Design and performance evaluation of a multilayer fixed-bed binder-free desiccant dehumidifier for hybrid air-conditioning systems: Part I – experimental

https://doi.org/10.1016/j.ijheatmasstransfer.2017.09.051Get rights and content

Highlights

  • A novel desiccant dehumidifier was proposed for energy efficient air-conditioning.

  • Sheet-type desiccant beds were designed for reduction of pressure drop.

  • M. S. Gel desiccant beads were fixed in metal meshes without polymer binders.

  • Dynamic heat and mass transfer performance of MFBDD was evaluated at various supply conditions.

  • Both a high dehumidification capacity and low pressure drop were achieved in MFBDD.

Abstract

A novel multilayer fixed-bed binder-free desiccant dehumidifier (MFBDD) was designed as part of this study, to be operated in combination with a conventional vapor-compression system to attain higher moisture removal capacity with lower electricity consumption. Silica-based high-purity spherical gel, Micro Sphere Gel (M. S. Gel), of 2.7 nm pore diameter and exhibiting an S-shape isotherm was used as adsorbents during the experiment. Transient adsorption, heat transfer, and pressure drop characteristics of the proposed device were evaluated experimentally for varying process air velocity, process air humidity, and desiccant bed thickness. The results revealed that while the pressure drop of the proposed device was approx. 98% lower, the average dehumidification capacity (during first 10 min of adsorption) was approx. 36% higher compared to a conventional desiccant wheel. Multilayer desiccant beds with adequate process airflow channel height and the usage of adsorbents without polymer binders in innovative sheet-type beds has led to such lower pressure drop and improved dehumidification capacity of the device.

Introduction

The primary objective of installing a heating ventilation and air-conditioning (HVAC) system in any type of building, either residential or commercial, is to maintain the indoor air temperature (sensible heat load) and humidity (latent heat load) within a certain range, for example, described in the ASHRAE Standard 55-2010: “Thermal Environmental Conditions for Human Occupancy” [1]. While temperature control is essential to provide higher thermal comfort for the building occupants, humidity regulation is pivotal to prevent “sick building syndrome,” which can adversely affect the health and productivity of the occupants [2]. Recently, indoor humidity regulation has become more critical as the new energy efficient buildings are highly favorable for the growth of airy habitats like molds, mildew, and other irritants owing to the tightly sealed and exclusively air-conditioned architecture [3]. In a conventional vapor compression system, the supply air is dehumidified in a cooling coil where the air temperature is lowered below its dew point. Such process lacks in efficiency resulting in poor moisture removal capacity and also consumes high electricity directly from the grid, which in a majority of cases accounts for more than 50% of total energy consumed by the building [4]. Therefore, developing highly efficient alternate HVAC technologies is instrumental to combat the ongoing energy crisis as well as to achieve the challenging low energy consumption goal of net zero energy building (ZEB) and net zero energy house (ZEH) [5].

Recently, desiccant based hybrid air-conditioners have emerged as a promising cooling technology since they can manage sensible heat load and latent heat load of supply air streams independently and thus can provide more precise control over humidity. In a hybrid air-conditioning system, the incoming moist air is first dried in a desiccant dehumidifier at a relatively higher temperature, and then, it is cooled in the evaporator section of a vapor compression system. Thus, such system eliminates the necessity of approaching the dew point which in turn reduces electricity consumption [6] and raises the thermal coefficient of performance (COP) [7]. As a result, there has been a growing interest in shifting from vapor compression systems toward desiccant based hybrid air-conditioning technologies.

Desiccants (either liquid or solid) are porous substances that can adsorb water vapor from moist air due to the water-vapor pressure difference between the surrounding air and desiccant bed surface. While liquid desiccants have lower pressure drop and regeneration temperature [8], they are more vulnerable to conditions that can acidify the desiccant, produce foaming, or precipitation into solid salts [9]. Crystallization of liquid desiccant solution stored at high concentration may also occur owing to decrease in temperature [10]. More severely, a majority of metal alloys interact with highly effective liquid desiccants, which may rapidly corrode the system components of HVAC plants [11].

Solid desiccant systems are advantageous owing to their compactness and lower corrosiveness [8]. Although it is feasible to design various configurations for solid desiccant dehumidifiers, e.g., rotary wheel [12], [13], [14], packed bed [15], and fluidized bed [16], [17], rotary wheels are most commonly used for HVAC applications as it can be operated continuously without altering the cycles from adsorption to desorption and vice versa [18]. Therefore, a great deal of research has been conducted in the last decade to improve the performance of rotary desiccant wheels. Angrisani et al. [14] experimentally investigated the effects of the rotational speed of desiccant wheel on adsorption performance and concluded that there exists an optimal rotational speed that maximizes the dehumidification capacity. Alili et al. [18] evaluated the performance of a desiccant wheel coated with a new type of porous zeolite aiming to augment the adsorption capacity of their system. Lee et al. [19] investigated the effect of integrating a desiccant wheel with a compressor based mobile air-conditioning (MAC) system and reported 26.3% less power consumption for their proposed design. However, a survey of the available literature revealed that the pressure drop across the desiccant wheel varied between 100 and 150 Pa [20], [21].

To obtain higher moisture removal capacity and enhanced thermal COP, staged cooling, e.g., two-stage desiccant cooling (TSDC) [22], one-rotor two-stage desiccant cooling (OTSDC) [21], and hybrid four-partition desiccant wheel [7], was investigated. To compensate for high adsorption heat release, other noteworthy concepts such as conjugate desiccant-heat pump system [23], [24] or desiccant-coated heat exchangers (DCHE) [25], [26], [27] were also proposed. Various numerical models were developed as well to predict the performance of desiccant wheel systems [13], [28], [29]. However, there are still several challenges need to be resolved prior to extensive adoption of desiccant wheels for commercial HVAC applications, such as high-pressure drop caused by the complex honey-comb structures of rotary wheels, inadequate dehumidification capacity owing to pore blocking effects caused by binders used to attach the desiccant substances within the wheels, challenges of integrating desiccant wheels with existing HVAC facility owing to their large size and lack of compactness, additional electrical energy consumed by the motor, and the noise produced by the rotating parts of the wheel.

While a large amount of research has been conducted on rotary desiccant wheels, only few experiments have been carried out to evaluate the performance of packed bed and fluidized bed desiccant dehumidifiers. Pesaran and Mills in Part-I [30] of their work modeled moisture transport in silica gel packed-bed based on solid side mass transfer resistance; and proposed a generalized diffusion equation to predict the transient performance of the bed. An experimental validation of the analytical model was presented subsequently in Part-II of their study [31]. Dupont and his co-workers [32], [33] experimentally studied the adsorption performance of silica gel and activated alumina with a solar-assisted packed-bed desiccant dehumidifier and revealed that silica gel was capable of transferring approximately 30% more water than activated alumina. However, the reported pressure drop for their system was approximately 196 Pa. Kabeel [15] experimentally investigated the effect of bed length on the adsorption/desorption performance of a multilayer desiccant packed bed and observed that an increase in bed length could enhance the adsorption/desorption rate; however, the pressure drop also increased owing to longer bed length. Although packed bed desiccant systems are more cost effective for residential air-conditioning applications over the desiccant wheel [34], they appear to be less appealing to HVAC designers owing to their large pressure drop caused by dense packing.

Fluidized beds are advantageous as they have a low-pressure drop and high heat and mass transfer rate; however, they cause air pollution as a result of mixing of desiccant particles and require extra power to fluidize the particles and maintain cyclic operation [34]. Sealability and air leakage are also critical drawbacks to be taken into account while using fluidized beds. Chen et al. [35] designed a funnel incorporated erect fluidized bed that could provide continuous cyclic operation between adsorption and desorption by using the fluidization energy of the particles and without the use of extra motor. Subsequently, Chiang et al. [34] modified the design of Chen et al. by introducing a circulating inclined fluidized bed (CIFB) with the aim of further augmenting the dehumidification capacity and lowering the pressure drop. However, the experimental pressure drop for their systems varied between 8183.64 and 8849.52 Pa/m.

In order to overcome the prevailing drawbacks of solid desiccant systems, in this study, we have introduced a novel Multilayer Fixed-bed Binder-free Desiccant Dehumidifier (MFBDD) using distinct silica-based high-purity micro spherical gels (M. S. Gel) of S-shape isotherm as adsorbents. MFBDD houses the adsorbents by means of two stainless steel meshes without the use of polymer binders; this configuration eliminates pore blocking effects and thus enhances dehumidification capacity. Moreover, multilayer beds are arranged parallel to each other to form air flow channels of adequate height between them to reduce the pressure drop. Unlike desiccant wheels, MFBDD provides noise-free operation without consumption of additional electricity since it does not require any additional motor due to having fixed beds.

A numerical model has also been developed to theoretically predict the performance of MFBDD, as presented in Part-II of this study [36]. This paper is the first part of our study, where we have experimentally investigated the transient adsorption, heat transfer, and pressure drop characteristics of the proposed design by varying the inlet air velocity, inlet air humidity, and desiccant bed thickness. The article is divided into sections as follows: Adsorption characteristics and other physical properties of M. S. Gel are briefly discussed in Section 2. Details of the MFBDD prototype, test facility, and test procedure are introduced in Section 3. The experimental outcomes as well as the adsorption performance indicators are the focus areas of Section 4, and Section 5 provides the concluding remarks.

Section snippets

Adsorption characteristics of M. S. Gel

In a typical desiccant dehumidifier, the adsorption/desorption process is mainly governed by the water-vapor pressure difference between the desiccant surface and incoming moist air. Therefore, a desiccant having S-shape isotherm (type IV) is highly suitable for similar applications as it exhibits rapid uptake and release of water vapor within a reasonable range of relative humidity. The M. S. Gel used in this study (manufactured by AGC Si-Tech Co., Ltd., Japan) also has a similar type of

Design and fabrication of MFBDD prototype

The concept of the desiccant bed incorporating wide process air flow channels lined with desiccant plates is extant in the literature. Gidaspow et al. [37] constructed a 0.71 mm thick desiccant plate using a Teflon web placed in a 3.1 mm wide air flow channel. However, owing to the higher resistance to mass transfer between the desiccant plate and flowing air, proper dehumidification could not be obtained [38]. Therefore, in this study we have introduced a novel multilayer desiccant dehumidifier (

Performance indicators

The performance indicators used in this study to quantify the transient adsorption and heat transfer performance of MFBDD are defined below:

Conclusion

A simple albeit effective solid desiccant dehumidifier was designed that is suitable for use in conjunction with a vapor-compression system to obtain enhanced moisture removal capacity in an energy efficient manner. The proposed device demonstrated better adsorption and pressure drop characteristics compared to extant solid desiccant systems. While the pressure drop of the proposed device was low owing to adequate process air flow channel heights that lessens the flow restrictions, the improved

Conflict of interest

The authors declared that there is no conflict of interest.

Acknowledgements

This work was supported by Asahi Glass Co., Japan, and JST-CREST program “Phase Interface Science for Highly Efficient Energy Utilization.”.

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