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[en] Goal of this contribution is to draw a picture about some general issues for using solar thermal energy for air conditioning of buildings. The following topics are covered:-A basic analysis of the thermodynamic limits for the use of heat cooling in combination with solar thermal energy is drawn; thereby fundamental insights about control needs for solar thermal driven cooling are obtained. -A short overview about the state-of-the-art of available technologies, such as closed thermal driven cooling cycles (e.g., absorption, adsorption) and open cooling cycles (e.g., desiccant employing either solid or liquid sorbents) is given and needs and perspectives for future developments are described. -The state-of-the-art of application of solar assisted air-conditioning in Europe is given and some example installations are presented. -An overview about new developments of open and closed heat driven cooling cycles for application in combination with solar thermal collectors is given and some of these new systems are outlined more in detail
[en] A man-portable personal cooling garment based on the concept of vacuum desiccant cooling (VDC) was developed. It was demonstrated with cooling pads that a cooling capacity of 373.1 W/m2 could be achieved in an ambient environment of 37 °C. Tests with human subjects wearing prototype cooling garments consisting of 12 VDC pads with an overall weight of 3.4 kg covering 0.4 m2 body surface indicate that the garment could maintain a core temperature substantially lower than the control when the workload was walking on a treadmill of 2% inclination at 3 mph. The exercise was carried out in an environment of 40 °C and 50% relative humidity (RH) for 60 min. Tests also showed that the VDC garment could effectively reduce the metabolic heat accumulation in body with subject wearing heavily insulated nuclear, biological and chemical (NBC) suit working in the heat and allow the participant to work safely for 60 min, almost doubling the safe working time of the same participant when he wore NBC suit only. - Highlights: ► Heat stress mitigation is important for workers health, safety, and performance. ► Vacuum desiccant cooling (VDC) a novel concept for personal cooling. ► VDC garment man-portable and more efficient than commercial ice/pad vest. ► VDC garment suitable for personal cooling with NBC suit.
[en] In this work a novel energy efficient air-conditioning system utilizing lithium chloride (LiCl) solution as liquid desiccant has been proposed and simulated. The simulation of this system is mainly formulated with two packed columns, one for regenerating the weak desiccant and the other for the dehumidification of ambient air. The air is first dehumidified in the dehumidifier and then sensibly cooled in the indirect and direct evaporative coolers. First and second laws of thermodynamics have been used to analyze the effect of five key variables on the performance of the system. High efficiency could be achieved if proper values of these variables are selected.
[en] Microwave heating is expected to be a novel regeneration method of desiccant rotor in desiccant humidity conditioner, because it has advantages of direct and rapid heating of material. In this study, desorption experiments were conducted by using the practical desiccant rotor coated with zeolite in the microwave power ranging from 200 to 800 W and desiccant rotor length of 60-180 mm. As a result, both desorption ratio in a desorption equilibrium and initial desorption rate were found to increase linearly with microwave power. Concerning the effect of rotor length, it was found that initial desorption rate and desorption ratio at 600 s also increased with rotor length. However, experimental desorption ratio in a desorption equilibrium was much lower than estimated value, which was calculated on the basis of change in relative humidity involved by temperature increase of rotor, and the deviation increased with microwave power and rotor length. From the temperature measurement in the rotor, a noticeable temperature distribution was observed in the radial and axial directions of rotor even though water desorption attained an equilibrium state. Consequently, it was indicated that nonuniform heating of the rotor mainly caused decrease in desorption ratio. - Research highlights: → Water desorption of zeolite desiccant rotor was conducted with microwave heating. → Initial desorption rate and equilibrium ratio rose linearly with microwave power. → Initial desorption rate and equilibrium ratio increased with rotor length. → Experimental desorption ratio was much lower than theoretically estimated value. → A big temperature distribution existed in the radial and axial directions of rotor.
[en] In this work a thermodynamic analysis of a desiccant wheel is proposed to investigate and identify the optimum size and operating regime of this device. A steady state entropy generation expression, based on effectiveness parameters suitable for desiccant wheels operability, is obtained applying a control volume approach and assuming perfect gas behaviour of the binary air–vapour mixture. A new entropy generation number NL is defined using a minimum indicative value of the entropy generation SL,min and investigated in order to obtain useful criteria for desiccant wheels optimization. The effectiveness-NTU design method is employed by combining solution of thermal exchange efficiency for rotary heat exchanger with the characteristic potential method, under the conditions of heat and mass transfer analogy. The analysis is applied to a specific desiccant wheel and NL variation with NTU is explored under various operative conditions and wheels characteristics in terms of dimensionless velocity and flow unbalance ratio. - Highlights: • Steady state entropy generation based on effectiveness parameters for heat and mass transfer. • Definition of a new entropy generation number NL for desiccant wheel. • Least irreversible features for a defined dehumidification rate of the desiccant wheel. • NL can be used as an optimization parameter for desiccant wheels design and control
[en] Highlights: • Preparation and characterization of clay additives based CaCl2 composite desiccant is described. • The exit air humidity ratio relative to inlet air humidity ratio is expressed in terms of percentage reduction in moisture. • Process air relative humidity, air velocity and bed weight influences the dehumidification performance. • Experimental results for percentage reduction in moisture are compared with theoretical mass transfer model. - Abstract: Transported clay suitable for pot making is used as desiccant carrier. Additives like saw dust and horse dung are considered in particle preparation. Particles nearly spherical in shape are prepared manually and are dried under shadow and subsequently the particles are dried at different temperatures. These burnt particles are characterized for pore volume and surface area. The BET test reveals that clay particles subjected to 500 °C possess higher pore volume but clay-horse dung particles exhibit higher surface area. Heat treated particles of clay with additives are impregnated with CaCl2 solution of 50% concentration. The ratio of desiccant water content to surrounding layer water content varies from 14.09 to 75.34 for CaCl2 based composite desiccants. One dimensional PGC mass transfer model for process air through burnt clay – additives - CaCl2 desiccant bed is adopted. The RMSE of measured and predicted results for reduction of moisture content from the process air by composite desiccant beds are in the range of 3.26–13.2%.
[en] Experimental investigations on several commercially available and newly fabricated rotors are conducted in two different laboratories to evaluate performance trends. Experimental uncertainties are analysed and the parameters determining the rotor performance are investigated. It is found that the optimal rotation speed is lower for lithium chloride or compound rotors than for silica gel rotors. Higher regeneration air temperatures lead to higher dehumidification potentials at almost equal dehumidification efficiencies, but with increasing regeneration specific heat input and enthalpy changes of the process air. The influence of the regeneration air humidity was also notable and low relative humidities increase the dehumidification potential. Finally, the measurements show that rising water content in the ambient air causes the dehumidification capacity to rise, while the dehumidification efficiency is not much affected and both specific regeneration heat input and latent heat change of the process air decrease. For desiccant cooling applications in humid climates this is a positive trend. - Highlights: ► New experimental results on a range of desiccant wheels. ► High dehumidification capacities and low enthalpy changes for process air high water content. ► Higher regeneration temperature increases capacity, but lowers energy efficiency.
[en] This paper presents a new approach based on artificial neural networks (ANNs) to determine the vapor pressure of three widely used inorganic desiccant solutions, namely, calcium chloride, lithium chloride, and lithium bromide. The vapor pressure of liquid desiccants depends on temperature and concentration. Empirical expressions generally provide vapor pressure with limited accuracy. Further, the expressions currently in use are tedious and valid for narrow ranges and must be adjusted constantly. In this paper neural networks were trained to predict vapor pressure of desiccant solutions with a reasonable accuracy without mathematical formulae. Trained neural network models provided wide ranges of vapor pressure for desiccant solutions without the need to cross reference several tables or charts. Results showed potential of using ANNs for the prediction of vapor pressure of desiccant solution for cooling applications
[en] Highlights: • The experimental operation of a solar driven hybrid DEC system is presented. • In situ monitoring data are analysed and compared to the system’s expected performance. • Drawbacks affecting the system’s performance are explained. • Fine-tuning of the control and maintenance strategy of the main system components is suggested. - Abstract: The aim of this paper is to report on a two-year operational experience with a solar driven desiccant and evaporative cooling (SDEC) system coupled with a vapour compression heat pump. The main objectives are to analyse the benefits and drawbacks of this innovative hybrid SDEC system, to compare the monitoring results against the expected theoretical ones, and to assess the system’s performance with respect to a reference air handling unit. The comparison focuses on the summer key operation modes using Primary Energy Ratio (PER) as indicator of the entire system performance. The results of the detailed analysis lead to the following conclusions: the specific design of the hybrid SDEC leads to high air quality, simpler control process and low electricity consumption for partial load conditions. The monitoring results show a summer mode PER 20% lower than expected due to underperformance of the desiccant wheel. Nevertheless, this innovative system is still very efficient as its PER is twice as high as the one of the considered reference system. Lastly, suggestions for optimization of the existing system through the fine-tuning of the control strategy of its main components are presented.
[en] Ongoing research and development works suggest that good system configurations have significant potential for improving the performance and reducing the cost and size of rotary desiccant dehumidification and air conditioning system. In this paper, a novel desiccant cooling system using regenerative evaporative cooling and a one-rotor two-stage desiccant cooling system are analyzed and compared under Air-conditioning and Refrigeration Institute (ARI) summer, ARI humid and Shanghai summer conditions. The objective of this paper is to compare the thermodynamic performance of the two systems and obtain useful data for practical application. It is found that compared with the conventional desiccant cooling system, the novel desiccant cooling system with regenerative evaporative cooling can handle air to a much lower temperature while maintaining good thermal performance. Under ARI summer, ARI humid and Shanghai summer conditions, the minimum attainable supply air temperatures are reduced from 13.5 °C to 7.9 °C, from 14.2 °C to 9.2 °C and from 18.0 °C to 13.0 °C respectively. It is suggested that the novel desiccant cooling system with regenerative evaporative cooling is beneficial to breaking the obstacle of limited temperature reduction encountered by conventional desiccant cooling system, especially in the case of extreme high humid conditions. - Highlights: ► Desiccant cooling system with regenerative evaporative cooling (REDC) has been studied. ► Comparison between REDC and conventional desiccant cooling system (DCS) has been performed. ► REDC is superior to conventional DCS in thermal utilization, air conditioning and energy saving. ► REDC has significant potential for breaking the obstacle of limited temperature reduction.