Effectiveness of indirect evaporative cooling and thermal mass in a hot arid climate

In this paper, we compare results of a long-term temperature monitoring in a building with high thermal mass to indoor temperature predictions of a second building that uses an indirect evaporative cooling system as a means of passive cooling (Vivienda Bioclimática Prototipo -VBP-1), for the climatic conditions of Sde Boqer, Negev region of Israel (local latitude 30°52′N, longitude 34°46′E, approximately 480 m above sea level). The high-mass building was monitored from January through September 2006 and belongs to a student dormitory complex located at the Sde Boqer Campus of Ben-Gurion University. VBP-1 was designed and built in Maracaibo, Venezuela (latitude 10°34′N, longitude 71°44′W, elevation 66 m above sea level) and had its indoor air temperatures, below and above a shaded roof pond, as well as the pond temperature monitored from February to September 2006. Formulas were developed for the VBP-1, based on part of the whole monitoring period, which represent the measured daily indoor maximum, average and minimum temperatures. The formulas were then validated against measurements taken independently in different time periods. The developed formulas were here used for estimating the building's thermal and energy performance at the climate of Sde Boqer, allowing a comparison of two different strategies: indirect evaporative cooling and the use of thermal mass.     

Energy saving potential of an indirect evaporative cooler as a pre-cooling unit for mechanical cooling systems in Iran

The performance of indirect evaporative cooling system (IEC) to pre-cool air for a conventional mechanical cooling system has been investigated for four cities of Iran. For this purpose, a combined experimental setup consisting of an IEC unit followed by a packaged unit air conditioner (PUA) was designed, constructed and tested. Two air simulators were designed and used to simulate indoor heating load and outdoor design conditions. Using of experimental data and an appropriate analytical method, the performance and energy reduction capability of combined system has been evaluated through the cooling season. The results indicate IEC can reduce cooling load up to 75% during cooling seasons. Also, 55% reduction in electrical energy consumption of PUA can be obtained.     

Performance analysis of a ground-assisted direct evaporative cooling air conditioner

In this paper, the results of performance analysis of a ground-assisted hybrid evaporative cooling system in Tehran have been discussed. A Ground Coupled Circuit (GCC) provides the necessary pre-cooling effects, enabling a Direct Evaporative Cooler (DEC) that cools the air even below its wet-bulb temperature. The GCC includes four vertical ground heat exchangers (GHE) which were arrayed in series configuration. In order to have an accurate prediction of the optimum performance of a GCC, a computational fluid dynamic simulation was performed. Simulation results revealed that the combination of GCC and DEC system could provide comfort condition whereas DEC alone did not. Based on the simulation results the cooling effectiveness of a hybrid system is more than 100%. Thus, this novel hybrid system could decrease the air temperature below the ambient wet-bulb temperature. This environmentally clean and energy efficient system can be considered as an alternative to the mechanical vapor compression systems.     

新疆地区三级蒸发冷却空调系统工程应用分析

介绍了三级蒸发冷却空调系统在新疆地区3个空调工程中应用的情况,对三级蒸发冷却空调实际应用的不同形式作了说明,给出了测试数据和分析结果,并简要介绍了三级蒸发冷却空调的自动控制系统。     

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.     

蒸发冷却制备冷水流程的热学分析

利用空气-水热湿交换过程湿(火积)和显热(火积)相互转换的原理,从(火积)的角度对蒸发冷却过程进行了重新认识和分析.比较了直接蒸发冷却和间接蒸发冷却制备冷水方式,从(火积)损失的角度指出了两种方式制取冷水极限温度不同的本质原因.在换热面积无限大和换热面积有限两类工况下,通过湿 (火积)-显热(火积)转换过程分析了两种不同结构的间接蒸发冷却制备冷水的流程.在换热面积有限工况下,通过显热(火积)和湿(火积)的转换得到了系统的显热(火积)损失平衡的原理,研究了流程内部各环节换热面积分配的优化方法;比较了间接蒸发冷水机的两种不同结构,尝试从系统换热量与制冷量之比、系统总传递(火积)损失的角度得出对流程结构的一些新的认识.     

直接蒸发冷却空调循环冷却水的臭氧净化

采用气液混合泵投加臭氧,限制引风机出口臭氧浓度在0.2mg/m3以下,在直接蒸发冷却空调样机上进行了循环水净化动态试验。32天的运行结果表明,臭氧投加量为0.03～0.09g/(m3.h),浓缩倍数由1.5逐步提高到4.5,杀菌、缓蚀、阻垢效果明显。对该方法的工业应用提出了建议。     

Thermal effectiveness characteristics of low approach indirect evaporative cooling systems in buildings

Meteorological enthalpy analysis of temperate and maritime climates above latitude 45°N suggests that the water-side evaporative cooling technique has considerable unrealised potential with contemporary “high temperature” building cooling systems—such as chilled ceilings and displacement ventilation. As low approach conditions are the key to exploiting the cooling potential of the ambient air, thermal performance at such conditions needs to be investigated. To address the research issues, an industrial scale test rig, based on a low approach open cooling tower and plate heat exchanger and designed to maximise evaporative cooling potential, has been constructed.     

含湿多孔介质应用于建筑墙体的制冷性能分析

在建立多孔介质墙体热湿平衡的基础上,采用描述非饱和多孔介质热质迁移的数学模型,分析了多孔介质墙体方位及结构、多孔介质层的材料及孔隙率对床层制冷特性的影响,为多孔介质蒸发制冷墙体的推广和应用提供理论依据.     

Energy savings in a building using regenerative evaporative cooling

This paper explores the potential of reducing the annual energy consumption of a central air-conditioned building through advanced evaporative cooling systems. The building considered is a typical three floor library building of a University. The regenerative evaporative cooling technology is coupled with the liquid cooled water chiller system to accomplish the energy conservation objective. Comparisons of the regenerative evaporative cooling are made with simple evaporative cooling to bring out the importance such a system. The well-known building simulation software, TRNSYS is used to carry out the heat load calculations and the dynamic simulations of the building. Annual energy consumptions of different components of the air-conditioning system are estimated for the existing water chiller system as well as for both coupled evaporative cooling systems (simple and regenerative). The annual energy consumptions, the indoor temperature, the relative humidity and the thermal comfort index ‘PMV’ are compared for all the three different air-conditioning systems. The coupling of direct and regenerative evaporative cooling technologies with water chiller system has shown, respectively, 12.09% and 15.69% savings in annual energy consumption of the building, while maintaining PMV between ?1 and +1 for most of the hours in the year.     

Investigation of a hybrid system of nocturnal radiative cooling and direct evaporative cooling

In this paper, the results of a study on a hybrid system of nocturnal radiative cooling, cooling coil, and direct evaporative cooling in Tehran have been discussed. During a night, the nocturnal radiative cooling provides required chilled water for a cooling coil unit. The cold water is stored in a storage tank. During eight working hours of the next day, hot outdoor air is pre-cooled by means of the cooling coil unit and then it enters a direct evaporative cooling unit. In this period, temperature variation of the conditioned air is investigated. This hybrid system complements direct evaporative cooling as if it consumes low energy to provide cold water and is able to fulfill the comfort condition whereas direct evaporative alone is not able to provide summer comfort condition. The results obtained demonstrate that overall effectiveness of hybrid system is more than 100%. Thus, this environmentally clean and energy efficient system can be considered as an alternative to the mechanical vapor compression systems.     

Impact of climate warming on passive night cooling potential

Night-time ventilation is often seen as a promising passive cooling concept. However, as it requires a sufficiently high temperature difference between ambient air and the building structure, this technique is highly sensitive to changes in climatic conditions. In order to quantify the impact of climate warming on the night-time ventilative cooling potential in Europe, eight representative locations across a latitudinal transect were considered. Based on a degree-hours method, site-specific regression models were developed to predict the climatic cooling potential (CCP) from minimum daily air temperature (T _min). CCP was computed for present conditions (1961–90) using measured T _min data from the European Climate Assessment (ECA) database. Possible time-dependent changes in CCP were assessed for 1990–2100, with particular emphasis on the Intergovernmental Panel on Climate Change (IPCC) ‘A2’ and ‘B2’ scenarios for future emissions of greenhouse gases and aerosols. Time-dependent, site-specific T _min scenarios were constructed from 30 Regional Climate Model (RCM) simulated data sets, as obtained from the European PRUDENCE project. Under both emissions scenarios and across all locations and seasons, CCP was found to decrease substantially by the end of the 21st century. For the six Central and Northern European locations (>47°N) CCP was found to decrease in summer (June–August) by 20–50%. For the two Southern European locations (Madrid and Athens), future CCP was found to become negligible during the summer and to decrease by 20–55% during the spring and the autumn. The study clearly shows that night-time cooling potential will cease to be sufficient to ensure thermal comfort in many Southern and Central European buildings. In Central and Northern Europe, a significant passive cooling potential is likely to remain, at least for the next few decades. Upper and lower bound estimates for future CCP were found to diverge strongly in the course of the 21st century, suggesting the need for flexible building design and for risk assessments that account for a wide range of emissions scenarios and uncertainty in climate model results.

La ventilation nocturne est souvent considérée comme un concept de refroidissement passif prometteur. Toutefois, cette technique nécessitant une différence de température suffisamment élevée entre l'air ambiant et la structure du bâtiment, elle est très sensible aux changements des conditions climatiques. Pour quantifier l'impact du réchauffement climatique sur les possibilités de refroidissement par ventilation nocturne, on a considéré huit emplacements représentatifs sur une transversale latitudinale en Europe. Sur la base d'une méthode ‘degrés-heures’, on a développé des modèles de régression spécifiques à des sites pour prévoir le potentiel de refroidissement climatique (CCP) à partir de la température quotidienne minimale de l'air (T _min). On a calculé le CCP pour les conditions présentes pendant la période 1961–90 en utilisant les données T _min de la base de données ECA (Evaluation du climat européen). On a évalué de possibles changements du CCP liés à la durée pour la période 1990–2100, en mettant l'accent sur les scénarios A2 et B2 du GIEC (Groupe d'experts intergouvernemental sur l'évolution du climat) relatifs aux futures émissions de gaz à effet de serre et aux aérosols. Des scénarios liés à la durée, avec T _min spécifiques aux sites ont été élaborés à partir de 30 ensembles de données de modèles de climats régionaux simulés obtenus dans le cadre du projet européen PRUDENCE. Pour les deux scénarios d'émission et pour tous les emplacements et saisons, on a constaté que le CCP diminuait de façon substantielle dès la fin du XXIe siècle. Pour les six emplacements en Europe centrale et en Europe du Nord (>47° nord), on a constaté que le CCP diminuait de 20–50% (juin–août). Pour les deux emplacements en Europe du Sud (Madrid et Athènes), on a constaté que le futur CCP devenait négligeable pendant l'été et diminuait de 20–55% pendant le printemps et l'automne. Cette étude montre clairement que le potentiel de refroidissement nocturne cessera d'être suffisant pour assurer le confort thermique dans de nombreux bâtiments construits en Europe du Sud et en Europe centrale. En Europe centrale et en Europe du Nord, un potentiel significatif de refroidissement passif va vraisemblablement durer pendant au moins les prochaines décennies. Il s'est avéré que les estimations supérieures et inférieures du futur CCP divergeaient nettement dans le courant du XXIe siècle, ce qui suggère la nécessité de concevoir les bâtiments avec souplesse et d'évaluer le risque en tenant compte d'une large gamme de scénarios d'émission et de l'incertitude des résultats des modèles climatiques.

Mots clés: changements climatiques, scénarios climatiques, potentiel de refroidissement climatique, ventilation nocturne, refroidissement passif, Europe     

基于蒸发冷却的辐射供冷/热实验台设计探讨

对基于蒸发冷却的辐射供冷/热半集中式空调系统实验台进行了简要介绍,并对实验台的新风系统和水系统设计进行了详细说明.分析了该实验台在中湿度地区不同季节的运行模式,指明了该实验台的特点.     

Energy performance evaluation of a novel evaporative cooling technique

High summer conditioning consumption is becoming a tough and critical issue and consequently there is a need to provide buildings with new technologies for energy saving. Current European and Italian legislation is also working in this direction. We present a preliminary experimental evaluation of the energy performance of a new technology which is capable of canceling conduction gains through walls: “water-evaporative walls”, which are not only able to prevent the entrance of energy fluxes from the exterior to the interior, but also to reduce wall temperatures to below the values found indoors. This solution basically suggests equipping standard ventilated façades with a proper water-evaporative system, which exploits the latent heat of water evaporation, in order to absorb summer cooling loads. From the technological point of view, it requires the insertion of a water spraying system and a proper insulating layer in the ventilated air chamber. The insulation will act not only as a standard insulating material, but also as a porous surface to store water sprayed by the system and then gradually release it when needed for cooling. The experimental analyses showed the effectiveness of this technology, which decreases the overall summer energy load in buildings by canceling conduction loads.     

Comparative study of heat and mass exchanging materials for indirect evaporative cooling systems

This paper investigated several types of materials, namely metals, fibres, ceramics, zeolite and carbon, which have potential to be used as heat and mass transfer medium in the indirect evaporative cooling systems, and from the investigation, the most adequate material and structure were identified. Magnitude of heat/mass transfer rates in relation to an air-conditioning application was analysed, and the results showed that thermal properties of the materials, i.e., thermal conductivity and water-retaining capacity (porosity), have little impact on system heat/mass transfer, and therefore, these two parameters play low keys in terms of material selection. Instead, shape formation/holding ability, durability, compatibility with water-proof coating, contamination risk as well as cost, are more important concerns in this regard. Each material type was then analysed based on the above criteria and the preferable structure and configuration of this type was illustrated. A comparative analysis into different material types was carried out, and the results showed that the wick (sintered, meshes, groves and whiskers) attained metals (cooper or aluminium) are the most adequate structure/material over the others. Wick-attained aluminium sheet is much cheaper than cooper with the same structure and therefore more suitable for this application.     

Passive downdraught evaporative cooling: performance in a prototype house

The design and performance of a passive downdraught evaporative cooling (PDEC) system are reported. This is the first application of this technique for housing in Europe. The system was integrated within a house designed and built by University of Nottingham students for the 2010 Solar Decathlon Europe event in Madrid, Spain. Testing of the thermal performance of the house occurred for a limited period during June 2010. Performance data have demonstrated that the combination of a high-performance building envelope and PDEC provided comfortable conditions inside the house over a period of increasingly warm weather. Estimates of the cooling achieved, based on the measured results, were compared with the energy required by the system to derive an indicative range of coefficient of performance (CoP) values under varying ambient conditions. The results suggest that PDEC can deliver significant energy savings and achieve comfortable thermal conditions without the need for additional mechanical cooling. This technique may therefore have wider relevance to housing in central and southern Spain, and other hot, dry regions of the world.

Il est rendu compte de la conception et des performances d'un système de refroidissement passif évaporatif à courant d'air descendant (PDEC). Il s'agit de la première application de cette technique dans le domaine du logement en Europe. Le système a été intégré à une maison conçue et construite par des étudiants de l'Université de Nottingham pour la manifestation Solar Decathlon Europe 2010 qui s'est tenue à Madrid, en Espagne. Les données de performance ont démontré que la combinaison d'une enveloppe de bâtiment haute performance et d'un système PDEC ont assuré des conditions agréables à l'intérieur de la maison sur une période où le temps est devenu de plus en plus chaud. Les estimations quant au refroidissement obtenu, sur la base des résultats mesurés, ont été comparées à l'énergie exigée par le système de façon à déterminer une plage indicative des valeurs du coefficient de performance (CoP) dans différentes conditions ambiantes. Les résultats suggèrent que le PDEC peut assurer d'importantes économies d'énergie et créer des conditions thermiques agréables sans qu'il y ait besoin d'un refroidissement mécanique supplémentaire. Cette technique peut par conséquent présenter un intérêt plus large pour le logement en Espagne centrale et du sud, ainsi que dans les autres régions chaudes et sèches du monde.

Mots clés: performance des bâtiments, refroidissement, courant d'air descendant, évaporatif, logement, innovation, passif     

Two-phase Euler-Lagrange CFD simulation of evaporative cooling in a Wind Tower

A new design of Wind Tower is investigated numerically under different structural parameters and environmental conditions. The new design is some wetted columns, consisting of wetted curtains hung in the tower column, which are modeled as surfaces that inject droplets of water with very low speed. The CFD open source package - Open FOAM - is used. The current three-dimensional CFD simulation has adopted both the Eulerian approach for the air phase and the Lagrangian approach for the water phase. The effects of water droplet diameter and water droplet temperature on the thermal performance of the Wind Tower are investigated at specific inlet air velocity and relative humidity and height of wetted columns. Also, the effects of wind velocity, temperature, and relative humidity inlet to Wind Tower are studied. Changing the height of the wetted columns and its effect on the evaporative cooling in other specific parameters is studied. The results obtained from the present CFD study are compared with the analytical data taken from the literature and a good agreement is observed. As a result, the height of 10 m of wetted columns decreases 12 K of the ambient air temperature and increases 22% of its relative humidity.     

Exergy analysis for day lighting, electric lighting and space cooling systems for a room space in a tropical climate

Turning off the electric lamp during available daylight will save electricity, while at the same time thermal energy from solar radiation transmitted through the window will increase the space-cooling load. Therefore, it is necessary to evaluate the whole system that includes not only the room space with the windows and the electric lighting systems, but also the air conditioning system. For analysis of the whole system using different types of energy (i.e. electricity, solar radiation, light emitted by lamps and thermal energy), it is important to take into account the quality of these different types of energy. The concept of entropy and exergy were applied in this analysis. The purpose of this study is to show the energy use for daylighting, electric lighting, and space cooling systems as a series of exergy input, output, and consumption and reveal how a daylighting system consumes solar exergy and how electric lighting and space cooling systems consume exergy from fossil fuel. The methodology to calculate the exergy consumption of the system during a given time was developed first. This method was then applied to the lighting and cooling for a typical room. The study found that electric lighting consumes the lowest amount of exergy while the space cooling consumes the highest amount of exergy for the system.