# Naked Science Forum

## General Science => Question of the Week => Topic started by: katieHaylor on 14/08/2017 18:05:42

Title: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: katieHaylor on 14/08/2017 18:05:42

If we could genetically engineer cells to make our own energy, like plants do using chloroplasts, how much extra skin surface area would we need in order to function with similar energy levels as today's humans?

What does everyone think?
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: RD on 14/08/2017 20:50:24
For a man, NHS recommended energy-intake is 2500 kCal per day.  That's ~3650MJoules per year.
Total solar energy in London over a year is ~1000kW per square meter.
=> 3650 square meters of green skin required*.  ( In ball-park figures, about 1/3 of a ball-park ).

[* assuming energy consumption was unchanged.]
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: Bored chemist on 14/08/2017 21:39:48
A different way to get a rough handle on the answer is that it takes (very roughly) a mature oak tree to produce enough oxygen for a person.
The energy released by combining food with that oxygen is roughly the same as was needed to produce it in the first place.
So you need roughly an oak tree's leaves worth of area.
Google tells me there are roughly 200,000 leaves on a tree. (I didn't check who counted them).
if a leaf is about  5cm square then that's about 500 square metres
More or less the same ball park.
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: chris on 14/08/2017 22:24:06
The saccoglossan sea slug has done this to good effect already, and is the animal equivalent of a Prius, running on a hybrid power supply.

It has evolved to steal the chloroplasts from its algal dinner and transport them through specialised tubules to the skin so that, supported by biochemistry supplied by genes also presumably stolen from algae earlier in evolutionary history, the pinched chloroplasts are press-ganged into operation and turn the slug into a solar panel.

But, if the Sun doesn't shine, the slug can carry on grazing and make up the energy deficit just by eating a bit more. Impressive!

[Images from en.wikipedia.org/wiki/Elysia_chlorotica]
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: chris on 15/08/2017 18:02:56
For a man, NHS recommended energy-intake is 2500 kCal per day.  That's ~3650MJoules per year.
Total solar energy in London over a year is ~1000kW per square meter.
=> 3650 square meters of green skin required*.  ( In ball-park figures, about 1/3 of a ball-park ).

I thought I'd build on the "ball-park" answer from @RD

I think the accepted value for solar intensity is actually about 1kW per square metre. (From wiipedia: " the direct sunlight at Earth's surface when the Sun is at the zenith is about 1050 W/m2").

If we optimistically assume the sun shines for about 10 hours a day at this rate, that's 10h/d * 3600 s/h * 1000 W/s = about 35MJ/d * 365 d/yr = 13GJ/yr.

From wikipedia, on photosynthetic efficiency, "the theoretical maximum efficiency of solar energy conversion is approximately 11%". So the useful energy in our sunlight to a photosynthesising plant is 13GJ * 11/100 = about 1GJ per year per square metre of collecting surface.

2500kCal is about 10MJ (megajoule), so 365 days/yr * 10MJ/day = about 4GJ/yr.

Energy required / energy available = 4GJ/yr / 1GJ/yr = 4 fold under-supply.

Surface area of a human is about 2 square metres.

Therefore we would need at least 4 x 2 = 8 times the surface area of skin we do currently, and live somewhere with 10h/day of constant strong sunlight, to soak up enough energy to survive on photosynthesis.

Have I got that right?
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: TimBandTech on 15/08/2017 19:09:04
For a man, NHS recommended energy-intake is 2500 kCal per day.  That's ~3650MJoules per year.
Total solar energy in London over a year is ~1000kW per square meter.
=> 3650 square meters of green skin required*.  ( In ball-park figures, about 1/3 of a ball-park ).

I thought I'd build on the "ball-park" answer from @RD

I think the accepted value for solar intensity is actually about 1kW per square metre. (From wiipedia: " the direct sunlight at Earth's surface when the Sun is at the zenith is about 1050 W/m2").

If we optimistically assume the sun shines for about 10 hours a day at this rate, that's 10h/d * 3600 s/h * 1000 W/s = about 35MJ/d * 365 d/yr = 13GJ/yr.

From wikipedia, on photosynthetic efficiency, "the theoretical maximum efficiency of solar energy conversion is approximately 11%". So the useful energy in our sunlight to a photosynthesising plant is 13GJ * 11/100 = about 1GJ per year per square metre of collecting surface.

2500kCal is about 10MJ (megajoule), so 365 days/yr * 10MJ/day = about 4GJ/yr.

Energy required / energy available = 4GJ/yr / 1GJ/yr = 4 fold under-supply.

Surface area of a human is about 2 square metres.

Therefore we would need at least 4 x 2 = 8 times the surface area of skin we do currently, and live somewhere with 10h/day of constant strong sunlight, to soak up enough energy to survive on photosynthesis.

Have I got that right?

I think that is a pretty good analysis. Good of you to include the theoretical efficiency of photosynthesis.

As I recall a human runs at about 100 watts steady state, with a high performance athlete able to deliver about four hundred watts for a limited time; maybe say an hour. Banking energy as fat ought to be in the system so a steady state figure should be OK, though the efficiencies matter tremendously and have to be multiplied.

I've done some photovoltaic design, and know that the 1000 watts per square meter figure is regarded as accurate as direct sun on a perpendicular plate; on a clear day. We then reuse this figure based on statistical sun hours in a given region. An old data set produced in the 1980's point to my location having about 4 sun hours per day averaged annually, but much shorter in the winter; down to around two sun hours per day as I recall.

The 11% efficiency shows that the 1000 watt figure happens to diminish to close to the 100 watt figure, so instantaneously a human could be solar powered at one hundred watts with just one square meter of green skin pointed straight at the sun. I won't do out the steady state figures, and would have to consider the efficiency of fat storage much like a battery backed system has to sacrifice about 15% to battery inefficiency. I wouldn't be surprised if we wound up at around ten square meters of green skin. But hey, that is way better than the acreage of space each human is consuming in food now. We've got to get this green skin thing going. Live for the sun. Stop the killing of our fellow sun worshipers.
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: cinka on 21/08/2017 01:25:19
I really don't know. Is there really a count?
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: katieHaylor on 23/08/2017 14:53:43
This question has now been answered in our show, and you can listen to it here - https://www.thenakedscientists.com/articles/questions/how-much-green-skin-would-we-need-photosynthesise (https://www.thenakedscientists.com/articles/questions/how-much-green-skin-would-we-need-photosynthesise)
Title: Re: QotW - 17.08.13 - How much green skin would we need to photosynthesise?
Post by: Danne on 04/10/2017 14:16:58
This question has now been answered in our show, and you can listen to it here - https://www.thenakedscientists.com/articles/questions/how-much-green-skin-would-we-need-photosynthesise (https://www.thenakedscientists.com/articles/questions/how-much-green-skin-would-we-need-photosynthesise)

Now that is a bold claim :D !