Aquamarine Power, based in Edinburgh, Scotland, is an alternative energy company with additional operations in Ireland and Northern Ireland. It is currently developing its flagship technology, a hydroelectric wave-energy converter called Oyster. Aquamarine Power’s goal is to develop commercial wave farms around the world to make renewable marine energy commonplace. Intended design life is about 25 years — a key reason designers chose hydraulics to transmit power. Developers don’t expect the systems to be maintenance-free for 25 years, so the design must allow divers to replace major components every 5 years.

Power-plant basics
The Oyster power plant sits on the sea floor and features a large (12 × 26 m), bottom-hinged buoyant oscillator. The plant would typically be located in 12- to 20-m of water to interact with surge forces encountered in near-shore areas. The surge forces move the oscillator like an inverted pendulum, which can pivot a full 180°. The oscillator is connected to a pair of hydraulic cylinders that transmit power from the waves as hydraulic power. Each stroke of the cylinders pushes hydraulic fluid (in this case, 95-5 water-glycol) to shore through piping, where the pressurized fluid drives a turbine to generate electrical power.

In 2009, Aquamarine deployed Oyster 1, a 315 kW near-shore wave energy prototype, at the European Marine Energy Centre (EMEC) in Scotland. The prototype became the world’s first grid-connected electrical power generator from a near-shore wave energy device. With a design life of two years, Oyster 1 was decommissioned in 2011.

One of the challenges encountered during the operation of Oyster 1 was reliability of the cylinder seals. Consequently, design efforts for subsequent systems have focused on longevity and ease of maintenance.

The next-generation unit, Oyster 800, represents a step change in design, size, and power output. The oscillator is about 50% wider than Oyster 1 but produces about 250% more energy because of hydrodynamic design optimization. Oyster 800 will form part of an array comprising three wave-energy converters which will be connected to a single onshore hydroelectric plant. The set-up will dramatically improve the plant’s overall economics.

Cylinder design
The hydraulic cylinders had to be designed to operate in an underwater environment where servicing or even a simple seal change is difficult. During the specified system lifetime of 25 years the cylinders will experience approximately 120 million cycles. But to keep the system running, a five year service period is planned for the final Oyster system — which corresponds to 24 million cycles. Therefore the typical wear parts of the cylinder have to be designed to last this long. Additionally, the design had to be easy to service and offer the possibility of refurbishing the main components.

System pressure under full load is 120 bar with pressure peaks up to 160 bar. This might not be extreme but considering the numbers of cycles, fatigue calculations led to a comparably strong design.

To protect the marine environment, the hydraulic fluid for the cylinders and onshore Pelton turbine is a 95% water-based hydraulic fluid. In addition, partial contamination of the fluid with seawater has to be tolerated by the entire system. This requires good corrosion protection of all inner cylinder parts. The piston rod is coated with an offshore-approved Ultraplate layer and the cylinder bore surface has a stainless steel liner for long-term corrosion and wear protection. Also the seal and bearing elements are designed to tolerate the limited-lubrication properties of a water based hydraulic fluid.