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outer sides of the dam is due to changes al., 2018), oscillating hydrofoils (MA et al., 2.3 Thermal Gradient Conversion at the mouth of a river. There are two
in the tidal regime. 2017; WANG et al., 2017; FILIPPAS et al., methods to generate electricity from the
The kinetic energy of tidal and ocean 2018), and effect turbines Venturi (CHAUD- Ocean thermal energy conversion salinity gradient: pressure retarded osmo-
currents can be extracted with hydroki- HARI et al., 2013). The currents passing (OTEC) is a technology that uses the wa- sis (PRO) (ALTAEE et al., 2017; Idem, 2019)
netic turbines, which have similarities with through the hydrofoils generate lift force, ter temperature difference between the and reverse electrodialysis (RED) (TUFA
wind turbines, however, operate under- which drives the movement of the hydrau- sea surface and great depths (about 1000 et al., 2018; AVCI et al., 2018). The PRO
water. Due to the greater density of water lic system to generate electricity. Venturi m) for heating, cooling, electricity genera- method is based on semipermeable mem-
compared to air, turbine blades for tidal effect turbines take advantage of the ki- tion or desalination purposes. The feasibil- branes that allow only water molecules
or ocean currents are smaller and move netic energy of the current, amplifying the ity of the technology requires a minimum to pass through. In this approach, water
more slowly than wind turbines. The main current velocity due to the reduction of the temperature differential of around 20 °C. flows from a dilute solution (freshwater) to
difference between tidal and ocean cur- flow cross-section (MOFOR et al., 2014). In the closed cycle, warm water from the a concentrated solution (seawater), raising
rents is that the former has bidirectional In addition to these types of converters, upper layers is used to vaporize a working the pressure of the saltwater, which drives
flows in contrast to ocean currents which there are ducted channels (KHAN et al., fluid, ammonia, for example, which drives a turbine, producing electricity. The RED
are unidirectional. The four main types of 2009; BELLONI et al., 2017; TAMPIER et an electrical generator. The resulting steam method is based on the transport of ions
current energy conversion devices, shown al., 2017) and tidal kite channels (TSAO et is then condensed by cold water, which is (salt) through membranes. Two fluids of
in Figure 3, are: horizontal axis turbine (SEO al., 2018; TAMPIER et al., 2018; TAMPIER lifted, by means of hydraulic pumps, from different salinities (fresh water and salt wa-
et al., 2019; SHIRASAWA, 2016), vertical et al., 2017), both under development. A the ocean floor and subsequently discard- ter) pass through a series of specific mem-
axis turbine (JING et al., 2014; FERNANDES review of tidal current technology is pre- ed. Some studies on closed-cycle OTEC can branes. The chemical potential difference
et al., 2015; WANG et al., 2018; CHEN et sented in (QUIAN et al., 2019). be found in the references (FAIZAL et al., between the membranes results in an elec-
2013; AYDIN et al., 2014; YANG et al., trical voltage. The brackish water is then

Figure 3 - Main categories of tidal current turbines 2014). Open-cycle technology uses heated discharged into the sea. In addition, some
water from the upper layers of the ocean hybrid processes such as electricity produc-
as the working fluid. The main benefit of tion from thermal energy using a closed-
the open cycle process is that it can pro- loop RED heat engine, low-energy desali-
duce both electricity and desalinated water. nation integrating RED with desalination
The hybrid cycle combines closed and open facilities, and the use of microbial RED cells
cycle technologies. In this process, heated with improved energy performance have
surface water is rapidly evaporated in or- been proposed for extracting energy from
der to vaporize ammonia from the working the salinity gradient (TUFA et al., 2018; MEI
fluid in a closed cycle, driving a turbine to et al., 2018).
produce electricity. Then the steam is con- 2.5 Offshore wind turbines
densed by a heat exchanger, producing de-
salinated water (OCTAVIANI et al., 2016). There are two main types of founda-
Such a cycle can generate electricity and tions for offshore wind turbines, fixed with
drinking water simultaneously. a support on the sea floor and floating.
Fixed foundation concepts are suitable
2.4 Salinity Gradient Conversion
for depths up to 60 m. However, for wa-
The salinity gradient takes advantage of ter-blades greater than 40 m, these struc-
the energy that can be generated by mixing tures suffer large hydrodynamic loads,
Source: SHADMAN et al., 2019 fresh water with salt water, for example, which leads to an increase in cost, due to

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