## 1 Introduction  
Marine Renewable Energy (MRE) is trending after solar and wind energy in the R&D sector and energy markets these years. This is due to the fact that solar and wind energy industries have reached the point of maturity and reliability. Moreover, MRE is an abundant source of untapped energy in many forms, in fact, it is estimated that harnessing merely 0.2% of the unused global ocean energy may provide sufficient power to meet power demands [1]. Additionally, a 529 MW of MRE installed capacity has been recorded as operational by the end of 2017 [2]. So, in the efforts of reducing dependency on depleting fossil fuel resources and utilizing an environmentally friendly resource of energy, it was inevitable to turn to ocean energy for countries with low solar and wind energy. This led authorities and policymakers to assess and invest more in MRE in places like Hawaii, India, Thailand, Brazil, and many others [3; 4; 5; 6]. Wave energy is considered the most exploited resource of MRE thanks to its availability and predictability. Many Wave Energy Converters (WECs) were developed, yet no particular concept has reached the commercial maturity [7]. The three major obstacles of the industrialization of WECs are associated with reliability, efficiency, and economic viability, but extensive efforts are being spent on the improvement of these aspects [8]. Reliability is achieved by refining the WEC designs and optimizing control strategies [9; 10], while economic viability and efficiency require the optimal selection of the site, the system, and materials in order to reduce costs and environmental impact while guaranteeing maximum energy absorption [11; 12]. Many ideas were investigated in order to conquer some of the obstacles, such as WEC integration in ongoing projects of maritime structure and breakwaters [13; 14; 15]. Other ideas proposed the multiple-use of offshore platforms by combining different energies such as the wind and wave energy converters in [16] or by combining different converters such as the Overtopping device and the Oscillating Water Column (OWC) in [17]. This paper discusses the airflow control of an example of WEC integrated into a breakwater which is the NEEIDA Multiple Oscillating Water Column (MOWC) [18]. This facility is installed in the breakwater of Mutriku in the north of Spain and it is an OWC system based on Wells turbine for wave energy conversion. The main drawback of Power-Take Off (PTO) systems equipped with Wells turbine is their power-limitation due to the occurrence of the stall effect [19; 20]. In this context, an airflow control based on a Proportional-Integral-Derivative (PID) controller has been proposed and the recourse to the Harmony Search algorithms and its variants has been proposed in order to facilitate the control design of the parameters. PID controller tuning is a delicate and complex task when lacking a systematic approach. To solve the problem design of PID controllers, the optimization theory has been proven to be an effective method to tune and optimize the controller parameters [21; 22]. Numerous optimization algorithms were investigated and tested with the PID controller in different applications, for instance, the Particle Swarm Optimization (PSO) [23; 24], the Water Cycle Algorithm (WCA) [21; 24], and the Harmony Search Algorithm (HSA) [22; 24]. The remainder of the article has been arranged as the following: Section 2 introduces the modelling section, describing all parts of the OWC system. Section 3 introduces the stalling behavior and the problem formulation of the proposed airflow control. Section 4 presents variants of the Harmony Search algorithm. Section 5 presents the Self-Adaptive Global-Best Harmony Search (SGHS) algorithm adopted to optimally tune the parameters of the Proportional-Integral-Derivative (PID) controller.