Advanced solar assisted desiccant and evaporative cooling system equipped with wet heat exchangers

This work concerns an innovative solar assisted desiccant and evaporative cooling (DEC) system for building air conditioning. In this system, air-to-air packaged wet heat exchangers are used to maximize the exploitation of the evaporative cooling potential associated with the exhaust air stream.The thermodynamic cycle is first theoretically described, and then an example of a real application is shown.The system presented is an experimental DEC system originally installed at the beginning of 2008 at the Solar Laboratory of the University of Palermo and recently modified and updated. These modifications have been implemented with the aim to foster some problems related to the conventional indirect evaporative process and to increase the overall performance of the process.Different energy performance figures, such as thermal and electrical COP, are presented and discussed.The obtained results are very positive, showing that DEC systems can benefit from the use of wet heat exchangers to efficiently cool the air stream after the adsorption process in the desiccant wheel without any change in moisture content.     

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.     

Thermal performance evaluation of heat exchangers fitted with twisted-ring turbulators

Heat transfer, friction factor and thermal performance characteristics in a tube equipped with twisted-rings (TRs) are experimentally investigated. The experiments were conducted using TRs with three different width ratios (W/D = 0.05, 0.1 and 0.15) and three pitch ratios of (p/D = 1, 1.5 and 2) for Reynolds numbers ranging from 6000 to 20,000 using air as a test fluid. The typical circular rings (CRs) were also tested for an assessment. The experimental results reveal that most TRs yield lower Nusselt numbers and friction factor than CRs, except at the largest width ratio (W/D = 0.15) and the smallest pitch ratio (p/D = 1.0). In addition, Nusselt number and friction factor increase as width ratio increases and pitch ratio decreases. However, a maximum thermal performance factor is associated by TRs with the smallest width ratio and pitch ratio. The empirical correlations of the heat transfer (Nu) and friction factor (f) are also included in this paper.     

Evaluation of plume potential and plume abatement of evaporative cooling towers in a subtropical region

Hong Kong is a typical subtropical region with frequently high humidity in late spring and summer seasons. Plume from evaporative cooling towers, which service air-conditioning systems of civil buildings, has aroused public concerns since 2000 when the fresh water evaporative cooling towers were allowed to be used for high energy efficiency and environmental issues. This paper presents the evaluation of the plume potential and its effect on the sizing of the plume abatement system in a large commercial office building in Hong Kong for practical application. This evaluation was conducted based on a dynamic simulation platform using the typical meteorological year of Hong Kong since the occurrence of the plume heavily depends on the state conditions of the exhaust air from cooling towers and the ambient air, while the state condition of the exhaust air is determined by the total building cooling load and the control strategies of cooling towers employed mainly for improving energy efficiency. The results show that the control strategies have a significant effect on the plume potential and further affect the system design and sizing of the plume abatement system.     

Characteristics and performance evaluation of surface-treated louvered-fin heat exchangers under frosting and wet conditions

Hydrophilic, hydrophobic, and dual (hydrophilic and hydrophobic) coatings were applied to louvered-fins typically used for heat pump heat exchangers, and the characteristics and performance of the heat exchangers under both frosting and wet conditions were compared according to surface treatment. The hydrophilic heat exchanger had the highest air-side pressure drop under frosting conditions. The hydrophobic unit had a lower air-side pressure drop than the others (hydrophilic and dual), due to frost retardation, and the reduction of the heat transfer rate was also smaller. The dual-fin heat exchanger exhibited frost retardation only in the early stage of the experiment, and the heat transfer rate was slightly greater than that of the hydrophilic unit. In wet-condition experiments for evaluating the evaporating performance of surface-treated heat exchangers, the hydrophilic unit had a lower air-side pressure drop than the others, due to the thin film of water condensation on the fins. The differences in the heat transfer rates of the heat exchangers were not significant.     

Second-law performance of heat exchangers for waste heat recovery

Exergy change rate in an ideal gas flow or an incompressible flow can be divided into a thermal exergy change rate and a mechanical exergy loss rate. The mechanical exergy loss rates in the two flows were generalized using a pressure-drop factor. For heat exchangers using in waste heat recovery, the consumed mechanical exergy is usually more valuable than the recovered thermal exergy. A weighing factor was proposed to modify the pressure-drop factor. An exergy recovery index (η_II) was defined and it was expressed as a function of effectiveness (?), ratio of modified heat capacity rates (C^∗), hot stream-to-dead-state temperature ratio, cold stream-to-dead-state temperature ratio and modified overall pressure-drop factor. This η_II–? relation can be used to find the η_II value of a heat exchanger with any flow arrangement. The η_II−Ntu and η_II−Ntu_h relations of cross-flow heat exchanger with both fluids unmixed were established respectively. The former provides a minimum Ntu design principle and the latter provides a minimum Ntu_h design principle. A numerical example showed that, at a fixed heat capacity rate of the hot stream, the heat exchanger size yielded by the minimum Ntu_h principle is smaller than that yielded by the minimum Ntu principle.     

Thermal evaluation of nanofluids in heat exchangers

Nanofluids, suspensions of nanoparticles (less than 100 nm) in a basefluid, have shown enhanced heat transfer characteristics. In this study, thermal performances of nanofluids in industrial type heat exchangers are investigated. Three mass particle concentrations of 2%, 4%, and 6% of silicon dioxide–water (SiO₂–water) nanofluids are formulated by dispersing 20 nm diameter nanoparticles in distilled water. Experiments are conducted to compare the overall heat transfer coefficient and pressure drop of water vs. nanofluids in laboratory-scale plate and shell-and-tube heat exchangers. Experimental results show both augmentation and deterioration of heat transfer coefficient for nanofluids depending on the flow rate and nanofluid concentration through the heat exchangers. This trend could be explained by the counter effect of the changes in thermo-physical properties of fluids together with the fouling on the contact surfaces in the heat exchangers. The measured pressure drop while using nanofluids show an increase when compared to that of basefluid which could limit the use of nanofluids in industrial applications.     

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.     

Performance evaluation of indirect evaporative cooling using whole-building hygrothermal simulations

In an indirect evaporative cooling (IEC) installation the return air is cooled by adiabatic humidification. In an air/air heat exchanger this air cools down the supply air. This paper presents a simulation methodology focusing on the interaction between the thermal performance of an IEC system and the heat and moisture balance of the building where it is applied. The thermal effectiveness of an IEC system is first studied by measurements. It was found that the effectiveness is independent of the air inlet conditions. In the second part the influence of the ventilation rate, the indoor moisture production and the moisture buffering capacity on the thermal performance were evaluated using dynamic calculations with the multizone building simulation program TRNSYS. Increasing the indoor moisture production and lowering the ventilation rate both reduce the thermal performance of the system. Hygroscopic materials may ameliorate the applicability of IEC.     

Study on the heat transfer characteristics of an evaporative cooling tower

In the present study, both experimental and theoretical results of the heat transfer characteristics of the cooling tower are investigated. A column packing unit is fabricated from the laminated plastic plates consists of eight layers. Air and water are used as working fluids and the test runs are done at the air and water mass flow rates ranging between 0.01 and 0.07 kg/s, and between 0.04 and 0.08 kg/s, respectively. The inlet air and inlet water temperatures are 23 °C, and between 30 and 40 °C, respectively. A mathematical model based on the conservation equations of mass and energy is developed and solved by an iterative method to determine the heat transfer characteristics of the cooling tower. There is reasonable agreement from the comparison between the measured data and predicted results.     

Modelling the thermal performance of earth-to-air heat exchangers

A new complete numerical model for the prediction of thermal performance of the earth-to-air heat exchangers is presented. The model describes the simultaneous heat and mass transfer inside the tube and into the soil accounting for the soil natural thermal stratification. The model is validated against an extensive set of experimental data and it is found accurate. The proposed algorithms are suitable for the calculation of the temperature and humidity variation of the circulating air and for the temperature and humidity distribution inside the ground. The presented model was developed within the TRNSYS environment and can be easily coupled with building or greenhouse simulation codes in order to describe the impact of the earth-to-air heat exchangers to indoor environments.     

冷却塔中微型混流式水轮机数值模拟及特性分析

基于RNG k-ε湍流模型、Simplec算法,运用数值模拟方法,对"S"型叶片微型混流式水轮机的工作特性及流场信息进行计算、分析。水轮机最优转速随水头升高向高速区域移动,转速增加引起功率、流量的线性减少,而水头升高使得功率、流量均线性增加,且效率对转速变化较敏感。设计工况下,水轮机空化性能良好,且满足冷却塔电机功率及循环水系统流量的改造要求。     

Experimental evaluation of solar thermosyphons with heat exchangers

The advantages of solar thermosyphons in terms of simplicity, reliability and cost have long been recognized. Recent studies have also shown their thermal performance to be comparable with that of equivalent active systems. When pump power is considered, the energy savings of domestic hot water thermosyphons can be significantly superior to active systems. In spite of these advantages, use of solar thermosyphons in the United States is almost negligible compared to their widespread use in other countries. A major limitation to the use of thermosyphons in the United States is lack of effective, reliable freeze protection. One technique for reliable, passive freeze protection is to use a heat exchanger in the storage tank and a nonfreezing fluid in the collector. Previous analytical work indicates that the performance penalty for these systems with practical-sized heat exchangers may be small enough to make these systems economically feasible. A full-scale, residential-size test facility has been constructed for testing this concept and validating the theoretical models. This paper describes results of testing comparing the performance of a horizontal tank with and without heat exchanger to a baseline case of a vertical tank without heat exchanger. An analytical expression for a “heat exchanger penalty factor” for these systems is derived and compared with the experimental results.     

Second-law-based optimization of heat exchanger networks using load curves

The problem of simultaneously optimizing heat exchangers for a number of hot and cold fluids is approached from the viewpoint of the second law of thermodynamics. If the total duty requirements are fixed, i.e. all inlet and outlet temperatures are established for the fluids, then the resulting entropy production rate is independent of the actual pairings of the fluids. Thus, optimization with respect to size becomes essentially a maximization of the temperature differences between the paired fluids for all of the fluids considered.Next it is shown that the same optimization can be obtained from the load curves where the temperature is plotted against the heat transfer for each of the fluids. If the film coefficient for any of the fluids is significantly different from the others, a simple shifting can be accomplished and a criterion is given to determine if such shifting improves the optimization. The method is extremely simple and can be performed most effectively on graph paper.     

Thermal evaluation of coaxial deep borehole heat exchangers

This paper presents a performance study of deep borehole heat exchangers. The coaxial borehole heat exchanger (BHE) has been selected because for the present conditions it has a better performance than the conventional U-tube BHE. A numerical model has been developed to study the coaxial BHE. The model predictions are compared to detailed distributed temperature measurements obtained during a thermal response test. The model is found to accurately predict the behavior of a coaxial BHE. The influence of the flow direction of the mass flow is studied for BHEs in the range 200 m–500 m. A parametric performance study is then carried out for the coaxial case with different borehole depths, flow rates and collector properties. The results clearly show a significant increase in the system performance with depth. In addition, it is shown that with increasing borehole depth, the heat load that can be sustained by the BHE is significantly increased. An overall performance chart for coaxial BHEs for the depths of 300–1000 m is presented. The chart can be used as a guide when sizing deep BHE installations.     

Twisted bundle heat exchangers performance evaluation by CFD (CJ12/5054)

Shell and tube heat exchanger with single twisted tube bundle in five different twist angles, are studied using computational fluid dynamics (CFD) and compared to the conventional shell and tube heat exchanger with single segmental baffles. Effect of shell-side nozzles configurations on heat exchanger performance is studied as well. Heat transfer rate and pressure drop are the main issues investigated in the paper. The results show that, for the same shell-side flow rate, the heat transfer coefficient of heat exchanger with twisted tube bundle is lower than that of the heat exchanger with segmental baffles while shell-side pressure drop of the former is even much lower than that of the latter. The comparison of heat transfer rate per unit pressure drop versus shell-side mass flow rate shows that heat exchanger with twisted tube bundle in both cases of perpendicular and tangential shell-side nozzles, has significant performance advantages over the segmental baffled heat exchanger. Optimum bundle twist angles for such exchangers are found to be 65 and 55° for all shell side flow rates.     

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.     

横流式冷却塔换热模型与影响因素研究

根据Merkel的冷却塔传热传质理论,推导了适应于横流式冷却塔的换热模型,通过理论模型正交试验和实测数据因子相关性分析,研究了横流式冷却塔换热性能的影响因素。实测数据因子相关性分析结果表明,风量对横流式冷却塔换热性能影响程度最小,与理论模型正交实验结果存在一定的差异。运行时应保证横流式冷却塔进水流量的分布均匀,才能更有效的利用换热面积,提高横流式冷却塔换热效率。     

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.     

Numerical and experimental investigations of heat transfer performance of rectangular coil heat exchangers

Both numerical and experimental investigations were conducted to understand convective heat transfer from a single round pipe coiled in rectangular pattern. The studied heat exchangers are composed with inner and outer coils so that the exterior flow is very similar to flow within tube-bundles. The inner and outer coils of the heat exchangers are in turn composed of bends and straight portions. Calculations and experiments were done for two cases with different outside flow arrangements. The results showed the effects of geometric arrangement with better heat transfer for the case 1 of staggered arrangement due mainly to its more tortuous flow characteristics and better mixing of the exterior fluid. The numerical and experimental results qualitatively agree well with each other. The numerical and experimental results showed that coiling a pipe so that an exterior fluid flows over or in tube bundle can help to induce the turbulence without increasing the velocity.