GOALI/EFRI-RESTOR #1038294 : NOVEL COMPRESSED AIR APPROACH FOR OFF-SHORE WIND ENERGY STORAGE Terry Simon U. of Minnesota Perry Li (PI) U. of Minnesota Eric Loth U. of Virginia Jim Van de Ven U. of Minnesota Steve Crane Lightsail Energy Inc. Near Isothermal Compressed Air Energy Storage Approach For Off-Shore Wind Energy using an Open Accumulator Perry 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 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 Acknowledgement: NSF-EFRI #1038294 UMN: IREE RS-0027-11; NSF-CCEFP2C.1

http://www.me.umn.edu/~lixxx099/EFRI_CAES 2 http://www.me.umn.edu/~lixxx099/EFRI_CAES Challenges of wind power: 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 Unused capacity Rated capacity Demand Generated power Generated power w/ow/ storage storage

Time Goal: Develop a scalable and rampable system for storing wind energy locally prior to electricity generation Benefits of local energy storage: Predictable, reliable output Increased energy capture Downsize components, increase capacity factor 3 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 4 Li et al. Near Isothermal Compressed Air Energy Storage Approach For Off-Shore Wind Energy using an Open Accumulator Stores energy locally before conversion to electricity Downsize generator and transmission line Open accumulator: Constant pressure Liquid port -> high power/low energy path Air port -> low power/high energy path => Downsize air compressor/expander Multi-Disciplinary Research Heat transfer Fluid Flow Nano-textured surfaces Machine Design Fluid power Systems dynamics & control Liquid Piston Near-isothermal air compressor/expander Active spray of tiny droplets:

very large h and A for HT Direct air/liquid interface Droplets, mist & vapor for HT Porous media/arrays of heat pipes Large HT surface area Sea/ocean as heat sink/source Hydraulic transformer: Efficient, power dense Storage vessel dual used as ballasts or integrate in tower @35MPa, Vol=500m3 for 3MW*8hrs, << $120/kWh Nano-texturing Super-hydrophobic Liquid drag reduction and augment heat transfer Systems Engineering & Optimal Control compression/expansion profile optimize plant wise control Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture Contact: Prof. Perry Li

Email: [email protected] 5 Project Challenge & Themes Major 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 6 Fundamental challenge due to Heat Transfer limitation Effective compressed air storage / regeneration requires air motor/compressor that is Powerful Efficient

Limited by heat transfer within Compact air motor/compressor Without HT Adiabatic compression to 210bar = 1260K Adiabatic expansion from 210bar = 60K Q: How to optimize efficiency / power-density ? 7 Problem setup Compression / Expansion Process 2 Assume heat source & sink at ambient temp T0 2. Cools down to T0 at constant P to (r P0 , T0) 3 Pressure 1. Compression in tf1

(P0 , T0) -> (r P0 , T1) 1 final pressure, Pfinal = r P0 Compression Expansion isothermal Initial pressure, P0 3. Expansion in tf2 : (r P0, T0) -> (P0 , T2) Volume 8 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

9 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 10 Low Pressure (10bar) Liquid Piston Experiment Test facility - cylinder filled with air and pressurized with a liquid piston 11 Micro-tube (large L/D) Upper plenum

Inner channel (c) t*=0.3 (a) t*=0.1 g Small L/D Outer channel Solid Tube (f) t*=0.4 (h) t*=0.8 Rich in vortices Strong 2nd ary flow (left) U(t)

Liquid level rise at different rates in inner and outer tubes Need interrupted channels 12 HT Surface Augmentation With HT augmentation Without augmentation, pressure decreases as air returns to Ambient temp Linear compression rate 89% Improvement! Without augmentation: With augmentation T = 111 +/- 3.5 KT = 111 +/- 3.5 K T = 111 +/- 3.5 KT = 12 +/- 2.2 K Result should be even better with optimal profile! 13

Optimal Compression/ Expansion trajectories Improves Efficiency/Power Trade-off Pareto optimal frontier 3 to 5 times increase in power for same efficiency over ad-hoc profiles ! 14 Multi-disciplinary Research Heat Transfer Machine Design Fluid Mechanics Surface !"#$"%& 'Texturing "()*+& , "-& . */ 0-1((*-23405+%1-6& ! ! "#$%&'() *%"+,--$*. (/ +&0, 1#

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Systems and Control A) . ($3'%*!+, - ! B- /+!C(/%4&- (! D $; - 0! @(- 447(- I!C- ) . - (/+7(- I!E$07) - ! 15 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 16 Li et al. Near Isothermal Compressed Air Energy Storage Approach For Off-Shore Wind Energy using an Open Accumulator Stores energy locally before conversion to electricity Downsize generator and transmission line Open accumulator: Constant pressure

Liquid port -> high power/low energy path Air port -> low power/high energy path => Downsize air compressor/expander Multi-Disciplinary Research Heat transfer Fluid Flow Nano-textured surfaces Machine Design Fluid power Systems dynamics & control Liquid Piston Near-isothermal air compressor/expander Active spray of tiny droplets: very large h and A for HT Direct air/liquid interface Droplets, mist & vapor for HT Porous media/arrays of heat pipes Large HT surface area Sea/ocean as heat sink/source Hydraulic transformer: Efficient, power

dense Storage vessel dual used as ballasts or integrate in tower @35MPa, Vol=500m3 for 3MW*8hrs, << $120/kWh Nano-texturing Super-hydrophobic Liquid drag reduction and augment heat transfer Systems Engineering & Optimal Control compression/expansion profile optimize plant wise control Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture Contact: Prof. Perry Li Email: [email protected] 17