West Kauai Energy Project

Natural Resources
Inspection/condition assessment, Alternatives analysis/feasibility study, Conceptual design, Engineering services, Final design, Environmental/permitting, Self-performed construction, Engineering during construction, Testing & commissioning
Alternative delivery (D-B, progressive D-B, CM/GC, CMAR, EPC), Professional services, Construction
Waimea, Hawaii
Kauai Island Utility Company
AES Corporation
why it matters

Clean energy transition is essential everywhere in the world, but it’s crucial for Hawaii. Between its renewable portfolio standard, oil dependance, and coal phase-out, Hawaii needs to quickly transition to a more significant percentage of renewable energy to meet its goals. The West Kauai Energy Project will be part of the solution. McMillen was retained to complete a new pumped storage project to decarbonize Kauai’s grid. The design uniquely integrates solar, hydroelectric, a battery energy storage system (BESS), and pumped storage to provide 24 MW of peak hydropower capacity, 340 MWh of daily pumped storage capability, and 97 GWh of hydroelectric generation annually while also providing a reliable irrigation water supply to support agriculture on the island.

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A new pumped storage project in Kauai requires rehabilitating existing infrastructure and new construction. We are modifying three reservoirs and dams and associated ditch systems, access roads, and diversion structures, and we are building six miles of pressurized penstocks, two new hydropower stations, gate and intake structures, and a pump station. McMillen is leading design, procurement, and construction to get the facilities up and running.

The project comprises three dams:

Puu Opae, Puu Lua, and Mana. Our team is rebuilding Puu Opae and Mana embankment dams following analyses of hydraulics, dam stability, loading conditions, seismic conditions, seepage, and drainage. We conducted a preliminary stability analysis on the Puu Lua earthen dam using a 2D model, drilling, and sampling.

These analyses guided our team in developing plans to modify each dam. Our design includes all civil, structural, mechanical, electrical, hydraulic, and geotechnical disciplines, and our team will self-perform construction and commissioning. Puu Lua will undergo construction of a new outlet structure and spillway, reconstruction of the embankment dam, sediment removal from the reservoir, addition of a toe buttress to address liquefaction concerns, and installation of new dam safety and instrumentation systems. The upstream face of the Puu Opae and Mana dams and the base of the Puu Opae and Mana reservoirs will be fully or partially lined with a geomembrane to help limit water loss via seepage.

In addition to the dam modifications, McMillen will build two new powerhouses and lay 32,000 feet of pressurized penstock. The Puu Opae powerhouse features a 4.25 MW Pelton turbine generator. The Mana powerhouse and pump station includes a 20 MW Pelton turbine and a 32 MW pump station consisting of ten 3.2 MW vertical turbine pumps. Our team is providing all electrical and control systems design and will be self-performing the systems controls and integration. In addition, our fire protection engineers provided a life safety analysis and detailed fire suppression and fire alarm designs. The design of the powerhouses accommodates challenging water supplies and a remote location from municipal utilities.

Other critical features of the project include:

• A 56 MW photovoltaic solar field to pump water uphill

• Underground medium-voltage power distribution along the penstock alignments

• A several-mile-long fiber optic ring-network backbone

• Power distribution and controls stations for the reservoir outlets, penstock intakes, and penstock flow


• Four rugged terrain stream diversion structure control stations that must operate fully non-grid powered

using solar arrays, batteries, and thermal-propane generators, communicating via satellite to the controls


• Grading, stormwater, and roadway design

• Inlet and outlet closure gates at all three reservoirs

• Installation of gauging equipment at each diversion location with remote monitoring

We anticipate completing construction in December 2027.


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