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.

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