# Solar panel related question -- help please

Discussion in 'Electronic Design' started by [email protected], Dec 17, 2012.

1. ### Guest

One important parameter related to solar panel is the
"panel generation factor" defined as:

Panel Generation factor(PGF) = Collection Efficiency x Average solar radiation in least sunny month.(kWh/sq m/day)

What exactly is "collection efficiency" ? Is it the solar
panel efficiency ? For example, the PGF of most of
Europe is 2.96 -- given that a standard solar pahel
can convert just 15% - 18\$ of light energy to electrical
energy, what exactly is the "collection efficiency"
Any hints, suggestions would be of immense help. Thanks

2. ### mikeGuest

suspect you'd get a better answer in a better group.
Anything with solar in the name
or alt.energy.homepower.
Or get an answer from the panel vendor.

I've gone looking for that definition in the past, and again today.
I never found anything that approached a definition.
I chose to ignore it and do the calculations.

Are you sure you're quoting the right number?
Your 2.96 sounds more like an annually averaged daily insolation than
the least sunny month. Multiply that by 20% collection efficiency
(round number)
gives more like 0.6 kWh/day/m^2 for energy generated averaged over the
year...much less in winter.

Given the way things tend to be measured at optimum, I'd expect that
you'd need MPPT and optimum sun tracking and no buildings or trees to
get that.
And remember that the panel is rated with no rain or snow or or dust on it.
And no bird crap or smog or smoke or nothin'.
And no losses in the conversion to what you actually need.
Local micro-climates can have a siginificant effect. Those won't show
up in average insolation numbers.

Solar sucks if you have any alternatives.

3. ### Phil AllisonGuest

"mike"

** The only fact you need to really know.

..... Phil

4. ### Robert MacyGuest

....my opinion:
Collection Efficiency = (Electrical Energy generated in kW) / (Energy
'available' in light per sq m).

Although, don't know why it is called 'collection efficiency' instead
of 'conversion efficiency'. Maybe there is one more factor involved
which would include reflectors and/or lensing.

There is a maximum amount of energy in sunlight per sq meter. From
memory, it was around 0.35 W per sq cm - measured for straight in sun
at the equator.

As I said, it is my opinion that CE relates to how well you can
convert that supplied energy into electricity. Also, from memory I
remember solar cells at the time being in the range of having 1-3%
conversion efficiency. But, absolutely not even near 10%. Had
something to do with the limited spectral sensitivity range of the
silicon. The sunlight had a lot of energy in frequencies the silicon
couldn't 'see'.

5. ### Robert MacyGuest

After reading wiki, it appears that the 0.35W number more reflects
what to expect from a solar cell.

6. ### George HeroldGuest

The number I always heard was 1kW/m^2 = 1mW/mm^2
(1 mm^2 is nice for laser stuff, since its about the size of a laser

George (millimeter) Herold

7. ### tmGuest

After reading wiki, it appears that the 0.35W number more reflects
what to expect from a solar cell.

+++++++++++++++++++++++++++++++++++++++++++++++++

That would give 3500 watts per m^2. Fails the reality check.

I think the insolation runs about 1 kw / m^2 at the equator (I did not

8. ### Jeroen BellemanGuest

That's about right, although I take issue with 1mW/mm^2. ;-)

Jeroen (SI) Belleman

9. ### Jeff LaymanGuest

Correct, but that's only at noon, with the sun directly overhead.

10. ### George HeroldGuest

Grin, no Jeroen was just poking fun at me for an earlier post about
units. (I was hoping to 'twig' him with mW/mm^2.)

George H.

11. ### Guest

A particular solar panel product will specify the output power , Pout, measured at standard test conditions (STC) which are defined as 1000 W/m^2, 25oC cell temperature, and 1.5 Atm air pressure. They also specify a derating factor for the variation of Pout with temperature, usually something on theorder of -0.5%/oC, and a tolerance for the output power. The PGF is an installation specific conversion factor specifying the daily panel output electrical energy in Wh, or kWh, per watt, or kW, unit of solar panel output power. The specifics used to compute the PGF are average cell temperature, average irradiation of the cell in units of 1000 W/m^2, average meteorological conditions, average insolation. Once the PGF is known, all you have to dois divide the minimum daily energy required of the solar (adjusted for obvious system losses like inverter efficiency, charging efficiency, panel pointing errors, panel dirt accumulation...) by the PGF to derive the total required panel STC power. Then divide by the per panel STC output to derive the number of panels.

12. ### Guest

On Monday, December 17, 2012 11:40:45 AM UTC-5, Jeff Liebermann wrote:

It's not that deep, it's a number used by installers in fixed geographic areas. They can lump all the stuff that stays the same on all their jobs intothe PGF. Things like the efficiencies of their inverters, presumptions of cable losses from just eyeballing the building dimensions, the low ball worst case insolation, the charging efficiencies of the battery system if any,typical panel power output reductions to be expected from soot, pollen or whatever in their area, etc... all that junk is pretty much the same every time. The only custom work that needs to be done is to size the energy requirements of the customer. Then PGF makes the energy->panel power conversion.. Manufacturers don't spec energy, they spec power, and systems are designed around energy and not power. So PGF gets them there.

13. ### Guest

Thanks for the links. As the 'panel generation factor' is
used to determine the number of solar panels to be used at
a given site, the following is an alternative scheme that
some of us have put together to determine the number of
panels at the same site.
Let us suppose that the electrical parameters for a given
panel are:
Wp=75 Watts
Isc=4.8 A
Voc=21.4 Volt
Im=4.4 A
Vm=16.8 Volt
Active solar panel area AA = 0.605 sq. meter

Define FF = (Vm*Im)/(Voc*Isc) = 0.718

Then, at STP
Efficiency= (Voc*Isc*FF)/(It*AA)*100% = 12.36% (at STP It=1000 Watts per sq. m)

Now let us suppose that the average insolation at the site
is: 500 Watts per sq. m
Actual efficiency at 500 Watts per sq. meter = 12.36*0.5/100.0 = 0.0618 = 6.2 %
Power produced by the single panel=0.062*500*AA(0.605) = 18.75 Watt per hr
So for a 8 hour period(brightest possible sunlight), the total
power generated by the panel = 8*18.75 = 150 Watts per day
So if the total power requirement is ww Watts, the total
number of panels is ww/150

How does the reasoning look like ? Are there any glaring

14. ### Robert MacyGuest

gaccckkkk!!!

slipped a 'few' decimal points.

15. ### Robert MacyGuest

Don't forget the cost of the security system to thwart 'solar panel'
thieves.

16. ### amdxGuest

Snip
Also use the proper derating factor on performance statistics gleaned
from any solar system owner, that spent his hard earned money.

Mikek

17. ### mikeGuest

Yeah, but, there are also those who just want a little target practice.

18. ### mikeGuest

Back when stimulus money was flowing like water down a rat hole,
I talked to a number of solar vendors.
None of the offerings stood up to simple questions.

The best looking one was a lease program. You saved money on
your electric bill and used that difference to pay the lease and
you came out ahead. No-Brainer...but wait.
Do you base it on actual savings?
NO, it's predetermined assuming you were one of the first 20 people
out of the 10,000 who applied for the subsidy within milliseconds of the
offering. And there was no guarantee that it would be available next year.
Took me half an hour of probing the vague deflected answers
to get to that point.

I asked for the free site audit...they must have lost my application.

Never trust the numbers from anyone who's trying to sell you something.

Talking to actual owners is way better...if you can find them.
But that's not foolproof. The placebo effect is very strong.
When I had my house weatherized, I noticed a great savings in energy.
Over time, as I settled back into my old ways, the savings decreased.
And I actually measured it. Most people are just guessing and feel
better if their guesses support their decisions.

There were a lot of weatherization subsidies.
I'm sure it's better for the planet, and the children, but based on the
payback period, I'd probably not have done it on my own dime.

Same for solar. It's great if you can get someone else to pay for it.

19. ### mikeGuest

Problem with solar is that it takes too much space. Even at 100%,
it's still too big to be practical...until we scale back demand