Ocean Thermal Energy Conversion, OTEC, is an emerging technology which generates electricity from the difference in temperature between tropical surface seawater and cooler deep water. Not only is it a renewable with baseload capability – but the power plants will also be able to produce fresh water as a by-product. The deep water used in the process is rich in nutrients, and the power plants can be combined with aquaculture.
The oceans are the largest solar collector in the world. The daily amount of thermal energy they absorb corresponds to 250 billion barrels of oil – one thousand times as much as the total Swedish annual energy consumption.
There are several well-known ways to harvest energy from the sun, such as photovoltaics and concentrated solar power. Energy can be gathered from the ocean in various ways as well – by tidal plants and wave energy converters, for instance. But there is also a way of doing both: tapping the oceans for stored solar energy.
This is the concept behind an interesting renewable energy technology, one that often has been overlooked. It is called OTEC (Ocean Thermal Energy Conversion). The temperature difference between surface water heated by the Sun and the cool water at the bottom of the sea is what drives the process (similar to how a ground source heat pump accesses stored solar heat energy in the soil.)
One kilometer into the depth
An OTEC plant could look a lot like an oil rig; a floating platform, pumping water both from the surface and from a depth of one kilometer. The temperature of the latter would typically be around 5 degrees Celsius. The temperature difference would power a heat engine, generating electricity with a steam turbine. Facilities could also be land-based, but they would require longer pipes to reach the sufficient depths.
Small-scale test facilites are operational in several places, for instance in Hawaii. Though the working principles have been verified, commercial plants are yet to be completed. Lockheed Martin is currently planning a 10 MW offshore facility in China.
Closed-cycle vs. open-cycle
There are two basic types of OTEC systems: closed-cycle and open-cycle. The closed-cycle process uses a fluid with a low boiling point, such as ammonia. Warm surface seawater pumped through a heat exchanger vaporizes the fluid. The vapor powers a turbine. Cold water, pumped through a second heat exchanger, condenses the vapor again.
Open-cycle systems use warm surface water directly. The warm seawater is pumped into a low-pressure container, which causes it to boil. The steam is desalinated in the low-pressure container, and when it is condensed by cool deep-ocean water (just as in the closed-cycle system), fresh water is obtained and can be used as drinking water.
Clean energy for tropical countries
The efficiency increases with the temperature gradient, and a differential of at least 20 degrees is needed for the technology to be viable. Therefore, OTEC is primarily an alternative in tropical waters close to the equator – about one third of the ocean surface.
The technology could be of special importance to a group of developing countries known as SIDS. These Small Island Developing States are particularly exposed to the dangers of global heating, and are often heavily dependant on imported fossil fuels for their energy security. But OTEC also has the potential to supply regions with a large and growing population, such as China, India and Brazil, with the energy they need to replace fossil fuels.
The by-products: drinking water, food and cooling
OTEC plants are able to produce a number of synergistic products beside the electricity. Desalinated fresh water is one of them – and the amounts can be substantial. Actually, an OTEC plant would likely be able to supply as many people with drinking water as with electricity. This is an important benefit, since water security often is a problem in tropical regions.
Once the cool deep water has been brought to the surface, it can be used for other purposes as well before being returned to the sea. To provide cooling, for instance – or to supply fish farming or microalgae culturing with nutrients. Since the deep sea water is free from pathogens and rich in nutrients and minerals, it makes sense to combine the power plants with aquaculture facilites. If the nutrients are not used purposely, the water might instead cause an unintended eutrophication of upper layers when it is released.
Continous power in pace with demand
Unlike many other renewable energy sources, OTEC is not intermittent but has potential to provide baseload electricity, day and night and year-round. The ocean serves as a levelling power storage facility, making the Sun’s energy available regardless of current weather conditions. This is a major advantage in comparison with solar cells, for instance. OTEC is actually most efficient at the same time that the electricity demand peaks – in the summer, when more air conditioning and cooling is needed.
Of course there are hurdles to overcome, as well – such as the sheer volume of water to bring up from the depths. A larger 100 MW facility may require a pipe with a diameter of 10 meters to reach a depth of a kilometer, while withstanding the aggressive conditions in the water. And the heat exchangers need to be dimensioned accordingly.
If these difficulties can be overcome, OTEC has a very large resource potential. More studies are needed to determine just how much power could be produced without beginning to affect the heat structure and currents of the oceans – but according to simulations, at least half of the current global energy demand could be met with no harm caused.
Future large-scale facilities might look like archipelagos of floating islands, where OTEC plants are combined with aquacultures, greenhouses and additional wind, wave and solar power generators helping to power the OTEC pumps.
OTEC is still an expensive technology in its infancy. It has to be developed further before it will be able to compete with the more established renewable energy technologies. Nevertheless, the large potential, the baseload capability and the possible synergies make a strong case for it. We are likely to see more and more small scale facilities pop up in the tropics in the years to come; it will be a chance for the technology to prove its worth.
The article was published in March 2016.