Renewable Energy - Energy and the Environment - page 180

Vapo r Vapo r

Vapo r

Turbine Turbine

L iquid

Evaporator Condenser Evaporator Condenser

(a) (b)

Figure 7.27 Diagrams of the fluid flow in an ocean thermal power plant. (a) An open cycle employs the
water evaporated from warm sea water as the working fluid in a low pressure steam cycle. (b) A closed cycle
uses ammonia as the working fluid, the latter being vaporized and condensed in heat exchangers supplied
with warm and cool ocean streams.

7.9 OCEAN THERMAL POWER

The concept of generating mechanical power using warm surface water from a tropical ocean
site was first advanced by Jacques d'Arsonval in 1881. The warm water would provide heat to a
Rankine cycle heat engine while cool deep water would supply the cooling needed to condense
the working fluid. In low-latitude regions of the ocean, within 20 degrees of the equator, water
near the surface, where sunlight is absorbed, is warmer by about 20 K than water at depths greater
than a kilometer. (The cool deep ocean water originates in the polar regions.) Commonly called
ocean thermal energy conversion (OTEC), such a plant would necessarily have a small thermo-
dynamic efficiency because it would operate with a temperature difference that is small compared
to the absolute temperature of the heat source. 27 Although the heat that can be extracted from
the ocean surface is very great, mechanical power is required to circulate the surface and deep
water through the power plant, subtracting from the power generated by the Rankine cycle tur-
bine. This and other practical difficulties have prevented OTEC plants from developing beyond the
demonstration stage.
Two types of OTEC plants have been tried. The first, an open cycle, utilizes water evaporated
from the warm stream under low pressure to supply a turbine discharging to an even lower pressure
condenser cooled by a spray of cool water [see Figure 7.27(a)]. The second, a closed or hybrid cycle,
employs heat exchangers that supply or remove heat from a closed Rankine cycle working fluid,
usually ammonia, permitting a more economical and efficient turbine than that for the open cycle,
but requiring a more expensive evaporator and condenser [see Figure 7.27(b)]. In both cycles, large
volumes of warm and cool water must be pumped from the ocean through long, large-diameter
ducts and then returned to the sea via a third duct. These systems must be constructed to minimize

27 A Carnot cycle operating with a temperature difference of 20 K and supplied with heat from a warm source
at 25 ◦ C would have a thermodynamic efficiency of 20 /( 25 + 273 ) = 6 . 7 %. A practical Rankine cycle would
have even lower efficiency than the ideal Carnot cycle.