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Author Topic: solar panels ,,worth the time and trouble??  (Read 4844 times)
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« Reply #30 on: May 14, 2011, 10:12:27 AM »

  Between falling tree branches and hail, without some kind of laminate over them I forsee some cell damage to contend with. But poking round on ebay, wow, looks like about $20-$25 square foot, averaging about 5.5 watts output per square foot. On a 35 footer thats almost $9000 in cells, and with perhaps 60% max output (thinking parked most un-optimally) just over 1100 watts of max energy per hour. Quite an investment. But only if you could use the available power to full advantage, and "if" the system had a reasonable and well known life expectancy could you determine the pay off time.

  More fuzzy math? Say you average 60% output 6 hours a day, so roughly about 6.6 KWH of energy. To accumulate the cost of electric ($9000) from the utility, using $0.10 per KWH, your system would have to provide 90,000 KWH. If your $9000 system only averaged 6.6 KWH per day, every day, it would have to run 37 years to match the same rate from the utility.

  And of course, were doing this to have the luxury of not plugging into utility power, and also in which case what were really trying to do is not run the generator. Were likely going to have a Genny anyway, so its aquisition cost is moot. So say were looking at providing the same energy from the Genny, 1.1 KWH, and figuring my lil 8KW genny will burn close to half a gallon an hour no matter what, and disregarding maintenence, the cost would be around $12 day. To break even on fuel, the system would have to produce power at that level for 750 days. Just past the two year point.

  These are all simple assumptions. Parking the Bus fully east/west in a clear area would provide more energy, and surely we would get more than 6 hours per day, and both would increase the available power that could be used. But also, when were not in the Bus, all that power is going to waste, eating up time, etc.. About the only real advantage is not running the genny, but your never going to save one penny doing it. OTOH your genny isnt paying off sitting idle either, its simply another form of investment. Now if the Bus is your home, and your living aboard year round, and spending a lot of time unplugged, it would make much more practicle sense.

  Very interesting and thought provoking discussion.
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Sean
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« Reply #31 on: May 14, 2011, 02:04:20 PM »

... with perhaps 60% max output (thinking parked most un-optimally) just over 1100 watts of max energy per hour. ... Say you average 60% output 6 hours a day, so roughly about 6.6 KWH of energy.

Whoa there.  First off, there is no such thing as "watts per hour" -- watts are already a rate, so it is just plain "watts."

As I wrote in the post that Jim linked, the average energy delivered by a single watt of PV is 3 watt-hours.  So your nominal 1,100 watts of PV panels will deliver around 3.3 KwH, not the 6.6 you suggest.  These are well-understood and well published numbers; there is no need to be guessing at them on this board.  The math does work out much better if you conveniently double the efficiency of the panels, though Smiley

Note the average of 3 watt-hours per watt of panel assumes completely unshaded panels set at optimal angles, so on a coach the number will actually be a fair bit less.

FWIW.

-Sean
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« Last Edit: May 14, 2011, 02:06:32 PM by Sean » Logged

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« Reply #32 on: May 14, 2011, 06:31:18 PM »

  Is a panel rated at 68 watts, not going to produce .068 KW? I assumed it would work the same way as figuring the KWH for a 60 watt light bulb, ie; 60 watts/1000=.06KW, and if used for one hour would equal .06KWH. Or is that wrong?

  Or did I word it wrong? Ive read this stuff and thought I had a handle on it, but maybe I missed something. Ive read panels produce around 8 to 10 watts per square foot, so 350 square feet of Bus roof (curvature is roughly 10 feet edge to edge), you should have potential of 3500 watts. However, the panel I was using for calculation purposes was a flexible panel rated at 68 watts, and covered 12.4 square feet, producing about 5.7 watts per square foot. I downrated it to 5.5, and calculated that on 350 square feet of roof it might produce 1925 watts. But as a curved roof will never fully face the sun, I downrated to 60%, or 1155 watts, and rounded down to 1100. I made the assumption that if the roof produced 1100 watts per hour, that would equal 1.1 KWH of potential output. No?
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« Reply #33 on: May 14, 2011, 07:25:14 PM »

Before you waste any more time, go find a graph or chart showing the efficiency loss as the solar panel is inclined away from optimal to the sun, both in the vertical and as the sun tracks across the sky.

And then you'll see why you are having trouble finding coaches covered in solar cells.

Arbitrarily choosing efficiency numbers without sound research doesn't inspire informed discussion, it's just noise.

Sometimes, the place to start is those researchers who have gone before using good scientific methods, not internet bulletin boards.

happy coaching!
buswarrior

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« Reply #34 on: May 14, 2011, 08:11:19 PM »

  Is a panel rated at 68 watts, not going to produce .068 KW?

No, it's not.  At least, most of the time it won't, and it might never.

That number is for "ideal" conditions, meaning at high noon, facing straight up, at the equator, on a completely clear day, with cool air (70F) flowing both above and below the panel.

In the real world, you are not at the equator, and the panels, if they ever face the sun directly (almost never on a coach roof), only face the sun exactly once each day.  The rest of the time the panel is producing a fraction of its rated output.

Between this and the fact that "average" conditions include partly cloudy days, the average PV panel produces, as I said, about 3 watt-hours per day for each rated watt (in the continental US).  IOTW, a 100-watt panel can be expected to produce about 300 watt-hours of electricity each day.  Even under the most ideal conditions, meaning a tracking panel at the equator on a clear day, the angle of incident radiation is only optimal for a few hours per day.

-Sean
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« Reply #35 on: May 14, 2011, 08:23:54 PM »

If you are wanting solar because you are concerned about generator noise, why not sound insulate the generator? I've stood beside one that made less noise than a box fan.

If you want to keep your batteries up while you are away, that is a different issue, but could be tied into a low battery start of the generator to charge them . . . .

The last time I dove into solar panel research, the cost vs benefit was way too high for my situation.

As for the guy building his home system as he goes - that sounds like a fellow I used to work with. Building that solar system was a hobby for him. It worked fine as long as he was there to tend to it - but output suffered greatly when he wasn't tinkering with it.

YMMV
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« Reply #36 on: May 15, 2011, 07:29:02 AM »

Before you waste any more time, go find a graph or chart showing the efficiency loss as the solar panel is inclined away from optimal to the sun, both in the vertical and as the sun tracks across the sky.

Arbitrarily choosing efficiency numbers without sound research doesn't inspire informed discussion, it's just noise.

happy coaching!
buswarrior

  Perhaps if you had that knowledge, or data, you could share it?
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« Reply #37 on: May 15, 2011, 08:17:18 AM »

Free lunch is over, you need to meet folks at least part way down the road.

Sometimes, the place to start is those researchers who have gone before using good scientific methods, not internet bulletin boards.

happy coaching!
buswarrior
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« Reply #38 on: May 15, 2011, 07:47:44 PM »

Because this discussion has polarised into two diametically-opposed camps, let's hear from anyone in bus-land who's actually using a solar system (either PV or water heating) on their bus.   We need to know if it meets their expectations (and what those expectations are), how much it cost, how much it produces, is it home-grown or off-the-shelf, where they are (the sunny Southwest or the Great White North?), if it's an adjunct to or a replacement of another setup, etc etc.   This whole subject reminds me of the bumblebee  -  shouldn't fly in theory, but somehow it does.

Something relevant here is whether one wants more-or-less conventional loads such as RV air-conditioners and standard fridges, or whether one will accept less-conventional alternatives such as simple evaporative coolers and chest freezers used as fridges ("fridgers").   In other words, what fundamental change of lifestyle is acceptable?   What's OK for one person may be laughably deficient or excessive to someone else.

John
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« Reply #39 on: May 15, 2011, 09:38:13 PM »

For what it's worth, we find that solar panels(electric) work for us. We started out to reduce generator hours, which are not real cheap, and we decided that the panels would not be expected to do heating or cooling directly other than occasional use of the microwave.

We installed four 55 watt panels that are 13" wide, which allowed us to have a path between them for us to walk on the roof of the coach. We use a Xantrex 2500 inverter/charger and a low cost MPPT charge controller with temperature compensation and we normally have 500 lbs. of batteries. Do not omit temperature compensation; you will be disappointed, if you do.

This gives us nearly a week between generator sessions, which is ideal. When we are in hot country, we figure on plugging into shore power when parked. In cold country, we can get by with a generator session every three days using a propane furnace.

It's not perfect, but it's a good compromise for us. Experimenting is pretty easy from this point.

Tom Caffrey
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« Reply #40 on: May 15, 2011, 09:53:45 PM »

sounds like a small amount of solar panels to supply energy for the basic electrical demands is okay, but for the heavy duty cooling or heating we just arent there yet...... and to cover the roof with cells would be overkill on the pocket book without a decent return....sigh
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« Reply #41 on: May 15, 2011, 11:26:12 PM »

Before you waste any more time, go find a graph or chart showing the efficiency loss as the solar panel is inclined away from optimal to the sun, both in the vertical and as the sun tracks across the sky.

And then you'll see why you are having trouble finding coaches covered in solar cells.

Arbitrarily choosing efficiency numbers without sound research doesn't inspire informed discussion, it's just noise.

Sometimes, the place to start is those researchers who have gone before using good scientific methods, not internet bulletin boards.

Free lunch is over, you need to meet folks at least part way down the road.


  In this entire thread youve added NOTHING to the discussion, your only comments were directed solely at me, and they are belittling and sarcastic. And its not the first time. But it IS the last time. Sir, from here on out, either put me on ignore, or simply ignore me, and I will do the same.
  
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« Reply #42 on: May 16, 2011, 12:45:32 AM »

Thanks for all the input from everyone ,I've been playing around with solar stuff on the bus for about  a yr and a 1/2..I started out with 2 100watt panels and a mppt controller ,3 rv/marine 105 amp hr batteries ..a 1000 watt inverter ,,,found out the batteries and the inverter could not keep up...now I have 7 100 watt panels on the roof ,,a 3000 watt inverter ,and 8 6v gel cell batteries ,,40 amp xantrex controller/charger ,,now after 2 grand kids and 10 days of sitting in the yard ,,everything works ,runs just fine ,frig ,lights water pump ,tv and I even made some coffee yesterday ,,,by about 3pm the batteries are charged up and the controller is on float,,we have not used the generater at all ,,no a/c,,I know I don't have enough power to run them,,As for money invested ,,I got most of the solar stuff from ?raigslist and ?bay ..if you shop around ,there are some pretty good deals out there,,I got the panels for 150 ea,,the batteries came from an elec contractor, used cell tower Deka 6v gel cells 35,dollars ea,,the inverter 120 dollars,,now hopefully ..I'll only have to run the gen for a/c..none of its very high tech ,,but it seems to be working for me ,,anybody else using solar for backup? Swap some ideas on differant ways to make it work better,,One thing that I have learned ,,use big wires from the panels to the controller and batteries ,,fuse everything ,,,still in the learning phase,,,thanks ,pete
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« Reply #43 on: May 16, 2011, 07:04:34 AM »

Sorry that my advice as to your next steps have been interpreted as offensive.

Perhaps in much the same manner, I have interpreted your posts as someone who is looking for answers without wanting to do the work, instead letting others invest the time to find them for you.

So, now we may both go away chastised and misunderstood.

happy coaching!
buswarrior
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