GOALI/EFRI-RESTOR #1038294 :NOVEL COMPRESSED AIR APPROACH FOR

OFF-SHORE WIND ENERGY STORAGE

Perry Li (PI)U. of Minnesota

Terry SimonU. of Minnesota

Jim Van de VenU. of Minnesota

Eric LothU. of Virginia

Steve CraneLightsail Energy Inc.

Near Isothermal Compressed Air Energy Storage Approach For Off-Shore Wind Energy using an Open AccumulatorPerry Y. Li (PI), Terry Simon, Jim Van de Ven, Eric Loth* and Steve Crane**University of Minnesota, *University of Virginia, and **LightSail Energy

Challenges:•Wind energy is intermittent, difficult to predict•Mismatch between supply and demand•Potential disruption of base power supply•Wind turbines are under-utilized: typical capacity factor < 50% •High cost of installation, transmission and interconnect for off-shore wind

Acknowledgement: NSF-EFRI #1038294UMN: IREE RS-0027-11; NSF-CCEFP–2C.1

http://www.me.umn.edu/~lixxx099/EFRI_CAES

Goal: Develop a scalable and rampable system for storing wind energy locally prior to electricity generation

Benefits:• Predictable output• Store energy when low demand/high supply &

regenerate energy during high demand/low supply• Downside electrical generator, transmission, and

interconnect• Increase capacity factor

Challenges of wind power:• Wind energy is intermittent, difficult to predict: disruptive to electrical grid• Mismatch between supply and demand• Wind turbines are under-utilized: typical capacity factor < 50%

Wind power

Demand

Time

Pow

er

Benefits of local energy storage:• Predictable, reliable output• Increased energy capture• Downsize components, increase capacity factor

Unused capacity

Generated powerw/o storage

Generated power w/ storage

Goal: Develop a scalable and rampable system for storing wind energy locally prior to electricity generation

http://www.me.umn.edu/~lixxx099/EFRI_CAES

Rated capacity

4

Approach

• Store energy in hi-pressure (300bar) compressed air vessel• High energy density relative to pumped-hydro• Not site specific, scalable and cost-effective

• Isothermal compression/expansion• Efficient operation

• Hybrid hydraulic-pneumatic operation• Rapidly rample, capable of capturing large transient power

Stores energy locally before conversion to electricity • Downsize generator and transmission line

Storage vessel dual used as ballasts or integrate in tower• @35MPa, Vol=500m3 for 3MW*8hrs, << $120/kWh

Open accumulator:• Constant pressure • Liquid port -> high power/low energy

path• Air port -> low power/high energy path => Downsize air compressor/expander

Liquid Piston Near-isothermal air compressor/expander

Direct air/liquid interface• Droplets, mist & vapor for HTPorous media/arrays of heat pipes• Large HT surface area• Sea/ocean as heat sink/source

Hydraulic transformer:• Efficient, power

dense

Nano-texturing• Super-hydrophobic• Liquid drag reduction

and augment heat transfer

Systems Engineering & Optimal Control• compression/expansion profile• optimize plant wise control

Multi-Disciplinary Research• Heat transfer• Fluid Flow• Nano-textured surfaces• Machine Design• Fluid power• Systems dynamics & control

Active spray of tiny droplets:• very large “h” and “A” for

HT

Li et al. Near Isothermal Compressed Air Energy Storage Approach For Off-Shore Wind Energy using an Open Accumulator

Contact: Prof. Perry Li Email: perry-li@umn.edu

Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture

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Project Challenge & ThemesMajor challenge: • System efficiency and power capability• Especially in the compressor/expander

Four thrusts:

1. Heat transfer augmentation – HT surfaces– Droplets, sprays and surface texturing

2. Efficient machines elements

3. Systems, Control and Optimization

4. Integration

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Fundamental challenge due to Heat Transfer limitation

Effective compressed air storage / regeneration requires air motor/compressor that is

• Powerful• Efficient• Compact

Limited by heat transfer within air motor/compressor

Q: How to optimize efficiency / power-density ?

Adiabatic compression to 210bar = 1260KAdiabatic expansion from 210bar = 60K

Without HT

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Problem setup

Assume heat source

& sink at ambient temp T0

1. Compression in tf1

(P0 , T0) -> (r P0 , T1)

2. Cools down to T0 at

constant P to (r P0 , T0)

3. Expansion in tf2 :

(r P0, T0) -> (P0 , T2)Volume

Pre

ssur

e

Initial pressure, P0

Compression

final pressure, Pfinal = r P0 1

2

Expansion

Compression / Expansion Process

isothermal

3

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Efficiency/Power trade-off in Compressor and Expander

• Deviation from isothermal compression/expansion wastes energy

• Multi-stage (n >> 1) approximates isothermal but more complex

• Slowing down process increases efficiency but reduces power density

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Thrust 1: Heat transfer augmentation

a) Liquid piston / surface area augmentation

b) Liquid Spray

Method: – Geometry of HT surfaces– Nozzle design– Control

Computation, Analysis,

Experiments

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Test facility - cylinder filled with air and pressurized with a liquid piston

Low Pressure (10bar) Liquid Piston Experiment

• Rich in vortices• Strong 2nd ary flow (left)

(a) t*=0.1 (c) t*=0.3

(f) t*=0.4 (h) t*=0.8

Upper plenum g

Inner channel

Outer channel

Solid Tube

U(t)

Small L/D

Micro-tube (large L/D)

• Liquid level rise at different rates in inner and outer tubes

• Need interrupted channels

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HT Surface Augmentation

Without augmentation, pressure decreases as air returns toAmbient temp

With HT augmentation

ΔT = 111 +/- 3.5 K

Without augmentation: With augmentation

ΔT = 12 +/- 2.2 K

89% Improvement!

Linear compression rate

Result should be even better with optimal profile!

Optimal Compression/ Expansion trajectoriesImproves Efficiency/Power Trade-off

Pareto optimal frontier

3 to 5 times increase in power for same efficiency over ad-hoc profiles !

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Multi-disciplinary ResearchHeat Transfer Fluid Mechanics

Machine Design Surface Texturing

Systems and ControlFluid Power

http://www.me.umn.edu/~lixxx099/EFRI_CAES

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Key areas of technology

• Near isothermal high pressure compression/expansion

• Heat transfer augmentation• Control to affect system trade-off between

efficiency and power• Efficient machine elements• Fluid mechanics of nozzle sprays• Hydro-phobic HT surfaces

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Stores energy locally before conversion to electricity • Downsize generator and transmission line

Storage vessel dual used as ballasts or integrate in tower• @35MPa, Vol=500m3 for 3MW*8hrs, << $120/kWh

Open accumulator:• Constant pressure • Liquid port -> high power/low energy

path• Air port -> low power/high energy path => Downsize air compressor/expander

Liquid Piston Near-isothermal air compressor/expander

Direct air/liquid interface• Droplets, mist & vapor for HTPorous media/arrays of heat pipes• Large HT surface area• Sea/ocean as heat sink/source

Hydraulic transformer:• Efficient, power

dense

Nano-texturing• Super-hydrophobic• Liquid drag reduction

and augment heat transfer

Systems Engineering & Optimal Control• compression/expansion profile• optimize plant wise control

Multi-Disciplinary Research• Heat transfer• Fluid Flow• Nano-textured surfaces• Machine Design• Fluid power• Systems dynamics & control

Active spray of tiny droplets:• very large “h” and “A” for

HT

Li et al. Near Isothermal Compressed Air Energy Storage Approach For Off-Shore Wind Energy using an Open Accumulator

Contact: Prof. Perry Li Email: perry-li@umn.edu

Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture