P. Kohlenbach*, D. Rossington and A. Weigand

1 CSIRO Energy Technology, PO Box 330, Newcastle, NSW, 2300, Australia
Corresponding Author, paul. kohlenbach@csiro. au

Abstract

CSIRO Energy Technology is developing a small-scale desiccant-based air-conditioning system for residential applications. In this context, a desiccant wheel made of a novel material has been experimentally tested for its dehumidification performance. The material is an iron-alumino-phosphate zeolite with an AFI structure and traded under the name of FAM Z-01. A 300mm diameter desiccant wheel was tested under varying inlet conditions of temperature and humidity with regard to its dehumidification performance. It was found that for constant regeneration humidity the maximum moisture removal capacity of the material is 17 grams of water per kg dry air at 50°C regeneration temperature and 24 grams of water per kg dry air at 80°C regeneration temperature from an inlet air stream of 40 °C and 95% relative humidity. At supply inlet temperature between 10 and 30°C and supply inlet relative humidity between 20 and 50% it was found that the difference in moisture removal at a regeneration temperature of 50 °C and at 80 °C is around 1 g/kg d. a.. At varying regeneration humidity (matching ambient conditions) it was found that the moisture removal is considerably lower, even though the regeneration air is supplied at the same temperature. Maximum moisture removal was 5.1 g/kg d. a. and 14.5 g/kg d. a. for supply inlet conditions of 40°C/95% RH at 50 degC and 80 degC regeneration temperature, respectively.

Keywords: Dehumidification, desiccant wheel, zeolite, FAM Z-01

1. Introduction

CSIRO is currently developing a solar-powered air-conditioning unit for residential houses, using a desiccant-evaporative process to provide cool and dehumidified air. This process is very well suited for the recovery of low-grade solar energy or waste heat. Thermal energy and water are used to provide air-conditioning, hence consuming only a very small amount of electrical power. As part of the development CSIRO is testing novel desiccant wheel materials for dehumidification purposes. The two most common materials for desiccant wheels are silica gel and LiCl due to their low cost and ease of handling. They are however limited in their moisture removal capacity, especially at regeneration temperatures below 80 degC. Recently researchers and manufacturers have been developing advanced materials to increase the moisture removal capacity and hence to allow for smaller unit size.

Jia et. al. [1, 2] describe a comparison between a novel composite desiccant wheel made of silica gel and lithium chloride and a conventional wheel made of silica gel only. They found that the composite wheel has a greater moisture removal capacity compared to the silica gel wheel, especially at lower inlet humidity. The regeneration temperature of the composite wheel was found to be lower than that of the pure silica gel wheel. Tokarev et. al. [3] have analysed a composite sorbent based on CaCl2 as an
impregnated salt and MCM-41 as a host matrix. At regeneration temperatures between 70 and 120 degC the moisture removal capacity of the composite was greater than of silica gel. Cui et. al. [4] investigated the properties of DH5, DH7 and 13x adsorbents with regard to their use in desiccant cooling systems. Their results show that DH5 and DH7 adsorbents have greater moisture removal capacity than silica gel when tested at a regeneration temperature of 100 degC. Restuccia et. al [5] also investigated a composite sorbent SWS-1L, a mesoporous silica gel KSK impregnated with CaCl2. It showed a promising moisture removal capacity of up to 0.7 g of water per 1 g of dry sorbent at regeneration temperatures of 90-100°C. Kakiuchi [6] presented the FAM-Z01 material used in this work as an application for adsorption heat pumps and proposed the application for desiccant wheels. This application was further investigated by Oshima et. al [7] who evaluated the performance of a desiccant rotor containing FAM-Z02 zeolite material. Various regeneration temperatures and air inlet conditions have been investigated in a parameter study. The moisture removal of the FAM-Z02 rotor was found to be 11-22% higher than that of a silica gel rotor at regeneration temperatures of 50-70°C.

One important aspect of using adsorbents in a solar cooling system is the long-term stability of the desiccant. Earlier investigations by Belding et. al [8] have shown that silica gel and 13x adsorbents can lose up to 63% and 13%, respectively, of their original moisture removal capacity after 50,000 cycles. The FAM Z-01 material used in this work has been tested by the manufacturer and has shown a 5% capacity loss after 50,000 cycles [6].

2. System and methodology

The experimental testing has been undertaken at CSIRO’s Energy Technology site in Newcastle, Australia. Figures 1 and 2 show the test rig used for experimental purposes.

As shown in Figure 1 and 2, the test rig consists of two separate conditioned air streams, one for desiccant wheel supply, and the other for desiccant wheel regeneration. Each of these two air streams enter via an intake filter (item 1 of Figure 1), and is then pressurised by a centrifugal fan (item 2) which is controlled by a variable speed drive to enable air volume control. Peak flow of 1000m3/hr is achievable with a 300Pa pressure drop across the desiccant wheel. The air stream then passes through two cooling coils. The first coil (item 3) is cooled with a 2°C chilled glycol solution capable of dehumidification of the leaving air stream to a moisture ratio of approximately 7 g/kg dry air. The second cooling coil (item 4) is cooled with a -5°C chilled glycol solution capable of further dehumidification of the leaving air stream to a moisture ratio of approximately 4 g/kg dry air. The dehumidified air stream then passes through a primary electric heater bank (item 5). This heater bank is capable of heating the air stream to 90°C in the case of the regeneration air stream, and 40°C in the case of the supply air stream. The air stream is then humidified as required using a steam injection lance (item 6). Low pressure steam at 1.5 bar is injected in the air stream via nozzles at a rate of up to 45 g/kg dry air. Finally the air stream passes though a secondary electric heater bank (item 7). The secondary heater bank is of similar capacity as the primary heater bank allowing for load sharing and fine temperature control. The supply and regeneration air streams are then ducted to the test desiccant wheel. This can be done in counter flow and parallel flow arrangement. The supply and regeneration air streams leaving the desiccant wheel are ducted from the wheel and exhausted outside. Temperature and Humidity are measured and recorded after each of the control elements described above. The temperature and humidity entering and exiting the desiccant wheel is calculated by averaging a number of sensors distributed over the cross section of the ductwork entering and leaving the desiccant wheel. Volume flow rate of the supply and regeneration air streams is calculated from velocity measured at the entering side of the desiccant wheel.