S3E10 – Solar Power Revisited

After four nights camping then a night at Sue’s parents to recharge the trailer batteries we are back at the State Forest campground for another five nights.

I was disappointed we had to leave to recharge the batteries as our experience last winter was that one sunny day would easily replace the energy we consumed over a 24 hour period. Last week, despite 3.5 of the 4 days being clear, we only able to offset about a third of our energy use with the sun.

It turns out that as with most technical issues, you have to do the math. I added the 100 watt solar panel just before we left for our trip out west to Texas and New Mexico. Our “solar” experience was in the desert – clear, dry, 5000′ altitude, no trees to shade the roof of the trailer. Pretty much perfect solar energy conditions. Also we were camping off the grid the first week of April at Big Bend National Park so it’s latitude of 29 degrees means the sun is 66 degrees above the horizon at noon. Here in Southern Florida at latitude 27 degrees the sun angle is 51 degrees in the middle of February. That angular difference means the horizontal solar panel got 20% more energy on our reference trip last April. Another factor is that April 5 is 12% longer than Feb 15 at this latitude. So date and location differences reduce the available solar energy by about one third.

The effect of the trees here is hard to quantify. I try to get the trailer in the sunniest spot but some shading is inevitable over the course of the day. Just a little shade kills the panel output so it’s not hard to see how here in Florida in the February we are getting only a third of the solar battery charging we got last April in Texas.

I anticipated camping in partial shade and built a nice aluminum frame to support the solar panel on the ground at the best angle to catch the sun and have a 25′ extension cord to give some ability to place the panel in the sun. We never had to use the frame last year in Texas and it was a pain to store so of course I left it home this winter. I built another cheesy plywood one a few days ago and we are using it now to see how much we can improve our solar performance by 1) keeping the panel in a sunny spot and 2) reaiming it every couple of hours as the sun moves from east to west. Compared to a flat (horizontal) panel one aimed at the sun at noon on this date and latitude will produce 30% more energy. The advantage is even greater as the sun moves lower in the sky. An experiment at 4:00 PM yesterday showed the panel aimed at the sun put out 5 times more energy than when it was flat on the ground.

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Our first day (100% clear sky) with the portable panel we were around the camp site most of the day and checked every hour or so to be sure it was aimed correctly and in a sunny spot. With this level of attention we were able to recover all the energy we used the previous night and finish the afternoon with a fully charged battery.

The second day (90% clear) we guessed where the panel should go at 9:00AM and left, so no adjusting until we returned at 2:00 PM. Then we moved to a more shady site but fussed the rest of the afternoon to make sure the panel was aimed at the sun. The battery meter showed -10 Ah at sundown so the panel provided about 75% of our days energy use.

The third day (60% clear) and still in the shady site we aimed the panel at 9:00 AM and didn’t return until 5:00 PM. We were -32 Ah in the morning and -26 Ah at sundown so this day the panel provided about a third of our daily energy use.

The conclusion is that it’s possible to fully meet our energy needs, even in the winter, with the solar panel if we can keep it aimed at the sun throughout the day. With the panel mounted on the roof four days camping used about 50% of the battery capacity. These three days camping with the solar panel in a portable frame used 20%.

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2 thoughts on “S3E10 – Solar Power Revisited

    • Hi Del, good to hear from you. Sue is slowly mending but it’s going to be another couple of months before she is 100%.

      A mirror would have to reflect the sun onto the roof solar panel, don’t know how that would work. The panel slides into a channel on the roof so it’s not too big a deal to take it down. Just leaving it at a 45 degree angle pointed south in a sunny spot roughly doubled output. Fussing around every hour or so to keep it pointed right at the sun roughly tripled daily output.

      Spring, summer or fall, the panel on the roof provides all the energy we need (about 500 watt-hours per day) if the trailer is parked in full sun. The battery bank holds 2600 watt-hours.

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