Experimental study of practical applications of a passive evaporative cooling wall with high water soaking-up ability

Aimed at controlling the increase in urban surface temperature and creating comfortable urban environments in summer, the authors have developed a passive evaporative cooling wall (PECW) constructed of porous ceramics. These ceramics enable their vertical surfaces to be wet up to a level higher than 100 cm when their lower end is placed in water. Our previous study has demonstrated the cooling performance and applicability of a prototype PECW constructed of pipe-shaped ceramics (ceramic pipes). The present paper presents a PECW unit system which can be easily installed for practical applications. Experiments were conducted using experimental PECW units. Experimental results show that the ceramic pipe developed in this study possessed a higher water-holding and soaking-up ability than the previous one. Wet surfaces of the new ceramic pipe reached a height of over 130 cm at an outdoor location exposed to solar radiation on sunny summer days. Furthermore, the air passing through the PECW unit was cooled, and its temperature can be reduced by around 2 °C during summer daytime. These results indicate that the proposed PECW can be broadly applied to various urban locations.     

Experimental study of the performance of porous materials to moderate the roof surface temperature by its evaporative cooling effect

The change of urban surfaces from permeable to impermeable materials, i.e. asphalt or concrete, has caused the rising of surface temperatures, particularly in densely developed cities. The consequences of this problem lead to higher energy consumption, especially for cooling purposes and other environment related issues. This paper aims to investigate the performance of several non-porous and porous potential roofing materials, to determine which ones might best be used to create a more effective system by utilizing their moisture absorption and evaporation capabilities. Here, four kinds of materials—pebbles, silica sand, volcanic ash, and siliceous shale—were tested to evaluate their moisture and thermal performance, including the effects from different particle sizes. First, the necessary physical properties and pore characteristics were obtained. Thus, each material, under simple boundary conditions, was evaluated in an evaporation experiment, to determine comparative moisture and thermal behavior. Next, cyclic experimentation was conducted, in which variations of temperature, relative humidity and simulated solar radiation were included. The measurement results showed that porous materials can satisfactorily lower surface temperature. Among the tested samples, siliceous shale of both small and large particle diameter was found to lower the daily average surface temperature by up to 6.8 and 8.6 °C, respectively. The better performance of large size particles could possibly be caused by the ventilation occurring within the material layers and high solar penetration through the large gaps between particles, which would release more latent heat when compared to materials of smaller particle size. Finally, analysis of surface energy balance suggested that water contents, solar absorptivity, and wind effects all have significant influences on cooling the surface temperature.     

An experimental study of a novel dew point evaporative cooling system

A novel dew point evaporative cooling system for sensible cooling of the ventilation air for air conditioning application was constructed and experiments were carried out to investigate the outlet air conditions and the system effectiveness at different inlet air conditions (temperature, humidity and velocity) covering dry, temperate and humid climates. The results showed that wet bulb effectiveness ranged between 92 and 114% and the dew point effectiveness between 58 and 84%. A continuous operation of the system during a typical day of summer season in a hot and humid climate showed that wet bulb and dew point effectiveness were almost constant at about 102 and 76%, respectively. The experiment results were compared with some recent studies in literature.     

Numerical study of a M-cycle cross-flow heat exchanger for indirect evaporative cooling

In this paper, numerical analyses of the thermal performance of an indirect evaporative air cooler incorporating a M-cycle cross-flow heat exchanger has been carried out. The numerical model was established from solving the coupled governing equations for heat and mass transfer between the product and working air, using the finite-element method. The model was developed using the EES (Engineering Equation Solver) environment and validated by published experimental data. Correlation between the cooling (wet-bulb) effectiveness, system COP and a number of air flow/exchanger parameters was developed. It is found that lower channel air velocity, lower inlet air relative humidity, and higher working-to-product air ratio yielded higher cooling effectiveness. The recommended average air velocities in dry and wet channels should not be greater than 1.77 m/s and 0.7 m/s, respectively. The optimum flow ratio of working-to-product air for this cooler is 50%. The channel geometric sizes, i.e. channel length and height, also impose significant impact to system performance. Longer channel length and smaller channel height contribute to increase of the system cooling effectiveness but lead to reduced system COP. The recommend channel height is 4 mm and the dimensionless channel length, i.e., ratio of the channel length to height, should be in the range 100 to 300. Numerical study results indicated that this new type of M-cycle heat and mass exchanger can achieve 16.7% higher cooling effectiveness compared with the conventional cross-flow heat and mass exchanger for the indirect evaporative cooler. The model of this kind is new and not yet reported in literatures. The results of the study help with design and performance analyses of such a new type of indirect evaporative air cooler, and in further, help increasing market rating of the technology within building air conditioning sector, which is currently dominated by the conventional compression refrigeration technology.     

Numerical study of a novel dew point evaporative cooling system

Dew point evaporative cooling system is an alternative to vapor compression air conditioning system for sensible cooling of ventilation air. This paper presents the theoretical performance of a novel dew point evaporative cooling system operating under various inlet air conditions (covering dry, moderate and humid climate) and influence of major operating parameters (namely, velocity, system dimension and the ratio of working air to intake air). A model of the dew point evaporative cooling system has been developed to simulate the heat and mass transfer processes. The outlet air conditions and system effectiveness predicted by the model using numerical method for known inlet parameters have been validated with experimental findings and with recent literature. The model was used to optimize the system parameters and to investigate the system effectiveness operating under various inlet air conditions.     

Innovative low-energy evaporative cooling system for buildings: study of the porous evaporator wall

To face the current increase of building cooling demand and the concerns related to climate changes, an energy-efficient evaporative cooling system using porous material has been developed. This article presents the innovative cooling system and a detailed hygrothermal analysis of its main element: the porous evaporator. A mathematical model and an experimental set-up are presented, which enable to determine the suitable material properties for the evaporator and its impact on the overall cooling system performance, focusing on the optimal use of both energy and water. A good agreement is observed between numerical and experimental results, and the evaporative cooling power is estimated from 12 to 72?W/m² of evaporator wall, depending on the evaporator characteristics. A parametric analysis is conducted to select the best material for the evaporative cooling system. An intrinsic permeability of the material of 4?×?10^?17?m² is recommended for this new cooling system.     

Study of cooling system with water mist sprayers: Fundamental examination of particle size distribution and cooling effects

A cooling system that sprays micro water droplets could prove useful in mitigating temperature increases in urban areas by using the heat of water evaporation, a process that consumes only small amounts of water and energy. If water mist is sprayed in a semi-outdoor area, for example, under a canopy, it could potentially improve conditions on hot days. However, there is little reference data concerning the design or control of such systems. In order to propose a method for designing and predicting the performance of a water mist system, we discuss differences in cooling effects in the context of particle size distribution of water mist. The results of numerical fluid analysis showed there is no significant difference in temperature reduction for different particle sizes. However, the water particles remained in a lower position with larger particles.     

Application of passive cooling systems in the hot and humid climate: The case study of solar chimney and wetted roof in Thailand

The thermal performance of two passive cooling systems under hot and humid climate condition is experimentally investigated. The experimental results were obtained from a test cell and a controlled cell with identical walls but different roof configurations. The passive cooling systems applied to the test cell are solar chimney and water spraying on roof. The experimental results obtained from the test cell are compared with the closed and no passive cooling controlled cell. In addition, the significant of solar-induced ventilation by using a solar chimney is realized by utilizing a wind shield to reduce the effect of wind-induced ventilation resulting in low measured air velocities to the solar chimney and low computed value of coefficient of discharge. The derived coefficient of discharge of 0.4 is used to compute Air Changes rates per Hour (ACH). The ACHs with application of solar chimney solely are found to be in the range of 0.16–1.98. The studies of air temperature differences between the room and the solar chimney suggest amount of air flow rates for different periods in a year. The derived relationships show that the air flow rate during February–March is higher than during June–October by 16.7–53.7%. The experimental results show that application of the solar chimney in the test cell could maintain the room temperature at 31.0–36.5 °C, accounting for 1.0–3.5 °C lower than the ambient air and 1.0–1.3 °C lower than the controlled cell. However, to make the test cell's room temperature much lower than the ambient temperature and increase the flow rate of air due to the buoyancy, the application of water spraying on roof is recommended together with solar chimney. The application of the two systems in the hot and humid climate are discovered to sustain the room temperature of the test cell to be lower than the ambient air by 2.0–6.2 °C and lower than the controlled cell by 1.4–3.0 °C.     

饱和砂土挡墙被动破坏超孔隙水压力试验研究

通过自制饱和砂土挡墙模型箱,对于不同挡墙位移模式和位移大小,测试了不同相对密度饱和砂土挡墙被动破坏过程中超孔隙水压力,揭示了饱和砂土挡墙后砂土破坏机理。对深入研究土与挡墙共同作用具有理论价值,对更加合理设计挡墙、控制稳定具有指导意义。