Meeting Global Energy Demands with the Power of the Oceans
Global energy consumption has increased year-on-year — with the exception of 2009 — since detailed records have begun, with 2018 seeing a 2.9 % increase from the previous year, to almost 14,000 million tonnes oil equivalent (BP 2019, p. 10). Million tonnes oil equivalent, abbreviated to ‘Mtoe’ is used invariably throughout this essay, and is an industry measure to try and standardise immense energy consumptions into smaller, more versatile values. With obvious concerns pertaining to the finite nature and environmental impacts of nonrenewable ‘fossil’ fuels there is an evident requirement to proliferate current renewable technologies, and explore methods of improving both their efficiency and capacity.
Ocean derived energy (and hydroelectric systems in general) are a massively unexploited resource for electricity generation. Hydroelectric systems have never accounted annually for more than 8 % of the global energy supply, or more than 3 % of the United States energy consumption (eia 2019). Moreover, the definition of hydroelectricity includes dams and other river-based turbine systems which comprise a majority compared to electricity obtained from the oceans. The European Commission estimated the total feasible amount of energy capable of being sustainably generated from the oceans to be 119.217 TWh/year, or 10250.817 Mtoe (SETIS 2013, p. 1). To put that in perspective, ocean power has the potential to deliver over 75 % of the global energy supply alone. Ocean-derived hydroelectric energy also has the distinct advantage over other non-renewables (such as solar or wind) of being extremely predictable with minimal deviation from predicted peak power output times.
There are four generally accepted methods of practically extracting power from the oceans. The first of these, wave energy, is heralded by Ocean Energy Europe as ‘The world’s largest untapped source of energy’ (OEE 2019). There are several mechanisms for extracting energy from waves, including overtopping, point absorbers, and water columns. In general, a wave transfers energy, but the average particle displacement of the water that propagates the energy remains stationary. This results in a circular or ellipsoidal motion of a water molecule as it transfers said energy. Point absorbers are floating buoys attached to the sea floor, and use the repeated vertical displacement of the water’s motion to produce electricity via a linear generator. Due to the minimal profile of a point absorber, there is little danger to marine life, and floating buoys can even provide a roosting site for aquatic birds.
Water columns contain an air chamber, which is compressed upon the incidence of a wave. The hydraulic force extracts work from the ocean using a turbine, which is directly converted into electricity. There are concerns regarding the obstructive profile of these chambers, and the potential for wildlife getting trapped or entangled in the internals of the device. An inherent inefficiency of the design is the conversion of kinetic energy into unwanted vibration; consequently, noise pollution is a factor that should be considered. Alternatively, gravitational potential energy can be used as a temporary storage medium to harness the peak power of a wave. An overtopping device takes the hydraulic pressure of a wave and uses it to force water up a narrow column, so it gains potential energy. This is then slowly converted into electricity through a turbine. Estimates for the annual potential energy extraction from waves vary from 343 Mtoe (Cruz, J. 2008, p. 141) up to 2536 Mtoe (Lewis, A. et al 2011, p. 504).
The concept of tidal extraction is markedly simple, and comparable to wind power in theory. However, in application, the intricacies of using water as a propagating fluid rather than air raise several Engineering challenges regarding placement and structural design, due to the far greater forces imposed.
The theory is simple; the attraction of celestial bodies such as the sun and moon exerts a force on the oceans, producing areas of high and low tide. As the Earth rotates, a mass net flow of water results, the motion of which can be harnessed by means of strategically placed turbines. Turbine technology has remained largely unchanged for the last decade, using either stream generators or barrages.
Dynamic Tidal Power (DTP) however is a promising area with many novel developments, using the same principle as a barrage but at a much greater scale. A ‘T’ shaped megastructure many kilometers long is constructed perpendicular to the coastline. For coast where the tide movement is perpendicular, the result is two segregated masses of water at a height differential, causing a massive disparity in potential energy between the two. The water masses can then be allowed to equalise by movement through turbines, producing large quantities of electricity. The specific geometries and dimensions of the DTP structure can be tailored so that each cycle coincides exactly with the movement of the tides. By using bidirectional turbines, there is potential to take advantage of both tidal movements through the ‘pseudo-dam’, making the process more efficient. Despite the design being patented in 1997 (Hulsbergen, K., Steijn, R. 1997) and all required technology in commission, no fully functioning project has yet been built. This is partially due to the anticipated disruption to coast-centric aquatic life (which would no doubt be widespread) but primarily due to the fact that small-scale prototypes of DTP are ineffective (Shao, D. et al 2017). This is because the power generated is a factor of both the head and volume of the ‘dam’ structure; hence power increases by a factor of x2, where ‘x’ is the length of the structure. Consequently, such a project is only feasible at lengths of over 30 km, and proof of successful operation can only be demonstrated using a full-scale prototype (Dai, P. et al 2018, pp. 220–228).
There are essentially three forms of harnessing power from the oceans; kinetically or by temporarily storing kinetic energy using a gravitational potential (which was discussed previously), chemically (by taking advantage of a salinity gradient) or thermally (which is discussed in this section). Ocean Thermal Energy Conversion (OTEC) uses the temperature differential between surface level water, and deeper, cooler water. Warm water can be pumped into a low pressure chamber, which causes it to boil and drive a turbine. Alternatively, in a closed system the warmer seawater is used to evaporate a fluid such as Ammonia, which drives the generator. It was previously predicted that just 7.5 Mtoe could be extracted from the oceans in this manner without noticeably altering the oceans thermal characteristics, but advancements have inflated this value to over 7100 Mtoe annually (Pelc, R., Fujita, R. M. 2002, pp. 471–479).
Using either osmosis or reverse electrodialysis between a mass of salinated (sea) water and freshwater, particles can be encouraged to cross a semipermeable membrane, and either directly produce an electric current or produce a potential difference by the osmotic force of particles driving a turbine. This isn’t new technology, but has only been proved relatively recently to be capable of achieving efficiencies that merit salinity gradients being used to produce energy on the macro scale (Yip, N. Y., Elimelech, M. 2012).
Out of all the advancements discussed in this essay, it seems unlikely that any single technology will provide a solution to the global shift into renewable energy sources. OTEC is only viable where there is a large oceanic temperature gradient, so is confined to tropical regions with high surface water temperatures like the East Indies and the Pacific ocean off the coasts of Indonesia, Papua New Guinea and the Philippines (Aldale, A. M. 2017, pp. 164–167). DTP is only suited to areas where the tides flow perpendicular to the mainland, and osmotic power requires a plentiful supply of fresh, desalinated water in order to function. It seems far more likely that individual technologies will be installed bespokely to suit local geography and aquatic conditions, in order to effectively take advantage of the massive untapped resource that is the ocean.
Reference List
- (2019), BP Statistical Review of World Energy, 68th edition. British Petroleum, p. 10 [11th October 2019]
- (2019), U.S. energy facts explained. U.S. Energy Information Administration (eia) [11th October 2019]
- (2013), Ocean Energy: Technology Information Sheet. European Commission, Strategic Energy Technologies Information System, p. 1 [11th October 2019]
- (2019), Wave Energy. Ocean Energy Europe, OEE [11th October 2019]
- Cruz, J. (2008), Ocean Wave Energy: Current Status and Future Perspectives, Chapter 5, p. 141, Springer [11th October 2019]
- Lewis, A. et al (2011), Ocean Energy, In IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation, Chapter 6, p. 504, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA [11th October 2019]
- Hulsbergen, K., Steijn, R., (1997), Tidal Current Energy Converter, Espacenet, Bibliographic data: WO9801670 (A1) ― 1998–01–15 [11th October 2019]
- Shao, D. et al (2017) IOP Conf. Ser.: Earth Environ. Sci. 63 012048 [11th October 2019]
- Dai, P. et al (2018), Numerical study of hydrodynamic mechanism of dynamic tidal power, Water Science and Engineering, Volume 11, Issue 3, Pages 220–228 [11th October 2019]
- Pelc. R., Fujita, R. M. (2002), Renewable energy from the ocean, Marine Policy 26 (2002) 471–479, Pergamon, Environmental Defense, 5655 College Avenue Suite 304, Oakland, CA 94618 USA [11th October 2019]
- Yip, N. Y., Elimelech, M. (2012), Thermodynamic and Energy Efficiency Analysis of Power Generation from Natural Salinity Gradients by Pressure Retarded Osmosis, Environ. Sci. Technol.20124695230–5239 [11th October 2019]
- Aldale, A. M. (2017), Ocean Thermal Energy Conversion (OTEC), American Journal of Engineering Research (AJER) e-ISSN: 2320–0847 p-ISSN : 2320–0936 Volume-6, Issue-4, pp-164–167 [11th October 2019]
