Hello, I built a small carport for my electric vehicle (Renault TWIZY). I fitted it with solar panels as the roof and side cladding. The used panels were cheaper than sheet metal at €10/m². The entire carport is covered by 8 panels rated at ~350 Wp each. The panels are mounted as follows:
I can post a picture later this evening.
Now to the problem: although it faces south, there is a house wall to the east, and to the south-west there is a large oak tree (~90 cm trunk diameter), which also casts shade.
As I don’t expect too much output, I initially bought an MPPT 75/15, which means I can connect a maximum of 2 panels in series.
So my question is: how should I split the 8 panels into 4*2?
That was a mistake! If possible, return it! You can then buy more suitable controllers there. To select a controller, the module specifications and the system voltage are required.
Most modules of this type have an MPP voltage of around 33V and a Voc of around 36V, which increases by at least 10% in freezing conditions. With a series connection, you therefore get at least 80V. That’s too much for the 75/15! You could only connect one module to it. The output current is a maximum of 15A. The current an MPPT controller can deliver depends on the module power and the system voltage. With a 12V system voltage, 14V is reached, allowing a power output of 14Vx15A= 210W. That isn’t even enough to utilize the rated power of a single module. With a 24V system voltage, it would be twice as much. The problem is that you cannot connect the modules in series. And in a 24V system, the module voltage is too low for a parallel connection. The output drops as soon as even a small part is shaded.
Thanks for the information, but I’m already aware of all that. I already have five systems up and running.
I don’t know the panels’ specifications off the top of my head, but 75 V is enough for two panels in series; I’ve worked it out. The panels are used and already a few years old, and I believe their Voc is 28 V.
I can’t return the MPPT either, as I bought it second-hand for €20.
As I said, I’m not expecting any yield from the system. The panels have been installed for a few weeks now and I have. So far, I haven’t seen a single panel that has been completely illuminated by the sun.
I see the panels more as roof covering that can still convert diffuse light into a few watts, particularly during the winter months.
In the summer months, I don’t really need any additional output, as I can’t consume any more in my island-like system.
I would measure the modules individually at the time of the estimated maximum output, preferably with a small load.
Afterwards, I would arrange the module pairs so that the four combined voltages are as close to one another as possible, allowing the solar controller to set the optimum operating point—at least at that time.
That’s already a good plan.
However, there probably isn’t really such a moment, since the sunlight moves across the panels throughout the day.
At the moment, I can’t say anything yet, as the panels are completely covered by the tree canopy.
In winter, there may be moments like that because of the low sun and the leafless tree.
But that shows that it might be best to arrange the pairs of modules vertically.
No, I have to disagree there.
It’s true that the Twizy is charged from the mains (via a Schuko plug), but I’m at work with the car from 7:00 to 17:00, so charging directly from solar power isn’t possible. The Twizy is automatically charged in the morning, either from the battery or, if little solar generation is expected, from the mains. That way, the battery is charged “warm” when I drive to work in the morning.
Besides, this isn’t about how the vehicle is charged.
Here’s another photo of the carport. The photo was taken on 14 September at around 17:00.
The upper panels have the same orientation and number as the lower ones.
As far as I can see, there are two (reasonably) sensible ways of wiring them.
Either the two panels in each row that are directly next to the house wall could be connected in series, along with the two panels on the left in the lower and roof rows. That way, around midday, when the sun is shining directly parallel to the house wall, there’s at least a chance that the panels directly next to each other will receive sunlight together.
The other option would be to connect the two horizontal panels in series, along with one panel from the lower row and the panel directly above it in the roof row.
However, I think that ultimately it won’t matter how the panels are wired, since one panel in each series circuit will always be in the shade, meaning that any yield will only come from indirect irradiance. Hence the relatively small MPPT.
Just as a reminder, the panels were an inexpensive way of roofing the carport; even sheet metal from a DIY shop costs more than the panels at €12/m².
If the whole thing still produces a little energy in winter, I’ll be completely satisfied.
if you can connect all the modules in parallel to one or more MPPTs, that should cause the fewest problems. However, to get the maximum output, each module needs its own MPPT, so you’d have to install PV optimisers!
I meant to provide the module data afterwards. However, my memory let me down. Contrary to what was stated above, they are not 350 W but only 200 W.
The exact designation is
Hyundai HiS M200SF.
Vmppt = 26.7 V
Voc = 33.6 V
Imppt = 7.47 A
Isc = 8.19 A
As the expected yield will be fairly low, I would like to keep the costs as low as possible. Multiple MPPTs or optimisers are therefore out of the question.
Have a look at how much a Hoymiles HMT or HMS 2250 costs.
It might be better to evaluate each module individually in view of the shading. But bear in mind that x solar controllers draw x A of quiescent current! What about microinverters in general?
The HMT can have 6 modules connected (3 phases), while the HMS can have 4 (1 phase).
When it comes to those modules and shading, I’m rather pessimistic about the power they’ll generate. These old 60-cell modules only have a single bypass diode. Regardless of the type of shade, their output always drops to a minimum. With modern half-cut modules, a little shading doesn’t make much difference. Even the 75/15 won’t reach its load limit in that case.
I’m speaking from experience here—unfortunately, the old modules aren’t much use in the shade, except as rain shelters.
So the modules are divided lengthwise into three strips.
But yes, the output won’t be particularly impressive, so there’s no point in buying any other controllers either. And Hoymils would be no use to me at all, as it’s an island-type system and the electricity can’t be used during the day. The investment in the small MPPT and the cabling is already more than enough.
Do you have a system with battery storage at home? If so, you could perhaps make use of that as a starting point.
But the shading could make things difficult. Your carport system as an island system isn’t really powerful enough for that, although it’s better than nothing. Connecting them in series is problematic. A series-parallel configuration might work, but it would be best if you recorded your consumption and generation for a year and then optimised things step by step.
Given the installation conditions, you can’t really expect much. There probably won’t be any alternative but to connect two groups of modules in parallel and then connect them in series. The series connection is important so that the battery voltage can be reached with fully illuminated substrings whenever possible.
The carport system is not intended to operate as an off-grid system. It is intended to expand the existing island-like system as its sixth part. The system has a 15 kw/h battery and already has 5 MPPTs with ~3.2 kWp.
I hadn’t even thought of that option. Until now, I’d only considered four fully populated 2-module strings. But that’s another possibility too: four modules in parallel for each pack, and then connecting the two packs in series.
Which option do you think would be more efficient given the circumstances?
