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 21st 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 20oC
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 21st
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 20oC. 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 20oC 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
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