Ocean Thermal Energy Conversion: A renewable source of base load power?
Posted by Alex Barrett on 29 August 2017 at 11:10 am
Ocean Thermal Energy Conversion (OTEC) is a renewable energy technology which generates electricity using sea water. 71% of the world’s surface is covered by ocean . This means that a large amount of solar energy is constantly warming the upper layers of the oceans. This creates a strong temperature gradient between the surface of the ocean, and the colder, deep waters, which rarely mix with the upper layers. This temperature gradient is most pronounced in the tropics, which is where this technology is most effective.
OTEC uses this temperature contrast to run a heat engine, generating electricity in a clean and sustainable manner [2, 3]. This technology has been in development for some time, and is now being implemented in tropical seas around the world. The large size of the oceans and the vast amount of thermal energy they store means that OTEC taps into a very large resource, and one which is constantly being replenished by sunlight.
A source of baseload power
This is also a renewable energy source that works all the time, unlike solar and wind power, where output can be dependent on the weather or the time of day. Although the energy that OTEC systems harvest ultimately comes from the sun it is stored as heat in surface waters, meaning that this sort of power station can be run all through the day and night. This means that it is a reliable way of producing “baseload” electricity through renewable means.
Base load electricity is the backbone of our energy system, the amount of energy that must constantly be generated in order to keep the world running. This is usually provided by fossil fuels or nuclear fission, since renewables tend to be less reliable. Only hydroelectric stations can provide baseload power since most other renewable sources are reliant on external conditions.
How does it work?
Ocean Thermal Energy Conversion works by extracting warm water from the surface of the ocean, and colder water from depth. The warm water is fed into a heat exchanger, where it is used to evaporate a “working fluid”, usually ammonia. The ammonia has a low boiling point, so the heat of the seawater is sufficient to turn it into a vapour. This vapour then passes through a turbine to generate electricity. The ammonia vapour is then fed into a condenser which cools it down using the cold water from the deep sea. This returns it to a liquid state so that the process can be repeated. Consequently the greater the temperature difference between the hot and cold water the more efficient the system.
It has been estimated that each square metre of ocean surface receives 175 watts of solar energy, meaning that over 90 petawatts of energy are stored in the sea (a petawatt, PW, is 1015 Watts, 1 followed by 15 zeroes) . Various studies have attempted to work out how much of this energy we could extract without disrupting the ocean’s currents and the temperature gradient on which this process relies. It seems likely that between 3-5 Terawatts (a terawatt, TW, is 1012 watts) could be extracted without jeopardising the system, or harming the environment. Some estimates, using more sophisticated models suggest that this could be as high as 30 TW .
This technology is particularly interesting for small island communities, many of which are located in tropical environments. These communities are isolated from larger power grids, and so cannot balance intermittent sources of power, by transmitting excess electricity from a productive region to one where less power is being generated, but more is needed. OTEC could allow these areas to become self-sufficient using renewables. Fresh water is also produced as a by-product of the OTEC process, so it can provide electricity and act as a desalinisation plant simultaneously.
1. Hawaii Pacific University Ocean Institute http://www.oceanicinstitute.org/aboutoceans/aquafacts.html
2. OTEC News http://www.otecnews.org/what-is-otec/
5. OTEC News: How much ocean thermal energy can be converted to electricity? http://www.otecnews.org/2012/10/how-much-ocean-thermal-energy-can-be-converted-to-electricity/
Image credit: arttmls
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