What is OTEC (ocean thermal energy conversion)?

Ocean thermal energy conversion (OTEC) drives a heat cycle on the temperature difference between tropical surface water at 25–30 degrees Celsius and deep water at about 4 degrees Celsius a thousand metres down. Its theoretical thermal efficiency is very low, only 3%–5%, so its real value lies less in the electricity than in the by-products: seawater air conditioning that cuts cooling power use by 75%–90%, and deep-sea-water agriculture.

Modern OTEC comes in two configurations. A closed cycle uses a working fluid with a very low boiling point, usually ammonia: warm surface water boils the ammonia, the vapour drives a turbine, and cold deep water condenses it again for the next pass. An open cycle instead flash-evaporates the warm seawater itself under low pressure, sends the steam through a turbine, and on condensing yields ultra-pure fresh water as a by-product — on water-scarce tropical islands, that by-product is sometimes worth more than the electricity.

The economics of OTEC do not rest on conversion efficiency. With barely twenty-odd degrees between the warm and cold reservoirs, the Carnot limit holds theoretical thermal efficiency to just 3%–5%, far below any other form of thermal generation. The value comes from the by-products. The cold deep water drawn up for the cycle can feed seawater air conditioning (SWAC), piped into the cooling systems of coastal buildings in place of conventional compressors, cutting electricity use for cooling by 75% to 90%. That same nutrient-rich cold water can also support fish farming and seaweed cultivation, a practice known as deep-sea-water agriculture.

Geography defines OTEC's market. The best sites are concentrated in the tropics — Pacific island states, Southeast Asia, the Caribbean and the West African coast — which are precisely the places with the weakest energy infrastructure and the highest cost of imported diesel. That overlap is why OTEC keeps being revisited: in small island states paying premium prices for diesel generation, a system that simultaneously delivers power, fresh water and cooling cannot be judged on cost per kilowatt-hour alone.

The same chapter section treats a second invisible potential: the salinity gradient. Where fresh water meets the sea, the osmotic pressure created by the salinity difference is equivalent to a head of about 270 metres, and the world's rivers carry a theoretical 2.4 terawatts to the ocean each year — more than 2,000 coal-fired power stations' worth — of which 100% currently dissipates as heat during mixing. Two extraction routes exist, pressure-retarded osmosis (PRO) and reverse electrodialysis (RED), both still experimental; the Dutch company REDstack is building a continuously operating RED demonstration project on the Zuiderzee.

The real value of OTEC is not its generating efficiency — the theoretical thermal efficiency is a mere 3% to 5% — but its multiple by-products.

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第十章 10.2