If I have a 1kW grid-connected system and I want to add panels to increase the output on cloudy days, do I need to protect the inverter from being overloaded on sunny days?
Extra Solar Panels
(26 posts) (15 voices)-
moz
MemberBut make sure that it's actual output vs actual input capacity, not rated output. Panels only give their SOC rated output in the lab, in real life you get ~80% of that at best (NOCT). But panels are sold based on SOC rating. So you can reasonably hook up (say) 1250W of panels to a 1000W inverter. Or just buy a maximiser to get the 10% boost from that and another 100W of panels (assuming that works with the current layout of your system. If you have (say) 12v/80W panels feeding 24V into your inverter you'll only be able to add pairs of panels in 160W steps).
You can possibly use even more if your panels are not optimally placed. The way to find out is to measure your peak output now and work out how much you can increase it. Ideally your system will record that but if not you will need to find a way to log the actual output if you want to go beyond the 80% estimate. Or do both - buy what extra panels you can now and a logging system that records solar panel output every few seconds and stored the peak. This time next year use that peak to decide whether you can fit more panels. There's probably even an ATA datalogger kit that would do this for you, or an Oatley Electronics one, or failing that even Jaycar.
Posted Wednesday 18 Mar 2009 @ 12:07:19 pm from IP # -
moz
MemberOh, and one other option is to add a reflector and boost your output that way. The further south you are the more benefit that will give you, and the less your risk of overheating your panels is. Since you probably only want a 20% boost, a white painted board or even a sunshade sail might be enough. If you're willing to move it twice a year, a winter reflector is a good way to boost effectiveness when insolation is low.
Posted Wednesday 18 Mar 2009 @ 12:09:56 pm from IP # -
rsigmund
MemberHey Moz,
Nice idea - it would be interesting to try that
You need to be careful here! Two points of caution...
1. Warranty (If your worried about that sort of thing)- A reflector may count as a concentrator of sunlight and therfore void the panel/s warranty
2. If the inverter ratings are already near their the Max limits you could perhaps exceed the allowable input voltage/wattage
Posted Wednesday 18 Mar 2009 @ 11:50:14 pm from IP # -
Sunshine
MemberI disagree with Moz that the panel output will generally be only 80% of the rated output.
I have a 3.06 kW solar PV system and have observed outputs of 3300-3600w on partially cloudy days, where the sun peaks in and out from behind the clouds. After 5 mins or so of continual sun, the output starts to drop, due to the panels heating up. I have only had the system for 3 months or so, I suspect the higher peaks will be even more common in the cooler months as the panel temperature will be lower, plus the angle of the panels are more suitable for the winter months, so I am expecting further higher peaks in the next 6 months.
As Rsigmund advises, if you have a fried inverter, the warranty will be most likely voided.
There is a program available for SMA inverters to work out what inverter/panel combination is suitable.
http://www.sma-america.com/en_US/products/software/sunny-design.html
It's a 37Mb file and Aust data file 12Mb is also required.
This program takes in consideration on where you are located in Australia, the tilt and orientation of the panels, plus the voltage outputs of the panels when they are in the temperature range from -10 to 70 degrees. This will show how much the voltage varies due to temperature.
For my system, the voltage variances due to temperature can range from 252v to 449v.Posted Thursday 19 Mar 2009 @ 12:03:26 pm from IP # -
petanque
MemberOn a cloudy day my system only gives 40-50% output.
I would imagine exceeding the capacity of the inverter by this amount on a sunny day is likely to cause problems.
Many panels are wired together to give an appropriate voltage.
So you may not be able to just put an extra 1or 2 panels in the array.
Posted Thursday 9 Apr 2009 @ 4:33:09 am from IP # -
rsigmund
MemberIf you are sure the extra panels don't exceed the input rating of the inverter as petanque said you still have extra things to consider...
If you connect the extra panels in series you will find the current limited to the lowest panels output e.g. if you go for lower wattage panels the current output will be limited to the output of the extra panel/s. This may mean the additional panels in this circumstance will not meet your expectations.
Posted Tuesday 14 Apr 2009 @ 7:10:33 am from IP # -
Lockie
MemberA reflector would be inpracical for my situation.
Let's say I were to add extra panels to boost output during the cooler months. Could I then put some form of regulator/limiter between the panels and inverter to avoid overloading the inverter during sunny periods?
Posted Wednesday 29 Apr 2009 @ 12:00:11 am from IP # -
rsigmund
MemberShort answer is yes-but
Any regulator between the panels and battery would increase the (resistance) losses.
You might be able to find 1Kw or 2Kw regulator but it would be very expensive.
And there might be other issues like standards
Posted Wednesday 29 Apr 2009 @ 12:17:05 am from IP # -
Sunshine
MemberLockie,
You may be able to add another inverter to the one you have. A lot of inverters out there allow other inverters (of the same brand make/model) to be plugged into the first one, hence upgrading rather than replacing your existing inverter. So if you want to go from a 1 kW inverter to a 2kW model, you may be able to just add another 1kW inverter...
There is no point in using the "extra" panels for only 6 months of the year. Waste of money, you will get better value using them every day.Posted Wednesday 29 Apr 2009 @ 9:55:12 am from IP # -
My Conergy inverter has all this in the manual.
*from memory* (probably wrong)
Conergy 1700
1500 W output
Up to 2kw of panels
Will dump excess of 1.5 as heat.My 1KW of panels normally sits on 750 - 850 watts on a clear day at midday.
I have seen 1150w just after rain, when the panels where cool.So a few more panels would be fine, and I would get their benefit 95% of the time, and for 5%, the inverter would dump the excess as heat.
Panels are hooked up in series, so adding more increases the voltage. There are voltage limits.
I'm not sure what happens if all the panels are not the same.
Paul
Posted Saturday 16 May 2009 @ 11:49:11 am from IP # -
I don't know anything about PV systems (besides the fact that my in laws' off grid house works fine) so reading all of these posts is very interesting.
The regular mention of PV systems spiking up to or in excess of their rated output after rain or cloud cover makes me think much more effort should be going into panel cooling - sounds like 10% - 20% extra power is available.
How much cooler would the surface need to be to give a decent performance boost?
Posted Saturday 16 May 2009 @ 10:57:40 pm from IP # -
rsigmund
MemberJohnB
If you can get hold of detailed specifications you should find a derating factor. I believe it a linear factor.
e.g -2w/C
Considering you have around 1000W sun energy per square meter it would take some cooling
me3neuralfibrecom
"I'm not sure what happens if all the panels are not the same" - The voltages will add together when connected in series, however the current in the string will be limited to the lowest wattage pannel.
Posted Sunday 17 May 2009 @ 11:33:37 pm from IP # -
Thanks rsigmund for that info. Is the -2W per degree C on a whole system or on a single panel?
If an 80W panel could gain back 8 lost W for a 4 degree surface temperature drop, say from 100 odd to 96 degrees, that can't be too hard to organise.
If a whole system only regains 8W for the same temp drop then I see your point - real gains would need serious cooling.Guess I should have been listening in Physics after all!
Posted Monday 18 May 2009 @ 6:40:33 am from IP # -
Sunshine
MemberRsigmund,
I don't believe the average temperature co-efficient is 2W/degree C.
Most poly/mono silicon panels are around the -0.47%/degree C value.
So a 170w panel, would lose around 0.799w/degree C, not 2W/degree C.Certainly, I have observed the extra power output when the panels are cool, say after a rain shower, the sun suddenly comes out. I have seen peaks of 3400w on a 3 kW system, when under a normal sunny blue sky, output is 2600w.
However the issue is, how to cool 18 panels and use way less than 800w of power to make it worthwhile? Certainly worth investigating.Posted Monday 18 May 2009 @ 11:12:37 pm from IP # -
ghostgum
MemberThe figure is not watts per degree C, but rather a fraction of the rated power per degree C. A quick search of the Internet gives figures of 0.5% and 0.7% per degree C. I suspect it changes between different panel types (poly-crystaline vs amorphous).
So at 50C you would lose 0.5% * (50C - 25C) = 12.5%. You lose another 6-10% in the inverter. Overall your output is about 20% less than the ratings on the solar panels. We are talking about the average output in sunny conditions, rather than the peak output just after the sun peeks out from behind the clouds and before the panel warms up.
Posted Monday 18 May 2009 @ 11:19:41 pm from IP # -
That'e very interesting.
Even at the lower 0.5% - 0.7% loss per degree C it's still a staggering loss.
Considering how much time, expense and energy the industry devotes to resarch and development to increase overall PV efficiency any 'free' performance gain should be persued vigourously.
Without ventilated 'heat sink' fins none of our modern computers' processors would function. Perhaps greater attention to heat dissipation would be cost effective. Surely an array over an open breezeway would run noticably cooler than the same array installed closely against a dark coloured roof.
The sunshine is free but the panels aren't, so anything that improves their output efficiency is a free gain - if you're getting an input tarriff too - so much the better.
Posted Tuesday 19 May 2009 @ 2:30:24 am from IP # -
Have you gone to the inverters website?
Posted Tuesday 26 Oct 2010 @ 6:39:44 am from IP # -
Inspector
MemberStretch, without knowing the spec's of the panels, it's impossible to determine how your system is wired. I suspect three parallel strings of 4 series-connected panels, with approx 180v open circuit and around 16A short circuit current. These values are based on 190W mono panels, but I'm not sure if the volts or current (or both) rises with 210w panels. They might even be poly panels (approx 36v and 8A ea). The system/array values should be marked on the switchboard label. Failing that, what are the spec's on the back of the panels?
That inverter has a max voltage of 200v I think, so adding one more panel to each of the three strings will exceed it's input voltage. You might need to wire up another string of 4 panels (assuming it's wired as previously described) and ensure the input current doesn't exceed the inverter's maximum input current (25A?).
By the way, if you're in NSW and you're getting the 60c tariff, adding 1 more panel to your system will mean you'll forfeit the 60c rate. I've seen a few sites where this has happened in the last 9 months or so.
Posted Tuesday 25 Oct 2011 @ 12:56:23 pm from IP # -
photonthief
MemberAdding extra panel(s) can be a cost effective way to gain additional output, or to compensate for shade or non-ideal alignment. Based on practical experience, reading of inverter manuals, and a fair knowledge of electronics, I offer the following points.
(a) In general, you will not damage your inverter simply because the nameplate power rating of the connected panels exceeds the nameplate rating of the inverter. Inverters are MUCH smarter than that, and simply 'throttle back' so as not to exceed their rated power. For example, in my case I have a 6kW Aurora inverter, with 8.4kW (44x190W) of panels connected. The inverter is completely unfussed, and precisely clamps the AC output power to no more than 6kW.
(b) While the inverter will look after itself re power, it is ESSENTIAL to make sure that the maximum open-circuit voltage output of the strings(s) does not exceed the maximium DC input voltage rating of the inverter. Here you need to know what you are doing, both to meet the Australian CEC regulations, and to stay within the inverter voltage ratings under ALL conditions. As the panel voltage increases at low temperature, the calculation is done for the worst case at the lowest temperature for your location, under open circuit conditions. For example, in Canberra, assuming -15 Degrees and 1000W/sqm, a standard 190W monocrytalline panel will produce around 49.8 volts, so a series string of 12 such panels could conceivably produce 598 volts. The absolute maximum rating for the Aurora PVI6000 inverter is 600 volts, so a string of 12 panels would (just!) be within ratings, though personally I would prefer to be conservative and opt for no more than 11 panels in series. Thus, my 44 panels are arranged as 4 strings of 11 panels each. As my particular inverter has two independent MPPT inputs, 2 strings of 11 panels each are connected to each input.
(c) The DC isolation switch(s) must be rated for the increased DC voltage or current resulting from an increased number of panels. Again, you need to know what you are doing, and preferably discuss with a CEC accredited installer.
(d) Do not mix panels with different power ratings or of different type, within the same series string. Subject to that, mixing different brands is usually OK, for example, all brands of 190W moncrystalline panels have simialr IV curves, and can in practice be mixed OK. Never parallel two strings having a different number of panels.
(e) Be aware that adding additional panels may invalidate your feed-in tariff.
In my view, if the panels are unshaded, it can make good economic and engineering sense to add panels to approximately 120% of the nameplate inverter rating.
In my case I went a bit further, to 133% (8.4kw panels for a 6kW inverter), for the following reasons and the system works like a dream. Firstly, I have significant shading in winter, and those extra panels boost my winter performance to what it should have been without shade. Secondly, part of my roof faces NW, and part faces NE, so I am unable to obtain an 'ideal' northerly alignment. However,, I have turned that to my advantage as follows. Half the panels face NW, and half face NE, with the two groups connected to independent inputs on the inverter. This 'flattens out' the power delivery over the course of a day. The peak output at midday is reduced somewhat so the inverter never sees an 8.4 kW peak. More importantly, the system output is very significantly boosted in the early morning and late afternoon, which is when you tend to want the most power. This is especially useful in a standalone system, but also nice in a grid connected system as well. Despite the fact that my inverter is only 6kW, and none of the panels face north, the yield is very respectable, typically giving between 40kWh and 56kWh per day at this time of year. It works for me
You need to look at the ratings for your particular inverter, and if you are not comfortable and confident in what you are doing, then don't do it.
Posted Wednesday 11 Jan 2012 @ 6:17:15 am from IP # -
Rafal
MemberI've installed simple black irrigation 19mm black hose with micro-jet sprinkles ( one per panel)on top of my panels. Build simple interval timer that gives me 10 second burst of water every 3 minutes.Connected by the 24 volt transformer to garden solenoid valve. It cools the panels and gives me extra ~200 watts on my 1.5 kw system. When the water if off on 32 deg day, I'm getting on average 1.12 kw and with the water on about 1.34kw.
I've calculated that this uses about 60-70 litters per day as I have all plugged in to basic timer between 11am-6pm.
I hope this info will help some of you guys improving your system efficiency.
RafalPosted Sunday 26 Feb 2012 @ 2:38:27 pm from IP # -
garysolar
MemberRafal, your idea sounds good in theory but the latest talk in solar is about the degrading of solar panels. As you may know most panels are guarenteed at 80% output after 20 years. What they are now finding is that one of main reasons for the degrading is when panels are cold and are heated quickly so by continually cooling and allowing the panels to heat you are actually speeding up the degrading of your panels. To what extend who knows but most manufacturers now are looking at how to combat this.
Posted Tuesday 28 Feb 2012 @ 10:17:01 am from IP # -
peter69_56
MemberJohnB said:
Thanks rsigmund for that info. Is the -2W per degree C on a whole system or on a single panel?If an 80W panel could gain back 8 lost W for a 4 degree surface temperature drop, say from 100 odd to 96 degrees, that can't be too hard to organise.
My system loses about 360w on a 26-32 degree day. I have 3080w and it drops to about 2720w or so due to panel losses. On cool days when the clouds partially shade heat but reflect the radiation I can make as much as 3100w for a short period.
The inverter operates at 97% efficiency and is a 4kw inverter.
Posted Thursday 22 Mar 2012 @ 11:22:41 am from IP #
