Advanced Liquid Desiccant-Based Air Conditioning Systems


ALDACS operate essentially on Thermal Processes, where heat and mass transfer dominate (ventilators, pumps and control devices play a necessary, but secondary role).

Here we shall describe in detail these processes as they unfold in the components of ALDACS equipment, to create and maintain comfort conditions to live and work. Strictly speaking, we sahll consider only the thermal and hygrometric conditions required, leaving out any mechanical (e.g. local air velocity) and surrounding aspects (e.g. views, colours, presences, etc.).

As is well known, comfort conditions (plural) are not defined by a single set of state values: e.g. a moderately warm air is comfortable at low humidities, while a cold-dry air is not comfortable at all. These conditions can be bounded inside a limited region of a h-x Mollier Diagram, for example, and may be specific to different climate regions of the Earth.

hx Comfort

The bounds of a Comfort Region1 are defined by the ABCDE polygon.

Components of ALDACS — Concise Description

Heat & Heat and Mass Exchangers

Heat Exchangers in ALDACS are designed either as Countercurrent, or as Cross-Countercurrent types.

Heat and Mass Exchangers, e.g. Indirect Evaporative Coolers, have more complex flow arrangements, since they are 3- or more-stream exchangers.

Absorber & Desorber

The Absorber and the Desorber are special cases of the Heat and Mass Exchangers, as they are design as stacks of 5-stream Membrane Contactor Elements.

Furthermore, as stacks of membrane contactor elements, they can be arranged in parallel (for large air volume rates), in series (for high changes in the Humidity Content of the air), and series and parallel, when simultaneous large air volume rates and high changes in Humidity Content are required.

Fans & Pumps

Compact Fans, driven by EC Motors, contribute to high energy efficiency of Air Handling Units (AHU), particularly in the case of Autonomous Air Handling Units.

Pumps used in ALDACS are of essentially two types: — Conventional circulator pumps for the water circuits, and Magnetic-Coupled Pumps for the liquid desiccant loop.

Furthermore, the casing and impeller of the desiccant loop pumps ought to be of a corrosion-resistant polymeric material.

Chemical Storage Vessels

Desiccant at 'Low' and 'High' concentrations is stored in separated vessels, allowing for independent operation of the Conditioner and of the Regenerator. This is particularly useful when a driving thermal energy resource is of intermitent nature (e.g. Solar Thermal). It also enables an optimum management of that resource.

Components of ALDACS — Principles of Operation

1.   Heat Exchangers

HRHX type heat exchangers are used for internal heat recovery between air streams, in both the Conditioner and the Regenerator Blocks.

HRHX heat exchangers shall have a high heat exchange area density (compact heat exchangers), and provide for a high temperature effectiveness. One method of attaining these two objectives, while avoiding high ventilator energy requirements, consists in dividing the HRHX,s into three sections in series, where entry and exit sections operate in crossflow, and the central section in counterflow.

Under certain conditions, air humidity condensation may take place on one side of the exchanger.

This arrangement of the streams, imposes that one stream flows unmixed in the entry and exit sections, and mixed in the central section, while the reverse is true for the other stream.

A schematic of the stackable elements illustrates this concept.

DHRHX type exchangers are used for internal heat recovery between desiccant streams in the Regenerator Block.

These are in general counter-current shell-and-tube exchangers. The flowrates of desiccant are generally low, requiring comparatively large exchange surface areas.

2.   Heat & Mass Exchangers

IEC type exchangers in ALDACS fulfill two fundamental functions:

— Indirect cooling of the Supply Air (SUP)2;

— Generation of sufficient cooling water to cool the absorption process.

Evaporative cooling of water is done by non-adiabatic saturation of the return air (ETA). This process is illustrated in a h-x Mollier Diagram.

3.   Absorber

For the purpose of Air Conditioning in ALDACS, the Absorber operates as an Air Dehydrator or Dehumidifier. Air and Liquid Desiccant contact across a membrane, usually a microporous membrane, from which there are several types.
Micoporous membranes with round entrance pores are the most favourable, since they offer the highest resistance to pore innundation, at equivalent pore sizes. The pores of commercial membranes have a complex geometry. And while this geometry (mean equivalent pore diameters and mean flow path length "tortuosity") is determinant for the mass transfer process, the pore entrance size determines how stable the membrane is against innundation. Pore density and pore entrance sizes can be determined by various methods, and obey an almost normal distribution.

Membrane materials (e.g. e-PTFE and PP-H ) play an important role in the performance and service life of membrane contactors: Wetability and Crystall Melting Temperature of the Polymer are limiting characteristics.

4.   Desorber

For the purpose of Air Conditioning in ALDACS, the Desorber operates as an Air Hydrator or Humidifier. As in the Absorber, air and liquid desiccant contact across a membrane. Membranes used for the Desorber are usually of the same type as those used for the Absorber, although the operating conditions in the Desorber are mostly more severe that those in the Absorber (higher temperatures and higher desiccant concentrations).

5.   High Efficiency Fans

Compact, High Efficiency Ventilators, driven by EC motors, provide for energy savings where it really counts in Air Handling Units — Electric Power. ALDACS driving energy is essentially a low Thermodynamic Availability thermal resource, with temperatures in the 40 ... ~80 °C range. Mechanically driven components, such as ventilators and pumps, become absolutely important energy savers.

6.   Magnetic Coupled Pumps

The most promising liquid desiccants are aqueous solutions of alkali halides, such as Lithium Chloride (LiCl). Aqueous LiCl is extremely corrosive to most metalls and their alloys. It is thus necessary that all parts of equipment, that contact liquid desiccants directly, be manufactured so as to avoid any and all possibility of contacting the liquid desiccant with metallic parts, to ensure a long service life. Desiccant circulator pumps are particularly exposed. They should, therefore, be chosen with this requirement in mind.

7.   Armatures and Sensors

Armatures, such as valves and other parts are also continuously exposed to the liquid desiccant in Open-Absorption Systems. They ought to be manufactured of a chemically resistant polymeric material, e.g. PP-H, and shall be designed for continuous operation at suitable temperature and concentration.

Sensors in direct contact with the liquid desiccant ought to be protected by at least a thin layer of an adequate polymer, e.g. PTFE.

Notes & References

[1] Steimle, F. 1995. Energy Demand for the Conditioning of the Supply Air in Ventilation, Proceedings of the 16th AIVC Conference, Palm Springs, USA, September 19-22.

[2] DIN EN 13 779 Standard.