Progress in research and application of oscillating water column wave power generation devices

Ocean wave energy reserves are huge, and the annual reserves of exploitable wave energy in the world can reach 2 TW[1]. Compared with renewable energy sources such as solar and wind energy, the energy density of wave energy is about 5 times that of wind energy and 15 times that of solar energy [2]. The total amount of wave energy storage is much greater than that of wind energy, with only small energy loss during transmission [3], and wave power can be generated 90% of the time in a year (20% to 30% for wind and solar) [4]. In addition, 37% of the world's population lives within 90 km of the coast, so the development of wave energy is of great practical importance. Many countries around the world have wave energy development test bases, and countries such as the United Kingdom, Portugal and Australia have established wave energy power plants and successfully supplied electricity [5]. These Wave energy generation devices can be classified according to their arrangement, such as the shore-based Terminator, the LIMPET that uses the oscillating water column, the Attenuator shaped like a sea snake, the Wave Dragon, and the point float AquaBuoy. Among them, the wave power generation device based on the principle of oscillating water column (OWC) is currently the most successful and widely used. In China, small OWC beacon lights have been mass-produced and exported, but the development of large OWC wave energy conversion devices has been slow and has not been connected to the grid. In this study, the basic principle of OWC wave energy conversion device is introduced, and the research and development of OWC wave energy generation device at home and abroad is summarized and prospected.

1OWC wave energy conversion principle and advantages

At present, the OWC wave energy generation device is the most widely used wave energy conversion device in the world, and its structure is shown in Figure 1. The working principle of wave energy conversion is to use the fluctuation of the wave to drive the oscillation of the water column in the OWC device (that is, the fluctuation of the free water surface in the device), so as to compress the air in the air chamber. In order to prevent the backflow of the exhaust port when the water column falls, the suction and exhaust valves can be set and controlled to open and close accordingly, or the Wells turbine can be used directly. The alternating airflow is rectified into a unidirectional airflow through the air turbine, driving the generator to generate electricity.

When the device is connected to the shore (FIG. 1), the rear wall is integrated with the coast and seabed, which is equivalent to a steel and concrete breakwater, protecting the coast from waves. When the device is combined with a floating structure, it can play the role of a floating breakwater when it is far away from the shore. Due to its special structural form, compared with other wave energy power generation devices, it has the following advantages: (1) simple structure, without too many moving components, can reduce the energy loss in the process of wave energy conversion; (2) The mechanical device is not in direct contact with seawater and is not easy to be corroded; (3) The adaptability of the device is strong, can be arranged on the coast, near the shore or offshore, can effectively use the Marine space; (4) The use of air turbines does not require gearboxes and other transmission devices, reliable performance, simple maintenance.

2OWC wave power generation device research progress

In 1974, Salter[6] first published a research paper on the wave energy power generation device, so far, human research on wave energy power generation has been more than 40 years. In 2008, Cruz[7] reviewed the research process of nearly 40 years.

2.1 Turbine device

The Wells turbine is an important part of the OWC installation. The invention of Wells turbine solved the problem that alternating air flow could not make the turbine rotate in one direction, and greatly simplified the structure of the device. Raghunathan[8], Curran et al. [9] conducted an in-depth study on the turbine and its operation effect, and Curran et al. [9] also calculated the turbine efficiency under different arrangement forms. Today, almost all OWC wave power plants use Wells turbines.

2.2 Structure and Form

In terms of theoretical research on OWC structure, Evans[10-11], Sarmento et al. [12] in the early stage simplified OWC into a rigid piston, and then studied the interaction between waves and OWC structure. The biggest disadvantage of traditional OWC devices is that their natural period is smaller than the wave period, resulting in low efficiency. In order to solve this problem, some improvements have been made on the original basis, so that the wave can reach resonance in the device [13]. A new U-shaped OWC structure (U-OWC) was proposed. U-OWC has a larger inherent period than traditional OWC and can make better use of wave energy, even under large period conditions such as surge or storm surge [14].

In the past decade, the combination of OWC and caisson breakwater has been widely favored by researchers. Boccotti[15] made a theoretical derivation of caisson breakwater as an OWC device, and completed related physical test research [16], which proved its feasibility. Huang et al. [17] designed a perforated breakwater caisson as an OWC gas chamber and conducted experimental research on it. Qin Hui et al. [18] improved on the ordinary OWC breakwater, proposed the structural form of caisson breakwater with shrinking channel as an OWC device, and conducted numerical and physical model tests.