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!

8 comments :

kristen said...

I warmly recommend the free Google Sketchup 3D modeling tool (which has built-in sun & shadows). If your straw-bale house design is still a work in progress, I think you'll love Sketchup. Check out their tutorial videos.

La Ferme de Sourrou said...

Great post folks!

This sort of thing is vitally important - it's good to see you're spending time on the planning stage.

I do know that feeling of getting on with something "productive" though...;-)

Anonymous said...

Don't talk to me about passive solar and solar hot water - since buying our house in Northern NSW 4 years ago, the trees in the northerly solar aspect have grown exponentially and are blocking out most of our winter sun....and 6 out of 8 of them are on our neighbours land...oooops! So do we negotiate felling these huge and glorious brush boxes and ghost gums or endure the winter shade? Billy, our lovely local timber fella, says we'll never win, unless we clear fell the entire area, expensive and not particularly ecofriendly. He recommends we insulate the house properly, get a very good slow combustion woodburner with back boiler and plant a sustainable timber lot in the paddock. Will check out the free Google Sketchup 3D modeling tool - sounds great. Love Jenx

Val Grainger said...

We have a similar problem to Jen!
We have a row of growing trees that should be a hedge.......problem is eccentric owner will not do anything nor let us do anything to them :-(
They shade my veg on one side more each year....now question is do i chainsaw the lot or shoot the owner (only joking!!!)

Stuart and Gabrielle said...

Thanks for the recommendation, Kristen. I've already looked at this but it's yet another thing to have to learn how to do, just on the back of learning how to write a website (checkout our new-look holiday gite website, all my own work) and putting in a planning application for a barn conversion. Perhaps you can spare a few minutes when you come to stay to give me a demonstration. I wonder whether one can adjust the shadows to suit a particular latitude or whether it's just for an aesthetic effect?
As for your comment on labelling our map, I can see another job for you in August!
Thank you to the Hard Working Hippy, Jen and Val for your comments. It's a shame your neighbour won't let Pete lay the hedge, isn't he interested in free firewood? And Jen, perhaps you could take out a few trees at a time, giving you that firewood you need, replanting in more suitable places? Living in Australia and no sun, what's going on!

kristen said...

You can change the latitude, the month and the time of day. You can even superimpose with google Earth scenery or blend with a real picture to see how the project fits in its surroundings.

I'll be very glad to help with tech stuff, but I expect to do my share of the hard day's work too.

I like the website. The code is neat (cannot help looking under the hood) and I especially like the sleek css photo gallery (at first, I thought it was javascript).

Anonymous said...

Did you remember that magnetic South is not the same as solar South when you did your diagram? If not: http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination

Stuart and Gabrielle said...

Sept. '11
I'm not working today, having hurt my back trying to help someone move a very heavy and very ugly jukebox. So I'm taking the opportunity to tidy up my inbox and a pile of paperwork.
So sorry for the very tardy reply and thank you 'anonymous' for your comment with the helpful link. We're in the other hemisphere, so it's magnetic and solar North!