Wednesday, July 14, 2010

Nice Rack (Solar PV of course)!

As we have talked about in other posts, the roof serves several purposes – capture solar energy, capture rain for the cistern, grow food, a spot for evening rooftop soirees, and if all goes well, keeps the house dry.

We have tried every conceivable way to capture more sun on the roof for energy production, but our roof is small, slightly off south orientation, with large beautiful  trees to the south making it very challenging. Makes me think of the recent article I read of a 'net zero' home being built in suburban Toronto where they added a four car garage to have more roof space for the PV.  The more solar we put up, the more it shades itself.  Below is an image from Google Sketch-Up of the roof taken from our shadow studies at noon on December 21st (sun at its lowest point during the year around 21 degrees) and at noon on June 21st (sun at its highest point at around 68 degrees).  The big cylinder is an approximation of our neighbor's tree with full leaves (trust me, the tree looks a lot better in real life).  This is not the case in December, but any shading hurts the solar performance.

What you see in the images is a large array of photovoltaic (PV) panels on the slope over the stairwell at 35 degrees, which is the ideal angle for PV on an annual basis in Ottawa.  10 panels at 230 watts per panel will fit on this slope, then another large rack at the back – at 60 degrees for the solar thermal, with another 5 PV panels above them at 35 degrees again.   Annually, we will generate roughly 5,900 kwh of useful heat from the solar thermal, and roughly 3,500 kwh of electricity from the 15 panel (3.45kw nominal output) PV array.  This is a total of 9,400 kwh/year of solar energy captured.

The capital cost of the PV system will be around $33k.  If you do some math for maintenance costs, and degraded panel performance over 20 years, the PV costs about 50 cents per kwh to produce.  This compares with the solar thermal at about 18 cents per hwh of heat.  The big difference is that the Ontario FIT program will pay us 80.2 cents for our PV generated – or roughly $2800/yr.  This certainly puts the economics in favour of PV. 


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