Client Eurus Energy and REC Solar
Location Waianae, Oahu, Hawaii
Scope 27.6 MWac single-axis tracker solar project

Waianae Solar LLC

Project Details

Westwood’s Role

  • Electrical Engineering
  • Civil Engineering
  • Hydrology and Drainage Design
  • Geotechnical Engineering
  • Structural Engineering
  • Production Pile Load Testing


Waianae Solar is a 27.6 MW project located on the Island of Oahu. Constructed and completed in 2016, it was the largest solar project on the Hawaiian Islands as of 2017.

With extensive renewable energy experience on the Hawaiian Islands and in-house civil, electrical, and structural design, Westwood proved to be a valuable partner for Eurus Energy, the owner, and REC Solar, the contractor. Westwood was brought on board during the permitting phase and quickly identified challenges facing the preliminary project design that needed to be addressed. Overcoming these challenges would be critical to ensuring a successful project.

Working with the Steep Terrain

This was a first-of-its-kind single-axis tracker (SAT) project, constructed on steep terrain up to 15% grade in the north-south direction and 10% in the east-west direction. (By comparison, the average SAT is installed at a grade less than 5%). Westwood recognized early on that the site faced several complex challenges due to the steep terrain, including the need for extensive drainage to control erosion and runoff as well as widespread terracing to manage the tracker design. Both issues typically require additional land, but on the island, land is at a premium.

Westwood began value-engineering the design by consolidating the site footprint, placing the drainage infrastructure underneath the racks, rearranging access roads, and reducing tracker row spacing in the north-south direction. These design modifications reduced terracing and stabilized the hillside by eliminating steep cuts, making the project feasible in the space available.   

Challenging Subsurface Conditions

The subsurface conditions on site were highly variable, ranging from high plasticity clay with cobbles and boulders to gravelly sand and coral rock. These conditions presented challenges to trenching and pile foundation installation. Westwood performed extensive pile load testing to evaluate pile drivability and develop solutions to overcome the subsurface obstructions, such as installing the piles in pre-drilled pilot holes and oversized holes filled with concrete. The project’s geotechnical team also contended with various geohazards: slope creep, expansive clay, highly corrosive soils, slope stability, and rock fall hazards posed by the adjacent mountain.

Solutions for Electrical Design and Cable Management

Westwood was responsible for the plant electrical concept and details, including the solar array design, AC and DC cable design, integration of the inverter stations with the plant, and electrical grounding. Our team produced the electrical construction plans as well as numerous studies and reports validating the design.

Due to requirements placed on the project by the local utility to energize the 34.5 kV collection circuits in two stages, it was necessary to install automated sectionalizing equipment in the circuits. Westwood specified the required switching devices and relays, integrating the equipment into the electrical and communication design of the plant. The team then coordinated with the substation engineer to determine the proper control sequence to meet utility requirements.

Westwood was tasked with integrating the numerous above-ground and below-ground wiring across the site, comprised of cables for DC power, medium voltage AC power, low voltage AC power, and communications. The team’s understanding of the site’s soil issues led to proposing an above-ground cable management system for portions of the DC and LVAC, using messenger wires and hangers attached to the tracker piles. This plan reduced costly underground trenching and limited the amount of cable trench crossings, as well as helped the contractor finish the project on time and within budget.