Sometimes an idea that seems great in abstract fails when you get into detailed drawings. Such is the case with our solar roof, now history. But as cool as it might have seemed (to some) we have a much better solution.
The array is now mounted all the way aft. The six panels shown here are mounted on shafts which rotate in bearings bolted to the transoms, and are set at slightly different heights.The pivots are at the third points rather than center.
In this screen shot the starboard panel is in inboard storage mode, the port is outboard.
Finally, inboard storage mode, with both sides inside of the rub rail. Mounted aft like this, if the booms are deployed, there will rarely be any shading.
Now the good news. It looks like our friend Bob Williams will be able to get us six of the much sought after 320 watt panels. Consider what this means. Six panels (320 watts) 1920 watts per hour at full output, less something for shading, dirt, heat, etc., so lets call this 1800. If you are in the Bahamas in May, multiply this by seven solar hours per day. That is a total of 12,600 watts per day. Divide by 27.8 (volts) to get an astounding 450 amp hours a day, fully 20% or more than we normally consume.
What can you do with an extra 2500 watts of power? How about running the forward cabin air conditioner all night, or heating a 20 gallon/75 liter hot water tank electrically, or making 100 + gallons of fresh water by desalinization.
One could argue to reduce the panels to four. But we have room for the six, and this leaves us with the ability to deal with overcast and lower sun angles that come with icebergs and high latitudes.
Posted by Steve Dashew (November 9, 2011)
November 10th, 2011 at 10:05 am
Steve,
Who makes the panels and MPPT which you will be using?
Thanks
Don Joyce
November 10th, 2011 at 5:55 pm
Don:
We have not selected the MPPT controller yet. We’ll see what Bob Williams and Corey McMahon have to say, both of whom have lots of real world experience.
The 320 watt panels are from Sunpower.
November 10th, 2011 at 11:07 am
Steve, will this arrangement work on the FPB 64?
November 10th, 2011 at 5:53 pm
Yes, Stan, it will, but maybe with four panels. Once we finish this we are going to look at the 64.
November 10th, 2011 at 12:58 pm
I assume they would be totally removable and stowable for passages? Would these positions get any wave slap or other negatibve environmental effects (strong wind maybe?)
November 10th, 2011 at 5:51 pm
Hi John:
Wave slap is not an issue for us, and they are too big and cumbersome to stow. But we will have sea going provisions for securing th em.
November 10th, 2011 at 5:46 pm
Can you provide profile and section view(s)?
Thanks,
Bill
November 10th, 2011 at 5:47 pm
We’ll have more detailed views, Bill, when we finish the drawings.
November 11th, 2011 at 12:35 pm
Ummmm…. Someone’s math is not good. 320W x 6 = 1920W. Max Power Point Tracking only assures you’ll get as much of that as possible. If someone says they can give you 2300W out of 1920W of modules, then they should patent it because they’ve discovered perpetual motion. But the patent office is going to say no.
And full rated module output is rare. Module wattage is based on 1000W/sq meter of direct irradiance, 1.5 Air Mass and 25 deg C cell temp. Standard Test Conditions (STC).
Irradiance is seldom at 1000W/sq mt, especially in a marine environment where there’s lots of moisture in the air to absorb the photonic energy. With your modules flat, you won’t benefit from any albedo (reflected) irradiance from the water surface either.
And 25 deg C cell temperature is seldom seen in real life. In partial to full sunlight, you can expect the module cell temperature to increase probably 20 deg C (think black car on a cool, sunny day). Why is this important? Because module voltage decreases and temperature increases. If voltage decreases, power decreases. The Sunpower module has a very good Pmax Temperature Coefficient, but a 20 deg C increase in temperature will still result in about a 7.5% power loss. And that’s when the ambient temperature is 25 deg C.
Hopefully your design guy is running 3 strings of 2 modules, instead of 6 modules in parallel. Heat will drop module output below 48V at about 37C ambient (yeah, I know… that’s hot).
While NREL doesn’t have data on the Bahamas, Weather Underground says the average insolation for the Bahamas is more like 5 peak sun hours per day, not 7.
Without knowing more about your existing equipment and losses, a fair estimate of average daily production in the Bahamas is 5.7kWh to 6.7kWh AC.
Brian
November 14th, 2011 at 6:03 pm
Hi Brian:
Thanks for the heads up. I misinterpreted Bob Wiliam’s comments on the MPPT controllers. What he was saying is they are substantially more efficient than the traditional type and thus got more of the panel output into the batteries. But they do not, as you said, increase output. I asked Bob to comment on the solar hours, and he indicated your figures are accurate, based on yearly averages. The numbers quoted in the post are for non-winter solar angles when we’d be in that area. As to other losses, Bob indicated :On most yacht applications there are so many variables, Azimuth, temperature, obscured to hard shadowing, salt/moisture build up on glass and irradiance that we typically allow for about an 8% loss on average in our area from the straight wattage value calculation.”
The panels will be wired in strings with separate controllers. When I get time this evening I’ll adjust the figures in the post.