Using Ocean Temperature Differences to Create Renewable Energy

Ocean Thermal Energy Conversion (OTEC) is an idea for creating renewable energy by exploiting the difference in ocean temperatures between the surface and the seabed. The OTEC permit office first opened in 1981 as part of NOAA, America’s National Oceanic and Atmospheric Administration, the marine counterpart to NASA. It was created after the oil price spike of the 1970’s when interest in alternative power sources rose. Oil prices eventually settled and as a result interest in the alternative power sources dwindled, so in 1994, just 13 years later the OTEC office was closed without ever having issued a permit. Good old American bureaucracy.

Now, again during times of high oil prices, alternative energy sources are back with vengeance. All options are being considered and one of them is OTEC. Luckily the concept is reasonably simple. A fluid with a low boiling point, such as ammonium, is vaporised in a heat exchanger using surface water from the sea with an average temperature of about 25°C. The resulting gas has a sufficient pressure to drive a turbine and create electricity. The gas is then cooled using seawater pumped up from a depth of about one kilometre and with an average temperature of about 5°C. The liquid ammonia can then be reheated and the whole process started again. Theoretically this means that OTEC plants can be built anywhere with a surface water temperature of 25°C and a depth of at least one kilometre.

One company pursuing OTEC technology is Lockhead Martin, which is collaborating with a smaller firm called Makai Ocean Engineering to build a 10 megawatt plant in Hawaii that is projected to open in 2015. Then if this plant is successful the idea is to construct a 100 megawatt plant by 2020.

Most of the technology necessary can be taken from existing areas of engineering, such as deepwater oil drilling, where the heat exchangers and pipework required to make a 10MW plant already exist. The 100MW facility however will need a pipe that is not only 1km long but also ten metres in diameter, in order to supply the necessary amount of water. It must also be strong enough to resist waves and ocean climates for decades. Kerry Kehoe, the current head of OTEC activities at NOAA, estimates such a facility could cost $1 billion.

Article by James Burgess, appearing courtesy Oilprice.com.