Economic Design of OTEC Power Plant with Concurrent Production of Desalinated Water – A Case Study

Ocean Thermal Energy Conversion (OTEC) power plants offer a green source of renewable energy. Since India is a tropical country and a peninsula, the prospects of OTEC power generation are extremely bright in India. Among the three modes of operation (open cycle, closed cycle and hybrid cycle) of OTEC system, the hybrid mode is most promising. However, one of the chief technical obstacles in OTEC power plant design is that since the temperature difference driving force available is of the order of 10-15ºC only, the size of the heat exchanger (evaporator / condenser) required becomes exorbitantly large. The use of variable area design, developed by the author and his co-workers, has been recommended in this connection. Such a design provides substantial increase in heat transfer coefficient (350 to 450% increase) with insignificant increase in the associated pressure drop penalty (118 to 120% increase). The required size of the heat exchangers thus gets reduced tremendously, while the operating cost does not increase materially, thereby making design and operation of OTEC power plants economical and cost- effective. The performance characteristics of such heat exchangers (Variable Area Heat Exchangers or VAEs) are discussed in detail in this paper. Further, in the hybrid mode of operation of OTEC system, low pressure steam is produced by the flash evaporation of sea water and this steam is used as heating fluid in the evaporator (of variable area design) to evaporate the working fluid (ammonia, freon). The condensate from this exchanger thus forms desalinated water, which constitutes a valuable by-product of the process. Apart from generating clean electric power around the clock (without consuming any valuable raw material), this power plant thus produces several gallons of desalinated water also per day.