A high-performance heat exchanger should be used in the evaporator and condenser of Ocean Thermal Energy Conversion (OTEC) since the utilizable temperature difference is small (about 20°C). It is generally said that the plate-type and the double-fluted tube-type heat exchangers are suitable for an OTEC power plant. In this paper, optimization carried out using a double-fluted tubetype heat exchanger, considering turbine configuration. Ammonia is used as the working fluid. The performance of an OTEC plant with a plate-type exchanger is compared with the performance of a plant with a double-fluted tube-type exchanger.

OBJECTIVE FUNCTION

The objective function g is defined Уby the following equation:

  • У= Total heat exchanger area/Net power = AT / PN (M2/kw) (1)

The net power is obtained by subtracting the power of the warm seawater pump, the cold seawater pump and the working fluid pump from the gross power. The total heat transfer area is obtained by using heat transfer coefficients of the seawater and the working fluid side.

FUNDAMENTAL EQUATION

The overall heat transfer area of the evaporator AE, and the overall heat transfer area of the condenser AC are given as follows:

  • AE= QE/{UE(ΔTm)E} (2)

  • AC= QC/{UC(ΔTm)C} (3)

where UE and UC are the overall heat transfer coefficients of the evaporator and condenser, respectively. QE and QC are the heat flow rate of the evaporator and the condenser, respectively. (ΔTm)E and (ΔTm)C are the logarithmic mean temperature differences of the evaporator and condenser, respectively. These overall heat transfer coefficients are calculated using the boiling heat transfer coefficient (Nakaoka and Uehara, 1988a; Rothfus and Neuman, 1977), the condensation heat transfer coefficient (Nakaoka and Uehara, 1988b), and the heat transfer coefficients of the seawater side (Nakaoka and Uehara, 1988b; Obana, 1974).

Keywords:
gas monetization, plate-type exchanger, double-fluted tube-type exchanger, exchanger, LNG, nakaoka, tube-type heat exchanger, heat exchanger, Uehara, liquified natural gas
Subjects:
Liquified natural gas (LNG), Natural Gas Conversion and Storage
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1999. The International Society of Offshore and Polar Engineers
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