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Sunday, December 15, 2013

Solar Power?

A number of people have asked if I've consider installing solar power on the boat.  The answer is Yes, but it's still evolving.

Initially I was planning to install solar panels right from the get-go.  According to my calculations, solar panels would allow the boat to remain powered indefinitely at anchor, unoccupied, with the fridge on but the freezer off.  And with the boat occupied, which means higher power use, I figured I could cut generator time in half if not better.

However, I have subsequently learned what sort of "idle" load power draw actual N55 and N60 owners are experiencing, and it is significantly higher than my calculations.  Like 5-6x higher.  With that kind of power load solar will have no meaningful impact and is likely not worth the effort.  But I haven't given up yet.  Here's more detail for those who are interested.

With any power system there are two key parts to the equation; how much power you are consuming, and how much power you are generating.  Let's look at the generation part first.

On most boats, and the N60 is no exception, there is very limited space suitable for solar panels.  You need an area that is large, flat, and has minimal shadowing over the course of the day.  On the N60, the only suitable location is on top of the flybridge hardtop.  With a little pre-planning, there is room on the hardtop for about 750 watts of solar panels.  The pre-planning involves keeping antennas, radar scanners, and other gadgets out of the way, including making sure there are minimal shadows cast over the panels.

Shadows are a big no-no with solar.  There is the obvious loss of light, but it's much worse than that.  For a variety of technical reasons, a little bit of shading caused a large loss of power output.  There are a few things that can be done to mitigate the problem, but it remains a key objective in locating panels and surrounding equipment.  The 6' radar scanner is a key challenge since it is located just aft of the panels.  Depending on where the antenna stops in it's rotation, it could be casting a big shadow.  But I lucked out on this one.  It turns out that the Simrad 6' radar has a park feature and can be set to part in any desired position.  Nice!  Other scanners that I'm familiar with can't do that and the antenna just stops where it stops.

With 750W of panels, the next question is to figure out how much power they will produce on a daily basis.  The rated output of 750W is based on full sunlight.  As we all know, the sun comes up gradually over the day then goes down gradually.  Plus, it's not sunny every day.  Sounds complicated, doesn't it?  Well, it is, but fortunately, other people have figured it out and distilled it down to a simple calculation.  The National Renewable Energy Lab has maps that provide the equivalent number of hours of full sun light across the US.  http://www.nrel.gov/gis/solar.html  And they even take into account historic weather patterns.  That said, boats move around so the location isn't fixed.  But in general they tend to follow better weather which will yield better sun exposure.  On the down side, panels should be tilted towards the sun (or close to it) for max output, and that's not practical on a boat, so they get placed horizontally.  If you are near the equator or during summer, flat is just fine.  But if you are at a high latitude in the local winter, performance will be impacted.  As you can see, there are lots and lots of variables, but the NREL numbers provide a really good estimation point, and in my experience have been quite good.  But even with a good number, output will still vary widely due to weather.  That's where having enough battery power to carry you thorough is important.

So what's the answer?  I've used 5hrs of equivalent full sun per day.  That gives 3750 watt-hours of power per day.  But wait, it's not that good.  The panel rating is under optimal conditions, plus there are losses in the power conversion and storage process.  I generally like to assume the whole process will be 80% efficient, so that 3750Wh turns into 3000 Wh, or 3 kwh.

Now let's look at the power loads.  First, there are a bunch of things that are just out of the question with solar.  High powered devices like air conditioning, water heating, ovens, clothes driers, etc. are not even wired up to be run off battery power.  They can only run when the generator is running or there is shore power,  But we can run everything else off batteries via an inverter.

On our Grand Banks, our at-anchor power draw is between 100-200W.  The lower end is with lights etc off and us asleep, but the anchor light is on along with the fridge and freezer plus basic electronics for an anchor alarm and weather.  The higher number additionally has a few lights on and us watching TV.  If we assume that our average over 24hrs is 150w, that's 3600wh which is about 20% more than the panels can produce.  That means solar will reduce generator run time, but not eliminate it.  But it will cut it down significantly.   Instead of creating a battery drain of 3600wh each day, we will only drain them 600wh because we are getting 3000wh from solar.  We could run for 6 days on the same battery drain as 1 day without solar.  That's pretty good.

Furthermore, if the boat were sitting without us on it, but with the fridge and freezer running, we would only consume 2400wh per day and the solar would carry the load day in and day out.

Based on this I was ready to install solar right away.  But then I talked to some Nordhavn owners.  They report idle power loads of 750-1000W.  Wow, that's a LOT more power, and means that solar would not make an appreciable dent in the battery drain, and consequently not reduce generator time significantly.

My first question is "why the big difference?".  Part of it is refrigeration.  The N60 has more cubic feet of refrigerated space, and that takes more power.  I originally made my calculations based on the Energy Star estimated power consumption for the fridge and freezer.  In the past I have found their estimates to be very accurate.  But maybe they are off in this case.

Also, the N60 has halogen lights which are pigs.  The Grand Banks did too, and I replaced them all with LED replacement bulbs.  It cut the power draw by a factor of 10.  On the Nordhavn, turning on the Salon and Galley lights is 300W.  That's part of the problem.  But until I'm actually on the boat I can't really figure it all out with any certainty.  I remain optimistic that I can get the power draw down to a point where solar can make a difference, but I'm going to see where it shakes out before installing solar.

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