Potential energy comes from the cold ocean, promising to one day provide endless electricity for island nations.

In the tropics, the sea surface is very warm and the temperature will decrease with depth. That temperature difference can be harnessed and turned into electricity. If we can improve the technology, this method of generating electricity could be a godsend for island nations that depend on expensive and polluting diesel-powered electricity.

For more than a century, scientists have studied the thermal conversion of the ocean. Basically, the idea of ​​​​harvesting electricity from temperature differences has been around for a long time. The technology is similar to how coal, gas and geothermal power plants generate electricity, by using steam to power turbines.

The challenge was to find the right location, where the more the temperature difference, the more worthwhile it becomes. Seas relatively close to the equator may satisfy this condition, such as northern Papua New Guinea, the Philippines and off the southern coast of Japan.

Currently, pilot plants can only generate a modest fraction of the energy compared to a large wind turbine. But if you look at it on the bright side, ocean thermal power plants (OTECs) can generate electricity 24/7.

These power plants operate on a liquid with a low boiling point, such as ammonia, through a closed loop. Heat from warm seawater (20 to 30℃) heats the liquid until it turns into steam and is used to drive turbines. The vapor is then exposed to cold seawater (about 5℃), turning it back to liquid so the cycle can continue. Factories need a pipeline extending 600 meters into the deep sea to exploit this cold water.

The system is a closed loop, heated and cooled by heat exchangers without discharging liquid into the sea. And this resource is readily available, which can eliminate the disruption challenges of longer-developed renewable technologies like solar and wind.

This technology also has the disadvantage that it is not yet ready for widespread adoption. A pilot plant in Hawaii installed by Makai Ocean Engineering in 2015 has a capacity of 100 kW. That’s 20-30 times less than a typical wind turbine and equates to around 12 solar panels installed in Australian homes or small businesses.

The main engineering challenge to overcome was how to get access to the large volumes of cold seawater needed. Makai engineers currently use a one meter diameter pipeline, which plunges to a depth of 670 meters on the ocean floor.

To scale up the plant to 100 MW, Makai estimates the pipeline would have to be 10 meters in diameter and go down to 1 km deep. This type of infrastructure is expensive and must be designed to resist corrosion and cyclones. Makai estimates 12 commercial-scale offshore plants that could meet Hawaii’s total electricity needs.

If oceanic heat conversion plants can be built large enough, costs will be reduced. But there is also another challenge. To get close to the cost of wind and solar, currently 1–2 cents/kWh, ocean thermal power plants would need about four times the amount of water flowing through the same system from Niagara Falls flowing through the same system. at the time.

Such a huge volume of water is needed because thermodynamics cannot convert all the heat into mechanical energy like spinning a turbine. This efficiency issue is a real challenge for marine thermal power plants, where warm and cool seawater has a relatively small temperature difference, meaning that only a very small percentage of the heat energy in seawater is absorbed. converted into electricity.

While these plants cannot compete with wind and solar power in major mainland markets, they can be used in small island nations scattered across the Pacific and the Caribbean. There are also islands far from the main grid, such as Norfolk Island or the many islets of Indonesia.

In particular, island nations often have high retail electricity prices, low electricity demand and dependence on imported diesel for power generation. Researchers from Korea and New Zealand have suggested that OTEC could be a viable primary energy source for the island nations, but only after more pilot plants are built to help complete design of larger factories.

If you want to help an island nation produce clean energy, you must first consider geothermal, a more mature and economic technology. That’s because the regions most favorable for OTEC plants often have significant potential for geothermal electricity, which is produced by drilling wells on land and using high-temperature fluids from those wells.

However, OTEC can play a useful role in addressing several challenges at once. People can take cool seawater and use it as a form of air conditioning, which is being used at two resorts in French Polynesia.

The water could also be used in aquaculture, farming cold-water fish such as salmon, or as a way to keep surface water cool during sea heat waves that threaten fish farming in New Zealand. It is even possible to use OTEC plants to produce hydrogen as an export commodity in the small island nations.

To meet urgent emissions reduction targets, we need to take advantage of all renewable energy options. However, at this stage, OTEC is unlikely to compete with better renewables such as wind, solar and even geothermal. This area is marked as “has potential, but needs more work”.