Liquid Desiccant Air Conditioning System

The individual validated desiccant dehumidifier and regenerator models have been evaluated in detail in Chap. 3, therefore only the incorporation of the dehumidifier and regenerator into a complete desiccant air conditioning system are provided in this section. Figure 4.1 shows a schematic of the entire tri-generation system, including the modelled desiccant air conditioning sub component. In order for the desiccant air conditioning system to operate continuously, the mass of vapour absorbed by the desiccant solution in the dehumidifier must be removed in the regenerator, as demonstrated in Eqs. 4.23 and 4.24. Thermal input to the desiccant air conditioning system is through the heating of the desiccant solution to a temperature such that the mass balances are satisfied.

To form a complete desiccant air conditioning system the dehumidifier and regenerator are combined, alongside an evaporative cooling tower, desiccant to desiccant heat exchanger and a regenerative heat source. The cooling tower is included to cool the liquid desiccant solution prior to dehumidification. The desiccant to desiccant heat exchanger is used to pre-cool and pre-heat the desiccant solution as it flows between the dehumidifier and regenerator respectively. The regenerative heat source is used to raise the temperature of the descant solution to allow de-sorption of the moisture added in the dehumidifier. In the tri-generation system model, the thermal energy input comes from the SOFC CHP system.

Chapter 3 provides the equations used to calculate the state points of the air (Eqs. 3.18 and 3.20) and the desiccant solution (Eqs. 3.19 and 3.21) for both the dehumidifier and regenerator. These equations will not be presented again; however, the desiccant air conditioning process in a tri-generation system context is described below. The system operation is described with reference to the numbered desiccant flows and lettered air flows shown in Fig. 4.1.

During dehumidification and cooling of the fresh air (process A-B), strong cool desiccant solution (point 6) is sprayed downwards through the dehumidifier membrane HMX. The desiccant solution absorbs water and becomes weak (point 1). In order for the solution to have its dehumidification capacity restored it needs to be regenerated. First, the desiccant solution is pre-heated by hot desiccant solution flowing from the regenerator in the desiccant to desiccant heat exchanger (PX2).

The desiccant solution outlet temperature (point 2) is calculated using Eq. 4.25. The concentration remains unchanged. C denotes the heat capacity rate in J K-1.

The weak warm desiccant solution (point 2) is then heated to a temperature (Tsol3) such that the mass balances in Eqs. 4.23 and 4.24 are satisfied (point 3). The thermal energy input at PX3 needed to raise the desiccant solution to the required temperature is determined using Eq. 4.26.

During regeneration, the weak hot desiccant solution is sprayed downward through the regenerator membrane HMX, and the moisture absorbed by the desiccant solution in the dehumidifier is desorbed into the air stream (process C-D). Following regeneration, the desiccant solution is strong and hot (point 4). Before it can be used again for dehumidification, it needs to be cooled. Initially the strong hot desiccant solution is pre-cooled by the cool desiccant solution flowing from the dehumidifier in the desiccant to desiccant heat exchanger (PX2). The desiccant solution outlet temperature (point 5) is calculated using Eq. 4.27. The solution concentration remains unchanged.

Following pre-cooling of the desiccant solution, it is further cooled in PX1 with water flowing from an evaporative cooling tower (process E-F). The cooling tower outlet water temperature (rcwout) is calculated based on wet bulb effectiveness (nwb) using Eq. 4.28. To replicate realistic operating conditions, the cooling tower inlet water temperature (rcw,;n) and the air wet-bulb temperature (Twba) are assumed equal to the ambient environment. nwb is assumed constant at 75 %.

The desiccant solution outlet temperature (point 6) is then calculated using Eq. 4.29. The solution concentration remains unchanged.

Following solution cooling, the strong cool desiccant solution flows back to the dehumidifier and the process begins again. During tri-generation system modelling the thermal energy output from the SOFC CHP system (<2wHr) must be greater than or equal to the thermal energy requirement of the regeneration process (<2reg) for the tri-generation system process to be possible. As highlighted in Sect. 4.1, the primary consideration for the selection of sub-component operating values is that the SOFC thermal output matches the thermal requirement of the liquid desiccant system.

As highlighted in Sect. 3.5.2, regenerator capacity increases as the temperature and relative humidity of the inlet air stream is reduced. Thus, for both the liquid desiccant air conditioning system and tri-generation system analysis presented in Sects. 4.2.2.1 and 4.3 respectively, it is proposed that the dehumidifier and evaporative cooler operate on fresh outside air, and the regenerator operates on extracted room air, with an assumed condition of 26 °C and 60 % RH, an appropriate summer operative condition for a domestic dwelling according to CIBSE Guide A (CIBSE 2006).

Next, the liquid desiccant air conditioning system parametric analysis is presented.

 
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