The world's oceans constitute a vast natural reservoir for receiving and storing solar energy. They take in solar energy in proportion to their surface area, nearly three times that of land. As the sun warms the oceans, it creates a significant temperature difference between the surface water and the deeper water to which sunlight doesn't penetrate. Any time there's a temperature difference, there's the potential to run a heat engine, a device that converts thermal energy into mechanical energy.
Most of the electricity we use comes from heat engines of one kind or another. The working principle of such an engine is very simple. It operates between two reservoirs of thermal energy, one hot and one cold. Energy is extracted from the hot reservoir to heat a working fluid which boils to form high-pressure vapour that drives a turbine coupled to an electricity-producing generator. Contact with the cold reservoir re-condenses the working fluid which is pumped back into the evaporator to complete the cycle.
The idea of building an engine to harness energy from the oceans, mainly to generate electricity, by exploiting the thermal gradient between waters on the surface and deeper layers of an ocean is known as OTEC—acronym for Ocean Thermal Energy Conversion. With OTEC, the hot reservoir is an ocean's warmer surface water with temperatures, which can exceed 25 degrees Celsius, and the cold reservoir is the cooler water, around five to six degrees, at a depth of up to one kilometre. The working fluid is usually ammonia, which vaporises and condenses at the available temperatures. This is analogous to choosing water as the working fluid matched to the temperature differential between a fossil-fuel-fired boiler and a condenser cooled by air or water.
The maximum efficiency of a heat engine operating between reservoirs at 25 and 5 degrees Celsius is 6.7 percent. This means efficiency of an actual OTEC engine will be much less, perhaps 2-3 percent. But low efficiency isn't the liability it would be in a fossil-fuelled or nuclear power plant. After all, the fuel for OTEC is unlimited and free, as long as the sun heats the oceans.
The greater is the temperature difference, more efficient an OTEC power plant would be. For example, a surface temperature of 30 degrees would raise the ceiling on efficiency to 8.25 percent. That's why the technology is viable primarily in tropical regions where the year-round temperature differential between the ocean's deep cold and warm surface waters is greater than 20 degrees. The waters of Bay of Bengal along the shores of Bangladesh, a country that enjoys a year round warm, and at times very hot weather, have excellent thermal gradients for producing electricity using OTEC technology.
The world's biggest operational OTEC facility, with an annual power generation capacity of 100 kW, was built by Makai Ocean Engineering in Hawaii. Tokyo Electric Power Company and Toshiba built a 100 kW plant on the island of Nauru, although as much as 70 percent of the electricity generated is used to operate the plant.
The US aerospace company Lockheed Martin is building an OTEC electricity generating plant off the coast of Hainan Island in China. Once operational, the plant will be able to generate up to at least 10 MW of power, enough to sustain the energy requirements of a smaller metropolis. India is building a 200 kW plant, expected to be operational before 2020, in Kavaratti, capital of the Lakshadweep archipelago, to power a desalination plant. Other OTEC systems are either in planning or development stage in Iran, Kuwait, Saudi Arabia, Thailand and several countries along the Indian Ocean, mostly to supply electricity.
Like any alternative form of energy, OTEC has its advantages and disadvantages, but the advantages outweigh the disadvantages. Among the advantages, the one that stands out is its ability to provide a base load supply of energy for an electrical power generation system without interruption, 24/7/365. It also has the potential to produce energy that are several times greater than other ocean energy options, such as waves and tides. More importantly, OTEC is an extremely clean and sustainable technology because it won't have to burn climate-changing fossil fuels to create a temperature difference between the reservoirs. A natural temperature gradient already exists in the oceans. The gradient is very steady in time, persisting over day and night and from season to season. Furthermore, the desalination technology as a by-product of the OTEC can produce a large amount of fresh water from seawater which will benefit many island nations and desert countries.
However, re-distribution of large volumes of water by OTEC power plants could have negative impacts on the aquatic environment. In particular, the introduction of nutrient-rich deep waters into the nutrient-poor surface waters would stimulate plankton blooms that could adversely affect the local ecological balance. Additional ecological problems include destruction of marine habitats and aquatic nursery areas, redistribution of oceanic constituents, loss of planktons and decrease of fish population.
Since OTEC facilities must be located closer to the shores due to cabling constraints, they could have significant effect on near-shore circulation patterns of ocean water. As a result, open ocean organisms close to the shores will be especially affected because they are known to have very narrow tolerance limits to changes in the properties of their environment.
The biggest drawback of OTEC is its low efficiency. This implies that to produce even modest amounts of electricity, OTEC plants have to be constructed on a relatively large scale, which makes them expensive investments. It's the price we should be prepared to pay to curb global warming. Industry analysts however believe that in the long run, low operation and maintenance cost would offset the high cost of building OTEC facilities.
The current effort, as agreed in the 2015 Paris Accord, to keep our planet lovable is like taking one giant step backward before trying to move one step forward. If technology for OTEC and other eco-friendly renewable sources of energy are fully developed and globally commercialised, it would indeed be one giant step forward in mitigating global warming. They would also equip communities worldwide with the self-empowerment tools that are required to build an independent and sustainable future.
Quamrul Haider is a Professor of Physics at Fordham University, New York.