About year ago I've posted a thread here (
http://www.cruisersforum.com/forums/f14/solar-battery-load-perfect-balance-136734-2.html) trying to share my experience with others. It was about solar-powered
electric propulsion. The question was simple: is it possible to move a vessel on sun-generated
power for several hours?
Although the
solar power issue seems to be extremely popular among cruisers, the disscussion was rather academic and not supported by field-tested systems. Perhaps because nobody else was ever considering such
project viable for many good reasons. Tempted by the challenge, I spent last year developing an experimental
solar system to push my 3-ton house
boat along the shores of spectacular mountain lakes in Canadian Rockies.
After 6 weeks long voyage this summer I'm very cautious - if not sceptical - about the idea in general. Yes, it is possible to build a vessel powered exclusively by the solar energy, but it will look like anything but the
boat. Like the aircraft carrier, perhaps. Hybrid power plants may represents more realistic approach, i.e.
diesel generator/PV panels/battery system.
The main challenge is to strike the practical balance between sun energy harvesting surface and the functionality of the vessel. Another words: to find this sweet spot when installed
solar panels are delivering just the right amount of energy and the boat didn't loose it's appeal completely.
Let's do some rough calculations with minimalistic approach. Smallest
outboard gasoline motors, able to push 2-3 ton
sail boat with few knots on calm waters, can deliver some 2HP of power. This translates to 1.5kW, within the range of Torqeedo 2.0
Cruise electric outboard. To run this
motor at half throttle 1kW of power need to be supplied, and 24V/200Ah
battery will be emptied very fast. Now, if you support the
battery by inserting solid stream of sun-harvested energy, the battery discharge
current drops dramatically, making much longer trips possible before lo-batt shut off.
The problem lies within limitations of PV technology. Certain average output can be achiewed with certain area of
panels, to put it simply. This is usually not a big issue for megawatt installations in Arizona desert or even for off-grid system on the roof of your cottage. But when you need to power an electric vehicle - car, boat, golf cart - the available harvesting surface is usually way too small to render practical, self-sustainable system. To harvest - say - average 1kW of real life output one must install at least twice that much of total Pmax! Assuming that an typical cruiser may offer enough space to install perhaps 300, unlikely 500W of Pmax worthy PV
panels, this is dead end already...
Because of limited sun-harvesting area,
electric motor must rely mainly on the capacity of on-board battery bank. This is yet another obstacle for the idea of any electric vehicle, because
batteries are heavy and must be re-charged. While filling your gas tank may take minutes, battery re-charging is a matter of hours. The most powerfull, commercially available electric motors may deliver 40, even 80 HP, but the boat must carry hundreds kilograms of
batteries, good for a hour long sunday afternoon trip around local pond.
When it comes to shop for an
electric motor for your boat, the choice is limited actually to one brand,
Germany made Torqeedo that is. Torqeedo motors filled self-created
marketing niche somewhere around 2005, when strict
regulations prohibit the use of gasoline fueled engines on many European waters. In such scenario an average user will consider the electric
propulsion as a poor substitute only. When special projects are in focus, however, the electric
motor may be much more seriously considered than just as a mediocre equivalent of gas or
diesel engine.
I've been able to taylor well balanced system, compromising the look and functionality of my vessel to a reasonable degree. One must realize, however, that what was tested and prooved on fresh waters and with narrowly specialized boat doesn't necessarily applies to all vessels. I hope the numbers mentioned below will be a good starting point for consideration.
So, this is basically an oversized pontoon boat on two 30" diameter tubes and 22' x 8'
deck, of approx.2750 L replacement. Not particularly slim or elegant, she was custom designed and build to undergo
extended cruising on
remote mountain lakes in
British Columbia. The supplies there are limited, therefore self-sufficiency was the main culprit and the solar power was a viable option.
Mercury 60HP Big Foot
engine supply the main propelling force, two Torqeedo
Cruise 2.0 motors were choosen for alternative electric propulsion. Although main engine will deliver about 11 knots at full throttle, 3-4 knots I'm considering just fine with electric motors, consuming roughly 1kW at this speed. Pushing the electric throttles full will drain 4kW, but the speed gain isn't really that dramatic or needed in this case.
Carefully tailored array of various PV panels consists 4 separate, independently controlled strings, 2 on each slope of
cabin roof and awning. This is accomplished with 2 double channel
MPPT chargers (Italian made Western WRM30) to eliminate conflicting
current flows between shaded and exposed sides, and to handle different characteristics of panels. This configuration equals 534W of Pmax per side, 1.064kW total. To my amazement the system is able to deliver 650-700W in a good day with both sides exposed to sun equally. The same days at evenings (with one side shaded) 400-450W contribution was typical. In this scenario the 24V lead acid battery bank was delivering remaining 500W or less of power, not really much of abuse for 225Ah capacity. As a result, several hours of peacefull, steady drive against 5-10km/hr
wind could render 50km long trip with enough battery energy left for overnight needs. The future replacement for bigger (and much lighter!) LiFePO battery will significantly improve the overall performance.
Personally I've found electric propulsion to be enjoyable almost as much as sailing with light breeze. No polution, no
noise, long hours of joyfull ride on secluded, pristine lakes. It may be perfect for
canal boating or barges, where speed is no issue and usually huge
cabin offers large sun-harvesting surface. Recent developments on lightweight flexible
solar panels,
lithium batteries and efficient charge controllers may render really nice systems. On the other hand such
installation is fairly complex, clumsy, heavy and prohibitevely expensive. Savings on
fuel? The cost of suitable solar power plant will cover probably a decade of diesel supply for your Volvo!
To hammer another nail into the coffin of electric propulsion idea, consider the following fact. By comparison of net calorific values, fossil
fuel carries roughly 100 times as much energy as mass equivalent of
Lithium battery. Another words: 100kg weighting Lithium battery will hold the same amount of energy as 1kg of gasoline (12,200 kWh). Unbelievable, rediculous, but true.
! See, my dear soul mate, I'm not THAT MUCH desperate to make my boat 100% green. Somehow I must control already insane amount of hassle and 
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