PV panels and Temperature/Shading
i also think the cold air going over the panel can stop it working but cant give definate proof yet. maybe mildred can let us know.
If I understand correctly you are asking about the effect of temperature on PV panels. I'm really a
battery and charge-controller specialist, but obviously that entails knowing a fair bit about the
sources of RE power.
I'll save people the mathematics, but it is interesting. (Where's the "geek" emoticon when you need one? other boards have them...)
Temperature has two effects on the output from PV panels - on the current and the open-circuit voltage. Lets get the least important one out of the way first - Current. There is a very small increase in the possible current - much less than 0.1% per degree C. The exact figure depends on the physical chemistry of the silicon. You'll never be able to make use of this though because:
Voltage
drops by 0.2% to 0.5% for every degree C rise, again depending on exactly what the physical chemistry of the cells is. I have graphs of performance in this area - it's not linear, but good enough for general discussion.
Next point - PV panels get
hot. I've recorded temperatures of 30C above ambient. so what may seem to be quite small losses according to the figures above really mount up. Typically, in domestic RE installations, we're dealing with no more than 48V configurations so these losses can become critical. When you consider that with "proper" charge controllers I'm setting fractions of a volt to control charging to batteries and to switch between sources you can see why temperature does get crucial.
What can we do? I work on RE systems installed on buildings and stand-alones, not vehicles, so there are some extra things available - forced ventilation and liquid cooling.
Whilst one could come up with some interesting ram-air aerofoils on a roof of a van to cool above and below, it's beyond my knowledge. I'm sure there will be some ideas put forward.
I'd be worried about ripping the panels off the fibreglass roof with the wind.
Certainly I believe that mounting the PV panels flush on the roof is doing you no favours at all, other than structural and anti-theft ones.
There's some interesting development going on with liquid calcium chloride cooling - I'm not involved with that, but probably will be at some time.
What is clear, is that combined
solar hot water and PV electric panels, have, shall we say, a lot of inherent inefficiencies to overcome.
Don't believe what the salesmen say!
Whilst forced coolng, whether air or liquid, does take power to run, indications are that on balance you come out ahead by several percentage points. Similarly with tracker systems - the gain achieved by accurately following the sun far outweighs the power taken to run the motors to move the panels.
Not that you have to have motors - we've seen one extremely effective system that is just a big panel supported on ropes from a convenient tree, that is then moved around by guy ropes to the ground. The children at that installation have been well trained!
It hasn't come up in discussions here - unless I've missed it - but it's very clear that intermittent shading is a killer to output. I'm talking about dappled sunshine through tree leaves and clouds. It has a far greater effect than one might think - especially as the panels appear to be well illuminated.
It's not trivial to analyse exactly what's going on with PV panels. A simple
battery and diode system just sinks the voltage and you end up measuring the plate voltage, introduce any ordinary charge-controller and you're measuring the characteristics of that.
Can't remember who said it, but for non critical use the idea of buying a couple of cheap batteries and charging them as best you can, knowing that they'll only give limited service, will be the answer for many, especially non full-timers who can get the batteries home and put them on a 50 quid "smart" charger between trips (remembering to check the "water" level of course!).
Hope this has either helped or interested some of the readers.
Mild Red
