Oceans cover 70% of the earth’s surface and occurs naturally. It is endowed with a vast range of resources ranging from sea animals, sea plants and water. These resources co exists in the ocean ecosystem.
Power generation in oceanic towns and countries poses a threat to both the plants and animals in the ocean either directly or indirectly. This can be envisaged from the available technologies such as diesel, coal and nuclear that pollutes both the air and the water by emitting heat wastes and toxic gases into the environment.
Moving to renewable energy can be a solution to the threats posed by the existing power generation technologies. Given that the oceans and seas are natural resources and combining the solar radiation, tidal movements, winds and ocean waves, a vast supply of renewable energy can be obtained from the ocean. However, these technologies are not free from pollution and adverse ecological effects. Hence a balance should be struck between power generation and ecological effects.
AVAILABLE OCEAN GENERATION TECHNOLOGIES
The currently available technologies include ocean thermal energy conversion, salinity gradient energy, solar energy, wind energy, tidal and wave energy.
Salinity gradient energy.
Salinity gradient energy is derived from two streams of water with different salinity levels mainly sea water and fresh water. The two streams are separated by a membrane that limits the movement of a particular type of ion. There are two methods that employ the use of membranes, that is, pressure retarded osmosis and reverse electro dialysis. Theoretically, salinity gradient plants can produce 0.8 kW/m3 though this value can be lower due to inherent irreversible energy dissipation.
Pressure retarded osmosis uses a membrane that blocks ion penetration but allowing water to diffuse through. The flow of water builds up a pressure that can be used to drive a turbine on the salty water side. A pressure retarded osmosis plant has been built in Tofte, Norway with a generating capacity of 4 kW.
Reverse electro dialysis requires two membranes, one that is selectively permeable to positive ions while the other is selectively permeable to negative ions. The charge separation creates a potential difference that can be directly be utilized as electrical energy. These is no existing reverse electro dialysis plant as it is still in the experimental stages
Ocean thermal energy conversion.
Ocean thermal energy conversion (OTEC) makes use of the temperature difference between the warm surface sea water and the relatively colder deep sea water. The warm sea water is pumped to a heat exchanger where it heats and vaporizes a fluid with a low boiling point mainly ammonia and the vapor use to produce electricity from a turbo generator. The vapor is then cooled using the cold sea water obtained from the deeper surfaces to liquid form then conveyed backed to the heat exchanger for a repeat cycle.
The largest existing ocean thermal energy conversion plant existing is the Makai Ocean Engineering power plant with a generation capacity of 100 kW and can supply 120 homes as a base load plant. There are plans to construct a 10 MW plant in Hainan Island which will be the first offshore OTEC plant.
Solar energy can be utilized in terms of direct conversion to electrical energy through photovoltaic cells or indirectly through concentrated solar power. To harness the ocean solar energy, floating solar technologies can be used where photovoltaic panels or solar concentrators are placed on a floating surface. Floating solar power plants reduces the cost of land and increases generation by 16% and limit the long-term heat induced degradation through the cooling effect of water.
The most common mode of application of solar is the use of solar PV panels. They use Cadmium Chloride to increase their conversion efficiency though it is highly toxic and expensive. Research is ongoing to replace Cadmium Chloride with Magnesium Chloride which is abundantly available in sea water hence cheap and less toxic.
Wind energy is harnessed by converting the kinetic energy in wind to electrical energy by rotating the rotor blades at about 10-25 rpm then a gear box is used to convert the slow speed to high speed of about 1500 rpm where they are coupled to an induction generator that produce electricity. Winds can either be harnessed onshore or offshore.
Offshore winds are more consistent, stronger and more abundant as compared to onshore winds. The equipment used to harness offshore wind however should be more robust to withstand the high corrosion potential they are exposed to due to sea water salinity.
Most offshore wind plants have been developed in Europe while Asian countries have increased their development of such plants.
Tidal energy can be harnessed using the potential energy in the tides or the kinetic energy in the moving water waves. To harness the potential energy, water is stored in an estuary using tidal barrage during high tides then released during the low tides. A barrage is a structure that spans across the inlet of a basin or estuary creating an enclosed tidal reservoir similar to a dam in hydro power generation. During the movement of water through the barrage, a turbine is rotated to generate power. The kinetic energy in sea water can be obtained using the same principle as that of wind. However, for the same turbine, sea water is expected to generate more power as compared to water due to the higher water density.
Tidal energy has been least exploited due to the numerous engineering challenges it possesses.
DATA ON THE CURRENT OCEAN POWER SITUATION
There have been various plants constructed in the ocean. Table 1 shows a list of the various oceanic plants that have been implemented, showing their capacities, power generated, cost and the technology used.
The cost of constructing a floating solar power plant per kilowatt is estimated to be 100 USD/kW based on the Three Gorges floating solar farm in China expected to generate 150 MW at a cost of $151 million.
Solar and wind have been implemented onshore in the past. For a shift to the ocean a comparison of the performance of solar and wind onshore and offshore is instrumental. Table 2 shows a comparison of ground solar PV and floating plant solar PV. Table 3 shows a comparison between Ocotillo Wind energy project, an onshore wind farm and the Walney wind farm, an offshore wind farm.
With the development of the various technologies, the emission of carbon dioxide has become a major issue. Table 4 shows the levels of carbon dioxide emission to help determine the technology with the least pollution levels.
Ocean power just like any other source of renewable energy is mainly faced with the challenge of cost and inefficiency. The cost of power production of ocean energy has been seen to be very costly as the return in terms of power generated has been observed to be low as compared to alternative sources such as coal and nuclear. For instance, the Statkraft Osmotic Power Plant was shut down in 2013, four years after being launched, after the investors realized that the devices utilized did not produce enough power to recover the construction, operation and maintenance costs. This thus makes salinity gradient technology the least feasible ocean technology that has been tested.
Ecological and environmental effects
Harnessing energy brings about environmental impacts such as pressure on land and pollution. Oceanic resources can be utilized to reduce pressure on land as an alternative to the anchorage of the plants and also since the resources are natural and renewable, they should reduce pollution. Construction of plants in the ocean temporarily disrupts the sea bed hence destroying habitats and reducing the sub surface visibility. The levels of ecological effects due to ocean power generation are however lower as compared to the impact of fossil fuels and nuclear. Extracting wind and tidal energy can also kill sea animals either on water or on the air when their turbines come into contact with the animals in motion.
Generally, pollution of the environment in terms of the emission of the greenhouse gases is reduced. For instance, the Yamakura dam floating solar PV plant in Japan has the capacity to offset 8170 tons of carbon dioxide a year that is produced when 19000 barrels of oil is used for power generation.
Ocean power generation can be multi-purpose such that they can not only produce power, but also be used for air conditioning, desalination and aquaculture. The power generated by the ocean resources are relatively low but their alternative uses can encourage increased investments and research on better ways of improving generation efficiency. Salinity gradient plants and ocean thermal energy conversion plants can be used for desalination and air conditioning. Floating solar plants provides a cover to the sea surface thus limiting evaporation of water.
The reliability of ocean power generation is dependent on the prevailing atmospheric conditions hence most of the available methods of ocean power generation can be used as peak load plants. For the modes to be used as base load plants, storage is necessary and this can increase the overall power generation costs. Salinity gradient and ocean thermal generation plants can however be used as base load plants since the salinity and ocean temperature difference is approximately constant throughout.
Comparison between onshore and offshore plants
The performance of both solar and wind plants is seen to be better offshore than onshore and according to Table 2 offshore solar plants are better than onshore power plants in terms of power produced. The efficiency for solar is increased when used offshore due to the cooling effect of water and the lifespan of the plant is increased due to a decrease in the rate of thermal degradation. Offshore winds are stronger than onshore winds thus for the same set of equipment used, more power can be generated offshore than onshore. Furthermore, offshore sites allow for the turbines that are much larger and higher thus allowing for more power generation.
Using data provided by National Renewable Energy Laboratory, the levelized cost of energy for onshore wind power is 61.03 $/MWh while that of offshore wind power is 181.36$/MWh as at 2015 . The price difference is attributed to the more advanced mounting structures employed at sea as compared to land to cope with the sea salinity and the ocean currents.
Ocean thermal energy conversion and salinity gradient plants are still in research and development stages hence they are very expensive. However, considering alternative uses, they can be viable hence research can be done to focus more on their alternative uses and power should be a secondary consideration for the technologies to develop further.
Wind and tidal power presents a relatively cheaper solution to the oceanic power generation. However, the interaction of their respective turbines poses a problem that should be put into consideration for the coexistence of the plant and the sea creatures. Tidal power production still poses great engineering challenges while the cost of constructing offshore wind power farms is still too high.
Solar is the best alternative given that the technology being developed to replace cadmium chloride with magnesium chloride will not only be a cheap alternative, but also environmentally friendly. The cost of solar competes favorably with both tidal and wind power generation but it has an upper hand given that other than the initial disruption of the sea bed during construction, its operation does not pose a threat to the sea animals.
Going blue will generally decrease the operational costs and reduce environmental pollution.
Table 1: Various plants capacities and costs
|Plant||Country||Technology||Capacity (MW)||Construction cost (million USD)||Construction cost per kW (USD/kW)|
|Statkraft Osmotic Power Plant||Norway||Pressure retarded osmosis||0.004||18||450000|
|Makai Ocean Engineering||USA||Ocean thermal energy conversion||0.1||4.6||4600|
|Yamakura Dam solar power station||Japan||Floating solar PV||13.7||Not disclosed||Not disclosed|
|Gode wind 1&2||Germany||Offshore wind farm||582||2400||412|
|La Rance||France||Tidal barrage||240||817||340|
|Sihwa Lake||Korea||Tidal barrage||254||298||117|
Table 2: Comparison of a ground PV with a floating PV
|Ground PV plant||Floating PV plant|
|Production of energy (GWh)||141.71||186.05|
|Annual insolation (kWh/m2/day)||5.02||6.17|
Table 3: Comparison between an onshore and offshore wind farms
|Capacity (MW)||Cost (million USD)||Energy produced (GWh)||Cost (cents/kWh)|
Table 4:Carbon dioxide emission levels for various technologies
|Technology||Carbon dioxide emission (t/GWh)|