XCT System for Harvesting In-Current Hydrokinetic Energy from Low-Velocity Sites

Case Study: XCT System for Harvesting In-Current
Hydrokinetic Energy from Low-Velocity Sites

Client: U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Water Power Technologies Office
Prime: Littoral Power Systems, Inc. (LPS), New Bedford, MA
Partners:

  • General Electric Research Corporation (GE), Niskayuna, NY
  • Alden Research Laboratory, Inc., Holden, MA and Fort Collins, CO
  • Turbo Solutions Engineering, LLC, East Thetford, VT
  • Marine Renewable Energy Collaborative (MRECo), Bourne, MA
  • RayBar Machine Company, Hebron, OH

Co-Funder: Massachusetts Clean Energy Center (MassCEC)
Timeline: October 2018 – September 2023

Project Summary

The XCT (Cross-Cutting Turbine) System project explored a fundamentally different turbine architecture to harness energy from low-velocity water flows—an area long considered impractical for hydrokinetic generation. The design centered on small, fast-spinning tip turbines mounted on rotating spars, enabling efficient power capture with minimal mooring loads and no reliance on shaft seals. Over multiple years, the system was advanced through detailed CFD and BEM modeling, iterative electromechanical prototyping, lab-scale flume testing, and full open-water deployment in the Cape Cod Canal. The result is a technically validated, modular system that maintains stability, withstands debris and sediment, and demonstrates performance metrics consistent with scalable, low-cost energy production.

The Challenge

An estimated 80% of global hydrokinetic potential exists in slow-moving currents (≤2 m/s), but most CECs are designed for higher velocities and require large, expensive infrastructure. Traditional axial-flow turbines face torque, size, cost, and gearbox limitations when operating at low speeds. LPS sought to demonstrate that a radically different approach—small, fast-spinning tip turbines driven along rotating spars—could achieve competitive performance and reliability while supporting a low Levelized Cost of Energy (LCOE).

LPS’s Role

The XCT turbine reconfigures energy capture by attaching small, direct-drive tip turbines to the ends of rotating spars. These tip turbines spin faster than traditional axial-flow blades and generate power through compact generators integrated into their nacelles. This configuration reduces torque loading on the system, enabling simpler mooring and support structures.

LPS built and tested two physical prototypes:

  • LSTA (Laboratory Scale Test Article) – A 2 m diameter XCT device tested in a flume to validate hydrodynamic performance and mooring stability under constrained conditions.

  • XCT-1 – A 3 m diameter open-water prototype tested at the Bourne Tidal Test Site in Cape Cod Canal to assess full-system integration, performance in bi-directional flow, mechanical resilience, and marine interactions.

Both devices operated without shaft seals, allowing LPS to assess real-world performance under water ingress conditions—a common failure mode in CECs.

Overall, LPS served as the prime contractor and systems integrator, leading the full-cycle R&D effort—from concept development and modeling through fabrication, testing, and LCOE modeling. LPS also coordinated test site logistics, partner engagement, and environmental evaluation, drawing on its experience in turbine design, field deployments, and marine systems engineering.

Engineering Highlights / Technical Solution

  • Hydrodynamic Performance: Blade element momentum (BEM) and CFD analysis, supported by flume and open-water testing, demonstrated power coefficients (Cp) of 0.35–0.40. Performance exceeded the design goal of Cp > 0.30 needed for economic viability in ocean currents.

  • Minimal Mooring Load: The near-zero net torque of the rotating XCT system allows for simple mooring with one or two lines. Tests showed stable operation in high degree-of-freedom configurations, reducing the need for rigid bottom structures.

  • Electromechanical Design:

    • Off-the-shelf AMK Automation DT4 motors were repurposed as tip generators, demonstrating feasibility of cost-effective integration.

    • A custom-built rotary transformer successfully transferred power from rotating components to the stationary frame, proving a sealed-less transmission concept. Though less efficient (peak 54–64%), it enabled crucial design learnings.

  • Structural Integrity: Despite debris impacts and sediment ingress, both prototypes remained operational. XCT-1’s aluminum components resisted corrosion in saltwater without seals. Sand-induced turbine seizure was reversible, highlighting mechanical resilience.

  • Environmental Performance: Acoustic imaging and modeling indicated fish avoidance and low impact. A protective rim further reduced potential fish mortality, and no direct fish interactions were observed during testing.

Results & Outcomes

  • Cp ≥ 0.30 Validated: Both LSTA and XCT-1 met or exceeded the target hydraulic power coefficient needed for scalable designs.

  • Stability in Flow: The turbine remained stable in both restrictive and minimal mooring setups, even during flow reversals at the tidal test site.

  • Sealless Operation Demonstrated: Both prototypes ran without seals and remained operational after prolonged submersion, addressing one of the top failure modes in marine energy systems.

  • Debris and Sediment Insights: Damage from floating debris and internal sand accumulation revealed key design considerations for future commercial units, such as protective coatings, redundancy, and sediment filtering.

  • Scalable LCOE Pathway Identified:

    • <$0.10/kWh at 2.13 m/s velocity and 26% capacity factor using raft-mounted configuration.

    • $0.16–$0.25/kWh at lower velocities or more complex mounting (e.g., triframe or submerged wing).

Impact

The XCT system expands the viable footprint of hydrokinetic energy by enabling economic energy capture in low-velocity currents previously considered uneconomical. With validated performance, simplified mooring, and scalable design, XCT represents a breakthrough approach that could unlock vast new deployment opportunities in rivers, ocean currents, and tidal passages.

Next Steps / Future Work

LPS is pursuing scaled prototype development (5–8 m diameter), site-specific feasibility studies, and commercial demonstration opportunities with industry and government partners. Future iterations will refine generator efficiency, reduce debris vulnerability, and validate multi-unit array performance.