Sustainable boating – starting from the top
IWA Sustainable Boating Group member Jonathan Mosse continues his new series on off-grid boating.
I MUST confess to having no firsthand experience of photovoltaic panels when I set out on this off-grid odyssey almost a year ago. But what I did know was that there was an awful lot of anecdotal information on solar PV floating about on the inland waterways system!
Starting with the basics, and some general details that are not in dispute, solar panels harvest two forms of the sun’s energy: direct and diffused.
Not only do I find it miraculous that when the sun shines it cooks my dinner, I’m even more amazed that when it doesn’t shine I can still enjoy a hot meal!
Received wisdom states that for most efficient operation, PV panels should be angled at 15 degrees greater than the latitude of their location for maximum mid-winter solar gain and 15 degrees less than latitude in mid-summer for similar, maximum efficiency. So, for my Edinburgh mooring located at almost 56 degrees of latitude, my panels should theoretically be angled at 71 degrees while a boat moored, say, on the Exeter Canal – sitting at a tad under 51 degrees of latitude – would settle for panels angled at 66 degrees to the horizon.
Similarly, the ideal mid-summer angles for my boat, and the hypothetical craft at Exeter, would be 41 and 36 degrees respectively. But it must be remembered that these angles are for the most efficient harvesting of direct sunlight; diffused sunlight can generally best be gleaned by panels fixed horizontally.
This then begs the question of whether it’s worth going to the trouble and expense of mounting solar panels in such a way that allows for angling them to the horizon, as it requires both greater capital expenditure and a regular human input. For this to make sense the boat needs to spend a good proportion of the year on a mooring with a reliable southerly aspect. Panels fixed at a rakish angle with the vessel on the move is hardly sensible and there is no guarantee that the course steered is going to be predominantly east/west (or vice versa) especially on a contour canal such as the South Oxford!!
I’ve just trialled my own panels lying flat over a 90-day period running up to the winter solstice and compared their output over the following 90-day period (so identical daylight hours) with them angled. The improvement was just a 6.4% gain from the angled panels over the panels fixed flat.
I also varied the angles as I can record data from each pair of panels separately. While the two fixed at the optimal angle gave a 7% overall improvement over the pair set 15 degrees lower, for 55 days (out of the 90-day total) the less than optimally angled panels actually produced a greater output, suggesting that there was more diffused, rather than direct, sunlight to be had.
I’ve yet to gather data for the full year, and for subsequent years, in order to start making statistically significant observations but even allowing for last November being very short of sunlight hours in this part of Scotland, I believe I can already fairly confidently say that angling panels to their theoretical optimum is looking like achieving no more than a 10% improvement over those fixed flat on a boat’s cabin top.
From this it’s possible to confirm that further received wisdom, suggesting that the actual angle itself (given that you’ve splashed out on a mechanism that allows variable fixing) will limit improvement in solar gain to a further 10%. Typically, panels fixed to a pitched house roof sit at between 30-40 degrees in the UK.
Ultimately, it’s a case of striking a happy medium between the harvesting of direct and diffused sunlight, for the best of both worlds. As for the construction of the panels themselves, monocrystalline panels outstrip their polycrystalline cousins, often approaching 22% efficiency and the bi-facial variants I have can up this limited output by a realistic 20%. This is achieved via sunlight that passes through the panels being reflected back and effectively harvested on its return. For this to happen, a solar reflective paint is applied to the roof beneath the panels.
As always, shading should be kept to a minimum, both from the boat’s surroundings and fittings on the roof. Attention to the vessel’s orientation goes without saying! I’m a gasless boat and my fundamental discovery is that I can live comfortably on an average consumption of 1kW per day. Future articles will explore in detail the nuts and bolts of this assertion.