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thataway



Joined: 02 Nov 2003
Posts: 20778
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
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PostPosted: Fri Jun 27, 2014 5:55 pm    Post subject: Reply with quote

I read last night that the panels are 10% more effective with an air space under for ventilation and cooling.
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Bob Austin
Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018 to Oct. 2021
Thisaway 2006 22' CDory November 2011 to May 2018
Caracal 18 140 Suzuki 2007 to present
Thataway TomCat 255 150 Suzukis June 2006 thru August 2011
C Pelican; 1992, 22 Cruiser, 2002 thru 2006
Frequent Sea; 2003 C D 25, 2007 thru 2009
KA6PKB
Home port: Pensacola FL
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journey on



Joined: 03 Mar 2005
Posts: 3593
City/Region: Valley Centre
State or Province: CA
C-Dory Year: 2005
C-Dory Model: 25 Cruiser
Vessel Name: journey on
Photos: Journey On
PostPosted: Fri Jun 27, 2014 7:53 pm    Post subject: Reply with quote

The comments here are interesting. What people are seeing is the way solar panels operate. Here's how solar panels output power and the consequences:

Pat, sorry to use your panels as an example. But you've given some numbers and a picture. Apologies in advance.

1. Heat decreases the solar cell efficiency. The rated output is tested at 20 deg C, 68 F. So if they aren't cooled by air flow or radiation and they get up to say, 100 F they lose power. And that's the cell temp, not ambient. Feel them when they're putting out power, they're warm.
.

2. They have to be pointed at the sun to put out maximum power. One may not remember trig, but the output falls off as the cosine of the angle between the sunliine and the perpendicular to the panel.. Looking at Pats installation, there's first ~30 deg of difference between the panel mounting from side to side. So, if one panel is pointed at the sun, the other is down by 14% (1-cos of the angle.) Also, the panels lie flat on the roof. To be perpendicular to the sunline they need to be tilted to the suns elevation. tis is the latitude (48 deg) + declination (23 deg at this time of year.) That's another 10% at this time of year when the sun is at its highest. In addition, Pat's installation is only getting max sunlight at noon, when the sun is directly overhead, say between 11 am and 1 pm. Again the cosine of the angle.

3. The solar panel rated power is given from flash tests and is at maximum power, which is for Pat's panels, 17 volts. By hooking them to a battery, you're reducing the power by decreasing the output voltage to 13.5 VDC. So those panels are 160 watt panels at battery voltage when cold. The current stays approximately the same. Solar inverters track the peak power point to run the panels at max output. The cheap regulators just turn the current on and off to maintain the battery voltage. That's what I have on the 25 watt solar panel which maintains the truck battery.

So Pat's numbers of 5 amps at 13.6 volts give 68 watts. Pointing those panels is one solution. Letting the breeze blow on them is another. Look at commercial solar farms. The panels are mounted on posts and track the sun. While we on boats can't do as well, we can do the best under the circumstances.

So, Pat's panels are doing what they're designed for at 68 watts.

Boris
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jkidd



Joined: 23 Oct 2006
Posts: 1609
City/Region: Northern, Utah
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Vessel Name: Voyager
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PostPosted: Sat Jun 28, 2014 7:05 pm    Post subject: Reply with quote

I've been experimenting with 4 Goal Zero portable panels that I take with me with the trailers or the boat. I have an Engel 45T Fridge. One panel will pretty much keep up with the fridge and two will keep the battery charged nicely. The only time I have to put all four out is when the weather gets bad. When I use them with the trailer they mount on a tripod so I can sit in the shade and move the panels to the sun. On the boat I have just been hanging them off of the boat some where. I made a board so I can put them on the bow and turn them at the sun. Of course at Lake Powell back in the backs of the canyons if your trying to hide from the sun there might be a small problem. You should be able to get enough time during the day to get the batteries topped off.








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