EVEN by the standards of American bureaucracy, an organisation that operated for 13 years without achieving anything is impressive. Yet that was the fate of the Ocean Thermal Energy Conversion (OTEC) permit office, which opened its doors in 1981 and closed them in 1994, having issued not a single OTEC permit.

The office was part of NOAA, America's National Oceanic and Atmospheric Administration—a marine counterpart of the country's space agency, NASA. And the idea of OTEC was to exploit the difference in temperature between the top of the ocean and the bottom, in order to drive turbines and generate electricity. The incentive was the oil-price spike of the 1970s. But once that incentive went away, so did interest in alternative sources of power and, eventually, so too did the office.

Alternative power sources are back in fashion, though, and OTEC is one of them. A range of companies, from giants such as Lockheed Martin to minnows like the Ocean Thermal Energy Corporation of Lancaster, Pennsylvania, are working on the technology, and this time it might actually come to pass. Most of the bits and pieces required can be borrowed from other areas of engineering, such as deepwater oil drilling. And the idea of a power station whose fuel is free is attractive, as long as the capital cost is not too high.

The most common OTEC design uses a fluid with a low boiling point—typically ammonia—which circulates through a network of pipes. First, it is vaporised in a heat exchanger that is warmed by surface water with a temperature of around 25°C. That puts the gas under sufficient pressure to spin a turbine and thus generate electricity. When it has done so, the gas is sent to a second heat exchanger, where it is cooled by seawater that has been pumped from a depth of a kilometre or so, where the temperature is about 5°C. That condenses it back into a liquid, and the whole process can be repeated. Theoretically, then, an OTEC plant can be built anywhere that the ocean has a surface temperature above 25°C and is more than 1km deep.

Fortunately for the technology's supporters, that state of affairs pertains in several places of interest to America's Defence Department. These include Guam, in the Pacific Ocean, and Diego Garcia, in the Indian Ocean. Both islands host American bases, and even in these straitened times the Pentagon's budget can stretch to an experimental technology that might reduce a base's fuel consumption.

The actual experiment, though, is on Hawaii, where Lockheed is collaborating with a smaller firm, Makai Ocean Engineering, to build a ten megawatt (MW) pilot plant that should be operational by 2015. If that goes well, the idea is to follow it with a 100MW power station by 2020.

For this, however, a new piece of kit will be needed. The heat exchangers and pipework required to make a 10MW plant already exist, but the 100MW facility will need a pipe that is not only 1km long (in order to reach the cold water at depth) but ten metres in diameter (in order to bring enough of that cold water to the surface). This is quite some pipe, and it will also have to be rugged enough to survive for decades in the open ocean. Nor will it be cheap. Kerry Kehoe, the current head of OTEC activities at NOAA, estimates such a facility could cost $1 billion.

A more modest project is planned by the Ocean Thermal Energy Corporation. It has signed a memorandum of understanding with the Bahamian government to build a fully commercial OTEC plant. Initially, cold water will be pumped from the ocean depths to provide cooling for a holiday resort—a project that will cost $100m. Eventually, the plan is to turn this into a full-fledged 10MW power station. Bolting cooling facilities onto an OTEC generator, and also using some of the resulting power for desalination on islands like the Bahamas that are short of fresh water, helps tip the economic balance in favour of OTEC.

The Caribbean, indeed, seems a popular place to try the technology out. The first OTEC plant, built in 1930, was at Matanzas Bay, just across the Florida straits from the Bahamas, in Cuba. That successfully produced 22kW, though it was eventually destroyed by wind and waves. A mere eight decades later, the technology may at last come to fruition.