Loading and Inverter performance

[Mickey]

I've bought qty 2 110w PV panels and need to think about optimum loading.    Do I connect say qty 1 or qty 2 Soladin 120?   By the way the Soladin 120 needs 24v so I need the two 110w PV connected in series.      Thanks  Mickey

[Ian]

Mickey - I think you have answered your own question.

The Soladin 120 comes in 4 different variants (and 1 of them operates over the range 18 - 30 v DC) - but as you say your unit requirs 24 v, then I assume you have one that operates over the range 24 - 50 volts. Note that the best range of voltage (under load) will be 24 - 40 volts and then the unit will try to maximise the power output using a very simple algorythm. 50 volts is the absolute maximum DC voltage you can present to the Soladin 120.

I do not have the specs for the Navitron 110 watt panels but on the back of the panel should be a sticker which states a number of numbers. The ones you are interested in here are (or similar words) "open circuit voltage" or Voc, and "peak voltage" or Vmax. (The actual words can be different but you should be able to determine the difference between the two numbers).

I will make some assumptions now - note that the panels will probably have different numbers. Lets assume that the panels say
Vmax = 18.8v
Voc = 23.8 v

So from these numbers you will need to put 2 panels together in series so the working voltage seen by the Soladin will be 18.8v x 2 = 37.6V DC. This is well within the working capability of the unit.

The other thing you have to look at is the V oc. If the open circuit voltages are added together (2 x 23.8 = 47.6 V) then this is still within the capability of the Soladin unit and you have a good balanced system.

Now, if the figures were different, say,
Vmax = 18.8v
Voc = 26v

Now, the normal voltage under laod would still be OK at 37.6v, but now the open circuit voltage is 52v DC which is ABOVE the safe working limit of the Soladin 120. It is a lottery whether you would want to get away with this. The chances are that it would be OK but maybe it would just blow the input stages of your inverter (say bye bye to your inverter). In this scenario, you are stuffed - because one panel on its own will not have enough oomph to keep the unit going, and 2 of them will blow it up. If this is the cae, you either need to mate your current panels with others of different characteristics, or find another inverter that can handle it.

I hope this helps.

Regards,
Ian

[Ian]

Oops - I think I missed one of your main points in the last post... that of whether you need to use 1 or 2 Soladin 120s to maximise the benefit.

Standing by everything in the last post - ie, making sure you have the right voltage levels and ensuring you do not allow the open circuit voltage to go above 50 volts, you should connect the series linked panels to 2 x Soladin 120s which are connected IN PARALLEL.

It is unfortunate that the Soladin 120 cannot be configured manually to make one inverter the "lead" and the other a "secondary". However, batch to batch variations in components are LIKELY to mean that when you have 2 Soladin 120s connected in parallel that one of them will automatically act as the lead and "steal" all the power available for itself. If the panels are providing more power than the one can handle, then the second one will synchronise and feed power into the grid.

As the power from the panels drops (say, due to cloud shade), ONE of the Soladins will drop out first. This means that there will probably be enough power left for the remaining Soladin to continmue feeding power to the grid.

When both Soladins are feeding power to the grid, it is likely that their MPP systems will fight each other. It is unlikely that anything untoward will happen but it may mean that you will see one or other of the Soladins dropping out without an apparent reason.

The Soladins are rated at 90 watts each. It is very unlikely (in standard UK weather conditions) that both panels will be supplying to the maximum. You may find in winter conditions that the optimum power output is achieved from only ONE Soladin being connected, so consider this option at times of continuous cloud or low angle incident sun.

I hope this helps.

Regards,
Ian

[Mickey]

Hi Ian,

Thanks for the very detailed replies - I've now got the picture.

I forgot to mention that the 110W PV panels have been ordered from Navitron and they have confirmed dispatch.  However no tech specs yet available and I'll ask Ivan what the Vmax & Voc rateings are.

Many thanks again for your help

Mickey

PS - 6 hrs later - the qty 2 panels have just arrived and it states on the back:   Voc = 22.0v,  Vmax = 17.50v.  Therefore, I can safely connect them in series to qty 2  parallel connect Soladin 120s ..   Also, according to the Soladin 120 spec sheet, the Maximum input voltage is 50Vdc.  All now looks ok and I'll order them and keep you posted

[furkin]

Hi again Ian,
As you know, i'm starting out with a 24v - 110w panel (or 2 x 55w) going through a soladin 120.

For future reference though, would you be kind enough to clarify the connections please ?

If I add another 24v - 110w panel and another Soladin later,,,,,, do I connect both panels into both soladins (where one or the other might act as master)  The soladins would be in a double socket or adapter.   I realise that your chats with Mickey would make this clear to people bighter than me !

cheers

[Mickey]

Hi Ian,

I made-up a temp installation as follows:

Qty 2, 110w panels – propped-up. S West facing, at  70 deg in the garden – connected in series to produce 24v
Qty 2,  Soladin 120 connected in parallel and plugged-into a multi way13 amp  mains distribution  block with a 13amp plug on the lead end.  This was plugged into the domestic ac mains via a power meter.

Weather:   High altitude clouds – giving defused, hazy but sultry hot sunshine.
Time: around 3:15pm

Performance: as measured on a 3 pin plug-in V, A,W,Hz  & Kwh meter.

Both  Soladins ON = 85w,  241v , approx 0.35a

Either Soladin only = 83w, 241v, approx 0.35a

So your prediction was spot on as it appears that, due to the weather conditions, the panels were not going flat-out.     There was not enough power to get both  Soladins Synchronised at the same time.

But I have a basic working configuration and can now plan a permanent roof installation

Overall the Navitron PV Panels and Soladins are impressive and well engineered

Mickey

[Ian]

Furkin - I put a diagram together and when I tried to attach it here, it was not accepted as it was too big. Sorry.

So I will try to explain in words.

Each item has a + connector and a - connector.

Series connection :
The - connection of the first panel is conected to the + connection of the second panel.
This now leaves the + connection of the first panel, and the - connection of the second panel free
Connect the + connection of the first panel to the + of the inverter; connect the - connection of the second panel to the - of the inverter.
You have successfully connected 2 PV panels in series to the inverter.

You can add more panels to the chain in the same manner; connecting the - of a panel to the + of the one next to it.

Parallel connection :
All the + connections of all the devices (panels and the inverters) are connected together.
All the - connections of all the devices (panels and the inverters) are connected together.
You have successfully connected everything together in parallel.

Series connected panels and inverters in parallel :
Take the panels and connect them together in series (as above); you now have the + of one panel at the end of the chain free and the - of the panel at the other end of the chain free.

Connect the free + of the panels to the + of inverter 1 AND inverter 2; connect the free - of the panels to the - of inverter 1 AND inverter 2.
You have now successfully connected the panels together in series and the inverters together in parallel.


Future expansion...

It would be easy to confuse here so I will try to explain a little more about my thinking...(but I bet I confuse anyway - sorry)

The Soladin is a great little inverter but it is a "small" inverter. It is designed to handle 90 watts of power. The "120" comes from its PEAK handling but it cannot run at peak for long before it overheats and throttles itself back. During its design certain assumptions had to be made and these are not user adjustable. The power tracking algorhythm operates between 24 - 40 volts. Any voltage below 24 volts fails to energise the unit. Anything over 50 volts blows it up. Anything between 40 volts and 50 volts will be handled as if the voltage was 40 volts (from a peak power tracking point of view). An inverter "manages" peak power tracking by sucking more current through or reducing current draw - with the objective of maximising the total power (from watts = volts x amps). The voltage resulting is whatever the panels can provide at that particular current; the inverter does not adjust the voltage, it alters the current drawn.

The Soladin 120 is rated at 90 watts. It has a peak power voltage at 40 volts. So it is designed to handle about 2.25 amps of current. (Note that the magic 120 number in inverter model number is, as near as dammit, 2.25 x 50 volts !)

So the maximum current the Soladin 120 can handle is 2.25 amps.

You should now be able to work out what is the maximum power output that can be taken by the inverter with different panel voltages - assuming the panel can provide 2.25 amps and most can!. What is really annoying is that you will rarely actually get the power output that the panel is rated at and this is all because of the basic design considerations of the inverter.

If we take the numbers in this thread as an example - from Mickey's posts. His 110 watt panel has an optimum voltage of 17.5 volts. So, if it is a 110 watt panel, then we can deduce that the maximum current it can provide is 6.3 amps when it is kicking out the full 110 watts.

Even if the 110 watt panel could energise the Soladin 120 (which it cannot because it needs 24 volts to do so), then the Soladin 120 could only "use" between 40 - 50 (from somewhere between the 17.5 volts working and 22 volts open circuit) watts of this 110 watt panel. In other words you can only ever get less than 50% of the panel capability squirted into the AC mains through the Soladin 120.

Now, lets look at what happens when reality kicks in. Because 1 panel will not energise a Soladin 120, we connect 2 of them together in series so as to provide 44 volts open circuit and 35 volts maximum power. The best we can get from a Soladin 120 in this case is somewhere between 79 watts and 99 watts (2.25 amps max). In reality, Mickey has found he only gets 80 - 85 watts from this thread - from a combined max panel output of 220 watts !

All of these figures are theoretical. No inverters are 100% efficient. The best (and including the Soladin 120) are in the low 90's in efficiency - so you could take 10% of the theoretical values off and this will be much closer to the actual power that gets squirted into the AC mains.

Now - to answer your question.... (I hope you have answered it already now....)

If you are really dedicated to the Soladin 120, then when you add another panel, feed it into a brand new Soaldin 120 which is totally independent of your existing panel system.

However, what I would do is to take my existing panels and any new ones purchased and feed them into a NEW inverter (not a Soladin 120). Exactly which inverter is dependent upon the power outputs of the panels, open circuit voltages, working voltages, and current delivery. In the grand scheme of things, inverters are generally chepaer than the panels driving them - so a new inverter SHOULD allow you to tailor maxium output from your panels and not the other way round.

Furkin, I do recognise that your 110 watts comes from 2 x 55 watt panels - which have different power characteristics to the 110 watt panels used by Mickey. Perhaps you could do the maths and see what the Soladin can squirt into the AC mains from your own panel configuration ?

Regarding the sizing of an inverter, I tend to suggest an inverter which is lower in power handling ability than the panels can theoretically provide - because most of the time in the UK environment the panels will not be working flat out. However, if the next inverter up (oversized) is not a lot more expensive, then there would be merit in going for that one as you CAN harvest more from the panels on a good day, but also, it allows room for future growth too.

Sorry for another marathon post...

I hope this helps.

Regards,
Ian

[Ian]

Mickey - the last post addressed to Furkin may be of interest.

Considering how much you have spent so far, I think the next spend you should consider is a new inverter.

So far, you have spent in the region of £800 on panels and around £300 on inverters. The inverters are holding back the maximum power delivery of your system.

Inverters cost around £ per watt. So you should be looking to spend around £250 to get an inverter which will allow you to benefit from the 220 watts of panels you have invested in.

Today, you are seeing 85 watts of the 220.

I think you are right that the weather on Wednesday 26th July would not allow full power output from your panels but I would not expect it to be a full 50% or more rduction. With weather like we have had and as you describe - i would expect to see somewhere in the region of 80 - 85% of full power output.

I recognise that you probably did not want to spend another £250 or so but, in my opinion, it would be money well spent as it will allow you to up your power delivery from 85 watts to 200 or so - doubling power output for £250.

I hope this helps.

Regards,
Ian

[furkin]

Many thanks Ian,

As usual, greatly explained,,, & just as usual, other readers will also benefit.

My initial reason for the soladin 120 - was price.  It allows me to get on the first rung of the solar ladder. I realise that Ultimately I have to strike a happy medium (poor woman) & that I/we have to weigh up the ratio of 'Outlay cost' to 'benefits' gained.

I already have the 120, power meter, cable & plug adapter - am just about to order the panel/s.

I don't have any figures for the 55w panels,,,, so knowing what i'm doing,,,, & if you know said figures,,, which would be the better choice - 1 x 110w or 2 x 55w ?  (Ivan has explained that the 110w has better connectivity !?)

When I do decide to upgrade, i'll get more advice from here first.

many many thanks

[Mickey]

Hi Ian - sorry for the late reply but I've been out  all day.    I understand your logic but the whole question of PV power and ac conversion  is very subjective. By this I mean it all depends on local circumstances rather than physical fact & formula.  I've tried the rationale approach and got completely lost.    One thing I keep coming back to is the theoretical maximum ( as stated by the panel manufactures) and what we could expect here in the UK at  >51 deg North. In my own case the local planning regs say I can put solar panels ( pv or water) on my roof providing I keep within existing contours.  This for me is 45 deg.    So you can see that my panels will be operating below max efficiency anyway.  All this does not take into question the winter/spring/summer/Autumn variation - so to cover this I can't adjust panel angles.

So I am going for a brute force approach  in that installing max power generation in the hope that it is sufficient over kill to allow the Soladin inverters to provide say around 100w average?  That's the question? Time Will tell? As always your thoughts appreciated.

Mickey

[Mickey]

A good session 28/7/06 - bright & clear sky all day.   

I set-up a trial rig in my garden again with , series connected,  qty 2 110w panels, aligned due south at 45degs off vertical connected to Qty 2 parallel connected Soladins 120.    The results at 13:00 BST ( high solar noon) as follows:-

DC = 32v,  DC current = 6.0 amps, AC volts = 242.00v,  AC amps = 0.63a, AC power = 150 watts, Hz =49.8, back of panel temperature = 42.8deg C

Readings were taken using a mixture of an AVO meter and a  3 pin plug-in performance indicator -plus a handheld temperature sensor.

I've been thinking about the max power as I have a potential generating capacity of 220w ie qty 2 x 110w PV.    I looked at the Soladin spec sheets and the excellent : http://www.zonnepanelen.wouterlood.com/index_uk.htm  .  Taking all this into consideration I make the following calculation:-

Total potential power generation = 220watts x .85 ( expected realistic PV performance) x 0.9 ( Soladin efficiency) .  This gives a practical max power I can expect as = 168 watts.

Conclusion - my reading today of 150watts compares very favorably with the theoretical max of 168watts.  The missing 16 watts can be easily explained away by, not ideal, panel inclination and cable losses as well as operating temperature degradation.

All comments appriciated?

Cheers Mickey

PS The next step is to run the trial again over the weekend and if the weather holds good I should be able to make a final conclusion, on my final set-up, by Monday

Good luck - Mickey

[Ivan]

Thanks for the feedback on a useful experiment. It has answered one of the questions I always wondered about - whether you could parallel 2 soladins on the same DC power source.

Watch out for shadows on the PVs, even a small shadow over once cell of the PV will bring down the power of the entire array. This may also account for some of the missing power.

Ivan

[Ian]

Mickey - Excellent detective work and great figures !

I agree with your conclusions and retract my previuos thoughts.

Ivan - I think you know your panels better than me - but surely the panels come ready fitted with a minimum of a single diode to mitigate the effect of shade on one panel bringing all the others down ? If not, then the installation of a diode would be a good idea and very low cost.

Regards,
Ian

[Mickey]

Hi Ian - thanks for your kind words.   I've just done a test on my evalution set-up and yes full or part shade on one panel shuts the lot down?  Mickey

[Bill H]

Mickey, 

Good news - sounds like a real result and pay-off for sticking with it !

I'm waiting for another panel from Ivan so that I can series-wire an 80 and 55 watt panel and get up to the trigger voltage for my single Soladin 120.  Will report results when the set up is in place.

Ordered the panel last week via the online store - so not sure if the order went through - fingers crossed !

best regards

Bill