Power Plant Engineering

100 POWER PLANT ENGINEERING

also results in very large warm and cold water flows and hence pumping power, as well as large heavy
cold water pipes.
The Closed or Anderson, OTEC Cycle Power Plant. d’Arsonval’s original concept in 1881
was that of a closed cycle that also utilizes the ocean’s warm surface and cool deep waters as heat
source and sink, respectively, but requires a separate working fluid that receives and rejects heat to the
source and sink via heat exchangers (boiler and surface con-denser). The working fluid may be ammo-
nia, propane, or a Freon. The operating (saturation) pressures of such fluids at the boiler and condenser
temperatures are much higher than those of water, being roughly 10 bar at the boiler, and their specific
volumes are much lower, being comparable to those of steam in conventional power plants.

Liquid

Vapour

Boiler

Surface water

discharge

Warm surface

water

Pump

Surface

condenser

T

Pump

Cool deep

water

Pump

Fig. 2.45. Schematic of a closed-cycle OTEC power plant.

Such pressures and specific volumes result in turbines that are much smaller and hence less

costly than those that use the low-pressure steam of the open cycle. The closed cycle also avoids the

problems of the evaporator. It, however, requires the use of very large heat exchangers (boiler and

condenser) because, for an efficiency of about 2 percent, the amounts of heat added and rejected are 50

times the output of the plant.

In addition, the temperature differences in the boiler and condenser must be kept as low as

possible to allow for the maximum possible temperature difference across the turbine, which also con-

tributes to the large surfaces of these units.

Barjot first proposed the closed-cycle approach in 1926, but the most recent design was by

Anderson and Anderson in the 1960s. The closed cycle is sometimes referred to as the Anderson

cycle. The Andersons chose propane as the working fluid with a 20°C temperature difference between

warm surface and cool water, the latter some 600 m deep. Propane is vaporized in the boiler at 10 bars

or more and exhausted in the condenser at about 5 bars.

In order to minimize the mass and the amount of material (and hence cost) used to manufacture

the immensely large heat exchangers, the Anderson OTEC system employs thin plate-type heat ex-

changers instead of the usual heavier and more expensive shell-and-tube heat exchangers. To help

reduce the thickness of the plates, the heat exchangers are placed at depths where the static pressure of

the water in either ex-changer roughly equals the pressure of the working fluid. Thus if propane is the

working fluid in the boiler at 26.7°C and 9.9 bar the boiler.