**2. Overview and State-of-the-Art**

Wind power or wind energy is an energy type that uses the kinetic energy of the wind and converts it to mechanical energy through wind turbines to turn electric generators for electrical power. The power content of the column of air is expressed by [2,3]:

$$P\_{wind} = \frac{1}{2}\rho A v^3 \tag{1}$$

where *ρ* is the fluid density, *A* the cross-sectional area and *v* the fluid velocity. Figure 1 illustrates this.

Betz's law indicates the maximum power that can be extracted from the wind, independent of the design of a wind turbine in open flow.

**Figure 1.** Schematic of fluid flow through a disk-shaped actuator. For a constant density fluid, the cross-sectional area varies inversely with speed.

The actual mechanical power *P* extracted by the rotor blades in watts is the difference between the upstream and the downstream wind powers:

$$P = \frac{1}{2}\rho A v (v\_1^2 - v\_2^2) \tag{2}$$

where *v* = *<sup>v</sup>*1+*v*<sup>2</sup> <sup>2</sup> . We can calculate the power obtainable from a cylinder of fluid with cross-sectional area *S* and velocity *v*1:

$$P = \mathbb{C}\_p \frac{1}{2} \rho A v\_1^3 \tag{3}$$

where *Cp* is the power coefficient.

An offshore and an onshore wind turbine have similar technology. They diverge on the fact that offshore ones might produce more energy: higher wind speeds are available offshore comparing to on land, so offshore wind power's electricity generation is higher per amount of capacity installed.

Portugal only has 2 MW of offshore capacity in the floating wind turbine WindFloat near the Aguçadoura Wave Farm in Povoa de Varzim.

Tidal energy is power produced by the surge of ocean waters during the rise and fall of tides, where the intensity of the water from the rise and fall of tides is a form of kinetic energy.

Tidal stream generators draw energy from water currents in the same way wind turbines draw energy from air currents. The water density is about 800 times the density of air. This means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speeds [5].

Piezoelectricity is the process of using crystals to convert mechanical energy into electrical energy, or vice versa, so there are two types of piezoelectric effect, direct piezoelectric effect and inverse piezoelectric effect. Since wind pressure is exerted on the wind turbine blades, the blades can be filled with piezoelectric materials to maximize the power obtained by wind force. When the wind impacts the piezoelectric panel, the change of pressure caused by the wind power is output as the voltage through the measuring meter and converted into the amount of energy [6].

The main objective of the co-location of offshore wind and tidal stream turbines is to reduce the cost of electricity generation from either technology separately [7–9].

Thus far, there is no yield that combines these two types of energy. However, there are some studies that simulate this installation.

A case-study site in the Pentland Firth that uses an eddy viscosity wake model in OpenWind to assess Wind energy, with a 3 MW rated power curve and thrust coefficient from a Vestas V90 turbine and to assess tidal energy, "is modeled using a semi-empirical superposition of self-similar plane wakes with a generic 1 MW rated power curve and thrust based on a full-scale, fixed-pitch turbine" [10].

The support structure loads due to wind, waves and current on a combined support structure featuring a single 3 MW wind turbine and 1 MW tidal turbine have been modeled for the same co-located farm case study of the Inner Sound of the Pentland Firth.

In this study, it has shown that the potential to share support structures by adding a tidal turbine to a wind turbine support looks promising.

This co-location results in a 70% increase in energy yield compared to operating the tidal turbines alone. It is found that, "within the space required around a single 3 MW wind turbine, co-location provides a 10–16% cost saving compared to operating the same size tidal-only array without a wind turbine. Furthermore, for the same cost of electricity, a co-located farm could generate 20% more yield than a tidal-only array" [7]. This also could help tidal stream technology move from being commercially uncompetitive alone to competitive when co-located with the wind [7,10].

There is other research that focuses on proposing and evaluating an optimized hybrid wind system and tidal turbines operating as a renewable energy generating unit in New Zealand.

It is known that using the capacities of wind and tidal power in renewable technologies would be a suitable alternative for fossil fuels and would help to prevent their detrimental effects on the environment. It is a cost-effective procedure for the power generation sector to maximize these renewables as a hybrid system.

This research indicates that Kaipara harbor has good potential for energy generation from a hybrid system (wind plus tidal) with good wind energy yield and additional energy from tidal energy.

It is concluded that the installation of a hybrid system with wind and tidal energy in a certain place in New Zealand is beneficial. However, prototypes would have to be installed to complete this study [11].

Another study, which uses the coast of the United Kingdom, is considered. This co-location of wind and tidal power will offer synergies of shared infrastructures that will help to reduce both capital and operational costs. With this purpose, a simulation and modeling of the system in terms of generation, structural forces, integration to the grid and economics is performed.

Although having Levelized Cost of Energy (LCOE) higher than a normal offshore wind farm, the co-location of both technologies are demonstrating feasible. The advantages of the use of tidal are that we can produce a lot of reliable energy with smaller turbines and seize more energy from the same location [1,12].

This study proposes a single structured tower with hybrid renewable energy cultivation on the southwest coast of Yemen.
