M. Jaradat*, R. Heinzen, U. Jordan, K. Vajen

Institut fur Thermische Energietechnik, 34109 Kassel, Germany Corresponding Author, Jaradat@uni-kasselde

Abstract

Liquid desiccant systems are used for comfort and industrial air-conditioning and represent a promising alternative when driven by solar heat, to vapour compression machines usually employed in these applications. A novel design of the most important component, the internally heated regenerator, is presented and experimentally examined in this paper. The main focus for reengineering the regenerator is to avoid carry-over of the salt-solution to the air which is accompanied with both, corrosion and environmental impact problems. Furthermore, an important issue is to realize an even distribution of the liquid desiccant inside the regenerator.

The experiments presented here will therefore concentrate on the liquid desiccant (LD) distribution system inside the regenerator which consists of perforated plexiglass tubes with different, equally spaced, throttling-points and a wick material attached to the transfer area between the desiccant solution and the air stream. In order to determine the evenness of distribution, experiments were conducted with different combinations of the throttling-points diameters with a range between 0.5 mm-0.9 mm, varying the LD volume flow in different ranges between 0.3 l/min-1.2 l/min. A second set of experiments concentrated on the distribution behaviour of different types of textiles (cotton, viscose, polyamide, polyester and wood-pulp based textiles). Each type of the textiles has been tested to measure the absorption capacity and the diffusion speed by simply pouring a quantity of lithium chloride (LiCl) onto the upper surface of a taut piece of textile, and measuring the time needed for the LiCl droplets to be completely absorbed by the textile fibers. Each type has been tested in both, dry and wet state. Then polycarbonate plates were coated with these textiles and exposed to the salt solution throttled through the perforated tube. A violet fluorescent light has been used to support the visual inspection of the LD diffusion through the textile fibers.

A factorial design analysis was carried out for the results gained by testing the perforated tube. The analysis shows the optimal throttling-points diameter and the optimal spacing that will give the optimal conditions for an even distribution over the whole length of the tube. Furthermore, the analysis shows the minimum volume flow rate that might be obtained and fulfils a fairly evenness distribution. The second set of experiments revealed that new fibres have the best absorption and diffusion characteristics among the tested textiles compared to the traditional textiles currently in use such as cotton. The obtained results are implemented to build a LD regenerator that will be used for air conditioning applications.

Keywords: Desiccant, carryover, factorial design analysis, throttling-points.

1. Introduction

Conventional vapour-compression air-conditioning systems are completely powered by electricity, which is often accompanied by peak load charges, carbon dioxide emissions into the atmosphere,

since the generation of electricity involves most often the utilization of fossil fuelled power plants, as well as high operational costs.

The main disadvantage of vapour-compression air-conditioning systems is that it is considered as an inefficient thermodynamic process. The handling of the latent load part requires cooling the air below its dew point which leads to an air temperature that is colder than the temperature needed to meet the sensible load. Thus, reheating the air is necessary.

Liquid desiccant air-conditioning systems remove the latent load directly from the air by absorbing the moisture by a hygroscopic salt solution e. g. lithium chloride (LiCl), calcium chloride (CaCl2). The main components of an open-loop liquid desiccant air-conditioning system are the absorber (dehumidifier) and the desorber (regenerator) shown in Fig. 1.

In the absorber, moisture absorbed from the conditioned air stream dilutes the desiccant solution loading the desiccant with water vapour. The dilute solution is reconcentrated in the regenerator, where it is heated to elevate its water vapor pressure. A scavenging air stream, usually ambient air, contacts the heated solution in the regenerator. There, water evaporates from the desiccant solution into the air and the solution is reconcentrated. A high impact on the system’s performance results from the design of the regenerator as the driving heat is consumed in this component.

In the literature concerning liquid desiccant systems, the most examined type of both, the absorber and the regenerator is the adiabatic heat and mass exchanger. A typical representative of an adiabatic heat and mass exchanger is a packed bed with both, regular and random structures. The history of packed bed heat and mass exchangers used in liquid desiccant systems dates back to

1930’s when Kathabar Inc. [1] produced the first LiCl system, primarily for industrial applications. Despite the intensive research that has been conducted to develop these systems, there are still disadvantages of packed bed structures in absorbers and regenerators, the main ones are high pressure drop, high auxiliary energy consumption, high liquid to air ratio, flooding risks and the entrainment of desiccant mists into the air stream are the main disadvantages of packed bed structures in absorbers and regenerators.

Newer studies [2-4] favoured internally heated regenerators designed with a parallel plate structure to obtain regular cross-sections for the air flow to prevent carry-over and to reduce essentially the desiccant flow rate. Out of this design the need for a sophisticated distribution system emerges as the low desiccant flow rate needs to be equally distributed over a large area. Bi-sectional or nozzle distributors have been proposed so far. They promised good results in the investigated prototypes but had problems during longer operation as crystallization of salt particles in the distribution channels occurred or air entered the distribution system.

Uneven horizontal distribution of the liquid desiccant as it enters the generator is undesirable because it reduces the effective area of contact between the liquid desiccant and air and thus decreases the mass transfer and heat exchange between the liquid and vapour. To ensure proper operation of the generator, it is also important to ensure that the ratio of liquid to vapour is constant over the cross-section of the plates. For this reason, it is important to have an even distribution of liquid as it enters the generator. A need has thus developed in this paper for a liquid distributor that is capable of facilitating uniform LD distribution at low flow rates.

A further part of the distribution system consists of textiles which are attached to the plate. Former studies revealed the necessity for closer investigations as the used materials e. g. cotton fleece did not distribute the desiccant as a uniform film over the vertical plate but formed runlets on the surface.