A multi-stage down-draft evaporative cool tower for semi-enclosed spaces: Experiments with a water spraying system

A novel down-draft evaporative cool tower (DECT) was developed which incorporates a secondary air inlet and a complex longitudinal section that comprises two partly overlapping cones. The objective of this design is to conserve water while providing equal or superior cooling to conventional DECTs, in which dry ambient air is drawn in through a single inlet at the top and cooler moist air is delivered at the bottom.The complex section and the addition of a secondary air intake near the middle required a sophisticated water spraying system. In addition to maximizing the cooling potential, the spraying system was designed to limit spray drift beyond the base of the tower, to reduce maintenance costs (especially due to clogging of the sprayers) and to minimize pumping energy. Analysis shows that maximum cooling may be obtained either by employing a very fine spray, requiring the introduction of a relatively small volume of water, or by spraying a larger volume of coarser drops. However, spraying fine drops requires more pumping power, finer nozzles are more likely to clog and small drops of water aggravate the problem of drift near the tower base. If full evaporation of the water spray is not required and excess water is collected for reuse, the second option is thus preferable. In addition to the theoretical analysis, the paper presents experimental findings on temperature reduction, water consumption and cooling output of an 8-m high prototype tower constructed at Sede-Boqer, Israel.     

Thermal performance of a building coupled to an evaporative cooling tower

The thermal performance of a building fitted with an evaporative cooling tower has been evaluated in terms of discomfort index for two climates, namely, composite and hot-dry, typified by New Delhi and Jodhpur, respectively. The effects of various evaporative cooling parameters (height and cross-sectional area of the tower, packing factor, area of the pads, resistance offered to the air flow and local wind conditions) on the performance of the building have been analysed. It was found that, for given parameters of the tower and wind conditions, there is an optimum height of the tower for which the thermal discomfort condition in the building is minimum. The optimum values of the tower height for comfort conditions in the building for various other tower parameters have been obtained for each climate.     

Cooling potential of an evaporative cooling tower: Parametric studies

Dependence of the cooling potential of an evaporative cooling tower on the tower parameters (height h, cross-sectional area A_t, evaporative pad area A_p, packing factor of evaporating pads F_p and flow resistance f) has been investigated. The performance of the tower is studied for two different climates, namely hot-dry and composite, typified by Jodhpur and Delhi.     

Theoretical and testing performance of an innovative indirect evaporative chiller

An indirect evaporative chiller is a device used to produce chilled water at a temperature between the wet bulb temperature and dew point of the outdoor air, which can be used in building HVAC systems. This article presents a theoretical analysis and practical performance of an innovative indirect evaporative chiller. First, the process of the indirect evaporative chiller is introduced; then, the matching characteristics of the process are presented and analyzed. It can be shown that the process that produces cold water by using dry air is a nearly-reversible process, so the ideal produced chilled water temperature of the indirect evaporative chiller can be set close to the dew point temperature of the chiller’s inlet air. After the indirect evaporative chiller was designed, simulations were done to analyze the output water temperature, the cooling efficiency relative to the inlet dew point temperature, and the COP that the chiller can performance. The first installation of the indirect evaporative chiller of this kind has been run for 5 years in a building in the city of Shihezi. The tested output water temperature of the chiller is around 14–20 °C, which is just in between of the outdoor wet bulb temperature and dew point. The tested COP_r,s of the developed indirect evaporative chiller reaches 9.1. Compared with ordinary air conditioning systems, the indirect evaporative chiller can save more than 40% in energy consumption due to the fact that the only energy consumed is from pumps and fans. An added bonus is that the indirect evaporative chiller uses no CFCs that pollute to the aerosphere. The tested internal parameters, such as the water–air flow rate ratio and heat transfer area for each heat transfer process inside the chiller, were analyzed and compared with designed values. The tested indoor air conditions, with a room temperature of 23–27 °C and relative humidity of 50–70%, proved that the developed practical indirect evaporative chiller successfully satisfy the indoor air conditioning load for the demo building. The indirect evaporative chiller has a potentially wide application in dry regions, especially for large scale commercial buildings. Finally, this paper presented the geographic regions suitable for the technology worldwide.     

Energetic and exergetic performance analyses of a combined heat and power plant with absorption inlet cooling and evaporative aftercooling

In this paper, exergy method is applied to analyze the gas turbine cycle cogeneration with inlet air cooling and evaporative aftercooling of the compressor discharge. The exergy destruction rate in each component of cogeneration is evaluated in detail. The effects of some main parameters on the exergy destruction and exergy efficiency of the cycle are investigated. The most significant exergy destruction rates in the cycle are in combustion chamber, heat recovery steam generator and regenerative heat exchanger. The overall pressure ratio and turbine inlet temperature have significant effect on exergy destruction in most of the components of cogeneration. The results obtained from the analysis show that inlet air cooling along with evaporative aftercooling has an obvious increase in the energy and exergy efficiency compared to the basic gas turbine cycle cogeneration. It is further shown that the first-law efficiency, power to heat ratio and exergy efficiency of the cogeneration cycle significantly vary with the change in overall pressure ratio and turbine inlet temperature but the change in process heat pressure shows small variation in these parameters.     

A new zero energy cool chamber with a solar-driven adsorption refrigerator

A new zero energy cool chamber (ZECC) consisting of two cooling systems, a solar-driven adsorption refrigerator and an evaporative cooling system, was developed and then evaluated as low-cost and eco-friendly cooling storage for storing fruit with moderate respiration rates. The solar-driven adsorption refrigerator, consisting of a solar collector containing activated carbon as an adsorbent, a condenser and an evaporator, cools water based by evaporating methanol and adsorbing it on activated carbon, and then makes ice. The methanol adsorbed on the activated carbon is desorbed by applying solar heat. The ice is then used to cool the storage space, which can be done for a long time without the need for electricity. The evaporative cooling system also cools the storage space by evaporating water from the wet walls containing wet filler. The combined use of two cooling systems reduced the average inside temperature of the new ZECC to 12.07 °C compared with an average outside temperature of 31.5 °C and extended the shelf life of tomatoes from 7 to 23 days. These results suggest that the new ZECC proposed here is low-cost and energy-saving and is useful for storing fruit and vegetables in areas where electricity is unavailable.     

Comparative study of the performance of the M-cycle counter-flow and cross-flow heat exchangers for indirect evaporative cooling – Paving the path toward sustainable cooling of buildings

This paper provides a comparative study of the performance of cross-flow and counter-flow M-cycle heat exchangers for dew point cooling. It is recognised that evaporative cooling systems offer a low energy alternative to conventional air conditioning units. Recently emerged dew point cooling, as the renovated evaporative cooling configuration, is claimed to have much higher cooling output over the conventional evaporative modes owing to use of the M-cycle heat exchangers. Cross-flow and counter-flow heat exchangers, as the available structures for M-cycle dew point cooling processing, were theoretically and experimentally investigated to identify the difference in cooling effectiveness of both under the parallel structural/operational conditions, optimise the geometrical sizes of the exchangers and suggest their favourite operational conditions. Through development of a dedicated computer model and case-by-case experimental testing and validation, a parametric study of the cooling performance of the counter-flow and cross-flow heat exchangers was carried out. The results showed the counter-flow exchanger offered greater (around 20% higher) cooling capacity, as well as greater (15%–23% higher) dew-point and wet-bulb effectiveness when equal in physical size and under the same operating conditions. The cross-flow system, however, had a greater (10% higher) Energy Efficiency (COP). As the increased cooling effectiveness will lead to reduced air volume flow rate, smaller system size and lower cost, whilst the size and cost are the inherent barriers for use of dew point cooling as the alternation of the conventional cooling systems, the counter-flow system is considered to offer practical advantages over the cross-flow system that would aid the uptake of this low energy cooling alternative. In line with increased global demand for energy in cooling of building, largely by economic booming of emerging developing nations and recognised global warming, the research results will be of significant importance in terms of promoting deployment of the low energy dew point cooling system, helping reduction of energy use in cooling of buildings and cut of the associated carbon emission.     

On the development, optical properties and thermal performance of cool colored coatings for the urban environment

This paper reports the measured solar spectral properties and the thermal performance of 10 prototype cool colored coatings, developed at the National and Kapodistrian University of Athens, using near-infrared reflective color pigments in comparison to color-matched, conventionally pigmented coatings. These coatings are developed to be used in the urban environment to fight the heat island effect. The spectral reflectance and the infrared emittance were measured and the solar reflectance of the samples was calculated. The surface temperature of the coatings when applied to concrete tiles was monitored, using surface temperature sensors and a data logging system, on 24 h basis from August to December 2005 in an effort to investigate the ability of the cool colored coatings to maintain lower surface temperatures than conventionally pigmented color-matched coatings. The data obtained has been extensively analysed and indicate significant success in the development of these cool colored coatings. It was found that all the coatings containing infrared reflective pigments have solar reflectance values higher than those of standard coatings. Furthermore, it was demonstrated that cool colored coatings maintain lower surface temperatures than color-matched conventionally pigmented coatings. This temperature difference is mainly due to differences in solar reflectance. These cool colored coatings can be used on buildings (roofs and walls) and other surfaces in the urban environment. Thus, at building scale, the use of cool colored coatings with increased solar reflectance can improve building comfort and reduce cooling energy use, and at city city-scale it can contribute to the reduction of the air temperature due to the heat-transfer phenomena and therefore improve outdoor thermal comfort and reduce the heat-island effect.     

A new approach to analysis and optimization of evaporative cooling system II: Applications

After introducing the concepts of moisture entransy, moisture entransy dissipation and thermal resistance based on moisture entransy dissipation (TRMED) in part I of this study, we further analyze several direct/indirect evaporative cooling processes based on the above concepts in this part. The nature of moisture entransy, moisture entransy dissipation and TRMED during evaporative cooling processes was reexamined. The results demonstrate that it is the moisture entransy, not the enthalpy, that represents the endothermic ability of a moist air, and reducing the entransy dissipation by both enlarging the thermal conductance of heat and mass transfer, and decreasing the temperature potential of the moist air, i.e. the difference between the dry-bulb temperature of moist air over its dew-point temperature, will result in a smaller system TRMED, and consequently a better evaporative cooling performance. Then, a minimum thermal resistance law for optimizing evaporative cooling systems is developed. For given mass flow rates of both moist air and water, with prescribed moist air and water conditions, minimizing the TRMED will actually lead to the most efficient evaporative cooling performance. Finally, the thermal conductance allocation for an indirect evaporative cooling system is optimized to illustrate the application of the proposed minimum thermal resistance law.     

A new approach to analysis and optimization of evaporative cooling system I: Theory

Using the analogy between heat and mass transfer processes, the recently developed entransy theory is extended in this paper to tackle the coupled heat and mass transfer processes so as to analyze and optimize the performance of evaporative cooling systems. We first introduce a few new concepts including the moisture entransy, moisture entransy dissipation, and the thermal resistance in terms of the moisture entransy dissipation. Thereinafter, the moisture entransy is employed to describe the endothermic ability of a moist air. The moisture entransy dissipation on the other hand is used to measure the loss of the endothermic ability, i.e. the irreversibility, in the coupled heat and mass transfer processes – this total loss is shown to consist of three parts: (1) the sensible heat entransy dissipation, (2) the latent heat entransy dissipation, and (3) the entransy dissipation induced by a temperature potential. Finally the new thermal resistance, defined as the moisture entransy dissipation rate divided by the squared refrigerating effect output rate, is recommended as an index to effectively reflect the performance of the evaporative cooling system. In the end, two typical evaporative cooling processes are analyzed to illustrate the applications of the proposed concepts.     

Effects of acoustic barriers and crosswind on the operating performance of evaporative cooling tower groups

Most evaporative cooling towers are arranged on building roof due to the limitation of space and noise, and acoustic barriers are always installed around cooling towers in practical applications. The existence of acoustic barriers and crosswind may affect the recirculation phenomenon which is directly related to the operating performance of cooling towers. In this study, a physical and mathematical computation model is proposed to research the crosswind and distance between acoustic barriers and inlet of cooling towers. Both sensible and latent heat are considered in this research. The reflux flow rate and performance ratio are obtained to evaluate the recirculation and operating performance, respectively. The results show that the higher the crosswind velocity, the larger the reflux flow rate, and the lower the performance ratio of cooling tower groups. For high crosswind velocity, the presence of acoustic barriers is useful to inhibit reflux and improve operating performance, especially for ICE cooling tower groups. In addition, the optimum values are recommended for LiBr/ICE cooling tower groups in the research cases The variation of reflux flow rate and performance ratio with the acoustic barriers’ distance presents a parabolic tendency.     

Thermal analysis on the cooling performance of a porous evaporative plate for building

In this paper, a wet porous cooling plate has been used for a building wall. Cooling can be achieved due to the evaporation in the porous layer. A mathematical model on the heat and mass transfer in the unsaturated porous media is developed to analyze the influences of ambient conditions and the porous layer thickness on the cooling performance of the porous evaporative plate. With a decrease in ambient relative humidity and an increase in ambient temperature, more cooling of the porous evaporative plate can be supplied for the inside of the room. The heat exchange between the inside surface of the porous plate and the air in the room should be intensified to achieve a higher cooling efficiency of the porous plate. The ambient wind speed and the thickness of the porous plate also have significant influence on the average temperature of the porous plate. All these results should be taken into account for the utilization of the porous evaporative cooling plate. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20284     

A solar cooling system for greenhouse food production in hot climates

This study is motivated by the difficulty of cultivating crops in very hot countries and by the tendency for some such countries to become dependent on imported food.

Liquid desiccation with solar regeneration is considered as a means of lowering the temperature in evaporatively-cooled greenhouses. Previous studies demonstrated the technical feasibility of the desiccation–evaporation process, but mainly in the context of human dwellings.

In the proposed cycle, the air is dried prior to entering the evaporative cooler. This lowers the wet-bulb temperature of the air. The cooling is assisted by using the regenerator to partially shade the greenhouse. The heat of desiccation is transferred and rejected at the outlet of the greenhouse.

The cycle is analysed and results given for the climate of the The Gulf, based on weather data from Abu Dhabi. Taking examples of a temperate crop (lettuce), a tropical crop (tomato) and a tropical crop resistant to high temperatures (cucumber) we estimate the extension in growing seasons relative to (i) a greenhouse with simple fan ventilation (ii) a greenhouse with conventional evaporative cooling.

Compared to option (ii), the proposed system lowers summers maximum temperatures by 5 °C. This will extend the optimum season for lettuce cultivation from 3 to 6 months of the year and, for tomato and cucumber, from 7 months to the whole year.     

Recent researches in indirect evaporative cooler V: relative thermal performance of buildings coupled to direct and indirect evaporative cooler

A mathematical model has been developed to evaluate the relative thermal performance of a building coupled with an indirect or direct evaporative cooler. Using periodic analysis for taking into account thermal storage of building envelope, explicit expressions have been obtained for room air temperature and room air humidity. For comparing their performance under different climatic conditions, numerical calculations have been made taking meteorological parameters for a typical day for Delhi (composite climate), Jodhpur (hot-dry climate) and Madras (hot-humid climate). It is found that the indirect evaporative cooler is a more effective and energy efficient system than the air-conditioner; it can hence be commercially used for computer and electronic exchange applications as well as for human comfort in a variety of climatic conditions, whereas direct evaporative cooler has limited use (only in hot-dry and composite climates). © 1997 by John Wiley & Sons, Ltd.     

Thermal performance of a closed wet cooling tower for chilled ceilings: measurement and CFD simulation

A closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested experimentally. The thermal efficiency of the cooling tower was measured for different air flow rates, water flow rates, spray flow rates and wet bulb air temperatures. CFD was also used to predict the thermal performance of the cooling tower. Good agreement was obtained between CFD prediction and experimental measurement. Copyright © 2000 John Wiley & Sons, Ltd.     

Short cut performance method for the design of flexible cooling systems

This paper considers the design of cooling systems in the context of piping costs, exchanger costs, pumping costs and its hydraulic and thermal performance. A methodology for designing coolers in the context of both process needs and cooling water system behaviour is introduced. It is recognised that cooling systems need to be flexible. One way of ensuring this is to design a system for the most demanding load and then use bypasses to control performance under reduced load.The hydraulic modelling is based on new formulations of flow resistance for pipes, pipe fittings and equipment items. By using volumetric flow rate rather than velocity as the prime variable it becomes possible to construct hydraulic models for cooling water systems quickly. These calculations then provide predictions of water flows to the individual heat exchangers in the cooling water network. Knowledge of these flows is fundamental to both the design of new coolers and the prediction of the thermal performance of exchangers of known geometry. Previous studies have ignored this aspect of design.     

侧风环境下湿式冷却塔性能参数变化规律的试验研究

外界侧风在很大程度上影响自然通风逆流湿式冷却塔的传热传质性能,根据相似理论,通过热态模型试验,以冷却数和刘易斯数为出发点,研究了外界侧风对冷却塔传热传质性能的影响.研究发现:冷却数和刘易斯数随着外界侧风的变化呈现先减小后增大的趋势,拐点风速值为0.4 m/s;当外界侧风风速小于0.7 ～0.8m/s时,侧风的存在严重恶...     

A decoupled method to identify affecting mechanism of crosswind on performance of a natural draft dry cooling tower

The natural draft dry cooling tower (NDDCT) has been increasingly used for cooling in power generation in arid area. As crosswind affects the performance of a NDDCT in a complicated way, and the basic affecting mechanism is unclear, attempts have been made to improve the performance of a NDDCT based on limited experiences. This paper introduces a decoupled method to study the complicated crosswind effects on the inlet and outlet of a NDDCT separately by computational fluid dynamics (CFD) modeling and hot state experiments. Accordingly, the basic affecting mechanism of crosswind on the NDDCT performance is identified. Crosswind changes the inlet flow field of a NDDCT and induces mainstream vortices inside the tower, so as to degrade the ventilation. Besides, low crosswind deflects the upward plume at the outlet to further degrade the ventilation, while high crosswind induces the low pressure area at the outlet to reduce the ventilation degradation.     

Prediction,potential and control of plume from wet cooling tower of commercial buildings in Hong Kong: A case study

This paper presents a case study of the prediction, potential and control of plume in wet cooling towers from a huge commercial building in Hong Kong based on the weather data available for a particular year. The power input is found to be lower and the coefficient of performance (COP) moderate when all the 10 towers with low speed are in use, while it is found to be reverse when there are five towers, especially, three low and two high‐speed towers are used. It is also found that the combined heating and cooling option can be a better approach than that of the heating option alone from the point of view of thermodynamics as well as from the point of view of economics. The COP of the chillers increases from 6.01 to 7.09 when the number of cooling towers increases from five to ten. On the other hand, the power consumption first decreases and then increases which is mainly due to the increment in the consumption of fan power from 270 to 900 kW for both options. The overall power consumption decreases slightly for the combined heating and cooling option, while in the heating option, the overall power consumption increases slightly. However, it is observed that a proper operation of cooling towers is an effective means to control and/or at least reduce the potential of visible plume generated by wet cooling towers at the existing chilling plant design for this particular building. Copyright © 2006 John Wiley & Sons, Ltd.