Benefits of tracking systems in solar PV

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Some interesting points on the “Gatton Solar Research Facility” from an Energy Express seminar over the weekend.

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Watt Clarity

On Thursday 2nd April, just prior to the Easter long weekend, I was fortunate to be able to attend an “Energy Express” seminar hosted by UQ Energy Initiative and the Global Change Institute – at which Professor Paul Meredith presented on the “Gatton Solar Research Facility”.

Though not as large as the Nyngan solar power station, which opened recently, this one is also opened recently to add to a pre-existing portfolio of solar facilities at UQ that have been developed to assist in meeting a number of coincident objectives, including:
1)  To assist UQ achieve some ambitious carbon reduction targets;
2)  To assist in energy procurement for the university; and
3)  To provide facilities for “learning via doing” enabling various disciplines to work together in a systemic way in areas relevant to where society is headed for the future.

Professor Meredith covered a broad range of information during the talk, but one thing he noted stood out for me – and, I thought, would be of interest to some our readers as well.

The following is my recollection of these particular points so, if there are any mistakes or omissions, please blame my memory and not Prof Meredith (for the benefit of our other readers, feel free to point these out below).

At Gatton there are three different types of solar PV technology deployed (by First Solar, in this case):
Section 1)  The bulk of the facility is “standard” fixed solar panels;
Section 2)  There is a smaller share of the “off-the-shelf” single axis tracking system; and
Section 3)  UQ has, in conjunction with its partners, installed a “first of its kind” dual axis tracking system to ensure optimal alignment of each panel in this section

With reference to the access provided here to the output data collected from the Gatton facility, I have copied in three static images that illustrate the particular point that caught my attention:

1)  Production shape for the fixed-axis array

Prof Meredith highlighted 19th March 2015, which appears to be only 13 days after the plant came online, as the day where he’d seen the text-book production shape for the solar array:

rsz_20150408gattonsolaron19march2015image1_thumb

Of the three banks of fixed-axis, I have selected the centre array.

2)  Production shape for the single-axis tracking array

Next, Prof Meredith highlighted the different production profile apparent when looking at the single-axis array:

rsz_20150408gattonsolaron19march2015image2_thumb

A “fatter” shape is clearly evident, which results from the ability of this array to change inclination to follow the sun’s path from east to west across the sky.

3)  Production shape for the dual-axis tracking array.

Extending the process further, Prof Meredith explained how UQ and its partners had implemented a “first-of-its-kind” dual axis array with a tracking system designed to maximise the output of the array at any given point in the day.

rsz_20150408gattonsolaron19march2015image3_thumb

The improvement in the shape here is not as marked (as from the 1st to the 2nd image above), but there does appear to be a higher capacity factor achieved.

Referring back to the title of this article “Benefits of tracking systems in solar PV” you will note no reference (until now) of the costs involved in their installation.  My memory is a little vague here, but I think it was mentioned that the cost of the dual axis tracking array was something like three times the cost (per Watt installed) of the standard fixed-axis array.

As we’ve noted on WattClarity recently, demand in Queensland is peaking later and later in the day – including well after the 6pm sunset time implied in the charts above.  Hence it does make it look difficult to achieve a commercially attractive payback on the fitting of tracking systems to increase the capacity factor beyond what’s possible with fixed axis arrays.

It does, however, show evidence of the potential network benefits of west-facing (e.g. residential) solar arrays – though, I would suspect, this might have the countervailing impact of reducing the capacity factor possible with the traditional north-facing inclination.

Perhaps those more knowledgeable in the solar space might like to help me understand better by adding comments below?

 

Source: Watt Clarity. Reproduced with permission.

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17 Comments
  1. Paul McLisky 4 years ago

    Why can’t the three graphs have the same y axis?

    • WR 4 years ago

      They are taken from the university website. http://uq.rise.ws/gci/ Click on the photovoltaics tab.
      The graphs autoscale, depending on output.

  2. David Osmond 4 years ago

    Using the PVWatts calculator, in Brisbane, using fixed panels with 20 degree tilt and no shading, you’d expect an annual average capacity factor of about 14% . Using 1 axis tracking (no tilt) you get a capacity factor of about 20%, while two axis tracking gets you up to 22%.

    I’d say it’s pretty clear that 2-axis tracking will struggle to compete financially with the fixed panel option. However I wouldn’t rule out the viability of (horizontal) single axis tracking. The tracking and mounting mechanism for horizontal single axis tracking is quite simple, much more so than the 2-axis system, and the 45% increase in energy might pay for the extra capex.

  3. Motorshack 4 years ago

    I’m not what one would normally consider an “expert” on solar PV, much less a professional, but I do have a lot of experience as a software developer designing industrial control systems, and last year I spent some weeks looking carefully into this question of costs versus benefits of tracking solar mounts.

    First, your graphs are what the geometry and astronomy would predict. In the ideal case the fixed-mount graph would be a cosine curve, and the two tracking-mount graphs would both be rectangles. Real world conditions, such as less than perfectly clear views of the horizon will tend to round off the actual graphs in the way you show. The cosine curve is, of course, already a smooth curve, just as your graph shows.

    Second, the difference between the single-axis graph and the dual-axis graph is not steeper sides, since, ideally, both would be rectangles with perfectly vertical sides. The actual difference is that the rectangle for the dual-axis graph will be slightly taller than that of the single-axis graph.

    Third, the dual-axis graph will not be all that much taller, because – if I did the math right – a well-designed single-axis tracker will already capture something on the order of 92% of the available sunlight. So, adding the second axis will have only a fraction of the increased yield that a good single-axis system will produce.

    Fourth, the total amount of energy available is fixed. Tracking makes a PV system more efficient, in that it lets less of the available input energy escape capture, but it cannot increase the total amount of energy available. That is physically impossible. There is no “volume control” on the sun (which is probably a good thing, given that politicians like Tony Abbot are presently running things).

    Fifth, given this last point about fixed supply, it is therefore clear that maximizing the profit from the system is purely a question of minimizing the costs of the system, and that means both the capital costs and the operational costs.

    Now, given all that, there is a huge variety of designs out there, and their cost-effectiveness varies dramatically. So, your rough figure of three times the cost of a fixed-mount system is the real weak point in your analysis. The least effective designs might well be that expensive, but the best designs are probably not anywhere near that.

    For utility-scale systems in the industrialized countries, where expensive manual labor needs to be minimized, and operations automated as fully as possible, the best choice, and the easiest way to make a profit, is probably to use very clever single-axis tracking mounts.

    It should also be borne in mind that adding a second axis of tracking not only involves increased costs directly attributable to the second axis, but also increases the costs of the first axis, since the design of the first axis must now accommodate the additional constraints in supporting the second axis.

    That is to say, in mathematical and astronomical terms the second axis is fully independent of the first axis, but mechanically, in a real-world system, they are necessarily enmeshed with one another to at least some degree. Thus, you cannot add tracking on the second axis without increasing the costs of tracking on the first axis.

    So, not only is the potential gain from a second axis proportionally smaller than from the first axis, but adding the second axis will increase the complexity and costs of the system in a way that is also disproportionately larger.

    In short, it is probably not too hard to make the first axis pay, but making the second axis pay is very challenging, and may not be worth the bother.

    Nevertheless, there do appear to be exceptions to that last statement, depending upon particular circumstances.

    For example, the second axis only needs to be adjusted a few times a year to be quite effective, so if you have a small, residential system, where that adjustment can easily be done by hand, then I think it is possible to make the second axis profitable.

    Better yet, in poor countries with lots of cheap labor and limited investment capital, both axes can be driven by hand, thus enabling a much greater ROI on the limited capital, and also offering a bit of steady employment where it is desperately needed.

    In fact, this was the original point of my little research project, and I did come up with a simple design for a two-axis system that could be built very cheaply out of a variety of local materials, and driven either manually or with rather inexpensive automation components.

    There is one other possibility worth mentioning.

    Adding fixed reflectors to the sides of the mounting rack can increase the amount of sunlight captured (by effectively increasing the surface area of the system), but without adding to the cost of the silicon or adding the cost of a second axis. The catch is that the wind-loading problem gets harder to manage, and you have to be careful not to overheat the panels.

    There are commercial systems now available with elegant single-axis tracking combined with reflectors. These should do as well as full two-axis tracking, or better, at a much lower relative cost.

    For what that might all be worth in the present context.

  4. David Hoadley 4 years ago

    Tracker costs are on the way down, even for dual-axis trackers. There are some interesting new designs out there which have not yet appeared on the market in Australia, but they will. Clearly, trackers that increase the total cost by as much as is suggested above will never be viable, and tracker developers know this.

    Just as an aside, the single axis tracker graph is for 19th March – pretty close to the equinox. If the single axis is mounted at the angle of Brisbane’s latitude (which is likely) then this graph is probably, approximately, the best result that will be achieved by this tracker all year. The dual-axis should give better relative performance in both July and December than it appears to give here.

    • Motorshack 4 years ago

      Excellent point. At the equinoxes the single- and dual-axis systems should produce the same results, and at the solstices the difference will be maximal.

      To see the full picture for an entire year, you would need to sum up 365 graphs, and to be perfectly precise it would require the summation of an infinite series, which is a problem in calculus.

      This is one reason there is so much confusion about these kinds of comparisons. They look simple to the untrained eye, but they are not.

      The other tricky bit is to correctly visualize the astronomical relationships involved. Once you get the right image in mind it is not too hard to understand, but a good description, with effective graphics, can be hard to find.

      In particular, the key to the best trackers is something called a “polar” mount, but to understand why that is so will take some study. Again, the right image is not all that complicated, but it is quite unfamiliar to the average citizen.

    • David Osmond 4 years ago

      Hi David, I suspect the single axis is mounted horizontally. This photo at this site supports this theory:

      http://www.utilitymagazine.com.au/shining-a-light-on-solar-research/

      If so, the single axis would produce most around the December solstice.

      • Motorshack 4 years ago

        I just looked at the picture in your link, and the inclination of the single axis will not change the basic annual pattern compared with a dual-axis system. It just means that this single-axis system is less efficient than one that it optimized for the particular latitude where it is located.

        Essentially any single-axis (i.e. east-west tracking) system will do its best on the equinoxes and its worst on the solstices.

        Also, the only place that a horizontal axis like the one pictured is best is at the equator, where, in fact, it is optimal. At the poles a horizontal axis will be maximally inefficient, although still able to gather significant amounts of energy.

        Again, the astronomy is a bit tricky to visualize.

        • David Osmond 4 years ago

          Hi Motorshack,

          I agree that horizontal single axis designs work best at the equator, and are near useless at the poles.

          The horizontal single axis designs are much cheaper to manufacture than non-horizontal single axis systems, assuming your are building on flat ground. I get the impression that nearly all single axis designs anywhere near the tropics will use horizontal axis designs.

          However I disagree that the horizontal axis will work best on the equinoxes. Outside of the tropics, they will work best near the summer solstice, as that is when the sun is tracking closest to the East-West tracking plane of the panels. This is readily verified using the monthly output of PVWatts:
          http://pvwatts.nrel.gov/pvwatts.php

          • Motorshack 4 years ago

            Point taken about low manufacturing costs, and I am sure that is why horizontal mounts are used, in spite of their inherent inefficiencies. It is always a question of relative costs versus relative benefits, and every situation is different.

            Anyway, at the equator, the sun is shining perpendicular to horizontally mounted panels only at the equinoxes, so that is when they will capture the most energy.

            On any other days, the sun will be slanting onto the panels from the north or the south, so they will gather less energy.

            At the solstices this inefficiency will be maximal, and the angle will be the same in both cases – either 23.5 degrees north or 23.5 degrees south.

            At other latitudes the pattern will be similar, but the best and worst days will depend upon the precise north-south angle at which the panels are mounted. If they are more vertical than the optimum, then they will be more efficient in winter. Conversely, if they are more horizontal they will be more efficient in summer.

            Finally, I misspoke when I said that a horizontal mount would be useless at the poles. Maximally inefficient, yes. Useless, no.

            At the poles the optimum angle would be vertical – unless I am having yet another senior moment – but other angles will capture some energy, so long as the sun is above the horizon at all.

            My previous comment has now been edited to avoid the blatant error.

          • David Osmond 4 years ago

            Cheers Motorshack. Agree with all you say, but just want to emphasize that horizontal single axis tracking generally has no tilt (other than the east-west tilt associated with the tracker). Thus, outside of the tropics they will perform best around the summer solstice.

          • Motorshack 4 years ago

            I’m sorry. Perhaps I misread something in your earlier posts.

            Yes, outside the tropics a horizontally mounted axis will do better in summer than in winter, and best at the summer solstice. No doubt.

            Somehow I got it into my head that you were saying that was true at the Equator, which is why I stepped through it the way I did.

            This stuff is tricky enough to discuss, even before adding a couple of random lapses of attention to the conversational mix.

            In any case, I am glad that Parkinson is starting to publish some articles on this subject, because, with clever designs, there is the potential to greatly leverage the world’s still-limited capacity for high-tech PV module manufacturing, and to do it with very low-tech materials and techniques.

            That is why I have been so voluble on the subject. The sooner that people realize what the opportunities might really be, the sooner that we can dump fossil fuels.

            I am particularly interested in what might be done to leverage limited capital in the Third World.

  5. WR 4 years ago

    Outside of the tropics, tracking systems have the greatest benefit in summer and the smallest benefit in winter.

    This is because, in winter, the sun’s path is a narrow arc that traces a path fairly low across the northern sky (as viewed from the southern hemisphere). So the sunlight’s angle of incidence on a flat, stationary panel doesn’t change much during the day.

    In summer, the sun follows a much wider arc, starting in the south-east, crossing almost overhead, and then setting in the south-west. A single axis tracker is very useful for following that large arc. So for regions with high summer electricity demands, single-axis tracking systems might be feasible.

    Dual-axis tracking systems are probably only ever going to be useful for systems that require focused, direct sunlight, such as CPV and CSP.

    • Motorshack 4 years ago

      Actually, you are onto a point worth considering, but I draw the opposite conclusion.

      Any mount with a fixed north-south axis (not at the equator) will have maximum exposure to the sun in the summer and minimal in the winter. As you (almost) say, this is because the summer days are much longer than the winter days.

      However, it is perfectly straightforward to pick a fixed north-south mounting angle that is more vertical, and therefore more optimized for winter days, and less so for summer days. So, right off this tends to minimize the difference between the seasons, regardless of whether or not any tracking is used.

      However, in the quest to get the maximum amount of energy at the time of year with the shortest day, east-west tracking will certainly improve the yield of the system, and therefore help to get the maximum yield with the minimum number of panels for any given application. So, I see short winter days as a very strong justification for the use of tracking, all other things being equal.

      Similarly, in the summer, east-west tracking will still produce enhanced yields, even though the system in question is optimized for winter.

      Also, in many places, summer loads, such as AC, are often much greater than winter loads, so the difference in winter and summer yields is not necessarily a disadvantage – again, independent of any use of tracking to increase overall yields.

      In short, tracking is simply one more item in the designers’ tool-kits that will help give them maximum chance of delivering cost-effective solutions.

    • Barri Mundee 4 years ago

      Not wishing to detract from the discussion regarding tracking systems, But I wonder if it is better to hold out for greater efficiency panels which are possibly on the way to commercialisation, given the lab efficiency improvements achieved over standard panels?

  6. Matt 4 years ago

    I would be very interested to see the economics of using a hybrid setup with the majority of fixed panels, and a smaller subset using dual axis tracking or a fixed east and west orientation. I suspect that the production curve for single and dual axis tracking could be reproduced at a much lower overall cost in this manner.

  7. Rex Elliott 1 year ago

    Hi, first let me clarify that I’m no expert; although I have built a single axis and a dual axis tracker. I’ve done it as a lay person. I will add I like the single axis tracker, but anything I have to say has been covered.
    BUT – A point that I have not seen raised is a comparison between a fixed axis tracker and a single axis tracker on an intermittently cloudy day. My experience, particularly in winter at the lower latitudes (Australia) is that the single axis tracker is much more efficient on cloudy days. For instance, if you only get sun in the morning and/or late afternoon, the fixed axis tracker is virtually useless. Whereas the tracker may replenish your batteries to last another day/night.

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