On the Winter Solstice for each degree of latitude south from here to Panama, one receives about 5% more insolation.
But aside from gross insolation increases there are some subtleties that bear consideration while thinking about adding a solar power capability to a boat.
First, we don’t cruise on the Chesapeake on the Winter Solstice. We winterize around the first week of December, but for solar purposes consider 1 November the shoulder of our season. So, the insolation for that date becomes our baseline. Why not the equivalent insolation day in the spring? In the spring the water is cold and our refrigeration is at its most efficient and its current draw the least. It can be a significant difference.
The graphic (hover or click) shows the Sun’s trajectory for Deale on 1 Nov and that becomes our reference. One can see that even in Titusville on the Winter Solstice we have more insolation than the Deale reference…but the water there is a bit warmer.
For instance today we have 62º water in Deale. Titusville, 1 January ’09, had 72º water. In the BVI it was 79º and in Colon, Panama, 81º. The air temps to match are/were 60º, 77º, 84º & 90º. So we can reasonably expect our amp-hour consumption to be higher. But how much?
With each 10º of water temperature increase from 60º our refrigeration load seems to go up by about 25% which means it would double at 90º water temps. (not that I would want to be where those are). So we might see an additional 15 amp-hours a day in warmer cruising waters. This would require an additional hour of payback from panels producing ~7-22 amps in the bulk charge phase which typically occurs before 1200 on a clear day.
In the Mid-Chesapeake area, we are generally around -25 amp-hours in the morning and “net=zero” by 1400 on brilliant October days [our only data sample so far]. So, perhaps, we are talking abut adding an hour to reach net=zero. Doubling that for margin of error would seem reasonable. That would still put the batteries into float phase well before the sunlight tapers off.
So, are the panels going to lie flat or be tilted?
The insolation map posted yesterday noted the reference panel was tilted. The rule of thumb for this tilt is often stated as “tilt the panel to numerically match the latitude.” But as you will notice from the graphic, the Sun’s trajectory across the sky changes shape as one moves south. So, the rule of thumb and forefinger is:
For the winter, use 90% of latitude, and add 29º
For Deale this would mean a tilt of 65º
and for the BVI 45º.
For summer, use 90% of latitude, and subtract 23.5º
For Deale this would mean a tilt of 17º
and for the BVI -7º [which proves rules of thumb sometimes have their limits]
There are other formula for other times of year — this site is a good source. From an engineering and installation perspective and from an aesthetic one as well, 17º is not too hard to do and not too hard on the eyes. 65º is another matter.
For Titusville, I expected we would want a bit of tilt but perhaps not. In the BVI, I have a hard time imagining 38% greater electrical consumption (The AC uses the genset or shore power.) Having been to Colon Panama, I wouldn’t go there again, but I can easily imagine going to Grenada (-2ºN of there) where the air temps are cooler and the crime rate lower. I really can’t imagine 75% greater electrical consumption.
For one reason, we have invested in converting the boat’s lighting, used when not under power, to LEDs (yes, “invested” — these little marvels, even from our friends in Fiji, cost mucho dinero* ). Today, the total of our reading lights consume less than a single one did when we bought the boat. Our main loads of consequence are radar, autopilot, stereo (when playing CD’s — so I am ripping the CDs to MP3s we can play from a memory device) and refrigeration (per above). Radar consumption will go up with the heat. The autopilot is something of an unknown.
So, if our 405 Watt array meets our needs in Deale without tilting, it should do fine down south oriented the same way.
So why did I build the array to be tiltable?
Cloudy days and marinas.
While cloudiness reduces the impact of the Sun’s elevation by diffusing the light, there is still a brighter side of the sky. When looking at percent cloudy days for Deale, Titusville, Key West, Tortola and St Georges, Grenada, they are 37, 13, 20, 14, “0”. So, (sorry, Judy C. and Joni M. ) I’ve looked at clouds from both sides now, from up and down, and still somehow, I don’t think we are likely to have to tilt the panels very often to overcome the impact of cumulus humulus.
Marinas, We all got ’em, we all want ’em. What do we do with [the solar panels while we are in] ’em? (Jimmy Buffett, Fruitcakes, 1994) Our slip in Deale faces very close to due west. This means we could tilt the panels up to port and improve output, and we well may when we move aboard. But since I can’t sell power to the grid that tilt will be enough to meet needs only. As we move south, based on our 2008-2009 experience , we will face mostly north or south. Here, tilting east for the three hours before solar noon or west for the three hours after could improve output. Since the insolance is greater than Deale this may (hopefully) be unnecessary — it just is too much like dragging a sprinkler around a lawn.
Did I make the panels tiltable for no good reason? Given I sized the array for the most I could possibly get from the real estate, time will tell. But I am glad I have them installed the way I do and probably would have done the same had I given tilt no consideration at all. One can buy a lot of panel watts for the cost of sun-tracking panel-mounting equipment that leaves the boat looking like a TV News van.
So, Here comes the sun, here comes the sun, and I say it’s all right… (thanks George Harrison)
*Recent LED purchases from Cruising Solutions have been pleasing as well. While the Sensi-bulbs from SailorsSolutions have performed well, they aren’t all that aesthetic and they need an occasional tap-in to keep them seated in the G-4 socket. The Dr LED masthead anchor light is doing fine, but when we inspect the rig next year we will replace it and the Aqua-Signal incandescent with brighter LED products -from whom we shall see.