Wednesday, July 16, 2008

Sun Path Charts / Diagrams: We have been beavering away behind the scenes on the design for our straw bale house for some considerable time. It seems to us now that having the “constraints” of the existing walls of a renovation project is, in some ways, an advantage. When you have a complete carte blanche, you increase the options and thereby multiply the decisions to be made … and coming to decisions slow things down! In the permaculture design process, the second stage is a site survey. Instead of having enough decisions to make based on limited knowledge, we have to go and measure stuff and so add loads more variables to the “things-to-be-considered-before-making-a-decision” pile. You might well imagine how taking off the planning cap and spending the day doing something physical like splitting and stacking logs is somehow very appealing.

In our ecological house built of straw, we’d like to maximise passive solar gain and also (pre-)heat our water by the sun … the jury’s still out on photovoltaic (PV) solar electricity for our situation (climate and available finances). Therefore, we need to know, by time and season, how much sun is going to fall, and where, on our proposed building site. We need a Sun Path Diagram. This sort of information is also very useful in planning a garden, so you can match plant sun / shade needs with available sun through the growing season.

A few years ago, I went on a Solar Energy course run by Damian Randle and Rob Gwillim (both, at the time, of the Centre for Alternative Technology). It was a very intense course, with lots of science but also very practical and we actually got to build two working solar water-heating systems: one gravity fed and one pressurised (see photos). It was there that I learnt about Sun Path Diagrams.

A sun path diagram is specific to your exact location (longitude and latitude). Thanks to the kind people of Oregon University, you can download a free sun path diagram. The chart will show the path the sun takes throughout its day by its horizontal (azimuth) and vertical (elevation) orientation. You then need a way of simultaneously measuring a compass bearing and elevation. You can buy ready-made devices, or fabricate one like I did (see photo). Once you’ve oriented the measuring device to directly face south, you begin to trace the top outline of all the obst-ructions, reading compass bearing and elevation off your device and noting that on the chart.

Once you have the complete silhouette drawn, you can read off the chart when, by date and time, you will be in shade (below) or sun (above) the line you’ve drawn. Remember that it's a diagram that shows about 270º on a flat piece of paper, so don't worry if the it doesn't look exactly like what you see. I hope that the various pictures make sense of what I’m saying. Important note: you are measuring what shade will fall on your measuring device, so you should ideally be measuring from the exact point you wish to install your solar panel. And if you’re doing this to learn where to put your vegetables, you need to be at vegetable height, i.e., flat on your belly!