Sizing
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Sizing
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DIMENSIONING OF THE ELECTRICAL SYSTEM: CONSUMPTION: The starting point for all our reasoning is the power-balance, i.e. determining the consumption of all on-board equipment; usually the computation is referred to a 24-hour interval, assuming that recharging will take place once daily because more frequent recharges would severely spoil the batteries' life, and recharging less often would require huge batteries and very powerful recharging means. On this page you will find examples of electrical balance, all based on Shaula4 electronics package (on Shaula3 consumption was even higher due to the greater complexity of the electronic devices of just 8 years before....). We considered 2 scenarios: boat idle (at anchor or moored with no shore-power available) and boat under sail on an extended passage (i.e. sailing through 24 hours); obviously the main difference is that when at anchor most electronics will be off and at night only 1 anchor light will be on, instead of the 3 needed for navigation (unless a tri-colour is used, of course). CASE 1, based on a conventional-equipped boat, shows daily consumption figures that are positively alarming: nearly 100 Amp-hours for the boat at anchor, and a whopping 250 Ah for the boat under-way! BATTERY DIMENSIONING: The commonly-accepted practice is to avoid discharging batteries below 50% of their capacity, to avoid spoiling them too quickly; furthermore, it's not realistic to assume being able to recharge the batteries up to 100%, because this would take too much time (the batteries accept less and less charge when they get near 100%): in practice, the assumption is to go from 80% down to 50%, that is only 30% of their nominal capacity. For example a 100Ah battery would output only 30 Ah. We apply this rule to CASE 1 and the result is that for the boat at anchor we would need 3 "service" batteries (to which another battery must be added for engine starting) and for the boat under-way 7+1 batteries would be required (or more likely 3+1 accepting to recharge twice per day.
Surprising as they may appear, these figures are quite normal,
and in fact it's very common to see large seafaring yachts hosting banks of 8-10
batteries: the problem is with medium-small boats, where there is absolutely no
way to fit all that stuff! Obviously, larger-capacity
batteries can be used, but that's not going to solve the problem, because
larger-capacity batteries are bigger!
The battery box (3+1) on Shaula3... and on Shaula4 (2+1) Besides the size problem, another one comes up, i.e.: DURATION OF THE CHARGING: Most "marine" engines are just modified automotive engines and are equipped with plain-standard automotive alternators, which are conceived to charge a battery...which is already charged! Furthermore, the problem is to not overcharge the battery during long runs. Not a surprise then, to see that after few minutes from starting the engine, the alternator's output is down to a value far below its nominal capacity (a typical value is 600 Watts, corresponding to about 50 Amps). Such an alternator would be adequate to recharge a 3+1 battery bank, but definitely not an 8-battery bank!
A typical value of charging current would be around 10% of the battery capacity (that is, for a typical 100 Ah battery, a charge flow of 10 Amps), so for a bank of 2 or 3 service batteries we may expect a charge flow around 20-30 Amps, which in turn means it takes 3 hours for a recharge back to 80% of battery capacity! THIS IS NO GOOD!! Running the engine 3 hours every day, practically at no load (the alternator would require less than 1 HP), is very bad for the engine, let alone if this practice is protracted over time, accumulating something like 1000 engine-hours and using 500 liters of fuel per-year!!
It's imperative to act on both sides of the equation, that is: 1) REDUCE CONSUMPTION: What can we do? First of all, there are some "technological" solutions, like adopting LED lamps for cabin-interior lighting and also for anchor- and navigation lights. Technology has quickly evolved over the last few years, and now LED lights provide an excellent illumination for a negligible consumption; in our example (see CASE 2) the saving would be around 30 Ah/Day. Another strategy is to rationalise the use of on-board electronics: for example, there is no need to keep radar constantly on when sailing offshore, and one of the two chartplotter displays can be switched off (I would prefer to keep on the one in the cockpit); furthermore, some last-generation instruments have an energy-saving mode: putting all these measures together, consumption from electronic instruments can be reduced to 30-35 Ah/day. One of the devices with the higher consumption is the refrigerator: not strictly essential, worst case it can be switched off, but a good insulation around the coolbox and an effective cooling of the compressor unit (perhaps by sea-water circulation, as it is advisable in tropical areas) can go a long way to help optimising the fridge consumption without resorting to more drastic measures! Another trick could be to switch the refrigerator off at night, especially if the fridge is equipped with an accumulator-plate. Finally, the other big power-hog: the auto-pilot! Indeed, the alternative of hand-steering under sail and using the autopilot only when motoring is not very attractive, albeit feasible. In fact, on a long-distance boat, the adoption of a wind-vane becomes an interesting alternative! All these measures lead us to a situation like our CASE 3, where 2 or maximum 3 service batteries would be plenty!!
There's still a problem though: it still takes 3 hours to perform
a daily recharge using the engine alone! 2) SPEED-UP THE RECHARGE: 2a) INCREASE THE ALTERNATOR OUTPUT, which can be achieved in two ways: - installing an "intelligent regulator" on the alternator, replacing the built-in automotive regulator (which is mainly designed to avoid over-charging batteries); these regulators provide a sophisticated charging cycle, ensuring a faster recharge and a safe hold stage when the batteries are charged and the engine is running. The problem with these regulators is that they cannot work miracles: they can just somewhat increase the regular alternator's output and possibly protect the batteries from overcharging, but that's basically all.
The ADVERC smart regulator installed on Shaula3
- replacing the alternator with a more powerful one (or perhaps
adding a second alternator, if space allows): space availability needs to be
checked acrefully, and extra belt and pulleys may be required to drive an
additional or more powerful alternator. In the end, it's an
expensive modification to keep charging the batteries running the engine at no
load, which is still undesirable. 2b) ADDING EXTRA CHARGING SOURCES:
- GEN-SETS: for many years, the only practical way to produce
electricity on-board has been with the use of a dedicated generator. At the moment the market does not offer small water-cooled, diesel-powered generators of few HP and suitable for a fixed installation; the few that exist are air-cooled portable ones, to be operated in open air, which could be useful for other tasks (e.g. to feed power tools during major works far from civilisation) but are not particularly convenient to the daily task of battery charging on a boat under way. - WIND GENERATORS: despite an output of only few Amps, over the 24 hours they can supply 40/60 Ah therefore giving an important contribution to the daily power balance, significantly reducing the battery output and therefore the recharge time. And all this for free! The downsides are not insurmountable either: - questionable looks and a large propeller which must be located high up where it cannot harm someone not paying enough attention - noise (this varies pretty much with the individual models, some being almost silent and others whistling like a banshee, which can be annoying at night while at anchor)
- output depends on wind strength
(not very effective when sailing downwind, totally useless in flat calm)
The wind generator and the solar panel installed on the stern
portal of Shaula3 - HYDRO-GENERATORS: similar to the wind generators, so much so that some can be operated in both modes, they replace the helicopter-style propeller with a much smaller one which is then trailed behind the boat on a long line or attached to some kind of shaft; the boat motion through water makes the propeller turn, the advantage on wind-generators being that they are more effective when sailing downwind, but: - there is some drag causing a speed penalty in the range of 0.5/1 Knot (not negligible over the 24 hours during a long passage) - obviously useless when the boat is not moving - on old-fashioned "trailed-propeller" models, recovering the rotating prop is a bit tricky - the trailed models do not mix well with fishing lines being trailed at the same time! In recent years some "double use" generators have been put on the market that try to address the switch-over from wind to water by simply rotating the pillar supporting the generator from the vertical position down into the water (replacing the propeller while doing that, of course!): interesting but fairly expensive and some have turned out to be rather fragile; a technology to be watched, nonetheless. - SOLAR PANELS: a seafarers' dream, electricity produced for free by a static device, no moving parts, no noise, almost no maintenance, and all this for an affordable cost. There are a few problems though: - it takes large panels to obtain a meaningful output (7-8 A per square meter?) - consequently problematic installation aboard due to size and need to face the sun (arguably the best position would be on an arch over the stern) - optimal output only in the central hours of the day (maximum 40-50 Ah/day for a 100-Watt panel?) - output is seriously diminished by clouds, shadows falling on the panel, orientation not square to the sun It is certainly not surprising that most ocean-going yachts sport both wind-generator and solar panels: the two combined may get very close to obtaining electrical self-sufficiency! PRACTICAL EXPERIENCE: On Shaula3 we had in fact wind- and water-generators plus a 100 Watt solar panel, and usually these combined allowed us to limit charging from the engine to one hour per day, sometimes even less; when we had to limit the use of the wind generator due to worn bearings causing serious vibration and at the same time using more frequently the autopilot instead of the wind-vane, we had to increase engine-charging time to two hours.
- NEW TECHNOLOGIES: in the last years new technologies have
appeared on the market; among the most promising, FUEL CELLS and new battery
technologies derived from rechargeable dry-cells. Still very expensive, but worth keeping an eye
on. GO BACK TO THE "ELECTRICITY" PAGE
CONTINUE READING THE "ELECTRICAL
SYSTEMS" PAGE
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Webmaster: Gianfranco Balducci - email: gfbalduc@tin.it Last Update: 07/09/2017
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