Solar Install. My goal is put this in as a very cost effective system. Right now the large solar panel are about $1/watt but the smaller one can be double that. I opted to use the largest panels I could get that still fit on my trailer roof. The difficulty with the large panels, is that they are large and can be very hard to fit between the various vents and A/C that populate an RV roof.
Background:
Solar isn't magic. Its a simple rule of, "closer to the sun you are" the more power you get. Up here in North Cannuckville the solar inclination value is .6, roughly 60% output is all I will get. Closest to the equator is the best. Shade is evil and a small bit of shade kills the panel output completely.
System:
An RV solar system consists of solar panels, an charge controller (read battery charger), a battery to store the energy, and the cabling to connect them with a couple of circuit breakers. You can get fancier and add a cool battery monitor. Of course I am. If you want to get extra from your system you can install a large inverter which turns the 12VDC into 120Vac for using common houshold items that plug in. The caveat here is that the AC power used at 120V uses 10X the electrical current from the 12V system. That power has to come from somewhere. For example a 1200W kitchen toaster uses about 10A at 120VAC. This turns into 100A at 12VDC. It takes some large wire to carry the 100A and a hefty set of batteries. I plan to use small 400w plug in type inverters for my tv etc.
The goals were:
1. Maximize solar panel wattage to accommodate less-than-optimum solar conditions. By over panelling I will not have to tilt them and will get as much energy as possible. I expect 50% output most times.
2. Low coast. I am cheap and should end up under $1k installed.
3. Optimum battery charging. My OEM onboard battery charger is 20' from my batteries and uses tiny #10 wire. I get a measured 2.5V voltage drop from charger to batteries, so when the charger puts out 14.5 V the batteries are only seeing 12V. That is not enough to reasonably charge the batteries. Uncharged batteries do not live a long life. This is a very common issue in all RVs as they use a cheap charger and small wire, to save cost. Check yours.
4. Zero maintenance.
Design rules of Thumb:
1. Voltage drop is the enemy. The problem with working at lower voltages is that the inherent resistance of the wire has more effect. Unfortunately the only cure is larger wire and shorter runs. The maximum voltage drop you should design for is about 1%. I am designing for 0.5% since I an an electrical weenie. I am using #2 welding cable to connect my solar panels to the solar controller. The smallest i could use is #6.
2. Solar charger must be close to the batteries. It can be further away, but then you have to spend $$ on larger cables. Mine will be about 6ft using #2/0 welding cable from the controller to the batteries.
3. Batteries. You need some decent battery capacity and the 6V golf cart battery is the best cost choice for most RVers. A pair of 6V batteries in series gives about 225 amp hours of capacity. Of course you can't use it all since going below 50% will damage the batteries. The beauty of a solar system is that it will recharge my battery bank in a few hours. I am using 2ea 6v batteries. 4 would be better but i will try 2 for a season.
Solar Panels:
I went with the Canadian Solar 270W units. These have a 31V output and 8.75A. They are big measure 39" x 65". Cheap and stocked in my home town. The downside of using the larger panels is that they are frame supported around the perimeter. The large expanse of glass flexes a fair bit in the center, and to survive the bumps of the road and the piles of snow accumulating in the winter, they needed some additional support. I built an aluminum frame to support the perimeter and came up with a custom torsion bar arrangement in the center to support the center of the solar panel glass. I haven't seen this done before but I always like to break new ground. This will be two panels, wired in parallel, so that if one panel is shaded the other may give some output. If wired in series, shading one panel kills the whole string.
Controllers:
There are 2 kinds of controllers, PWM and MPPT. I chose the MPPT for a particular reason, although they are more expensive. MPPT work on the basis of power. On a good day, 30V and 9A output yields 270W. The controller converts this to 15V and 18A, same 270W. The PWM controller works on current. The 30V and 9A (same 270W) gets converted to 15V and 9A, it can't produce more amps than it gets. So we are at 135W and have lost 50% of our potential battery charging. This is a simplistic example but you get the idea. Also, the higher the panel voltage, the smaller the wire needs to be. For example, a 10KW system I built for Nepal used 900V solar panel arrays.
Batteries need about 15V to be able to charge properly, so the controller needs more than 15V at the supply. Typical RV panels are 12V rated but actually have an OC voltage of about 17V. This gives a 2V headroom for battery charging under bad sun conditions. The panels I am using are about 31V, so I have 16V headroom to accommodate for bad sun conditions. I like my chances better. If I lived in Florida the choice of controller may not be as important. Cam.
