Estimation of passive cooling efficiency for environmental design in Brazil

An estimation of passive cooling techniques was conducted for 14 cities in Brazil, using a fairly accurate algorithm that accounts for heat conduction, convection, radiation, and evaporation; this was done to determine the amount of heat gain/loss of room air, defined as a particular quantitative index for passive techniques. Heat gains and losses were calculated for four envelope conditions - namely, insulated, high-albedo, wet surface, and a combination of the previous two - and compared to a case assumed to be the standard condition. A conclusion drawn was that a passive design is efficient in decreasing the need for cooling in typical dwellings in Brazil; solutions should differ with regional climate characteristics. In semi-arid areas, evaporative cooling showed the best results. Reduced heat gain was found during the warm seasons for all cities, along with increased heat gain during the cool seasons for mid-latitude cities. In particular, a combination of high-albedo enveloping and evaporation can greatly decrease heat gain in building walls. High-albedo surfaces in the sub-tropical areas found in southern Brazil are more efficient. It is suggested that passive techniques should be conceived in such a way so as to work during the cooling season and be disabled during mild ones.     

Modeling and analysis of a fan–pad ventilated floricultural greenhouse

The application of greenhouse technology in the field of floriculture is rapidly expanding worldwide. In India solar radiation is abundant and the climate in the plains are rather hot and dry in summer months while the coastal parts witness a hot and humid climate. For greenhouses in such climates, cooling and ventilation are major factors influencing the production of quality flowers. In the present paper a thermal model of a greenhouse is presented based on fan–pad evaporative cooling. Thermal performance of the greenhouse, as predicted by the model under different climatic conditions is analyzed and compared with a reference study available in the literature. The analysis reveals that a suitable combination of evaporative cooling, shading and ventilation arrangements can effectively maintain the inside microclimate of the greenhouse within permissible limits throughout the year.     

Performance evaluation of misting fans in hot and humid climate

Singapore experiences a hot and humid climate throughout the year. This in turn results in heavy reliance on mechanical systems especially air-conditioning to achieve thermal comfort. An alternative would be the use of evaporative cooling which is less energy intensive. Objective and subjective measurements were conducted at an experimental setup at the National University of Singapore (NUS) to evaluate the thermal conditions and thermal sensations brought about by misting fans. Field measurements were also conducted at food centres in Singapore to determine if they are coherent with the objective and subjective measurements conducted. Analysis of objective and subjective data showed that the misting fan was able to significantly reduce the dry-bulb temperature and thermal sensation votes. This is consistent with field measurements taken, where regression analysis showed that with the misting fan, thermal neutrality can be obtained at a higher outdoor effective temperature (ET*). However, the reduction in temperature comes at the expense of higher relative humidity which results in consistently greater biological (bacterial and fungal) pollutants being enumerated from samples collected under the misting fan system. In some samples, the bacteria count is very much greater than samples collected under the non-misting fan, illustrating the potential for a substantial increase in biological pollutants due to the generation of mists.     

The effect of urban evaporation on building energy demand in an arid environment

An open-air scaled urban surface (OASUS) was used to physically model the influence of urban structure on microclimatic conditions that affect the cooling requirements of buildings. The OASUS scale-model consists of an extensive urban-like building/street array constructed at an open site in the arid Negev region of southern Israel. Building rows are comprised of hollow concrete masonry blocks and have thermal and optical properties analogous to common local construction materials. Previous experiments with the scale-model were limited to “dry” conditions, with only negligible exchanges of latent heat. Considering that one of the main advantages of using the scale-model facility is to be able to control factors affecting microclimate, this study analyses the impact of adding moisture to the scaled “streets” between “building” rows, and gauges the impact of outdoor evaporative cooling on the energy demand of adjacent buildings. Measurements carried out during the summer month of August 2006 at the scale-model facility were used to obtain street canyon air temperatures, which in turn provided input for a dynamic energy simulation of indoor cooling loads in an actual building. The simulation model was calibrated with simultaneously measured data from a nearby residential building. Results suggest that the cooling factor in a street canyon is a direct function of the relative availability of moisture, with respect not only to horizontal area but also to the “complete” three-dimensional urban surface. In addition, simulation results of building energy demand show the importance of accounting for urban density when planning the disposition of vegetated surfaces for cooling purposes.     

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.     

Roof pond cooling of buildings in hot arid climates

The weather in Baghdad, Iraq is hot dry in the summer while water is plentiful and cheap. These are conditions which encourage the use of evaporative cooling. A building with one space in it was used to test the effect of a roof pond which was ventilated mechanically for summer cooling. Thermal measurements were taken for the room in normal conditions without a pond, with a lone pond and no mechanical ventilation, and then finally with mechanically forced ventilation. The results showed a marked improvement in the space temperature with a significant reduction during the peak time outside temperatures at 3 O’clock reaching 6.0 °C between the room without the pond and with a ventilated one and 6.5° at 18:00 during peak inside temperatures. The study also showed that improvements would be better in real-life conditions where exterior wall area is less than the test room.     

填料式喷水室空调机组的节能及净化研究

本文针对现在空气污染严重,空调系统能耗大等问题,提出了一种新型填料式喷水室空调机组。该空调机组与传统式空调机组相比,具有节能、环保、净化等优点。空气与水直接接触,其热湿交换效率高且净化效果好,可达中效过滤器水平。风机及水泵能耗小,冷水机组制取高温冷冻水,耗电量小,节能效果明显。     

Experimental study of cooling effects of a passive evaporative cooling wall constructed of porous ceramics with high water soaking-up ability

As a passive cooling strategy aimed at controlling increased surface temperatures and creating cooler urban environments in summer, the authors developed a passive evaporative cooling wall (PECW) constructed of porous ceramics. These ceramics possess a capillary force to soak water, which means that their vertical surface is wet up to a level higher than 100 cm when their lower end is placed in water. The present paper describes an experiment that clarifies the cooling effects of a prototype PECW constructed of pipe-shaped ceramics. The PECW is capable of absorbing water and allows wind penetration, thus reducing its surface temperature by means of water evaporation. Passive cooling effects such as solar shading, radiation cooling, and ventilation cooling can be enhanced by incorporating PECWs into the design of outdoor or semi-outdoor spaces in parks, pedestrian areas and residential courtyards. The following findings were understood from an experimental data collected over a summer period. Wet vertical surfaces of the ceramic pipe reached a height of over 1 m at an outdoor location exposed to solar radiation. Wet surface conditions can be maintained throughout successive sunny days during summer. A slight difference in the vertical surface temperatures of the ceramic pipe was found. The air passing through the PECW was cooled, and its temperature can be reduced to a minimum value by several degrees during summer daytime. It was also found that the surface temperature of the shaded ceramic pipe can be maintained at a temperature nearly equal to the wet-bulb temperature of outdoor air.     

Spatially-resolved spray measurements and their implications

Fire sprinkler manufacturers have developed a great variety of application specific sprinkler designs. Advances in product development practices, performance based design, risk analysis, and fire suppression research have increased stakeholder interest in quantifying the spray produced by these devices. These sprinkler spray patterns consist of complex spatio-stochastic features originating near the sprinkler head. A Spatially-resolved Spray Scanning System (4S) has been developed to capture the complete spatio-stochastic nature of the spray at its point of origin for documentation and analysis. The 4S synthesizes spray measurements, transport analysis, and statistical representation frameworks providing high-fidelity spray characteristics suitable for evaluation of component-level performance (e.g. sprinkler spray pattern uniformity) or system-level performance (e.g. fire suppression system simulations). Each sprinkler's unique spray pattern is captured through optical and mechanical probing of the spray over a measurement (or initialization) surface close to the sprinkler head (0.4−0.8 m) and analyzing local drop characteristics (e.g. drop size, velocity, and volume flux). These high-fidelity 4S spray measurements consisting of terabyte scale data densities present remarkable challenges with regard to data management, test repeatability, and test timing. These challenges are addressed through integration of automation, flow control, data acquisition, and data analysis systems. Spatially-resolved sprinkler spray measurements are presented providing insight into the sprinkler spray patterns and their connection with deflector geometry. Comparisons between far-field spray dispersion measurements and simulations initialized with 4S measurements demonstrate unprecedented agreement further highlighting the value of spatially-resolved sprinkler measurements for modern suppression analysis.     

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.     

冷却顶板的测试标准及相应的实验研究

该文介绍了德国冷天花元件的两个主要测试标准FG和DIN4715。天津大学根据FGK标准建造了冷天花性能测试实验台,对不同冷天花元件进行了测试,得到了较为满意的结果。     

Field study on indoor thermal environment in an atrium in tropical climates

Solar penetration through the transparent envelope can severely deteriorate indoor thermal environment inside an atrium building particularly in tropical climates. This paper reports the application of two low-cost measures, namely high level internal solar blinds and water spray, to minimise overheating problems on the three levels inside the atrium of a guesthouse in Southern China, where summer is hot and humid. The blinds reduce direct solar penetration at the top of the atrium whilst the evaporative spray system cools down the glazed surfaces of the atrium envelope. A site test was undertaken over 10 consecutive days covering both overcast days and clear days in July 2004. Measurement of indoor thermal environmental parameters was conducted on three levels in the atrium and the recorded data represent the internal conditions: with and without internal blinds protection from solar, and with and without water spray. This study has shown that on hot and clear summer days, with water spray and without blinds the average air temperature difference from 1200 to 1800 h between both first floor and second floor, and second floor and external were 5.7 and 1.7 K, respectively; whilst with blinds and without water spray the average air temperature differences were 8.7 and 4.8 K, respectively.     

Development and evaluation of two new droplet evaporation schemes for fire dynamics simulations

The evaporation of sprinkler droplets is an important phenomenon in fire simulations both for heat removal from the gas and for heat removal from surfaces. In this paper, we address the problems of potential numerical instability and super-saturation that may occur in explicit time integration of the droplet equations. Two novel numerical approaches are developed and evaluated. The first is based on an analytical solution that relaxes the cell composition and temperature toward the equilibrium values. The second method is an implicit solution to the droplet equations. The two approaches are implemented in the Fire Dynamics Simulator (FDS) and verified and validated using both single droplet and practical sprinkler calculations. Ultimately, the implicit approach is deemed the most cost effective for practical fire simulations.     

Speculation in the feasibility of evaporative cooling

The use of evaporative air cooling, for residential air conditioning, cannot be taken for granted in all situations. It depends on the climatic conditions and the specific nature of application. This work establishes a general foundation for judging the feasibility of evaporative cooling with different evaporative-system configurations, under different climatic conditions and for different applications. Two feasibility criteria were stipulated; the rate of air supply to space and the indoor relative humidity. Systematic procedures are presented for evaluating the required air-flow rate and predicting the achievable indoor condition. Explicit mathematical expressions are derived to define the limitations on outdoor conditions for any allowable specific air flow. The impacts of various pertinent factors are investigated. These include the required indoor temperature, the quality of space load represented by its SHF and the performance index of the system. Computer programs were devised to automate, hence facilitate, the repetitive computations and to evade the graphical work on the psychrometric chart. Samples of program results are graphically displayed.     

Feasibility study of a novel dew point air conditioning system for China building application

The paper investigated the feasibility of a novel dew point evaporative cooling for air conditioning of buildings in China regions. The issues involved include analyses of China weather conditions, investigation of availability of water for dew point cooling, and assessment of cooling capacity of the system within various regions of China. It is concluded that the dew point system is suitable for most regions of China, particularly northern and west regions of China where the climate is hot and dry during the summer season. It is less suitable for Guangzhou and Shanghai where climates are hot and humid. However, an air pre-treatment process involving a silica-gel dehumidification will enable the technology to be used for these humid areas. Lower humidity results in a higher difference between the dry bulb and dew point of the air, which benefits the system in terms of enhancing its cooling performance. Tap water has adequate temperature to feed the system for cooling and its consumption rate is in the range 2.6–3 litres per kWh cooling output. The cooling output of the system ranges from 1.1 to 4.3 W per m³/h air flow rate in China, depending on the region where the system applies. For a unit with 2 kW of cooling output, the required air volume flow rate varies with its application location and is in the range 570–1800 m³/h. For a 50 m² building with 60 W/m² cooling load, if the system operates at working hours, i.e., 09:00 to 17:00 h, its daily water consumption would be in the range of 60–70 litres. Compared with mild or humid climates, the dry and hot climates need less air volume flow rate and less water.