How to use the ocean as a giant battery!

There is a way to turn the ocean into an unlimited power source. All you need is a really warm and cold water. Now, you can produce renewable energy around the clock. And the basic principle has been around for more than a hundred years. Can this really satisfy our thirst for more renewable energy? This seemingly miraculous technology is called Ocean Thermal Energy Conversion. Short: OTEC "And thermal means: it takes advantage of the temperature difference between warm and cold water." When you look at the surface of the ocean, the water here can be quite warm because it's heated up by the sun.

In tropical waters, that can be around 26°C. "But the deeper you go – and everyone who swims in a lake or dives in an ocean experience that – the colder the water gets." At 1000 meters deep, the temperature reaches roughly 4°C. And this difference in temperature is what Ocean Thermal Energy Conversion utilizes. "So how do you generate energy from that?" It's quite simple. You need a heat exchanger. The warm surface water heats a fluid that has a low boiling point. Usually that's ammonium. That fluid evaporates, creating a steam – and that steam runs a turbine, generating electricity – similar to a regular steam engine. Then the steam gets cooled by the deep-sea water back into a liquid and the cycle repeats. "Well, the seawater itself doesn't boil.

This process can run for 24 hours, 7 days a week. Unlike other renewables such as wind or solar. "And when you have a greater temperature difference, with hotter surface water like this one here... the process can run even quicker and create more energy." The groundwork for this technology was laid in 1881 by French physicist Jacques-Arsène arsenal. His student, Georges Claude, then actually built the first but unsuccessful OTEC plant in Cuba in 1930. Later, interest in OTEC peaked when oil prices exploded during the oil crisis. In 1980, US president Jimmy Carter signed a law to ensure the production of 10,000 MW of electricity from OTEC in the next two decades. The problem here is that you need to get that cold deep-sea water up to the surface or onshore where the heat exchanger is. And that is done with pipes that are more than two kilometers long.

Currently, there are two onshore research plants. One on the island of Kume in Japan with 100 kW and the other one in Hawaii, USA, with 105 kW. French developers were set to launch a 16 MW plant in 2020 in Martinique, but the project has reportedly been shelved due to technical difficulties. There were other research plants around, but not for very long. "This also means, that there is little to no continuous energy production from OTEC until now. And just for comparison, the 100 kW pilot plants that we talked about earlier, one offshore wind turbine is a hundred times bigger when it comes to capacity." So, despite not being a new idea, this technology is still very much in its infancy.

The pilot plants are mostly set up onshore. "To make OTEC commercially viable at a large scale, you need to go offshore." said Hermann Kugeler from Makai Ocean Engineering. The company has been developing OTEC parts in Hawaii since 1979. "You know the size and number of pipes that you need for your cold water and even for your return, your discharge pipeline as well. The amount of trenching and shoreline crossing for a commercial scale plant would just be infeasible and cost-prohibitive as well." Going offshore makes it possible to install multiple OTEC platforms next to each other, similar to offshore wind parks. "We do see a need and opportunity for certain locations that make sense to have an onshore OTEC facility."

Around the world there's many different uses for deep-sea water. They use it for desalination, for seawater air conditioning, cooling, for fishery use, for drinking water and other kinds of needed industry supplies, cosmetics. So, it's a resource and there's multiple uses. And so, if it can be adapted to the needs of that local community, it's a huge benefit." But even with multiple revenue streams, costs are still more than double the price of other renewables. The tricky construction of the deep-sea water pipes is a turn off for major investors. But before we get into that... "Let's look at where this technology can be used.

The major limiting factor is that we need a big temperature difference between the surface and the deep-sea water." The warm surface water is available all year round in the tropical equatorial zone. That's crucial because a smaller temperature difference means less power generated. "For us, our mission is to address a real problem in tropical islands where there are 600 million people who are unable to generate clean, affordable baseload power." Said Dan Grech. His company is looking to build the biggest OTEC platform to date – a 1.5 MW unit – in São Tomé and Príncipe. "What we have with OTEC is because we're deploying this really across the developing world, that band 20 degrees north and south of the equator, we are supplying markets that have a higher risk. Higher political risk, lower credit ratings." And many of the islands near the equator rely on diesel generated power. OTEC would change that. Older studies estimate that if you disregard practical and financial hurdles, OTEC could power the entire the world. Hypothetically. But before OTEC can even produce a fraction of a terawatt hour, it needs to overcome major hurdles. The potentially huge and hard to calculate price tag being number one.

Today, estimates for a 100 MW OTEC plant can range from $780 million to $1.5 billion. A big unknown is... "The cold water pipes. The cold water pipes." Today, hard plastic pipes two to three meters in diameter are pretty standard. But for a 100 MW plant you'd need pipes almost four times that size. And pipes that big haven't been developed yet. "It is complex because the pipe needs to be stable and flexible at the same time. To not break apart when it gets hit by waves or currents." It has proven so tricky that a floating OTEC near the Indian state of Tamil Nadu did not succeed – because of cold water pipe failure.

And even if it works, storms can destroy the entire thing. That's what happened to one of the first pilot plants in 1930. This uncertainty has even driven away companies with more than 40 billion USD in revenue. Lockheed Martin was set to develop the biggest OTEC plant to date in China, but dropped the project because of the costs. But there is scope for improvement. For example, with the heat exchangers. "To give you a bit of perspective, the heat exchangers on a commercial scale OTEC plant, are about a third of the entire project costs. So, the reason that these are so expensive for commercial OTEC is that you have to use titanium. Deep-sea water is very corrosive. We've developed what we call the thin foil heat exchanger and as the name implies, we're using thin foils.

And the purpose of that is basically we're trying to reduce the amount of material and also the size." "Another big question mark is the actual effect on the environment. Because you're moving insane amounts of water." We are talking about 4.3 billion kg of warm water and 2.2 billon kg of cold water per day for a small plant. That’s about 16 million bathtubs of water. "There are a lot of question marks about OTEC: the economics, the environmental side effects, the cold-water pipe and today’s plants are just way too tiny to figure anything of that out. And in the last 10 years, not much did happen." So, until serious money is poured into it, OTEC probably won’t be taking off any time soon.