SDCS Dehumidifier Inlet Air Condition Effect
Figure 6.6 presents the effect of inlet air temperature and relative humidity on dehumidifier performance. The temperature and relative humidity range studied was 25-35 °C and 60-80 % relative humidity respectively. Figure 6.6a shows moisture removal rate increases as both the inlet air temperature and relative humidity increases. Over the temperature range 25-35 °C and at constant RH of 60 %, the moisture removal rate increased from 0.1655 to 0.2258 g s-1. At a
Fig. 6.6 SDCS dehumidifier performance with inlet air conditions constant RH of 70 % moisture removal rate increased from 0.2195 to 0.3203 g s-1 and at a constant RH of 80 % moisture removal rate increases from 0.2823 to 0.3813 g s-1. Similarly, Fig. 6.6b shows that the change in absolute humidity of the air across the dehumidifier increases as the inlet air temperature and relative humidity increases. For a constant RH of 60 % the change in absolute humidity of the air increases from 0.002007 to 0.002876 kg/kg, for a constant RH of 70 % the change in absolute humidity increases from 0.00268 to 0.004095 kg/kg and for a constant RH of 80 % the change in absolute humidity increases 0.003474 to 0.004953 kg/kg. Figure 6.6c shows that latent (dehumidifier) effectiveness increases as both the inlet air temperature and relative humidity increases. For a constant RH of 60 %, latent effectiveness increases from 30.64 to 34.7 %, for a constant RH of 70 % latent effectiveness increases from 32.66 to 36.51 % and for a constant RH of 80 % latent effectiveness increases from 34.55 to 38.75 %.
Because the entire SDCS is placed within the environmental chamber, the water for the evaporative cooler and desiccant solution for the dehumidifier are at ambient temperature i.e. the temperature inside the chamber; this had an impact on the moisture absorption capacity of the desiccant solution and the potential for sensible cooling (little or no sensible cooling was achieved). Therefore, the cooling output reported for the dehumidifier is focussed on the latent cooling potential. When the entire SDCS is placed within the chamber, the evaporative cooler only provides around 80-150 W of cooling over the studied range. This is because the direct evaporative cooling process is not as effective under humid climatic conditions, due to a higher wet-bulb temperature. As a result, the evaporative cooling provided is not enough to produce a sufficient solution temperature decrease and to provide sensible cooling to the supply air in the dehumidifier. Figure 6.6d shows that the dehumidifier cooling output increases as both the inlet air temperature and relative humidity increases. Over the temperature range 25-35 °C and at a constant RH of 60 % the dehumidifier cooling output increases from 414 to 565 W. At a constant RH of 70 % the dehumidifier cooling output increases from 550 to 801 W and at a constant RH of 80 % the dehumidifier cooling output increases from 706 to 954 W. The COPei of the dehumidifier increases with increasing inlet air temperature and relative humidity, from 3.8 at 25 °C 60 % RH to 8.7 at 35 °C 80 % RH. At the conditions investigated it is concluded that the dehumidifier performance increases with increasing temperature and relative humidity.
The partial pressure of water vapour in air increases as the inlet air temperature and relative humidity increases, leading to a greater vapour pressure differential between the air and desiccant solution. An increase in the vapour pressure differential between the air and desiccant solution leads to increased mass transfer and thus a greater moisture removal rate and change in absolute humidity of the air across the dehumidifier. The desiccant system is therefore well suited to hotter more humid climates such as China or costal Mediterranean regions. However, an increase in dehumidification capacity will result in a greater dilution of the desiccant solution. For successful tri-generation system integration, consideration needs to be given to whether the regenerator moisture addition rate achievable with the
SOFC CHP thermal output can match the mass removal rate in the dehumidifier. If it cannot, the operation of the dehumidifiers needs to be limited in order to achieve balanced continuous operation.
