Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Terry McHugh on 28/11/2011 15:30:02

Terry McHugh asked the Naked Scientists:
A question that I cannot find an answer to and which I believe is currently relevant refers to solar PV panels.
Light (photons) strike a solar PV panel which, for a given size emits watts.
How can the light striking the panel be measured against the wattage output. (X photons in = Y watts out) What is the correlation between these two factors.
Regards
Terry
What do you think?

You cannot directly equate photons with Watts there is a time element involved, Photons are carriers of energy measured in Joules while Watts are units in which the rate of flow of energy is measured i.e 1 Watt = 1 Joule per second.
You must talk of Photons per second or energy accumulated in units such as KWh (3,600,000 Joules)
If you are interested in solar panels The figures are that Solar Photons are assumed to carry 1.1 Ev which converts to 1.76 * 10^19 Joules while the typical efficiency of a solar panel would be 15% while the incident energy in sunlight at ground level is quoted as 1KW per meter^2.

Why not simplify the maths and ask how many photons of white light [ whose frequency average is say 6 *10^14 ] would be required to give a magnetic flux output of 1 watt per second.
Answer is I think 1.76*10^19/6*10^14 = 3*10^4
So it takes about 30 thousand photons hitting a one square metre solar panel per second at right angles to produce 1 Watt of energy at 100% efficiency. You would need 1000 wattsecs for a kW at typically 20% efficiency then you would need 5 times as many making it 1.5*10^8 photons! Someone should check the maths to confirm the figures.
CliveS

"1 Watt of energy at 100% efficiency" ???
I clarified the matter of units without actually doing the detailed arithmetic for the guy and here you are talking abouts Watts as energy.
energy is measured in Joules watts is the measure of how fast energy flows i.e Joules per second.

acsinuk there are several fundamental errors in your calculation of the number of photons per second required to give an output of one watt (oops! error deleted [:(!] should have said joule per second) so other people should disregard this very low number that he suggests as it is many orders of magnitude greater than this.
Directly from syhprum's previous answer one typical solar photon equals 1.76 * 10^19 Joules so you need 1/ (1.76 * 10^19 ) = around 6*10^18 per second to produce 1 Joule per second of incident power and five times more if the quantum efficiency is only 20%

Watt is it about units? ;)
"number of photons per second required to give an output of one watt per second "

Watt is it about units? ;)
"number of photons per second required to give an output of one watt per second "
Maybe the joules are multiplying? That would solve our energy problems PDQ.

What a strange mistake from a scientifically trained correspondent "one watt per second" this can only be the rate at which the power is changing !!!

syhprum that's what comes of doing things too quickly and working in mixed units I have now corrected my error thanks for pointing it out [:)]

I knew we should have stuck with BTUs/month.

An interesting point geezer! A bit off topic but Have you tried to understand the relationship between the numbers in your gas meter and your gas bill. It is way beyond most experts in quantum physics and relativity. [:D]

Early gas meters had alternate dials reading clockwise and anti clockwise making it very tricky to read.
Geezer
I think we should abandon foreign units like Joules and use proper Scottish units like Watt seconds.
Deduct one brownie point I did not realise he came from Salford !!

Our gas is liquid propane pumped in US gallons from a tanker truck. They know when to deliver it based on the average daily temperatures. We never have to call them.
I seem to remember the UK gas bill referred to therms, but I would assume the units are a bit more SI these days?
I think the air conditioning folks in the US still use BTUs. They also rate AC units in "tons". It took me a while to figure out what on earth they were on about.

(...snip...)
I think the air conditioning folks in the US still use BTUs. They also rate AC units in "tons". It took me a while to figure out what on earth they were on about.
1 ton = 1019 kg
speed of light in vacuum = 2.998E8 m/s
E=mc^{2} = 9.0E19 J
No wonder the electricity grid struggles on hot days!
[;)]

1 ton = 1019 kg
speed of light in vacuum = 2.998E8 m/s
E=mc^{2} = 9.0E19 J
No wonder the electricity grid struggles on hot days!
[;)]
Gotcha!
"Air conditioner equipment power in the U.S. is often described in terms of "tons of refrigeration". A ton of refrigeration is approximately equal to the cooling power of one short ton (2000 pounds or 907 kilograms) of ice melting in a 24hour period. The value is defined as 12,000 BTU per hour, or 3517 watts.[10] Residential central air systems are usually from 1 to 5 tons (3 to 20 kilowatts (kW)) in capacity."
http://en.wikipedia.org/wiki/Air_conditioner

That may well take over from the barn yard atmosphere as my favourite "ridiculous unit".
The clo gives it a run for its money.
Anyway the answer to the original question is something like 3X 10^ 19 photons of visible light per second for each Watt of power output.
Who is counting?

Don't atom bashing physicist's talk about effective cross sectional area in barns

That may well take over from the barn yard atmosphere as my favourite "ridiculous unit".
It does seem a bit bizarre, but I just realized it might have a very practical origin. The US railroads shipped vast quantities of perishable produce in "reefers"  refrigerated box cars. The early versions had ice loaded through their roofs at stops along the way. It was probably important to know how many tons of ice should be loaded into each car for the next part of the journey.

http://en.wikipedia.org/wiki/List_of_unusual_units_of_measurement
Wikipedia has a fine collection of odd units, some in genuine use and other just a farce.
One of the oddests still in common use is "horse power" 746 watts.

The barn is a crosssectional area unit of 1.0E28 m^{2}
It is very much on the scale of an atomic nucleus; nuclear collision crosssections range between about one thousandth and one thousand barn.
You can find it fairly easily in wikipedia.
What you may not find, and I cannot reference (knowing it only through my 1960s Physics lecturer and a few particle physicists of my acquaintance from the late 60s and early 70s) is that it was whimsically named after the size of barn door that would need to be closed to prevent a nucleus from straying.

is a Barnyard the imperial measure of nuclear cross sectional area i.e one Barn yard equals 0.8359 * 10^28 square meter

Going back into ancient history the barn (note no yard) is another good example of humour on science when the the first scientists doing nuclear science needed a measure for the sort of target that an atomic nucleus presented. Now a big nucleus is about 10^14 meters across and so its cross sectional area is about 10^28 sq metres when they first realised that that was the size of their target someone said its as big as a barn door (from the saying that its poor shooter that couldn't hit a barn door) and the name stuck. There are a whole load of other target sizes for lower probability events see http://en.wikipedia.org/wiki/Barn_(unit)
I have also just done a search on another interesting but old unit the "hide" which is the area of land needed to completely support an extended family unit and is around 30120 acres. This could come into modern units on population and self sufficiency.

I know the barn is not a S.I unit but it is tolerated but in a recent Phsyorg article I was reading a millibarn was quoted as being 10^27 cm^2 which I think is stretching our toleration a bit far

The barn yard atmosphere is a unit of energy (a rather small one).
An area multiplied by a distance gives a volume, and a volume multiplied by a pressure gives energy.
BTW if you don't like the "milli barn" as a unit for capture cross section you would really hate the alternative. Cross sections for improbable events are sometimes quoted in sheds.

There is actually a perfectly convenient and respectable SI alternative to millibarn.
1 barn = 1.E28 m^{2}, therefore 1 millibarn = 1.E31 m^{2}
1 fm = 1.E15 m, therefore 1 fm^{2} = 1.E30 m^{2} = 10 millibarn

There is actually a perfectly convenient and respectable SI alternative to millibarn.
1 barn = 1.E28 m^{2}, therefore 1 millibarn = 1.E31 m^{2}
1 fm = 1.E15 m, therefore 1 fm^{2} = 1.E30 m^{2} = 10 millibarn
Yes, but how many sheepies would that hold?

There is actually a perfectly convenient and respectable SI alternative to millibarn.
1 barn = 1.E28 m^{2}, therefore 1 millibarn = 1.E31 m^{2}
1 fm = 1.E15 m, therefore 1 fm^{2} = 1.E30 m^{2} = 10 millibarn
Yes, but how many sheepies would that hold?
approximately 0.314 attosheep

Have just looked up Oliver Boyd and found that a 1 kWh is equivalent to 2.247 * 10^25 electronvolts which means that as average light photons have 1.1 eV each then 2*10^25 is good aproximation for a one square metre PV panel which gets 1 kW peak at noon on equator after allowing for third haze reduction. The power is received per square metre but to change it into energy we must multipy by time. So each photon is not a magic massless ray but a volume of 3 dimensional magnetic flux energy which is absorbed at the surface of the PV cell.
CliveS