In the tropics, the deep sea is cold and the sea surface is very hot. This temperature difference can be utilized and converted into electricity. If we can improve the technology, this method of power generation could be a gift for island nations dependent on expensive and polluting diesel for their power.
For over a century, researchers have been exploring the idea of ocean thermal energy conversion. There is nothing fundamentally new about the idea of extracting power from temperature differences. In fact, the underlying technology is similar to the way coal, gas and geothermal power plants create electricity, using steam to spin a turbine.
The challenge is to find the right place where the temperature differences make it worthwhile. That means relatively close to the equator – think north of Papua New Guinea, the Philippines and off the coast of southern Japan.
Currently, pilot plants are only capable of generating a fraction of what a large wind turbine can. But on the plus side, ocean thermal plants can generate power 24 hours a day.
How does it work?
These power plants operate by running liquids with low boiling points, such as ammonia, through a closed loop. The heat of hot sea water (between 20 and 30 ℃) heats the liquid until it becomes steam and can be used to spin a turbine. Then, the steam is exposed to cold seawater (about 5 ℃), which changes it back into liquid so the cycle can continue. To get this cold water, these plants have tubes extending down 600 meters into the deep sea.
The advantages of the system are clear: it is a closed loop, heated and cooled by heat exchangers without the flow of fluid into the ocean. And it is always available, in contrast to the well-known intermittent challenges of better-developed renewable technologies such as solar and wind.
The downside is currently, the technology is not ready for prime time. A pilot plant in Hawaii installed by Makai Ocean Engineering in 2015 has a capacity of 100 kilowatts. This is 20–30 times less than a typical wind turbine in operation, or the equivalent of about 12 solar panels in homes or small businesses in Australia.
The main technical challenge to overcome is to gain access to the large volumes of cold seawater needed. The Makai pilot uses a one-meter-diameter tube that plunges 670 meters into the ocean floor.
To climb to a more useful 100 megawatt plant, Makai estimates that the tube should be ten meters in diameter and go as deep as one kilometer. This type of infrastructure is expensive, and needs to be built to withstand corrosion and cyclones.
If the plants are built overseas, the cost of transmission lines adds to total cost. Makai estimates 12 commercial offshore plants could cover Hawaii’s total electricity needs.
If OTEC plants can be built large enough, the cost will go down. But there is also another challenge. To get close to the cost of wind and solar – now even 1-2 cents per kilowatt hour – ocean thermal plants would need around. four Niagara Falls value of water flowing through the system at any time.
Why is such a huge amount of water needed? In short, a thermodynamic bottle. The physics of any energy conversion means that it is impossible to convert all the heat energy into mechanical work like spinning the turbine. This efficiency problem is a real challenge for ocean thermal plants, where the energy conversion process has a relatively small temperature difference between hot and cool seawater. In turn, this means that only a very small percentage of the heat energy in the seawater is converted to electricity.
Could OTEC find use despite the cost and technical challenges?
While these plants could not compete with wind and sun in large continental markets, they could play a role for the small island states bordering the Pacific and Caribbean, as well as islands off the main grid, such as Norfolk Island or many of the. smaller Indonesian islands.
Island nations, in particular, tend to have high retail electricity prices, low electricity demand and dependence on imported diesel for electricity generation. Researchers from Korea and New Zealand have made the case that OTEC could be a viable source of base load power for island states – but only after more pilot plants are built to help perfect the design of larger plants.
If I were tasked with helping an island state produce its own clean energy, I would first look at geothermal, a more mature technology with a better economy. This is because the areas most favorable for OTEC plants typically have significant potential for geothermal electricity, produced by drilling wells on the ground and using high-temperature fluids from those wells.
However, OTEC could play a useful role in addressing several challenges at once. Take cooling. You can take the cool seawater and use it as a form of air conditioner, like two resorts in French Polynesia does. You can also use this cold water in aquaculture to grow cold-water fish such as salmon, or as a way to keep surface water cool during warm seas. threatening fish farming in New Zealand. It may even be possible to use OTEC plants for produce hydrogen as an export commodity in small island states.
To meet our urgent emission reduction targets, it is worth exploring all renewable energies.
We should not remove OTEC yet. At this stage, however, it is difficult to see how ocean thermal plants can become competitive with better established renewables, such as wind, solar and even geothermal, given the vast volumes of cold seawater needed. File this under “has potential, but needs more work”.