Experimental investigation on desiccant air-conditioning system in India

An experimental investigation in India was presented to evaluate the performance and energy saving capacity of a desiccant air-conditioning system composed of a silica gel bed, a split type air-conditioner (1.0 ton refrigeration) installed in a room with a volume of 86.4 m³, air ducts and a blower. The experiment was made in such a way that the percentages of return air, outdoor air and indoor air mixed with the air leaving the desiccant and desiccant bed thickness could be adjusted. Tests were conducted on several days with relatively similar ambient conditions. Under the test conditions in this experiment, a 7cm bed thickness is recommended with a maximum adsorption rate of 403g/h. The optimum percentages of air ratios were as follows: 10% of outdoor air, 10% of return air (mixed together at the desiccant bed inlet) and 80% of indoor air mixed with the dry air leaving the desiccant. The corresponding electricity saving was about 19%. As expected, simple economic analysis indicates that the desiccant air-conditioning is not viable for smaller cooling capacities.     

Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

吸湿供冷系统的空气处理方式分析

温度和新风量的大小在影响室内环境的舒适性方面起着重要的作用。为了改善室内空调环境，ASHRAB规定设计温度应该保持在30％-60％之间，且需要把原有的设计新风量增大2倍至4倍。显然，降低温度和增大新风会加大空调的冷负荷，传统的空调供冷系统在满足上述条件方面显示出了自身的一些缺点。吸温供冷系统把除温和冷却降温供冷解耦分开处理．不但能把空气温度处理到一个比较低的程度，而且在节能方面也显示出很大的潜力。对吸温供冷系统的几种空气处理方式进行了分析和比较，从而可根据不同的情况为吸温供冷系统的使用选择比较理想的空气处理方式，以达到节能目的。     

转轮复合式空调系统的数值计算及能耗分析

主要研究了以太阳能作为再生热源的转轮除湿和蒸气压缩制冷相结合及转轮除湿、蒸气压缩和蒸发冷却相结合的2种复合式空调系统,同时对电能作为再生热源的上述空调系统进行研究,建立了系统的物理模型,并对系统性能参数进行数学描述。通过与相同条件下常规蒸气压缩空调系统的比较分析,得出复合式空调系统制冷剂质量流量分别减少50.20％和66.67％；压缩系统性能系数COP分别提高了26.49％和32.16％；压缩机能耗分别节省了62.64％和76.92％。电能作为再生热源时,总负荷能耗分别节省了32.68％和42.00％；当采用太阳能作为再生热源时,总负荷能耗节省更多的能量,分别为61.86％和71.16％(认为1kW电能等价于3kW热能)。研究还发现,室内相对湿度相同,随室内设计温度的提高,复合式系统压缩机能耗明显减少,节能率呈上升趋势；相反总负荷能耗的节能率呈下降趋势。干热气候条件下,系统节能较为明显：71.75％和85.96％(电能再生)。热湿气候条件下,系统节能不明显,甚至消耗更多能量,而采用太阳能时,复合式系统均具有明显节能效果。     

Energy saving strategies in air-conditioning for museums

In the museum environment a strict thermal-hygrometric control is necessary primarily for the correct artwork conservation and then for the visitor thermal comfort. Considering that the air-conditioning system has to operate constantly, suitable techniques permit to obtain useful energy savings, allowing, however, a good dynamic microclimatic control.In this paper a case study is presented about various strategies used to reduce energy requirements for HVAC systems in an exhibition room of a modern museum. Using the dynamic simulation code DOE 2.2 and typical climatic hourly data sets, the annual energy use for an all-air system has been calculated, as well as the savings obtainable using different techniques, such as dehumidification by adsorption (desiccant wheel – saving equal to 15% with respect to a base configuration), total energy recovery from the relief air (passive desiccant – 15%), outdoor airflow rate variation (demand control ventilation – 45%). Moreover, the correspondence has been analyzed between the energy request and the admitted variation of indoor temperature and relative humidity: changing the admitted indoor RH range from 50 ± 2% to 50 ± 10%, energy savings around 40% have been obtained. As regards the thermal-hygrometric performance, an optimal control of temperature has been guaranteed with all the configurations, while the best performance in RH control has been obtained with the desiccant system.Considering a simple payback analysis, if the artworks preserved in a museum are particularly sensitive to indoor humidity variation, a desiccant system should be properly used; on the contrary, when the indoor humidity control is not strongly needed, the use of a HVAC system with demand control ventilation is advisable, because of the lowest payback value. The system with total energy recovery presents intermediate features.     

Simulation–optimization of solar-assisted desiccant cooling system for subtropical Hong Kong

Solar cooling is a novel approach, which primarily makes use of solar energy, instead of electricity, to drive the air-conditioning systems. In this study, solar-assisted desiccant cooling system (SADCS) was designed to handle the cooling load of typical office in the subtropical Hong Kong, in which half of the building energy is consumed by the air-conditioning systems. The SADCS mainly consisted of desiccant wheel, thermal wheel, evaporative coolers, solar air collectors and gas-fired auxiliary heater, it could directly tackle both the space load and ventilation load. Since the supply air flow is same as the outdoor air flow, the SADCS has a feature of sufficient ventilation that enhances the indoor air quality. Although it is inevitable to involve the auxiliary heater for regeneration of desiccant wheel, it is possible to minimize its usage by the optimal design and control scheme of the SADCS. Through simulation–optimization approach, the SADCS can provide a satisfactory performance in the subtropical Hong Kong.     

Control performance of a dedicated outdoor air system adopting liquid desiccant dehumidification

Liquid desiccant is energy efficient for dehumidification in air-conditioning systems. In this study, a novel dedicated outdoor air system (DOAS) adopting lithium chloride solution as liquid desiccant is proposed to process supply air. The DOAS mainly consists of a membrane-based total heat exchanger, a liquid dehumidifier, a regenerator and a dry cooling coil. It can realize independent temperature and humidity controls for supply air. Control strategies for the supply air dehumidification and cooling process as well as the desiccant solution regeneration process in the DOAS are developed and verified. The control performances of the proposed dedicated outdoor air system are investigated at different operation conditions by simulation tests. The results show that the DOAS is more suitable for hot and humid climates. The effects of the total heat exchanger on the performance of the DOAS are also evaluated. It can improve the system energy performance by 19.9–34.8%.     

Integration of variable refrigerant flow and heat pump desiccant systems for the cooling season

Energy saving and indoor air condition enhancing potentials by integrating the variable refrigerant flow (VRF) and heat pump desiccant (HPD) systems were investigated in a field performance test during a cooling season. Three different operating modes: non-ventilated, HPD ventilation assisted and HPD ventilation–dehumidification assisted VRF systems were investigated. The HPD systems operated in the ventilation–dehumidification mode dehumidify the outdoor air and supply it to the indoor air during the ventilation. It was found that the VRF systems provided an average of 97.6% of the total cooling energy for the HPD ventilation assisted mode. The remainder was the recovered cool by the HPD systems during ventilation. The VRF systems provided an average of 78.9% of the total cooling energy for the HPD ventilation–dehumidification assisted mode. The remainder was covered by the HPD systems which provided additional sensible and latent cooling. Overall, among the three operating modes, it is concluded that the HPD ventilation–dehumidification assisted VRF outdoor units consume less energy than the HPD ventilation assisted ones, but more than the non-ventilated ones, while providing the best indoor thermal comfort and indoor air quality conditions. For the total system, the HPD ventilation–dehumidification assisted VRF systems consume less energy than the HPD ventilation assisted ones.     

Case study and theoretical analysis of a solar driven two-stage rotary desiccant cooling system assisted by vapor compression air-conditioning

In this paper, a solar hybrid desiccant air conditioning system, which combines the technologies of two-stage desiccant cooling (TSDC) and air-source vapor compression air-conditioning (VAC) together, has been configured, experimentally investigated and theoretically analyzed. The system mainly includes a TSDC unit with design cooling capacity for 10 kW, an air-source VAC unit with 20 kW in nominal cooling capacity, a flat plate solar collector array for 90 m², a hot water storage tank and a cooling tower. Performance model of the system has been created in TRNSYS simulation studio. The objective of this paper is to report the test result of the solar hybrid air conditioning system and evaluate the energy saving potential, thereby providing useful data for practical application. Experimental results show that, under typical weather condition, the solar driven desiccant cooling unit can achieve an average cooling capacity of 10.9 kW, which contributes 35.7% of the cooling capacity provided by the hybrid system. Corresponding average thermal COP is over 1.0, electric COP is up to 11.48. Under Beijing (temperate), Shanghai (humid) and Hong Kong (extreme humid) weather conditions, the solar TSDC unit can remove about 57%, 69% and 55% of the seasonal moisture load, thereby reducing electric power consumption by about 31%, 34% and 22%, respectively. These suggest that the solar hybrid system is feasible for a wide range of operating conditions.     

溶液除湿的地源热泵毛细管顶板空调系统

提出了一种基于溶液除湿的地源热泵毛细管顶板复合空调系统。该系统采用了溶液除湿承担潜热负荷,地源热泵制取的高温冷水承担显热负荷的方式,达到了节省高品位电能、减轻大气污染、减少运行费用的效果,与传统的空调系统相比具有节能、环保、高舒适性的特点。     

Active desiccant integration with packaged rooftop HVAC equipment

Current research indicates a direct correlation between indoor air quality and fresh air ventilation rates which supports requirements for building ventilation standards calling for continuous supply and increased amounts of ventilation to help assure safe and healthy interior air environments [O. Seppänen, W.J. Fisk, M.J. Mendell, Ventilation rates and health, ASHRAE Journal (August) (2002) 56–58; C.C. Downing, C.W. Bayer, Classroom indoor air quality vs. ventilation rate, ASHRAE Trans., 1993, Vol. 99, Part 2, Paper Number DE-93-19-1, pp. 1099–1103. [1] and [2]]. Off-the-shelf, packaged rooftop equipment used to air condition most facilities is not designed to handle the increased or continuous supply of outdoor air necessary to comply with building ventilation codes written to this new standard [American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc. (ASHRAE), Ventilation for acceptable indoor air quality, Standard 62-1989, 1791 Tullie Circle, NE, Atlanta, GA 30329. [3]].Integration of a rooftop, unitary air conditioner with an active desiccant module (ADM) allows the use of a standard rooftop air conditioner with a thermally regenerated active desiccant component to provide a compact, cost-effective, and simple-to-use packaged system for efficiently pre-treating and supplying ventilation air adequate to ensure healthy indoor environments. By designing a combined vapor-compression/active desiccant system with the desiccant component positioned after a conventional cooling coil, the dehumidification effectiveness of the desiccant is significantly enhanced because it operates on cold, saturated, or nearly saturated, air leaving the evaporator. “Post-coil” rather than the normally used, “pre-coil” desiccant arrangement also minimizes the regeneration temperature required for the active desiccant, allows for partial bypass and post-cooling of the desiccated air after recombination, and dramatic decreases in the overall size for the pre-conditioning unit.     

Research on ratio selection of a mixed liquid desiccant: Mixed LiCl– CaCl2 solution

Liquid desiccant cooling system is a new type of air-conditioning system capable of saving energy. The dehumidification process dominates the performance of this system, while the thermal properties of the liquid desiccant play a key role in improving dehumidification effect. However, there is little work about how to choose a proper liquid desiccant that has a better performance. To settle this problem, a novel method is proposed to search an ideal liquid desiccant by applying the nonrandom two-liquid equation (NRTL equation). This idea is further applied to mixed LiCl and CaCl₂ solution to work out the right mixture ratio with a better dehumidification effect under certain working conditions. Moreover, the related experiments were carried out. The results show that: compared to single LiCl solution, the dehumidification effect could be raised by more than 20% with mixed LiCl and CaCl₂ solution.     

On the performances of a hybrid air-conditioning system in different climatic conditions

In previous papers the authors demonstrated that significant energy savings can be achieved in air-conditioning through the use of a hybrid plant in which a vapor-compression inverse cycle is integrated with an air dehumidification system working with hygroscopic solution and hydrophobic membrane. The advantage of this system lies in the fact that the refrigeration device operates at a higher evaporation temperature than that of a traditional system, in which dehumidification is achieved through condensation.In the proposed hybrid system the supplied air is simultaneously cooled and dehumidified in an air-solution membrane contactor. The LiCl solution is cooled by means of a vapor-compression inverse cycle. The solution is regenerated in another membrane contactor by exploiting the exhaust air and the heat rejected by the condenser.The paper reports a study of the steady-state behavior of the system in summer climatic conditions on varying some significant climatic parameters, such as the latent load of the conditioned space and the outdoor and indoor relative humidity. The performances of the hybrid system are compared with those of a traditional direct-expansion air-conditioning plant. Results of the simulations reveal that energy saving may exceed 60% when the latent load in the conditioned environment is high.     

Effect of regeneration mode on the performance of liquid desiccant packed bed regenerator

The regenerator is one of the key components in liquid desiccant air-conditioning systems, in which desiccant is concentrated and can be reused in the system. The regeneration heat is supplied into the regenerator by either hot air or hot desiccant. The heat and mass transfer performances of these two regeneration modes are analyzed and compared in detail. In the hot air driven regenerator, the parallel-flow regenerator has the best mass transfer performance and the counter-flow performs poorest under the same conditions, because the heat transfer process is the governing process and the mass transfer performance depends on the promotion of the heat transfer to the mass transfer process. In the hot desiccant driven regenerator, counter-flow configuration has the best mass transfer performance and parallel-flow is the poorest at the same conditions, since mass transfer is the governing process. Regeneration heat should be chosen to heat the desiccant instead of the air in the packed bed regenerator, since the hot desiccant driven regenerator has apparent better mass transfer performance. The proposed regeneration mode and flow pattern will be helpful in the design and optimization of the regenerators.     

Analysis on a hybrid desiccant air-conditioning system

Hybrid desiccant-assisted preconditioner and split cooling coil system, which combines the merits of moisture removal by desiccant and cooling coil for sensible heat removal, is a potential alternative to conventional vapor compression cooling systems. In this paper, experiments on a hybrid desiccant air-conditioning system, which is actually an integration of a rotary solid desiccant dehumidification and a vapor compression air-conditioning unit, had been carried out. It is found that, compared with the conventional VC (vapor compression) system, the hybrid desiccant cooling system economizes 37.5% electricity powers when the process air temperature and relative humidity are maintained at 30 °C, and 55% respectively. The reason why the hybrid desiccant cooling system features better performance relative to the VC system lies in the improvement brought about in the performance of the evaporator in VC unit due to desiccant dehumidification. A thermodynamic model of the hybrid desiccant system with R-22 as the refrigerant has been developed and the impact of operating parameters on the sensible heat ratio of the evaporator and the electric power saving rate has been analyzed. It is found that a majority of evaporators can operate in the dry condition even if the regeneration temperature is lower (i.e. 80 °C).     

过渡季节隔板式工位空调房间气流组织的数值模拟

现代办公楼由于室内发热设备增多,部分房间在过渡季节也需要制冷,传统空调按夏季工况设计选用的设备容量较大,不能适应过渡季节小负荷运行的需要。工位空调是一种人性化的空调形式,直接将新鲜空气送到人的呼吸区,在过渡季节通过工位空调,充分利用室外新风消除室内负荷是一种可行的空调方式。利用CFD仿真软件对一间使用隔板式工位空调办公室的过渡季节的温度场、速度场进行仿真,得出了空气温度、流速的分布图,并对模拟房间进行了实验测试,对模拟结果与实测结果进行了分析比较。分析结果表明模拟值与实测值的吻合度较好,认为该空调房间的气流组织是符合要求的。最后指出,仿真应该和实验有机的结合起来,才能更有效的解决专业问题。     

Performance analysis of dehumidification rotating wheel using liquid desiccant

In the present work, theoretical and experimental evaluation of the effect of bed configuration and operating conditions on the performance of desiccant dehumidification system has been carried out. A new rotating absorption disk has been designed and constructed to be tested in the experimental work. The desiccant wheel has a cylindrical shape of 50-cm diameter and 10 cm thickness. The flow area of this bed is consisted of 350 narrow slots, which are uniformly distributed over the cross section of the cylindrical bed. Each slot has a cylindrical shape and constructed from a steel spring of 100 mm length and 20 mm inside diameter. To form the absorbing surface in the bed, each spring is coated with a thick cloth layer impregnated with lithium chloride solution, which is used as the working desiccant in these experiments.In the theoretical part of this study, a mathematical model has been developed where its output results are compared with the experimental data. The effect of different design parameters and operating conditions on the absorption and regeneration processes is discussed. The effect of regeneration air temperature, the process air and regeneration air inlet humidity, the rotational speed, the process and regeneration air velocity (or flow rates), the bed length, etc. on the amount of water absorbed/desorbed in a cycle is investigated.For the specific bed design parameters, actual recorded data show that an amount of 95 g of water can be absorbed in the absorption cycle per hour. This value changes with varying the operating conditions. From the theoretical investigation, it is found that at regeneration temperature of 85 °C, the amount of water absorbed is nearly equal to the amount of water desorbed (i.e. equilibrium condition) for a complete cycle. It is seen also that for moderate operating conditions (50% RH, 30 °C) and lower regeneration temperature which is suitable for solar energy application, the reduction in the humidity ratio of the process air reaches about 13% of its initial value. Finally, comparisons between theoretical and experimental results show good agreement.     

低温全新风系统的应用分析

简述了低温全新风系统的组成,分析了低温全新风系统在实际应用中的优势和不足,分析结果表明：与常温一次回风系统相比,低温全新风系统具有改善和稳定室内空气品质、避免病菌重复感染、保证空调系统安全性、节约风道材料、对于新风比大于等于30％的建筑可以节能、适合与冰蓄冷系统的联合应用等优点,具有过滤器易堵塞、滤料消耗多、运行管理任务繁重等缺点,低温全新风系统的应用利多弊少,应该扩大范围推广应用。     

Solar cooling systems: Mass transfer studies for a liquid desiccant dehumidifier

Liquid desiccant cooling systems have the advantage over conventional compression systems of being able to operate with largely solar thermal energy sources, and of efficiently handling the latent load. The solar energy is used to regenerate the liquid desiccant by removing the water absorbed from air in the dehumidifier. A packed-bed liquiddesiccant (LiBr) dehumidification unit has been operated with varying air conditions and liquid streams and with three levels of packing (0, 28 and 40 cm). Number of transfer units (NTU) values of 2–2.5 were obtained in the best performing configuration; the corresponding height of transfer unit (HTU) values were 0.25–0.31 m. Overall, gas-side mass transfer coefficients calculated for the dehumidifier are made up of contributions from the packed bed and spray sections of the tower. With full packing and a higher solution flow rate, the overall K_ya was 151.3 g mol/sm³ contact-volume log mean concentration driving force. Spray-only contacting at the higher solution flow rate gave a K_ya of 15.7 g mol/sm³ contact-volume log mean concentration driving force. The individual mass transfer coefficients for the two sections have been separated; to the authors' knowledge, this is the first time the separate contributions of spray and packing have been quantified in a composite dehumidifier tower. Spray contributions were found to contribute from 10 to 70% of the mass transfer occurring in the dehumidifier, the higher percentages being found for a very inefficient deep bed and low liquid flow conditions.