INGRESOS TRIBUTARIOS DE LOS MUNICIPIOS Capítulo I

The use of solar energy to regenerate liquid desiccant has attracted much interest. Currently, solar desiccant regeneration systems are mostly driven by solar thermal energy. Solar electrodialysis regeneration is also being investigated as a means of new solar regeneration method (Cheng et al., 2013).

A common type of solar thermal regeneration system for LDAC is shown in Figure 3.16. Weak desiccant solution flows from the dehumidifier into the solar collector and soaks the heat in there, which causes to increase in the temperature. Then, hot but still dilute desiccant solution is discharged to the regenerator and contacts with the air passing upwards. Water molecules in the desiccant solution get absorbed by the air stream and the dilute solution becomes regenerated. The concentration of the

85

desiccant solution is adjusted and strong desiccant solution is obtained as a result of this process. The strong solution is introduced to the dehumidifier from the strong solution storage to carry on the dehumidification cycle.

Figure 3.4.1 Schematic of a solar thermal regeneration system for LDAC (Cheng et al., 2013)

Li and Yang (2008) performed investigations on the energy performance of using exhaust air for the regeneration of the dilute desiccant solution and heating the

solution beyond the equilibrium value of various lengths of the

collector/regenerator(C/R) panels. Findings revealed that the performance was substantially improved and implementing this technique would allow shortening the length of the solar collector/regenerator without degrading the performance considerably. Energy saving in the range of 25-50% compared to a traditional vapour compression system was also reported. Yin and Zhang (2008) proposed a new internally heated regenerator which was expected to achieve a better performance compared to conventional packed regenerators. The results of the experimental test indicated that not only higher regeneration rate was achieved, but also higher energy utilization was monitored. Elsarrag (2008) experimentally tested a novel regeneration system altered from solar tilted still. During the experiments, the effect of the liquid to air flow rate ratio, the desiccant temperature and concentration, and the inlet air

86

humidity ratio on the evaporation rate were monitored. Peng and Zhang (2008, 2009) proposed a novel solar air pre-treatment collector/regenerator for solar liquid desiccant air conditioning system. This system was designed with the objective of operating in high humidity region. The simulation results unclosed the great potential of the novel solar air pre-treatment collector/regenerator to improve the solution regeneration performance. Xiong et al. (2010) investigated the performance of two kinds of desiccant solution (lithium chloride and calcium bromide) in the two separate dehumidification stages of a two stage solar assisted liquid desiccant dehumidification system. During the operation, air moisture (latent) load is separately pre-dehumidified by low-cost calcium chloride and then dehumidified via stable lithium bromide in a main dehumidifier. It was deduced that the effect of pre- dehumidification on the performance was remarkable in a high humidity ambient. Although solar regenerators are successful in producing intended outcomes, the corrosion problem related with the liquid desiccant solution on the energy absorbing surface cannot be disregarded. Luo et al. (2012) studied low corrosive ionic liquids as a substitute of common liquid desiccant solutions. The experimental findings demonstrated that [Dmim]OAc and [Emim]BF4 with inlet mass concentrations of 81.7% and 85.5% could achieve same dehumidification rates as LiCl and LiBr with the mass concentrations of 40.9% and 45.0%, respectively.

Bassuoni (2011) compared the performance of the structured packing dehumidifier/regenerator in a liquid desiccant system with vapour compression system. The performance of the system was assessed using the mass transfer coefficient, moisture removal rate, effectiveness and coefficient of performance (COP). According to economic analysis, the payback period of 11 month was

87

attained with annual running savings of 31.24% compared with vapour compression system.

3.4.2 Photovoltaic Electrodialysis (PV-ED) Regeneration

Apart from the thermal regeneration technique, a recent solar method for the liquid desiccant regeneration is being studied. Electrodialysis (ED) is a technology based method that ions transport through the selective membranes under the influence of an electrical field (Li et al., 2009; 2011). Cation and anion exchange membranes are alternately set in between a cathode and anode within an electrodialyzer. Under the electrical field, the anions and cations inside the electrodialyzer cells move towards the anode and cathode. During this process, the anions and cations pass through anion exchange membranes and cation exchange membranes, respectively. This flow causes a rise in the ions concentration in the concentrate compartments and fall into the dilute compartment. By this way, both the concentrated desiccant solution and pure water can be acquired. A schematic diagram of the PV-ED regeneration method is presented in Figure 3.17.

88

In a PV-ED regeneration process, the dilute desiccant solution is drained from the dehumidifier to the regenerator. The PV cells driven regenerator is made of ED stacks formed of a mass of cells located in parallel between two electrodes. As shown in a schematic of an ED regenerator (Figure 3.17), the cells a, b and e are the concentrate, dilute and electrode rinse cells, respectively. The weak solution is introduced to the dilute cell and regenerated solution to the concentrate cell. After this process, the dilute solution becomes regenerated which is sent to the dehumidifier unit of the LDAC system (Cheng et al., 2013).

As regards the PV-ED regeneration concept, Li et al. (2009, 2011) investigated this new regeneration method for the liquid desiccant cooling system. In this novel regeneration method, the regenerator was formed as an ED stack, and the required electric power for the regeneration process was provided by the photovoltaic cells. In addition, new double stage photovoltaic/thermal ED regeneration (PV/T-ED) system was introduced in (Li et al., 2009). The results of the performance analysis remarked that double stage system outperformed the single stage one in terms of efficient regeneration of liquid desiccant solution.