Ocean Thermal Energy Conversion: The Future of Energy Production?

Ocean Thermal Energy Conversion: The Future of Energy Production?

A long-rejected form of clean energy has now been revived. Ocean Thermal Energy Conversion (OTEC) provides renewable, non-polluting power, day and night and could theoretically supply  ‘4000 times the world’s energy needs in any given year’ (Knight, H., 2014, pp.49).

Two centuries ago Jules Verne mused about the idea of OTEC in Twenty Thousand Leagues Under the Sea and ever since there have been lofty claims for its potential. So why OTEC’s 21^st century renaissance? The oceans are the world’s largest energy system and OTEC is a technology that harnesses this potential energy source. The temperature difference between cold deep-ocean water and warm surface water is exploited to generate steam energy. A mechanism pumps warm surface water past pipes that contain a liquid with a low boiling point, such as ammonia. The liquid subsequently boils and the steam powers a turbine connected to a generator in order to produce electricity. The steam is then condensed by seawater pumped from the deep-ocean so it can be reused.

 

Fig. 1. Knight, H., 2014, p.51

In the real world the concept has long been deemed as impractical and a major setback was accessing the cold deep water. Pumping the vast volume of water warrants the need for 1000-metre-long pipes that are both wide and resilient enough to withstand ocean currents in order accommodate the necessary temperature difference of at least 20^oC between the surface and deep water for the system to work. To overcome this complication, developers looked to wind-turbine manufacturing for their inspiration. Pipes can now be made from fibreglass and resin composites that are both strong and flexible enough to endure the pressures of ocean currents whilst also being cost effective. The advantage of this innovative development is that the entire network of pipes can now be ‘assembled on the ocean-surface platform of the OTEC plant itself’ (Knight, H., 2014, p.50), thus eliminating the risk of dropping a pre-assembled structure into the ocean.

Aside from the technical difficulties the other major concern is that the technology is 6-8% efficient. The most pressing question is whether it realistically compete against the giants of energy production like coal and natural gas in terms of profit made. The recent emergence of fracking has also pushed OTEC to the back of our minds and it seemed unlikely we would ever see it in actuality. However, unexpected collaborations with other renewables as well as myriad lessons from the oil and gas industry have potentially allowed OTEC to become a genuine contender in the 21^st century’s renewable energy mix. Currently, there are even projections that it could eventually become 60% efficient (Masutani, S.M, AND Takahashi, P.K., 2001. pp.1994) and recent advancements mean that now, the construction of a 100-megawatt floating plant would cost $790 million (Vega, L.A., 2010, p.11). Such a plant would produce electricity at a price of 18 US cents per kilowatt hour which is not far short of other main non-polluting competitors - ‘the US Department of Energy estimates… 14 cents for coal with carbon capture and storage, and 14 to 26 cents for solar energy’ (Knight, H., 2014, p.50).

OTEC’s resurgence is timely and was spearheaded last year by Lockheed Martin following their announcement that they were to begin construction of a 10-megawatt plant off the coast of southern China. Unstable oil prices, elevated pollution from non-renewables and the threat of climate change make this alternative form of clean energy truly an attractive option. Indeed, just this month a UN-backed expert panel declared that, ‘if the world is to avoid dangerous climate change’, the ‘unrestricted use of fossil fuels should be phased out by 2100’ (McGrath, M., 2014).

The need for OTEC is furthered as a result of the limitations of intermittent renewables such as wind and solar energy. Their electricity production is unreliable and climate change is proving to be a serious issue: ‘higher temperatures could reduce the amount of fresh water for both hydropower generation and concentrated solar power plants’ (Knight, H., 2014, p.51). Conversely, not only can OTEC produce electricity day and night but global warming might even increase the technology’s global output by expanding the “equatorial waistband” for productive OTEC plants by warming surface water found further north or south of the equator.

The technology is most effective in equatorial areas due to a constant temperature differential of at least 20^oC. Whilst 98 nations or territories could presently accommodate OTEC, in reality only Kume Island in Japan (50-kilowatt plant) and South Korea (20-kilowatt plant) have working plants. Having said this, Hawaii has a 100-kilowatt plant in development whilst India, Bahamas and Curacao have plans or proposals for OTEC (Knight, H., 2014, p. 50). Likewise, China has planned the creation of a 10-megawatt plant which is certainly a welcome development. In 2009 China surpassed the USA as the world’s biggest energy consumer, in turn leading to rising carbon dioxide emissions which pose a serious threat to the environment.

OTEC also offers the possibility of generating other products: ‘fresh water, nutrients for enhanced fish farming and seawater cooled greenhouses enabling food production’ (OTEC foundation, 2000-2003). This would be a godsend for poorer nations suffering food shortages and drought, hence Tanzania’s proposition for an OTEC plant - a 50-megawatt plant could support a developing community of approximately 300, 000 (Vega, L.A., 2010, p. 15). For developing nations, a more tantalising prospect is using the cold water for refrigeration systems as well as in air conditioning units.

Figure 2. Knight, H., 2014, p.50

So, in truth can OTEC truly become the future of energy production without any impact on the environment? OTEC plants could purportedly extract ‘7 million megawatts before they would have any noticeable effect on ocean temperatures’ which is equivalent to ‘nearly 50 percent of global energy consumption’ (Knight, H., 2014, p.51). Indeed there are concerns that environmental damage could be caused by leakage - ammonia is toxic in moderate concentrations and there are fears that water packed with nutrients being introduced to algae in warmer waters could cause algal blooms. However, this could be avoided if the cold water is returned to the ocean at depths lower than 60 metres. OTEC could in fact yield environmental benefits: organisms sustained by nutrient-rich, deep ocean water could enhance uptake of atmospheric carbon dioxide whilst ‘energy extraction and the artificial upwelling of deep water’ (Masutani, S.M, AND Takahashi, P.K., 2001. p.1994) could preserve diminishing coral populations by limiting temperature rise at the surfaces of oceans.

The future seems bright for OTEC.  Combining OTEC with other power sources appears the most promising idea. A study has investigated using solar power to increase the temperature of surface water before it is used to boil the ammonia and has found it could triple a plant’s daytime electricity output (Bombarda, P., AND Invernizzi, C., AND Gaia, M., 2012, p. 42302). Similar techniques are being developed at the Korean Ocean Research and Development Institute. South Korea’s 20-kilowatt plant can only operate during summer when the temperature differential exceeds 20^oC so in order to make it work year-round there are plans to harness heat from solar power and waste incineration to pre-heat the surface water.

More promising is the concept of coupling OTEC with another 24 hour power source - suggestions have been made for a ‘GeOTEC’ plant that would use geothermal energy to heat the seawater used to boil ammonia. Nonetheless, the greatest prospect lies in OTEC ships which could search for spots with best temperature ratios in order to make the technology even more cost effective. The electricity generated could be used onsite to split seawater into hydrogen and oxygen – the hydrogen being stored in fuel cells before being transported for use around the world. A 100-megawatt ship could produce over 1 tonne of liquid nitrogen per hour and whilst this would currently be expensive, the hydrogen economy is just emerging so the cost will surely decrease (Knight, H., 2014. p.51).

To conclude it is clear that Ocean Thermal Energy Conversion could provide significant global benefits and merits serious consideration from governments and businesses as a viable, future solution to energy production.

Bibliography

Knight, H.K., 2014. Sea Change. New Scientist, 2958, pp.49-51

OTEC Foundation, 2013. What is OTEC [online]. Available from: http://www.otecnews.org/what-is-otec/ [Accessed 2 November 2014]

Masutani, S.M, AND Takahashi, P.K., 2001. Ocean Thermal Energy Conversion. pp.1994-1999. Available from: http://curry.eas.gatech.edu/Courses/6140/ency/Chapter2/Ency_Oceans/OTEC.pdf. [Accessed 3 November 2014]

McGrath, M., 2014. Fossil fuels should be phased out by 2100 says IPCC. Available from: http://www.bbc.co.uk/news/science-environment-29855884. [Accessed 1 November 2014].

Vega, L.A., 2010. Economics of Ocean Thermal Energy Conversion (OTEC): An Update, pp.1-18. Available from: http://hinmrec.hnei.hawaii.edu/wp-content/uploads/2010/01/OTEC-Economics-2010.pdf. [Accessed 4 November 2014]).

Watts, J, 2010. China overtakes US as world's biggest energy consumer. Guardian [online], Tuesday 3 August. Available from: http://www.theguardian.com/environment/2010/aug/03/china-overtakes-us-energy-consumer

Bombarda, P, Invernizzi, C and Gaia, M, 2012. Performance Analysis of OTEC Plants With Multilevel Organic Rankine Cycle and Solar Hybridization. Journal of Engineering for Gas Turbines and Power [online], vol. 135, p.42303. Available from: http://gasturbinespower.asmedigitalcollection.asme.org/article.aspx?articleid=1671877. [Accessed 2 November 2014]