IDCS Dehumidifier Component Analysis

The role of the dehumidifier is to cool a supply air stream through the lowering of its enthalpy. Enthalpy reduction is achieved primarily through the removal of moisture from the air stream to a liquid desiccant solution. Depending on the desiccant solution temperature, a reduction in the supply air temperate may also occur.

This section presents the results and analysis from the IDCS dehumidifier component evaluation. The IDCS dehumidifier component evaluation has assessed the impact; inlet air temperature, inlet air relative humidity and volumetric air flow has on dehumidifier performance. The key dehumidifier performance metrics considered in this section are: moisture removal rate, change in absolute humidity of the air across the dehumidifier, latent (dehumidifier) effectiveness and dehumidifier cooling output. Table 5.3 lists the operating values used in the dehumidifier testing.

IDCS Dehumidifier Inlet Air Condition Effect

The IDCS dehumidifier performance has been evaluated over a 50-70 % relative humidity range at a 30 and 35 °C inlet air temperature. The data presented in Fig. 5.8 shows that dehumidifier performance improves with increasing inlet air temperature and relative humidity. Figure 5.8a shows that over the investigated relative humidity range the moisture removal rate from the supply airstream increases from 0.1541 to 0.4395 g s^-1 for the 30 °C inlet air condition and 0.23540.4682 g s^-1 for the 35 °C inlet air condition. As the relative humidity and temperature of the inlet air increases, its vapour pressure increases, and thus a greater vapour pressure difference between the humid air and desiccant solution exists, driving greater mass transfer. Figure 5.8b shows that over the investigated relative humidity range the absolute humidity difference of the supply air stream increases, i.e. more dehumidification occurring, from 0.001988 to 0.005728 kg_vapour/kg_dryai_r for the 30 °C inlet air condition, and from 0.003073 to 0.0062 kg_vapour/kg_drya;_r for the 35 °C inlet air condition. Figure 5.8c shows that over the investigated relative humidity range the latent (dehumidifier) effectiveness increases from 29.91 to 38.39 % for the 30 °C inlet air condition, and from 32.32 to 46.78 % for the 35 °C inlet air condition. Figure 5.8d shows that over the investigated relative humidity range the cooling output from the dehumidifier increases as the inlet air relative humidity and temperature increases. The dehumidifier cooling ranges from 570 to 1084 W at an inlet temperature of 30 °C, and from 1059 to 1362 W at an inlet temperature of 35 °C. The increase in cooling output with air relative humidity and temperature is due to greater moisture removal rate and thus greater latent cooling achieved plus a greater temperature difference between the air and desiccant solution, leading to increased sensible cooling.

Fig. 5.8 IDCS dehumidifier performance with inlet air conditions

At the 30 and 35 °C inlet air condition, the average supply air temperatures across all relative humidity tests is 28.81 and 31.97 °C respectively. From Fig. 5.8 it is evident that the IDCS dehumidifier performance improves with an increase in inlet air temperature and relative humidity. The system is therefore well suited to hotter, more humid climate such as Southeast Asia. However increased performance will result in a greater dilution of the desiccant solution. For tri-generation system integration, consideration needs to be given to whether the regenerator moisture addition rate achievable with the SOFC thermal output can match the mass removal rate in the dehumidifier.

The IDCS evaporative inter-cooler is included to enhance performance by providing sensible cooling to the dehumidification process. The evaporative inter-cooler is operated on laboratory air. Figure 5.9 shows the relationship between the inlet air absolute humidity to the evaporative inter-cooler and the cooling effect it provides. The cooling output is determined based on the enthalpy difference of the inter-cooler’s inlet and outlet air. At an inlet air condition of 0.007 kg_vapour/kgd_ryai_r around 800 W of cooling is achieved, this reduces to around 400 W at a 0.011 kg_vapour/ kg_dryair inlet air condition. At lower inlet air absolute humidity values, the evaporative cooler produces a greater cooling output due to the inlet air having a lower wet-bulb temperature and thus greater evaporative potential. As a result, in a building application, it would be recommended to operate the evaporative inter-cooler on drier return room air, as opposed to fresh outside (humid) air, to increase cooling output.

Fig. 5.9 IDCS evaporative-inter cooler output with inlet air absolute humidity