ABSTRACT
The ocean is the world’s largest solar collector. In tropical seas, temperature differences of about 20−25 Cmay occur between the warm, solar-absorbing near-surface water and the cooler 500-1000m depth ‘deep’ water at and below the thermocline. Subject to the laws and practicalities of thermodynamics, heat engines can operate from this temperature difference across this huge heat store. The term ocean thermal energy conversion (OTEC) refers to the conversion of some of this thermal energy into useful work for electricity generation. Given sufficient scale of efficient equipment, electricity power generation could be sustained day and night at 200kWe from access to about 1km2 of tropical sea, equivalent to 0.07% of the solar input. Pumping rates are about 6m3 s−1 of water per MWe electricity production. The technology for energy extraction is similar to that used for energy efficiency improvement in industry with large flows of heated discharge, but on a much larger scale. The attractiveness of OTEC is the seemingly limitless energy of the hotter
surface water in relation to the colder deep water and its potential for constant, base load, extraction. However, the temperature difference is very small and so the efficiency of any device for transforming this thermal energy to mechanical power will also be very small. Even for heating, warm seawater cannot be spilt on land due to its high salt content. Moreover, large volumes of seawater need to be pumped, so reducing the net energy generated and requiring large pipes and heat exchangers. There have been hundreds of paper studies, and a few experimental
demonstration plants, with the first as far back as 1930. These were mostly resourced from France (pre-1970s) and then the USA, Japan and Taiwan in the 1980s, but less activity since then; see Avery and Wu (1994) for a detailed history. This experience confirmed that the cost per unit of power output would be large, except perhaps on a very large scale, and led to other justifications for pumping up the cold, deeper waters, which contain nutrients and therefore increase surface photosynthesis of phytoplankton and hence fish population. It now appears that OTEC could be at best a
secondary aspect of systems for deep-water nutrient enrichment for marine fisheries, for cooling buildings or for desalination (see Section 14.5). Such integrated technology is called Deep Ocean Water Application (DOWA).
