Regenerator Desiccant Solution Temperature Effect
Figure 3.24 shows the impact increasing desiccant solution inlet temperature has on the performance of the regeneration process. Figure 3.24a demonstrates that as the desiccant solution temperature is increased, the outlet concentration rises. This is because a higher solution temperature creates a higher vapour pressure, and thus a greater vapour pressure differential between the regenerator airstream, resulting in a greater mass of vapour transferred from the desiccant solution to the air. As the inlet desiccant solution temperature increases, so does the outlet solution temperature. Figure 3.24b demonstrates, for the stated inlet conditions, the air absolute humidity difference and latent effectiveness increases with solution temperature. It is evident that the regeneration process, indicated by a positive absolute humidity difference/latent effectiveness, begins at an inlet solution temperature of 47.5 °C at a 0.6948 solution mass concentration. This is a realistically achievable temperature when integrated with an SOFC CHP system. Above
47.5 °C the regeneration capacity increases. Re-concentration of the desiccant
Fig. 3.24 a, b Desiccant solution temperature effect on regenerator performance
solution back to the dehumidifier mass concentration of 0.7 is achieved at an inlet solution temperature of 58.5 °C. Above 58.5 °C, the desiccant solution is re-concentrated above 0.7.
A higher inlet solution temperature means greater re-concentration of the desiccant solution, representing a greater cooling potential in the dehumidifier. However a higher inlet solution temperature means a greater thermal energy input is required, potentially lowering the COPth of the system. Furthermore, in a desiccant air conditioning system the outlet solution from the regenerator needs to be cooled to an acceptable level prior to it re-entering the dehumidifier. If the solution outlet temperature from the regenerator is too high this may (a) incur a large energy penalty for cooling, or (b) not be possible with the available means i.e. evaporatively. It is therefore critical not to operate the regenerator at too high a temperature as this can have an adverse impact on the total system performance.
Figure 3.25 presents a plot of the change in absolute humidity (A&>a) of the regenerator airstream with respect to inlet desiccant solution mass concentration and temperature. It is evident that the regeneration process, indicated by a positive absolute humidity difference (air is taking on water vapour from the desiccant solution) begins at a lower inlet solution temperature at lower solution concentrations. This is a very important observation when considering the integration of a liquid desiccant air conditioning system in a tri-generation system set-up. The expected flow temperature of the water in the WHR loop of the prime mover, i.e. SOFC, will dictate what solution concentration the desiccant system should be operated at. If suitable matching is not achieved, the liquid desiccant system may not operate in a balanced manner i.e. mass balance in the dehumidifier and regenerator. Regeneration of the CHKO2 solution begins at 44.5 °C for a 0.65 solution mass concentration, 48 °C for a 0.7 solution mass concentration and 52 °C for a 0.75 solution mass concentration.
Section 3.5.2 has presented a regenerator parametric analysis investigating the effect inlet air relative humidity, inlet air temperature, desiccant solution mass flow rate, desiccant solution temperature and desiccant solution mass concentration have on the performance of the regenerator. Using the input conditions described in the dehumidifier and regenerator parametric studies, along with a desiccant to
Fig. 3.25 Desiccant solution mass concentration and temperature effect on regenerator performance
desiccant heat exchanger, the desiccant air conditioning system achieves a COPth and COPei of 0.65 and 16.1 respectively. The liquid desiccant air conditioning system electrical requirement of 110 W has been gained from an experimental investigation presented in Chap. 6. The COPth and COPel calculation method is provided in Eqs. 4.30 and 4.31 respectively. Conclusions from both the dehumidifier and regenerator parametric analysis are summarised in Sect. 3.5.3, with particular reference to their importance to the SOFC tri-generation system.