Temperature differences, which naturally occur in the ocean, underpin an emerging green technology. Photo credit: Ant Rozetsky via Unsplash
At the forefront of efforts to mitigate further effects and save the planet is a consideration of our energy production methods [but] the challenge is threefold…
The very real effects of climate change have never been more apparent. Each year, humanity is faced with unprecedented extreme temperatures, catastrophic meteorological disasters, and the heart-breaking news of ecological destruction. At the forefront of efforts to mitigate further effects and save the planet is a consideration of our energy production methods.
The challenge is threefold: an increasing demand for energy with the growth of populations and industries; our dwindling reserve of fossil fuels; and the catastrophic impact of non-renewables on the planet. The race to find sustainable fuel sources that are up to the task of fulfilling society’s demands is on, pressured by international treaties like the Paris Agreement—an agreement to a 1.5°C limit on global warming, which is becoming increasingly unlikely with our continuing use of oil and gas.
A handful of countries have already met their 2050 goal of Net Neutrality, with many more well on the way. This means that overall, their carbon emissions are entirely offset by positive contributions, like planting trees. Net Neutrality is largely achieved by relying on hydroelectric systems or nuclear power plants.
Notably, Portugal has recently set an example by running on entirely renewable energy sources for six straight days in November 2023. This was made possible by the immensely windy and rainy conditions over that week, allowing the country’s wind and hydroelectric infrastructure to support the energy needs of a population of 10 million. However, their success might not have been so great had calm weather prevailed.
Unlike these meteorological-dependant sources that are already well established, our efforts must turn to new renewable systems that can operate year-round if we are to meet out targets. Enter OTEC.
How it works
Ocean Thermal Energy Conversion as a concept has been around as early as the late 19th century. The principle is fairly simple, and functions on floating platforms or “barges”. Heat energy from a hot reservoir of surface sea water is transferred to an intermediate refrigerant liquid which evaporates, and the vapour produced will spin a turbine. A cold reservoir of deep-sea water, pumped up to the surface in pipes, then cools the vapour down to recondense it and complete the cycle. The heat energy (or, specifically, the gradient in heat energy) is harnessed as mechanical work by the turbine to then be converted into usable electricity. This is the same basic heat engine technologies used in powerplants all over the world.
The driving force of this process lies in the difference in thermal energy between the water at the surface and in the depths. As guidance for development, studies have found that the monthly average temperature difference between surface and depths must always be 18°C or greater. This obviously imposes a significant constraint on regions where OTEC is viable, resulting in a zone of around 114 million square kilometres where using this technology may be possible—equating to 30% of the ocean’s surface, encompassing over 100 countries and territories.
These tropical waters have an inviting surface temperature of 25–28°C all year-round, but drop to a chilly 4°C 800m below the surface due to deeper polar currents. This temperature gradient remains fairly constant throughout the year, meaning OTEC promises to be more robust than other renewable technologies.
Unlike solar panels, which are rendered useless on a cloudy day, or wind turbines, which produce little energy on calm days and have to be shut off in high winds, OTEC maintains as a consistent baseline production rate in favourable regions.
Where it works
You can interrogate the distribution of these potential OTEC regions for yourself, using an interactive map on Global OTEC’s website. Many studies have taken place and are underway to assess the suitability of OTEC in locations such as Barbados, Florida, and Indonesia. In particular, OTEC may be particularly beneficial to Small Island Developing States (SIDS). The energy demands of 600 million people living on these tropical islands are met by importing expensive fossil fuels, sacrificing precious land space to solar fields, or hoping for a particularly gusty day to power their wind farms. The OTEC barges require significantly less space and can provide a steady output of power 24/7, 365 days a year, as well as creating jobs via their installation and operation.
An example
A pioneer in this technology is the UK firm Global OTEC, who unveiled their next-generation platform at the international Vienna Energy and Climate Forum in November 2023. Their first-of-a-kind barge, named Dominique, will be installed in São Tomé and Príncipe off the west coast of Africa, making it the first country to host an OTEC system at a commercial scale.
Global OTEC said, ‘This is set to be a shining example to the rest of the world of how diesel fuel imports can be replaced with clean energy from the ocean’. National Energy Director of São Tomé and Príncipe, Gabriel Maquengo was equally excited about the prospects of the OTEC programme, which offers the opportunity to explore new energy and possibility of meeting the nation’s climate ambitions and goals.
Drawbacks
Despite these promises, this technology is still in its infancy. Thermodynamic efficiency (how much of the energy is successfully captured) is only a mere 3%. As such, OTEC cycles must counteract this by increasing their seawater flow rates, up to several cubic meters per second per megawatt of net electricity produced. This limits the density of OTEC systems in tropical regions, to avoid creating large-scale disturbances to ocean currents.
Unlike solar panels, which are independent and have no effect on neighbouring panels, one-dimensional models of water columns tend towards a self-limiting maximum, where the intrinsic properties of the technology and its impact on the surrounding area create a physical limit to how many systems you can put in proximity. These findings are confirmed by more advanced studies using ocean general circulation models (OGCMs), that also predict potential drastic temperature changes in the surrounding waters. Cooling is observed in regions of OTEC deployment, and other places experience increases of up to 6ºC—whereas the ideal renewable energy source would have zero impact on the environment.
Impact and future prospects
Among emerging green technologies, the ocean’s thermal energy is an often-forgotten renewable source. The 60 million square kilometres of sea surface absorb an estimated one quadrillion megajoules of solar energy on a typical day. To generate this amount of energy from fossil fuels, you’d need to burn a staggering 170 billion barrels of oil. The functional global resource of OTEC is estimated to be more than 7 Terawatts, even with the spatial limits imposed to ensure minimal effect on the oceanic temperature field. That’s around 60 trillion kWh of energy per year—to put this in context, the average UK home uses only 3000 kWh.
Among emerging green technologies, the ocean’s thermal energy is an often-forgotten renewable source.
If we hope for further deployment of OTEC platforms, further studies will be required to assess the potential impacts of large-scale installation. Even small changes in ocean heat and current can affect marine life in unpredictable ways, well-known to be sensitive to ocean conditions. An additional worry is the potential impact on tropical storms, as they also intricately depend on sea temperature.
Yet even with these drawbacks considered, OTEC provides a fantastic alternative energy source. Unlike classic renewable sources, OTEC can provide a year-round constant output of energy. Companies like Global OTEC are pioneering the way forward and opening doors to new climate-friendly possibilities. With significant deployment of barges and improvements to the thermal efficiency, OTEC promises to make a real impact against our energy crisis, with particular benefits to small island nations.
