Rainwater Harvesting With Solar PV Panels

A Marriage Made in Heaven!

As the project progresses, we've been blessed with making connections with some excellent contributors:

I've mentioned some of them before, but most recently we've been connected to DTE Solar as well as Boss Solar, and Trevor at EcoInsulation, Reiner (the Reno Coach) all excellent people.

One connection was with John Paul from DTE Solar, who has put me onto rainwater collection as an energy-related component of our project.  He's written an excellent article on the topic, discussing rainwater collection's potential impact on the City of Toronto's energy, waste, environmental and financial costs.  It certainly opened my eyes to the amazing, yet highly under-appreciated costs of water treatment, distribution to and from, etc.  These are costs easily offset by rainwater collection if implemented ubiquitously.  Not to mention water costs in Toronto have increased about 8% per year for some time now.

After some discussion we've together arrived at tantalizing possibilities for our own project.  The fact that PV panels are glass-clad means they present a premium surface for rainwater collection.  The trouble is that they are not designed for this task - which I certainly feel they should be.  In fact, I would like to see PV panels made as large interlinking panels that shed water - This way they can be installed on the roof without plywood (so using PV panels, one eliminates not only the shingles but also the plywood layer, although the roof shape must be planned with this in mind), perhaps directly to the rafters or purlins.  The benefit is they can be wired or accessed from the backside (from inside the attic), they can serve as the roof, and shedding water all the way down the roof, rainwater collection becomes a breeze - not to mention the quality of the collected water is also improved.  In any case, given the current, less-perfect situation of panels on rails on shingle roof, JP's thought was to add troughs underneath the panel gaps to collect the water - in our case 3 long troughs, and direct it to the collection reservoir.  However, what if we also introduced pipes onto the roof to pump that rainwater from the holding tank onto the solar panels?  I believe by dousing the panels with rainwater we can cool them dramatically, increasing electricity yields significantly - perhaps as much as 10%, even after pumping losses.  Some of the water will evaporate, but much of it will just flow back into the collection system, where the heat may again be made use of - a heat pump can take the heat from the rainwater collection tank and port it to the DHW (domestic hot water) system.  Even if not, an un-insulated rainwater tank in the ground is constantly being cooled by the soil, so that over the hot summer season, there will be a significant cooling capacity available for the solar panels.  This cooling action will also lengthen the lifespan of the solar panels.
This is in a way similar to the new techniques appearing whereby the heat from the solar panels is being captured by flowing air over/under the panel backsides.  The heated air is then used in the building.  I've read this can increase by some 90% the energy harvested via electricity alone.  But air is generally a lesser medium than water - which is compact and easily filtered and transported in pipes.  Air carries dust, and requires larger ducts to move it.  And the fact that it is a (compressible) gas means there are a lot more losses related to its handling (pumping).
Mike at Boss Solar has devised a water coil to attach to the backs of solar PV panels to both cool them and capture their heat - but this seems a lot more difficult and costly than the 'rainwater collection' idea, - which appears to be - a marriage made in heaven!
Notice how there are no photos or sketches here.  The ideas are very simple.  This seems to be the reality of rainwater collection as it is with other aspects of super energy efficient houses - not glamorous - just important.