Can Partial Shading Seriously Impact PV Array Output with Feed in Tarif Scheme?

[sophiecentaur]

I am looking into the possibility of getting a PV array fitted on my roof and taking advantage of the (ludicrous) Feed in Tarif scheme that the UK government is offering. A bloke came around and we looked in some charts to find the likely yearly yield and there are some issues of partial shading. He clearly didn't understand the technical details so it was pointless pressing him for more information.
He seemed to imply that a small amount of partial shade would seriously compromise the output of the whole system. I understand that the PV panels are 24V and I would guess that they are connected in parallel and the power is fed to a 24V inverter. This seems a poor approach and not very 'intelligent' as only panels producing the highest voltage would contribute anything to the power output.

If the panels were connected in series then I should have thought that you'd get a contribution from all panels. I appreciate that this would mean the use of a different inverter for each panel arrangement but there aren't that many combinations of panel layout in practice.

The Voltage / Current characteristics of the panels may not be simple, though, and my worries about 'sharing' may not be well founded .

Has anyone looked into this in more detail that I have been able to? Any ideas?

 

[Mike_In_Plano]

I think solar cells are essentially diodes that feature an equivalent current source from the photon generated hole-electron pairs. If one attempts to "force" this current when insufficeint light is available, the cell reverse biases and becomes a voltage drop. Thus, the worst cell in the bunch limits the others.

As to the voltage, that is a "nominal value." The panel's optimum operating voltage changes conserably with temperature (i.e. diode), and it may change with light, though I don't recall. In my career, I've made a couple of "trackers." They are power converters that change the loading on the panel between two values that stride your nominal value. Whichever value produces the most current to your batteries becomes preffered and the nominal operating point moves towards it.
It's interesting to watch, especially as a clound comes. Clouds play havoc as the output suddenly increases and then falls - every cloud has a silver lining...

 

[vk6kro]

The solar arrays that I have seen always connect them in series to give about 300 volts. This is then converted to 240 volts AC to feed into the mains.

I have experimentally shaded just one cell and it is like switching the whole array off. The affected cell just acts like an open circuit and no current can get past it.
Seems like a heap of Schottky diodes could compensate for this if one was placed across each cell.

We get AUD$0.47 for a KWH (Or 0.30 GBP), but even this doesn't pay off the cost of the hardware in a reasonable time or less than the lifetime of the hardware.

What part of the UK are you in?

 

[sophiecentaur]

I am on the South Coast.
Interesting that the series connection is used with a current source model (!?). That would, surely, favour a parallel connection for unequally performing panels. Much more opportunity for intelligent control.
Bearing in mind that fuel costs are sure to rocket, there is a lot more incentive than the simple FIT.

 

[Studiot]

Two months ago I visited some friends in Germany who had fitted these panels to their roof just over 6 months ago.

The deal they received in Germany seemed almost identical to the one I was offered in the UK. Indeed I think it may be an EU wide deal.

Discussing matters further revealed that:

The owner is paid a state subsidised rate for the electricity fed into the grid at a rate well above (several times) the current purchase price of electricity from the grid.

This subsidy decreases every year for about 10 years until it will disappear alltogether.

Entry into the scheme brings in a decreasing start rate = the current subsidised rate ie last year's rate was £0.42 per unit.
So everyone receives the same rate in any given year, regardless of when they start.

This will clearly make scheme entry less attractive as time goes on.

I said the owner receives the rate and this raises a further issue. Who is the owner of the panels?

My friends had the capital to spend the E10,000 - E20,000 costs and so are the owners.
Not everyone is so lucky and there are many schemes out there offering a no initial capital basis. The providing company then effectively receives the money for the electricity and pays rent to the building owner for the roofspace used.

A recent financial investigation of these schemes thew up some interesting issues.

Banks & Building Societies are very unwilling to lend on a property with someone elses installation on the roof. In effect the propety has become unmortgageable.

Then there is the question of what happens if the roof leaks or whatever and needs repair.

go well

 

[sophiecentaur]

A recent financial investigation of these schemes thew up some interesting issues.

Banks & Building Societies are very unwilling to lend on a property with someone elses installation on the roof. In effect the propety has become unmortgageable.

Then there is the question of what happens if the roof leaks or whatever and needs repair.

go well

Thanks for those thoughts. The above is particularly interesting!

 

[Studiot]

The part about mortgages was a BBC (money I think) programme.

 

[Mike_In_Plano]

Interesting side

I hear on National Public Radio that some companies were using PV systems for financial breakthrough. Of course, they are located in favorable conditions, but this also implies that the prices commonly seen for PV arrays are very different than those being offered at corporate rates.

I am not at all surprised, having spent time designing OEM goods. Little $3.00 power supplies quickly reach $25 after passing through a few hands and being offered to individuals. If you came in wishing to buy 500kw capacity and you refuse to deal at the distributor level, I'm confident that the prices would drop markedly.

- Mike

 

[sophiecentaur]

Thanks for the comments chaps. At the moment, the choice of FIT seems to be a no-brainer. The cost of the scheme to the Supply companies, who pay you well over the odds just for generating the stuff, is very high and a net loss to them. This means they need to load everybody else's power bills to make up for it. You can hardly afford not to go for it if you are eligible. And that is another issue because you need to be a Home Owner in a Single Occupancy Dwelling, with a suitably uncluttered South Aspect. That means wealthier than average and with access to credit. Unfair or what?

 

[vk6kro]

I found this site which gives the hours of sunlight for Southhampton:

http://weatherhobo.com/united-kingdom/southampton/

Seems fairly promising for England. The winters are not very productive, but a long bright summer makes up for the winters.

I put those figures into a spreadsheet and came up with some possible benefits:

[PLAIN]http://dl.dropbox.com/u/4222062/solar%20panel.PNG

If you had a 2000 watt system and managed to put all this into the mains and got 40p per KWH, you would make about £1364 per year.

Since this would require tracking the panels to face the sun, maybe 700 watts average would be more likely. And 40p seems a bit high, so try 30p. This would give you about £358 per year.

Even that would pay off a £2000 system in 6 years. After that, the electricity produced would be free.

 

[mheslep]

...
He seemed to imply that a small amount of partial shade would seriously compromise the output of the whole system...

He's right, at least to a degree. Not just panel to panel, but since each panel is made up of multiple PV cells a given panel's entire output is compromised from shading of individual cells, because:

I think solar cells are essentially diodes that feature an equivalent current source from the photon generated hole-electron pairs. If one attempts to "force" this current when insufficeint light is available, the cell reverse biases and becomes a voltage drop. Thus, the worst cell in the bunch limits the others...

Exactly. At least http://www.tenksolar.com/technologyCell.html" , and with some other enhancements lead it to claim its product will produce amortized power as low as $0.08 per kWh.

Even with more traditional PV, if the UK tariff pays you four times the going rate of electricity http://solarfeedintariff.co.uk/the-feed-in-tariff/" for solar generated power, the highest in the US, and consequently are on track in 2011 to (ridiculously) install more solar PV than California (~19 times larger.)

The problem of course is that your solar 'business' has only one customer via that tariff, the govt/utility, and at any time in the future they can rescind that tariff. Apparently they have http://www.guardian.co.uk/environment/2011/apr/19/solar-legal-action-feed-in-tariffs" it is not even close to being economically viable to power the home.

 

[mheslep]

Thanks for the comments chaps. At the moment, the choice of FIT seems to be a no-brainer. ...

Again, I agree if the FIT stays in place. The FIT may go away, but you will still be paying the note on £6000 of solar array (2KW).

 

[mheslep]

...

If you had a 2000 watt system and managed to put all this into the mains and got 40p per KWH, you would make about £1364 per year.

Since this would require tracking the panels to face the sun, maybe 700 watts average would be more likely. And 40p seems a bit high, so try 30p. This would give you about £358 per year.

Even that would pay off a £2000 system in 6 years. After that, the electricity produced would be free.

Math check on the cost? A 2KW crystal PV system would cost at least $5/W (maybe $6/W) for residential installation, or ~£6000

I agree with your production estimate. In Southhampton, the solar insolation is reported as http://www.contemporaryenergy.co.uk/solarmap.htm" At 20% efficiency for PV, that's 220 kWh/M^2/year, unshaded. A 2KW rated panel array might be realized as ten 200W panels over ~15 M^2, which woulc collect 3300 kWh per year, or £1320 at 40p per kWh.

 

[Studiot]

or £1320 at 40p per kWh.

But that's only the first year. The premium over standard tariff decreases to zero over a 10 year period.

See the rest of this post

Entry into the scheme brings in a decreasing start rate = the current subsidised rate ie last year's rate was £0.42 per unit.
So everyone receives the same rate in any given year, regardless of when they start.

 

[mheslep]

But that's only the first year. The premium over standard tariff decreases to zero over a 10 year period.

See the rest of this post

Then this likely won't pay off. Stick with off shore wind.