Technical: The electrical ship
Hybrid propulsion, Hybrid yachts, electrical propulsion: there’s often a great deal of confusion on what should be understood by this wording. Not to mention the propaganda which at times is carried through when switching from “hybrid” to “electric” to mean “green” or eco-friendly, eco sustainable, oreven environmental impact and so on as if they are all the same thing. “Green”, “hybrid”, “electric”, are certainly words which link with one another, but are not synonyms, nor are they used to mean the same things. Let’s start from the beginning: what does hybrid mean, or better what does hybrid propulsion mean? In nature the term “hybrid” is used to signify union between two diverse species.when talking about “hybrid” in naval propulsion terms, we mean there are different ways of getting propellers to rotate: an electric engine and a diesel one are no doubt the most popular systems installed on board.the same goes for the automotive industry when talking about hybrid cars since they too deploy two engines: an electric one and a four stroke petrol one which develop a working system set within defined parameters which define the level of their hybridisation.
Instead when the engines are not working together the correct definition for this mode is: diesel electric where the diesel (generator set) supplies sufficient energy to work the electric engine which makes propellers rotate. This system is commonly found on cruise ships where a power house, made up of a series of gen. sets supplies needed energy to work all the on board systems, services and above all the electric engines. The electric engines’ high efficiency means they’re mostly exploited since they can perform over very long periods above the 90% mark with a max. torque curve close to zero rpm. Furthermore there is no need for separate drive units as they can easily work from gen. sets or from engines. It is also a known fact that thermal 4 stroke engines are not equally efficient at low rpm. and their efficiency is rated at about 35% only. By working these two engine types accordingly together or, in the case of a hybrid propulsion unit by working them together it is however possible to reduce fuel consumption and consequent environmental pollution which translates into a series of advantages which enhance versatility, and flexibility of use and the quality of life on board. The systems to make the two work together are diverse just as much as and more importantly so are the ones deployed to manage and monitor. Here is where understanding how a given hybrid system really works becomes more difficult and we fall into the kind of confusion we mentioned in the beginning. We invited Alberto Amici founder of Auxilia Electric Propulsion namely RTN’S brand ( a company which has been designing and delivering propulsion systems for ships for 50 years) and has been entirely involved in supplying ships’ on board electric propulsion systems for ten years, to shed some light on this topic. This company has also been delivering very important hybrid and /or diesel electric installations to large and small yachts.
Mr. Amici, let’s start by clearing the first point: is electric propulsion a synonym of green, meaning that it always guarantees a lesser environmental impact?
Well let’s start by asking ourselves the following question: “How will the energy needed to run the ship be stored?”. With the available technology, today we have three different ways of doing so. Liquid forms, (diesel fuel, petrol, LPG, natural gas, Gas forms, (methane gas, hydrogen) or battery banks (dry cell, deep cycle, lithium etc.) In the first two cases Liquid and gas forms) the supply is delivered by transferring the products from land based stations to ships (in the first case with pumps, and with compressors in the second). Instead as for batteries they’re powered up or filled by electricity from quay side shore lines or from on board gen. set plants. This premise is important in an effort to clarify things. If I were to produce electricity on board through the deployment of a generator set (a diesel engine coupled to an alternator) which then is cabled through to propeller via an electric motor, even if electric efficiency is higher when electric devices are used the loss of energy passing through the cables is summed, I will lose effective efficiency (and then will increase diesel fuel consumption to make up for the loss) by comparison to traditional mechanical drive systems which are in the main made up of in line shafts accordingly coupled to mechanical gear boxes. Therefore allow me to be provocative, electric propulsion can be a solution which in fact increases fuel consumption and is more polluting ( less eco friendly) than traditional in line drive shafts. Only thanks to working intelligent synergy between a diesel engine with an electrical one is it possible to produce energy in a cost effective and eco friendly way on board ship with the notable advantages that go with it in the likes of fuel reduction, extra comfort and less noise. As for the environment (less NOX and SOX or possibility of advancing in ZEM mode (Zero Emission Mode). This has been the main reason that has spurred us to develop our AUXILIA system.
The figures show loss of efficiency (graph below) that occurs when exploiting conventional mechanical drives (fig 01 a), a “full” diesel–electric system (fig 01 b) while considering a hypothetical power of 1000KW. A “full” diesel-electric system as can be seen will in the end yield less than a conventional mechanical engine and drives.
Alberto Amici, RTN owner anf founder of AUXILIA ELECTRIC PROPULSION (www.auxilia-propulsion.com)