from a freewheeling propeller. In the
future, fuel cells may be a viable energy
source.
Every non-engine kilowatt-hour
(kWh) of energy reduces the average
fuel consumption rate for propulsion
and house power. In certain niche
applications, it is possible for the entire
energy budget to be met from non-engine
sources. An example of this is short-haul
ferry boats that can plug into shorepower
between trips, storing sufficient energy in
the batteries to complete the subsequent
trip. Catamarans with high sailing
speeds and intermittent propulsion
needs are another example. These boats
can produce sufficient energy through
regeneration to meet normal propulsion
needs; they also benefit from having
a large surface area available for solar
power.
In both these examples, a serial
hybrid installation would, in effect,
operate as an electric boat, with the
generator held in reserve for emergency
purposes or longer trips that exhaust
the batteries. The lower the generator-run hours in relation to the propulsion
hours, the lower the average fuel
consumption rates. These rates are
likely to be well below those in an
optimized conventional installation—
even if the fuel consumption rates when
the generator is running are higher than
in the conventional installation.
The core problem with relying
on non-engine energy sources for
propulsion is the low levels of energy
that are available at sea, and the low
amounts that can realistically be stored
on board. In practical terms, leaving out
regeneration, which will not be available
when in propulsion mode, the sources
are solar and wind.
Any time a boat has to be operated
at speeds above the crossover point,
the propulsion loads will be relatively
high and, as such, will overwhelm these
energy sources, forcing the system to
draw off the batteries, which will be
rapidly depleted. Once the batteries are discharged, the serial-system generator must be cranked, typically with a loss of
efficiency as compared with an optimized conventional system.
In other words, any extended period of operation above
the crossover speed will sooner or later force the generator
to be cranked, resulting in overall efficiencies below those
of a conventional system. In contrast, a parallel system will
now operate as a conventional system, with the same level
of efficiency.
A potential breakthrough technology that will
fundamentally change this situation, when and if it matures,
is reformulating fuel cells—fuel cells that will run directly
from diesel, but with energy conversion levels from diesel to
electricity well above those achieved by generators.
Figure 4 Fuel Consumption Detail
This expanded version of the lower end of Figure 3 shows that if you are using
batteries for electric propulsion, a conventional diesel engine becomes more fuel
efficient at around 4. 5 knots.
Figure 3 Fuel consumption
Fuel consumption versus boat speed for an optimized conventional system versus
diesel-electric and battery-powered hybrid modes. TPPL stands for thin plate
pure lead, a type of AGM battery. The chart shows that at higher boat speeds a
conventional diesel engine is more efficient than an electric motor powered directly
or through batteries.
