The panels get hot and drop efficiency . Most panel supplies have a chart where output fills off as heat rises . Why there should be an air gap under panel . At 28* the panel can lose efficiency around 14% . Take this into account when fitting ? Allow 20% more solar . This doesn't take cloudy day either !
A hot topic :lol: The gap under solar panels is sort of important but mostly when the panels are mounted at an angle like on a house roof. There the cool air can enter at the bottom and flow out the top hopefully dragging some of the heat away from under the panel at the same time. This way the panel is cooled on both sides. On an RV roof the panels are flat mounted. Solid panels have an aluminium frame to hangs 25mm to 35mm below the back side of the panel. This traps the air under the panel, heat rises so the hot air remains against the underside of the panel and there is nothing to make that air move away from that area unless a wind is blowing in the right direction. This means the panel will only be cooled by ambient temp from the top. Panel temp is measure inside the panel at the actual solar module, not at the glass top but a reasonably accurate temp can be measured by sticky taping a thermocouple to the underside of the panel some where around the centre. Panel internal temps can rise to more than 70*C, the STC (standard test conditions) marked on the back of the panel are measured at 25*C so an adjustment to the advertised panel output needs to be adjusted down by 0.46% for every *C above 25*C. 0.46% doesn't sound like much but with the panel internal temp at 70*C that's 45*C above the test temp so the percentage is now 0.46% x 45 = 20.7% The other bit of information given is the Isc (short circuit amps) only drops by 0.06% per *C above 25*C, so this means the big drop is in the voltage where the maximum amps are produced drops a lot in comparison to the amps. Watts = voltage x amps, the panel is sold as "x" watts so that relies on the voltage being high x the amps being high, say 17.8v Vmp x 5.62 amps (Imp) = 100w, the advertised panel output What does all that mean? 17.8v reduced by 20.7% = 17.8 x 0.207 = 3.6846v = 17.8v - 3.6846v = 14.1154v. So if the point the panel makes the most amps is 14.2v then getting the battery up to 14.8v will mean the amps are a lot less than the 5.62 amps advertised.
A rather complex explanation as to why solar doesn't seem to work very well with flooded cell lead acid batteries when it gets real hot. It also goes a long way to explain yet another reason why lithium batteries seem to charge so much faster than lead acid batteries, 14.2v is still plenty to fully charge a lithium battery so the full amps available from the solar can go straight to the battery.
T1 Terry
-- Edited by T1 Terry on Monday 24th of April 2017 02:32:26 PM
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hi As Aus Kiwi said Yes u do loose some wattage to temp just add more solar eg 1x 100watter My mppt controller gives between 13% extra amps and a lot more if I have accidently run the battery right down low. This is living North Qld . Weather 30-37 c deg , high humidity . The humidity acts as a extra insulation and adds to the high heat also. Personally 2x 200watts & [2x] 120ahr batt never a problem in hot weather . But I am over done if you considered my low electrical loads .
-- Edited by swamp on Monday 24th of April 2017 02:45:10 PM
-- Edited by swamp on Monday 24th of April 2017 02:45:39 PM
-- Edited by swamp on Monday 24th of April 2017 02:46:29 PM
Great info Kiwi, Terry & Swamp.
This is exactly what I needed as I'm about to place an order for my panels.
Originally I was going to order 4 x 300w panels but now it will be 5 x 300w. The extra panel will be to offset any losses due to weather changes.
It may seem overboard but I need a good supply of electricity for air con and my computers. All else is more than manageable.
Thank you.
ps: will pose another question re solar in another post.
I'm assuming this is for an RV and not a fixed house set up. 1500w is a decent size array but I'd steer away from big panels like the 300w units in preference to 15 x 100w panels for a number of reasons:
1) Large panels have a large heat absorption area, flat mounted they would be very hard to keep cool enough to give an acceptable output.
2) Shade across 1 x 300w panel results in the loss of 300w, across a 100w panel results in 100w loss
You can fit a lot more 100w panels than you can 300w panels so spots where shading is likely can be left empty or used if the shading will only be for part of the day, like beside the air con, one side works in the morning and the other side in the afternoon.
3) 100w panels are much easier to handle, easier to mount and cheaper if you break a panel on an over hanging tree branch
There is quite a bit more wiring involved but that is a one time thing, the benefits are for as long as you use the system.
T1 Terry
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I'm assuming this is for an RV and not a fixed house set up. 1500w is a decent size array but I'd steer away from big panels like the 300w units in preference to 15 x 100w panels for a number of reasons: 1) Large panels have a large heat absorption area, flat mounted they would be very hard to keep cool enough to give an acceptable output.
2) Shade across 1 x 300w panel results in the loss of 300w, across a 100w panel results in 100w loss You can fit a lot more 100w panels than you can 300w panels so spots where shading is likely can be left empty or used if the shading will only be for part of the day, like beside the air con, one side works in the morning and the other side in the afternoon.
3) 100w panels are much easier to handle, easier to mount and cheaper if you break a panel on an over hanging tree branch
There is quite a bit more wiring involved but that is a one time thing, the benefits are for as long as you use the system.
@T1 Terry . . . yes you are correct, they will be mounted on bus/rv but I suppose I'm planing my set up in a less conventional way and I shall explain it in my next post.
-- Edited by Dr Sable on Wednesday 26th of April 2017 04:48:58 AM
-- Edited by Dr Sable on Wednesday 26th of April 2017 04:49:23 AM
I agree completely with Terry about the size of panels More smaller ones are better IMHO. Big ones like 250-300W are also more prone to glass cracking from flexing too, some people have found, as they are not designed for mobile use ! They are certainly hard to handle fitting to a roof.
Bigger is not better in this case. Perhaps an in-between size would suit.
Jaahn
-- Edited by Jaahn on Wednesday 26th of April 2017 09:21:38 AM
Wont it be great when the manufactures use aluminium extrusion (for the side bracings) WITH SOME HOLES IN IT!!!
Requires the brain being in gear before drilling the holes, seen more than one where the hole was drilled into the side of the glass panel resulting in a panel that no look so good
25 x 70 x 1.6mm aluminium angle makes a great mounting bracket. Glue the 25mm side to the roof and rivet nuts in the solar panel so the bolts go through the 70mm side of the angle and tighten into the solar panel. Placing a bolt very close to each end allows the panel to be tilted up using the end bolt each side as a pivot allowing for wiring inspection/testing and cleaning the mud wasp nests out.
T1 Terry
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They actually make the ally extrusion with nice big slots (gets punched out @ manufacture) which would be more suited to panels. But never seen the extrusion on panels. Oh well, some lead, some follow.
Hi all, just a quick update.
I cancelled my order for 5 panels just in time.
I think I need to re-prioritize what size of battery bank I will need to run a fridge, aircon and pc's.
5 x 300w panels may not be the answer.
Aircond's are NOT normally viable with solar.
Thnough my 4kw panels on house runs our 7.2kw Inverter unit.
Hence the reason most of us older travellers run a Genny big enough.
I have 3.8kva. (Overkill. used for other things too)
2.4 is comfortable for most.
340w of panels with 2 x 110a AGM's ran my van apart from Aircond.
But we live, travel. ONLY in tropics and Territory.
Get a LOT more advice b4 going further I reckon mate.
And get power to suit. NOT want you want in total power.
But what you'll NEED on average usage at any time.
There's a huge difference.
BIG panels DO crack.
i've stuffed 85w on corrugations yrs ago.
Mount flat. and 100\110w size MAX.
Ambient temp and radiation from sun can be much hotter .. Although they "usually" charge your batteries by the time heat builds up . Ours is fully charged between 10!/ midway !! I may fit another battery and use more at night ? Less overall battery drainage ..
In the dark and distant past air con required a generator, but we are in the 21st century now and that is no longer the case. There are a lot of people out there living full time in their RV running the air con off solar. Battery chemistry is the key, you need to actually be able to get the batteries full using all the solar or combined charging output you have available, that is charged to 100% SOC, not 70% before the battery voltage rises to the point the solar regulator needs to taper back the charging amps to stop the voltage running away. The point a lead acid battery reaches 14.4v is not fully charged, it is the point where absorption charging starts and that takes a lot more hrs to get the lead acid battery to 100% SOC than there are sun hrs in a day. The longer it takes to get to the absorption stage voltage point the less likely you are to get the batteries anywhere near fully charged, this means there is no spare solar available to power an air con until well into the absorption stage. With the right choice in battery chemistry the air con load can be shared between the solar and battery until the air con cycles off, then all the solar goes to charging the battery. This process can continue all day and without the problem of current tapering to control battery voltage at 70% SOC all the solar available goes into recharging the battery, right up until sun down.
T1 Terry
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In the dark and distant past air con required a generator, but we are in the 21st century now and that is no longer the case. There are a lot of people out there living full time in their RV running the air con off solar. Battery chemistry is the key, you need to actually be able to get the batteries full using all the solar or combined charging output you have available, that is charged to 100% SOC, not 70% before the battery voltage rises to the point the solar regulator needs to taper back the charging amps to stop the voltage running away. The point a lead acid battery reaches 14.4v is not fully charged, it is the point where absorption charging starts and that takes a lot more hrs to get the lead acid battery to 100% SOC than there are sun hrs in a day. The longer it takes to get to the absorption stage voltage point the less likely you are to get the batteries anywhere near fully charged, this means there is no spare solar available to power an air con until well into the absorption stage. With the right choice in battery chemistry the air con load can be shared between the solar and battery until the air con cycles off, then all the solar goes to charging the battery. This process can continue all day and without the problem of current tapering to control battery voltage at 70% SOC all the solar available goes into recharging the battery, right up until sun down.
T1 Terry
"right up until sun down."??????
Seems we have magic panels, where light intensity on the panel is not a factor!!!
-- Edited by oldtrack123 on Sunday 14th of May 2017 09:42:14 PM
I did say "all the solar available goes into recharging the battery, right up until sun down." at no stage did I mention that the panels would be putting out the full output from sun rise to sun set, what I did say was the controller would not be blocking any of the potential charging current due to the inability of the battery to absorb that current until the battery reached 100% SOC.
I don't want to have a head to head battle with you Peter so close to the loss of your wife, my thoughts are with you and your family.
T1 Terry
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Don't feed the trolls . Proof read everything !! Dot the T's etc !! Lol
Yes & make sure you actually know what you are talking about, especially on technical subjects & is worth reading & really contributes something to the subject
-- Edited by oldtrack123 on Tuesday 16th of May 2017 10:51:48 AM
Don't feed the trolls . Proof read everything !! Dot the T's etc !! Lol
Yes & make sure you actually know what you are talking about, especially on technical subjects & is worth reading & really contributes something to the subject
-- Edited by oldtrack123 on Tuesday 16th of May 2017 10:51:48 AM
Hmmmm....... I've had a rethink and won't bite back, my thoughts are with you Peter.
T1 Terry
-- Edited by T1 Terry on Tuesday 16th of May 2017 02:34:21 PM
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More smaller ones are better IMHO. Big ones like 250-300W are also more prone to glass cracking from flexing too, some people have found, as they are not designed for mobile use ! They are certainly hard to handle fitting to a roof.
