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GOALI/EFRI-RESTOR # : NOVEL COMPRESSED AIR APPROACH FOR OFF-SHORE WIND ENERGY STORAGE Perry Li (PI) U. of Minnesota Terry Simon U. of Minnesota Jim Van de Ven U. of Minnesota Eric Loth U. of Virginia Steve Crane Lightsail Energy Inc.

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

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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 Power Benefits of local energy storage: Predictable, reliable output Increased energy capture Downsize components, increase capacity factor Unused capacity Generated power w/o storage Generated power w/ storage Goal: Develop a scalable and rampable system for storing wind energy locally prior to electricity generation Rated capacity

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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

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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 Vol=500m 3 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 HT Porous 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 Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture

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6 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

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7 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 = 1260K Adiabatic expansion from 210bar = 60K Without HT

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8 Problem setup Assume heat source & sink at ambient temp T 0 1.Compression in t f1 (P 0, T 0 ) -> (r P 0, T 1 ) 2.Cools down to T 0 at constant P to (r P 0, T 0 ) 3.Expansion in t f2 : (r P 0, T 0 ) -> (P 0, T 2 ) Volume Pressure Initial pressure, P 0 Compression final pressure, P final = r P Expansion Compression / Expansion Process isothermal 3

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9 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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10 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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11 Test facility - cylinder filled with air and pressurized with a liquid piston Low Pressure (10bar) Liquid Piston Experiment

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Rich in vortices Strong 2 nd 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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13 HT Surface Augmentation Without augmentation, pressure decreases as air returns to Ambient 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!

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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 !

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15 Multi-disciplinary Research Heat Transfer Fluid Mechanics Machine Design Surface Texturing Systems and Control Fluid Power

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16 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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17 Stores energy locally before conversion to electricity Downsize generator and transmission line Storage vessel dual used as ballasts or integrate in Vol=500m 3 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 HT Porous 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 Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture

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