Experimental study on silica gel-LiCl composite desiccants for desiccant coated heat exchanger

Desiccant coated aluminum-fin is an important part in desiccant coated heat exchanger (DCHE). It will affect the overall performance of this novel solid desiccant component. In the paper, different silica gel-LiCl composite desiccant coated aluminum sheets were fabricated and some key parameters were studied. Nitrogen adsorption suggested that texture properties of composite desiccants were different from silica gel due to impregnated salt and combined action of silica sol and silica gel. The highest thermal conductivity of composite sheets was 5.8 Wm⁻¹K⁻¹, which is twice of silica gel coated one. Sorption kinetics indicated that composite sheets exhibited higher dynamic sorption quantities and faster sorption rates. Sorption isotherms were also obtained and fitted with Dubinin–Astakhov equation. Besides, dehumidification capacity of composite DCHE was tested and compared with silica gel one. Results indicated that enhanced dehumidification capacity can be achieved with use of composite desiccant in the DCHE system.     

Experimental investigation of solar driven desiccant air conditioning system based on silica gel coated heat exchanger

A silica gel coated heat exchanger based air conditioning system driven by the evacuated tube solar water heater has been experimentally investigated. The system has been operated for two different modes namely cooling with dehumidification mode and heating with humidification mode in summer and winter season respectively. The system performance is analyzed in terms of regeneration rate, dehumidification rate and thermal coefficient of performance (COP_th). Experimental results demonstrated that, for cooling and dehumidification mode, the process air is cooled by an average temperature of 8.5 °C. A better dehumidification rate can be achieved by using pre-cooling before dehumidification process. Post-cooling after dehumidification process is found to be advantageous for cooling capacity and COP_th. For heating with humidification mode, the process air is heated by an average temperature of 13.3 °C with an average increment in humidity ratio of 1.9 g/kg. It is found that the average COP_th of the system is 0.45 and 0.87 for cooling and heating mode respectively.     

涂覆MIL-101(Cr)干燥剂的翅片管换热器的实验研究

干燥剂涂层换热器(DCHE)是一种新开发的翅片管表面带有干燥剂涂层的换热器,它可以同时处理潜热和显热.为了获得更高的性能,高孔隙度的金属有机框架(MOF)被引入到设计中.本文通过水浴法成功合成MIL-101(Cr)材料,并对材料进行X射线衍射(XRD)、N2吸附-脱附测试、水蒸气吸脱附曲线、电镜扫描等系列物理表征.制作...     

Experimental study on a two-stage rotary desiccant cooling system

The objectives of this study were to evaluate the performance of a novel two-stage rotary desiccant cooling (TSRDC) system and to obtain useful data and experiences for practical application. Newly developed compound desiccant (silica gel–haloids) was adopted in the system. An experimental set-up was built and used to test the system performance under three typical environmental conditions. System performances were evaluated in terms of moisture removal D and thermal coefficient of performance COP_th. It has found that the required regeneration temperature of TSRDC system is low and COP_th of the system is high. Regeneration temperatures from 65 °C to 80 °C, 65 °C to 75 °C and 80 °C to 90 °C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study.     

Theoretical analyses of a new two-stage absorption-transcritical hybrid refrigeration system

In this context, a two-stage absorption-transcritical hybrid refrigeration system is proposed. R744 is chosen as a refrigerant for the transcritical heat pump subsystem and LiBr-H₂O working pair for the two-stage absorption refrigeration subsystem. Based on the mathematical and physical models, theoretical investigation is carried out on its performance. The main effects are discussed on COP_net (the ratio of cooling capacity powered by low-grade heat to the low-grade heat consumption for the hybrid system) and COP_mt (the ratio of cooling capacity powered by mechanical work to the mechanical work consumption for the hybrid system). Comparing with the normal two-stage absorption refrigeration system, theoretical results show that COP_net could be improved up to about 55% when the refrigeration temperature is 7 °C. In addition, COP_mt are more than 50% higher than that of the conventional transcritical refrigeration system. It is also found that both 45–55 °C low-grade heat and condensing heat could be used as actuating heat of the two-stage absorption refrigeration subsystem.     

离子改性的聚丙烯酸除湿换热器系统性能研究

干燥剂吸附性能对除湿换热器系统的除湿性能有重要影响。本文通过改进的ASAP2020气体吸附仪和恒温恒湿箱,先后对Na+和K+改性的聚丙烯酸聚合物的水蒸气平衡吸附性能和吸附动力学特性进行了测试研究,并与硅胶相对比。研究结果表明:平衡吸附测试显示Na⁺改性的聚丙烯酸在整个相对压力区间性能优于硅胶,K⁺改性的在高相对压力区间(>0. 8)逊于硅胶;基于Polanyi吸附势理论对干燥剂吸附特征曲线进行拟合,以用于后续的除湿换热器模拟研究;动态吸附性能表明聚丙烯酸干燥剂的吸附速率均低于硅胶,但Na⁺改性的聚丙烯酸整个吸附过程的动态吸附量仍多于硅胶;通过模拟除湿换热器除湿性能,在ARI summer、ARI humid和上海夏季这3种典型室外工况下,Na⁺改性的聚丙烯酸换热器可以从待处理空气中去除更多的水分,较硅胶换热器提升49%～118%,适用于室外湿度较小的气候工况。     

Investigation on capacity matching in a heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system

The heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system is promising recently because solution droplets can be prevented from crossing over into the process air. The quasi-isothermal processes are realized by two-stage dehumidification processes and the system performance is improved. In this study, a novel capacity matching index (CMI) is introduced to evaluate the energy capacity matching of the system through modeling study. It is found that CMI is usually lower than 1 under the typical hot and humid weather condition like South China and the demand and supply of energy in the system is mismatching. As inlet air temperature rises, the dehumidification rates, CMI, EER and COP all decrease. But CMI is almost constant with different inlet air humidity. The influence of air inlet temperature to dehumidifiers and regenerators on the system performance is also investigated. The higher the inlet temperature of dehumidifiers is, the larger the CMI, EER and COP are. The dehumidification rates and CMI both grow with an increase in the inlet temperature of regenerators. It is beneficial for energy balance of the system and high moisture loads, but the side effect is that the EER and COP both decrease.     

除湿换热器串联换热器强化除湿降温性能实验研究

除湿换热器可以同时处理显热与潜热负荷,但由于吸附热的影响,存在热湿负荷处理不同步及显热负荷处理能力不足的问题。本文提出了在除湿换热器后面串联一个显热换热器对空气进行二次处理,搭建了实验台对除湿换热器串联换热器情况下除湿降温过程的动态性能进行测试,并且在实验中分析了水温、进风温度、湿度、速度等主要参数对除湿量、降温量、制冷功率、COP的影响。结果表明:增加显热换热器可以大幅度增加处理空气的平均降温温差,在除湿初期阶段效果尤为明显,同时系统的制冷量也明显提高。此外,分析各参数对实验结果的影响可知,冷水温度与热水温度升高都可以有效提高系统制冷量与COP,空气的温湿度升高会提升系统性能,空气流速变慢对系统平均除湿量与有效除湿时间有明显的提升。     

Theoretical study on a frost-free household refrigerator-freezer

This study deals with a frost-free household refrigerator-freezer, in which frosting can be retarded by dehumidifying the air before it enters the evaporator of the refrigerator-freezer via a desiccant-coated heat exchanger. Because the desiccant can be regenerated via the condensation heat of the refrigerant (which is exhausted into ambient air in conventional household refrigerator-freezers), the proposed system can achieve high energy efficiency. Calculations show that the coefficient of performance of this system (COP) is within the range 1.5–2.5 at an ambient temperature of 15–35 °C. Moreover, it is found that the relative humidity of the refrigerator air (RH_RA) and the temperature of freezer air (T_FA) have a significant effect on COP: COP decreases by about 13% when RH_RA varies from 0.4 to 0.8 and by 10% when T_FA runs from −18 to −23 °C.     

Performance comparison between enthalpy recovery wheels and dehumidification wheels

A mathematical model of desiccant wheels is established and validated by experimental results. The different effects of substrate, air mixing, and rotation speed on the performance of enthalpy recovery wheels and dehumidification wheels are analyzed. Substrate with high heat capacity and low heat conductivity is recommended for enthalpy recovery wheels, while substrate with low heat capacity and low heat conductivity is suitable for dehumidification wheels. Air mixing should be considered in evaluating recovery wheels' performance but can be ignored for dehumidification wheels. For recovery wheels without considering air mixing, higher the rotation speed, higher the recovery efficiency; when considering air mixing, the optimal rotation speed is lower than the corresponding rotation speed at a 5% air mixing rate. There also exists an optimal rotation speed to obtain better dehumidification performance for dehumidification wheels. Meanwhile, the reasons behind the discrepancies between recovery wheels and dehumidification wheels are discussed.     

Characterization of desiccant wheels with alternative materials at low regeneration temperatures

A number of new desiccant materials have been proposed which have the potential to improve the performance of desiccant wheels being regenerated at low temperature. Desiccant wheels containing two such desiccant materials (zeolite and superadsorbent polymer) were compared with a conventional silica gel desiccant wheel. The superadsorbent polymer desiccant wheel achieved greater dehumidification than the silica gel wheel when dehumidifying high relative humidity air with low temperature (50 °C) regeneration air. The temperature of dehumidified air exiting the polymer wheel was also lower. The zeolite desiccant wheel was generally less effective at dehumidifying air and had a higher pressure drop.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.     

Performance analysis of desiccant dehumidification systems driven by low-grade heat source

If a desiccant dehumidification system can be driven by a heat source whose temperature is below 50 °C, exhaust heat from devices such as fuel cells or air conditioners can be used as its heat source, thereby saving energy. Therefore, in this study, we used a previously validated simulation model to determine the minimum heat source temperature for driving a desiccant dehumidification system. We considered four desiccant dehumidification systems that can be driven by waste heat—conventional desiccant-type systems (wheel type and batch type with only desiccant), a system with a precooler, double-stage-type systems (a type with two desiccant wheels and a four-partition desiccant wheel type), and a batch-type system with an internal heat exchanger. We found that among these systems, the last system can be driven by the lowest heated air temperature—approximately 33 °C—which is considerably lower than that of the conventional system.     

Use of compound desiccant to develop high performance desiccant cooling system

The paper is aimed to develop a high performance rotary solid desiccant cooling system using a novel compound desiccant wheel (DW). The unique feature of the desiccant wheel is that it can work well under a lower regeneration temperature and have a higher dehumidification capacity due to the contribution of the new compound desiccant materials. Experimental results indicate that the novel desiccant wheel under practical operation can remove more moisture from the process air by about 20–40% over the desiccant wheel employing regular silica gel. A mathematical model that is used to predict the system performance has been validated with the test results. By integrating the desiccant wheel with evaporative cooling, heat recovery and heating for regeneration sections, a solid desiccant cooling system can be formed. Simulation results show that because of the use of the new compound desiccant, the desiccant cooling system can work under much lower regeneration temperature and have a relative high COP, thus low grade thermal energy resources, such as solar energy, waste heat, etc., can be efficiently utilized to drive such a cooling cycle.     

Simulation based identification of the ideal defrost start time for a heat pump system for electric vehicles

Resistance heating with PTC elements to cover the heat demand of electric vehicles reduces significantly the cruising range at low outside temperatures. Reversible heat pump systems are one of the most promising solutions for this problem. However, in heat pump mode the frost formation on the exterior heat exchanger reduces the performance and efficiency of the system. Therefore, an efficient defrost method is crucial to benefit from the heat pump also under frosting conditions. In the present paper, a transient Modelica simulation model of a reversible CO₂-heat pump system with hot gas defrost was set up in order to assess the impact of different defrost start times. The model is able to handle frost growth on the exterior heat exchanger as well as defrosting. The simulation results showed an optimal point of time to conduct defrost at chosen operating conditions in order to maximize the average COP including the frosting and defrost period.     

Experimental and theoretical study on a novel double evaporating temperature chiller applied in THICS using R32/R236fa

A novel chiller with double evaporating temperatures is proposed in this paper, which can be applied in temperature and humidity independent control system (THICS). A zeotropic mixture R32/R236fa is selected as the refrigerant, and chilled water with two different temperatures is produced. The experimental coefficient of performance (COP_exp), theoretical coefficient of performance (COP_th), and second law efficiency (η) of the chiller are studied. The performance of the chiller is studied by varying the mass fraction of R32 in the R32/R236fa (W(R32)), chilled water temperature, and the flow rates of the heat transfer media (chilled water and cooling water). The results show that the high temperature chilled water (T_H,out) can be at 15–18 °C, and the low temperature chilled water (T_L,out) can be at 6–8 °C. When T_H,out is 17 °C and T_L,out is 7 °C, the maximum COP_th and COP_exp are 4.73 and 3.97, respectively. Second law efficiency, η, increases to 31% as W(R32) increases from 0.3 to 0.6.     

Effects of metal mass on the performance of adsorption heat pumps utilizing zeolite 4A coatings synthesized on heat exchanger tubesEffets de la masse métallique sur la performance des pompes à chaleur à adsorption utilisant des revêtements en zéolite 4A synthétisé sur les tubes de l'échangeur de chaleur

The effects of the wall thickness of stainless steel heat exchanger tubes on the performance of adsorption machines, employing zeolite 4A coatings synthesized on metal heat exchanger tubes, are investigated. A recently developed mathematical model is used to determine the cycle durations when various wall thicknesses of the heat exchanger tubes as well as different zeolite layer thicknesses are utilized. For each case, the power and the COP_cycle values of the system are estimated. In general, very high power and quite low COP_cycle values are obtained when the proposed arrangement is utilized in the adsorption heat pumps. The zeolite layer thicknesses that may result in obtaining high COP_cycle values are generally much higher than the optimum layer thickness value that maximizes the power and the utilization of layers thicker than the optimum value may lead to significant extensions in the cycle durations and hence to a decrease in the power obtained from the system. Decreasing the wall thickness of the heat exchanger tubes increases both the power and the COP_cycle values when the optimum zeolite layer thickness for each wall thickness is taken into account. The possibility of such an enhancement will most probably be limited by the minimum wall thickness value that can actually be obtained by the available technology. The COP values of adsorption heat pumps may also be increased by using regenerative processes. Due to the generally low COP values obtained, the proposed arrangement seems especially suitable to be employed in adsorption machines utilizing energy sources of low economical value, such as waste heat. An optimum compromise between the COP value, which is closely related to the operating costs, and the power of the system should be provided, in case more valuable energy sources are utilized.     

Experimental study on a new internally cooled/heated dehumidifier/regenerator of liquid desiccant systems

For providing good performance of dehumidifier and regenerator with certain dimensions, a new type of internally cooled/heated dehumidifier/regenerator based on the plate–fin heat exchanger (PFHE) was designed. To investigate the behavior of the new equipment, an experimental setup was established in an environment chamber with regulable temperature and humidity air. By the internally cooled dehumidification testing, effects of the cooling water temperature, the air flow rate and the desiccant temperature on the dehumidification performance and the cooling efficiency were presented. The behavior of internally cooled dehumidification process was compared with that of the adiabatic dehumidification process. The results suggested that the cooling efficiency decreased with the increasing of the cooling water temperature and desiccant with low temperature could bring more mass transfer coefficients. There is an optimal air flow rate to achieve the maximum absolute humidity decrease of the air. By the internally heated regeneration testing, effects of the air flow rate and the desiccant inlet temperature on the regeneration performance and air outlet parameters were discussed and also compared with those of the adiabatic regeneration process. It was concluded that the regeneration efficiency of internally heated regeneration was more than that of the adiabatic regeneration, and the internally heated regenerator could offer better thermal performance.     

Exergoeconomic performances of the desiccant-evaporative air-conditioning system at different regeneration and reference temperatures

This paper presented the exergoeconomic evaluation of the developed desiccant-evaporative air-conditioning system. The developed system was evaluated based on the steady-state conditions at different regeneration and reference temperatures. The exergoeconomic evaluation method was implemented to the system components and the whole system to evaluate the exergy efficiency, exergy destruction ratios, cost rates, relative cost differences and exergoeconomic factors. The regeneration and reference temperatures affected the exergy efficiencies, exergy destruction ratios, cost rates, relative cost differences and exergoeconomic factors. The desiccant wheel, heating coil and evaporative cooler had a high cost rate (investment cost, operation and maintenance cost, and exergy destruction cost). The exit air fan, outdoor air fan and evaporative cooler had a high relative cost difference. The exit air fan, outdoor air fan and secondary heat exchanger had a high exergoeconomic factor. Replacement of the desiccant wheel with a higher dehumidification performance could decrease the high cost rate. A higher efficiency evaporative cooler and heating coil were needed. Cheaper air fans (outdoor air fans and exit air fans) were needed.