Identification of future emerging technologies in the ocean energy sector
Part 3 - Power Take-off
As alluded to in our previous post power take off (PTO) technology is a key factor in wave energy device development. It should also be noted that significant research has gone into refining PTO systems for tidal turbines too. This week we take a look at the findings of the European Commission Workshop (link) on this topic.
The broad range of PTO systems considered in this study largely stems from the attempts that have been made to find a PTO system that is suited to wave energy converters. Typically, tidal energy converters rely on rotational PTOs with the rotor blades driving a generator via a gearbox. The main emerging technology areas for tidal are in the application of direct drive PTOs. The advantage here is the removal of a gearbox system, which requires, cooling, lubrication and alignment upon assembly. Not to mention the weight, efficiency and vibration damping requirements. It's just that, to date, cost effective direct drive generators for the typically slow rotor speeds associated with tidal turbines, have not been forthcoming. Multiple projects are in progress to attempt to address this, such as Nova Innovation's TiPa Project.
As we have discussed in previous blogs (link) due to the irregular oscillatory input from the resource wave energy converters face a challenge in finding an optimal PTO. Typically, a rotational PTO based upon a gearbox and a rotational generator like those found in tidal and wind turbines would not perform well will varying speed and directional inputs from a typical wave energy device; hence the search for something more suitable.
Due to the large variety in wave energy devices it is very much "horses for courses" regarding the choice of PTO system. Some projects are developing more sophisticated mechanical conversion technologies to include power smoothing in the conversion from linear to rotational power (such as Corpower's cascade gearbox).
An interesting development is in the area of dielectric polymers which can be used as flexible membranes in wave energy devices. There is a compromise to be made between durability and flexibility of these materials, but they can directly generate electrical current from wave motions (application dependent).
Hydraulic PTOs seem well suited to the high forces and lower frequencies of wave energy, but they are accompanied by challenges in sealing and wear as well as poor efficiencies away from peak power. As a result pneumatic PTOs are being explored in a number of forms. Whether that be replacing hydraulic fluid in PTOs, or being used to drive an air turbine. There is always a balance with these PTOs in terms of complexity and ultimate efficiency.
In essence there are many avenues being pursued, each suited to a different type of device. It is thought that once a developer establishes an energy conversion principle coupled with an efficient PTO the sector may start to converge onto a single device type, as has been seen in wind and tidal.