When Green Yachts Ain't So Green

A BOAT OR A YACHT MAY CARRY A "HYBRID" LABEL, BUT THAT DOESN'T MEAN SHE'S TRULY ENERGY-EFFICIENT

Green is in when it comes to marine marketing. Which means a whole lotta builders are rushing to label their vessels as "hybrids". Unfortunately, not every hybrid configuration is, in fact, energy-efficient.

It's commonly assumed that, whatever else a hybrid might be, it's more energy-efficient than a non-hybrid. But whether or not it is, in fact, depends heavily on how you define “hybrid” and critically on the hybrid configuration exhibited by the vessel in question.

A while back, I was talking to a yachtsman about his custom "hybrid" vessel. It utilized some of the power developed by her propulsion engines to generate electricity by means of a couple of high-capacity, engine-mounted alternators. The electricity produced was stored in a large bank of batteries to provide propulsion power entering and leaving harbors and overnight as silent house-power without resort to a genset. Because the vessel employs both direct diesel power and electric power from batteries, the owner was explaining to me that she is an "advanced hybrid" vessel.

What I tried to explain was that his use of the term "hybrid" in this context is, at best, a stretch and, at worst, an outright misrepresentation. Because the system he was describing to me actually uses more, not less energy to service his vessel's needs than would be the case for a configuration that utilized direct-drive diesel for propulsion and an appropriately-sized diesel genset to provide ship's current at anchor. Here's why.

Energy is neither created nor destroyed, but only transformed from one form to another — for example, from mechanical energy to electrical energy, or from electrical energy to chemical energy, and vice versa. A perrenial obstacle in achieving energy-efficiency is that transformation always and inevitably involves the loss of a portion of the energy being transformed. And almost always that loss is to a disordered form from which the lost energy cannot be recovered.

In other words, the useful energy output in any transformation is necessarily always smaller than the energy input.

Consider that the energy supplied to an electric motor — whose output is mechanical energy of the kind that turns a propeller — is generally only about 90% efficient, with 10% being lost in the form of heat that is dissipated into the surrounding environment. Consider further that the transformation from electrical energy to chemical energy (battery storage of electricity) is, in the case of conventional lead-acid batteries, only about 85% efficient. All of which means if you generate electricity that is then transformed into chemical energy stored in a battery, then recovered from the battery and used to drive an electric motor to turn a propeller — the total energy loss in the course of this transformational "round trip" can be as much as, or even more than 25%. Consequently, for a given amount of propulsion energy absorbed, the kind of "hybrid" system described by the yachtsman with whom I was speaking uses some 25% more fuel than mechanical power derived directly from a diesel (or gasoline) engine.

To be sure, the "round-trip" efficiency (from electrical energy to chemical energy and back again) of lithium batteries is much better at approximately 95%, than for conventional lead-acid batteries. But that is still significantly less than 100%. Which means that storing electrical energy in batteries and retrieving that energy to use when needed incurs a loss to heat of at least 10%.

The bottom line is a "hybrid" vessel that uses only diesel or gasoline power to generate electrical power which is stored in batteries, actually uses more, not less fuel than a vessel driven directly by a diesel or gasoline engine.

If you store in a bettery electrical energy which has been transformed from mechanical energy, then recover and use that electrical energy to propel your boat or yacht, the loss of energy to entropy may be as much as 25% or more.

Consequently, to my mind, there may be valid reasons for employing diesel-electric propulsion power — for instance, the ability to soft-mount electrical generators for use in lieu of conventional propulsion engines in a way that seriously minimizes transmission of sound and vibration — but fuel-efficiency, per se, is not among those reasons.

The physics of transforming one form of energy into another invariably involves energy losses to entropy. The reason diesel-electric is not more widely employed in yachts is that it is less, not more fossil-fuel efficient than direct diesel mechanical drive. The only way a hybrid system can be designed to reduce fossil-fuel consumption is to introduce one or more additional sources of energy production (e.g. solar, wind, or electrical generatoion via water-driven impellers) with the energy so produced stored in batteries when available and used for propulsion when needed.

One of the best hybrid power configurations to come onto the market is the PHEV, which is a road vehicle that combines a fossil-fuel engine with an regenerative braking energy recovery system, both supplemented with the ability to plug into land-based electrical charging stations. Unfortunately, this configuration is simply not viable for vessels (except, perhaps, those which operate away from their home dock only for short, clearly defined periods).

Alas, there is no magical energy hen to lay golden energy eggs.

Using the diesel propulsion engine to generate electrical energy won't create new energy, battery-stored or otherwise. Nor will a diesel genset. In fact, using a diesel engine or genset to produce electrical energy that is then converted to mechanical energy to drive a propeller will use more fossil-fuel than simply using the diesel engine to drive the boat's propeller.

Please understand, I am not arguing against hybrid propulsion in boats and yachts. I am simply saying we need to understand what form(s) "hybrid" has to take if it is to be correctly associated with "fuel-efficient".

— Phil Friedman

Note: An earlier version of this article was originally published on my LinkedIn blog.

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