Shade Tolerant Solar Panels?

Shade Tolerant Solar Panels?

I have heard a number of travelers repeat the great “shade tolerant” sales pitch given them, when selecting solar panels for their motorhome or caravan. Much of this is just that – sales pitch.

Firstly, nobody in their right mind is going to intentionally locate their expensive solar panel in the shade. Solar panels use the photovoltaic effect to generate electricity when exposed to light, their output is somewhat (but not entirely) proportional to the intensity of that light. If you want your roof mounted solar panels to charge your batteries, you are going to need to park in full sun.

So what do the sales people mean by the term “Shade Tolerant”?  Generally they refer to the performance of a panel when it is partly shaded. Let’s look at the performance of three different panels.

  1. A monocrystalline panel with no bypass diodes
  2. A monocrystalline panel with bypass diodes fitted to each cell
  3. An amorphous panel (Unisolar) – “Shade Tolerant”

(polycrystalline panels are very similar in construction and performance to monocrystalline, so for the purposes of this discussion may be considered the same)

flexable_solar_panel

All solar panels are made up of strings of cells – each of these cells is connected in series and contributes 0.4v – 0.5v to the output of the entire panel. When in full sun the 36 cells of a so called 12v panel will produce about 18 volts.

The monocrystalline panels have square cells that are easy prey to shading from leaves and bird droppings. When a single cell of the first monocrystalline panel (no bypass diodes) is shaded, the cell acts like a block. The shaded cell stops almost all of the current from the other cells from flowing out of the panel and the entire panel stops working.

In the case of the monocrystalline panel with the bypass diodes – a single shaded cell will reduce the output of the panel (by 0.4v – 0.5v) but will not stop the entire panel from working.  This type of panel is correctly labeled “Shade Tolerant” – however, it will not produce its rated output unless all cells are exposed to full sunlight.

Now – the amorphous panel.  These panels have their cells arranged in long strips – this has the advantage of not being easily fully shaded. Furthermore, amorphous panels have a far more impressive lower light performance (for example in partly cloudy conditions) and they have bypass diodes fitted to each cell – so if a cell is shaded, the remainder of the panel will contribute some useful charge.

These panels are almost always referred to as “Shade Tolerant” – and correctly so.  So when you consider the amorphous panels impressive low light performance, its good high temperature performance, its ability to survive massive hail storms and even being driven over, along with this great shade tolerance, why are we not seeing every caravan and motorhome clad in these impressive solar panels?  The simple answer is efficiency!  Amorphous solar panels are extremely inefficient when compared to other types of panels.   You will need to reserve almost twice as much roof space for the same output if you decide to use amorphous solar panels.

As our power requirements have slowly increased over the years that we have been living in our motorhome, roof space has become a very precious commodity. If we wish to add still further solar capacity to our roof, we are going to be forced to replace some or all of our Unisolar amorphous panels with the more efficient poly or monocrystalline panels – an expensive exercise indeed.

It is important to note that shade-tolerant doesn’t mean shade-proof. A shade-tolerant panel will perform exactly the same as a conventional panel when under full shade. The only gains come when the panel is partially shaded.

Conclusion

In my opinion, shade tolerance is a very desirable attribute to look for when buying solar panels – however not at the expense of extremely poor efficiency.  poly or monocrystalline panels equipped with bypass diodes provide a good degree of tolerance to partial shading and maintain a far better  overall efficiency than amorphous panels.

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6 Responses to “Shade Tolerant Solar Panels?”

  1. Antoine Says:

    Yes shaded performance of mono-crystalline can be increased by adding a bypass diode to each cell, but this is not commonly done due to the extra expense of the diodes. In general they are fitted about one diode to a string of 15 cells. And even then the main advantage of the bypass diode in this scenario is to reduce ‘hot-spotting.’ Which is when a PV cell is reverse biased and acts as an inefficient conductor; hence heating up.
    Amorphous Cells do not require additional bypass diodes as they are ‘in built.’ The average efficiency of an amorphous panel is between 6-8% whilst a polycrystalline cell is usually 11-12%, but also usually about twice the price. So if you have to room amorphous is a reasonable alternative.

  2. Hobo Says:

    Hi,
    Thanks for your comment.
    I am not sure that amorphous panels are a reasonable option for use on a caravan or motorhome. Energy needs have a tenancy to grow over time and if (like us) you put large, less efficient panels on your roof, and then later want to add more, the extra size is major issue. Also it is not a great idea to mix panel types (due to the different voltage output and temperature coefficients. For these reasons I would strongly recommend against amorphous panels for motorhomes and caravans.

    Gavin

  3. mick Says:

    ive been looking at these 24v uni solars , they deff take up some room ,5.5m x 400mm each. i can get 2 of on my roof , so im assumming once the 236watt of 24v comes to a mppt and converted into 12v it would be the same as having 512watt of 12v panels
    http://www.ebay.com.au/itm/191679949317?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

    cheers mick

  4. Hobo Says:

    Hi,
    Watts is calculated using the following formula W = V X I (I is current in amps and V is volts). This means that watts is watts no matter what the voltage is (watts is a unit of energy). An MPPT regulator can lower the voltage and raise the current – but it can not make more energy (watts). 236watts at 24v will be LESS THAN 236watts after conversion to 12v by an MPPT regulator (less due to the losses in the process).
    Example
    236watts of panel at 24v (assuming ideal conditions) will produce 9.83 amps. If we convert this to 12v (again ignoring losses) this will become 19.66 amps. It is no coincidence that 19.66 amps multiplied by 12 volts equals 236watts. As you can see – the volts and the amps change … but the resulting watts remain the same.

    BTW – In my humble opinion, unless you have some really compelling reason to use flexible panels, this is NOT an economical setup. At about $1.80/watt they are almost twice the price and twice the size of standard glass panels. For most applications 236w is not enough and if the roof is covered in these panels you have nowhere to expand to. I have pulled all except two of my Unisolar panels off our motor-home to make room to much more efficient panels.

  5. mick Says:

    u always make sence hobo 🙂 , thank you . ill give my idea a miss ,
    what would u suggest to keep 2x220ah @12v charged up reasonbly quick daily on normal days ?
    cheers mick

  6. Hobo Says:

    It really depends on what you are running from it. Take a look at my article on calculating the number of solar panels.
    The general rule of thumb since solar became so cheap (< 1$/watt), is "as much as will fit on the roof". Avoid panels larger than 200w if you can - they are too large for use on vehicles that will be driven on less-than-perfect roads.

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