DIY solar panel: figuring out panel layout
And now for the juicy stuff… Yes there is no chance this will be approved for microFIT but it will work great for my future LED garden light system – but the main purpose here is to get a little more educated on the subjects of electricity and solar panel technology, to hopefully improve my decision making during the microFIT installation.
Today I laid out my 40 solar cells on the floor, just so I can take a look at them. As expected, they have some chips. 4 of them are chipped pretty badly. Another 4 are chipped noticeably but it is negligible.
Each 6×6 inch cell is expected to pump out around 3.8 – 4.0 Watts. All of them produce around 0.5-0.6 V. So that means each cell is pushing around 6.5 Amps on a good sunny day around noon.
I measured a somewhat chipped cell around 4 PM on May 10, and registered 1.7 Amps at 0.58 V = 0.99 Watts. I am hoping this is around 25-30% of the full potential and I will try again at noon, tomorrow if we get a sunny day.
So I am looking at the cells and a few questions popped up.
Where do I start?
First, I had to remind myself that power = voltage * current, or Watts = V * A
Series vs. Parallel solar cell wiring
Looking at each solar cell, there are two sets of contacts: two bands on the “sunny” side, and 12 contacts on the back side. Sunny side is negative and backside is positive. Electrons are moving away from the sunny side, creating a flow of electrons. Obviously, the cells need to be put on a wire string (tabbing wire).
Not being much of an electrical person, I asked the next logical question: how? Do I connect all sunny sides using the same tabbing wire, and all backside contacts using another tabbing wire? Or as suggested by most sources on the Internet, run the tabbing wire from the sunny side of the first cell, to the backside of the next cell?
I had to look up the difference between wiring in series vs. wiring in parallel. Essentially, connecting all sunny sides using the same wire string would be connecting all cells in parallel. The other scenario, where sunny side of one cell is connected to the backside of the next cell, is a series circuit.
In parallel circuits, we add current generated by each cell, while holding voltage steady. Example:
- 20 cells in a parallel circuit
- 0.5 volts per cell
- 6.5 amps per cell
- total volt output = 0.5 volts per string of cells
- total amp output = 130 amps
- total power output = 65 Watts
In series circuits, we add voltage generated by each cell, while holding current steady. Example:
- 20 cells in a series circuit
- 0.5 volts per cell
- 6.5 amps per cell
- total volt output = 10 volts per string of cells
- total amp output = 6.5 amps
- total power output = 65 Watts
As you can tell, on paper the output is the same. Why should we care?
Cell wiring depends on application
Without an inverter, we get direct current out of the cells – a steady stream of electrons. The next question we need to answer, is what is this panel going to be used for?
According to sources on the internet, one would need DC current of * 1.5 of the battery voltage. So to charge a 12-volt car or garden light battery, we need to feed 18 volt DC current from the panel to the battery.
That instantly eliminates parallel circuitry, which is a damn shame. I was reading this insightful paper about maximum power point tracking (MPPT) that inverters are supposed to do, and about scientifically observed efficiency of using parallel setup at the PV system level. That is all fine, but the same principles must apply to cells within a single panel. We are just working on a micro scale – cells in a PV panel are like PV panels in a grid-connected PV system. Conclusion of the paper is that even under perfect operating conditions, parallel setup came out on top in terms of power production.
I’ll pretend for a second, that 6% performance gain is immaterial to my garden light setup scenario. What isn’t immaterial, is parallel circuit’s inability to add voltage between cells. If I want to charge batteries in the backyard, my only option is to wire in series – or buy a $200 microinverter (I am not there yet).
Disadvantages of tabbing solar cells in series
On top of output efficiencies of the parallel circuit, we get disadvantages of wiring in series… Great!
First, the whole panel will be like an old Christmas tree light – if one bulb goes, so goes the whole light. If one of the cells breaks down, or tabbing wire gets disconnected due to aging, corrosion, or wear and tear, the circuit will open and there will be no joy at all. Zero watts for a long, long time.
Second, apparently cells of all (well, most) sizes produce 0.5 – 0.6 volts. But depending on their sunlight exposure, they can produce more or less current. Which is bad news for two reasons: 1) chipped cells have a permanent handicap, they will never produce as much amps as a whole cell; 2) bird goo and other exogenous factors will also reduce output at a cell level.
Both 1 and 2 above are really bad news for a series circuit of solar cells: the entire circuit will operate only at the amp (current) level of the weakest cell. Which means, if one cell gets half-shaded, I get 50% output drop at the whole panel level. But again, the only option, if battery charging is the goal, is to wire all 40 cells in series and get some 20 volts out – and then keep the panel clean and out of the shade.
This knowledge will be very useful during microFIT installation.
Why use parallel circuits
Solar cells even under ambient light conditions will produce 0.5-0.6 volts. So when there is no direct sunlight (cloudy day, permanent obstruction, or early/late in the day) panels still produce 0.5 volt output. There will be little current (amps will be low). In a parallel circuit, voltage is unchanged at 0.5 volts no matter how many cells are added to the string. But current generated by each cell is added. So if you have cells with no sunlight exposure, there would still be some output from the cells that are catching some light.
Net result is that power is flowing out of the panel, and output level does not dive to the level of the lowest producing cell (which is potentially zero). Panels with parallel circuitry handle regular or permanent shading significantly better, than a series circuit would.
But… in a commercial panel / grid-tie setup, we use inverters. Inverters are able to take high-amp, low-voltage output from a parallel circuit and flip it to AC current to match output of other panels. Low voltage isn’t relevant with inverters – at least this is my understanding of it, based on some reading. However, for my application (off-grid, garden light charging, without inverter), this is a problem.
I am looking forward to measuring advantages of a parallel circuit myself, with a soldering iron and a multimeter in the backyard.
Mixing parallel and series circuits
I’ll admit (again) that I am not an electrician and I am lazy to Google right now. What that leaves me with, is a semi-educated guess that if I put 20 cells on one series circuit, and the other 20 cells on the other, I will get 10 volts times 6.5 watts out of each, but when connecting them to the main panel bus in parallel, I would get 13 watts times 10 volts = 130 watts total. This is basically the same output, except 10 volts isn’t enough to charge a 12-volt battery. So in my garden light scenario, it won’t work.
In general though, it sounds like a good idea, hypothetically speaking, due to tabbing being a small wire that should not be used to carry entire panel’s output. Smaller tabbing wire should be kept short, and should carry each string only as far as the bus wire. Shorter strings should be connected to the bus in parallel. A wider bus wire should be used when power output is accumulated – otherwise we will be running a risk of losing more power to resistance. Remember analogy of water flowing through a pipe in one of the earlier posts. I’ll investigate this further before tabbing the cells together.
Next: what to do with chipped cells?
Armed with this parallel/series knowledge, I am looking again at the picture showing all cells (see above), and notice that 4 of the cells are chipped pretty badly. Worst chip, I estimate, took out 10% of the cell. Here’s some basic math:
- 40 cells * 4 W = 160 watt of capacity at a cell level (best case performance, of course)
- Requirement to maximize voltage, so we are in a string circuit
- In string circuits, current (Amps) falls to the lowest producing (smallest) cell
- Largest chip on the worst cell amounts to about 10%
- This cell will reduce panel output by 10%, so -16 Watts down to 144 Watts
- It does not make sense to keep a 3.6W cell (4W -10%) to lose 16 Watts of panel output. This cell is out.
Using the same logic,
- Cells with 5% chips are expected to produce 3.8 W output
- One cell with a 5% chip in a series circuit drops whole panel output by 5%, or 8 Watts. This cell is out.
- Three cells are eliminated in this category, leaving 36 cells in working pool
- Last cell eliminated was giving us 3.8 Watts but causing a loss of 5% from a 148 Watt circuit, or 7.4 Watts
So we arrived to 36 cells… some of them probably still have chips, but likely in a 2-3% range, which may reduce the output of the panel by 4-5 additional Watts, but will really screw its symmetry. I either lose 4 Watts by tossing a cell with a minor chip, or or lose 5 Watts by leaving chipped cell in a pretty array. So to keep things pretty, we will work with 36 cells.
For a 36 cell PV panel, there are basically two choices: 4 strings of 9 cells, or 6 strings of 6 cells. Again, this is a matter of personal preference and geographical arrangements. I am guessing that 6×6 arrangement would make more sense for a mixed circuit (3 parallel strings * 12 cells in series). I personally have a feeling that 4×9 arrangement is going to pay off… I’ve seen a lot more solar panels that are rectangle rather than square. Not to say that shape has anything to do with efficiency.
So now I know that I need some building materials, potentially some wood to use as a base. Each cell is 6×6 inches, so I need something that is at least 4’6″ feet long and 2″ wide.
I also need 80 feet of tabbing wire, and I only have 20…
Off to Home Depot, before they close it!