What is the next big thing in Energy?
By: Harish Maddukuri
Tags: Research, Renewable Energy, Science, Technology, Future Technology

Types of Marine Energies

The Renewable energy resource in the ocean comes from five distinct sources mainly, each with different origins and requiring different technologies for conversion. These sources are:

1. Waves

Ocean wave energy is the energy that has been transferred from the wind to the ocean. As the wind blows over the ocean, air.sea interaction transfers some of the wind energy to the water, forming waves, which stores this energy as potential and kinetic energies.

Global offshore annual wave power level distribution (Source: Cornett, 2008)

Recent reviews have identified more than 50 WEC (wave energy converters) at various stages of development. They are classified based on their operating principle and depending upon the installation site. Some of the technologies are shown below.

created by the author ( pc: AQUARET)

2. Tidal Range

Tides are the regular and predictable change in the height of the ocean, driven by gravitational and rotational forces between the Earth, Moon and Sun, combined with centrifugal and inertial forces. Many coastal areas experience roughly two high tides and two low tides per day (called ‘semi-diurnal’); in some locations, there is only one tide per day (called diurnal).

World map of tidal amplitude (Source: NASA)

3. Tidal and Ocean currents

Tidal currents are generated by horizontal movements of water, modified by seabed bathymetry, particularly near coasts or other constrictions (e.g., islands). Tidal current flows result from the rise and fall of the tide; although these flows can be slightly influenced by short-term weather fluctuations, their timing and magnitude are highly predictable and largely insensitive to climate change influences.

In addition to near-shore tidal currents, significant current flows also exist in the open ocean. These currents flow continuously in the same direction and have low variability. Some of these offer sufficient current velocities (~2 m/s) to drive present-day technologies.

Surface ocean currents, showing warm (red) and cold (blue) systems(Source: Bahaj 2013)

Several classification schemes for tidal and ocean current energy systems have been proposed. Just like the Wave energy converters, these are also classified based on the principle of operation, such as axial-flow turbines, cross-flow turbines, and reciprocating devices. Some are shown below.

created by the author ( pc: AQUARET)

4. Ocean Thermal Energy Conversion

About 15% of the total solar input to the ocean is retained as thermal energy, with absorption concentrated at the top layers, declining exponentially with depth as the thermal conductivity of seawater is low. Sea surface temperature can exceed 25°C in tropical latitudes, while temperatures 1 km below the surface are between 5°C and 10°C.

A minimum temperature difference of 20°C is considered necessary to operate an OTEC power plant. Both coasts of Africa and India, the tropical west and south-eastern coasts of America, and many Caribbean and Pacific islands have sea surface temperature of 25°C to 30°C, declining to 4°C to 7°C at depths varying from 750 to 1,000 m. The OTEC resource map showing annual average temperature differences between surface waters and the water at 1,000-m depth shows a wide tropical area with a potential greater than 20º C temperature difference is shown below.

Worldwide avg. ocean temperature diff. between 20°C and 1,000 m water depth(Nihous, 2010)

Ocean thermal energy conversion (OTEC) plants have three conversion schemes: open, closed, and hybrid. the open conversion cycle, about 0.5% of the warm surface seawater is flash-evaporated in a vacuum chamber. This steam is the cycle’s working fluid, which passes through a power-generating turbine before being condensed by deep cold seawater. Desalinated water can be obtained as an additional product by employing an appropriate cycle.

A hybrid conversion cycle combines both open and closed cycles, with steam generated by flash evaporation acting as the heat source for a closed Rankine cycle, using ammonia or another working fluid. A simple working scheme has been shown below.

Simple OTEC plant (Source: Makai)

5. Salinity Gradients

The mixing of freshwater and seawater releases energy as heat. Harnessing the chemical potential between the two water sources, across a semi-permeable membrane, can capture this energy as pressure, rather than heat, which can then be converted into useful energy forms. Since freshwater from rivers discharging into saline seawater is globally distributed, osmotic power could be generated and used in all regions wherever there is a sufficient supply of fresh water. River mouths are most appropriate, because of the potential for large adjacent volumes of freshwater and seawater. Recently, the technical potential for power generation was calculated as 1,650 TWh/yr. (6 EJ/yr.)Salinity gradients could potentially generate base-load electricity if cost-effective technologies can be developed. Two technologies were shown below.