Impact of Solar Panel Orientation

7 Posts
Aug 4, 2006 08:02 am
Impact of Solar Panel Orientation

I am looking for some advice regarding the impact of solar panel orientation and its impact on power generation.  Just to give you some idea of the situation, my current solar array was installed flush mounted to the roof.  Fortunately our house happens to be nearly perfectly oriented to the cardinal directions (it's not exact, but for luck of the draw you couldn't ask for closer).  At the same time, unfortunately, the house is rectangular and the end which faces North/South is the shorter side.  The roof is not a standard v-shape, but rather made up of 4 triangles, the two on the ends being smaller.  The end result of this is the longer sides of the roof face nearly east/west, while we have one shorter side facing south. 

The panels on the southern portion of the roof already have a southern orientation as a natural result of the roof's inclination, but the others on the west and east sides are flush mounted so they are flat, facing slightly west or east with no southerly orientation.  My question is, whether there would be a significant advantage to reinstalling these panels on tilted racks to orient them towards the South?  I suppose it may be important to note that I live in Southern California at approximately 32 degrees latitude. 

I am trying to determine how much difference orienting the panels to the South would make, in order to decide whether it is worth going to the trouble and expense of reracking and reorienting them, rather than leaving them as they currently are. 
 
351 Posts
Aug 5, 2006 01:22 am
Re: Impact of Solar Panel Orientation

You are losing a lot of power. IF it was me, I would definitely put them on racks facing south. I would make them adjustable racks so that I could change the tilt angle a couple of times a year.

Here is a good article on tilt optimization.

http://www.macslab.com/optsolar.html

Ken
 
Aug 6, 2006 04:54 am
Re: Impact of Solar Panel Orientation

I agree, the orientation of a PV array in relationship to our Sun is very important, concerning the number of full rated hours of output per day. Here is another site to look at where you can experiment with a lot of options.

http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/
 
Aug 6, 2006 08:28 pm
Re: Impact of Solar Panel Orientation

Oops! That should read - "number of hours of full rated output."
I miss the editing part of this forum. Geuss I will have to start reading what I type more closely.
Something else to consider id the deviation of magnetic south from true solar south if you really want to "dail in" that PV array. Set them as close as possible to true solar south on Dec. 21.
Cut a piece of 2x4 square on both ends about 8 inches long and place one end of it on a moveable flat surface. Move the surface until the 2x4 does not cast any shadow at all on Dec.21 at 12:00 noon. Thats the direction and angle you want your PV array in. If the direction is fix and the PV array is lowered for a summer Sun this will not happen until about 1:30 on June 21.
 
Aug 6, 2006 08:51 pm
Re: Impact of Solar Panel Orientation

Other times to check are the Autumnal and Vernal Equinox's (fall and spring). March 20 and Sept. 23. These two angles will be the same.
By the way, the other two dates are known as the Summer and Winter Solstice. Or the longest and shortest days and well... I guess equinox speaks for its self. Do you know about the Analemma?
On the same day of each month for a whole year push a rod,  eleven in all of equal lengths, into the ground at a point where the tip of its shadow at 12:00 noon touches the same point as the other months. If done with a little patience what you should have at the end of the year is a stretched out figure eight. This works best on flat level ground and the tops of the rods are the same height from the ground.
 
7 Posts
Aug 6, 2006 10:21 pm
Re: Impact of Solar Panel Orientation

Thanks for the links to those sites guys, those should help a lot.  I am seriously considering reorienting the panels, but I have two concerns. 

I looked at the adjustable mounting racks on this site.  I was thinking the low profile design might be best, as they will be on a roof, and I think having solar panels sticking 5 feet up in the air might be kind of weird.  But I was wondering just how easy it is to adjust these racks.  It looks like they are of fairly basic design (I won't go so far as to say crude), and I was wondering with the weight of several panels on them how hard it would be to adjust them.  Would it require two people, one holding the panels in place while the other unbolts and rebolts the rear support in order to adjust its length? 

My other concern is how to figure out how far apart I'd need to place the racks so they don't end up shading each other in the winter (when there will be the combination of steepest angle of panels, and lowest angle of sun).  There is limited room on the roof, so if I have to space the racks out too much it might be counter productive.  As an example, if I lose too much space, I might be better off just resorting to brute force by putting up more panels, albeit in terms of expense that option will probably be a lot higher. 

I would suppose in terms of shadowing, I should be most concerned about the middle of the day when most generation occurs.  The whole fact that the roof is not completely flat so there will be some east/west tilt of the installation makes figuring out shadowing a little more difficult. 
 
462 Posts
Aug 7, 2006 02:11 pm
Re: Impact of Solar Panel Orientation

If you mount them on the ground, you can orientate them in any direction and change the pitch throughout the year....
 
30 Posts
Aug 10, 2006 02:19 pm
Re: Impact of Solar Panel Orientation

Hi David - I ran the model for a home in Memphis (you didn't say where you were located, but my uncle has a home similar to yours in Memphis!).  Anyway, it turns out that, on an annual basis, panels facing due east or due west in Memphis at a tilt angle of 35 degrees will have an overall net efficiency of 80% of those facing due south. Definitely not as bad as I thought before running the model.  While purists will argue that this is unacceptable, all the other issues mentioned (and a bunch not mentioned) probably make the case for staying with simple, low profile, aesthetic flush mounting on the west and/or east facing roofs.  With (relatively) low tilt angles, be sure to keep all the collectors clean so that you maximize the performance with what you have.  I would also recommend evaluating overall energy use and work to increase efficiency (you are using CFLs everywhere for example?) before I would start working up a rack system for the panels.

Best regards -

Tom Hardy, PE
HVTA, Inc.
 
Aug 11, 2006 05:51 am
Re: Impact of Solar Panel Orientation

I think a lot of the concern over maximizing PV module output is cost efficiency. A single 175 watt module optimized might yield an average per day over the year of 1000 watthours a day. With the cost of utilizing this power added this could cost around 1000 dollars. When compared to coal fired generation costs to the customer of 0.09 per 1000 watthours. Of course there are, more or less, unrevealed costs for coal fired generation that the customer never sees on their bill but pays all the same. This is why I think optimizing PV output over the year is very crucial.
 
7 Posts
Aug 11, 2006 09:09 am
Re: Impact of Solar Panel Orientation

I am in San Diego, just about as far South and West as you can get, without either ending up either in Mexico or the Pacific.   Smiley

Some of the links provided above have been very helpful in assessing my situation.  I tend to agree that being in a relatively southern latitude, the less then ideal placement of my panels is not that dire.  I do believe that reorienting the panels to the south would be worthwhile, and it's something I plan to do eventually, but I am not going to treat it as an emergency. 

I have already more or less optimized my power usage with CFLs, energy efficient appliances, etc, so there is not much more I can do in that area (although always striving for improvement).  The balance of my generation to usage is already generally positive, but then again it never hurts to have a little bit of extra cushion, and in the end if I don't use the power I suppose I can rest easy knowing I have displaced a little bit of polluting power from the grid. 
 
462 Posts
Aug 11, 2006 12:43 pm
Re: Impact of Solar Panel Orientation

Or you may just consider moving all the panels off the hot roof to a location on the ground that faces due south. Heat reduces the efficency of PV panels and ground installations reduce cost of wire runs and grounding equipment and allow you to tilt the panels year round. Also the use ground reflection can increase incoming light.
 
7 Posts
Aug 14, 2006 06:03 am
Re: Impact of Solar Panel Orientation

I would consider putting them on the ground, but it simply is not practical due to space constraints.  I would not be struggling with the whole east/west orientation issue if the ground was an option.  Unfortunately, it's just isn't. 
 
578 Posts
Aug 14, 2006 10:15 am
Re: Impact of Solar Panel Orientation

hey folks,

I was thumbing through an old home power magazine looking for something else when I stumbled upon this gem of an article.  Home Power #93 page 24.  The article is titled PV Orientation and is written by Zeke Yewdall.  It has some good info including charts and graphs that may help you.  enjoy.

james - Alt-E staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
 
3 Posts
Aug 14, 2006 03:50 pm
Re: Impact of Solar Panel Orientation

It has been my experience that orientation doesn't matter as much as the pleasent look on the roof. Why align the panels at latitude plus 15 degrees to maximise the sun for the one day of the year that the angle will be perfect (dead of winter). My modules work great and perform above the rated amps set 5 degrees below latitude at 4500 ft ASL. Rain won't wash them as well at the lower angle but I just spray them with a hose from the ground.
more later..
 
30 Posts
Aug 15, 2006 03:43 pm
Re: Impact of Solar Panel Orientation

Hi David - I ran the models with your actual location (SD vs. Memphis, I should have seen your mention of southern California in your first post) and get pretty much the same results as before.  The model takes into account long term observed weather etc. so it can result in interesting numbers (as you'll see).  Some models do not take into account long term observed weather and only look at the potential solar radiation based on geometry and lat/long.  Naturally those will show that facing due south is the best. Anyway....

For the San Diego area, panels facing due east, over the course of a year will provide you about 76% of the power of a due south orientation. For the panels facing due west, you get about 83.6% of the power of a due south orientation.  If you dig out the detailed numbers, the lower average of the due east setup looks like the result of more cloudy/foggy weather in the morning hours (again on a long term average), which reduces how much power the east facing panels can gather.

I would say that anyone building a new home or installing a system from scratch look at facing the collectors due south (actually the model shows that in SD  about 195 degrees azimuth is optimum).  In your case, "if it ain't broke don't fix it" comes to mind. However, if you had some room on the west facing roof, you might want to move some of the panels from the east roof to the west roof. Otherwise I wouldn't mess with a good thing. 

Tom Hardy, PE
HVTA, Inc.
 
2 Posts
Jul 1, 2007 08:07 am
Re: Impact of Solar Panel Orientation

You might want to check out my free solar panel simulation on YachtSoftware.org.  It will predict the output of an array for a user specified orientation at any time of the day, for any cloud cover, and for any latitude.  Although it was designed for a boat installation you can simply set the boat heading to the orientation of the house.  If after using it you find the software useful we ask for a small donation.
 

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