Pulsed Power Loads Support and Efficiency Improvement

on Navy ShipsR. E. Hebner, J. D. Herbst, A. L. Gattozzi

Center for ElectromechanicsUniversity of Texas, Austin

May 20, 2010

Statement of the Problem

• Increasing demand for reliable electric power• Projected expansion of pulsed loads• Rising fuel costs

Technical Solutions

• Advanced power generation• Energy storage technologies

Study for the DDG51 Destroyer

• High speed generators at 15,000 RPM 3 MW can be coupled directly to the gas turbine– Elimination of gear box

• New class of power electronics allows decoupling of the 60 Hz distribution frequency from the generated frequency– Turbine speed can be adjusted to maximize SFC

• Energy storage provides additional benefits– (details later)

Notional 3 MW Power Module

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

Storage Technologies Considered

• Capacitors– Low energy density – not considered further

• Batteries– Li-ion technology

• Flywheels

– Batteries and flywheels competitive evaluation on several points follows

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

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

S&C PureWave UPS System2.5 MVA, 60 s, Lead-Acid

Li-ion equivalent at 2.5 MW, 10 minutes = 121 m3

Alaska Golden Valley Cooperative Project27 MW, 15 min, NiCd

Li-ion equivalent at 2.5 MW, 10 minutes = 116 m3

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)

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

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

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

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

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

Physical Characteristics Flywheel Flywheel Motor/Generator

Length, Width, Height 90" L, 41.5" W, 41.5" H 33.1" L, 25.8" W, 25.8" H

Maintenance Envelope (L x W x H) 130" L, 81.5" W, 81.5" H 73.1" L, 65.8" W, 65.8" H

Weight, Center of Gravity14715 lb,

COG*: 45.75"x, 0"y, 0"z

2368 lb, COG*: 16"x, 0"y, 0"z

RPM 9573-19146 rpm 9573-19146 rpm

Equipment Rating52 kWh/FW deliverable

(208 kWh per skid)625 kW/MG

(2.5 MW per skid)

Thermal Cooling Fluid, Type, Volume, Pressure

water, 10 gpm, 60 psi/skid

water, 28 gpm, 60 psi/skid

Thermal Discharge Fluid, Type, BTUs, Vol, Press, Temp

water, 20 kW (1137 BTU/m), 10 gpm, 60 psi,

42°C outlet/skid

water, 56 kW (3185 BTU/m),

28 gpm, 60 psi, 42°C outlet/skid

Expected Mounting Location of Components – include Type of

Mount

Pedestal mounted to modified AG9140 skid,

shock isolated

Flange mounted to FW on modified AG9140 skid

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

Electrical Characteristics Flywheel Flywheel Motor/Generator

Time to full power from standby 500 ms 500 ms

Time to full power from secured 1245 s (21 min) 1245 s (21 min)

Noise Frequencies and Level (dB) 160-319 Hz, 75dB (est) 160-638 Hz, 85 dB (est)

Operating Temp Range: Internal and External

100°C int., 49°C ext. 140°C int., 49°C ext.

Electrical Power Inputup to 10 kW control power

& auxiliaries

up to 19 kW charge maintenance/5 kW excitation,

and auxiliaries

Input Volts, Amps, Phases, Freq 220 V, 45.5 A, 1 ph, 60 Hz460 V, 23.8 A, 3 ph, 60 Hz220 V, 22.7 A, 1 ph, 60 Hz

Input Harmonic Limits MIL-1399 MIL-1399

Backup SourceBattery UPS for controls

and bearingsBattery UPS for field excitation

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

Simulation Study of Common DC Bus Topology

Simulation Studies: UPS Function

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))

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