Investigation on solar assisted liquid desiccant dehumidifier and evaporative cooling system for fresh air treatment
Introduction
The semi-centralized air conditioning (A/C) system consists of independent fresh air system and fan coil system are more widely adopted in office buildings, hotels and some commercial buildings compared with all-air/centralized A/C system because of its simplicity, flexibility, economy and easy-controllability. The energy consumption for operating A/C system is huge, especially for treating the hot and humid fresh air [1]. It is reported that 20%–40% of the overall building energy consumption is consumed by fresh air handling process [2]. Generally speaking, the lower the fresh air ratio, the smaller the total energy consumption. However, in recent decade, much more attention is being paid to the topics of improving indoor air quality (IAQ) since the large-scale outbreak of SARS virus and SBS (Sick Building Syndrome) in air-conditioned buildings [3]. Thus, there is contradictory between IAQ improvement by increasing fresh air ratio and energy conservation by reducing fresh air ratio. A high efficient fresh air handling scheme is urgently expected to balance the contradictory, in other words, to cool and dehumidify fresh air with less energy consumption.
The hybrid liquid desiccant dehumidification and regenerative indirect evaporative cooling system (LDD-RIEC) is therefore proposed as a promising energy-saving scheme for fresh air treatment in a semi-centralized A/C system. The hot and humid fresh air is firstly dehumidified by a LDD and then sensibly cooled by RIEC. In a LDD-RIEC system, there is no electricity-intensive compressor but only low-energy-consumed solution pumps and water pumps. Therefore, the energy consumption of dehumidification and refrigeration is much less than a conventional Mechanical Vapor Compression Refrigeration (MVCR) system. The heat captured by solar collector is used for desiccant solution regeneration, which further improves the system's efficiency.
Under the trend of energy saving worldwide, the A/C system incorporated with dehumidification and evaporative cooling technology has drawn great research attentions in recent years [4], [5], [6]. Overall, the hybrid A/C system can be classified into solid desiccant-enhanced MVCR system [7], [8], [9], liquid desiccant-enhanced MVCR system [10] and hybrid desiccant + evaporative cooling system [11]. Solid desiccant is usually used in a desiccant wheel which is compact but has high pressure drop [12], [13]. Liquid desiccant has lower regeneration temperature and pressure drop but has corrosion problem [14]. Evaporative cooler (EC) can be classified into direct evaporative cooler (DEC) and indirect evaporative cooler (IEC) base on whether the primary air has contact with water. The IEC includes traditional plate type and tubular type cooler and advanced dew-point cooler [15]. The dew-point IEC, includes RIEC, M-cycle IEC and multi-stage IEC, is able to cool the primary air below the web-bulb temperature of inlet working air, and close to its dew point temperature.
The hybrid system, in its various aspects, has been intensively investigated theoretically and experimentally. The reported works are related to feasibility study, regional applicability study, performance prediction, parameter optimization and new material's development. The desiccant-enhanced MVCR system is proved to be superior to the conventional MVCR system in terms of energy saving and possibility of low-grade energy utilization [16]. The research shows that the system performs the best in high humid areas [17]. Among all hybrid A/C systems, the hybrid desiccant and evaporative cooling system seems to be the most promising one because it can handle both the sensible load and latent load without using MVCR system. La et al. [18] experimentally studied a novel rotary desiccant cooling system consisted of two-stage dehumidification and regenerative evaporative cooling. The system can produce 15–20 °C high temperature chilled water and dry air at the same time. Enteria et al. [19] evaluated the desiccant-evaporative A/C system using the exergetic method. Percentage contributions of exergy destruction of system components at different regeneration temperatures and reference temperatures were determined. El Hourani et al. [20] optimized the design and operation of a 100% fresh air A/C system consisted by a solid desiccant dehumidification system and a two-stage evaporative cooling system. Regeneration temperature, air flow rate and air fraction entering the evaporative cooler are optimized with respect to energy and water consumption. Table 1 lists out some other representative studies related to the hybrid desiccant and evaporative cooling A/C system.
It can be seen that most existing studies related to modeling of hybrid liquid desiccant and evaporative cooling system are based on the multi-parameter fitting formula of components. Although accurate differential equation models have also been used in some studies, they were mainly open-loop based system simulation, which not consider the close-loop relationship of solution temperature and concentration between the dehumidifier and regenerator. In addition, in previously reported studies, only 100% fresh air A/C system or all-air A/C system was discussed, without much attention paid to the semi-centralized A/C system. Actually, the semi-centralized A/C system, which delivers cooling by both air and water, is much more widely used in hotels and office buildings with the high demand for flexibility and easy-controllability. Therefore, a LDD-RIEC semi-centralized A/C system is proposed. The fresh air treated by LDD-RIEC system can remove part of indoor cooling load. The rest of cooling load is handled by return air which is cooled by fan coils using chilled water from a MVCR system. Besides, the existing research on optimal extraction ratio of RIEC focuses on a stand-alone RIEC system [26]. The optimal extraction ratio in hybrid LDD-RIEC system considering the interacted influence of dehumidifier, regenerator, RIEC and solar collector has received few research attentions.
In this paper, the solar assisted LDD-RIEC semi-centralized A/C system is studied. Firstly, the working principles of the solar assisted LDD-RIEC semi-centralized A/C system is reported. Then, the development of the system model is presented by detailing the sub-models of each component. By using the validated model, a typical air handling process can be illustrated in the Psychrometric chart. The extraction air ratio of RIEC is optimized and the influences of solar collector area and inlet air conditions on system performance are analyzed. Finally, the energy saving ratio is quantitatively evaluated with respect to a conventional MVCR system under various operating conditions.
Section snippets
LDD-RIEC semi-centralized air-conditioning system
Fig. 1 shows the system diagram of a LDD-RIEC semi-centralized A/C system. It consists of an independent solar-assisted LDD-RIEC system for fresh air treatment and a fan coil unit (FCU) for return air treatment. The RIEC is used in the proposed system because of its much higher cooling efficiency compared with traditional IEC although a part of air is sacrificed as secondary air [27]. Besides, the price for producing additional dehumidified air is low because the driven heat is ‘free’ solar
Model and validation
Each component model was established as follows to facilitate the system simulation.
Air handling process
As previously mentioned, the FCU could be operated under dry-coil and wet-coil conditions. In this section, an example of air handling process of LDD-RIEC semi-centralized A/C system under FCU dry-coil condition is illustrated in a Psychrometric chart. The total sensible load is set to be 1239 W and total latent load is 585 W. As shown in Fig. 6. The fresh air (30 °C, 20 g/kg) is firstly dehumidified by the LDD, then pre-cooled by HE4 and finally supplied to RIEC. In the RIEC, 30% of the outlet
Conclusion
The solar assisted liquid desiccant dehumidifier and regenerative indirect evaporative cooling (LDD-RIEC) semi-centralized air-conditioning (A/C) system was investigated in this paper. The system model was developed by solving the heat and mass transfer equations of each component integrally in a closed loop. The main results are as follows.
- 1.
The proposed system can provide supply air of 17.2 °C and 9.8 g/kg when ambient air is 30 °C and 20 g/kg.
- 2.
The optimal extraction air ratio of RIEC is 0.3
Acknowledgement
This research is financially supported by the Research Institute of Sustainable Urban Development of The Hong Kong Polytechnic University and the Housing Authority of the Hong Kong SAR Government with account No. K-ZJK1.
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