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.     

Evaluation of buoyancy-driven ventilation in atrium buildings using computational fluid dynamics and reduced-scale air model

This research focuses on developing a reliable methodology for predicting the performance of buoyancy-driven ventilation in atrium buildings during the design stage using both computational fluid dynamics (CFD) and scale model tests. The results show several features. First, the agreement between CFD simulation and measurement results in the heated zone is better with rng k–? and zero-equation turbulent schemes; whereas, in the atrium space, the laminar and zero-equation CFD models provide better results. Second, the external ambient temperature has a larger effect on the temperature distribution in the atrium space than the thermal load inside the building. Third, the position of the stack openings that create a direct ventilation path can improve the internal thermal environment. The size of the stack openings also affects the temperature distribution in the atrium space. Lastly, due to the small temperature difference in hot and humid climates, a buoyancy-only ventilation strategy is not very effective in such a situation. That is, when a low-rise atrium building is situated in a hot and humid environment, additional efforts such as wind-driven ventilation, wind-buoyancy ventilation or mechanically driven ventilation will be necessary to achieve the thermal comfort desired.     

Experimental investigation of thermal and ventilation performances of stratum ventilation

Stratum ventilation has been proposed to cope for elevated indoor temperatures. Air speed, temperature and CO₂ concentration of a stratum ventilated office are investigated experimentally. The data obtained under well defined conditions and therefore can be used for validating numerical models. Thermal comfort conditions and ventilation efficiency are studied based on the experimental results of four experimental cases. Thermal comfort indices, i.e. PMV, PPD and PD are calculated from measured data. The values of these indices are found to satisfy the requirements of ISO 7730, CR 1752-1998 and ASHRAE 55-2010. In terms of thermal comfort, the two cases with supply air temperature of 21 °C are found to perform better compared with the two cases with supply air temperature of 19 °C. For all the cases, the ventilation effectiveness is close to 1.5. This ventilation method could therefore be expected to provide indoor air quality in an efficient way.     

Effects of double skin envelopes on natural ventilation and heating loads in office buildings

The influence of natural ventilation on heating load and energy savings in a building with a double skinned envelope (DSE) was examined in this study. Field measurements and computer simulations were performed under various weather conditions. The DSE was effective for saving energy and creating natural ventilation rates under clear and partly cloudy skies. Due to insufficient irradiance, the DSE was not effective for reducing the heating load under overcast sky conditions. When natural airflow rates from the cavity space between internal and external skin to the indoor space were controlled, the southwest-facing DSE effectively reduced heating loads due to the accumulation of solar irradiance.Regression models showed that outdoor air temperature was the most significant factor governing variations in cavity temperature under all sky conditions. Computer simulations indicated that natural ventilation was practical at an appropriate supply temperature only when the sky ratio was less than 0.7. The airspace in cavity of the DSE provided additional natural ventilation rates to the indoor space and effectively reduced heating loads. Natural ventilation was available for 135 h during three winter months without consuming additional energy to heat the outdoor air. The heating load was reduced by the DSE ranged from 17.98% to 18.7% depending on the airflow control options for the cavity space.     

Application potential of solar air-conditioning systems for displacement ventilation

Solar air-conditioning can have higher application potential for buildings through the strategy of high temperature cooling. In recent years, displacement ventilation (DV), which makes use of the indoor rising plumes from the internal heat gains, provides a more effective supply air option than the traditional mixing ventilation (MV) in terms of both thermal comfort and indoor air quality. As it is possible to raise the supply air temperature to 19 °C for DV, it would enhance the competitive edge of the solar air-conditioning against the conventional vapour compression refrigeration. Through dynamic simulation, a solar-desiccant-cooling displacement ventilation system (SDC_DV) was developed for full-fresh-air provision, while a solar-hybrid-desiccant-cooling displacement ventilation system (SHDC_DV) for return air arrangement. The latter was further hybridized with absorption chiller (AB) to become SHDC_AB_DV, or adsorption chiller (AD) to be SHDC_AD_DV, in order to be wholly energized by the solar thermal gain. Benchmarked with the conventional system using MV, the SDC_DV had 43.3% saving in year-round primary energy consumption for a typical office in the subtropical climate; the SHDC_AB_DV had 49.5% saving, and the SHDC_AD_DV had 18.3% saving. Compared with their MV counterparts, the SDC_DV, the SHDC_AB_DV and the SHDC_AD_DV could have 42.4%, 21.9% and 30.3% saving respectively.     

An analytical and numerical study of solar chimney use for room natural ventilation

The solar chimney concept used for improving room natural ventilation was analytically and numerically studied. The study considered some geometrical parameters such as chimney inlet size and width, which are believed to have a significant effect on space ventilation. The numerical analysis was intended to predict the flow pattern in the room as well as in the chimney. This would help optimizing design parameters. The results were compared with available published experimental and theoretical data. There was an acceptable trend match between the present analytical results and the published data for the room air change per hour, ACH. Further, it was noticed that the chimney width has a more significant effect on ACH compared to the chimney inlet size. The results showed that the absorber average temperature could be correlated to the intensity as: (T_w = 3.51I^0.461) with an accepted range of approximation error. In addition the average air exit velocity was found to vary with the intensity as (ν_ex = 0.013I^0.4).     

The impact of temperature on mean local air age and thermal comfort in a stratum ventilated office

The influence of the supply air temperature on the mean local air age and thermal comfort of a typical individual office under stratum ventilation is investigated by a numerical method, which is validated by an experiment carried out by the authors. The results show that for an office, when the supply air temperature is increased from 19 °C to 21 °C, the corresponding mean occupied zone temperature rises from 24.5 °C to 26.5 °C. The inhaled air quality for the occupant is improved when supply air temperature rises from 19 °C to 21 °C. Also, the thermal comfort indices (predicted mean vote or PMV, predicted percentage of dissatisfied or PPD and predicted dissatisfied or PD) fulfill the requirements of ISO 7730 and CR 175 1998. For summer cooling operation, stratum ventilation may offer a feasible solution to elevated indoor temperatures, which are recommended by several governments in East Asia.     

A study on the potential of natural ventilation and cooling for large spaces in subtropical climatic regions

The potential of natural ventilation and cooling due to stack effects was investigated for large spaces with high ceilings. Different opening area ratios with respect to floor area were studied. Parameters of stack effects that consider floor heights were analyzed. Performance of natural ventilation was evaluated with cooling effects and indoor air quality for different months of the year. Three cities in northern, middle and southern Taiwan were used to represent typical subtropical weather types. It was found that opening ratio above 0.9％ is sufficient to provide fresh air to meet IAQ requirement. Two different temperature control strategies, fixed indoor temperature (FIT) and operative indoor temperature (OIT) were proposed and studied. A sensible cooling potential, f_pc, was proposed. Different levels of f_pc, namely, strong, medium, weak and not available were used to evaluate the number of days for which natural cooling, hybrid ventilation, mechanical air conditioning are to be applied to satisfy the cooling requirement. The research results presented can be used in the design of openings for large spaces, and also the air-conditioning control strategies for different seasons of the year.     

The feedback of heat rejection to air conditioning load during the nighttime in subtropical climate

Taipei City, located in the subtropical zone, has a basin landform. The summer is always hot and humid and the air temperature right after sunset is typically higher than 30 °C. Heat rejection from residential buildings in urban area, equipped with lots of the window type air conditioners, not only increases the air temperature outside, but also burdens the cooling load. Based on the time schedule of air conditioner use of Taipei citizens, the heat rejection/building energy use and the air temperature distribution were evaluated, and finally the additional electric consumption of air conditioners was predicted. Two software, EnergyPlus (building energy program) and Windperfect (CFD, computational fluid dynamics software) were employed in this study. In the CFD simulation, the geometry of buildings that covers 700 m in diameter was created with GIS (geographical information system) and the total mesh number was more than 3 millions. Three specified temperatures (T_am, T_bu and T_ac) were used to describe the temperature distribution within the urban canopy by hourly time variation and spatial distribution with height and horizontal profile. The results revealed that the temperature gradually increased with height and the temperature next to the buildings was always higher than the ambient air. The feedback (penalty) of heat rejection to cooling load was found 10.7% during 19:01 to 02:00 h on the following day.     

Thermal characteristics of a double-glazed external wall system with roll screen in cooling season

A double-skin system (double-glazed external wall) is an effective passive system that can be used to decrease solar heat gain into buildings. Detailed information on the thermal distribution of double-skin facades is necessary to design better systems that can provide thermal comfort and conserve energy. In this study, the three-dimensional thermal characteristics of double-skin facades that had the ventilation opening installed partially and were screened partially by the adjacent buildings were investigated by field measurements. To that end, field measurements were carried out on the double-skin exterior wall (9.4 m high and 27.0 m wide) installed in an atrium located in the west of an existing building during cooling period for typical summer conditions. Maximum air change rate of natural ventilation through the bottom opening up to the top opening is about 20–25 [1/h], the reduction ratio of total solar heat gain compared with those of non-natural ventilation is about 25%. The exhaust solar heat gain is about 100 W/m² per inner glass surface area of the double-skin facades. Air temperature distribution of air space in the double skin was ranged from 30 °C to 44 °C, and heat gain difference ranged from 50 W/m² to 130 W/m². The influence of the ventilation openings and the shade conditions on temperature distribution of double skin is found to be significant and the double-skin system was verified to reduce the cooling loads effectively.     

Space temperature difference,cooling coil and fan—energy and indoor air quality issues revisited

In designing an energy-efficient air-conditioning system that also simultaneously addresses the needs of adequate ventilation and acceptable indoor air quality, several factors begin to play an important role. Among several others, the cooling coil, the fan and the temperature difference between the space and the supply air (commonly known as the Space ΔT) can be considered to be crucial. For a given space cooling load, the choice of a particular Space ΔT has an implication on the amount of supply air required, which further has an impact on the performance of the cooling and dehumidifying coil as well as the fan. Inherent in these implications are issues related to energy, ventilation and indoor air quality. This paper investigates these implications and quantifies them by considering a hypothetical building in a tropical climate and subjecting the design to several parametric variations involving different Space ΔTs for a given space temperature and humidity condition. The total power requirements, comprising additional cooling, reheating and higher fan power, for a design involving a Space ΔT of 5 °C can be as high as a factor of 2.2 of the total power for a design with a Space ΔT of 8 °C. The implication of higher supply air flow rates on duct design is qualitatively discussed. For a given space cooling load and a given Space ΔT, the implication of increased design ventilation rates to address part-load ventilation requirements can lead to an additional installed cooling capacity of 17.5%. Finally, emerging technologies that are aimed at addressing both energy efficiency and IAQ are discussed.     

Modeling sky luminance using satellite data to classify sky conditions

A model for calculating sky luminance is presented. The earth–atmospheric reflectivity obtained from satellite data was used to classify sky conditions. The proposed sky luminance at a specific point in the sky hemisphere is a product of two functions, namely F₁ and F₂. For any point in a sky hemisphere, F₁ varies with the sky zenith of the considered point (Z) and F₂ is a function of the angular distance between the considered point and the sun (χ). The model uses simultaneous measurements of sky luminance and GMS-5 satellite observation, both taken at Nakhon Pathom, Thailand (13.70°N, 103.10°E), during November 2002 to May 2003. Both F₁ and F₂ are derived as polynomial functions of χ, Z and the solar zenith angle (Z_s) for each sky condition. The classification of the sky condition is based on the satellite-derived earth–atmospheric reflectivity. This sky luminance model was validated against independent measurements from the Asia Institute of Technology (AIT) (14.08°N, 100.62°E). The root mean square difference (RMSD) between the relative sky luminance calculated from the model and that obtained from the measurement is 0.133.     

Air movement acceptability limits and thermal comfort in Brazil's hot humid climate zone

In hot humid climates, natural ventilation is an essential passive strategy in order to maintain thermal comfort inside buildings and it can be also used as an energy-conserving design strategy to reduce building cooling loads by removing heat stored in the buildings thermal mass. In this context, many previous studies have focused on thermal comfort and air velocity ranges. However, whether this air movement is desirable or not remains an open area. This paper aims to identify air movement acceptability levels inside naturally ventilated buildings in Brazil. Minimal air velocity values corresponding to 80% and 90% (V80 and V90) air movement acceptability inside these buildings. Field experiments were performed during hot and cool seasons when 2075 questionnaires were filled for the subjects while simultaneous microclimatic observations were made with laboratory precision. Main results indicated that the minimal air velocity required were at least 0.4 m/s for 26 °C reaching 0.9 m/s for operative temperatures up to 30 °C. Subjects are not only preferring more air speed but also demanding air velocities closer or higher than 0.8 m/s ASHRAE limit. This dispels the notion of draft in hot humid climates and reinforce the broader theory of alliesthesia and the physiological role of pleasure due to air movement increment.     

Effect of warm air supplied facially on occupants’ comfort

Human response to air movement supplied locally towards the face was studied in a room with an air temperature of 20 °C and a relative humidity of 30%. Thirty-two human subjects were exposed to three conditions: calm environment and facially supplied airflow at 21 °C and at 26 °C. The air was supplied with a constant velocity of 0.4 m/s by means of personalized ventilation towards the face of the subjects. The airflow at 21 °C decreased the subjects' thermal sensation and increased draught discomfort, but improved slightly the perceived air quality. Heating of the supplied air by 6 K (temperature increase by 4 K at the target area) above the room air temperature decreased the draught discomfort, improved subjects' thermal comfort and only slightly decreased the perceived air quality. Elevated velocity and temperature of the localized airflow caused an increase of nose dryness intensity and number of eye irritation reports. Results suggest that increasing the temperature of the air locally supplied to the breathing zone by only a few degrees above the room air temperature will improve occupants' thermal comfort and will diminish draught discomfort. This strategy will extend the applicability of personalized ventilation aiming to supply clean air for breathing at the lower end of the temperature range recommended in the standards. Providing individual control is essential in order to avoid discomfort for the most sensitive occupants.     

A numerical and field investigation of underground temperatures under Urban Heat Island

This paper introduces a simplified model for underground temperature prediction in summer hot weather. The data of 14 observation sites show that the surface temperature curves are close to trapeziums, and surface temperatures are related to air temperatures. Therefore, approximated temperature trapeziums that are determined by high- and lowest air temperatures can be used to simulate the underground temperature variation. Two observation sites respectively in the urban and suburban areas were used as examples. Good agreement was obtained between simulated- and measured temperatures. Measured data indicate the average temperature under urban concrete surface is 3.70 °C greater than that of suburban bare surface. The deviation is due to the heat urban environment effect and different surfaces effect, which are about 1.68 °C and 2.02 °C, respectively. Combined with soil volumetric water content (w_v), ‘Heat’ Islands associates with ‘Dry’ Islands, which means urban soil moisture is lower than suburban soil moisture (13.9%). According to the variation of w_v and temperature deviation graphs, Urban Heat Island, ground surface types and rainfall are important factors that influence the underground soil moisture and temperatures.     

Daylighting control performance of a thin-film ceramic electrochromic window: Field study results

Control system development and lighting energy monitoring of ceramic thin-film electrochromic (EC) windows were initiated at the new full-scale window systems testbed facility at the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, CA. The new facility consists of three identically configured side-by-side private offices with large-area windows that face due south. In one room, an array of EC windows with a center-of-glass visible transmittance T_v range of 0.05–0.60 was installed. In the two other rooms, unshaded windows with a T_v = 0.50 or 0.15 were used as reference. The same dimmable fluorescent lighting system was used in all three rooms. This study explains the design and commissioning of an integrated EC window-lighting control system, and then illustrates its performance in the testbed under clear, partly cloudy, and overcast sky conditions during the equinox period. The performance of an early prototype EC window controller is also analyzed. Lighting energy savings data are presented. Daily lighting energy savings were 44–59% compared to the reference window of T_v = 0.15 and 8–23% compared to the reference window of T_v = 0.50. The integrated window-lighting control system maintained interior illuminance levels to within ±10% of the setpoint range of 510–700 lx for 89–99% of the day. Further work is planned to refine the control algorithms and monitor cooling load, visual comfort, and human factor impacts of this emerging technology.     

Monitoring results of an interior sun protection system with integrated latent heat storage

An interior sun protection system consisting of vertical slats filled with phase change material (PCM) was monitored from winter 2008 until summer 2010. While conventional interior sun protection systems often heat up to temperatures of 40 °C or more, the monitoring results show that the surface temperature on the interior side of the PCM-filled slats hardly ever exceeded the PCM melting temperature of 28 °C even in case of long-term intense solar radiation. As long as the PCM is not fully melted, the latent heat storage effect reduces the solar heat gain coefficient (g-value) of the sun protection system to 0.25 for a totally closed blind, and 0.30 for slats set at 45° (the g-values of the same system without PCM are 0.35 and 0.41, respectively). This reduced the maximum air temperature in the offices by up to 2 K in contrast to a reference room with a comparable conventional blind. The sun protection system with PCM therefore considerably improves thermal comfort. In order to discharge the PCM, the stored heat must be dissipated during the night. In climates with sufficiently low outside air temperatures, this is best achieved using a ventilation system in combination with tilted windows.     

Experimental and numerical study of room airflow under stratum ventilation

This paper investigates the air movement, air temperature profile and gaseous contaminant transportation in an individual office with stratum ventilation. The room temperature is elevated compared with conventional standards. The experimental investigation is carried out in an environmental chamber with the presence of heat generating rectangles used to simulate an occupant and a computer. Measurements of temperature, velocity, and CO₂ concentration are carried out for nine plumb lines in the chamber. Up to sixteen points are measured along each plumb line. The experimental data of the aforesaid three parameters of the individual office in warm condition under stratum ventilation are presented. The experimental data collected are used to validate a re-normalization group (RNG) k–? turbulence model used for the warm condition. The agreements between the predicted values and experimental results are acceptable, which demonstrates the feasibility of simulating indoor airflows at elevated room temperature under stratum ventilation by the RNG k–? turbulence model.     

Improvement of natural ventilation in a large factory building using a louver ventilator

When heat generated from facilities inside a large factory building is not discharged outside the building due to a stagnant ventilation flow, the working environment of workers becomes worse, and the cooling of high-temperature products is delayed. In this study, wind tunnel tests were conducted to investigate the natural ventilation of entrained air inside a large factory building. The scale-down factory-building models were embedded in a simulated atmospheric boundary layer (ABL), and the mean and fluctuating velocity fields were measured using a two-frame particle image velocimetry (PIV) technique. For the original factory model, some of the outdoor air came in the factory building through the one-third open windward wall, while the stagnant flow region existed in the rear part of the target area. In order to improve the indoor ventilation environment of the present factory building, three different types of the louver ventilator were attached at the upper one-third open windward wall of the factory model. Among the three louver ventilators tested in this study, the ventilator model ?3 with the outer louver (θ_o=90°) and the inner louver (θ_i=−70°) was found to improve the natural ventilation inside the target factory-building model. It increased the flow rate of the entrained air by aligning the outer louver blades with the oncoming wind and guiding the entrained air down to the ground surface with the elongated inner louver blades.     

An analysis of climatic parameters and sky condition classification

In modelling solar radiation, of the required data daylight illuminance, luminous efficacy, radiance and luminance sky distributions, are always analysed under various sky types. Sky conditions are frequently categorised into overcast, partly cloudy and clear using some common climatic data including cloud cover (CLD), sunshine hour (SH) and solar radiation. This paper presents the study of these climatic parameters and their applications in the classification of sky condition. Frequency of occurrence and cumulative frequency distribution of each sky indicator have been established to interpret the prevailing sky conditions in Hong Kong. Special features on defining the sky clearness are highlighted and the implications for energy-efficient building designs are discussed.