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5.2. System configurations
in a hotter climate. If T DES2 is increased from 150 ◦ C to 190 ◦ C the limit of
the ambient temperature is increased from around 40 ◦ C to 50 ◦ C for the
wet tower. For the dry tower, the limit is increased from 20 ◦ C to 30 ◦ C.
But in order to avoid corrosion in the desorber, 150 ◦ C is generally
considered to be the maximum allowed desorber temperature [27]. So
for the very hot climate applications (>40 ◦ C) an option could be to use
more expensive more corrosion resistant desorber materials and increase
its temperature, or a single cycle could be used, which of course would
be cheaper but less efficient. The same tendencies go for the dry tower.
η sys,el ,net
For the dry tower, the electrical efficiency (black curve in figure 5.8A) is
quite steady except when the ambient temperature becomes high, which
leads to an efficiency increase due to lower cooling tower fan power since
the tower needs less cooling when the evaporator cooling production
decreases.
For the wet tower (figure 5.8B) the electrical efficiency increases
steadily with the ambient temperature. This is because of the following:
when the air temperature is high (and the relative humidity constant
at 40%), one kg of air can absorb a bigger amount of water since the
absolute humidity difference between the air inlet and the (saturated)
outlet becomes larger. This means that the air flow through the cooling
tower is lower at high ambient temperatures thereby reducing the fan
power consumption.
η HW
The hot water heating(η HW , red curve in figure 5.8) is bigger for low
ambient temperatures than for high ambient temperatures. But this is
because the temperature of the water to be heated up is assumed to be the
same as the ambient temperature. So when the ambient temperature is
low, the exhaust gas can be cooled down to a lower temperature whereby
more energy is transferred to the water.
5.2.3 ∆T min,Tower
The influence of the closest approach temperature difference on the two
different types of towers is examined in appendix E.1.3.1 page 280.
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