تجزیه و تحلیل عملکرد سیستم خنک کننده چرخشی جذب چندبستر پیوسته
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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Applied Thermal Engineering, Volume 25, Issues 2–3, February 2005, Pages 393–407
A multi-bed adsorption rotary cooling system can make use of the concept of heat regeneration cycle and recently developed technology of heat enhancement in adsorbent bed. In the present study, the performance parameters such as COP and specific cooling power (SCP) of the rotary system are initially analyzed based on heat recovery temperature difference and counter-flow heat exchanger, and meaningful parameters related to the heat transfer fluid (HTF) and the system design are identified. Then, an integrated thermodynamic and heat transfer model of the system is developed and used to evaluate the effect of important design and operational parameters. Results show that the performance of system with certain adsorbent/adsorbate pair is sensitive to Reynolds and Prandtl number of heat transfer fluid. Also, the module number and module heat transfer area play an important role in improving the system COP and SCP. This study shows that, the performance of the system could be improved further by optimizing the cycle time.
Adsorption refrigeration systems present the advantages of being absolutely benign for the environment: zero ozone depletion potential (ODP) as well as zero global warming potential (GWP). Compared to the existing absorption systems, adsorption systems can be built in small scale which can be operated with no moving parts, without any need for a rectifier or solution pump. Also, such adsorption systems are corrosion free. However, certain disadvantages of adsorption systems have become obstacles for real mass production commercialization. They are: (1) long adsorption/desorption time; (2) small refrigeration capacity per unit mass of adsorbant, i.e., low SCP; (3) low COP. Meunier  has summarized the performance of various solid sorption systems based on closed cycles, and has concluded that if cooling COPs of approximately 1, with cooling rates between 300 and 1000 W/kg of sorbent can be obtained, such systems will most probably be an alternative to CFCs for refrigeration. To enhance the performance of adsorption system, recent research focus on two aspects: (i) to enhance the heat and mass transfer in the adsorbent bed; (ii) to apply new cycle which can recover more heat. Several researchers have studied the newly patented rotary system  and , which can achieve more heat recovery, and also can make use of the recently developed technology on heat and mass transfer enhancement. Llobet and Goetz  have proposed a thermodynamic model on such rotary system for the continuous operation using the concept of heat regeneration in steady state. The energy performance of the continuous system is measured in terms of COP and cold production capacity. It is reported that the number of transfer units (NTU) of the heat exchanger is the main parameter that conditions the value of COP. A similar study has been carried out by Critoph  who has developed a one-dimensional radial conduction model to simulate a continuous multiple-bed regenerative adsorption cycle. By using simple governing equations, the performance of the system consisting of 32 tubular adsorption modules has been predicted. The effect of key parameters such as thermal capacity ratio, number of modules, generator heat transfer coefficient, evaporator air inlet temperature on the system performance has been also studied. However, some assumptions on which their model is based are so ideal that can not reflect the realistic situation, such as that the heat transfer coefficient of heat transfer fluid is independent from the fluid velocity and the contact thermal resistance is neglected. Another problem is that though NTU is used as a characteristic parameter in analyzing adsorbent bed  and , it is an indicative parameter of the size of the heat exchanger, which includes the combined influence of heat transfer coefficient and the velocity of heat transfer fluid, and cannot be varied independently. Hence, NTU cannot be considered as an independent value which influence the performance of the system in terms of individual parameters. In this paper, the performance parameters COP and specific cooling power (SCP) are analyzed based on temperature difference during heat recovery and cycle time, to identify the important parameters that influence the system performance. Then, an integrated thermodynamic and heat transfer model of the system is developed and used to evaluate the effect of the identified parameters.
نتیجه گیری انگلیسی
The multi-bed adsorption rotary cooling system using the concept of heat regeneration cycle, is proposed to attain the objective of enhancing the performance of adsorption system. In this paper, the important parameters that influence the system performance with active carbon/ammonia as working pair are first identified, based on energy balance and counter-flow heat exchanger performance. The analysis shows that besides the detail geometry of the system, the Re, Pr, actual heat transfer area, cycle time, and number of modules are important parameters that influence the COP and SCP of the system. An improved numerical model is developed to study their influence on the rotary system and results indicate that there exists optimal values of Re number, number of modules, cycle time to obtain maximum COP being around 0.65–0.7 and SCP being above 180 W/kg adsorbent. A heat transfer fluid with low Pr number is recommended for multi-bed adsorption rotary cooling system so that the system could act as a counter-flow heat exchanger. The fins effect is significant in certain range, however, when the effective fin area increases beyond 16 times of the smooth tube area, the fin has a negative effect on SCP.