GOALI/EFRI-RESTOR #1038294 : Novel Compressed Air Approach for Off-Shore Wind Energy Storage

1. GOALI/EFRI-RESTOR #1038294 :Novel Compressed Air Approach for Off-Shore Wind Energy Storage Terry Simon U. of Minnesota Jim Van de Ven U. of Minnesota Perry Li (PI) U. of Minnesota Eric Loth U. of Virginia Steve Crane Lightsail Energy Inc.

2. 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 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-CCEFP–2C.1 http://www.me.umn.edu/~lixxx099/EFRI_CAES

3. http://www.me.umn.edu/~lixxx099/EFRI_CAES 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% Rated capacity Power Wind power Demand Time Generated power w/ storage Generated power w/o storage Goal:Develop a scalable and rampable system for storing wind energy locally prior to electricity generation Unused capacity Benefits of local energy storage: • Predictable, reliable output • Increased energy capture • Downsize components, increase capacity factor

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

5. 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 • Multi-Disciplinary Research • Heat transfer • Fluid Flow • Nano-textured surfaces • Machine Design • Fluid power • Systems dynamics & control • Open accumulator: • Constant pressure • Liquid port -> high power/low energy path • Air port -> low power/high energy path • => Downsize air compressor/expander • Active spray of tiny droplets: • very large “h” and “A” for HT 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 • Nano-texturing • Super-hydrophobic • Liquid drag reduction and augment heat transfer • Hydraulic transformer: • Efficient, power dense • Systems Engineering & Optimal Control • compression/expansion profile • optimize plant wise control • Storage vessel dual used as ballasts or integrate in tower • @35MPa, Vol=500m3 for 3MW*8hrs, << $120/kWh Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture Contact: Prof. Perry Li Email: perry-li@umn.edu

6. Project Challenge & Themes Major challenge: • System efficiency and power capability • Especially in the compressor/expander Four thrusts: • Heat transfer augmentation • HT surfaces • Droplets, sprays and surface texturing • Efficient machines elements • Systems, Control and Optimization • Integration

7. Effective compressed air storage / regeneration requires air motor/compressor that is Powerful Efficient Compact Fundamental challenge due to Heat Transfer limitation Limited by heat transfer within air motor/compressor Adiabatic compression to 210bar = 1260K Adiabatic expansion from 210bar = 60K Without HT Q: How to optimize efficiency / power-density ?

8. Compression Expansion isothermal Problem setup Compression / Expansion Process Assume heat source & sink at ambient temp T0 • Compression in tf1 (P0 , T0) -> (r P0 , T1) • Cools down to T0 at constant P to (r P0 , T0) • Expansion in tf2 : (r P0, T0) -> (P0 , T2) 2 1 final pressure, Pfinal = r P0 3 Pressure Initial pressure, P0 Volume

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

10. Thrust 1: Heat transfer augmentation • Liquid piston / surface area augmentation • Liquid Spray Method: • Geometry of HT surfaces • Nozzle design • Control Computation, Analysis, Experiments

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

12. Micro-tube (large L/D) Upper plenum g Inner channel (c) t*=0.3 (a) t*=0.1 Small L/D Outer channel Solid Tube (f) t*=0.4 (h) t*=0.8 U(t) • Liquid level rise at different rates in inner and outer tubes • Need interrupted channels • Rich in vortices • Strong 2ndary flow (left)

13. 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 ΔT = 12 +/- 2.2 K ΔT = 111 +/- 3.5 K Result should be even better with optimal profile!

14. 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 !

15. Multi-disciplinary Research Heat Transfer Fluid Mechanics Surface Texturing Machine Design Fluid Power Systems and Control http://www.me.umn.edu/~lixxx099/EFRI_CAES

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

17. 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 • Multi-Disciplinary Research • Heat transfer • Fluid Flow • Nano-textured surfaces • Machine Design • Fluid power • Systems dynamics & control • Open accumulator: • Constant pressure • Liquid port -> high power/low energy path • Air port -> low power/high energy path • => Downsize air compressor/expander • Active spray of tiny droplets: • very large “h” and “A” for HT 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 • Nano-texturing • Super-hydrophobic • Liquid drag reduction and augment heat transfer • Hydraulic transformer: • Efficient, power dense • Systems Engineering & Optimal Control • compression/expansion profile • optimize plant wise control • Storage vessel dual used as ballasts or integrate in tower • @35MPa, Vol=500m3 for 3MW*8hrs, << $120/kWh Hydrostatic Transmission: Reliable (no gearbox), tunable, optimize turbine speed for energy capture Contact: Prof. Perry Li Email: perry-li@umn.edu