Care to clarify that comment for those of us who frankly haven't got a clue. Or perhaps just me.
Not at all. They're very valid questions and I should be able to support any comments I make anyway. Especially a bold one such as this. Just hope you don't get bored. And I have to talk in metric so you may have to go to
www.onlineconversion.com if you want to check what I'm saying.
Let me answer this one first.
“I'm thinking at a PV panel of that size should be at least double the output, but that may be totally off base.”
OK IIRC at the earth's surface a we get about 1 kw of solar energy per square metre and in space it's about 1.33kw per sq.m. Therefore if a solar panel is about 1 sq.metre and is rated output is 80watts, it is said to be 8% efficient. If it is more like 130watts it could be said to be 13% efficient. This is a rough guide but you should get the idea.
BTW When I say if a solar panel is a certain size I mean the area of the cells on the panel not the length and width of the outer frame. There are two trains of thought here. One is to maximize the area used by filling the entire panel with cells so no space is wasted. The other is to give the individual cells a border around them of a reflective material to aid in cooling the overall panel and reduce the cell temps. More about that in a sec but you often see this on more efficient and costly mono crystalline panels.
So typically Amorphous cells are 8 or 9% efficient, poly crystalline are 13 or 14%. And mono maybe 17 to 19% plus. I haven't looked this up. Just what I remember from early study so I hope it's still accurate. I often have people in the solar industry say “why do you use the most inefficient panels” (probably because they have their own sales agendas). “Panel efficiency” as a term is only relevant to the size of the panel, not how much they put out or more importantly how they actually perform on the roof of a camper in offroad conditions.
“what is a temp de rating?”
Ok this may not sound logical but solar panels put out more power in more sunlight but they put out less power as the temperature increases. Crazy, I know. Luckily the solar energy is usually going up at the same time. But the big thing is that if you can lower a solar cells temp you can increase the output. Actually I talked to a guy here who is developing a solar mat that has glycol tubes running through it. The idea is that the panel is cooled slightly by the glycol and the heat is transferred to the hot water tank. So more efficient solar electricity and solar hot water in one mat. He's hoping to have one available for the RV market in the not too distant future.
(BTW if you are using polys or monos I always thought you should leave a space under them to allow airflow to reduce temps.
Anyway this is probably not critical to the system design of an Expedition truck, so don't get too wrapped up in it. An Expo trucks electrical system usually has very fluid needs and multiple charging sources. At the very least, an expo truck with solar will also have the ability to charge from the engine and / or possibly a small generator. So worst case scenario, if your solar can't keep up due to the conditions then you may have to run the genset or if you are moving on that day, the truck will charge your house batteries without any dramas. However if you are designing a remote area power system, say for a shack in Baja, things may need to be a little more carefully thought out. Actually if you aren't getting the rated output from a panel on your truck and you want to know why, cell temp de rating is one factor that needs to be considered.
So to calculate cell temp de rating
F temp = 1 - (y x (ECT day – T stc))
f temp = temperature de rating factor
y = power temperature co-efficient per degree Celsius (typically 0.5% for poly crystalline, 0.4% for mono crystalline, where as amorphous are less than 0.2% ).
ECT day = average daily cell temperature in degrees Celsius
T stc = cell temperature at standard test conditions in degrees Celsius
BTW as a general rule (from the Aust Standards) the average daily cell temp is the daytime average ambient temperature plus 25 degrees C. Tstc should be given on the solar panel data and on the BP panel I looked at recently it was 25 deg. C. That is also a general rule Tstc = 25deg C.
So to give you an example, on a 30deg C day (86deg F) with a 100watt poly panel
F temp = 1 – (0.5% x ((30 + 25) – 25))
F temp = 1 – ( 0.5% x 30) = 1 – 15% = 85% = 0.85
So theoretically your 100watt panel can now be reduced to 85watts through temperature. For the same conditions an amorphous panel might go like this
F temp = 1 – (0.1% x ((30 + 25) – 25))
F temp = 1 – (0.1 % x 30) = 0.97
Or worst case maybe F temp = 1 – (0.2% x 30) = 0.94
So the amorphous on the same day is only temp de rated down to between 94 to 97 watts.
To sum up, cell temperature is only one of half a dozen different factors to de rate for and consider when comparing different types of panels but still worthy of note. Remember factoring this into a design is far more critical in a fixed system purely dependent on solar and not a camper. If the solar on the truck isn't right, well it means you will just spend more on fuel. However I hope this backs up my original claim about an amorphous panel outperforming others when mounted flat on the roof of a truck out in the hot desert.
