Pacific 2010: Hybrid propulsion not just for cars | ADM Dec 09/Jan 10
Katherine Ziesing | Canberra
The propulsion solutions available today cover a wide performance spectrum and display considerable flexibility; this can be seen with the example of the F125 frigate.
In the past the prevailing mixed propulsion solutions usually consisted of diesel engines and a gas turbine (e.g. CODAG, COmbined Diesel and Gas turbine).
Today some navies are replacing their conventional diesel engines with electrical propulsion motors, while others are moving towards full electric propulsion.
The US Navy intends to backfit its DDG-51 Arleigh Burke-class guided-missile destroyers with a hybrid electric drive propulsion motor to gain fuel savings when the ships operate at speeds of roughly 13-14 knots, the director of science and technology for Naval Sea Systems Command's surface warfare directorate Glen Sturtevant said late last year.
By installing an electric-drive motor aboard a DDG-51 destroyer, the shipdriver can shut down the gas engines and operate more efficiently at low rates of speed, Sturtevant said.
The final design was slated for approval after ADM went to press.
The RAN's first investment into electrical propulsion will be with the new LHD's which will be fitted with Siemens SSP podded electrical propulsors.
The choice of system depends on the hierarchy of boundary conditions such as the service profile, flexibility in setting up the ship, redundancy concepts, the noise signature and fuel consumption reduction possibilities, to mention a few.
The latest frigate design F125 is the German Navy's first frigate to be equipped with a CODLAG (COmbined Diesel-eLectric And Gas) hybrid propulsion system, i.e. a partly electric propulsion system that essentially consists of two electric propulsion motors and one gas turbine (combined diesel electric and gas turbine) where the electrical propulsion system component is designed and delivered by Siemens.
Here, the advantages of the high performance density of the gas turbine for propulsion at high deployment speeds are combined with the considerable flexibility of a diesel- electric propulsion solution during cruising.
In the electric propulsion mode, only the required number of power generating sets (diesel engines) need to be connected to the master network.
These generating sets can then operate in the optimum efficiency range with the remaining diesel generators at standstill.
Additionally, while in the electric propulsion mode, the complete gear set and gear stages are at standstill, ensuring lower noise signatures and losses optimising the efficiency of propulsion system.
This delivers significant savings during deployment, therefore achieving an optimisation of the integration, flexibility and efficiency of the system.
"The CODLAG propulsion solution for F125 consists of two shaft systems with controllable-pitch propellers driven by two electric propulsion motors and / or one main gearbox plus a cross-connect gearbox driven by an LM 2500+ gas turbine.
"The electrical propulsion motors are only 4.5 MW each as they are sized to meet the operational profile of the vessel," Michael Wycisk, a technical director with Siemens AG, said.
"Associated converters are included for closed-loop control of the electric propulsion motors' speeds with the electronic propulsion system control (E mode), and the 6.6kV medium - voltage system and the generators for power generation."
Modes of operation
In the E propulsion mode, the vessel is propelled via the two electric propulsion motors only.
With a constant propeller pitch, the vessel's speed is controlled by the speed of the electric motors, which can be varied by means of the converters.
"The number of generators connected to the master network is optimised to the demand requirements," Wycisk explains.
"Here the gearbox is not engaged resulting in a low noise signature."
The GT mode is a pure gas turbine mode for higher speeds, where the propulsion is provided by the gas turbine only.
The vessel's speed is initially controlled via the propeller's pitch and then via the engine speed.
Maximum speed is achieved in the CODLAG mode where the gas turbine and the two electric propulsion motors work together.
In CODLAG mode with both the gas turbine and the two electric propulsion motors engaged, operation of the system can be started from standstill and the full speed range can be achieved in a continuously variable fashion.
The two electric propulsion motors and/or the gas turbine demand corresponds to the requested propulsion output.
"To ensure optimum operation and the most economical solution, the power management system ensures that only the required electric output actually needed is provided to the propulsion system," Wycisk said.
"Therefore, only the required generating sets are connected depending on the propulsion mode and the control lever setting.
"The generating sets can then operate in their optimum efficiency range, ensuring that the lowest fuel consumption is achieved.
"In the case of F125, the SINAVY Drive MV electric propulsion system is based on a modular structure and can therefore be adapted to all of the project-specific propulsion requirements.
"The main benefits of today's navies to the broad range of modern hybrid propulsion solutions available are not only the significant savings in fuel cost and life cycle costs, but also the increased flexibility in vessel design that independent generator sets and reduced gear sets create."
To maximise the benefits offered by a hybrid electrical propulsion system, the operating profile needs to be considered early in the vessel design to identify and match the propulsion requirements.
With this approach the navy vessels can meet the requirements of tomorrow, increase the optimisation of vessel design and significantly reduce the fuel and minimise the life cycle cost of the platform.
"Reduced drive train complexity and optimisation of the total installed Power (Prime Mover) through the Load Balancing of Ships E-Power Service Network and Propulsive Power increases the overall efficiency of the system, with the added benefit of reduced signatures," Wycisk explains.
HTS machines
So what of tomorrows' technology?
With the emergence of HTS (High Temperature Superconducting) motor technologies and the migration of vehicle hybrid electrical technologies into the marine market, the opportunity to benefit from these propulsion solutions is becoming realistic for a broader range of vessel designs, where today space limitations are restricting the opportunity.
Up to now, most of the efforts for developing HTS technology have been directed to devices for grid applications.
However, HTS synchronous machines as motors and generators are becoming increasingly interesting within worldwide development programs.
Since 1999, Siemens has been developing a future class of externally excited synchronous machines utilising the benefits of HTS.
In general, HTS machines combine increased efficiency with extreme compactness for electric machines.
"By replacing the copper winding of the rotor with an HTS rotor and introducing an ironless airgap stator winding, the HTS machine's volume and weight are significantly reduced by ≈40 per cent," Wycisk said.
"At the same time, losses are reduced while higher efficiency and excellent operational behaviour is achieved in comparison to conventional devices."
The first HTS machine in product scale within Siemens was a 4 MVA generator which was tested in 2005.
This machine was the first HTS generator ever tested in the grid and now is undergoing a long-term grid test to obtain operating hours.
Siemens research into HTS is now developing even further with a 320 kNm ship propulsion motor presently undergoing comprehensive trials (system test with a standard PWM converter) and due to be proven by mid 2010.
Here, the major focus is the 40 percent reduction in weight and space gained through HTS technology with the additional benefits in reduced lifecycle costs as a result of these reductions.
"This space saving allows not only destroyers and frigates to take advantage of hybrid electrical solutions, but also OPV and Combatant classes," Wycisk explains.
"Historically the smaller vessel classes have had difficulty in finding available space on ship for the electrical propulsion motor.
"The space and weight savings that a HTS solution provides now makes this possible, which leads to major costs savings to navies on these high power demanding platforms where the operational profiles can be even more varied."