Diesel-electric power has powered ships and submarines for nearly a century. Today, it is the subject of more discussions between owners, captains, designers and builders as a cleaner, more efficient source of on-board power and propulsion. It has been hyped as a possible ‘greener’ way forward, but is it?
What it is and how it works
When Rudolph Diesel obtained the patent for his 1.58kW Rational Heat Engine with its 10ft diameter flywheel in 1892, he never could have predicted that what came to be called the diesel engine would be as popular as it is today. Nor could he have foreseen the many uses to which the engine has been put. He might, however, recognise the diesel-electric drive train.
Such a system, quite crude by today’s standards, was installed in 1903 in the Russian tanker Vandal, while Diesel was still alive. _The Vandal’s _diesel-electric drive was a far cry from today’s systems where the driving engine and its electric generator (often called by its abbreviation, genset) might be on one part of the vessel and the motor driving the propeller might be elsewhere, even outside the hull.
There are arguments both for and against diesel-electric drives. Proponents say that emissions are lower because only the power needed is used and fuel consumption is reduced, while those against suggest that the weight of a genset and driving motor increases the overall vessel weight, which requires more horsepower to drive the yacht. Additional electrical systems also add complexity, which can cause interference with other electrical systems.
From the naval architect’s standpoint, a diesel-electric system allows the drive engine to be moved away from the end of the propeller shaft. The diesel generator can also be located away from guests to reduce noise and vibration and placed in areas of the hull not normally used for engines or people.
For example, on a 40m yacht, the area under the cockpit may not have adequate headroom for crew cabins, but a pair of diesel generators might fit easily. On vessels over 150m, a gas turbine located in the stack where hot gases can be directed up and away from the deck may be used to generate power, moving power generation completely out of the hull.
A further advantage is that a yacht equipped with several generators can carefully optimise power levels between the house and drive loads. For example, when the yacht is under way, house loads tend to be lower than when the yacht is sitting in harbour with guests aboard. This enables the electronics system to adjust generator power levels between house and powering modes so that both are operating with minimum emissions and fuel consumption.
Under way, the electronic controls monitor propulsion performance as well as house loads and adjust the generators accordingly. This eliminates running high-horsepower main engines continually and still having to adjust generator loads. The end results are lower emissions and improved fuel performance. In addition, an electric propulsion motor can easily be controlled in terms of speed and be reversed without the need for a large gearbox or transmission.
The end result is lower emissions and better fuel performance. In addition, an electric propulsion motor can easily be controlled in terms of speed
An added benefit is the fact that the drive motor can be located outside the hull in a pod drive of the type that Siemens and Schottel have developed, or an azimuthing pod drive can also be used. Using pod drives allows seawater to cool the drive motors, reduces the size of the engine compartment and cuts down on vibration and noise inside the hull. By turning the actual drives, propeller thrust is directed where it can do most good, rather than being deflected off rudders. In addition, rudders and their complexity are eliminated, improving hull drag. In addition, because thrust lines are horizontal, propeller efficiency is considerably improved.
Gensets and batteries
The number of gensets required for a successful diesel-electric installation is critical. If, for example, three large generators are used, at least one will have to operate continuously unless the vessel is on shore power. In addition, one might be under repair. Considering that an 80m yacht might require 300kW when in port, and an additional 400kW when under way, it might seem sensible to have two 400kW generators. But at anchor at night, the generator loads might go down to 100kW, thus both generators would be oversized. In this case it might be more feasible to have a smaller genset of 100kW, two of 300kW, and one of 400kW or 500kW to enable loads to be matched to power consumption to minimise emissions and fuel costs.
For silent operation, say at night, the diesel-electric system could be combined with an energy-dense lithium-ion battery bank or two. In operation, the batteries could be used to drive the pod motors or to power the house lighting for totally silent, night-time operation. The battery banks would have to be quite large to store enough power – probably around 800 to 1,000 amp-hours at 600 volts for a 60m to 70m yacht – but with today’s lithium-ion batteries, such a system is not beyond the realm of possibility.
Diesel-electric and emissions regulations
For all its appeal, diesel-electric has a few disadvantages. To start with, instead of a mechanical engineer, an owner will require an electrical engineer.
As mentioned above, the increased weight of a diesel-electric system can be a drawback. With the new emissions controls that have just come into force in various parts of the world, each genset will require its own controls, should each set be in different parts of the yacht.
Throwing a wrench into the development of diesel-electric yachts are ECAs, or Emission Control Areas. These areas are intended to cut down on NOx (nitrogen oxides) and SOx (sulphur oxides). ECAs, which recently were put in place around the coasts of North America and affect every large vessel in those waters, will necessitate the use of Selective Catalytic Reduction (SCR) to reduce NOx and low-sulphur diesel fuel to reduce SOx. Generator manufacturer Northern Lights has just introduced what it calls its Diesel Exhaust Cleaning System (DECS). The system is based on DCL International’s Marine-X system, which removes particulates as they are emitted from the engine.
Throwing a wrench into the development of diesel-electric yachts are ECAs, or Emission Control Areas. These areas are intended to cut down on NOx (nitrogen oxides) and SOx (sulphur oxides)
In a non-Northern Lights installation, to reduce NOx, a diesel exhaust fluid (DEF) tank containing urea and an atomiser pipe would need to be located near the generators. The DEF is injected into the exhaust stream to react with NOx to create nitrogen and water. To simplify this installation, the entire generator compartment may have to be located in an easily accessible place where the DEF tanks and atomiser can be used on all the generators.
Because of SCR and its associated piping, engine rooms will grow larger, but by placing the drive motors on pods outside the hull, engine room size increases can be minimised. However, this will require widespread adoption of diesel-electric drives, instead of diesels with transmissions and shafts.
Alternative fuel systems
In order to reduce emissions, commercial ship owners are experimenting with using fuels that reduce costs and do not produce much in the way of emissions. Often, where commercial shipping leads, recreational vessels follow, thus a superyacht of the future might be powered by one of these fuels. However, terminals where ships using liquefied gas can refuel may be far and few between and until the infrastructure improves considerably yachts will not follow this trend.
A hydrogen-fuelled superyacht has been proposed. It is a 64m hydrogen diesel-electric yacht from Pharos Marine in Egypt. Given the explosive nature of hydrogen and the lack of refuelling facilities, though, this is more a marketing concept than a real project.
In the far longer term, nanotechnology could lead to major advances in solar and wind power generation. Combined with advances in electric motor and battery technology, nanotechnology could change the look and style of superyachts as we know them today.Superyacht Owners’ Guide to Propulsion 2012