Friends Wilderness Center
Solar DC Voltage System

Background: The goal of this project is not so much to provide solar power at 12 volts in some of the FWC overnight camping areas, the Geodesic dome, the Yurt and the Treehouse, but to show the value of solar power to a clean renewable energy future. Two panels have been donated through the Annapolis Friends Meeting camp solar initiative to FWC to provide about 80 watts of solar for night lighting, cellphone and lantern charging and other 12v DC applications. The goal is to use solar power to keep the 12v battery in each structure charged for nighttime use.

The SWER Power Distribution Concept: The main challenge in providing solar power to any camping area east of the Mississippi is that we generally put summer camps in shade, under trees for maximum enjoyment out of the hot sun. At FWC 95% of the camp is shaded and the only sunny place to put solar panels is by the lake which is hundreds of yards from the overnight structures. Then the next hurdle in power distribution is the losses in long wires. The original idea had been a Single-Wire-Earth-Return (SWER) by distributing the power at a higher voltage where the losses are less, and then downconverting at the load as shown here.

Distribution losses: In this system, the output of the solar panels is at 140 VDC but we only need 14 VDC at the overnight structures. Since current is the inverse of voltage, this means that the current in the wire at 140 VDC is 1/10th of the current needed at 14 volts to charge the battery. And since power loss in the wire is proportional to current squared, then the losses when using 140 VDC are only 1% of the losses that would be at 14 volts. This would allow us to use small hook-up wire over hundreds of yards to distribute the solar power to the camping structures about the FWC property. The 14 VDC then is used to fully charge any number of 12 volt car batteries at the load sites using any readily available DC/DC switching power supply such as the 15 Volt 3 Amp power supply shown above For about $9 each.

Gound Resistance too high! But it did not work. The gournd resistance was almost 1000 ohms compared to the 20 to 40 Ohms we had measured in Annapolis. The difference is the rocky soil which is much higher resistance. So we abandoned that approach. We found that one of these solar panels even in the shade near the dome could produce the same power and so we put one panel at the Dome and the other at the Tree house. Neither ever gets full sun but maybe a few minutes a day, but all the rest of the day, the panel does put out about 10% of rated power in the shad which is enough to keep the 12v battery charged. With 10 hrs a day of light, 7 days a week that is about 2 Amp hours of energy which is enough to run the lights for 4 hours a week.

Dome Now has 12v and 120 VAC Power: On 25 May 2016, the two 40W solar panels were installed as shown below, but on 12 June we moved them to the Dome and Tree house as noted above. In the Dome there is a 12v-to-120 VAC inverter to power the two lamps. Just turn on the inverter and the lamps come on. If you want half the light, and to last twice as long, unplug one of the lamps.

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Direct 140 VDC lighting: Although almost any modern dimable LED bulb rated for 100-240 volts can run directly off of the 140v distribution line, the problem is that we do not need much lighting during the day, and the 140 volts disappears at night when the sun goes down. The only use of such bulbs might be in the darkness of the Yurt or other daytime enclosed space. But again, the 140VDC system was abandoned due to the high groiund resistance.

USB Charging: Another very frequent application in camp is cellphone and other small device charging. This is easily accomodated with any number of USB power supplies. See this a multi-USB box with its own switching power supply to provide six one amp USB sockets (3 on the front and 2 on the back).

Camp Lighting: This system can, of course, power almost TEN modern standard 120 volt CFL or LED 40W equivalent lamps that operate just fine from 120 VDC. The problem is, of course, that they would only light in the day. This might be useful in dark places,and can also serve as a "pilot light" to give confidence that power is being delivered. But at night, all we have is the 12 volts from the solar charged camp batteries. But still, the 12 volts then can be further up-converted to 120v AC for all applicaitons or downconverted to 5 VDC where it can be used to power dozens of small garden LED night lights or for USB chanrging.

Take a look at the rating on most LED screw-in bulbs for the home. You will see many of them are stamped as 100-240VAC just like any other universal electronic item. These bright 40, 60 and 100W equivalent bulbs only draw 9, 11 or 22 Watts and will work just fine on this system (but only in the daytime). At night, you will need 12v DC LEDs.

Trail Moon Lighting: Sometimes, a bunch of flashlights flashing around the trail canbe distracting and less than peaceful. One possiblity is using simple garden solar lights to light the trails to the camps. The goal is to provide sufficient "moon"light to avoid stumbling but not so much as to lose night vision.

But they are useless in the shade, so the best way to use these garden lights is to bring them all to the sunny field in the morning, and leave them there all day to get sun, and then when you go back to the cabin at night, take the lamps with you and post them along the trail. Reverse the process every morning and night. A great example of solar energy! THe original idea was to power the garden lights again with ground return, but the fact that the grouind was so lossy, we abandoned that idea. At night,we can use the same wire to carry +12 volts to light as many garden lamps as needed. A simple diode at each solar light isolates the light during the day as shown in the schematic below. See also the Relay schematic.

Return to Camp Solar Page.

Bob Bruninga, WB4APR,
US Naval Academy
lastname @ usna dot edu