Advanced Liquid Desiccant-Based Air Conditioning Systems


The simulation of thermal processes has become air for life in equipment design and in plant performance analyses. In particular, the development of new types of equipment requires a good mix of computational and laboratorial experiments.

Here we shall offer an approach (suggest models) to the simulation of ALDACS, going from the global to the detail, that is from the Unit to the individual components1.

The Conditioner


Figure 1 – The conditioner – Its main components and the variables characterizing their inlets and outlets.

The Conditioner, as depicted above, works as a DOAS: No recirculation, though free-cooling and simple evaporative cooling are possible. For Winter operation, the ABS can be turned into an air humidifier, with heating contribution as well.

The position of the exhaust ventilator could be changed for some applications. In general, the position depicted is the best for Summer operation and for hot climates.

The Regenerator


Figure 2 – The regenerator – Its main components and the variables characterizing their inlets and outlets.

The Regenerator as shown, includes the desiccant Storage Module. The DES component may operate as the Absorber in Winter, wherever SUPply air humidification is required. Humidity is transferred by the desiccant.

The Five-Stream Heat & Mass Exchangers — ABS & DES


Figure 3 – The ABS & DES – Physical and Impedance Diagram models. Note the symmetry of the Impedance Diagram.

The Simulation of the ABS & DES is best carried out Cellwise (see Figure 4) as the fluid properties in the five streams involved vary strongly. Symmetry of the streams permits considering them as were they three only.

In thermal steady state, the total variation of the water load in salt must equal the total variation of the air humidity content.


Figure 4 – The ABS & DES – Basic Model Equations.

The Indirect Evaporative Cooler — IEC


Figure 5 – The Indirect Evaporative Cooler (IEC) – Physical Diagram.

In this concept of the IEC, the air streams are in crossflow in the entry and exit sections, and in counterflow in the central section. Water, on the other hand is in crossflow with the ETA Air. Determinant for the SUP Air cooling process, is the temperature in the falling film.

Contrary to common opinions, the process is not adiabatic: – Thermal energy is delivered to the water all along.


Figure 6 – The Indirect Evaporative Cooler (IEC) – Transport Process Diagram.


Figure 7 – The Indirect Evaporative Cooler (IEC) – Impedances Diagram.

Air-to-Air Heat Recovery Heat Exchangers — HRHX


Figure 8 – The HRHX Heat Exchangers – Geometry of the stackable Parts.

Air-to-Air Heat Recovery Heat Exchangers are used in both the Conditioner and the Regenerator. Although they may be designed as single-phase heat exchangers, they may, under certain circunstances, work as two-phase. This depends essentially on the warmer air stream.

When designing the simulation model, this possibility shall be taken in good account, since condensation of air humidity provides for a much higher heat transfer rate than sensible heat transfer. The mathematical solution becomes more complex, as condensation onto non-wetable surfaces, e.g. most polymeric surfaces, does not lead to continuous condensate films. Large, unstable, dry patches alternate with a few, smaller, weted ones.

Notes & References

[1] Images in the diagrams do not necessarily reproduce images of actual components.