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The Science of Hybrid Propulsion: Part 2

In the second half of Nigel Calder's Hybrid series, the author argues the hybrid technology is not just about propulsion, but supporting comforts.
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In the previous issue of PassageMaker, I defined “crossover speed”—a theoretical speed below which a hybrid propulsion system is more efficient at moving a boat than a conventional propulsion system. Above this speed, a conventional system is more efficient.

Click Here To Read Hybrid Science: Part 1

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.

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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.

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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 conventional power is more efficient. Figure 4 represents a revised version of the previously developed theoretical crossover map used in Part I. It is still optimistic in practice, representing what we think are achievable targets—not what has already been achieved.

In Figure 3 I have plotted fuel consumption versus boat speed for our test boat, a 48-foot, 18-ton sailboat with an optimized conventional propulsion system. This system was made as efficient as possible to raise the bar as high as possible for the hybrid systems. Figure 3 also shows two other curves: one for a serial hybrid system in diesel-electric mode and one for either a serial or a parallel system in battery-driven mode using TPPL batteries. The crossover point for diesel-electric mode occurs at 5.8 knots. In battery-driven mode, the crossover speed drops to 4.55 knots. Figure 4 is an expanded version of the lower end of Figure 3.

When looked at in percentage terms, below 3.5 knots, both diesel-electric and battery-driven hybrid systems achieve massive gains in fuel efficiency compared with the optimized conventional system. If we extrapolate down to 3 knots (i.e., harbor maneuvering speeds), hybrid fuel consumption is less than half that of the conventional system, and there will be additional gains through the elimination of all dockside idling.

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Achilles’ Heel

Above the crossover speed, in percentage terms, the increase in fuel consumption of the hybrid system is nowhere near as great as the percentage gains below the crossover speed. However, in absolute terms (liters per hour), the hybrid losses rapidly dwarf any gains below the crossover speed. Parallel systems do not care, because when the vessel is operating above the crossover speed, it will be under engine power, whereas serial systems pay the full penalty.

This is the Achilles heel of a serial system. If vessel operation requires sustained running above the crossover speed—and often, cruising speed will be above the crossover speed—the losses will rapidly overwhelm any gains below the crossover speed, for a net loss of efficiency.

To avoid this serial-system efficiency penalty, you have to raise the efficiencies in the serial system to a level that shifts the crossover speed in diesel-electric mode to cruising speed or higher. Then, the serial system only pays an efficiency penalty on those occasions when it is operated above cruising speed. I have seen a number of efficiency claims for larger serial systems that fit this scenario, but I have not seen data to determine whether this has been achieved in practice.

The parallel system must pay the battery loss penalty, which lowers the crossover speed, but as long as electric operation is used only below the crossover speed, the parallel system will always show a net gain in efficiency. This may be quite substantial in percentage terms, although, as we have seen, it will typically be modest in absolute terms.

The electrical testing required multiple custom shunts for measuring amperage, which were also specifically developed for the project.

The electrical testing required multiple custom shunts for measuring amperage, which were also specifically developed for the project.

There is something important missing from this analysis. Up to this point, I have presumed that all propulsion energy comes from an engine on the boat—either the main engine in the conventional installation, and at higher loads in the parallel system, or a generator supplying the energy for electric propulsion. This is necessarily true in the conventional system, but not necessarily true for the hybrid. At the core of any hybrid system (serial or parallel) is an electric bus. Energy for propulsion (and house) loads can come from any electrical source. In practical terms, on boats this means shorepower, solar, wind and, on a sailboat when under sail, regeneration 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.

Beyond Efficiency

So far, my focus has been on propulsion efficiency. It is clearly important to understand these efficiency issues, but in reality I do not believe this will be the driving force behind hybrid adoption by recreational boaters. The core benefits of hybrid propulsion are quiet operation and freedom from exhaust emissions. Every time we took our test boat off the dock with no noise and no exhaust, all those around stopped and stared.

With hybrid power there is no more dockside idling, or idling in locks on a canal system. There is no more engine run time while in a harbor, because low-speed maneuvering is performed under electric power. Our powerful generator enabled us to consolidate six hours of engine run time for maneuvering in harbor into one hour of generator run time (or an overnight charge from shorepower, when available, with no generator run time at all).

Our experiments required sophisticated data monitoring, including a custom-built datalogger,  which is here being programmed and tested by people from TML, its developer.

Our experiments required sophisticated data monitoring, including a custom-built datalogger, which is here being programmed and tested by people from TML, its developer.

A hybrid battery pack will be relatively large. For the hybrid to function, the battery pack will need a high charge acceptance rate. As a result, a considerable amount of energy can be put into the batteries in a short space of time, and the battery pack will support substantial house loads between recharges. On our test boat, in an extreme case we could put four days of house power into the batteries in 15 minutes of generator run time. On many boats it will be possible to power overnight air conditioning without running a generator.

Unlike propulsion power, at all times the generation of house power is far more efficient than in a conventional system, a topic for some future issue.

The powerful generator and large battery pack of hybrid systems are enablers for more extravagant lifestyles. It is these lifestyle issues—lots of house power and substantially reduced engine run times, along with the concomitant silence and freedom from exhaust emissions—that will ensure the success of hybrid systems. The greater the house loads and the more extravagant the onboard lifestyle, the more attractive a hybrid becomes. Any efficiency gains will simply be a bonus.

Whether or not a serial or parallel system is more appropriate in any given application will remain a complex calculation based on projected propulsion duty cycles and the current state of technology.