Shadows on PV panels

Shadows on PV panels are a problem, but just how significant are they? The time is 16.45 and it's two months before the summer solstice.

Naturally this will look very different in winter and at this stage I'm unsure of the effect. I should have been taking photos year round.




I am planning to put three large 300W photovoltaic cells up on this west facing wall for extra energy in the late afternoon; that's when the boss is cooking. But how significant is the shading due to the roof and the chimney going to be?

Elsewhere we'll discuss the merits of siting the panels where they'll only get direct bright light for half the day. Perhaps some mirrors to reflect the morning sun onto them might be an idea; that's how the large scale commercial solar plants work.

From noon until 16.00 hours these three panels will get full sunshine but then the silhouette of the roof and the chimney will start affecting the lower right panel. How significant will it be?




Conventional wisdom is that shadows drastically affect the performance of PV panels. Shall we test that belief?

We will do two little experiments.

  1. How does increasing shadow affect the output of one panel?
  2. How does a shadow across one panel affect two photovoltaic cells in series?

Shadows on PV panels

First then shading part of this small panel with a sheet of paper; how does it affect the voltage and amperage?


Voltage

Shading

Unshaded

25% shading

50% shading

75% shading

Voltage

23V

22V

21.5V

20V

We can conclude that shading has remarkably little affect on the voltage.

Amperage

Shading

Unshaded

25% shading

50%

75%

Current (mA)

75

54

45

13

Shading up to about half the panel has more or less the expected result; the current roughly drops accordingly.

But after that increased shading dramatically affects the output of a PV panel.




Of course, in the late afternoon the intensity of the overall light decreases anyway, so one is expecting a fall off in amperage.

I'm quite intrigued to see just how much that will be with these directly west facing panels.

Setting the PVs west instead of north has raised some eyebrows; is it totally dumb? I don't think so as we have a surplus of power in the middle of the day, but too little in the late afternoon; in the morning too. In due course I may consider three east facing solar panels.

More storage would be another option but, with a 48V system, that means four more batteries at around 500 dollars each. Adding a few west facing PV panels is to my mind a better option.


Shadows on two panels in series

Here are two old small PVs that we are using for testing the affect of shading on the performance.

One is cracked after being hit by a hailstone (I think) and surprising it hasn't affected the generation of current.

What is the affect of shading one panel?

Shading 1 panel

Unshaded

1 panel 50% shading

75%

100%

Amperage (mA)

140

100

60

0

Shading one panel by 50% has a relatively small affect on the performance of two panels in series.

But shading one panel completely negates the effect of the other panel, even though it's in full sun. If one panel is shaded significantly, the output of the whole string drops to zero.

What is the effect of shading both panels?

Shading

None

Both 50%

Current (mA)

140

85

Shading both panels by 50% has roughly the expected affect. The current drops by slightly less than half.

However, if we increase the shading on either or both more than 50% we can expect a dramatic drop off in performance of the combo.

I'll be using three panels in series, and expect the result to be more or less the same.


Conclusions

I've decided that the shadows falling across one panel, but not affecting the other two, will be significant but not disastrous until the very late afternoon when the shadows start to cover more than half that one right lower panel.

Let see if practice confirms the theory?

This page is in development. The brackets for the three panels are complete, but need to be painted and erected.

I'm expecting a big increase in mid to late afternoon power with these three panels directly facing the sun. That's when she who must be obeyed will be cooking the evening dinner.

Further conclusions are that the effect of shadows is very panel dependent; on the larger panels even small shadows had quite a serious detrimental effect on the power outage.

For example, on this mark I solar car, the relatively small shadows from the transverse bar and handles dropped the output by more than 15%.

So, Mark II has a removable handle, and the bar has been moved behind the pivot point. These three mobile panels are providing between 700-800 watts for much of the day; more important, on the steepest setting they provide much needed power just after sunrise, and as the sun is waning in the late afternoon.

The frustration of these is that despite being relatively cheap overall, as a stand alone they will only supply LED lights and a laptop and TV. That means you will have power but the overall reduction in your utility bill will be small; they will do little for the environment.

But as an add-on to your existing system, without a great deal of extra infrastructure, the payback time is much shorter, and they provide much needed energy early and late in the day. In our own case the addition of these mobile panels means that we could seriously consider going off the grid.

Update on shadows on PV panels

After noting the shift of shadows on this wall as winter advances, I've made an about turn. Instead I've opted for west facing solar panels on the roof.

Life is full of compromises; flat on the roof is much simpler and more attractive, but I was looking for late afternoon sun; these optimise around 2pm but are less effective later; they should really be at 60 degrees.

I may yet opt for two panels on that wall on the left, the shadows on the third made it a non starter; or build a mobile panel that can be manually turned during the day; that you have seen above. It's very effective but requires moving two or three times a day.

Update: The three mobile solar carts with the same panels as those above, fixed on a west-facing roof, are providing a lot more energy because they can be turned, and the angle altered as the sun dips towards the horizon.

We are working on making it more user friendly, and then will come out with do it yourself plans; hopefully by early 2018.

If you're a greenie, life can never be boring; there's always something new to be tested, built or tried. Right now it's mobile solar panels and breeding chicks; life's fun!


Solar power energy

Solar power energy comes for free, courtesy of Mr Golden Sun. Plants collect it by virtue of photosynthesis, and humans using photovoltaic cells and vacuum tubes; shadows on PV panels are a very important consideration. 

Shadows on solar panels in series are only a major headache when they start to cover more than half the area of one panel.


» » Shadows on PV panels

Bernard Preston

Bernard Preston is a semi retired chiropractor with a passion for healthy living and maintaining the environment for our children's children; shadows on PV panels is just one tiny diversion from his many interests.

Too many 'she who must be obeyed' would say!


Plastic is overwhelming our beaches and oceans; the greenhouse effect of carbon dioxide in the air and seas is causing devastation of the weather patterns; toxic chemicals and the injudicious use of medication are poisoning our bodies.

Learning to wean off plastic, giving consideration to shadows on PV panels and related topics, and how to adjust to a life without medication are the only way to a healthy future.

Bernard Preston says do it for your children, do it for yourself if you want to reduce the risk of cancer and painful autoimmune diseases; do it for the preservation of two of the greatest of God's gifts to us, our planet and our bodies.

Is a beach like that above the legacy you want to leave to your children? It's time each and every one of us stood up to the mark and take responsibility for the planet; one small start would be to recycle or reuse a plastic bag or bottle today.

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