Thursday, September 06, 2007

Solar Powered Cabin

In the Spring of 2005 we embarked on a project to build a cabin here in Canada that would be off grid and be able to sustain it's electrical power needs with both solar and wind generation. We also wanted to make use of passive solar and solar air heating to supplement some of the heating costs especially when winter temperatures dropped to or below -25 deg C. The result was a relatively inexpensive house built from a well insulated garage package and some simple techniques to keep the cabin warm and cozy during the long winter months. We are paying about half the heating costs of a similiar sized house in the city and we pay the taxes on the property, that's it, there are no other bills!

This is a short overview of some of the "learned" knowedge that we acquired about solar energy. The decision to go solar on the cabin was an easy one. To bring power into the cabin from the utility was going to cost $26,000 and to go solar was going to cost around $10,000 so the cost in many ways justified the decision (also there wasn't going to be a monthly bill). One of the first decisions that we had to make was what voltage did we want to have for a final output.

There are 3 main or common "voltage standards" currently available to most consumers they are 12 volt, 24 volt and 48 volt DC systems and there are a wide choice of inverters that will run on these voltages. We chose 12 volts for the cabin because of the fact that our propane fridge would be running on 12 volts and there is a wide variety of 12 volt appliances (from the RV industry) that we could run on the system without any need for step down transformers or other converters. With each voltage system there are pluses and minuses but if you decide to "live off the grid" and you want to have a system capable of doing a fairly large home then a 24 or 48 volt system is your best choice. Here are some examples of why these are the best choices. A 12 volt system using one charge controller (we'll use the Outback mx-60 as an example) is really only capable of receiving and delivering a maximum of 720 watts. Why is that? Well the Outback mx-60 is a very good charge controller from our experience, its rated to output a maximum of 60 amps. So here is how it works. If You have a 12 volt system then 12 volts X 60 amps = 720 watts of output to charge your batteries. If I have a 24 volt system then 24 volts X 60 amps = 1440 watts. And of course a 48 volt system X 60 amps = 2880 watts.

Now if we apply a simple example and let's say we had 6 hours of day light our total KW for the day on a 12 volt system would be 4.32 KW. For the 24 volt system it would be 8.64 KW and for the 48 volt system it would be 17.28 KW which would easily power most housholds. There are other advantages of going to a higher voltage system such as smaller guaged cables from the batteries to the Inverter(s) but you also pay more for the number of batteries required so again a trade off.
One of the things that we learned about most solar powered system that are in these northern latitudes of Canada is that during the winter months, the amount of power that can be gained from solar becomes quite limited. Essentially this is because the hours of daylight are less and the sun is very low on the horizon and low in intensity for most of the day. The reduction in intensity makes a big difference in the amount of power that can be gained with your solar panels. Here at the cabin we are at 53 deg north latitude and we find that from about the first of November to about mid-February the intensity of the sun is greatly reduced. In fact if we are here for 2 or 3 days in this winter period and it is cloudy or overcast we will run the generator to charge up the batteries. So bottom line Solar works here for about 9 months of the year. I made a quick calculation one time and it turns out that if you live south of 40 deg north latitude in the winter, solar should still be viable for your power needs (that of course is if you live in an area where the sun is shining most of the winter).

Another problem that we have run into is snow accumulation on the panels in winter. As you can see from our pictures we opted to mount our panels on the roof (for security reasons) rather than mounting them on a pole or a ground mount. The optimum angle for roof mounted panels is at an angle that is roughly the equivalent to the angle of the sun when it is at the spring or fall equinoxes. In this part of the world that angle is somewhere around 37 degrees, unfortunately that means that snow will accumulate on your panels and you can spend a great deal of time on a ladder sweeping away the snow. What we have learned is that it is better to buy a few extra panels and mount them almost vertically and about 12 inches or 25 centimeters higher than the surface of the roof. In fact the angle that we chose is 13 degrees which is the height of the sun's angle at noon on December 21. This works much better and the snow does not accummulate on the panels. Some of the literature that I have read has said that there is only a decrease in performance of about 10 percent in the summer months with the panels at this angle. Our observations looking at the readings on our charge controller seems to bear this out and considering that the summer months have a "lighter" electrical load than our winter months, we'll stick with this setup.

The cabin has a wall construction that consists of R-22 insulation and the ceiling is at R-40. In many ways I wish that I had waited and learned more about NetZero housing construction which has R-56 in the walls and R-100 in the ceiling before I had built the cabin. But none the less the construction is not bad and we have many windows on the south side that make use of passive solar heating.

We also installed a solar air heater that has proven to be quite effective. The amount of heat that we are able to attain from it is around 9000 Btu/Hr between 10:00 am and 3:00 pm on a nice sunny day. I have measured the temperature at the outlet with a digital thermometer at 165 deg F with a fan delivering 110 cfm.