Hello from Tucson: PV newbie

[posplayr]

I have been doing some research on PV Solar Systems and plan a DIY install. With BSEE/MSECE my background is in DoD systems engineering and controls systems. While by thermodynamics background is weak at best (I think I got a "D" about 30 years ago), the tracking, dynamics, software and electronics is much more closely aligned with my skill set.

After looking at the selection of systems at Wholesale Solar, I almost became convinced to follow conventional wisdom and not even pursue a tracking system. However with the vast experience of the last week, I feel I have the basis for a cost effective dual axis tracking system for ground or roof mount residential applications. I'm starting off with an array of panels of approx 10'x10' dimension. Subject to local zoning restrictions this design seems to be working (economically), but with the dramatic drop in the costs of the panels eeking out additional performance seems to be getting harder and harder.

Anyway, there seem to be some bright and experienced members here and well worth the time I plan to spend browsing around. I found this site researching PV panel cooling.

[inetdog]

Hello posplayr, and welcome to Solar Panel Talk!

I have not seen any descriptions of either one or two axis tracking for roof mount (at least not for a sloped roof.) A significant problem with tracking is that it requires a larger spacing between panels to avoid shading and if the mounting area is limited you may end up with less production per unit area even though you get more watt-hours per panel. Tracking for a pole mount or, for low latitudes, single horizontal axis tracking for a row of panels oriented with the axis running north-south is practical in some situations.


PS: In this context, all you need is to remember the three Laws of Thermodynamics:

1. You can't win.
2. You cannot even break even.
3. You can't get out of the game.

[SunEagle]

Hello posplayr, and welcome to Solar Panel Talk!

I have not seen any descriptions of either one or two axis tracking for roof mount (at least not for a sloped roof.) A significant problem with tracking is that it requires a larger spacing between panels to avoid shading and if the mounting area is limited you may end up with less production per unit area even though you get more watt-hours per panel. Tracking for a pole mount or, for low latitudes, single horizontal axis tracking for a row of panels oriented with the axis running north-south is practical in some situations.


PS: In this context, all you need is to remember the three Laws of Thermodynamics:

1. You can't win.
2. You cannot even break even.
3. You can't get out of the game.

Reminds me of the "rules" back up in NJ when you became "part of the family".

[posplayr]

Hello posplayr, and welcome to Solar Panel Talk!

I have not seen any descriptions of either one or two axis tracking for roof mount (at least not for a sloped roof.) A significant problem with tracking is that it requires a larger spacing between panels to avoid shading and if the mounting area is limited you may end up with less production per unit area even though you get more watt-hours per panel. Tracking for a pole mount or, for low latitudes, single horizontal axis tracking for a row of panels oriented with the axis running north-south is practical in some situations.


PS: In this context, all you need is to remember the three Laws of Thermodynamics:

1. You can't win.
2. You cannot even break even.
3. You can't get out of the game.

Now that is funny. If I had not ran into the thread last night about water spraying panels I would not have gotten it

I may be just intoxicated, but after doing some reading I posted this at a motorcycle forum I frequent. I started a thread there to get a gauge on general interest on the subject of DIY solar panel installs.

Here is a pretty decent Thesis on Solar Trackers.

There is a summary analysis on Page 57 showing a 47% gain of dual axis tracking over as fixed install. The Matlab code is included in the appendix.

The analysis assumes ideal solar conditions (i.e. no cloudy days) which will offer much more energy to be collected overstating the benefit of tracking as this cloudless sky will not always be available.

http://www.slideshare.net/SaiRam49/dual-axis-trackerfinalreport?related=2

This is a more scholarly reference with simulated and measured values over multiple years and it reports about 31% increase of a dual tracker v.s. a fixed tilt array.

http://www.researchgate.net/publication/236969054_Performance_comparison_of_a_double-axis_sun_tracking_versus_fixed_PV_system

Here is a monthly summary and it explains the discrepancy in the ideal data and the real life results. In the summary when the solar energy available is high, the tracking systems gain increases. When there is less sun available owing to low elevation angles the gain drops off. You can see that in May through August the gains are in the region of 37-38% even exceeding 40% in one month.

Having reported that, according to the 1st and 2nd laws of "Thermodynamics"(reference your post) I might just be plain delusional .

My starting point(dream or vision??): I have about 3000 ft^2 of essentially flat roof only partial obstructed with the odd heating and cooling adornments. So 80% of the roof is available with almost no sun shading. The initial (and maybe final) plan is to make use of a 30'x45' detached shop roof; 50% of 1350 ft^2 at 15 watts / ft^2 gives me a 10KW capability without tracking. Figure 13KW with tracking.

Local Pima County (North Tucson) codes will allow permitted installations without even a stress analysis if it follows the following:

• Less than 50% roof coverage for solar equipment.
• Less than 4 lbs/ft^2 load.
• Max installed height Less than 8" above a flat roof and 6" above a gabled roof.
• Designed for 95 mph winds which I'm nominal interpreting as 10 lbs-ft^2 in live load.
• Below 4000 ft alt (Tucson is about 2200 ft) there is no snow load requirements.
• At least 1 attachment point per 10 ft^2 of equipment. This is basically a minimum 20 lbs/ft^2 holding force.

Within those zoning parameters, I'm looking at mounting approximately 10'x10' panel arrays that can achieve 45 degree (subject to roof constraints) variability in attitude. I have been working getting the servos and mounting down close to a fixed installation weight although as per the 2nd law of Thermodynamics I'll never get there.

[J.P.M.]

Posplayr:

Respectfully: Start by determining what your loads are, how much you pay for the power, and decide what you want to accomplish. Then start with the basics: Buy a solar for dummies book. If you are still serious, buy a copy of Duffie & Beckman. You will not be sorry.

Dual axis tracking will intercept more energy than other schemes, but is usually not worth the extra material, maint. and hassle. KISS applies here.

On available irradiance: NREL suspects a panel on a dual axis tracker in Tucson will intercept about 35-40% more solar energy over a year than a south facing panel fixed at a tilt - local latitude. However, the practical aspects and lower cost, usually make the fixed panel a no brainer. That 47 % # is kind of high for most anywhere, Tucson and your backyard included it would seem.

FWIW: your wind loading seems light.

PVpanel cooling is also an idea whose time has probably not yet come. Like dual face panels and close to perpetual motion machines, Caveat emptor.

[posplayr]

[QUOTE=J.P.M.;161571]Posplayr:

Respectfully: Start by determining what your loads are, how much you pay for the power, and decide what you want to accomplish.

Just because I did not post that information, do not assume that I don't know what it is. I as well aware of top down systems thinking as well as having practiced the discipline for more years that I should mention. I have been doing system performance trade and sensitivity analysis using spreadsheets since the late 1980's when all SE's at the company I worked were barred from using any programming language except to use Lotus 1-2-3 on your newly issued 286 PC.

Then start with the basics: Buy a solar for dummies book. If you are still serious, buy a copy of Duffie & Beckman. You will not be sorry.

I think the dummies book might be a waste of time. Duffie & Beckman...... Here is a link to the 4th edition of D&B. Something specific you want to point out? This is the closest thing I could find about trackers; it is sparse.

23.5 Controls and Maximum Power Point Trackers 763

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&es_th=1&ie=UTF-8#

Dual axis tracking will intercept more energy than other schemes, but is usually not worth the extra material, maint. and hassle. KISS applies here.

There are many published results (I referenced two of them) documenting productivity gains from tracking. If I am to understand your statement, while there is more opportunity (than anything else) to enhance power harvesting from tracking, no one has figured out how to do that in an economically feasible way. Does that imply a violation of thermodynamics to required to make tracking economical or does it imply an economic opportunity?

On available irradiance: NREL suspects a panel on a dual axis tracker in Tucson will intercept about 35-40% more solar energy over a year than a south facing panel fixed at a tilt - local latitude. However, the practical aspects and lower cost, usually make the fixed panel a no brainer. That 47 % # is kind of high for most anywhere, Tucson and your backyard included it would seem.

Well 30-40% more solar power is even higher than I have been presuming (sources quote 30% for single plus 4-5% for dual tracking but YMMV), but not out of line with my expectations (at least 30%). Thank for confirming that with the NREL. I have a large roof, area of high specular irradiance and a certain expertise in this area (i.e. tracking).

FWIW: your wind loading seems light.

I quoted Pima County Zoning requirements for Solar Installs.

http://webcms.pima.gov/cms/One.aspx?portalId=169&pageId=62064

http://webcms.pima.gov/UserFiles/Servers/Server_6/File/Government/Development%20Services/Building/Equipment%20on%20roofs%20.pdf

I can't find the 95 mph reference but it is consistent about 10 lbs /ft^2 and the 1 attachment per 10 ft^2 requirements.

http://www.awc.org/pdf/ASCE7-10WindChanges.pdf

http://www.awc.org/pdf/ASCE7-10WindChanges.pdfASCE_7_10_Minimum_WindProvisions.jpg

PVpanel cooling is also an idea whose time has probably not yet come. Like dual face panels and close to perpetual motion machines, Caveat emptor.

I saw the thread (panel misting) and assortment of remarks and recognized the parallels to tracking. Fortunately I don't have to do an thermal energy balance to control a panel in two axis. The economic viability will be in the BOM.


I found a couple of interesting equations on pg 164 eqs 3.15.2 and 3.15.3

Apparently Pg 263, Eq 6.7.6 is the most important in the book (author's opinion) and I'm finding interesting as well.

[inetdog]

[QUOTE=posplayr;161576]

I saw the thread (panel misting) and assortment of remarks and recognized the parallels to tracking. Fortunately I don't have to do an thermal energy balance to control a panel in two axis. The economic viability will be in the BOM.

Not just in the BOM. You also need to consider maintenance costs.

[posplayr]

[QUOTE=inetdog;161577]

Not just in the BOM. You also need to consider maintenance costs.

Obviously maintenance can not be a big cost driver, but it also does not necessarily have to be 25Y life if a replacement cost is low enough. I think you would have to agree there are few absolutes (other the the laws of Thermo ) and that there are a variety of trades all that can be measured in $. Nothing says you can't change a motor out after 10-15 years it it doesn't cost too much.

The BOM is really the realization of the tracking system design and all the characteristics it will have. (including cost to produce and cost to install and indirectly through reliability analysis maintenance costs ). Having to go on the roof periodically to inspect and squirt a couple of drops of oil are probably not going to be seen as "prohibitive". I can see washing solar windows as a much bigger impediment.

Solar panel efficiencies or all the science won't really matter because what it will get down to is Collection boost (in Kw-Hr) per $ spent.

[inetdog]

Most trackers are ground mount/pole mount, both for ease of access and because of the clearances between panel arrays needed for rotation and to avoid shading of one panel by another.
Do you have a flat roof, making it very similar to a ground mount system?

[posplayr]

Most trackers are ground mount/pole mount, both for ease of access and because of the clearances between panel arrays needed for rotation and to avoid shading of one panel by another.
Do you have a flat roof, making it very similar to a ground mount system?

Posted just above.

My starting point(dream or vision??): I have about 3000 ft^2 of essentially flat roof only partial obstructed with the odd heating and cooling adornments. So 80% of the roof is available with almost no sun shading. The initial (and maybe final) plan is to make use of a 30'x45' detached shop roof; 50% of 1350 ft^2 at 15 watts / ft^2 gives me a 10KW capability without tracking. Figure 13KW with tracking.

[inetdog]

Posted just above.

Got it.

The forces on a pole type tracker mount are pretty large, including the overturning moment. A ballasted rack on your flat room may not handle them well. And making a connection to the roof structure risks major leaks.
A tilted single angle rack, on the other hand, lends itself well to flat roofs.
I am sure you can do the calculations, just do them before committing yourself to one approach.

And remember that, as long as you are not limited in area, you need to compare the cost per watt of panel plus tracker not just to the same panels without tracking but also to the larger fixed panel array you can buy with the money you would have spent on tracking hardware.

[posplayr]

The forces on a pole type tracker mount are pretty large, including the overturning moment. A ballasted rack on your flat room may not handle them well. And making a connection to the roof structure risks major leaks.
A tilted single angle rack, on the other hand, lends itself well to flat roofs.
I am sure you can do the calculations, just do them before committing yourself to one approach.

Off hand I don't really see the wind loading for a fixed panel (on a flat roof) is any different to a tracking panel on a flat roof as long as the panel is close to the roof. If the tracking panel is 10 foot above it will be, but mounted close to the roof I don't see much of a derating on attachment points or lift loads for fixed mounts. The same requirements seem to apply uniformly. For example, the Pima county criteria do not make any distinction between fixed and tracking roof mount systems as well neither does ASCE7-10 I referenced above.

I think the overturning moment (is design specific) as the real requirement is the wind loading (pressure - psf) as embodied in the lifting force requirements (holding force lbs).

And remember that, as long as you are not limited in area, you need to compare the cost per watt of panel plus tracker not just to the same panels without tracking but also to the larger fixed panel array you can buy with the money you would have spent on tracking hardware.

With the cost of a panel being about $1 per watt and the cost installed $3-4 per watt, there is question as to whether a 30-40% boost in output really makes any sense. However as long as the installed cost is $3-4 per watt, a 30-40% increase in output for the same equipment ratios to a 30-40 % cost savings for those installed equipment costs that either, a.) scale with panels or that b.) are fixed. These two items account for most of the $3-4/watt cost (most of it). I'll leave the spreadsheet to calculate that ratio. This assumes that the overhead of tracking can be kept to a minimum.

As mentioned above, I'm looking at a point design for 10x10 panels with tracking to 45 degrees in two axis to see what that would cost to work on this flat roof common in Az. If that design can be extended is possible but not my focus at the moment. As per your comment there are a variety of sensitivities and options to explore. The primary one is fixed mount vs. tracking (that is the whole point of the exercise).

I have a variety of small spreadsheets I have been constructing. At some point it will be worth cobbling them together into a total ROI roll-up but at the moment I'm still trying to understand the costs and PV electronics side of this.

[inetdog]

You are discussing all of the right questions and comparisons, but I wonder whether you have calculated the distance off the roof that you will need for clearance for your two axis trackers? And the fact that the azimuth axis mount pretty much makes the weight of the array a point load and all of the wind moment is concentrated there too.

If you have only a number of rows of single panels at a fixed angle and spacing you can keep the whole array a lot closer to the roof.

A system with the azimuth tracking supported by circular rails at the edges instead of a pole at the center would be interesting to think about. I have not seen anyone doing that.

[posplayr]

You are discussing all of the right questions and comparisons, but I wonder whether you have calculated the distance off the roof that you will need for clearance for your two axis trackers? And the fact that the azimuth axis mount pretty much makes the weight of the array a point load and all of the wind moment is concentrated there too.

If you have only a number of rows of single panels at a fixed angle and spacing you can keep the whole array a lot closer to the roof.

A system with the azimuth tracking supported by circular rails at the edges instead of a pole at the center would be interesting to think about. I have not seen anyone doing that.

I will admit to being coy on the description of the tracker( I don't know yet if this is patentable or not or if it is even worth pursuing). It is not a typical azimuth tracker however it is being designed according to whatever I do post. The PIMA county guidelines are my starting point. If I can't do that then it probably won't work anywhere else. There are zoning restrictions of 8' for flat roofs and 6' for gabled roofs.

The design is based on an analytic model of shading, angular tracking and physical constraints, square foot of production space, weight of installation and loads etc,..... so with respect to your question about what I'm considering, I hope that answers it. If you review the zoning requirements for the "no stress analysis waver" (I summarized them), you will see the basic operating parameters for a fixed install that will get rubber stamped at the building department.

If you follow those criteria, a variety of constraints follow much of what you mentioned. They seem to apply universally whether tracking or fixed.

[posplayr]

BTW, I have some energy efficiency projects to do around the house this weekend, but as I get more familiar around here, I think I'll probably start a thread on panel cooling (and try and violate some more laws of thermodynamics).

I have an idea what I think would provide harvesting gain for little cost (at least around here).