System and Control

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Hybrid hydraulic/pneumatic power paths provide extra DOF to operate components more efficiently (Similar to hybrid vehicle operation)

 

 

Essential Considerations in this Design:

Ø  Improving heat transfer capability in liquid piston air  compressor/expander

Ø  Efficient power flow (machine design, sizing)

Ø  Control to achieve high efficiency and performance

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

Ø  Accumulator size ≈500 m3 for 24MWhr at 35 MPa

Advantages:

       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

 

 

 

Disciplines:

Heat Transfer and Fluid Mechanics

System and Controls

Machine Design

 

 

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