You don't need to sit and track the sun's path for a year to determine your location's solar window. Modern technology provides several tools to help you work out the path you need to clear in the air to optimize your solar collectors' efficiency.
If any of the methods given below show more than 10% obstruction to sunlight during the prime hours of the day, you may need to reconsider the collector site.
Solar Pathfinder
This device uses a sun path chart based on your location and reflects the solar window through a plastic dome. The idea is to show you what obstructions there are to the sun at your site, so that you can remove them.
A neat addition to the system lets you upload a digital image of your reading and analyze it with assessment software to work out what percentage of the potential annual solar radiation your selected site will receive. If you decide to do this, bear in mind that analysis software is usually written for PV panel implementations. This gives rise to two limitations that are important when considering a solar thermal installation:
1. PV panels usually track the sun, so they generate power from rise until set. Solar thermal collectors do not track and only collect heat in the six hours around midday (three before, three after). Consequently, calculations based on tracking panels are not applicable to collector arrays.
2. PV panels need completely unobstructed locations. Even a tiny 10% shade can cause them to stop generating power. Collector arrays are less picky: as long as there's some sunlight, they will heat and the fewer obstructions there are, the more efficient they will be. If you enter an amount of shading into an assessment application, make sure it's not considered as an on/off proposition but as a percentage shading which affects radiation collection efficiency.
The best way to work out your solar window and the amount of energy you can harness is to ask a professional installer for their experienced advice.
Solar Site Selector
Although it looks less technologically advanced, a Solar Site Selector has one big advantage over a Solar Pathfinder: it lets you see the solar window and any obstructions directly, rather than reflected.
The Solar Site Selector uses a semicircular sun path chart (chosen according to your location) which fixes into place on top. You then point the device directly south and look through the eyepiece to see an actual visual representation of the solar window, complete with any obstructions. The down-side of this is that there's no way to record the outcome or analyze the situation.
Old School Method
If you don't want to buy one of these tools or just prefer a traditional DIY method, you can work out your solar window with a sun angle table (see image), a protractor and a piece of straight wire.
• Bend the end of the piece of wire and push it through the hole in the zero point of the protractor (in the middle at the bottom, where all the lines meet).
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