Solar panel orientation

[102 Inspector]

I have a single panel, 20" x 30" for a remote sign illumination powering a LED fixture. Located in NE Indiana, the panel is located behind the sign and angle at approximately 70 degree tilt. Problem is the sun only hits the panel from about 1PM till dusk. Can the panel be mounted plumb on the end of the sign and still collect properly? does the angle impact the charging output that angle cannot be modified. Any input appreciated.

 

[Smart $]

Yes, the panel angle affects output. Optimum is to have two-axis tracking so the panel is always normal (i.e. perpendicular) to the sun... or direction of most intense light in cases of part-day shading, which appears to be what is occurring if sunlight doesn't strike panel until 1PM.

Without tracking, you'll likely have to use a software based calculator that includes shading inputs to determine optimum orientation. Perhaps the more experienced PV guys will provide additional recommendations...

 

[jaggedben]

Plumb on the end of the sign, if facing south, may be better than angled facing west.

 

[BillK-AZ]

Assuming that your PV module is rated at about 40 watts with a Vmp of 16.5 to 17.5V, you can power a nighttime only load that does not exceed 0.27A, provided that the PV module is south-facing at a tilt of 55? (optimum) and is not shadowed at any point of the day. Increasing the tilt to 70? reduces the output in December by 6%. If you have shadows after 1pm, you loose about 50% of the possible output.

You did not state how much current your LED light requires, hopefully you can ratio from the above.

For your climate, you need a battery that power the LED for 90 hours. (15 hrs in Dec x 5 days /80% for life of the battery)

This will work well only if you provide enough charging of the battery, use a controller with low voltage cutoff, and adequate battery capacity. Best to bury the battery if possible to avoid low temperatures.

 

[GoldDigger]

The PVWatts website will give you the average daily production for fixed angle or one or two axis tracking under the assumption of no shade but the horizon.
It will also allow you to guesstimate the effect of your pre-1PM shade, but you will certainly need at least twice as large a panel as otherwise.

 

[Anode]

Problem is the sun only hits the panel from about 1PM till dusk. Can the panel be mounted plumb on the end of the sign and still collect properly? does the angle impact the charging output that angle cannot be modified. Any input appreciated.

Inspector, take a look at page 89 of Nrels Solar Radiation Data Manual, old yes, but still a great tool today. http://www.nrel.gov/docs/legosti/old/5607.pdf

You are looking at the first table for flat plate fixed collectors. All the way to the right shows the total average sun hours per day. It appears that both latitude and latitude -15 are the same overall total average, but have different variations throughout the year. Based on what you've said, if you are looking to optimize the panel for maximum output, you should point the panel to the soutwest 200-205? azimuth at about a 22-26 degree tilt. I say this because if your exposure window is actually between 1 and evening, you are better off putting the modules where they can have a more direct window.

I also ran some quick pv watts simulations here and found that if you just point it to the south and plumb (90? angle) you will lose about 60% from optimal. By my math, for the total yearly production, each Watt of your module will produce about 450 - 750 Wh in the course of a year. The 450 Wh is for plumb orientation, and the 750 is for 200? at 26? (or latitude - 15). A shallower angle favors production when azimuth is oriented slightly east or west, so I wouldn't go with latitude. I accounted for significant shading in the numbers.

Cheers!

 

[jaggedben]

The PVWatts website will give you the average daily production for fixed angle or one or two axis tracking under the assumption of no shade but the horizon.

Actually you can incorporate shade into the derate factor.

 

[GoldDigger]

Actually you can incorporate shade into the derate factor.

You can incorporate it arbitrarily, but can you add it analytically by specifying shade times, whose effect will vary with orientation.
For example, shade till 1PM may not make much difference on a west facing panel but be a disaster for an east facing panel.

 

[keepsake]

building new ground mounted 10k array

building new ground mounted 10k array

Being ground mounted in pasture leaves orientation choices wide open. Thought hear is splitting the bearing 50/50 between the panels. Half facing 150 deg true and half 210 deg true. Tilt still undetermined but likely to be between 15 deg and 28 degs. I am latitude 28 deg.
Benefit I see to split orientation is it spreads the production across a wider part of the day, eliminates the current peak at solar noon and yet doesn't sacrifice any total output.
Also better if charging batteries as charge last long at less current.
Grid tie benefit spans a larger portion of the 24 hour day -- better for utility averaging.

Thoughts?

 

[ggunn]

Being ground mounted in pasture leaves orientation choices wide open. Thought hear is splitting the bearing 50/50 between the panels. Half facing 150 deg true and half 210 deg true. Tilt still undetermined but likely to be between 15 deg and 28 degs. I am latitude 28 deg.
Benefit I see to split orientation is it spreads the production across a wider part of the day, eliminates the current peak at solar noon and yet doesn't sacrifice any total output.
Also better if charging batteries as charge last long at less current.
Grid tie benefit spans a larger portion of the 24 hour day -- better for utility averaging.

Thoughts?

If the concern is simply kWh generated, then for many/most locations a 180 degree orientation will be best. PVWatts is pretty good at showing this, as well as the effect of other orientations with local conditions under consideration such as a predominance of cloudy mornings or afternoons. For example, if most mornings are cloudy, a more westerly orientation will produce more. I don't think, however, that an array split between two orientations in any case will produce more gross kWh than will an array all at whatever is the locally determined optimum orientation.

Of course, there are other factors. If you are working with a grid tied array in a utility district with a time of use (TOU) tariff in place, it is more important to match your peak output with the time of most expensive electricity; in most places I have investigated this favors a more westerly orientation. Also, if you are installing an overloaded central inverter it may be advantageous to split the array as you describe to widen the power curve and reduce clipping.

 

[keepsake]

2 inverters

2 inverters

"Also, if you are installing an overloaded central inverter it may be advantageous to split the array as you describe to widen the power curve and reduce clipping."

I do plan on two 5kw inverters, one for each orientation.

With 150 deg and 210 deg I expect associated peaks at close to 1030a and 230p and never hitting a simultaneous 10kw condition. Solar noon be 1230 ish EST.

 

[ggunn]

"Also, if you are installing an overloaded central inverter it may be advantageous to split the array as you describe to widen the power curve and reduce clipping."

I do plan on two 5kw inverters, one for each orientation.

With 150 deg and 210 deg I expect associated peaks at close to 1030a and 230p and never hitting a simultaneous 10kw condition. Solar noon be 1230 ish EST.

If gross kWh production is your main concern you would probably be better off putting the whole array at 180 degrees. Run PVWatts simulations for two systems as you describe and combine them, then for one of them at 180 degrees and double it, and see what you get when you compare the results. It also may be that for where you are 180 degrees isn't optimum, but you can do a few runs changing the orientation and see where your max point is.

If you were running a heavily overloaded 10kW inverter, then there might be some benefit to orienting half your strings east of south and half west of south, but you'd need more sophisticated software than PVWatts to quantify it.