Integrated electric propulsion (IEP),full electric propulsion (FEP) orintegrated full electric propulsion (IFEP) is an arrangement ofmarine propulsion systems such thatgas turbines ordiesel generators, or both, generatethree-phase[1]electricity that is then used to powerelectric motors[2] that turn eitherpropellers orwaterjetimpellors. It is a modification of thecombined diesel-electric and gas propulsion system forships which eliminates the need forclutches and reduces or eliminates the need forgearboxes[2][3] by using electrical transmission rather than mechanical transmission of energy; so, it is aseries hybrid electric propulsion, instead ofparallel.
Some newer nuclear-powered warships also use a form of IEP. A nuclear power plant produces the steam to operate turbine generators; these in turn power electric propulsion motors.
| Combined marine propulsion |
|---|
Combined diesel or gas (CODOG) |
Eliminating the mechanical connection between engines and propellors has several advantages including increased freedom in placement of the engines,[3]acoustical decoupling of the engines from thehull, which makes the ship less noisy, and a reduction in weight and volume.[3] Reducingacoustic signature is particularly important tonaval vessels seeking to avoid detection and tocruise ships seeking to provide passengers with a pleasant voyage, but is of less benefit tocargo ships. Because ships require electricity even when not underway, having all of the engines produce electricity reduces the number of engines needed compared to more traditional arrangements in which one pool of engines provides electricity and another pool of engines provides propulsion, reducing capital and maintenance costs.[1][3]
A typical integrated electric propulsion arrangement on larger (e.g. cruise ships) and naval vessels includes both diesel generators and gas turbines. On smaller vessels (which make up the majority of IEP vessels) the engines are typically just diesel. The advantages of gas turbines include much lower weight.[1][4] and smaller size[1] than diesels of similar power, with much less noise andvibration;[4] but gas turbines are efficient only at or near maximum power. Diesel generators have the advantage of high efficiency over a wide range of power levels. Using both types in combination allows for the benefits of a full range of operational efficiency, a low-vibration quiet mode of operation, and some reduction in weight and volume relative to a diesel-only arrangement. In naval vessels, a pool of diesel generators are typically used to provide a base load and enough power to achieve cruise speed. The gas turbines are used to provide peak power for higher speeds and may be required to operate weapon systems that have high power demands. In passenger ships, one or more gas turbines are used for fast cruising. The diesels provide reliable redundancy and an efficient source of electricity when in port, at anchor, or drifting.
Adiesel–electric system is an integrated electric propulsion system in which no gas turbines are used and all of the engines are diesel.
Aturbine–electric system is possible using only gas turbine generators. Someyachts use only gas turbines for integrated electric propulsion without any diesel engines. If electric propulsion is used via electric motor on shaft, or integrated into the main reduction gear driving the shaft, greater power is realized faster than using diesels. In addition, an on-shaft permanent magnet motor drive system, utilizing gas turbine prime movers on the main reduction gear, can also provide electricity when driven by the prime movers. The on-shaft permanent magnet electric motors provide propulsion at lower speeds via on-board electrical power generation by gas turbine or diesel, at significant fuel savings. If fleet-wide usage is analyzed, significant logistical advantages are realized over time. Compared to diesel, a gas turbine system increases flexibility, versatility, and efficiency, with the capability of transforming to provide either propulsion or electrical power more rapidly.
In Norway, gas electric hybrid propulsion is being used on a fleet offully electric and plug-inhybrid ferries that cross fjords. Capable of travelling at 17–18 knots, these ships reduce total NOx by 8,000 tonnes, and CO2 emissions by 300,000 tonnes, per year. While saving a million litres of diesel per year per ferry, the ferries recharge their batteries overnight and top them up fromshore power at each port of call.[5][6][7]