Low-friction bearings reduce energy losses in free-stream turbines

A hydro turbine company based in Portland, Maine, USA, has chosen Vesconite Bearings’ lowest-friction material for the thrust bearings on its free-stream turbines.

The material, Vesconite Superlube, has one of the lowest coefficients of friction of any plain bearing material available, with a coefficient of friction lower than virgin PTFE (polytetrafluoroethylene).

“Thrust bearings made from this material were first ordered in June 2022 and installed in November 2022,” states renewable energy application developer Petrus Fourie.

These initial bearings will be assessed in December 2023, but the client, which has been satisfied with how they were installed and how they have operated to date, does not anticipate any problems, he adds.

The company has also ordered additional sizes of Vesconite Superlube thrust bearings, so it is believed that the thrust bearings have now been installed in multiple devices.

The renewable energy company wanted a water-lubricated bearing because of the environmental benefits of not having grease in water systems.

It also required low friction, since this would result in less friction losses and therefore greater overall system efficiency, and low wear rates, so bearings would last longer and require less maintenance — which is desirable due to the difficulty and cost of doing maintenance on turbines that are deployed in the ocean or rivers, since they either float on the surface of a river, sit at the bottom of a river or they can be placed in the sea where there is high tidal flow so that electricity is generated from the flow of water.

For this reason, the thrust bearings, which are wear rings made of plate to absorb the axial forces on the turbine, are required to have low wear rates to reduce maintenance requirements.

“The thrust bearing rings rotate against a stainless steel counter-surface and absorb any axial loads on the main shaft of the hydro turbine,” describes Fourie.

“If any axial load is applied, the friction on the sliding surface results in a torque on the main shaft acting in the direction opposite to the rotation of the shaft, effectively braking the turbine and causing energy losses in the system. The energy losses due to this effect are directly proportional to the friction coefficient, hence the importance of minimizing friction to improve system efficiency.”