Ask a renewables installer...

Energetic_2

My previous thread ‘Solar Professional, here to help ‘ I started to provide info on the technical and pricing aspects of domestic renewables projects has been completely hijacked by altogether another argument so I have started again here. Microgeneration is a great way to reduce your energy bills, reduce your carbon footprint and attach an income stream to your home. I am happy to assist if I can.

My previous thread started like this:

Hi Money Savers,

I am a renewable energy installer. I want to stress that I am not here to try to get work directly or promote my company. I'm a bit frustrated that the national media has become so anti renewables so I am here to answer questions, promote renewable technologies generally and hopefully dispel myths that renewables are 'so 2011'...

Lots of people, including many people who had PV installed last year believe solar has been killed off by the current feed in tariff. I am here to tell you that this is NOT THE CASE. Returns are still in the 10 - 15% region as installation prices have dropped significantly. We've recently requoted a customer from early summer last year and the returns are better now than they were then.

I am happy to answer any questions about any renewable technologies or power management systems that can be used in conjunction with them. If I do not know the answer I will say so...

Cardew

A question about this thread in the Gas and electricity forum.

https://forums.moneysavingexpert.com/discussion/4326505

The poster in Eastbourne has a 2kWp array with a Sunnyboy 1700 inverter.

There has been discussion in this part of MSE about sizes of Inverter. It seems to be a trade off between the slightly higher efficiency of a smaller inverter against it limiting output on days of blazing sunshine.

Cynics would also say that a smaller inverter is cheaper, and most customers wouldn't realise the implications!

What is your view of that size of inverter for a 2kWp system way South in UK?

The total output in exactly a year is excellent at 2,140kWh - but could it have been higher with a larger inverter?

Yut_Man

Good question Cardew. I have a 3.33kWp system with a 3.1kWp invertor and have found my generation was capped because of this. but only for a few days in the last year and probably for only a hour or two at that. So I suspect the loss is actually very small. However when this does happen the invertor does become very hot. I guess the excess electricity needs to go some where. Is this correct?. If so is the excess heat that this generates going to shorten the life of the invertor. Is a higher rated invertor going to last longer than a lower one?.

jamesingram

Strangely UK MCS states(demands?) that installers should size inverter to 80% of total installed panel wattage. Most manifacturers suggest 90-110%.
I believe most other regs. usually defer to manifacturers instructions.

I like to size for maxium generation and generally size inverters via the manifacturers software ( 90-110%)
Clouding the water further, unfortunately the DNO G83 limit of 16A ( 3.68kW DNC) and the seperate 4kW FITs banding also create restrictive/inefficient design practice.

grahamc2003

Yut_Man wrote: »

Good question Cardew. I have a 3.33kWp system with a 3.1kWp invertor and have found my generation was capped because of this. but only for a few days in the last year and probably for only a hour or two at that. So I suspect the loss is actually very small. However when this does happen the invertor does become very hot. I guess the excess electricity needs to go some where. Is this correct?. If so is the excess heat that this generates going to shorten the life of the invertor. Is a higher rated invertor going to last longer than a lower one?.

I would think with that small margin between the inverter and the nominal panel output, the clipping would be rare, as you noticed. It's likely that the max output would anyway be moderated by thermal effects, with the likely high panel temperature at that time causing the output to be lower than the nominal max.

There is a small gain at the low end to offset (or perhaps exceed) the loss at the high end. The small rated inverter will synchronise at a lower open circuit voltage than a higher rated one. So while you're generating, a higher rated inverter will still be waiting for a higher voltage (and this at both ends of the day). A small gain, except when the weather is marginal and the smaller one just synchronises while a larger one wouldn't.

I'd expect the inverter to moderate the voltage away from the mpp when approaching it's higher output limit to keep the panel output within range. This would dissipate heat in the panels themselves rather than in the inverter itself. Only a small portion of the almost 1kW max falling on a sq m of panels gets converted into electricity, so a little extra wouldn't make much difference to the panels themselves.
So no, there is no 'extra' heat dissipated in the inverter - the inverter itself ensures the panels don't generate 'excess' electricity. At those conditions though, the inverter inefficiencies will mean it's producing a fair amount of heat, and heat is what causes the life of electronic components to shorten.

Yut_Man

Thanks grahamc2003, It all makes sense now.

Energetic_2

Hi Guys, thanks for the questions and responses, pretty much already answered but I’ll give you my take on it.

The rule of thumb for inverter /panel sizing in the UK is 80%. The reason for this is that the inverter is operating under peak more that it operates at peak (due to UK climatic conditions) and hits a better efficiency in that range of the power curve. You would not size like this in North Africa for example...

Interestingly our design software issues warnings outside of a range of 85-115% but inverter manufacturers have a larger range. I know of a system locally which was designed on SMA's own sunny design program is a 3.92kWp system on an SMA Sunny boy 3000HF inverter. This gives a 131% sizing factor but the system is performing brilliantly. The output is clipped on a sunny summer day but the annual figures are very impressive. The systems are at most likelihood of limiting during sunny cool weather in spring and autumn when panel efficiencies are highest due to temp.

There is no danger for the inverter in doing this provided that the DC input Pmax, Imax and Vmax are not exceeded and that you are within the Vmpp band. I have recently been approached by a client of ours to add panels to his system, it's currently a 2.5kWp system on a 2.6kw inverter, adding two panels would bring it to 3kWp. This is higher than I would normally push it but I spoke to Frons’s Tech who reassured me that no harm would come to the inverter. Whether it's worth doing financially is another question entirely.

The current G83 rules about 16A now limits systems to a 3.68kW inverter output but we are seeing no appreciable difference between the outputs of the 4000TL's installed last year and the 3600TL's installed this year. Oversizing the inverters seems to make little difference either.

The problem is that the limiting is very visible on the power output charts the inverter and the efficiency gains lower down the curve are completely invisible so people worry. The best advice I can give is to go for the best quality, highest efficiency TL inverter you can afford, as long as it's pretty close to the panel size it will work fine.

Martyn1981

Hiya Energetic, any chance you could have a quick look at this thread, and my post #14

https://forums.moneysavingexpert.com/discussion/4281581

perfectly happy for you to have a laugh at my expense, just interested to know if exceeding the max MPPT voltage is ok, bad or really bad!

Many thanks.

Mart.

Energetic_2

Martyn1981 wrote: »

Hiya Energetic, any chance you could have a quick look at this thread, and my post #14

https://forums.moneysavingexpert.com/discussion/4281581

perfectly happy for you to have a laugh at my expense, just interested to know if exceeding the max MPPT voltage is ok, bad or really bad!

Many thanks.

Mart.

Hi Mart

Best to keep within the MPPT band for maximum efficiency, there is a Max DC input power also, over this could damage the inverter.

The system on the thread you linked would be split into even strings of 8 panels each. This would halve the voltage and double the current going into the inverter keeping it in the MPPT range.

Not all MPPtrackers are created equal, rumour has it that SMA's was originally created for wind turbines and has/had deficiencies with PV. There was a spate of incidents last year all over the country of SMA inverters matched to Sanyo (now Panasonic) panels dropping down to a couple of hundred watts on sunny days from 3000W or more, it was due to minor amounts of shading from chimneys etc. activating a single blocking diode on a single panel. The high voltage of the Sanyo panels meant that knocking out 1/3 of the voltage from one panel was enough to shift the Voltage sharply away from the MPPT point and the inverter was unable to scan the range far enough to pick up the new peak. We had a couple of systems suffering from this. SMA developed optitrac which rescans the range at set intervals to ensure that the tracker is tracking the correct peak.

The micro inverters mentioned in the other thread mitigate the problem as they optimise each panel individually, as do the power optimisers. We’ve done only one system with micros but have avoided the PO’s as our feeling is if something goes wrong with them it would mean scaffolding etc, we would rather have easy access to the inverters.

dannyboyroberts

Ok a little background, it would be a 3kw, 30 degree pitch and the shading is only a problem for about 2-3 hours a day from November the 10th until Febuary the 10th, I have some pictures of what the shading would look like.

It's my neighbors roof, assuming that from december - febuary it is these pictures in reverse.

Because the panels are arranged in series, and one panel has been completely covered from December to January, in the morning, it would mean that the whole system would output nothing ? Or am I confusing things ?

What I'm wondering is how much money/kw do you think would be lost from this, and would it still be worth the time. Thanks a lot everyone

Oh and one last thing, the last picture says Febuary 2013, it should be January 2013










BTW, I'm newish to the forum, hi everyone :T

Martyn1981

Energetic wrote: »

Hi Mart

The system on the thread you linked would be split into even strings of 8 panels each. This would halve the voltage and double the current going into the inverter keeping it in the MPPT range.

Thanks E. It was the string 'bit' that concerned me, as the tech sheet seemed to suggest only one input. I thought that odd, but the max voltage being so, very, very high (top of head I think the bigger SMA TL's are 550V?) made me ponder if it was a single MPPT and single string inverter.

I really shouldn't keep looking for stuff like this, can't help it.

Mart.