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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Renewable Energy, Volume 57, September 2013, Pages 101–110
A new type of outdoor air dehumidification processor using solid desiccant is proposed, in which a heat pump and square desiccant plates are combined. Each desiccant plate consists of an air channel with a honeycomb structure that is coated with desiccant material. The square desiccant plates change positions between the processed air duct for dehumidification and the regenerated air duct for regeneration. The cooling capacity of the heat pump is utilized to cool the processed air, and the exhaust heat of the heat pump is used to provide regenerative heat to the desiccant. Several stages can be combined together to gain higher efficiency. The proposed desiccant dehumidifier can achieve a low humidity ratio of the supplied air and provides low-temperature regeneration. A mathematical model is established to predict the performance of this desiccant processor, and the model shows good agreement with the experimental results. The factors that influence the performance of the processor are then analyzed in order to maximize performance. The simulation results show that the proposed desiccant processor provides regeneration at a low temperature (40–50 °C), and the COP can surpass 4.0 at different processed air inlet states.
Along with condensation dehumidification, solid desiccant dehumidification is commonly used in industrial and commercial buildings. The latter method utilizes the adsorption characteristics of solid desiccants such as silica gel, molecular sieves, and activated alumina to dehumidify the processed air. After adsorbing the moisture from the processed air, the desiccant must be regenerated before being reused. The regenerative heat can come from solar energy, steam, etc. Solid desiccant devices are classified into two main types: rotary desiccant wheels , ,  and  and desiccant beds , ,  and . A desiccant wheel is a rotor covered with solid desiccant material, which slowly rotates between the processed air and the regenerated air . During the dehumidification process, the processed air operates close to an isenthalpic process; thus, the outlet temperature of the processed air will be very high, and assisted cooling must be implemented to cool down the dried processed air before it is introduced into occupied spaces. The cooling source involves a mechanical chiller  and  and direct or indirect evaporative cooling modes ,  and . The advantage of desiccant wheels is that they can reach a lower and more constant humidity ratio of the supplied air compared to desiccant beds. However, the required regeneration temperature of desiccant wheels is very high (usually 80–130 °C), which hampers the utilization of waste heat. Jia et al.  adopted compound desiccant materials, which can absorb 20–40% more water than silica gel, to reduce the regeneration temperature to 80–90 °C. In two-stage desiccant wheel systems, cooling water from either direct cooling or indirect cooling systems is used to cool down the processed air after each stage of the desiccant wheel  and . Two streams of regenerated air enter separately into each stage and are discharged to the outdoor environment. The regeneration temperature of these two-stage desiccant wheel systems can be reduced to below 90 °C. Due to the strength of the rotary structure, the thickness of the desiccant wheel is difficult to reduce, and multi-stage desiccant wheel systems described in previous research have been limited to two stages . Desiccant beds represent another kind of desiccant dehumidification system in which a fixed bed is covered with desiccant materials. There are usually two beds, one for dehumidification mode and the other for regeneration mode, and the modes of dehumidification and regeneration alternate. The dehumidification process is discontinuous, and the humidity ratio of the supplied air always varies as the dehumidification process moves forward. The performance of inner cooling/heating desiccant beds appears to be superior to that of adiabatic desiccant beds. In inner cooling/heating desiccant beds, cooling fluid is supplied into the dehumidification bed, and hot fluid (50–70 °C) is supplied into the regeneration bed . The desiccant bed can be combined with a heat pump, in which the evaporator and condenser are coated with solid desiccant and serve as the dehumidification bed and the regeneration bed, respectively. A product that utilizes a heat pump system has been developed for home use , and its COP can exceed 4.0 using indoor exhaust air as regenerated air. However, the processed air duct and the regeneration duct have to alternate every 3–5 min, as do the evaporator and the condenser in the heat pump with four-way valves. This makes the system complex, and it suffers from the cooling-heating offset of the refrigerant inside the heat pump cycle as well. A new type of solid desiccant dehumidification processor combined with a heat pump is proposed in this paper. In this system, the square desiccant plates change positions between the processed air duct and the regenerated air duct. The cooling capacity of the heat pump is utilized to cool the processed air before it enters the desiccant plates, and the exhaust heat of the heat pump is used to provide regenerative heat to the desiccant. Multiple stages can therefore be combined together. The proposed desiccant dehumidifier can be operated easily, and a low regeneration temperature can be achieved. The heat pump system does not need to change its refrigerant direction or air ducts to meet the alternation demand of the dehumidification and regeneration modes, and it avoids the cooling-heating offset of the refrigerant inside the heat pump cycle. In this paper, the performance of this desiccant dehumidifier is experimentally and numerically analyzed, and the main factors that influence its performance are discussed.
نتیجه گیری انگلیسی
A multi-stage desiccant processor is proposed in this paper, which is suitable for areas with hot and humid climate. It integrates a heat pump system that avoids the need to switch from dehumidification mode to regeneration mode, which results in a low regeneration temperature. And the structure is simple which guarantee low investment and maintaining costs. The new model was examined numerically and then validated with experimental results. The main conclusions can be summarized as follows: 1) The mathematical model showed good agreement with the experimental results, with an error of less than 10%. 2) A low regeneration temperature can be achieved with the multi-stage design. The condensing temperature of the heat pump was about 50 °C, which is high enough to dehumidify the processed air from 22 g/kg to 10 g/kg with a COP greater than 4.0. 3) The factors that influence system performance were analyzed in detail, especially switching time and stage number. The optimal switching time was 3–5 min. Increasing the number of stages improved the dehumidifier's performance, but the improvement decreased as the number of stages increased. 4) The proposed multi-stage dehumidifier has a COP over 4.0. For the same humidity ratio of supplied air, the lower the inlet air temperature and the humidity ratio are, the higher the COP will be.