Sun hours

[bcroe]

As a 1st approx., all the irradiance hitting a panel that is projected into the plane of the array is either reflected or absorbed with the absorbed portion then either turned into electricity or rejected as excess heat to the environment via conduction through the frame, convective loss to the ambient air or radiation loss to the surroundings. All the energy is accounted for via an energy balance on the cell, panel or array. The amounts/%ages will vary as the panel and environment warrant to maintain an energy balance.

In other words, you agree.

I am doing some model array testing, and rather than set up MPPT equipment, think I'll just measure the short circuit
current which is pretty much proportional to Imp. I'll be suffering about a quarter more heating. Bruce

[J.P.M.]

In other words, you agree.

I am doing some model array testing, and rather set up MPPT equipment, think I'll just measure the short circuit
current which is pretty much proportional to Imp. I'll be suffering about a quarter more heating. Bruce

Respectfully, somewhat, but not entirely. Under MPP conditions, there is still the reflectance loss so everything not converted to electricity does not get rejected as heat.

Assuming steady state conditions, whatever energy does not leave as electricity leaves either as reflected irradiance or gets dissipated as heat. A panel that turns 20% of the POA irradiance into electricity and loses, say, 5% off the glass from reflectance will reject (100-20-5) = 75% of the POA irradiance as heat. What goes in must go out for steady state conditions to prevail. If not steady state, the panel temp. will chance in response to the changing conditions until a new steady state is reached.

As Sensij notes under Isc, or Voc all the POA irradiance will be turned into heat or lost via reflectance (assuming steady state conditions).

Panels often perform less well under low irradiance levels so that will change the numbers under low light conditions from high irradiance levels.

Also, high irradiance levels, low wind and high(er) ambient temps. will also change the heat balance because their effect on panel temps., through the negative coefficient of power with respect to panel temps. That will lower electrical output and increase the portion of the POA irradiance that gets rejected to maintain the energy balance. The percentage of POA irradiance converted to electricity will never be the same from min. to min. for a variety of reasons.

FWIW, the amount and type of dirt on a panel will also have a slight effect on the reflectance, probably lowering it slightly, but not enough to make up for the irradiance loss from the dirt. That's beginning to separate fly crap from pepper however, and the stuff of papers for the journals.

[bcroe]

Respectfully, somewhat, but not entirely. Under MPP conditions, there is still the reflectance loss so
everything not converted to electricity does not get rejected as heat.

Then, would you say the reflectance loss will change as panel electrical loading changes? Bruce

[J.P.M.]

Then, would you say the reflectance loss will change as panel electrical loading changes? Bruce

No. The reflectance losses for glass used as panel glazing are f(glazing characteristics, incidence angle, nature of the irradiance). See Duffie & Beckman for details. Electrical loading has no influence on those things.

[bcroe]

Here is a measurement question. Lets say a string (720 cells total in this case) has the inverter in clipping. The inverter will tell me
the power delivered. I would like to look at the voltage, which will rise with clipping limiting, and estimate how much power is
clipped instead of delivered. I could have a conversion chart of voltage vs % delivered/not delivered. To make this actually work
the panel temperature would be needed to make a temperature correction. Bruce Roe

[sensij]

I think you would also need to know how the current would change with voltage under those specific conditions; it isn't constant. Not an easy problem to solve.

[bcroe]

MODELING

The array here started as a classic SW desert design. In 3 years of studying
performance, I have a list of things it doesn't deal with very well. Like, CLOUDS,
SHADING, and SNOW. The thought here is to improve these things by changing
the placement of panels. At this stage some model panels could be placed in the
field to compare actual performance to ideas.

3 panels have been set up to model any new design over an operating day, starting
in summer. Maybe they will still be there in winter. In fact about as much energy
can currently be collected on clear day in April, May, or June; this seems to be
because shadowing here increases as the sun moves north. The placement of these 3
test panels will check for any remaining shading. Notice the locating stakes, measured
out within inches. The angle of elevation is adjustable.

Output of a panel recorded over a day will give a curve of power vs time.
Adding together these curves on an hour by hour basis over a clear day, will
show how well total power output is (or is not) maintained at a fairly constant level.
Some fine tuning of the orientation will be involved.

I didn't really want to bother with an MPPT control for each of the 3 test panels,
representing the 3 tilts and locations of a complete array. I decided to just wire
a shunt to each panel (0.1 ohm 10 watt), putting them in the shorted output mode.
This will give a current nearly proportional to the MPPT value. Here is one plugged
into its panel, with meter jacks. Power of a full sized array is just a matter of
estimating the Vmp & Imp, multiplying them together and by the number of panels.

Here are some curves made on recent sunny days. Note the highest curve is the
summation of the (1)W and (3)E facing panels, elevated 58 degrees on 13 June.
The implication is that enough of these will produce excess power at noon without
any help from the (2)S panel. The 17 June curve attempts to level things with an
elevation of 61 degrees; the E-W sum is quite different for a small change. I'm
considering just using E & W facing strings, but some winter curves are still
needed. Bruce Roe

[sensij]

Here are some curves made on recent sunny days. Note the highest curve is the
summation of the (1)W and (3)E facing panels, elevated 58 degrees on 13 June.
The implication is that enough of these will produce excess power at noon without
any help from the (2)S panel. The 17 June curve attempts to level things with an
elevation of 61 degrees; the E-W sum is quite different for a small change. I'm
considering just using E & W facing strings, but some winter curves are still
needed. Bruce Roe

I hope that if you have the terrain / space for it, you aren't overthinking yourself out of simply putting as many panels facing south as possible. 3 east + 1 west might make sense off-grid, or *severely* inverter limited, but otherwise, putting them south will surely generate the most over the course of the year (especially with adjustable tilt).

[DanKegel]

Do any utilities incentivize morning and evening generation yet?

[bcroe]

I hope that if you have the terrain / space for it, you aren't overthinking yourself out of simply putting as many panels facing south as possible. 3 east + 1 west might make sense off-grid, or *severely* inverter limited, but otherwise, putting them south will surely generate the most over the course of the year (especially with adjustable tilt).

If its cloudy, it doesn't matter which way the panels face. Sunny, I'm already in clipping at noon, so more south facing panels
won't contribute. Anyway best daily sun here frequently isn't around noon. So I'll try to even out power, to minimize clipping
and maximize my "sun day". Tilt to vertical is in the plan for the snow months only. Bruce Roe

[sensij]

If its cloudy, it doesn't matter which way the panels face. Sunny, I'm already in clipping at noon, so more south facing panels
won't contribute. Anyway best daily sun here frequently isn't around noon. So I'll try to even out power, to minimize clipping
and maximize my "sun day". Tilt to vertical is in the plan for the snow months only. Bruce Roe

At the end of the year, one azimuth will have outgenerated any of the others. Most likely, that ideal azimuth is something very close to south, unless the cloud patterns are very strong and consistent. The outperformance of a south facing panel is not just during noontime, and even with some clipping, south can still outperform east or west (over the course of the year). With heavy clipping, it becomes more complicated. Certainly, over short periods of time, east or west could be better, but tools like TMY files can help narrow down the range of likely possibilities.

[bcroe]

With heavy clipping, it becomes more complicated.

Exactly

tools like TMY files can help narrow down the range of likely possibilities.

My "tool" here is dependent on clear weather. But it includes things a pure simulation might miss, like temp or shade. Bruce

[bcroe]

I don't really understand my graph on PVOUTPUT, under SUN HOURS. Sometimes they show a sun symbol on that EFFICIENCY
curve. It plots the number of KWH per KW capacity. Today the array managed 9 KWH/KW capacity, but it did't rate a SUN. How
much more can an array put out in a day?

I'm also not pleased that they keep changing the KWH scale, and its scaled so it never goes up up more than half the space. I scale
plots to fill the page. Of course they do it so that it never overlaps the EFFICIENCY curve (that I don't need), entirely above KWH. Bruce Roe

[ButchDeal]

Turn off eficiency if you don't want it.

[bcroe]

I tried all the buttons at the top, but none listed a way to turn off EFFICIENCY. Bruce Roe

[ButchDeal]

I tried all the buttons at the top, but none listed a way to turn off EFFICIENCY. Bruce Roe

Second bullet.... Random site in image....

Or were you talking about the daily view? If so click on the word efficiency to turn it off