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THERMODYNAMICS - CASE STUDY

    Introduction


Schematic of an OTEC Power Plant

 

A town located on an island near the equator needs a new power plant to generate additional electricity to meet the increasing demand for electrical power. An Ocean Thermal Energy Conversion (OTEC) power plant is considered since it operates between the ocean surface and the deep ocean near the equator of the earth. For economic consideration, the thermal efficiency of the power plant can not be lower than 5%. The ocean surface water, which flows through the evaporator (a heat exchanger) leaves with a temperature of 22 oC due to the vaporization process of the working fluid (propylene) used in the OTEC power plant. The deep ocean water used in the plant is exhausted at 10 oC. The 10 oC water will be used by a local bottling company to help refrigerate drink. In order to design the pipe used to draw water from the ocean, the flow rate of the ocean surface water and deep ocean water that the power plant consumes needs to be determined.

What is known:

  • The average temperature at the ocean surface is Tsurface= 27 oC.
  • The water exhausted from the evaporator has a temperature of 22 oC.
  • The temperature in the ocean changes with the depth. The relation is:
    T = -0.005 h + Tsurface
    where T is temperature (oC) and h is the depth (m).
  • Deep ocean water from the depth of 1,000 m is sent to the power plant and left with a temperature of 10 oC.
  • The power required from this power plant is 15,000 kW.
  • The thermal efficiency of the power plant is 5%.
     
    Question


 

Determine the volumetric flow rate of the ocean surface water and the deep ocean water

     
    Approach



 
  • The power plant is a heat engine since it generates power at the expense of heat input.
  • The ocean surface is a source and the deep ocean is a heat sink.