How much ocean thermal energy can be converted to electricity?
The conversion of ocean thermal energy into electricity (OTEC) relies on the availability of temperature differences of the order of 20°C in the upper water column. The area of interest covers about a third of all oceans. Intense solar radiation keeps the surface layer of most tropical seas warm, as large surface heat fluxes between the ocean and the atmosphere reach a subtle balance. The existence of a pool of deep cold seawater at low latitudes is less obvious, and was not discovered until the 18th Century. It actually takes a vast network of planetary currents to transport sinking polar water virtually everywhere. Because OTEC seawater temperature differentials are small, their maintenance is essential, while large seawater flow rates must be used in OTEC plants.
This brings out an interesting question about the size of the OTEC resource. Could a massive deployment of this technology affect ocean temperatures on which the process itself depends? In other words, could OTEC be self limiting? Some years ago, I attempted to answer this theoretical sustainability question with very simple models. Results suggested that OTEC resources might indeed have a limit of about 3 to 5 TW [Nihous, G.C., Journal of Energy Resources Technology, 129(1), 10-17, 2007]. Although such estimates may seem disappointing when weighed against environmental fluxes, they still represent an enormous potential given mankind’s total energy use of 16 TW today.
To frame the problem correctly, however, the complex interplay between planetary heat fluxes, a fully three-dimensional oceanic general circulation and potentially large OTEC intakes and discharges spread over more than 100 million square kilometers would have to be captured with state-of-the-art analytical and numerical tools. First steps in that direction were taken over the past two years, with support from the U.S. Department of Energy’s HawaiiNationalMarineRenewableEnergyCenter, and results were recently published [Rajagopalan, K. and G.C. Nihous, Renewable Energy, 50, 532-540, 2013].
This effort confirmed a maximum for global OTEC power production, but a significantly higher one (≈ 30 TW). As OTEC flow rates increase, the erosion of vertical seawater temperature gradients is much slower in three-dimensional ocean models, because any heat locally added to the system can be horizontally transported and re-distributed at a relatively fast rate. Another distinctive feature of the model results is the persistence of slightly cooler surface waters in the OTEC region. This is compensated, however, by a warming trend at higher latitudes. A boost of the planetary circulation responsible for the overall supply of deep cold seawater is also shown. Taken at face value, predicted environmental effects at maximal OTEC power production suggest that lower outputs should be considered. On a positive note, a more modest OTEC scenario with a global potential of the order of 7 TW showed little impact. The corresponding net power density is shown in the Figure, and should be interpreted as cautiously conservative. Work with better numerical resolution and improved physics is under way.
By Gérard C. Nihous
Dept. of Ocean and Resources Engineering
University of Hawaii, Honolulu, Hawaii