Electricity from the cold depths of the ocean could someday power island nations

In the tropics, the deep sea is cold and the sea surface is very warm. This difference in temperature can be harnessed and converted into electricity. If we can improve the technology, this method of energy production could be a godsend for island nations that depend on expensive and polluting diesel for their power.

For more than a century, researchers have explored the idea of ​​ocean thermal energy conversion. There is nothing fundamentally new in the idea of ​​extracting energy from temperature differences. In fact, the underlying technology is similar to the way coal, gas and geothermal power plants generate electricity, using steam to spin turbines.

The challenge is finding the right spot, which is where the temperature differences make it worthwhile. That means it’s relatively close to the equator – think north of Papua New Guinea, the Philippines, and off the coast of southern Japan.

At present, pilot plants can only generate a fraction of what large wind turbines are capable of. But on the plus side, ocean thermal plants can generate power 24 hours a day.

Deep seas overlooking the surface

The depths of the sea are much colder than their surface.
stock struggle

How it works?

These power plants work by running liquids with low boiling points, such as ammonia, through a closed loop. The heat from warm sea water (between 20 and 30 °C) heats the liquid until it turns into steam and can be used to spin turbines. Next, the steam is exposed to cold sea water (about 5 ℃), which turns it back into a liquid for the cycle to continue. To get this cold water, these stations have tubes that extend 600 meters deep into the deep sea.

The benefits of the system are obvious: it is a closed loop, which is heated and cooled by heat exchangers without draining fluid into the ocean. It is available at all times, unlike the well-known intermittent challenges of cutting-edge renewable energy technologies such as solar and wind power.

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The downside at the moment is that the technology is not ready for prime time. a pilot plant In Hawaii it was installed by Makai Ocean Engineering in 2015 with a capacity of 100 kW. This is 20 to 30 times less than a typical wind turbine when operating, or the equivalent of about 12 solar arrays in homes or small businesses in Australia.

The main technical challenge to overcome is access to the large volumes of cold seawater that is needed. Mackay’s pilot uses a one-meter-diameter tube that plunges 670 meters into the depths of the ocean.

To expand to a more useful 100-megawatt plant, Mackay estimates that the tube would have to be ten meters in diameter and one kilometer deep. This type of infrastructure is expensive, and must be built to withstand erosion and hurricanes.

If the stations are built overseas, the cost of transmission lines add to the overall expense. Mackay estimates that 12 marine stations on a commercial scale could cover Hawaii’s total electricity needs.

If large enough OTEC plants can be built, the cost will come down. But there is another challenge, too. To get close to the cost of wind and solar power – now as low as 1-2 cents per kWh – ocean thermal plants would need about Four Niagara Falls The value of the water flow through the system at any time.

Why is this huge volume of water required? In short, a thermodynamic bottleneck. The physics of any energy conversion means that it is impossible to convert all thermal energy into mechanical work such as spinning a turbine. This efficiency issue presents a real challenge for ocean thermal plants, as the energy conversion process is characterized by a relatively small temperature difference between warm and cold seawater. This, in turn, means that a very small percentage of the heat energy in seawater is converted into electricity.

Ocean Thermal Power Plant

The Mackay Ocean Thermal Conversion Experimental Plant in Hawaii can produce 100 kilowatts of power.
Wikimedia CommonsAnd the CC BY

Can OTEC find benefit despite cost and technical challenges?

While these plants cannot compete with wind and solar power in the large mainland markets, they can have a role for small island states scattered in the Pacific and Caribbean, as well as islands far from the main grid, such as Norfolk Island or several islands . The smaller Indonesian islands.

Island nations, in particular, tend to have high retail electricity prices, low electricity demand, and dependence on imported diesel to generate electricity. Researchers from Korea and New Zealand have case making that OTEC could be a viable source of base load power for island nations – but only after more pilot stations are built to help perfect the design of larger stations.

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If I am tasked with helping an island nation produce its own clean energy, I will first look at geothermal energy, a more mature technology with better economics. This is because the areas most suitable for OTEC plants typically have significant potential for geothermal electricity, which is produced by drilling wells on the ground and using high-temperature fluids from those wells.

However, OTEC can play a useful role in addressing several challenges simultaneously. Take cooling. You can take cold sea water and use it as a form of air conditioning, such as resorts In French Polynesia they do. You can also use this cold water aquaculture to grow cold water fish like salmon, or as a way to keep surface waters cool during marine heat waves. Threatening fish farming in New Zealand. It may even be possible to use OTEC stations to hydrogen production As an export commodity in small island states.

To achieve our urgent emissions reduction goals, it is worth exploring all renewable energy options.

We shouldn’t write off OTEC just yet. However, at this point, it’s hard to see how ocean thermal plants can become competitive with more established renewables, such as wind, solar, and even geothermal energy, given the vast amounts of cold seawater required. Put this under “He has potential, but needs more work.”