This document discusses using pulsed power loads and energy storage technologies to improve the efficiency of electric power generation on Navy ships. It analyzes using high-speed generators directly coupled to gas turbines, eliminating gear boxes. Energy storage like flywheels could support intermittent high power loads, allow more efficient turbine operation through load leveling, and provide power quality and stability benefits. The document evaluates lithium-ion batteries and flywheels for a potential application on DDG51 destroyers. It determines that flywheels are the preferred technology due to their higher technology readiness, longer lifetime, reliability, and safety compared to lithium-ion batteries. Simulation studies show flywheels could effectively provide UPS functionality and improve fuel efficiency on DDG

1. Pulsed Power Loads Support and Efficiency Improvementon Navy Ships R. E. Hebner, J. D. Herbst, A. L. Gattozzi Center for Electromechanics University of Texas, Austin May 20, 2010

4. Notional 3 MW Power Module

5. Benefits of Storage Support of intermittent duty high power loads Load leveling (more efficient turbine operation) Power quality and stability improvement Stiffer power bus Single turbine at near full load instead of two turbines at fractional loads Higher efficiency & expanded engine operational hours Reduction of turbine thermal cycling Maintenance reduction and operational life extension

6. Storage Technologies Considered Capacitors Low energy density – not considered further Batteries Li-ion technology Flywheels Batteries and flywheels competitive evaluation on several points follows

7. 1. Technology Readiness Level (TRL) Li-ion batteries: Preferred technology for low power electronics Some developments in the kWh and kW (electric vehicles) No MW level application identified low TRL Flywheels: UPS system up to 1 MW in commercial use 20 MW system being planned

8. 2. Scaling Li-ion batteries: 3 MW 10 minute power delivery is difficult Practical packaging of large scale array is challenging Lacking direct examples at these power levels, projections were made from installations with other battery chemistries

9. S&C PureWave UPS System2.5 MVA, 60 s, Lead-Acid Li-ion equivalent at 2.5 MW, 10 minutes = 121 m3

10. Alaska Golden Valley Cooperative Project27 MW, 15 min, NiCd Li-ion equivalent at 2.5 MW, 10 minutes = 116 m3

11. 3. Performance Degradation Li-ion batteries: Capacity fade (temperature and depth of discharge cycles) Energy capacity typically based on 1 hour discharge (1C rate) In our case 10 min discharge = 6C rate Higher internal resistance than other chemistries (higher heating)

12. 4. Life Li-ion batteries: Short useful life relative to ship’s service life May need to replace 3-4 times over 35 years Support of pulsed loads and load leveling function will require frequent cycles Asymmetrical charge / discharge rate Flywheels: Independent energy stored and power delivery NASA study found no significant degradation after 110,000 deep discharge cycles Can be designed for 35 years life

13. 5. Reliability Li-ion batteries: Low voltage of 3.6 V/cell 188 cells needed for 680 Vdc bus to generate 450 V 60 Hz Many strings in parallel to supply needed current Several thousand cells needed on board Failure of single cell impairs the whole system Flywheels: Based on standard rotating machine technology

14. 6. Safety Li-ion batteries: Demonstrated catastrophic failure mode Very sensitive to charging voltage (4% maximum overcharge limit) New non-flammable electrolytes reduce energy and power by ~30% Complex cell monitoring system (eliminates failed cell from array) Based on all the issues above, flywheels are preferred technology

15. Flywheel Storage Upgrade main generator: Package the system in the current volume of the AG9140 Remove low speed generator and gearbox Use high speed generator and power electronics Integrate independent flywheel storage modules into existing power system: Flywheel + motor/generator + power electronics + auxiliaries

16. Stand-alone Flywheel Storage System(8 needed for 10 min. discharge)

17. Table 1. Physical Characteristics for 2.5 MW, 10-minute UPS Energy Storage System

18. Table 2. Electrical Characteristics for 2.5 MW, 10-minute UPS Energy Storage System

19. Simulation Study of Common DC Bus Topology

20. Simulation Studies: UPS Function

21. Response of AC Grid to Loss of Gas Turbine Generator Set at t = 0.75 s Flywheel Discharge and Recharge Cycles (Discharge (0-7 s) and Recharge (7-10 s))

22. DDG51 Fuel Saving Estimate Baseline parameters taken from BAA07-029: 4,000 hours of operation per year with a ship service power of 2525 kW (electrical) and a fuel cost of $100 per barrel Turbine specific fuel consumption for the AE1107 engine provided by Rolls-Royce Baseline fuel consumption using current DDG51 CONOPS with two AG9140RF units providing the required 2525 kW Projected resulting fuel savings are $1.25 million per ship per year