Power Plant Engineering

NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION 99

Vacuum

pump

Dissolved

gases

surfaceWarm

water

(^12)
4
Warm-water
discharge
Evaporator
Direct
contact
condenser
Low-pressure steam
T
5
7
Cold-water
discharge Cold deepwater
Pump
6
Powerplant
13° C
Surface water
27°C
Deep
water 11°C
Fig. 2.44. Flow diagram and schematic of a Claude (open-cycle) OTEC power plant.
In the cycle warm surface water at 27°C is admitted into an evaporator in which the pressure is
maintained at a value slightly below the saturation pressure corresponding to that water temperature.
Water entering the evaporator, there four, finds itself “superheated” at the new pressure.
This temporarily superheated water undergoes volume boiling causing that water to partially
flash to steam to an equilibrium two-phase condition at the new pressure and temperature. The low
pressure in the evaporator is maintained by a vacuum pump that also removes the dissolved
noncondensable gases from the evaporator.
The evaporator now contains a mixture of water and steam of very low quality at 2. The steam is
separated from the water as saturated vapor at 3. The remaining water is saturated at 4 and is discharged
as brine back to the ocean. The steam at 3 is, by conventional power plant standards, a very low-
pressure, very high specific-volume working fluid (0.0317 bar, 43.40 m^3 /kg, compared to about 160
bar, 0.021 m^3 /kg for modern fossil power plants). It expands in a specially designed turbine that can
handle such conditions to 5. Since the turbine exhaust system will be discharged back to the ocean in
the open cycle, a direct-contact condenser is used, in which the exhaust at 5 is mixed with cold water
from the deep cold-water pipe at 6, which results in a near-saturated water at 7. That water is now
discharged to the ocean.
The cooling water reaching the condenser at 13°C is obtained from deep water at 11°C (51.8°F).
This rise in temperature is caused by heat transfer between the pro-gressively warmer outside water and
the cooling water inside the pipe as it ascends the cold water pipe.
There are thus three temperature differences, all about 2°C: one between warm surface water
and working steam, one between exhaust steam and cooling water, and one between cooling water
reaching the condenser and deep water. 'These represent external irreversibility’s that reduce the over-
all temperature difference between heat source and sink from 27 – 11 = 16°C (28.8°F) to 25 – 15 =
10°C (18°F) as the temperature difference available for cycle work. It is obvious that because of the
very low temperature differences available to produce work, the external differences must be kept to
absolute minimum to realize as high efficiency as possible. Such a necessary approach, unfortunately,