Solar-desiccant air-conditioning systems - Solar Energy Sciences and Engineering Applications - page 517

Application of the desiccant wheel as the air dehumidifier has factors to be
considered. Kang and Maclaine-cross (1989) show that the performance of the
desiccant-based air-conditioning system relies much on the desiccant material's mois-
ture sorption capacity. Kodama et al. (2001) show that there is an optimal speed
at which a high sorption rate occurs in the rotating desiccant wheel. Optimal speed
increases with increasing regeneration air flow rate, decreasing desiccant wheel depth,
and decreasing bulk density of the rotor. Optimal wheel speed decreases with higher
humidity and lower regeneration temperature. They also show that the sorption rate is
relative to the relative humidity of the air. Zhang and Niu (2002) show that the sorp-
tion performance of the desiccant wheel depends on the wheel rotational speed and the
number of transfer units; they suggest that the desiccant wheel should have 2.5 transfer
units. Subramanyan et al. (2004a) show that increasing the air flow rate reduces the
specific cooling load (difference in the enthalpy of the outdoor air and of the processed
air). The cooling load increases due to the amount of air mass flow rate. In addition,
Subramanyan et al. (2004b) show that increasing the air flow rate increases the supply
air moisture content. Harse et al. (2005) show that for higher humidity air the opti-
mal speed of the wheel is greater than for the air with lower humidity content. They
show that at higher regeneration temperature, the performance of the desiccant wheel
improves. The depth of the wheel affects the dehumidification rate and as the depth
increases the dehumidification rate also increases, resulting in a lowering of the opti-
mum wheel speed. Gao et al. (2005) show that the thickness of the desiccant material
affects sorption capacity. At higher desiccant material thickness in the channel, higher
sorption rate is attained owing to more time to reach the steady state. In addition, a
lower desiccant rotor speed makes for optimum wheel speed. Their study shows that
channel shapes affect rotor sorption capacity. Hence, for the same cross-sectional area,
sinusoidal channel is the best performer due to its lower hydraulic diameter, resulting
in higher air velocity and heat transfer coefficient. Furthermore, the study shows that
increasing the outdoor air relative humidity increases the processed air temperature.
However, the humidity content of both the processed and the exit air increases as rela-
tive humidity of the outdoor air increases. Enteria et al. (2010a) presented parameters
affecting the performance of the desiccant wheel and performance evaluation for the
desiccant wheel dehumidification capability. La et al. (2010) reviewed the development
of the rotary desiccant wheel-based system.
16.3.3 Modified systems
In most designs, operation of the solid desiccant air-conditioning system is through a
dehumidification-humidification process. In this process, air dehumidification is done
at very low humidity content to achieve evaporative cooling. For this, the required
regeneration temperature is increased. Accordingly, application of constant dehumid-
ification will help to prevent the deep dehumidification in regions with hot and humid
climates. Enteria et al. (2010b, 2010c) looked at the constant humidity air cooling
cycle of the desiccant, as presented in Figure 16.3.2. Ando et al. (2005) show the
double stage dehumidification process, in which two desiccant wheels are employed
(Figure 16.3.3). The main purpose of this process is to reduce the air moisture con-
tent in the case of humid air with lower regeneration temperature requirements. Ge
et al. (2009) investigated the two-stage desiccant air-conditioning system. They show