UCAES Undersea Compressed Air Energy Storage
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UCAES Undersea Compressed Air Energy Storage


Brayton Energy received SBIR Phase-1 and Phase-2 awards, to advance the development of compressed energy storage, using an innovative undersea air storage system.

Period of performance DOE (2010-2015) and US Navy (2015-2016). The project was performed in cooperation with the Hawaiian Electric Company (HECO) and First Wind. There was numerous contributions from specialists in the areas of ocean engineering, marine construction, and deployment, as well as coastal permitting that participated in the program as sub-contractors to Brayton.

A detailed report has been delivered to the DOE and HECO, focusing on the description of a 60 MWe energy storage system which is compliant with Hawaiian Electric Company. Energy Storage RFP. April 29, 2014.

UCAES Features

  • Efficient Power Generation / dispatchable power plant:
    • Fuel-to-electric efficiency over 41%; substantially better that the average existing Hawaiian peak power generation assets.
    • Operable with a range of liquid or gaseous fuels and will meet the most stringent California (CARB) emission standards.
  • Peaking power with very fast response
    • Power ramp-up equivalent to 215% of continuous load rating within 1 second (e.g. 5.7 MWe to 12.3 MWe); accomplished by simply turning off the electric compressor(s).
  • Rapid energy absorption rates as a ‘battery’
    • The electric compressors may be sequenced on to absorb power within seconds.
    • Continuous power generation rating can drop by 215% in seconds (e.g. 5.7 MWe net generation to 6.6 MWe consumption); accomplished by shutting of expander unit(s).
  • Superior round-trip efficiency and durability as an energy storage module
    • ‘Round-trip efficiency’, as calculated by industry standard methods, is in the 90% range.
    • The PCS equipment has a mean-time between overhauls of 80,000 hours, with maintenance costs of less than $0.005/kW-hr of electricity delivered.
    • The underwater ACS structures and piping employ materials and designs specified to achieve a 75 year life.

Optimized structure for air pressure and overburden

Undersea Air Storage Vessel

  • Proven monolithic concrete dome construction
    • Dome Technology, LLC providing project planning assistance
    • Engineering System Solutions (ES2) is providing design consultation
  • Negatively buoyant, eliminating the need for anchors
  • Majority of ballast from low cost earth-fill
  • Rigid concrete skeleton in pure compression
  • Static structure designed to receive artificial reef classification
  • Thick earth-fill outer layer for external protection, and low cost

Air Transport Pipe

  • Proven fused HDPE construction and deployment strategy
    • Makai Ocean Engineering provided project plan and cost estimate.
    • Hawaiian Tug and Barge provided validation of equipment cost and availability.
  • Similar to existing pipes at NELHA but shorter and smaller diameter
  • Pipe is staged at appropriate on-shore construction site
  • Sections are fused as the pipe is pulled into the water
  • Concrete weights are added to each segment after fusion
  • Central air pipe, two ballast pipes, and optional slurry pipe
  • All pipes are initially filled with air and positively buoyant
  • Completed unit is capped and floated to the deployment site

Case Study – 100 MW-hr Plant on O’ahu

The ‘normalize cost of storage’ is based on the capital cost to upgrade a standard industrial gas turbine to accommodate energy storage and includes the underwater air containment system (ACS).

Proposed CAES Cycle