تجزیه و تحلیل عملکرد از یک سیستم تهویه هوا غشایی خشک کننده مایع

A new membrane liquid desiccant air-conditioning (LDAC) system is proposed and investigated in this paper. Liquid-to-air membrane energy exchangers (LAMEEs) are used as a dehumidifier and a regenerator in the proposed membrane LDAC system, which can eliminate the desiccant droplets carryover problem occurring in most direct-contact LDAC systems. A parametric study on steady-state performance of the membrane LDAC system is performed using the TRNSYS energy simulation platform. The impacts of various climatic conditions and key system parameters on the system performance are evaluated. Results show that the proposed membrane LDAC system is capable of achieving recommended supply air conditions for productive, comfort and healthy environments if the key system parameters are effectively controlled. The system coefficient of performance (COP) at the design condition is 0.68, and the sensible heat ratio (SHR) for the dehumidifier lies in the range between 0.3 and 0.5 under different climatic, operating and design conditions. The proposed membrane LDAC system is able to effectively remove latent load in applications that require efficient humidity control.

The world energy consumption has increased significantly in past decades, due to population growth and economic development. Since air-conditioning (AC) systems make up 50% of building energy consumption [1], there should be energy efficient AC systems that are able to provide healthy environments with acceptable indoor air quality (IAQ) in order to improve the health and productivity of building occupants. Desiccant dehumidification AC systems show promise as energy efficient AC systems [2]. Although there are several advantages for the widely used conventional AC systems and they are able to effectively remove sensible loads within conditioned spaces, they are inefficient in terms of conditioning latent loads. When indoor humidity control is required in some cases, the cooling coil temperature in conventional AC systems must be lower than the dew point temperature of the process air stream in order to remove moisture by condensation. This results in wet cooling coil surfaces that may lead to the growth of mold and bacteria; consequently lead to undesirable health issues and poor IAQ within conditioned spaces [3]. Moreover, after moisture is removed from the process air stream, the overcooled air often needs to be reheated before it is supplied to the occupied spaces. A large amount of energy consumed in the overcooling and reheating processes makes conventional AC systems energy intensive [4]. It is clear that the latent load treatment is a challenge for conventional AC systems. Since latent load is dominant over sensible load in ventilation air in hot and humid regions, according to the ventilation cooling load index developed by Harriman et al. [5], efficient AC systems that can handle the latent load effectively are required. The aforementioned drawbacks of conventional AC systems can be avoided by using liquid desiccant air-conditioning (LDAC) systems. LDAC systems are considered as a promising alternative to other AC systems especially in the applications that require efficient humidity control such as: supermarkets, green buildings and greenhouses [6], [7] and [8]. Although many studies have been performed on this topic since the 1950s, most of them focused on direct-contact liquid-to-air conditioners [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and [19]. These systems have been found to be more energy efficient than conventional AC systems, but entrainment of desiccant droplets in the air streams is a significant drawback of the direct-contact LDAC systems. The carryover of liquid desiccant by the supply air stream can lead to the corrosion of downstream ducting and equipment which results in high maintenance requirements, short life cycles and high costs. In addition, desiccant carryover may affect IAQ within the conditioned space and health of occupants. These drawbacks have limited the widespread use of LDAC systems in civil/domestic applications [20]. Different liquid desiccant dehumidifiers that are able to overcome the droplets carryover problem have been designed and developed recently [21]. One design uses an internally cooled/heated (isothermal) low flow rate flat-plate exchanger that is able to significantly reduce or eliminate the carryover of droplets by the low-speed air stream [22], [23], [24] and [25]. Another design is the indirect-contact liquid-to-air energy exchanger, where the liquid desiccant and air stream are separated by semi-permeable membranes which eliminate the liquid desiccant carryover problem. The energy performance of using these membrane energy exchangers in a hybrid membrane LDAC system was studied by Bergero and Chiari [20] and [26], and it was found that energy savings may exceed 60% in humid climates compared to a conventional AC system. However, the characteristics of the membrane LDAC systems have not been extensively studied. The aim of the present study is to analyze the characteristics of a membrane LDAC system that uses flat-plate liquid-to-air membrane energy exchangers (LAMEEs) to serve as the dehumidifier and regenerator [27], [28], [29] and [30]. The proposed system is modeled using TRNSYS [31] and [32]. The performance of the system is investigated under different climate conditions (i.e. outdoor temperature and relative humidity), design conditions (i.e. number of transfer units and solution heat exchanger effectiveness) and operating conditions (i.e. liquid desiccant flow rate and temperature of solution entering the regenerator/dehumidifier).

In this study, the performance of a membrane LDAC system using two LAMEEs as the dehumidifier and regenerator was investigated under various climatic, design and operating conditions. It is found that the proposed membrane LDAC system is more energy efficient (has higher COPs) in hot and humid climates. Results show that the dehumidification capacity of the system can be effectively controlled via regulating the temperatures of the liquid desiccant solution entering the regenerator and dehumidifier. The performance of the system significantly improves when NTU increases until NTU = 10, while the performance improves only slightly when NTU is beyond this value. Also, increasing Cr* is beneficial for the cooling capacity and COPs of the system, as they increase over the entire range of the tested Cr*, especially when Cr* increases from 2 to 4. It is found that the COP and SHR of the proposed membrane LDAC system are 0.68 and 0.38, respectively, at the design condition. The proposed membrane LDAC system can be more energy efficient if low-grade heat-sources (e.g. solar energy or waste heat) are available to cover a portion or the whole of the thermal energy required for solution regeneration, as this will lead to the increase of COP and TCOP, and the reduction of heating equipment capacity. According to the results obtained in this study, it is recommended to set the solution inlet temperatures to the dehumidifier and regenerator at 15–20 °C and 45–55 °C, respectively, in order to achieve good performance for the proposed membrane LDAC system. The NTU and Cr* are recommended to be within 5–10 and 3–5, respectively. A significant reduction in operating costs may be achieved during certain conditions (i.e. higher NTU), but it will be accompanied by an increase in the capital costs. Therefore, both the capital and the operating conditions have to be considered in selecting the optimal values for these parameters, and the payback period should be evaluated based on the actual application and operating condition.