About a year ago I laid out the range situation for my electric lobster boat Eleccentricity. I speculated about what the range would be if I added solar panels, and in other posts I wondered what speed I could attain under sunlight alone. Now I have added 480 watts of panels and can now update the speculation with hard data. I made this test today, the 25th of August, 2013. Not exactly the height of mid-summer solar radiation, but good for averaging over the boating season. First, the sunlight-only question. This was funny because I noticed that I could get an extra 36 watts or so by orienting my boat’s roof toward the sparkling water (i.e., the sun), where the headwind was unfortunately, strongest. With the motor off I could see 10.2 amps coming from the panels. Since the system voltage at the time was about 38 volts this works out to 388 watts. Not too near the rated 480 watts, but what I didn’t know at the time was that there were two blobs of gull poop on the panels, and rated wattage is just an ideal anyway. Now the speed test. I plugged in my car’s GPS and waited while it found satellites. Then I headed into the sun and wind to measure the speed while adjusting the throttle so that my amps were zero (inflow equaled outflow). I tried this in several directions and ended up with an average of 3.5 MPH. Imagine, moving a 1000 pound boat through the water at a slow jogging speed on sunlight alone! I expect I could hit 4 MPH at the summer solstice at midday, with calm wind but I’m pretty happy with this. I increased the speed to 4 MPH and checked the ammeter: 2.2 amps, or 83 watts from the battery. So I could travel at 4 MPH for 7200 (my batteries’ capacity in watt hours)/83=86.75 hours for a range of 347 miles! Now, the clever reader will note that the sun will be not only weakening but also going down, so that 86.75 hours would be a fantasy, but it’s fun to do the math.
With the panels’ contribution I throttled up to 5 MPH using 16-20 amps or 608-760 watts, giving a range of 60-47 miles. At 6 MPH I was using 52-70 amps or 1872-2520 watts yielding 23 to 17 miles. All of these computations are of course subject to reductions due to wind, waves and clouds.
I would not want to drain my batteries down to zero. In fact, going down to 50% is about the lowest I would ever go because deeply draining batteries shortens their lives, so the above range numbers should be cut in half. However, the nice thing about going somewhere is that when you get there, you let the boat sit in the sun until you go back. There are no trees or buildings blocking the sun on the water! If you voyage two hours at 5 MPH and use 1300 watt hours, the charge percentage would be 1300/7200 = 18% , 100%-18%=82% in reserve. Let’s say you do something on shore for 2 hours. During that time you get 380 solar watts, so that’s 380*2=760 watt hours. 760/7200=10.5%. So when you shove off your state of charge is now 82% + 10.5%= 92.5%. So you get home from your 20 mile round trip with 92.5%-18%=74.5%. Real numbers would be lower due to reduction in solar radiation over the six hour time, but this is the kind of planning a solar boat owner has to go through. This virtual trip brings up an important point, that a trip should be planned around the strongest sunlight (midday) for the greatest range. So that six hour trip left us with 74.5% at the end. If we wanted to be left with 50% instead, we could use an extra 24% (1728 watt hours, or 2.2 hours at 5 MPH with no solar help because the sun is weaker) This would add 2.2*5 MPH or 11 miles to our 20 mile trip, for a total of 31 miles.
Now the down side: Rain and clouds. It’s a day later and the sky is as dark as predawn. These panels use monocrystalline silicon cells and they’re supposedly the best, converting 18% of the sun’s energy to electricity. Will there be any watts flowing into my batteries at 8:30 AM on a very cloudy day?
No! What was I thinking? Still, I came back with six lobsters, so the trip was not wasted. The battery level started at 100%, so the sun did its job yesterday.
I think owning an electric boat will not require complex projections every time I want to make a trip. Eventually I will develop rules of thumb regarding the range question like, if it’s sunny I can make a 20 mile round trip at 5 MPH, no problem. It won’t be more complicated than figuring out where the next gas station is, and a lot cheaper. And the nice thing about a solar electric boat is that I can slow down or stop in the sun to extend my range, and best of all, it’s a zero-carbon trip.