What's the difference between polycrystalline and monocrystalline solar panels?
Posted by Stuart Elmes on 25 February 2015 at 9:55 am
When I agreed to write an article on the difference between the two main types of photovoltaic solar panel, those based on monocrystalline and polycrystalline silicon, I thought what could be easier? You can find numerous articles on websites, all saying the same thing. It usually goes something like this.
1. Polycrystalline cells are square shaped, monocrystalline cells are square with missing corners.
2. Polycrystalline cells are blue-ish in colour and have a characteristic ‘metal shard’ pattern on the surface. Monocrystalline cells are black and even in colour.
3. Polycrystalline cells are of lower efficiency than monocrystalline cells.
4. Polycrystalline cells are more sensitive to heat, losing efficiency more quickly as temperatures rise, and so produce slightly less energy each year.
5. Monocrystalline panels are more expensive, polycrystalline panels are cheaper.
Rather than just repeat what people have written before, I thought I’d better do some research first.
I’m glad I did.
Turns out that none of the simple ‘facts’ about solar panels above is really correct. The truth, as is the case so often, is more complex and nuanced.
Monocyrstalline cells are made from single crystals, grown in the shape of a round pillar. When these are sliced to make the cells, the resulting circular shape needs to have the edges trimmed off to pack neatly into a solar panel without gaps. It wastes too much material if you cut a square from the circle, so manufacturers go for a square shape with missing corners.
By contrast, a polycrystalline silicon wafer is made by pouring molten silicon into a cube shaped mould and letting it cool and solidify. The solidified block of silicon is sliced into pillars and these are in turn sliced into perfectly square cells.
The trimmings from cutting and slicing the monocrystalline silicon are no longer thrown away; they can be re-cycled into making polycrystalline cells. Some manufacturers now offer mono crystalline panels with full square cells.
When polycrystalline blocks are moulded from a molten silicon, crystals start to form in many, many different places. They grow until they reach other crystals. The direction of the crystal structure is random and each is likely to be different from the adjacent crystals. When the block is sliced to make the wafer this creates a pattern, like a metal flake effect, with each crystal reflecting the light differently. The colour of thec ells is a deep blue.
Mono crystalline cells have an even black colour.
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.
The joins between the crystals in a polycrystalline cell impede the flow of electricity, so polycrystalline cells have lower efficiency.
Polycrystalline cells have been closing the gap in recent years. The highest performing polycrystalline cells have a higher efficiency than the lower performing monocrystalline cells. So, while it’s true that the maximum efficiency for monocyrstalline cells exceeds that for polycrystalline, it’s not true to say that monocrystalline is always more efficient than polycrystalline.
Add to this that polycrystalline cells are fully square and mono have corners missing and we’ve reached the point where the additional surface area from the square cell shape can makes up for a lower efficiency in the cell itself.
High temperature performance
I looked at a sample from one manufacturer than makes both types of solar panels and their polycrystalline panels did lose their power output more quickly, by about 0.02% more per degree C above 20C.
If you work this through you find that this only amounts to a difference of 1% less power at an elevated temperature of 70C, which only occurs in direct bright light on a hot day. But it’s not hot and sunny all day every day; in fact conditions to produce a 70C operating temperature are rare. The energy loss from choosing polycrystalline solar panels over monocrystalline would depend on where you are in the world, but will be much less than 1%.
Monocrystalline panels do command a price premium (on average). The reason for this is that people prefer the way they look (compared to the blue polycrystalline cells) and the panels have a higher power (on average).
This higher power panel means you save money on other costs because you have to fit and fix down less panels. It also means that you can get more energy out when the available area for panels is limited or expensive.
We can only really say that on average monocrystalline solar panels are more expensive on a per-watt basis. It’s absolutely possible to buy polycrystalline panels at the higher end of the market for a much higher price than monocrystalline panels at the lower end of the market.
At the top end of the market, monocrystalline and polycrystalline solar panels are becoming more and more alike in both power output and looks. If this trend continues, with blacker polycrystalline cells and squarer monocrystalline cells of similar performance, then the price for the two will line up.
In more developed solar markets (as the UK is now becoming), the domestic rooftop market starts to demand good looking solar panels, and solar panels with black cells and black frames are becoming the most popular for discerning customers. More and more customers are considering integrated solar panels that replace the roof covering to preserve the looks of their property (and its resale value!)
Your choice of solar panel is becoming far more about choosing a quality brand from a good installer that you trust.
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About the author:
Stuart Elmes is founder and CEO of Viridian Solar, a UK manufacturer of beautiful roof integrated solar PV panels and matching solar thermal panels.
If you have a question about anything in the above blog, please ask it in the comments section below.
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