Viability of wind towers in achieving summer comfort in the hot arid regions of the middle east

Operation of conventional wind towers, or Baud-Geers, are described. Wind towers maintain natural ventilation through buildings due to wind or buoyancy effects. The tower structure is cooled externally through radiative transfer with the sky, and internally with the cool ambient air, circulated through the building and the tower during the night. During the day, the warm ambient air is partially cooled by the tower structure before entering the building. When passed over moist surfaces, air is cooled evaporatively. However, sensible and evaporative cooling potentials of conventional wind towers, which depend on the tower design, are limited. Another disadvantage of the conventional wind towers is the admittance of dust into the building.Two modern designs of wind towers are considered which eliminate the above disadvantages. One design incorporates one-way dampers in the tower head and a wetted column in the tower. This design, which is particularly suitable in areas with good winds, evaporatively cools the hot-dry ambient air before admitting it into the building. The other design incorporates evaporative cooling pads at the tower entrance. This design is particularly suitable in areas with very little or no winds.This latter design may be incorporated into the existing minarets of the mosques and shrines, church towers, or other existing tall towers, to supply evaporatively-cooled air into the space.Conventional and the modern versions of wind towers can be incorporated aesthetically into the designs of modern buildings in the hot-arid regions of the Middle East, and other areas of the world with similar climate, to provide summer thermal comfort with little or no use of electricity.     

A natural ventilation wind tower with heat pipe heat recovery for cold climates

Commercial wind towers are passive ventilation technology based on traditional wind towers of the Middle East. Typical operation of wind towers in cold – mild climates is generally limited to summer-seasons as the outdoor air is too cold to be introduced into spaces for the majority of the year. In addition, the use of natural ventilation solutions has been seen to increase heat loss in buildings and lead to increased energy cost. Wind towers are normally shut down for the sake of avoiding indoor heating energy losses during winter months. Consequently, the concentration of pollutants has seen to rise above the guideline levels, which can lead to ill health. To improve the year-round capabilities of wind towers, a heat recovery system utilising the combination of heat pipes and heat sink was incorporated into a multi-directional wind tower. This study investigates the potential of this concept through the use of numerical analysis and wind tunnel experiments for validation. The findings showed that the wind tower with heat pipes was capable of meeting the required ventilation rates above an inlet air velocity of 1 m/s. In addition to sufficient ventilation, the integration of heat pipes had a positive effect on thermal performance of the wind tower; it raised the supply air by up to 4.5 K. The technology presented here is subject to a patent application (PCT/GB2014/052263).     

Energy performance evaluation of an evaporative wind tower

The aim of this paper is to optimize the energy performance of cylindrical cross section evaporative wind towers as passive systems for thermal conditioning of urban spaces. Two theoretical models, a thermal model and a fluid model, have been developed to characterize the evaporative system and the tower design respectively. The thermal model evaluates the tower operation when the fan and the nozzles are working, giving as result the difference between the outlet temperature and inlet temperature. This model has been used to analyze the thermal response of the system to fluctuations in design parameters (water flow, air flow and absorption coefficient of the plastic). To that effect, three one-parametric and one multi-parametric optimization have been done. The fluid model describes the tower operation when the fan and the nozzles are not working, giving as result the wind behavior through the tower. Additional configurations of the wind tower have been evaluated: changing the number of the wind catcher openings, varying the height of the internal walls of the tower and modifying the geometry of the lower ventilation apertures.     

Airflow in courtyard and atrium buildings in the urban environment: a wind tunnel study

A wind tunnel study was carried out to investigate the airflow through courtyard and atrium building models located within an urban setting and exposed to an urban atmospheric boundary layer. Ventilation strategies resulting from the use of different courtyard and atrium pressure regimes (positive pressure and suction) were examined. The model buildings were monitored both in isolation and in idealised urban environments of varying group layout densities. The effect of wind direction was also observed. The results from the study suggest that the open courtyard in an urban environment had a poor ventilation performance whilst an atrium roof with many openings operating under a negative (suction) pressure regime was the most effective. Changing the wind direction from perpendicular to the building façades to a 45° incidence angle had the effect of making the differences in the observed flows between all the models much smaller.     

Performance prediction of a multi-stage wind tower for indoor cooling

A theoretical model is developed to establish an indepth understanding of the performance of a three-stage wind tower with a bypass system for indoor cooling in rural dry and hot climates. Model simulations are presented for a wide range of ambient conditions that include inlet wind speed, inlet temperature and relative humidity. Simulation results provide an insight into the desirable water flow rates and air-to-water loadings for comfort zone tem-peratures and relative humidity levels at the exit of the wind tower. Simulations show wind towers with variable cross-sections provide an increase in the cooling power for the same inlet wind speed, inlet air temperature and relative humidity when compared to wind towers with a constant cross-section. The study shall lead to a better understanding to designing wind towers that are both environmentally friendly and energy efficient.     

A ventilated courtyard as a passive cooling strategy in the warm humid tropics

The paper investigates the potential of a courtyard for passive cooling in a single storey high mass building in a warm humid climate. The inclusion of an internal courtyard in building design is attributed to the optimization of natural ventilation in order to minimize indoor overheating conditions. However, the efficiency of this strategy greatly depends on the design details of the building composition in providing appropriate airflow pattern to the courtyard. From the results of thermal measurements, a significant correlation between wall surface temperatures and indoor air temperatures is evident. A reduction of indoor air temperature below the levels of ambient is seen as a function of heat exchange between the indoor air and high thermal mass of the building fabric. However, this behavior is affected by indoor airflow patterns, which are controlled through the composition between envelope openings and the courtyard of the building.From a computational analysis, several airflow patterns are identified. A relatively better indoor thermal modification is seen when the courtyard acts as an air funnel discharging indoor air into the sky, than the courtyard acts as a suction zone inducing air from its sky opening. The earlier pattern is promoted when the courtyard is ventilated through openings found in the building envelope. The computational simulation utilizing the standard k-ε turbulent model with isothermal condition agrees closely with the measurements taken from the field investigation.     

间接空冷机组空冷塔塔群内空气流动及传热性能研究

由于具有巨大的节水优势,间接空冷机组在我国富煤少水区域得到广泛应用。研究环境风对间接空冷系统的影响机理对指导电厂运行具有重要意义。以某电厂间接空冷机组为基础,构建水平布置散热器的空冷塔群物理和数学模型,通过数值模拟方法分析环境风对塔内空气流场及空冷散热器换热性能的影响。结果表明:环境风对空冷系统塔内空气流场影响较大,进而影响空冷散热器的散热性能。随着风速的增加,空冷塔的换热性能不断恶化。在临界风速时额定负荷下,下游空冷塔换热量比上游空冷塔减少2.5%。     

Numerical simulation of cooling performance of wind tower (Baud-Geer) in hot and arid region

In the present study, an attempt is made to study the cooling performance of a wind tower in a hot and dry region, Yazd, in Iran. For the relevant experiments and numerical studies, at first, the temperature and wind velocity inside and outside of the wind tower measured. Based on four-day measurements during last summer, a computer program was designed with language C⁺⁺ to solve the equations. Also in the study the effects of parameters including wind tower height, variety of the materials used in the wind tower walls, the amount of vaporized water, the temperature of input and output air, the wind velocity and the relative humidity were investigated.Furthermore, to develop, a natural flow of air, for days without blowing a wind the role of solar chimney was considered.Finally, to evaluate the method of integral view and take information about streamlines of airflow in wind tower (Baud-Geer), velocity, pressure, humidity, temperature and density profile of fluid, Fluent software is applied to analyze the air flow in the wind tower in differential view for three-dimensional and steady state conditions with water spraying at the top of wind tower.The results indicate that the evaporative cooling is very effective in a hot and dry region. The temperature decreases considerably, if the wind towers are equipped with the water vaporization system. This causes the air becomes heavier and a natural motion of air through downside of wind tower to be produced.     

Performance analysis and improvement of the use of wind tower in hot dry climate

Wind towers for passive evaporative cooling offer real opportunity for improving the ambient comfort conditions in building whilst reducing the energy consumption of air-conditioning systems.This study aims at assessing the thermal performance of a bioclimatic housing using wind towers realized in a hot dry region of Algeria. Performance monitoring and site measurement of the system provide data which assist model validation. The analysis and site measurement are encouraging, and they confirm the advantage of the application of this passive cooling strategies in hot dry climate.A mathematical model is developed using heat and mass transfer balances. For a more effective evaporative cooling, a number of improvements on wind tower configurations are proposed.     

An improved design of wind towers for natural ventilation and passive cooling

A design is proposed to improve the performance of wind towers (or Baud-Geers) for natural ventilation and passive cooling. Under similar climatological and design conditions, the new design is capable of delivering air to the building at higher flow rates. It can also cool the air evaporatively to lower temperatures. Higher airflow rates and the evaporative cooling capability of the new Baud-Geer design can be fully utilized at nigh in summer to cool the building mass to lower temperatures.Momentum, mass and energy analyses are carried out for the proposed design. The results are presented in graphical forms which may be used as guidelines for employing the design for specific applications in the hot, arid areas of the world. An example is worked out to show the use of the results.     

Numerical simulation of solar wind energy towers

A numerical investigation is conducted to study the performance of solar wind energy towers. The two-phase flow of air and water droplets in the tower is modeled following an Euler–Lagrange approach with air representing the continuous phase and water droplets the discrete phase. Results demonstrate that energy towers perform best in hot and dry environments. Water injection at the inlet to a tower increases the strength of the downdraft current with the rate of increase diminishing as the flow at exit approaches saturation. At a given water injection rate and tower diameter the downdraft strength increases as the height increases, while it is independent of the diameter at constant height. Energy analysis shows that for towers of low height the cost of electricity is expensive and commercially unfeasible, while it is cheap for towers of heights higher than 100?m.     

Natural production, storage, and utilization of ice in deep ponds for summer air conditioning

A design is proposed to improve the performance of wind towers (or Baud-Geers) for natural ventilation and passive cooling. Under similar climatological and design conditions, the new design is capable of delivering air to the building at higher flow rates. It can also cool the air evaporatively to lower temperatures. Higher airflow rates and the evaporative cooling capability of the new Baud-Geer design can be fully utilized at nigh in summer to cool the building mass to lower temperatures.Momentum, mass and energy analyses are carried out for the proposed design. The results are presented in graphical forms which may be used as guidelines for employing the design for specific applications in the hot, arid areas of the world. An example is worked out to show the use of the results.     

Study of thermal comfort in courtyards in a hot arid climate

The outdoor thermal comfort in an enclosed courtyard has been studied numerically by the three dimensional prognostic microclimate model, Envi-met 3.1. The effect of wind, and shading by different means – galleries, horizontal shading or trees – has been examined. The effect of wind is evaluated by allowing cross-ventilation through openings at 3 and 5 m height above ground level, designed according to the prevalent wind direction. The study was conducted for the hours 11–17 LT during June assuming average climate conditions. The thermal comfort is evaluated by the Predicted Mean Vote (PMV) index.During hot summer days, outdoor comfort is mainly dependent on solar radiation; hence, shading is the best means to improve comfort, while the contribution of wind under all configurations studied was limited and much smaller than the shade contribution.The amount of shade is mainly determined by the courtyard orientation. Inspection of empty enclosed courtyards has shown that an elongated E–W rectangular courtyard has the least shade, and therefore it is the most uncomfortable.When the courtyard is ventilated by openings, hot air and radiation penetrate through them increasing the air temperature and the radiation temperature in the courtyard relative to the conditions obtained in a closed courtyard. Higher openings are less comfortable to stay under, and further decrease the comfort in the courtyard. The addition of trees or/and galleries to the closed courtyard significantly improves the outdoor comfort. Under the assumption of constant building temperature of 25°, the addition of galleries is the most efficient shading strategy.Quantitative results exhibiting these trends are presented for specific configurations and orientations of ventilated and/or shaded courtyards.     

Experimental investigation of new designs of wind towers

Two new designs of wind towers were tested side by side with a conventional wind tower in the city of Yazd, Iran. All the towers were of identical dimensions. The two new designs were one with wetted column, consisting of wetted curtains hung in the tower column, and the other one with wetted surfaces, consisting of wetted evaporative cooling pads mounted at its entrance. The air temperature leaving the wind towers with evaporative cooling provisions were much lower than the air temperature leaving the conventional design, and its relative humidity much higher. The air-flow rate was reduced slightly in these new towers. It was found that the wind tower with wetted column performs better with high wind speeds whereas the tower with wetted surfaces performs better with low wind speeds. It is recommended that these new designs of wind towers should be manufactured in different sizes and incorporated in the designs of new buildings. They can replace the evaporative coolers currently employed in Iran, and other hot arid regions, with considerable saving in electrical energy consumption.     

冷却塔外复杂流场的数值模拟与实验研究

对某能源中心楼顶的冷却塔组、烟囱、通风孔等散热设备及整个楼顶空间的流场与温度场进行了整体数值模拟,探讨了高温烟囱帽引起的排烟方向改变对冷却塔组进风处空气温度场及流场的影响,并进行了实验测试。结果表明,烟囱帽使排烟向下折转,导致冷却塔进风处下部位置空气温度显著升高;较低的烟囱烟气直排时,高温烟气会先抬升一段距离,然后受冷却塔进风处负压区吸引进入冷却塔内。     

中国输电线路规范的风荷载计算比较

  [目的]  在台风袭击下,一些20世纪80、90年代建设的110 kV和220 kV输电线路经常出现铁塔损坏的现象,为了加固和改造事故线路,需要分析铁塔损坏的原因。  [方法]  介绍了中国1979、1990、2002和2012四个版本规定的输电线路风荷载计算方法,比较了设计风速、风压高度变化系数、体型系数、风荷载调整系数和覆冰风荷载增大系数等主要计算参数的差异,计算了某铁塔节段和导线在四个版本规定中的风荷载。  [结果]  计算结果表明:12规定的设计风速相比79、90和02规定增大5%～10%;79规程中60 m以下铁塔不考虑风振系数的规定严重低估了塔身风荷载;四本规定中110 kV和220 kV输电线路不采用导地线风荷载调整系数的规定严重低估了导地线风荷载。  [结论]  各规定的铁塔和线条风荷载差异的主要影响因素是风速重现期、铁塔或导地线风荷载调整系数,这也是风灾倒塔的主要原因。     

侧向风对自然通风直接空冷塔性能影响的数值分析

采用多孔介质简化模型分析了在不同风速情况下300 MW自然通风直接空冷系统空冷塔内的流场和各凝汽器换热量.结果表明:在没有侧向风时,流场是均匀对称的,此时塔内空气流量最大,而各凝汽器换热量均匀;当有侧向风时,由于空冷塔底部气流存在漩涡,使空冷塔内空气回流,空气流量减小;在风速大于4 m/s时,各凝汽器换热量差异较大,随着风速增加,这种现象加剧.     

Development of a modified stochastic subspace identification method for rapid structural assessment of in‐service utility‐scale wind turbine towers

The strong drive to harness wind energy has recently led to rapid growth of wind farm construction. Wind turbine towers with increased sizes and flexibility experience large vibrations. Structural health monitoring of wind turbines is proposed in the wind energy industry to ensure their proper performance and save maintenance costs. This study proposes a system identification method for vibration‐based structural assessment of wind turbine towers. This method developed based on the stochastic subspace identification method can identify modal parameters of structures in operating conditions with harmonic components in excitations. It benefits wind turbine tower structural health assessment because classical operational modal analysis methods can fail as periodic rotation excitation from a turbine introduces harmonic disturbance to tower structure response data. The effectiveness, accuracy and robustness of the proposed method were numerically investigated and verified through a lumped‐mass system model. The method was then applied to an in‐service utility‐scale wind turbine tower. The field testing campaign and modal parameter identification as well as structural assessment results were presented. Copyright © 2017 John Wiley & Sons, Ltd.     

Wind-induced natural ventilation analysis

The paper presents and discusses internal pressure and discharge coefficients for a building with wind-driven cross-ventilation caused by sliding window openings on two adjacent walls. The study found that both coefficients vary considerably with the opening area (porosity of wall(s)) and the inlet to outlet ratio. Comparisons with previous work were also carried out. Experimental results verify the unsteady pressure and velocity field, particularly in the case of cross-ventilation with large opening areas. For such cases, a simulation sensitivity analysis of wind-induced building ventilation confirms that airflow rates vary considerably when different discharge coefficient values are used.     

The influence of wind speed,terrain and ventilation system on the air change rate of a single-family house

When the heat balance of a building is assessed, the heat needed for the ventilation air is usually calculated according to the intended ventilation rate. However, in order to calculate the air change rate accurately several aspects have to be considered. One important parameter is the ventilation system. Whether the building has a mechanical exhaust-only, supply-only, balanced exhaust–supply or natural ventilation system will influence the air infiltration rate through cracks in the building envelope. High infiltration rates lead to an increase in the heating demand and can result in an inadequate capacity of the designed heating installation. In this paper, computer simulations of the air change rate for a detached single-family house are presented. The house is simulated in different topographical surroundings, equipped with a mechanical exhaust-only, or a balanced exhaust–supply, ventilation system. In addition, the airtightness of the building is varied, from very tight, 1 air change per hour (ACH), to quite leaky, 6 ACH, when pressurized to 50 Pa. Results from the simulations show that the same house has quite different air change rates in different surroundings with different airtightness.