Performance analysis of four-partition desiccant wheel and hybrid dehumidification air-conditioning systemAnalyse de la performance d'une roue déshydratante à quatre segments et d'un système de conditionnement d'air à déshydratant hybride

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Abstract

A desiccant dehumidification system with air can decrease energy consumption because it can be driven by low-grade waste heat below 80 °C. If this system can be driven by low-temperature heat sources whose temperature is below 50 °C, exhausted heat from fuel cells or air conditioners that exist everywhere can be used as heat sources. This could lead to considerable energy saving. This study provides a detailed evaluation of the performance of a four-partition desiccant wheel to make a low-temperature driving heat source possible and achieve considerable energy saving by the simulation and experiment. Further, the study investigates the in-depth performance of a hybrid air-conditioning system with a four-partition desiccant wheel by simulation. As a result, it was clear that there exists an optimum rotational speed to maximize the dehumidification performance and that the hybrid air-conditioning system improves COP by approximately 94% as compared to the conventional vapour compression-type refrigerator.

Introduction

Since a desiccant dehumidification method can dehumidify air by converting latent heat into sensible heat, it is unnecessary to supercool and to reheat the air on a mechanical dehumidification system such as a compression-type refrigerator (ASHRAE, 2001). Therefore, this method attracts attention as the air-conditioning system from the view point of energy saving and amenity. In the normal desiccant dehumidification system (Harriman, 1994, Meckler, 1994), an approximately 80 °C heat source is directly required to dehumidify the outdoor air. If an electric heater or boiler is used as this driving heat source, it runs counter to energy saving. In order to solve this problem, we have investigated the desiccant wheel dehumidification method with the multistage adsorption and regeneration process to make it possible to use the low-temperature heat source that is approximately 40–50 °C from the compression-type refrigerator (Inagaki et al., 2004; Shibao et al., 2006).

As a result, it turned out that we could construct a highly efficient air-conditioning system by combining the multistage desiccant dehumidification system with the compression-type refrigerator. However, the system increased in size because of the multiple desiccant wheels. Accordingly, in a previous report (Shibao et al., 2007), we suggested the four-partition desiccant wheel to downsize multiple desiccant wheels. This wheel can realize the double-stage adsorption and regeneration process in only one desiccant wheel. Lately, Ge et al. (2008) also experimentally investigated its performance under various operation conditions for a one-rotor two-stage rotary desiccant cooling system. However, as it was not discussed theoretically, the performance of the four-partition desiccant wheel system was not totally evaluated considering driving heat source.

For the hybrid air-conditioning system with desiccant, Yadav, 1995, Dhar and Singh, 2001 and Jia et al. (2006) investigated a performance of a hybrid desiccant cooling system comprising the conventional vapour compression-type refrigerator coupled with a desiccant dehumidifier. However, as the conventional single stage desiccant was used for this system, the condensation temperature of the vapour compression-type refrigerator increases greatly. And, sometimes, an electric heater is also used to compensate the shortage of the driving heat. This system can't increase the system performance. To accomplish high performance hybrid desiccant air-conditioning system with only vapour compression-type refrigerator, it needs to decrease the driving heat source temperature for the desiccant regeneration. Hence, the four-partition desiccant wheel has been investigated to decrease the driving heat source temperature as hybrid system.

To construct the hybrid system with the four-partition desiccant wheel, this study investigates the in-depth performance of the four-partition desiccant wheel with the simulation and experiment to clearly evaluate the characteristics of a four-partiton desiccant wheel; effect of the regeneration temperature, rotational speed of desiccant wheel, cooling temperature, process air flow rate, and humidity of the ambient air. Moreover, to realize a high efficiency for the air-conditioning system and amenity in the room at the same time, we further discuss the performance of a new type of hybrid air-conditioning system that combines the four-partition desiccant wheel with the compression-type refrigerator by simulation as well.

Section snippets

Four-partition desiccant wheel and hybrid air-conditioning system

Fig. 1 shows a hybrid air-conditioning system that combines two conventional desiccant wheels with the compression cycle, and Fig. 2 shows the hybrid air-conditioning system with a four-partition desiccant wheel is divided by the equal area ratio on each flow path. The four-partition desiccant wheel used in the experiment is shown in Fig. 3. Fig. 4 illustrates the driving points of each cooling and dehumidification system, such as mechanical dehumidification

including a reheat process with the

Detailed structure and mathematical model of desiccant wheel, and model of other elements

Fig. 5 shows the detailed structure and model of the desiccant wheel. The structure of the wheel is usually realized by arrayal made up of a corrugated lamina and a plane that are sheets of the glass fiber impregnated with desiccant. The diameter of the desiccant wheel that is made of silica gel is 250 mm, and the thickness is 200 mm. These are the same as those of the actual desiccant wheel; HY-SG. Specifications of this desiccant wheel and property of desiccant are listed in Table 1. A

Experimental method of four-partition desiccant wheel

We have conducted an experiment and simulation in order to examine the actual performance of the four-partition desiccant wheel and investigated the validity of the simulation model with the experiment in detail by fully examining the effect of many parameters – the regeneration temperature, rotational speed, cooling temperature, process air volume flow rate, and ambient air humidity ratio – on the performance of the four-partition desiccant wheel. This is because the model and the detailed

Validity of experiment accuracy and definition of parameters

Fig. 9 for example shows the total dehumidification rate of the moisture in the dehumidification side and the regeneration rate in the regeneration process. From this figure, the experimental accuracy for the moisture balance of four-partition desiccant wheel is satisfactory level in less than approximately 10% error.

To show the simulation and experimental results, the following parameters are used: cooling temperature Tco is the temperature of the process air at the entrance of the desiccant

Simulation method of conventional compression-type refrigerator and hybrid air-conditioning system

The performance of the hybrid air-conditioning system with the four-partition desiccant wheel is compared with the conventional vapour compression-type refrigerator by the simulation. Fig. 15 shows the flow diagram of the conventional compression-type refrigerator as mechanical dehumidification, which reheats the process air by its condensation heat. The numbers in Fig. 15 correspond with the numbers in Fig. 4. To evaluate the performance of each system, pressure drops in the evaporator and the

Effect of SHF

Fig. 16 shows the effect of SHF on the characteristics and performance of the system, for instance, the refrigerant condensation temperature, the regeneration air temperature, the refrigerant evaporation temperature (the refrigerant evaporation temperature needed for the mechanical dehumidification is also shown), the heat in the condenser and the regeneration air, the compressor power, and COP (COP of the mechanical dehumidification is also indicated). In this simulation, SHF is changed by the

Conclusions

This paper evaluates the characteristics of the four-partition desiccant wheel by experiment and simulation, and the performance of the hybrid air-conditioning system that combines the four-partition desiccant wheel with the compression-type refrigerator by the simulation. The results are summarized as follows:

  • 1)

    Experimental results of the four-partition desiccant wheel were in good agreement with the simulation results even though the experimental conditions changed greatly. Therefore, the

Future plan

The pressure drop of the hybrid system with four-partition desiccant wheel is considered larger than the conventional system due to multiple air flow paths. Therefore, it is necessary to consider the pressure loss for the efficiency of the hybrid system. Now, we are investigating the influence of pressure drop. We will report a detailed effect of pressure drop on four-partition desiccant system as well as the conventional system.

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