Friday, 5 December 2014

Mono vs Polycrystalline Solar cells - Myths Busted

Customers often ask what's the difference, but the old certainties have gone. 




Monocrystalline have missing corners, polycrystalline cells are square : Myth


Monocrystalline solar cells are cut from a large single crystal of silicon. The process by which this crystal is grown is remarkable. It is drawn from a molten crucible of liquid silicon by dipping in a 'seed' crystal and then slowly pulling this away from the liquid surface and rotating it.  By carefully controlling the temperature gradient in the crucible and the speed of withdrawal it is possible to create a solidified single crystal with the same atomic orientation as the seed.

If this cylindrical crystal were sliced to produce silicon wafers, they would be round and this would leave gaps when you tried to assemble them together into a solar panel.  So the cylinder is first cut along its length on four sides to make its shape closer to a square in cross-section.

There's a compromise here. The more you slice off, the closer to a square shape you get, and the more working area you can squeeze into your monocrystalline PV panel. The less you slice off, the less material you waste and the cheaper are the cells to manufacture.  The compromise that most manufacturers have reached is to make a shape that was a square with rounded corners (pseudo-square).

By contrast, a polycrystalline silicon wafer is made by melting the silicon feed stock, pouring it into a cube shaped mould and letting it cool and solidify.  The resulting block of silicon is sliced into pillars and these are in turn sliced into perfectly square cells.

So one difference between mono and poly is the characteristic shape of each; Poly are square and mono have missing corners.

Not any more!

The trimmings from cutting and slicing the silicon are no longer wasted; they are re-cycled as a material input for polycrystalline cell production. Some manufacturers now offer mono crystalline panels with full square cells.
  

Monocrystalline cells have an even black colour, polycrystalline are patterned and blue: Myth


When the polycrystalline ingots solidify in their mould, crystals start to form in many, many different places (nucleation sites) and grow until they meet up with other crystals.  The orientation of the atomic structure in each crystal is random and is different from its neighbours. When you slice though the ingot to make the wafer this creates a characteristic pattern, a kind of metal flake effect, on the surface of the solar cell because each crystal reflects the light differently. The cells also have a bluish colour. By contrast, mono crystalline cells have a homogeneous atomic structure throughout and have an even black colour.

Not any more!

High performance solar cells are now treated during processing to create pyramidal micro structures on the surface which improves light absorption.  Anti-reflective coatings are added to reduce light reflection from the surface. Both polycrystalline and monocrystalline cells can be made to look matt black with an even colour.

Monocrystalline panels are more efficient : True - well, sort of


The boundaries between the crystals in a polycrystalline cell (grain boundaries) can impede the flow of electricity, so mono crystalline cells (which have no grain boundaries) have always had higher efficiency. However, polycrystalline  cells have been closing the gap in recent years and the point has  just about been reached where the additional active surface area from the square cell shape in a polycrystalline panel makes up for the lower efficiency in the cell itself.

Check out this table.




It shows the product range from one of the world’s largest manufacturers.  Power is given in Watt-peak (Wp), the power output under standard test conditions. 

If you compare the standard mono and poly products (code 6/60 models), you can see the range of peak power output runs from 250 to 270Wp for the mono panel and from 245 to 265Wp  for the poly panel.  The difference is 5Wp, or 2% less power for the polycrystalline.

Monocrystalline/Polycrystalline  panels work better in low light conditions : No evidence


I have read many claims that one type of panel works better than the other in low light conditions, and writers on other websites seem to be evenly split in whether it is monocrystalline or polycrystalline that is best (presumably depending on which they sell).

I have been unable to find evidence to support these claims (in either direction…).

Until I see some evidence, I’m going to mark this one down as a myth!  Please let me know in the comments below if you know about this.


Monocrystalline panels have better high temperature performance : True – though marginal


Looking again at the table, the right hand column shows the Power Temperature Coefficient.  This is the rate at which the panel power output falls as its temperature rises.

Polycrystalline panels do indeed lose their power output more quickly, by about 0.02% more per degree C.  But what does this mean in practice? 

If, for example, a Monocrystalline solar panel were operating at 70C on a hot and sunny day, it would be producing 0.41 x (70-20) = 20.5% less power than is measured under standard test conditions (20C).  By contrast a polycrystalline solar panel could be producing 0.43 x (70-20) = 21.5% less power.

All other things being equal, polycrystalline panels would produce 1% less power at the elevated temperature.  But that is a very different thing from saying it would produce 1% less energy over a year of operation.  It’s not hot and sunny all day every day; in fact conditions to produce a 70C operating temperature are rare.  The energy penalty from choosing polycrystalline solar panels over monocrystalline would depend on climate, but will be far less than 1%.   Although there would be a penalty, it’s pretty marginal.

Polycrystalline panels are cheaper, monocrystalline are more expensive  : True, on average


The argument often goes that because the process of producing monocrystalline cells is more complex and involves more wasted material, they’re more expensive to make.

However, just because something is more expensive to make, doesn’t make it worth more to the customer.  The reason that monocrystalline panels command a price premium is that more people prefer the way they look and the panels have a higher power.  Having a higher power panel means you save money on other costs like racking and fixings for the same total energy output.  It also means that you can squeeze more energy out of situations where the area to place the panels is limited or expensive.

The PHOTON module price index report for November 2014 has average spot market prices for solar panels in Europe as follows.  (Prices are always given per watt-peak, Wp, so you can compare based on the power output).

Monocrystalline solar panels   0.65 EUR/Wp           (Range 0.48 – 0.95)
Polycrystalline solar panels     0.55 EUR/Wp           (Range 0.40 – 0.82)

So yes, on average monocrystalline solar panels are 18% more expensive on a per-watt basis, but the range of prices show that it’s perfectly possible to buy polycrystalline panels at the higher end of the market for a much higher price than the monocrystalline panels at the lower end of the market.


Conclusion


The old certainties are disappearing.  At the high end of the market, monocrystalline and polycrystalline solar panels are becoming more and more alike in aesthetics and performance.  If this trend continues, with black polycrystalline cells and square monocrystalline cells of similar performance, then average prices will converge too.

In mature solar markets, the domestic rooftop market starts to demand good looking solar panels, and has settled on solar panels with black cells and black frames with improved aesthetics.  For this market, your choice of solar panel will be far more about choosing a quality brand that you trust than worrying about whether those panels followed a polycrystalline or monocrystalline manufacturing route.

11 comments:

  1. interesting, and what about Thin Film panels? I have installed a number of them, they were invented here in south Africa, and would like to know your thoughts and comparisons on this 3rd type of panel.

    ReplyDelete
  2. Very informative article on the difference between polycrystalline and monocrystalline solar panels. It was interesting to learn that the monocrystalline is only .10 more expensive then the polycrystalline materials. I would like to learn more about how to get the most out of my solar panels, especially when dealing with heating. air conditioner repair

    ReplyDelete
  3. (Unclear if this posted before - I hit "publish", logged into google and then was directed back here).

    Nice post but I have two comments.

    1. STC is for 25 degC cell temperature, no?
    2. Axitec AC-245M mono modules seem to do much better at low light (250 W/m2) than the similar AC-245P poly modules. See below:
    http://www.axitecsolar.com/tl_files/pdf/DB_60zlg_poly_power_MiA_GB.pdf
    http://www.axitecsolar.com/tl_files/pdf/DB_60zlg_mono_premium_MiA_GB.pdf

    Thoughts? Thanks.

    ReplyDelete
    Replies
    1. Hi Michael

      1. STC is for 25C cell temperature, 1000W/m2
      2. Yes, although I assume you're referring to the I-V curves at different light levels, so the comparison is between an AC-240P and an AC250M, (with a 10W difference at STC). Unfortunately the document doesn't give MPP power output for each curve, so you're left to assess it by eye, but yes, it does look like (for these 2 products) the mono might have fallen by less at lower light levels as a proportion.

      Delete
    2. This is interesting. As the polycrystalline modules should have a higher series resistance (due to more grain boundaries) I presume that the performance at lower irradiances than 1000 W/m2 should be better for polycrystalline.
      As you say though, I've seen people affirming and rejecting this view!

      Delete
  4. Great article ...Thanks for your great information, the contents are quiet interesting. I will be waiting for your next post.

    ReplyDelete
  5. Very interesting and informative post.

    Thanks a lot.

    ReplyDelete
  6. I know this is an older article, but as I'm sure you know scientists recently constructed an entirely transparant solar cell. My newbie question for you is would something like that be constructed of Polycrystalline panels or monocrystalline?

    I'm assuming that they are the same substance yet stacked differently? Somehow?

    ReplyDelete
  7. Very informative, difference is clear on fundamental grounds. Just want to ask the difference in actual on cost basis or performance for 60 Cell and 72 cell modules.

    ReplyDelete