– which type is best?
by Ray Prowse, SEE-Change inner north convenor
You know that saying about questions and dollars, i.e. I’d like a dollar for every time I’ve been asked that question? Well this is mine. Over the years I’ve lost count of the number of times I have been asked to make a recommendation between different types of solar panels.
The simple answer is that they’re all pretty much equal. The more detailed answer is below and there are some fundamental differences which may give preference to one or the other in some circumstances.
The three common types of solar panels are mono-crystalline (sometimes called single crystal), poly-crystalline (multi crystal) and amorphous (sometimes called thin film). In common usage they are predominantly made out of silicon (chemical symbol Si), plus a few impurities (dopants) to give them their electrical characteristics.
A note about silicon first:
It is the second most abundant element on the surface of the earth, so it’s in plentiful supply. Think rocks and sand. Chemically and mechanically it is a very adaptive element and can be configured for a wide variety of uses. Electrically it is a semi-conductor. It has also been calculated that a kilogram of silicon, when converted into solar cells, will produce more energy over its life than a kilogram of enriched uranium!
As the name suggests these cells are made from single crystals of silicon. Each cell is made from layers of silicon doped with either boron or phosphorous. When these layers are brought together an electric field is established between the layers and, when sunlight falls on the cell and releases electrons, they are forced to move under the influence of that electric field. We now have an electric current — moving electric charges.
The electric charges are collected at the surface of the cell by a grid of metallic contacts and, when connected into an electric circuit, will produce a current in that circuit. The image to the right shows a mono-crystalline cell. Note the typical colour and slight curves in each corner — these are the best ways to tell them from the other types of cells. The curves in the corners are due to the cell having been cut from a circular cell. This is done to decrease the wasted space between the cells when they are connected together to form a panel.
Current and voltage
Each cell by itself will produce approximately 0.6 volts and the current will depend on the area of the cell. To produce a useable output the cells are connected in series to form a panel. The image below shows 72 cells connected together to form a panel.
43 V. This voltage is still not high enough to run household appliances, but these panels can then be connected into an array with other panels to produce the required voltage. The specifications provided with the panels you buy (and obtainable from specification sheets from the supplier) will show different voltages.
They will typically be: Open circuit voltage (Voc) ≈ 44 V, and Voltage at maximum power (Vmp) ≈ 36 V. The current at maximum power (Imp) depends on the area of each cell and therefore the overall area of the panel. For a 180 W panel, Imp will typically be ≈ 5 amperes (A).
Note that all panels will vary a little, but the voltage and current will be around these figures.
The efficiency for mono-crystalline panels will vary from one manufacturer to another but will typically be around 14.5–15% when installed.
Because the cells are connected in series, if you shade one cell you will cut out the current from not only that cell, but the whole module. This is prevented to a slight extent by the use of bypass diodes. Each diode offers an alternate current route around the shaded cell so that, when the cell is shaded, there is a path for the current to take and the rest of the module will still produce current. The problem with bypass diodes is that they also reduce the normal current by a small amount. Hence the number of diodes in each panel is normally restricted to two i.e. half the panel is protected by each diode. This means that if you shade one cell you will lose the output from half the panel, but not the whole panel.
Contrary to what you would expect from a solar device, the output of a solar cell actually decreases the hotter it gets. The internal cell temperature for a panel under operation is typically around 200C higher than the ambient temperature. There is normally a very slight increase in the current, but there is a more significant decrease in the voltage. Because the power of a module is the product of voltage x current there is an overall decrease in the power when temperature rises.
For most mono-crystalline panels the temperature effect is a reduction of approximately 0.5% per degree of temperature increase, i.e. for a 200C increase in the temperature there is a 20 x 0.5 = 10% decrease in the power from the panel. If the power was, say, 180 W then under the increased temperature the power would be 180 — 18 = 162 W. The specification of panels is normally given under standard test conditions and the temperature used for testing is 250C.
The opposite is also true. The cooler the panel is, the more power you would expect from that panel. It is important to install the panels so that there is a gap behind them to allow for cooling breezes.
A 1 kW system will normally take around 6m2 of roof space.
These are fundamentally the same as mono-crystalline cells but, as the name suggests they are made from multiple crystals instead of a single crystal. They are produced using a casting process and are made with square corners; hence no gaps between the cells. The following image shows a poly-crystalline cell.
The characteristic visuals for poly-crystalline cells are the colour (typically blue) and the fact that you can actually see the individual crystals in each cell.
Current and voltage
Each cell will produce the same voltage as mono-crystalline cells (that is a characteristic of silicon), however the current will be slightly less. Since power is the product of voltage and current, the power will also be slightly less for cells of the same area.
This slightly lesser current comes about because the cell boundaries present an opportunity for reverse current. The typical characteristic of a 72 cell panel will be: Voc ≈ 44 V and Vmp ≈ 36 V. Imp will also be around 5 A for a 180 W panel.
The efficiency of poly crystalline panels will also vary from one manufacturer to another but will typically be around 13.5–14 %.
Poly-crystalline panels use the same strategy as mono-crystalline panels, i.e. the use of bypass diodes. This also means that if part of the panel is shaded you will typically lose the output from half the panel. Shade is a real killer!
Same as for mono-crystalline panels.
The only real difference between a mono-crystalline and a poly-crystalline panel of the same power rating will be the area. The poly-crystalline one will be slightly larger — although you might find it difficult to notice any marked difference.
If you are comparing mono and poly-crystalline panels you won’t find any significant differences and there is no reason to recommend one over the other. The only difference might come about when you start comparing the manufacture’s and performance warranties. More about that in the next newsletter.
Note that for both mono and poly-crystalline panels, in recent years, we have seen the introduction of panels with different numbers of cells. The number of cell determines the output voltage and panels in grid connected applications that we see today are not bound by the requirements of battery charging voltage as panels were when solar energy was first used for electricity generation.
Amorphous means ‘without shape’. For both mono and poly-crystalline cells the silicon is ordered, i.e. the silicon atoms demonstrate clear crystal structure. In amorphous solar panels there is no crystal structure. The outcome is that the efficiency of the solar panel is lower and to achieve the same array output the area must be much larger.
The cost of electricity from solar panels should be considered in terms of price per unit of power or price per unit of energy delivered, i.e. $/W or $/kWh. Using either of these measures amorphous panels hold their own with both mono and poly-crystalline panels because, even though their efficiency is lower, they are cheaper to produce.
If you look at an amorphous panel you will not see the clearly defined cells in mono-panels, nor will you see the multiple crystals of the poly-crystalline panels. You will see a uniform, generally black, cover of silicon material. There will be many very fine lines across the panel. These are the cell boundaries and connections between cells which are not visible to the eye. However, the cells connect as do the cells in both mono and poly-crystalline panels to build up the voltage to that required for the panel as a whole.
Current and voltage
All panels, mono, poly and amorphous will have very similar current and voltage values if the panel has the same power, i.e. a 180 W panel would have a current of around 5 A, no matter what its construction. However, amorphous panels are typically lower power for the same physical size.
The efficiency of amorphous panels is increasing with greater research and development, but is still quite low compared with mono and poly panels. Expect efficiencies in the range
One advantage of amorphous panels is that they have a bypass diode built into every cell. These diodes are important in that they offer an alternate current route past a shaded cell if shading is present. This means that the output of the panel will only be reduced by the proportion of the panel which is in the shade.
This is often confusing as some retailers of amorphous panels suggest that they work in the shade. Nothing could be further from the truth. Any solar panel will be affected by the shade and if there is the possibility of shade on your site you should either get rid of the shade by removing the obstruction or move the array to a place which is free of shade. That said, amorphous panels are tolerant of partial shade and their output will only be reduced by the fraction of the panel which is shaded — just make sure that the fraction isn’t too high.
Amorphous panels may also tolerate increased internal cell temperature better than mono or poly-crystalline panels. This argument is also used by the proponents of amorphous to claim that their product is better than the others. I have seen reports suggest that this advantage is over-emphasised and other reports which clearly give an advantage to amorphous. In any case the advantage appears to be minimal, particularly when installation techniques are used to create a cooling air gap between the panels and the roof for all types of panels. This is generally the case because the array frame used to support the panels creates a natural air gap in practically every installation.
This is one aspect where amorphous panels struggle. Because of their lower efficiency they need a greater area to produce the same output as mono and poly panels. You may need as much as 2–3 times the area on the roof to produce the same total array power. This may not be a problem if you have plenty of available roof space with the correct orientation and shade free and for smaller arrays (say less than 1.5 kW) this might not be a problem on most roofs. However, when you consider the area required for, say, a 3 kW array or larger, most residential roofs don’t have enough space available. There should be plenty of space on commercial and industrial roofs so amorphous panels still have a place in the market.
Note that amorphous solar is much more flexible in its application than both mono and poly crystalline panels. Amorphous, or thin film, solar cells can be applied to a variety of surfaces and we are seeing amorphous applied to the roofing material itself.
When it all boils down, the type of panel that you choose doesn’t make that much difference. Amorphous panels will require a larger area, but if you have plenty of available roof space then amorphous will do just as well as either of the others. Remember if the specified output is, say, 180 W then you will get 180 W from a mono, poly crystalline or amorphous panel. The other things to consider when buying panels are the warranties.