean replicating what a plant does exactly, it's using sunlight to turn carbon dioxide and water into fuels like hydrogen or ethanol. The sun puts out enough energy in an hour to power the entire planet for a year.

Specialised/GM bacteria might be able to do it... but the process might use more energy than it makes, I don't know.

[ATG]

Simpo Two said:

Specialised/GM bacteria might be able to do it... but the process might use more energy than it makes, I don't know.

Of course it won't convert all of the energy put into the system into fuel. Energy is conserved in the system as a whole and entropy increases. That's true for any system ... fusion, a match, a plant, me digesting a mars bar, etc. It doesn't matter unless the process is so inefficient that you'd need huge tracts of land to gather a meaningful about of energy.

If you make a bacteria that excretes oil at 1% efficiency, you might capture something like 10W/m^2. Let's say you drive 2 hrs a day at an average power of 2kW, then you'd need something like 1/80th of an acre to run your car. Even if some of those numbers are out by a factor of 10, it still is clearly vaguely plausible.

Edited by ATG on Thursday 11th June 14:48

[otolith]

You've then got to extract, refine and distribute it, though. And you've probably got some energy input into running the system.

I do think biofuels from genetically engineered organisms have a great deal of potential, though.

[ewenm]

otolith said:

You've then got to extract, refine and distribute it, though. And you've probably got some energy input into running the system.

I do think biofuels from genetically engineered organisms have a great deal of potential, though.

As long as the genetic engineering includes the ability to grow where food-crops cannot.

[otolith]

Yep. But that's most of it, if you design your biofuel farm to operate at sea.

[Simpo Two]

ATG said:

Simpo Two said:

Specialised/GM bacteria might be able to do it... but the process might use more energy than it makes, I don't know.

Of course it won't convert all of the energy put into the system into fuel. Energy is conserved in the system as a whole and entropy increases. That's true for any system ... fusion, a match, a plant, me digesting a mars bar, etc. It doesn't matter unless the process is so inefficient that you'd need huge tracts of land to gather a meaningful about of energy.

If you make a bacteria that excretes oil at 1% efficiency, you might capture something like 10W/m^2. Let's say you drive 2 hrs a day at an average power of 2kW, then you'd need something like 1/80th of an acre to run your car. Even if some of those numbers are out by a factor of 10, it still is clearly vaguely plausible.

My point was really that if it took two litres of ethanol to make one litre, then it would be pointless to do it. We know yeast can make ethanol - man's been doing it for millennia - however hydrogen would be a lot more specialised/difficult/complex.

[otolith]

Simpo Two said:

My point was really that if it took two litres of ethanol to make one litre, then it would be pointless to do it. We know yeast can make ethanol - man's been doing it for millennia - however hydrogen would be a lot more specialised/difficult/complex.

There are bacteria which will produce hydrogen from a suitable substrate, so the possibility of direct production from biomass exists. It's also possible to produce it industrially from hydrocarbon gases, which again are relatively easy to produce by fermentation - though I'm not sure I see the point when you could just use the gases as fuel. One of the issues with ethanol production is the energy required to separate it from the dilute aqueous solution produced by fermentation - so fuels which are easier to separate, gases, oils, etc, may have advantages there.