Review of Wave Energy Converter Power Take-Off Systems, Testing Practices, and Evaluation Metrics
Authors: Tom Nathan 
Published: February 2023

Abstract
While the field of wave energy has been the subject of numerical simulation, scale model testing, and precommercial project testing for decades, wave energy technologies remain in the early stages of development and must continue to prove themselves as a promising modern renewable energy field. One of the difficulties that wave energy systems have been struggling to overcome is the design of highly efficient energy conversion systems that can convert the mechanical power derived from the oscillation of wave-activated bodies into another useful product. Often the power take-off (PTO) is defined as the single unit responsible for converting mechanical power into another usable form, such as electricity, pressurized fluid, compressed air, or others. The PTO — and the entire power conversion chain — is of great importance, as it not only affects how efficiently wave power is converted into electricity, but it also contributes to the mass, size, structural dynamics, and levelized cost of energy of the wave energy converter (WEC). Because there is no industrial standard device or devices for wave energy conversion in the marine energy industry, PTO system designs are highly variable. The majority of current WEC PTO systems incorporate a mechanical or hydraulic drive train, power generator, and an electrical control system. The challenge of WEC PTO designs is designing a mechanical-to-electrical component that can efficiently convert irregular, bidirectional, low-frequency, and low-alternating-velocity wave motions. While gross average power levels can be predicted in advance, the variable wave elevation input has to be converted into smooth electrical output and hence usually necessitates some type of energy storage system, such as battery storage, accumulators, super capacitors, etc., or other means of compensation such as an array of devices. One of the primary challenges for wave energy converter systems is the fluctuating nature of wave resources, which require WEC components to be designed to handle loads (i.e., torques, forces, and powers) that are many times greater than the average load. This approach requires a much greater PTO capacity than the average power output and often leads to a higher cost. In addition, supporting mechanical coupling and or gearing can be added to the power conversion chain to help alleviate difficulties with the transmission and control of fluctuating large loads with low frequencies (indicative of wave forcing) into smaller loads at higher frequencies (optimal for conventional electrical machine design). But these additions can quickly increase the complexity of the power conversion chain, which could result in a greater number of failure modes and increased maintenance costs; therefore, it is important to balance complexity and ruggedness. All of the previous points demonstrate how the PTO influences WEC dynamics, reliability, performance, and cost, which are critical design factors. This paper further explores these topics by providing a review of the state-of-the-art PTO systems currently under development, how these novel PTO systems are tested and derisked prior to commercial deployment, the evaluation metrics historically used to differentiate PTO designs, and how PTO systems can be improved to support the development of wave energy systems focused on control co-design.