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Photovoltaic systems to work at much lower energies

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Edi
#1
Apr26-13, 03:52 AM
P: 176
Ordinary photovoltaics work by depleting phosphorus un boron doped silicon in eatch other and creating a permenant electric field, which then seperates any pair of hole and electron induced to the conducting band by a incoming photon, creating an aditional elekctric potential difference, which can discharge only trough an external circuit. Right, close, correct?

Ok, so, from what i have been told and understand, the electrons in a phosphorus doped silicon crystal are, at room temperature (even lower, but how much lower?) or higher, are close to or already at the conduction band energy level (as the so called "free electrons", swimming from atom to atom..). That is, effectively, that low energy photons, room temperature kinetic enerby is enough to rise them to the conduction band (?)
So, why deplete the doped silicon to throw the electrons in lower energies, when we could leave them at high energy and, potentially, generating usable electric energy from low level temperature, IR and even room temperature itself?

How to do that? Well, I want to ask about one arrangement here I thought up.
Simply, charge the boron doped silicon from another source of electrons, instead of connecting it to the phosphorus doped silicon, creating a negatively charged block of sklicon. Now, just take the charged block and connecg it to the phosphorus doped silicon with a thin, isolating layer betwene them (or, maybe, even without isolating layer?) --- would theelectric field from charged block be enough to sepparate the hole from electron (push away electrons and attract holes) doing kinda the same as an ordinary PV cell, but at much lower energies (much more effective and useful)?
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mfb
#2
Apr26-13, 12:52 PM
Mentor
P: 11,576
You cannot violate the second law of thermodynamics, and reduce entropy.
If excitations are so low-energetic that thermal fluctuations are sufficient to generate them, they will fall back again in the same way, in the same rate.
Edi
#3
Apr26-13, 01:03 PM
P: 176
Quote Quote by mfb View Post
You cannot violate the second law of thermodynamics, and reduce entropy.
If excitations are so low-energetic that thermal fluctuations are sufficient to generate them, they will fall back again in the same way, in the same rate.
How is it really reducing entropy? Is it reducing it in the whole universe or just the localized field? It is possible to reduce entropy in a localized field while increasing the overall entropy, witch we do all the time. Or, in a closed system, perpetual motion is also possible (not that I am suggesting perpetual motion.)
Why will they fall back? Why wont the electric field push away electrons and attract holes?

mfb
#4
Apr26-13, 04:55 PM
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P: 11,576
Photovoltaic systems to work at much lower energies

You want to get electricity out of heat, without a colder object to dump energy into, if I understood your post correctly.
Edi
#5
Apr27-13, 04:58 AM
P: 176
Quote Quote by mfb View Post
You want to get electricity out of heat, without a colder object to dump energy into, if I understood your post correctly.
Well, sort of. Actually, there is no such thing as uniform heat, practically and there is always some heat/ energy difference.. But, yes. Out of heat itself, out of IR radiation.
Can you explain me how and why this setup would not work?
mfb
#6
Apr27-13, 06:12 AM
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P: 11,576
It would violate the second law of thermodynamics. The most appropriate description here is probably this:
It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects.
This applies to electric energy as well.
Edi
#7
Apr27-13, 06:54 AM
P: 176
Ok, you quote me the law, but what exactly in the scenario prevents it from happening? Not that there is a policeman standing, who says: "you shall not work!"
I want to know the "why?"

.. and, by the way, when I red that law you quoted, it suggested even more that the scenario could work - because temperature temperature we measure is only the average and in practically any system there are some particles moving faster than others and some moving much slower than average, so .. why cant it be cooled to the point of the coolest/ slowest moving particle?
mfb
#8
Apr27-13, 07:04 AM
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P: 11,576
but what exactly in the scenario prevents it from happening?
As I don't understand how your device is supposed to work, I don't know. And the nice thing of conservation laws: I don't have to know.
It looks like you use an external power supply to separate charges. This won't give electricity, it needs electrocity and heats the object.

because temperature temperature we measure is only the average and in practically any system there are some particles moving faster than others and some moving much slower than average
That is not how temperature works. There is no temperature of individual particles.
Edi
#9
Apr27-13, 07:38 AM
P: 176
Quote Quote by mfb View Post
As I don't understand how your device is supposed to work, I don't know. And the nice thing of conservation laws: I don't have to know.
It looks like you use an external power supply to separate charges. This won't give electricity, it needs electrocity and heats the object.

That is not how temperature works. There is no temperature of individual particles.
mm, no, it is said in my first post about the charge that separates the electron form hole - in a normal PV cell there are boron-doped part of silicon that is negative and phosphorus-doped part that is positive (as one takes electrons from the other), but in this case there would be a previously charged boron-doped silicon and another, un-charged, "normal" part of silicon, separated by an insulator, if necessary.

Yes, there is no temperature of individual particles, but there is kinetic energy for individual particle. The average kinetic energy of the particles is what we feel as heat. Of course I know that.
mfb
#10
Apr27-13, 08:32 AM
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P: 11,576
Well, you cannot reduce this average kinetic energy in your device - therefore, you cannot extract energy out of the heat.
Edi
#11
Apr27-13, 08:43 AM
P: 176
Quote Quote by mfb View Post
Well, you cannot reduce this average kinetic energy in your device - therefore, you cannot extract energy out of the heat.
Well, if I put it in a box with uniform temperature and close the circuit - I will get a colder side and a hotter side, but the average temperature in the box will be the same :D
Yes, here comes the entropy law, but that does not really explain anything. It just says: "No!"
mfb
#12
Apr27-13, 11:55 AM
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P: 11,576
Quote Quote by Edi View Post
Well, if I put it in a box with uniform temperature and close the circuit - I will get a colder side and a hotter side
That is impossible.
Yes, here comes the entropy law, but that does not really explain anything. It just says: "No!"
Indeed, and that is the most general statement you can do here. A detailed analysis of your setup (by someone who understands it) will eventually show where it does not work as intended.


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