PV Gap

Pedro Said:

Thermodynamics problem?

We Answered:

(1)

Mass of water to be heated = Volume*Density
M = 1.2*1.0x10^3 kg
M = 1.2x10^3 kg

Temperature Rise = 40-10 = 30 Degrees C or 30 K (same amount of rise - although 30K is not 30 degrees C)

Energy Needed = specific heat capacity*mass to be heated*change in temp.
E = 4.2x10^3*1.2*10^3*30 J = 1.512x10^8 J
That's close enough to 1.5x10^8 J

(2)
Time required to heat water in seconds = 2*60*60 = 7200 s
Power Needed = Energy/time
P = 1.5*10^8 / 7200 = 20.833x10^3 W = 20.833 kW

Area of solar panels needed = Power / average power per unit area
Area = 20.833/0.8 = 26.04 m^2 (about 5m by 5m - The roof of the house should do it)

Herman Said:

how do I recharge my battery bank?

We Answered:

CCA spec is used for starting engines; 20 hr capacity is used for deep discharge batteries. I hope you got deep discharge batteries, not starting batteries, as they will last much longer. Marine deep discharge are usually between the two types.

I would start with chargers for use dockside, and maybe a small backup generator with 12V output to run the chargers when underway.

A battery bank with 5kA CCA probably has about 400A*hr (up to 1000A*hr for good deep discharge batteries), you need to check battery capacity to be sure. The best charging rate is usually C/20 to C/10 or 20A to 40A (up to 50 to 90A for deep) or 280 to 1300W. This takes 24 to 12 hours to charge the banks.

You need about 35A (all currents at 12V, would be 1/2 as much for 24V) to do your steady state drain. You should be able to get by with about 1kW of solar for $1000 or so. This theoretically generates some 70A; you will be lucky to get half that on the average, when you have strong sunlight. This will barely keep up with your steady state load when operating, much less charge anything. The panels must be unshaded and stay perpendicular to the sun to charge the batteries. This is difficult to do on a moving sailboat, but if you are tied up at a dock, you should be able to position it reasonably well if you make it as a moveable panel, though the sun moves during the day also. As Bill mentions, if part of a panel is shaded, it won't generate enough voltage, and won't charge the batteries at all. Thus, better to have a bunch of small (say 50W/4A panels) connected in series. That way, if some are shaded, the unshaded ones will still charge.

Wind is tough to harvest on a sailboat. The windmill needs room for the blades, and needs to stay perpendicular to the apparent wind. It will also slow the boat some.

A 1kW generator runs $3k, has 8.2 feet prop diameter, and requires 25mph to run at capacity. http://www.bergey.com/
Only decent place to mount windmill is on top of mast, assuming mast is sufficiently strong for it. If you could mount a 6 ft dia windmill on top of the mast, it could make about 500W with a 25mph wind, or 60W/4A with an 8mph wind. This should run about $1500 or so, should make about 2.5kW*hr/day, using the manufacturer's probably optimistic numbers.

There may be water turbines you could lower away and tow. Not sure how heavy, bulky or expensive they are, sources are hard to find. They will slow the boat, but can work.

A 2ft dia water turbine to tow would generate about 600W or 40A at 5 mph, requiring about 75 lb of drag. Best position is off the bow, as the water is disturbed off the stern. Towing off stern is problem, because cannot allow generator to spin. Up and down motion of bow will cause problems tho.

I would google different sources, price the systems and compare the power generated with what you need, select the one that is most economical for you, and consider making do with supplementary power from the chargers and generator when needed.