Tropical juice: ocean thermal energy conversion

US company to tap into energy from the world’s warm oceans.

A contract to build and operate a floating 100MW power plant that uses tropical seawater to generate electricity could be announced within the next two months, its developer has claimed.

Designed by Baltimore-based Sea Solar Power, the system is based on technology known as Ocean Thermal Energy Conversion (OTEC), a principle by which solar energy trapped in the warm waters of tropical oceans is converted into useable energy.

Company president Robert Nicholson claimed that more than 300 times the world’s electricity needs could be met by harvesting this energy which is stored in the upper layers of the tropical oceans.

The technology works by exploiting the temperature differential between surface water, typically around 27°C, and the 4°C water 900m below. This temperature difference is used to operate vapour turbines, which drive generators and produce electricity.

A typical plant is made up of a series of cycles using heat exchangers, pumps, vapour turbines, compressors and generators. The system is charged with propylene, a refrigeration fluid that boils at 19°C at 10 bar.

The 27°C surface water is pumped through the heat exchangers where it boils the propylene into a vapour that expands through turbines to drive generators and produce electricity.

To complete the cycle, cold water is pumped up from deeper down to condense the propylene vapour back into its liquid state.

As well as generating electricity, the technology is made doubly attractive by the fact that one of its by-products is a huge amount of fresh water. Indeed, a 100MW plant ship could produce 320 million gallons of fresh water a day claimed Nicholson.

However, he added that the technology’s advantages also conspire to make commercialisation a complicated process. ‘we have to sell electricity to one company and water to another,’ he said.

Despite high levels of scepticism and a frustrating political landscape in which ‘Tony really needs to talk to George’, Nicholson said that the company has benefited from $300m (£157m) of funding from the Baltimore-based Abell Foundation. As a result it is on the verge of signing a contract for a 100MW plant ship and has also signed an agreement to build and operate a smaller land-based 10MW plant.

Nicholson said that he is negotiating with organisations based on UK-owned islands as well as US territories such as Guam and the Virgin Islands.

While the sceptics are yet to have their doubts blown away by full-scale demonstrators, Nicholson claimed that the engineering principles are sound and simple. He pointed to the fact that the technology is fundamentally similar to that used at the heart of most of the world’s binary cycle geothermal power plants.

And if fresh water and electricity are not enough, the system could also provide a valuable food source. Nicholson claimed that the nutrient-rich, pathogen-free water drawn from the deep sea would enable each floating plant to support an estimated $100m worth of fish per year. ‘It’s precisely what the world’s most populated region needs — electricity, fresh water and food.’