Sunday, August 17, 2008

Experiments in off-grid Solar

Anyone reading this blog or our other blog (project solar) knows that we put up a grid-tied PV system that has been running flawlessly since we commissioned it back in May of 2007. Since this system was installed by a professional installation team, I didn’t get much of a chance to do any hands-on work on this project (although the team was answered all my questions and let me be a pest). Well, late last year (2007) I had put together a couple small 15 watt panels that I was using to charge 2 75 amp/hour batteries. Although these panels were small, they did the job. Unfortunately, they were not build to withstand a New England winter and cracked under the load of snow we had on our deck. That left me with 2 75 amp/hour batteries that I couldn’t get recharged. As I shared earlier, we bought a bike generator that works very well and kept these batteries charged through the winter. The problem with the bike is the rate of charge is slow, and is solely dependant on how long you can stay on the bike.

With that as the backdrop, I decided to try my hand at designing a small off-grid PV system. Obviously when I started I had visions of being able to run the whole basement off battery power. As we all know, visions generally have to be scaled back when cost and time are factored into the equation. The project ultimately became a simple 2 panel replacement for the 2 panels that were destroyed but set up permanently and wired in such a way as to allow for expansion without a total redesign. What I ended up with after MANY, MANY design changes was 2 125 watt Mitsubishi panels feeding a Xantrex C35 charge controller. The panel selection was based purely on these panels having a high amp to volt output which I needed since I did not want to add the additional cost of a MPPT charge controller. The next challenge for me was wiring size and configuration. I wanted the charge controller to be in my basement, but my best sunny spot was almost 50 feet away which meant large wires (#4 AWG). I also had the constraint that I could only charge 1 battery at a time at 12 volts and the batteries could NOT be put in parallel since they had never been used together, nor was I planning on keeping them together. The rule with these batteries is the same as any other batteries, don’t mix old and new, or even old and old if they have been used differently. This meant I had to run the 2 panels separately and could not combine them until I got to the charge controller. This meant 200 feet of #4 wire (4 runs of 50 feet). After finding suppliers for everything I needed it took me the better part of a few weekends and weeknights to get everything put together and wired up, making lots of mistakes, but learning and redesigning along the way. After all was said and done though, we now have a system that on a nice sunny day can charge both 75 amp/hour batteries, and even on a cloudy day can bring one of the batteries up to full charge in a couple days.

So, what do we do with these batteries? One of the batteries we use in the kitchen to primarily run a drinking fountain for the cats, but it is also used for the small kitchen appliances such as the food processor and blender. This battery lasts 3-4 days before it needs a recharge. The other battery is in the bedroom and is used to power the TV and DVD player and occasionally a table lamp when the TV is not in use. This battery also lasts 3-4 days and that includes watching at least 1 movie and maybe a couple hours a TV per night.

I am very happy with how the small PV system turned out and right now (in the summer) we are producing so much power that I don’t always have a battery that needs charging. That will change in the winter, but we also have the charging bike to help supplement in the winter.

So, what are the next steps in this little experiment? I am thinking of adding a couple additional batteries that could be used to power our computers in our home office. This may totally wipe out my generation capacity since these computers will need to run for at least 8 hours a day on the days when we are working in our office. That means adding one, maybe two more panels which fortunately the current design should be able to handle. Before I do that though, I need to make sure of what this system will do over the short low-sun days of winter. I’ll blog again in the dead of winter on this topic to let everyone know.

This system was extremely fun to design and build, and I would encourage anyone out there with interest in PV to not be afraid to experiment and see what you can do because there is lots of information out there, and lots of people willing to help. Then again, this is just life according To Troy.

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