Naked Science Forum
Non Life Sciences => Technology => Topic started by: Scott on 08/04/2010 17:30:03
-
Scott asked the Naked Scientists:
I came across a statement that I was hoping you could shine some light on.
It said that the laws of physics determine that when solar panels are connected together (I'm figuring in parallel) each can only work as well as the lowest power panel in the system. Could you briefly explain why this is so?
What do you think?
-
Solar cells individually generate around 0.45V, and at a short circuit current that depends on the area exposed to light and the intensity of the light ( it also needs to be shorter than the critical wavelength, otherwise the photons have insufficient energy to liberate an electron, but that is a topic for later, here we assume all light will give a current, at least sunlight will).
This means a panel of around a square metre will give about 200W roughly. Normally a panel has many single cells in series, to give around 18V per panel. If you shade any cell, then it will only deliver the current it is capable of, and will limit the current for the whole panel. It makes no difference if the rest are capable of current, the cell with the lowest current generating capability sets the output for the whole panel.
If you connect multiple panels in parallel, like in a solar cell array on a house, then each string of cells delivers the maximum it can, and a shaded cell string just delivers less current, the others are unaffected.
-
When you connect them in parallel, cells that output a lower voltage will dissipate power from the cells that output a higher voltage. As the voltage output of any cell is a function of many variables, no two cells are likely to output precisely the same voltage, so this condition is unavoidable. This is why it's best to connect them in series.
Large banks of series connected panels can be connected in parallel with other banks by including diodes or other means to prevent some banks draining current from others.
Parallel connected batteries suffer from a similar problem.
-
I too thought that diodes were incorporated in each cell, and between each panel in an array, to handle the partial shading issue.
From http://en.wikipedia.org/wiki/Photovoltaic_array#Performance (http://en.wikipedia.org/wiki/Photovoltaic_array#Performance)
Many Photovoltaic cells' electrical output is extremely sensitive to shading. There are some non-traditional solar cell manufacturers, thin-film a:Si, that have installed bypass diodes between each cell that minimize the effects of shading and only lose the power of the shaded portion of the array. When even a small portion of a cell, module, or array is shaded, while the remainder is in sunlight, the output falls dramatically due to internal 'short-circuiting' (the electrons reversing course through the shaded portion of the p-n junction).
and from http://en.wikipedia.org/wiki/Solar_panel#Theory_and_construction (http://en.wikipedia.org/wiki/Solar_panel#Theory_and_construction)
Diodes are included to avoid overheating of cells in case of partial shading. Since cell heating reduces the operating efficiency it is desirable to minimize the heating.
-
Because the voltage produced by individual cells is less than the voltage drop through a diode, I think they only use them with an array of series connected cells. Do you think there is a problem if some of the cells in a series array are shaded? I would not have thought so, but perhaps there is.
If so, parallel would be the way to go, but the low cell voltage makes that problematic too.
EDIT: Wait a minute. I read the above again. They include a bypass diode. I suppose that's to limit the voltage across the internal resistance of "dark" cells in series connected arrays.
-
I've got to admit that when we get down to specifics it's not entirely clear to me either.
Perhaps the wording is just a bit ambiguous, but a bypass diode between cells doesn't make sense; if it's between cells then it'll be a link diode and not a bypass diode. A bypass diode would be one that routed around the cell (and what good would it do there?).
Also too, if diodes are placed between each cell in a string of series linked cells, and one of the cells becomes shaded, then the diodes either side of the shaded cell can prevent reverse flow through that cell. However, in doing so, the diodes will have broken the series string, so you'd get no output at all from that string unless there is some sort of switching circuitry to bypass the shaded cell when its output drops relative to the others. That sort of electronics would be a bit more complex than just a few simple diodes though.
-
I've got to admit that when we get down to specifics it's not entirely clear to me either.
Perhaps the wording is just a bit ambiguous, but a bypass diode between cells doesn't make sense; if it's between cells then it'll be a link diode and not a bypass diode. A bypass diode would be one that routed around the cell (and what good would it do there?).
Also too, if diodes are placed between each cell in a string of series linked cells, and one of the cells becomes shaded, then the diodes either side of the shaded cell can prevent reverse flow through that cell. However, in doing so, the diodes will have broken the series string, so you'd get no output at all from that string unless there is some sort of switching circuitry to bypass the shaded cell when its output drops relative to the others. That sort of electronics would be a bit more complex than just a few simple diodes though.
This is likely baloney, but I wondering if they put a forward biased diode in parallel with each cell. That might seem strange, but it will not conduct when the cell generates power, because the cell voltage is less than the voltage drop across the diode. However, when the cell is dark, the diode will limit the voltage across the internal resistance of the cell and limit the power that the cell can dissipate.
I think I'll need to draw some diagrams to check this out though [:D]
-
So If one were to put a series of panels across ones roof,to generate power, from what is being said, do I understand right that one should use the best producer size panels and keep them all the same in size, none being smaller in size to keep the current at a higher output?
Am I understanding then, that a smaller panel would then cancel out the higher potential the other panels in the series might have if they were to have an equal panel replacing the smaller one in the series...?
-
Hi here is a tutorial videos in which you can learn about the calculations required using python. Solar panels in series or parallel are calculated with pc, programs or calculators. Hope this help. The youtube is Tutorialesdecalculadoras or you can find it on google with Foros Calculadoras.com.mx