Desiccant Solution Evaluation

Liquid desiccant solutions used for air conditioning applications should, according to Jain and Bansal (2007) possess a range of physical characteristics; low vapour pressure, low density and viscosity, good heat transfer characteristics and favourable surface tension as this influences the wetting of contacting media. There are a variety of liquid desiccant solutions available for air conditioning applications; they can be split into three broad groups: halide salts, glycols and weak salt solutions. Application of a desiccant material is dependent upon cost, operation and source of thermal energy (Enteria and Mizutani 2011). In this section three desiccant solutions are evaluated with regards to their potential for dehumidification and suitability for application in a SOFC tri-generation system. The solutions considered are:

• (1) Lithium chloride (LiCl)
• (2) Calcium chloride (CaCl2)
• (3) Potassium formate (CHKO2)

LiCl and CaCl₂, known as halide salts, have been widely used as working fluids in liquid desiccant air conditioning systems. The advantages of these materials are that they are strong desiccants; LiCl can dry air to 11 % RH (Lowenstein 2008). However, the halide salts are extremely corrosive and cause significant damage to air conditioning equipment (heat exchangers, pipes etc.). Titanium is one of the few materials that can be used, however it is very expensive. In response to the shortcomings of the halide salt desiccant solutions, other options have been explored. Salts of weak organic acids such as potassium or sodium formate have been used. These solutions have low toxicity, density and viscosity, are less corrosive, and they can, at the correct concentration achieve sufficient dehumidification potential. The concentrations of a CHKO2 solution for an air conditioning application needs to be greater than that of halide salts, for example, in terms of dehumidification potential, CHKO₂ at a 50 % concentration by mass is roughly equivalent to a 27 % LiCl solution by mass. Although it is a weaker desiccant than the halide salts, for air conditioning applications its ability to dehumidify air below 30 % RH makes it an attractive desiccant solution (Lowenstein 2008).

As discussed in Sect. 3.1.1, dehumidification in liquid desiccant air conditioning systems is driven by a vapour pressure differential between the hot humid air (high vapour pressure) and the liquid desiccant solution (low vapour pressure). An assessment of the dehumidification potential of the LiCl, CaCl₂ and CHKO₂ desiccant solutions is achieved by comparing the vapour pressures of the different solutions over a range of solution concentrations and comparing the results. The vapour pressures of the LiCl and CaCl₂ solutions have been calculated using a routine based on extensive empirical data presented by Conde (2009). The vapour pressure of the CHKO2 solution is determined using a routine, presented in Appendix A (A1.1), taken from the work of James (1998). The routines presented are valid in the 0-100 °C and 0-0.8 solution mass concentration range; thus covering the requirements of a typical air conditioning application. The three desiccant solutions have been evaluated at a set solution temperature of 20 °C, over a concentration range of 0.2-0.8. Figure 3.2 shows the vapour pressure comparison of the three solutions. Identification of the solution operating concentrations is based upon values which will provide a dehumidification potential across a range of summer air conditions. At an air dry bulb temperature of 20 °C, the ranges selected will dehu- midify air from 65 % RH upwards at the lower solution concentrations, to 30 % RH upwards at the higher solution concentrations. The ranges selected also agree well with published literature and do not pose issues of crystallisation.

Table 3.1 summarises the typical operating concentrations LiCl, CaCl₂ and CHKO2 are used at in liquid desiccant air conditioning systems. Other important considerations such as corrosiveness, cost, regeneration temperature and density are also detailed.

It is evident from the data presented in Fig. 3.2 and Table 3.1 that the LiCl and CaCl₂ solutions operate at lower concentrations comparted to the CHKO₂ solution. LiCl and CaCl2 show, based on vapour pressure, better dehumidification

Fig. 3.2 Vapour pressure comparison for LiCl, CaCl2 and CHKO2

Table 3.1 Typical operating conditions of different desiccant solutions

performance compared to CHKO₂. However, the CHKO₂ solution shows better environmental performance, is less harmful to human beings, and is cheaper. As a result, the desiccant air conditioning equipment can be cheaper as there is less need for expensive materials highly resistant to corrosion, also desiccant entrainment into supply airstreams poses less of a threat. Furthermore, the temperature at which the desiccant solution may be regenerated (water vapour driven off) is critical, particularly in a tri-generation system, as it needs to match the thermal output of the prime mover, in this case the SOFC. In general the halide salts (LiCl and CaCl₂) require higher regeneration temperatures in the range of 70-90 °C. As demonstrated in Sect. 3.5.2, the CHKO₂ solution may be regenerated anywhere between 45 and 70 °C. The CHKO2 solutions regeneration temperature is therefore in better thermal agreement with the SOFC thermal output which is usually around 60 °C in a SOFC CHP building application (Pilatowsky et al. 2011; Jradi and Riffat 2014a, c).

Section 3.2 has evaluated three desiccant solutions that could be employed in the novel tri-generation system. Compared to CHKO₂, LiCl and CaCl₂ show better dehumidification potential at their typical operating concentrations. However, the CHKO₂ solution possesses many characteristics that make it favourable, particularly in a trigeneration system application. Positive attributes include low density and viscosity, a lower regeneration temperature requirement, negligible environmental impact, low corrosiveness and cost. As a result, a CHKO₂ solution is the preferred working fluid to be used in the SOFC tri-generation system. As a result, Sect. 3.2.1 will investigate the dehumidification performance of the CHKO2 solution in more detail.