Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 464

Table 14.5.2 Comparison between air cooled and evaporative tower condenser.

Air cooled condenser Evaporative tower condenser

Ambient temperature ( ◦ C) 23

Humidity (%) 60

Air dry bulb temperature ( ◦ C) 23

Air wet bulb temperature ( ◦ C) 17.74

Heat rejected (MW) 86.2

Air flow (kg/s) 5880.5 1386.9

Air outlet temperature ( ◦ C) 37.41 32.74

Condensing pressure (bar) 0.096 0.0689

Water make-up (kg/s) - 36.84

Fan consumption (kW) 842.2 346.6

Pump consumption (kW) - 298.3

of the steam cycle, the evaporating pressure is in the range of 100 bar and RH around
25 bar. The pressures are set in order to limit moisture content in the last stage of of the
steam turbine because water droplets during expansion can be detrimental for turbine
blades.
The steam cycle, as a closed loop, requires a heat exchanger to discharge the
heat: in Rankine cycle this heat exchanger is called a condenser because the steam
condenses. Condensers can be based on evaporative cooling towers as well as dry
coolers. Considering that solar thermal plants are usually placed in arid places with
high solar radiation and low precipitation, dry cooling condensers are more typical.
Air condensers are more expensive than evaporative cooling systems and are more
sensitive to the ambient conditions: the air temperature at the condenser inlet is the
dry bulb temperature, and the entire heat rejected is transferred as sensible heat to
the air. Conversely, evaporative towers transfer the heat rejected from the condenser
also as latent heat by evaporating water in the air, requiring lower amounts of air.
In addition, the air temperature at evaporative tower inlet is the wet bulb, which is
lower than dry bulb particularly in dry conditions.
Comparisons between evaporative tower and air cooled condenser conditions
are summarized in Table 14.5.2. The evaporative tower has lower fan consumptions
(346.6 kW vs. 842.2 kW) because of the lower air mass flow. This advantage is par-
tially balanced by the consumption of the recirculating pump, which is not present in
air condensers, and the required water make-up of 36.8 kg/s; the cost of water replace-
ment can be significant because of solar field location. Both condensate extraction and
feedwater compression are performed by dedicated pumps which have a motor driver.
The number of regenerative bleedings can be variable (between four and seven)
with the pressure selected in order to optimize heat exchange in the regenerators. The
target is to minimize the temperature difference between the hot stream and the cold
stream in order to limit entropy production, hence maximizing efficiency.
When direct steam generation configuration is adopted, the power section has a
much simpler layout since the entire heat exchanger section can be avoided. The layout
of a DSG configuration and the thermodynamic properties of main streams are shown