Tropical idea: Ocean thermal energy conversion
Technology that taps into the solar energy stored in seawater may prove to be a valuable source of power
Thanks to the development of systems that can harvest energy from its waves, tides and currents, the ocean is viewed as in increasingly valuable source of power. But according to the proponents of technology that exploits the sea’s ability to trap solar energy, we haven’t seen anything yet.
The technology, Ocean Thermal Energy Conversion (OTEC), could, they say, transform the way we use energy, allowing us to supply huge quantities of non-polluting, baseload power. It could also help desalinate water and provide refrigeration to local communities.
According to some estimates, more than 300 times the energy we currently consume is available from the solar energy that is constantly stored in the upper layers of the tropical ocean. On an average day, 60 million square kilometres of tropical seas absorb an amount of solar radiation equivalent in heat content to around 250 billion barrels of oil. Tapping into that potential has been the goal of OTEC scientists for more than a century.
Invented in 1881 by a visionary French scientist, Jacques-Arsène d’Arsonval, OTEC uses the temperature difference between the warmer water of oceans and the colder water that can be found up to 1km below the sea’s surface. In some tropical regions, this difference is enough to operate vapour turbines that drive generators and produce electricity and fresh water as a byproduct.
In the 19th century, d’Arsonval couldn’t prove the viability of OTEC. But its potential captured the imagination of a small group of researchers. The oil crisis of the 1970s brought the radical technology into the mainstream, and work in Europe, the US and Japan began to gain momentum. No sooner had it started, however, that petroleum prices dropped and investors lost interest.
But now, major players are planning to revive the technology. Two years ago, the US Navy awarded defence group Lockheed Martin an $8m (£5m) contract to develop critical OTEC components and to mature its design for a pilot plant. Since then, the navy has awarded a further $4m to build on its research. The hope is that OTEC will provide a consistent energy source for naval bases.
More than 300 times the energy we consume is available from the solar energy in the tropical ocean’s upper layers
’Some of the large bases, such as Pearl Harbour, are ideal locations,’ said Robert Varley, OTEC project manager at Lockheed Martin. ’Our fiscal-year defence programme has a separate area specifically to develop OTEC. That will be a signal to industry that the navy is serious about going forward with the technology. Of course, congress has to approve it, but I think the chances are good.’
As the technology requires a difference of 20°C in water temperature, OTEC can only operate between latitudes of 20°N and 20°S. This area includes 29 territories and 66 developing nations, as well as OTEC’s main investor: the US. The first markets are likely to be the small island nations of the South Pacific and Molokai in Hawaii. By 2015, Lockheed Martin hopes to have developed a 5MW pilot plant in this area in collaboration with Hawaii’s Makai Ocean Engineering.
’The challenge is designing the plant for minimal capital cost,’ said Joe Van Ryzin, vice-president of Makai. ’We’ve spent some time developing large low-cost and corrosion-resistant heat exchangers. Makai has a heat exchanger test facility in Hawaii where we are testing various heat exchangers in both surface and deep seawater.’
In Lockheed Martin’s planned closed-cycle system, warm water from the surface of the sea is pumped into a heat exchanger that acts as a boiler for the system’s working fluid, ammonia. As the ammonia vapour expands, it spins a turbine coupled to a generator to produce electricity. The vapour is condensed in another heat exchanger using cold seawater from below the surface to remove the heat. Then, the working fluid is pumped back to the evaporator to repeat the cycle.
Varley believes that, while the process is non-polluting, the movement of the seawater could cause concern among environmental groups. ’We’re pumping about 1,000 gallons per second per megawatt. That’s a lot of water, and the impact needs to be assessed. In the next five to 10 years, I would like to see a full-scale OTEC system in the water that can make these environmental measurements.’
In some cases, the change in seawater could benefit the environment. The cold water is rich in nutrients and could help to culture marine and plant life near the shore. An open-cycle system that uses warm seawater as the working fluid instead of ammonia could also produce desalinated water using surface condensers. The water could be used by local communities where freshwater supplies are limited. In addition, the cold water (around 4ºC) could provide cooling near the plant and supply air conditioning in buildings.
“We’re pumping a lot of water, and the environmental impact needs to be assessed”
ROBERT VARLEY, LOCKHEED MARTIN
’Despite its benefits, OTEC is a hard sell in the US,’ said Jan War, operations manager of the Natural Energy Laboratory of Hawaii Authority. ’For the 5-10MW demonstration plant, we are talking at least $200m of investment, and the return of investment is going to be quite bad because it is not on an economical scale…It’s a hard pill to swallow by just one entity, so we’re working with Japan and Taiwan to see if they want to share the IP.’
The future for OTEC, however, could be in open grazing plants. If the technology can be proven on a full-scale floating plant, engineers will develop a system that is free to drift across the ocean. The plant would use the power generated from thermal differences to split seawater into liquid hydrogen and liquid oxygen. The hydrogen could be offloaded into tankers and taken to countries that need liquid hydrogen as part of their infrastructure.
’This could happen within the next 10 years,’ said Varley. ’While the UK’s waters aren’t warm enough for direct use, a grazing plant could help to import much-needed alternative fuels for transport.’ If this happens, it would require huge investment at the cost of other renewable technologies. But Varley believes the return will be worth it. ’The real advantage is in autonomy,’ he said. ’Do you want to import oil from our friends - or not-so friends - in other countries, or from developments that Great Britain controls? That’s the question that needs to be asked.’
Good for the solar
From a radical concept to a serious idea awaiting congress approval
- 1881 French scientist Jacques-Arsène d’Arsonval reveals his idea to tap into the thermal energy of the ocean.
- 1926 Georges Claude, one of d’Arsonval’s students, begins research into OTEC for commercial use.
- 1935 Claude attempts to build an open-cycle plant aboard a 10,000-tonne cargo vessel, but fails.
- 1956 French researchers design a 3MW open-cycle plant for Abidjan on Africa’s west coast. The plant was never completed owing to high costs.
- 1974 The Natural Energy Laboratory of Hawaii is established as a test facility for OTEC technologies.
- 1981 Japan demonstrates a shorebased, closed-cycle plant in the Republic of Nauru in the Pacific Ocean.
- 2009 The US Navy awards Lockheed Martin $8m to develop a 5MW OTEC pilot plant.