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In the previous issue of PassageMaker, I defined “crossover speed”—a theoretical speed below hich a hybrid propulsion system is more fficient at moving a boat than a conventional propulsion system. Above this speed, a conventional system is more efficient. The definition of crossover speed is based on the following assumptions: 1) The engine driving a generator in a hybrid system can be
kept within 5 percent of its peak efficiency. 2) The generator
will have an electrical efficiency (converting engine power
into electrical power) of 90 percent. 3)
The electric motor driving the boat will
be 90 percent efficient at converting
electric power back into mechanical
power (turning the propeller shaft). 4) If
batteries are used to store energy from
the generator and then supply it to the
electric motor, the efficiency losses will
not exceed 15 percent.
For the past several years, I have
participated in extensive testing of
engines, generators, electric motors,
propellers and thin plate, pure lead
batteries (TPPL), a variant of AGM
technology. I can say with certainty
that other than in large hybrid systems
( 100 hp and up), it is difficult to achieve
any of the four efficiency targets defined
above.
We tested a generator that met the
efficiency targets, but it required design
compromises that would make its
price uncompetitive in the real world.
Furthermore, to maintain high-cycle life
with any form of lead-acid battery, and
even with some lithium batteries, they
must at times be given an extended charge at low rates. If the
charging current has to come from a generator and there are
no other loads on the generator, the efficiency of the engine
driving the generator and of the generator itself will fall below
the target efficiencies, dragging down the overall efficiency of
the hybrid system.
We tested electric motors that met the efficiency target,
but only over a narrow propulsion range. In particular, we
found that just as with engines, electric motors suffer a loss
of efficiency at low speeds and light loads. This is the area of
operation in which the hybrid theoretically gains the most
over the conventional system, but now we find the hybrid
cannot realize some of these gains.
Serial hybrid systems require powerful electric motors to
deal with worst-case propulsion demands, whereas parallel
systems rely on an engine for high propulsion loads and,
as such, have smaller electric motors sized to handle light
propulsion loads. Given the powerful electric motor in a
serial system, the inefficiencies of low-speed operation are
likely to migrate into harbor maneuvering speeds, negating
some of the presumed efficiency benefits as compared
with a conventional system. In contrast, given the much
smaller electric motor in a parallel system, even at harbor
maneuvering speeds, the load should be high enough to drive
the electric motor into its efficient region of operation.
We tested the charge and discharge efficiencies of TPPL
batteries. (Thin plate pure leads are a form of AGM battery.)
At the high charge and discharge rates likely to be found
in a hybrid system, the batteries fell short of the efficiency
target. We could have used lithium batteries, which have
cycling efficiencies above 90 percent, but we felt their cost
is too high at present for much of the market to bear, and
there are unresolved management and safety issues that
need to be sorted out before deploying these batteries on
a widespread basis (as Boeing has discovered, to its cost,
aboard its 787 airliners).
As a result of what we learned, we lowered the estimated
crossover speed below which hybrid propulsion is more
efficient than conventional propulsion, and above which
Figure 1 Propeller Curve
Propeller curve plotted on the efficiency map for a 16kW electric motor.
The colored squiggles are labeled with the percentage of efficiency as RPM
increases. It shows that at no point is the motor more than 90 percent efficient,
and even looses more efficiency at harbor maneuvering speeds.
