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.     

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.     

Novel design and modelling of an evaporative cooling system for buildings

Passive evaporative cooling has great potential as an alternative to conventional air‐conditioning in arid hot climates because of its low cost and zero pollution. This paper describes a novel evaporative cooling system with an automatic wind‐tracking device to improve its operating efficiency. The design and operating principles are discussed. A mathematical model is simplified by the assumption of convective heat and mass transfer of staggered streamlets of water. A computer program has been developed to calculate the deflection and length of spray water streamlets, as well as evaporative water mass, minimum cooled water temperature and required cooling time. A typical example illustrates that approximately 20 kg water are evaporated and around 26 min are required for 980 kg of water to be cooled from 28°C to the wet bulb temperature of 19.2°C of ambient air in a typical arid hot climate (relative humidity = 0.30, dry bulb temperature = 32°C and wind velocity = 4 m s^?1). The application of adsorbents, would allow the evaporative cooling system to be applied in hot, humid climates, in addition to hot climates with low humidity. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

Experimental study on the heat exchange effectiveness of a dry coil indirect evaporation cooler under various operating conditions

In this study, a pilot Dry Coil IEC unit, an indirect evaporation cooler established by connecting a direct evaporation cooler and a sensible heat exchanger (SHE) in series, was made, and the effectiveness data of the pilot unit were acquired under various operation conditions in an environmental chamber realizing a wide-range of indoor and outdoor air conditions. The test result shows that over 40% effectiveness can be acquired even in hot and humid climates by using the Dry Coil IEC proposed in this paper. The Dry Coil IEC reduces the cooling coil size by pre-cooling the process air during the cooling season. It can also be used as an SHE reclaiming the sensible heat from the exhaust air during winter operation. The pilot unit recovered over 60% of sensible heat in the test. In addition, a simplified model of Dry Coil IEC returning the various operation conditions was developed based on existing models of an SHE and a direct evaporative cooler. A polynomial equation returning the effectiveness of the Dry Coil IEC was derived as a function of seven independent variables highly influencing the performance of the unit. The experimental data acquired by the pilot unit operation agree well with the effectiveness values of the Dry Coil IEC predicted by the proposed model. It was also identified that the proposed equation agrees well with the existing model of the Dry Coil IEC applied to the energy simulation program.     

Recent researches in indirect evaporative cooler III: optimization of the cooling potential of a room–coupled indirect evaporative cooler

A mathematical model based on heat transfer principles, for characterizing the cooling performance of a room coupled indirect evaporative cooler (tube type) has been developed. Two dimensionless parameters, i.e. environment factor, ϕ and cooling factor, CF have been defined to characterize the performance of IEC coupled with a room. The optimum values of these parameters have been obtained for different environmental and thermal load conditions. In addition to this, a linear relationship has been obtained for the optimum size of a cooler to remove maximum heat from a room of given size. © 1998 John Wiley & Sons, Ltd.     

Performance evaluation and comparison of multistage indirect evaporative cooling systems in two operation modes

The evaporative cooling technique is an efficient approach for cooling application. This study aims to establish a performance evaluation method to advance the appropriate design for multistage indirect evaporative cooling systems. A mathematical formulation has been developed for the indirect evaporative cooler (IEC). After the validation, the mathematical model was used to analyze the evaluation criteria by considering the simultaneous influence of the cooling effectiveness, the pressure drop, and the cooling capacity of the multistage IEC operating in two modes. The Mode-1 IEC is a conventional counterflow unit, while the Mode-2 IEC employs a regenerative M-cycle arrangement. The IECs are operated in a tandem arrangement. The multistage system is capable of improving the cooling performance and reducing the outlet air temperature. In addition, the multistage system displays a higher pressure drop resulting in a lager consumption of fan power. The analysis of performance evaluation criteria indicates that the appropriate maximum stage is suggested to be three-stage and two-stage for the Mode-1 and the Mode-2 IEC, respectively.     

CFD方法与间接蒸发冷却换热器的三维数值模拟

采用计算流体力学(CFD)和数值传热学方法,对间接蒸发冷却器内流体流动与热质交换过程进行简化和假设,建立了换热器内三维层流流动与传热的数学物理模型。采用交错网格离散化非线性控制方程组,编制了三维simple算法程序。对间接蒸发冷却器内的流场、温度场及浓度场进行数值模拟研究,得到换热器内的流体流动状态和热流分布,并分析了通道宽度变化对换热器内流体流动与换热的影响。     

板式间接蒸发冷却器热工特性的实验研究

对自行研制设计的间接蒸发冷却换热试件开展了实验,研究了影响换热器换热性能的因素。结果表明：板式间接蒸发冷却器换热效率随二次空气入口的速度升高、一次空气入口的温度、二次空气入口的湿球温度升高而变大,随一次空气入口的速度变大而变小。实验结果对于深入认识间接蒸发冷却器的换热机理及开展换热器的优化设计有着很大的指导意义。     

蒸发冷却技术在我国非干燥地区的应用研究

介绍了蒸发冷却的原理与种类 ,通过分析计算 ,得出除湿蒸发冷却技术在我国非干燥地区存在应用的可行性。采用该技术不但可以保护环境 ,提高室内空气品质 ,在有余热废热利用的场合 ,还能大大节省系统耗电量 ,起到很好的节能效果。建议应在我国推广应用这一技术并制定相应的标准规范。     

Recent research on an indirect evaporative cooler. Part VI: evolution of design pattern for indirect evaporative cooler

A rule of thumb for indirect evaporative cooler has been derived in terms of the size of floor area to be cooled and design parameters of tube type IEC, viz. process stream air flow rate and number of tubes; the three Indian climate zones, namely hot–dry, composite and warm–humid, represented by three cities of Jodhpur, Delhi and Madras, respectively, have been considered. Copyright © 1999 John Wiley & Sons, Ltd.     

Recent research on an indirect evaporative cooler (IEC) Part 1: optimization of the COP

The paper presents an analysis for optimizing the coefficient of performance of an tube type indirect evaporative cooler (IEC) in terms of process stream air velocity and wet stream air velocity. Theoretical predictions of the optimum COP value have been compared with the experimental results obtained for an IEC at Indore in the summer months of May and June; the agreement is satisfactory. © 1997 John Wiley & Sons, Ltd.     

Experimental validation of a thermal model of an evaporative cooling system

A periodic thermal model for an evaporative cooling system over the roof has been presented. Open roof pond, water film and flowing water layer are the the special cases of the analysis. The time dependency of solar radiation, ambient air, sol-air and room air temperatures has explicitly been taken into account by expressing as a Fourier series of time for a 24 h cycle. Experimentally observed air temperature of rooms, treated with and without evaporative cooling over the roof, has been found in good agreement with theoretical results.     

Thermodynamic performance assessment of an indirect intercooled reheat regenerative gas turbine cycle with inlet air cooling and evaporative aftercooling of the compressor discharge

This study provides a computational analysis to investigate the effects of cycle pressure ratio, turbine inlet temperature (TIT), and ambient relative humidity (φ) on the thermodynamic performance of an indirect intercooled reheat regenerative gas turbine cycle with indirect evaporative cooling of the inlet air and evaporative aftercooling of the compressor discharge. Combined first and second‐law analysis indicates that the exergy destruction in various components of gas turbine cycles is significantly affected by compressor pressure ratio and turbine inlet temperature, and is not at all affected by ambient relative humidity. It also indicates that the maximum exergy is destroyed in the combustion chamber; which represents over 60% of the total exergy destruction in the overall system. The net work output, first‐law efficiency, and the second‐law efficiency of the cycle significantly varies with the change in the pressure ratio, turbine inlet temperature and ambient relative humidity. Results clearly shows that performance evaluation based on first‐law analysis alone is not adequate, and hence more meaningful evaluation must include second‐law analysis. Decision makers should find the methodology contained in this paper useful in the comparison and selection of gas turbine systems. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

Experimental study of a direct evaporative cooling approach for Li-ion battery thermal management

A desirable operating temperature range and small temperature gradient is beneficial to the safety and longevity of lithium-ion (Li-ion) batteries, and battery thermal management systems (BTMSs) play a critical role in achieving the temperature control. Having the advantages of direct access and low viscosity, air is widely used as a cooling medium in BTMSs. In this paper, an air-based BTMS is modified by integrating a direct evaporative cooling (DEC) system, which helps reduce the inlet air temperature for enhanced heat dissipation. Experiments are carried out on 18650-type batteries and a 9-cell battery pack to study how relative humidity and air flow rate affect the DEC system. The maximum temperatures, temperature differences, and capacity fading of batteries are compared between three cooling conditions, which include the proposed DEC, air cooling, and natural convection cooling. In addition, a DEC tunnel that can produce reciprocating air flow is assembled to further reduce the maximum temperature and temperature difference inside the battery pack. It is demonstrated that the proposed DEC system can expand the usage of Li-ion batteries in more adverse and intensive operating conditions.     

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.     

Investigation on performance of an integrated solid oxide fuel cell and absorption chiller tri-generation system

An integrated tri-generation system incorporating a solid oxide fuel cell (SOFC) and a double-effect water/Lithium Bromide absorption chiller is presented in this paper. The proposed tri-generation system can provide power, cooling or heating simultaneously with a typical gas produced from a gasication process. The system conguration and design are discussed, and the energy and mass balances are obtained through the matrix representation method and integrated into a simulation program by MATLAB soft package. The developed model comprises of three modules: SOFC module, exhaust combusting and HRSG module, and the absorption chiller module. Validation of the SOFC model is performed by comparison with a single tubular cell of Siemens-Westinghouse, and a specific case study of the system is presented. For parametric analysis, the fuel utilization ratio, fuel flow ratio and air inlet temperature are investigated and the results are discussed in detail.     

Experimental study of effect of an inner thermal curtain in evaporative cooling system of a cascade greenhouse

In this paper experimental study has been carried out in a cascade greenhouse with inner thermal curtain to see the effect of thermal curtain. A thermal model has also been developed to predict the air temperature in a cascade greenhouse. The fan-pad system has been used for evaporative cooling and an inner thermal curtain has been used to divide the greenhouse in two zones. Experiments have been conducted in hot summer conditions at Solar Energy Park, IIT Delhi, New Delhi, India for empty greenhouse. Statistical analysis has been carried out to validate the agreement of experimental observations with predicted values. The values of the root mean square percent deviation and coefficient of correlation has been found out 9.0%, 0.90; 5.0%, 0.95 and 7.0%, 0.97 for April, May and June in case of evaporative cooling without curtain in greenhouse-2. The degree of freedom for the experimental work is 10.0. It is found that the use of evaporative cooling with a thermal curtain reduces the temperature of greenhouse by 5 °C and 8 °C in the second zone of greenhouse-1 and 2 in comparison to greenhouse without curtain in May.