Offshore Ocean Energy
Global demand for energy continues to rise with prosperity in the emerging and developing economies particularly in China, India, Brazil, South Africa, Japan and Korea. At the same time, the severity of environmental degradation and climate change associated with conventional sources of energy in the form of increased CO2 emission, acidification in oceans, etc. is greatly acknowledged by the high-energy consuming nations. In the blue economy paradigm, the tolerance level for higher emissions will no longer be acceptable to the countries adopting it in their development models. Since demand for energy would remain high in the growing economies in the future, the reliance on alternative non-conventional renewable sources of energy would remain high. Oceans are a vast source of energy particularly for the renewable energy. As per an estimate, electricity generation from oceans could range from 20,000 terawatts to 80,000 terawatts per year which is equivalent to 100 to 400 per cent of the current global demand for energy. Further, sea can produce 250 billion barrels (oil equivalent) of energy every day. Ocean energy could be classified based on the different sources such as tides, tidal currents, solar, waves, salinity gradient and thermal gradient. Renewable offshore energy provides energy security, climate change mitigation and provides wider access to resources for energy production for a nation. In addition, it also provides job opportunities. The Indian Ocean Renewable Energy Ministerial Forum held in January 2014 launched several initiatives to explore renewable energy resources in the Indian Ocean region.
Power drawn from waves occurs when wind blows across the sea surface and water act as carrier for the energy by using wave energy converters. The quantity of energy generated varies with height and the time gap between successive peaks. Energy drawn from waves is considered to be the most promising energy as it is less variable in comparison to other sources of energy like wind and solar. Generally, western coast of the continents and extreme latitude are the places where best wave energy is found. There are many different technologies presented globally to capture energy from waves. The most well-known example of this kind of technology is Pelamis, where there is long series of cylindrical floating devices connected to each other with hinges and anchored to the seabed. Wave energy is the least mature energy among the offshore ocean energy technologies. Globally, there is no consensus over any design for wave energy technology as it is still in testing phase. Wave energy technology has higher potential than tidal energy.
Ocean Thermal Energy
Ocean thermal energy is a reliable source of offshore ocean energy. The difference between the temperature of warm and cold sea water at around 800-1000 meters depth is used to run heat engine and hence produces electricity. This technology needs an environment where the thermal gradient between the surface and depth is at least 22 degree Celsius, which makes it only viable in tropical seas. The difference in temperature is converted into electrical power through turbines. On one hand, vapors from the warm sea water are used as working fluid to drive the turbine, on the other hand, cold water is used to condense the vapor. This difference in vapor pressure drives the turbine, hence produces energy. The resource can be utilized in either side of the equator (latitude 0 to 30 degrees). The capacity factor of OTEC is in between 90-95 per cent which is among the highest in all power generation technologies in both renewable and non-renewable sector. Ocean thermal energy can also be used for seawater air conditioning, seawater district cooling and aquaculture. Moreover, OTEC plants can also produce fresh water with the desalination technologies.
Offshore Wind Energy
It is an indirect form of solar energy, resulted due to unequal heating of various parts of Earth thereby replacing warmer air by cooler air and hence causing wind. It is estimated that around 1-2 per cent of solar radiation is converted into wind. With the growing concern of land acquisition, noise pollution, visual impact problems and many other problems with onshore wind energy, this vital role of offshore energy has been recognized worldwide. The most developed technology in offshore wind energy is installed on either gravity foundation or sited on monopolies. There are other turbines used at different water depth levels to harness wind energy from the oceans. Improvement in technologies such as ‘Smart Turbine Blades’, use of material like carbon fibre, direct-drive generator technology would expand the harnessing of offshore wind energy.
Tidal cycles are caused by rise and fall of the tides occurring every 12 hours due to gravitational force of the moon. Water flowing in and out of estuaries carries energy and the amount of energy extractable depends on the area intercepted and speed of flowing stream. Tidal range can be forecasted with a high level of accuracy and is highly predictable when compared to wave, solar and wind energy. The current in tidal flows is constant over the water depth, hence it makes tidal energy to be a greatest opportunity to harness energy. There are mainly two methods for generating energy from tidal: Tidal Barrages and Tidal Stream Turbines. Variation in tides ranges from 4.5m to 12.4m; however, to harness tidal energy economically at least 7m high tide is required to head start the turbine. The cost efficiency of tidal power plant depends on the ‘Gibrat’ ratio, which is the ratio of the length of the barrage (in meters) to annual energy production in kWh. The smaller the ratio the better the site is for harnessing energy. The harvestable tidal energy resource is estimated at 1 terawatts (TW) globally. Extensive plans exist for tidal barrage projects in India, Korea, the Philippines and Russia adding up to around 115 gigawatts (GW). The sites such as Severn Estuary between Southwest England and South Wales are examples of favorable sites which could provide 12 GW (approximately 10% electricity need of the country) of energy. Several sites in the Bay of Fundy, Cook Inlet in Alaska, and the White Sea in Russia are found to have the potential to
Saline Power is the energy created from salt concentration between fresh and salt water places where the river flows into the ocean (river mouths). The energy density from saline gradient can be measured as osmotic pressure between two saline solutions. The global estimation of the saline gradient energy is 3.1 TW, where Asia has the highest potential, followed by South America and North America. The projected cost for standalone installation for the year 2020 ranges from EUR 0.09/kWh to EUR 0.28/kWh for producing energy from saline gradient (IRENA). Reversed electro dialysis (RED) and pressure-retarded osmosis (PRO) is among the two technologies identified for converting power. PRO is often called osmotic power. PRO creates power by utilizing the difference in the pressure of salt water and fresh water. Seawater is pumped into a pressure exchanger and freshwater flows through a semi-permeable membrane towards the sea water, which increases the pressure within the chamber, followed by spinning the turbine and hence generating electricity. On the other hand, in RED, ions (salt) are transported through the membrane, which essentially creates a salt battery
Offshore Solar Energy
Solar energy can be tapped directly by Photovoltaic (PV) cells or indirectly by concentrating solar power. PV cells produce direct current electricity by releasing the electrons when they are exposed to sun, which can be stored or transferred into the grid with an inverter. On the other hand, concentration solar power (CSP) requires a large flat area to heat a liquid substance which is then used to drive an electric generator. This method generates alternating current (AC) which can be easily distributed into the grids. The use of offshore solar power plants would increase the resource power for countries like Europe, where landscape for CSP is scare. Offshore solar power plants offer two technical advantages. Firstly, the implementation of vertical axis to track sun heat is easy which not only simplifies the requirement of CSP but also avoids shading between collector rows. Secondly, the cooling water needed for solar plants is available easily which would increase the efficiency in thermodynamic cycle. Offshore solar power plants reduce water requirement for cleaning up the panels due to concentration of dust, especially in desert areas. The performance of offshore solar plant depends basically on solar irradiation and the status of the sea which includes wave and wind motion and their variability.