Combined Heat and Mass Transfer Characteristic in Air Handling Process Using Liquid Desiccant wVV'9pw}
Liu Xiao-hua 8@Y]dz gjj
ABSTRACT [ 7Q |vu
Liquid desiccant air-conditioning system, in which air can be processed by directly contacting liquid desiccant, is one of the most efficient ways to deal with latent load (or moisture) in buildings and realizes a separately control of temperature and humidity. Comparing with conventional air handling process, the liquid desiccant air-conditioning system can dehumidify air without cooling down to its dew point. Meanwhile, counteraction cooling and heating can be avoided, which usually happened in conventional air handling process and led to huge energy waste. In addition, during the air handling process no condensation water separates out of the air in the liquid desiccant air-conditioning system, thus, avoid fungi thrives in moist environment of traditional air handling process. Not only being applied in dehumidification process in summer, the system can also realize humidification process in winter conditions efficiently. Furthermore, liquid desiccant system can be driven by low grade energy, such as solar energy and waste heat. ,Q 2` N{f
The liquid desiccant system has already been applied for more than 70 years. However, due to unsolved problems existing in the air handling process which leads to a poor energy performance, the traditional liquid desiccant system was far from wide application. The study of this thesis focuses on the most elementary heat and mass transfer processes between air and liquid desiccant, especially dealing with the coupling impact of heat and mass transfer processes, which will contribute greatly to improve the energy performance, indoor air quality and promote liquid desiccant air-conditioning system in practices. #.#T+B+9
The main works of this dissertation are shown as following: S'vrO}yU
Firstly, the combined characteristic of heat and mass transfer processes between air and liquid desiccant is carefully investigated. Heat and mass transfer models at parallel-flow, counter-flow and cross-flow configurations are proposed and validated by large numbers of experimental results including dehumidification and regeneration processes. The numerical results show good agreement with the experimental results when Le number is assumed as one. The key physical parameters representing heat and mass transfer such as temperature and humidity ratio are investigated and one finds that these parameters of outlet air and desiccant can even exceed those of the inlet air and desiccant, due to the two driving forces of heat and mass transfer are combined. The combination of heat transfer driving force and mass transfer driving force can be presented as uncoupled driving forces of differential enthalpy and differential relative humidity . Thus, differential enthalpy presents the combined heat and mass transfer ability, which is the content of nearing equilibrium for total heat exchange. And differential relative humidity presents the content that moisture near the equilibrium status. From a perspective of differential enthalpy and differential relative humidity , the parameters of outlet air and desiccant are within those of inlet air and desiccant. c9r, <TR 9
Secondly, based on the uncoupled driving forces analysis, the reachable handling regions of heat and mass transfer between air and liquid desiccant are proposed. The parameters of outlet air are within a triangle area shown in the psychrometic chart, which is composed of the three boundary lines: (1) isenthalpic line of inlet air, (2) iso-concentration line (or iso-relative humidity line) of inlet desiccant, and (3) the linkage between the inlet status of air and that of desiccant. The boundary lines (1) and (2) are same to the uncoupled heat and mass transfer driving forces. There is a clear physical explanation of boundary line (3), that the parameters of outlet air will locate in the boundary line (3) when the flow rate of desiccant is essential large comparing with the flow rate of air, thus, the influence of heat and mass transfer on the variance of desiccant parameters can be neglected. The reachable handling region is universal applied that means it is suitable for any air and desiccant status, including cooling and dehumidification process, heating and dehumidification process, cooling and humidification process, heating and humidification process and so on. Thanks to this reachable handling region, the direction of liquid desiccant air-conditioning processes and the maximal /minimal reachable parameters of outlet can be easily determined. $oe:km1-D
Thirdly, a zonal method is proposed for liquid desiccant air-conditioning processes analysis. Other than the well-known factors such as flow pattern, flow rate ratio, heat and mass transfer coefficients, mass transfer performance is also influenced significantly by inlet status of the air and that of the desiccant, including temperature and humidity ratio of the air, temperature and concentration of the desiccant. Four zones are divided in the psychrometic chart according to the relative inlet position of air to desiccant including two dehumidification zones, zone A and zone D, and two regeneration zones, zone B and zone C. In zones A and C, the direction of mass transfer is same to that of the combined heat and mass transfer, which is opposite in zones B and D. With the same operating conditions, the dehumidification performance in zone A is much higher than that in zone D, and the regeneration performance in zone C with hot liquid desiccant is much better than that in zone B with hot air as heating source. In zones A and C, counter-flow configuration leads to the best mass transfer performance; on the other hand, in zones B and D, parallel-flow configuration performs best when the desiccant concentration changes little (common condition). l<! ?`V6}
Fourthly, similarities and differences between the combined heat and mass transfer processes in dehumidifier/regenerator and the sensible heat transfer process in heat exchanger are analyzed. The total heat and mass transfer process (denoted as enthalpy “h”) in dehumidifier/regenerator is almost same to the sensible heat transfer process (denoted as temperature “t”) in heat exchanger. The solutions of the combined heat and mass transfer process in dehumidifier/regenerator can then be derived theoretically with that of sensible heat exchanger. One finds that the commonly used logarithmic mean differential moisture method may fail in prediction the mass transfer performance in dehumidifier/regenerator, or even meaningless in some situations, due to the influence of heat transfer on mass transfer process. New logarithmic mean differential enthalpy method is proposed which is applicable and adequate in prediction the coupled heat and mass transfer performances in air and liquid desiccant contacting processes. 45q-x_
Fifthly, performance optimization in different zones is processed on single module considering of both heat and mass transfer performance and pressure drop. The ideal efficiency of air and liquid desiccant contacting process is a function of heat capacity ratio m*, air and desiccant inlet dimensionless parameters and . The availability content is then defined and used to evaluate the performance difference of current devices to the ideal condition. The module sizes before and after optimization are 0.5×0.5×1.2 m3 and 0.6×0.35×0.83 m3 respectively. Comparing to the original module size, the optimized packing volume decreases to 58% with almost same heat and mass transfer performance as well as the packing pressure drop. i|y8n7c
In addition, new types of liquid desiccant air-conditioning devices are proposed based on the aforementioned studies and applied in practical projects. Thanks to the more uniformly heat and mass transfer driving forces, total heat recovery device based on the single module have better mass transfer performance with larger stages though the overall heat and mass transfer volume keeps constant. The COP of heat pump driven liquid desiccant outdoor air processor is over 5 both in summer for dehumidification and in winter for humidification. The COP of heat (70~75ºC) driven liquid desiccant outdoor air processor is 1.2, which provides an effectively mode to use the low grade heat in summer and benefits to the urban energy configuration. The performance of current heat pump driven and heat driven liquid desiccant outdoor air processor is 28% and 50% higher respectively, compared with other available liquid desiccant devices. Furthermore, liquid desiccant based temperature and humidity independent control air-conditioning (THIC) system is introduced, in which liquid desiccant system removes the entire latent load and other chilled water system (about 17ºC, instead of 7ºC in conventional air-conditioning system) removes the remained sensible load. The temperature requirement of chilled water makes the utilization of “free” cooling sources possible. The operating cost of the THIC system is only 50% of conventional air-conditioning system when regeneration heat of liquid desiccant is free, and the operating energy will also save 20%~30% even if regeneration heat can not be freely gained. " :@5|4qK
The main innovative works are summarized as following: [*]&U6\j
Fundamental researches: 1) to propose the reachable handling region of air and liquid desiccant contacting process based on the decoupling analysis of heat and mass transfer driving forces; 2) to provide the handling zone dividing method only according to inlet parameters of air and desiccant; and 3) to analysis similarities and differences between the combined heat and mass transfer processes in dehumidifier/regenerator and the sensible heat transfer process in heat exchanger. The author has published 8 SCI-cited papers and 6 EI-cited papers. ?tV $o,11
Application researches: 1) to develop new liquid desiccant air-conditioning devices based on the above theory with higher energy-saving potential, including total heat recovery device, heat pump or heat driven liquid desiccant outdoor air processors, and provide liquid desiccant device based temperature and humidity independent control air-conditioning system; and 2) to develop a series prototypes with the cooperation of the research team members, and now the produces have already commercial available in the market and widely applied in Beijing, Tianjing, Shanghai and Shenzhen, and the performances are obviously higher than those of other available liquid desiccant products. The author has been authorized 7 patents, published 1 book as the first author, and awarded the Second National Prize for Technological Invention in 2007. [N'YFb3"O
\O m.pOz
<TC\Nb$~
Key words: liquid desiccant dehumidification, regeneration, independent temperature and humidity control, heat and mass transfer device, and air-conditioning
