2. Energy Capture from Wave to Structure
The wave energy utilization can be decomposed into two critical processes: (wave) energy capture and PTO.
Figure 2 depicts the generic working mechanism of a wave energy marine buoy. Energy capture is upstream of wave energy utilization, which refers to the critical process of harvesting the wave (fluid) energy with the main structures (solid) and achieving the mechanical energy for PTO. Therefore, energy capture is governed by the fluid–structure interactions (i.e., hydrodynamics), which are subject to the inputs (e.g., wave height, period, water depth, etc.) and the main structure design (such as floating structure dimension, geometry, mass, moment of inertia, center of gravity, mooring configuration and stiffness, etc.). Though they have not converged, mainstream designs of energy capture include three categories: oscillating water columns (OWC), oscillating bodies (OB) and overtopping devices
[29][31].
Figure 2.
A mechanism diagram of a wave energy marine buoy.
Overtopping devices are structures that “trap” the water from the run-up waves
[30][32]. The trapped water provides a water head higher than the mean sea surface that turns into water flow out through a duct, which drives an axial flow turbine and, in turn, the generator in order to produce electricity
[31][33]. Wave Dragon is the world’s first grid-connected floating overtopping device
[32][34]. Overtopping devices are advantageous in that they largely stabilize the unstable input waves to the PTO and they usually work with low-head hydro turbines (standard for hydroelectric stations)
[31][33][33,35]. Overtopping devices usually involve larger dimensions (e.g., Wave Dragon
[32][34]) or fixed infrastructures (e.g., OBREC
[33][35]), as they work in a similar way to a reservoir/hydroelectric station. Therefore, an overtopping device is not considered for marine buoys and the following discussions concern the other two types: OWC and OB.
3. PTO from Structure to Wire
The PTO is the other critical process that essentially “converts” the mechanical energy into electrical energy, which is governed by structure–generator interactions (i.e., electrodynamics). The performance of the system is highly dependent on the interactions between the wave environment, main structure and the generators. Wave-to-wire modelling (W2W) is being developed to address these interactions with high fidelity
[34][35][66,67]. The PTO determines not only the output power, but also the dimension, cost and operational life of the system. Based on the generator mechanism, this research classifies wave energy PTOs for marine buoys according to the generator type: electromagnetic generator (EMG) and triboelectric nanogenerator (TENG).
4. Applications of Wave Energy Marine Buoy
The functional loads onboarding the marine buoys include navigation beacons, sensors, data acquisitors and communication devices, etc. With the expansion of buoy functions, the number of electronic equipment carried is gradually increasing, and the power consumption of the buoy is also increasing. In order to ensure the normal operation of marine buoys, solar battery systems need to mix other energy sources, such as wave energy, microbial fuel and wind energy, in order to prolong the service span of the buoy system and improve the charge/discharge efficiency
[36][106]. According to the specific application requirements, a variety of buoy systems have been developed. The representative ones include power supply buoys, data buoys, navigational buoys, drifter buoys and aquaculture buoys
[37][107]. Some of them are listed in
Table 1.
Table 1.
Typical applications of marine buoys.
With the progress of wave energy technologies, some attempts have evolved into large, non-cylindrical platforms, while many still fall into the buoy scale. Large buoys can output the electricity to the grid or to other marine structures instead of serving themselves only. Wavebob is a two-body heaving buoys system developed for the sheltered waters of Ireland. The rated power of Wavebob reaches 1000 kW and is considered highly adaptive to Mediterranean environments
[39][109]. Ocean Power Technologies developed the first commercial WEC in the U.S., PowerBuoy, which acts as an uninterruptable power supply (UPS) that constantly recharges itself by harvesting wave energy. Deployed to supply devices on-board or underwater, the PowerBuoy3 incorporates a redesigned PTO, a battery pack, a higher voltage power management and distribution system and a novel auto-ballasting system
[40][110]. Other power supply buoys include the OEbuoy
[41][111], AquaBuOY
[50][120] and AWS
[51][121]. Generally speaking, power supply buoys yield quite good performances, yet their survivability and financial feasibility are subject to examination and improvements
[39][109]. This is why the power supply buoys are more inclined to step into the segment market (e.g., PowerBuoy in the offshore applications) instead of the general power grid, for now.
In terms of offshore applications, wave energy buoys are very promising. In fact, the earliest successful application of wave energy technologies is realized on a navigation buoy (probably the most straightforward mission for marine buoys). Masuda’s navigation buoy captures wave energy with an OWC and converts it to electricity through a turbine-drive rotational generator. The buoys were commercialized in large numbers in Japan and the U.S. as navigation equipment, and proved to be the first successful wave-powered devices in real applications
[52][44]. The first commercially manufactured wave energy device in China also turned out to be the navigation buoy developed by Guangzhou Energy Research Institute. Since the late 1980s, around 800 wave energy navigational buoy products have been purchased by clients in China, Singapore and the U.K
[53][122]. The Chinese wave energy navigation buoys also adopt the combination of an OWC and turbine-drive rotational generator, while the PTOs are becoming more powerful and more mature. On top of this, the buoy-based PTOs developed by the Chinese Academy of Science have evolved into multiple models (10 W, 100 W and kW). In 2020, a comprehensive wave energy data buoy, “Hailing”, operated without any failure for one year in South China Sea. “Hailing” implemented two 60 W wave energy pneumatic generators, one 30 W solar panel and a complementary power management system
[54][123]. This means that wave energy could become the major renewable energy source for the mid-scale buoy.
As nerve nodes to the ocean, marine sensors perceive all sorts of valuable physical quantities, such as conductivity (salinity), temperature, depth (pressure), wave, wind, current (tide), radiation, turbidity, potential of hydrogen, dissolved oxygen and nitrogen concentration
[55][124]. In many occasions, the signals from the sensors need to be delivered to data acquisitors, in which, they are turned into time series in certain steps to be stored, transmitted or processed
[56][57][58][125,126,127]. The data are used to predict the weather
[59][128], hurricanes and cyclones
[60][129] and monitor the environment
[61][130]. The earlier representative of the data buoy is the McLane moored profiler designed by Woods Hole Institute (with an auto-lifting function)
[62][131] and that designed by Norway SAIV AS with an electrical winch
[63][132]. The international Argo project has deployed over 3200 oceanographic data buoys to increase sampling quantities and coverage in time and area
[64][133]. Other data buoys could be the buoys carrying GNSS receivers for geological monitoring
[65][134] and the drifting buoys with INSAT communication for the sea surface observations
[45][115].
Compared to power supply buoys or navigation buoys, data/sensor buoys do not require much volume. In fact, data/sensor buoys can be relatively small-scale. The Seahorse buoy is an autonomous profiler designed by Bedford Institute of Canada that consists of a buoy, jacketed wire, suspended weight and buoyant instrument package. The Seahorse buoy utilizes wave energy to deliver the buoyant instrument downward along the mooring line
[66][135]. The successor of Seahorse, Wirewalker, follows a similar wave-powered mechanism, but makes the device even simpler and cheaper
[67][136]. The U.S. Navy’s sonobuoy AN/SSQ-101 is an air-deployable active receiver. It is said that AN/SSQ-101 is powered by converting wave energy through an integrated linear magnetic generator
[68][137]. Wave energy greatly increased the mission endurance. In turn, the unit cost of AN/SSQ-101 is significantly reduced so that it can be extended to civilian purposes, such as monitoring marine mammals, port security and seismic activity
[69][138].
Due to the limitations with the battery of the buoy, the service availability of the functional device on-boarding a buoy is largely determined by its standby time and its temporal resolution
[70][139]. The power requirement of the functional devices involved with buoys ranges from 10
−3 W to 10
2 W. Approximately a quarter of them (mostly small-scale, single-function sensors) have a power consumption of less than 1 W. Over half of them have a power consumption within 1–10 W (e.g., camera). Approximately 20% of the functional devices require a power of 10–100 W (e.g., beacon light), whereas the rest (requiring more than 100 W) are some larger-scale, comprehensive systems
[71][140].
The wave energy technologies could be extended to other marine buoys. In fact, as the world population and economy grow, the demand for marine protein has increased rapidly in the past few decades. Aquaculture buoys are effective equipment used to increase aquaculture production and, at the same time, protect the environment. Echo-sounder buoys could reduce the number and impact of fish-aggregating devices
[47][117]. Low-cost aquaculture buoys could collect physical, chemical and biological data from marine farms, which help to determine whether the area is suitable for activities such as lobster breeding
[13]. The finfish-breeding buoy could store different types of feed for a long time
[46][116], whereas the feed buoy could feed fish autonomously
[72][141]. Generally speaking, small-scale buoys such as aquaculture buoys follow a design philosophy of being low-cost and robust. Therefore, sensor buoys and aquaculture buoys have become an appropriate application scenario for the TENG-based PTO. A combination of single-body OB and flexible track nanogenerator could power these buoys in a robust way
[73][74][142,143].
Dynamic environments are usually the negative factors for solar panels, but, to a certain extent, they can supply more energy to wave power systems
[75][144]. Many studies on self-powered buoys are attempting to shift the buoys’ power source from solar energy to wave energy in order to reduce weight and to increase the power capacity
[76][145]. For instance, a position-tracking buoy powered by a wave-drive EMG-TENG hybrid generator has been developed by Chandrasekhar et al. Sea trials revealed that the wave-powered buoy realized GPS position tracking for itself a few kilometers away from shore
[77][146]. Li et al. developed an EMG-based wave energy powered buoy that could automatically charge a lithium battery and discharge external loads. In sea trials in the Yellow Sea, it yielded a power density of 210 W/m
3, which is adequate for supporting many low-power sensors
[78][84]. A modular wave-energy-powered buoy (developed by Vella et al.) went through a series of model tests under both regular and random waves. The buoy generated an average power output value of around 0.9 W under a mild sea state of a 0.2 m wave height, meaning that it could become an observational buoy with a longer lifespan
[79][147]. Chen et al. developed a wave-energy-powered buoy by integrating an EMG/TENG hybrid generator. The buoy served as a self-powered sensing node and transmitted the sensing data over a distance of 300 m in real sea trials
[80][148].
It is found that the above wave energy marine buoys can be categorized into “wave energy converter buoys” (such as Ocean Power Technologies’ PB3, AWS’ Archimedes Wave) and “wave energy powered buoys” (such as Masuda’s navigation buoy, AN/SSQ-101 sonobuoy) depending on whether they can output electrical power to exterior payloads not on-boarding the buoy. There is not a solid boundary for the two buoy types. In fact, PB3 can be scaled down (at a reduced cost) to supply power only to on-board payloads
[81][56]. AWS’ Archimedes Wave can be scaled up to over 500 kW per unit, making it closer to a power station
[51][121].