08 June 2021:
The rated power of solar PV panels has climbed steadily over time. This has been driven in large part by innovative new processing techniques for the cells themselves, although improvements to the technology of panel assembly has also played a role. Over the decade from 2010, customers of the panel manufacturers came to expect higher and higher module powers each year.
Because all panels were the same size, the panel power was a good shorthand measure for how advanced the cell technology was. If a panel was rated at 320Wp then it would generate 14% more energy per square metre of space than a 280Wp module.
Squeezing more power (measured in Watt-peak – or Wp per panel) into the same footprint tended to drive down the cost per installed unit of rated power ($/Wp). Since the cost of the glass, frame and other components of the module and all the installation materials remained the same for modules of any power. The inexorable rise in power density of the available cells was a significant factor in helping the industry achieve the amazing feat of cost reductions we have seen over that decade.
In addition, new techniques for squeezing every drop of performance out of the cells in solar modules have also allowed the accumulation of small gains. Using a greater number of conduction wires (bus bars) with a more slender width on the top face of the cells reduces power lost due to shading of the cell below and also reduces losses from electrical resistance. Cutting the cells into halves, or thirds, or quarters and wiring all these fragments together into parallel circuits again reduces resistance losses as well as reducing the sensitivity of the module to shading. The efficiency gap between measuring the cell in isolation compared to the assembled panel has reduced over the years.
However, gains from improving cell powers have reached a plateau. See my blog on why solar cells are not getting more powerful (coming soon).
So manufacturers reaching for new ways to keep the story of ever more powerful modules at ever lower cost per Wp going have found a simple answer – just make the cells and the panels bigger.
Breathless excitement from credulous industry commentators as announcement of modules exceeding 400Wp, then 500Wp barriers misses the point. A not-so sleight of hand is evident as soon as you look at the product behind the headline number. Panels are not getting better, they’re just getting bigger.
You can have any (size), so long as its….
When it came to solar PV panels (modules) we all used to know where we stood. A solar PV panel was just under 1m wide and around 1.65m long. It had each of its 60 cells were 156mm square. A defacto standard for PV panels emerged around 2010 and manufacturers stuck to it.
Using the Waybackmachine internet archive I accessed historic web pages of industry stalwart Trina Solar. For each year where I could find the information, I downloaded product information for the highest power module that was being offered for sale at that time. See the graphic at the top of the page.
You can see that from 2009 to 2018, its module powers increased from 230Wp to 315Wp while their size remained the same. The corresponding module power density increased from 141 to 192Wp/m2.
Today Trina Solar offers modules with powers ranging from 320Wp to 600Wp by virtue of offering modules of much larger dimensions.
There were many advantages that came from working to a standard size.
In manufacturing – everything from the sheets of glass to the width of the encapsulant film and backing sheet to the pallets and packaging could all be made to the same size. Economies of scale for the whole industry drove down costs and raw materials are readily available from many different suppliers.
Downstream, manufacturers of so-called ‘balance of materials’ equipment could also standardise their products.
The electrical characteristics of the modules were all in a relatively small range and manufacturers of module level power electronics (MLPE) such as power optimisers and microinverters could easily cover the whole market with a small range of products.
Since the physical dimensions of modules varied so little between models, manufacturers of roof mounting kits also benefitted. The wind loading per module was the same for all modules, allowing optimised designs for fixings. The spacing of point loads on rails was consistent for any design. Roof integrated BIPV products emerged that worked with these standardised modules and relied on their predictable format for wide interoperability and a single wind resistance value (though fire classifications proved less simple).
When designing with standard sized modules, solar installers had confidence that the system they were designing would be available both now and into the future. This is especially important for new build projects where the design may be priced for construction many months or even years in the future. Even if the specific module selected with was no longer available, an alternative of the same dimensions could easily be found.
Finally customers benefitted from the knowledge that in the event that a single panel in a system was to fail, that a replacement of the same format would be available in future and could simply drop into the mounting system.
Battle of the Formats
A standards war has broken out with TrinaSolar, Risen Energy, Canadian Solar and others lining up against Hanwha Q Cells, REC Group, LG on the one hand and Longi, Jinko and others on the other – each calling for the industry to focus on cells of the dimensions they prefer. Will the industry settle on 158.75, 166, 182, or 210mm? At this point nobody knows.
(See my guide to different PV cell size formats here)
In any case the cell size no longer quite defines the module size. Since manufacturers have started slicing up cells into smaller sub-units (half-cut , third cut cells) there is greater freedom to choose different module sizes. Dizzy with this new found freedom, module designers are expressing their creativity with the results of a a wide range of module sizes now available in the market (see graphic).
Even within single manufacturers a range of many different panel sizes are now on offer. The graphic shows a sample of products available from four of the larger industry players. Panel powers range from 320Wp to 800Wp, but as can be seen the power density (Wp/m2) ranges only from 193 to 212 Wp/m2.
This is because the cells are pretty much the same but the packing efficiency is ever so slightly higher in a larger panel (because the edges are a smaller proportion of the whole).
An arms race for ‘plus-sized’ size panels is in progress, with JA currently in the lead offering the Jumbo Blue at 2.2m x 1.8m and 800Wp. These panels are completely impractical for rooftop applications, but are aimed instead at utility scale ground mount.
It seems that module manufacturers have decided that the cost and convenience benefits of standardisation of module sizes can go hang. Instead they are pursuing a product strategy of scaling up panel dimensions to increase panel power that is leaving the rest of the solar supply chain scrambling to catch up.
Where will this end? There are clearly practical limits. How big a module can customers handle? The answer to this question depends on where you are trying to install it. If you’re on top of a windy roof you don’t want to be humping round a 4m2 monster, but maybe that’s perfectly fine with mechanised lifting for a ground mount system.
The economics of solar PV has changed. Modules now dominate the cost structure of a solar PV installation to a much lesser extent. While it was the case that the module was the principal cost, it made sense to work with a standard format and drive out as much cost as possible. As panel prices have fallen, maybe it does make sense to design specialist modules for different jobs. Naturally manufacturers are focussing on their most important markets – solar farms and hence the rush to produce the bigger formats, which reduce other costs in installation and clamps.
Could this trend extend beyond different panel sizes to panels with special design features for different applications? Maybe in future the panels used for flat roofing installations will be different from those used for fixing to corrugated metal roofs and have special features that make installation easier? Or maybe the industry will step back from format wars and settle on a few standardised sizes for different applications. Some folks in the business of making mounting kits for the solar modules made by these manufacturers must certainly be hoping so!