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University of Minnesota  
College of Science & Engineering > Department of Mechanical Engineering
Research: A Compressed Air Approach to Off-shore Wind Energy Storage   
       Research > System and Control  
Essential Considerations in this Design:

Improving heat transfer capability in liquid piston air compressor/expander

Efficient power flow (machine design, sizing)

Control to achieve high efficiency and performance

Control objective is to maintain accumulator pressure and shaft speeds (generator and wind turbine) in normal operating condition

Accumulator size ≈500 m3 for 24MWhr at 35 MPa

• Improved energy density and high power density through combined pneumatic/hydraulic open accumulator approach.
• High efficiency and low volume conversion through a near isothermal liquid piston air compressor/expander.
• Generator and Transmission sized for demand, not peak supply (Capture energy before electricity generation (more efficient, and reduced generator size)).
• Components can be at sea level instead of in nacelle.


Open Accumulator:

First Issue: In pure compressed air energy storage system, pressure decreases as energy depletes. This leads to difficulty maintaining compressor/expander’s efficiency and power density. The solution is to maintain constant accumulator pressure by increasing or decreasing air volume as energy depletes/increases:


Second Issue: Power input/output relies on pneumatic compressor/motor which can be power limited. It can be solved by introducing two power paths. First, High power transient events are efficiently handled hydraulically by increasing and decreasing oil volume and pressure (like a closed accumulator); Second, Lower power pneumatic path is used to re-acquire optimal pressure for high energy density storage. The benefits are:

  1. Easy to introduce hydraulic load
  2. Downsize pneumatic compressor/expander


Heat Transfer and Fluid Mechanics

System and Controls

Machine Design

      Contacts: Prof. Perry Li and Prof. Terrence Simon  
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