Numerical analysis of temperature non-uniformity and cooling capacity for capillary ceiling radiant cooling panel

Capillary ceiling radiant cooling panel is a high temperature cooling system, which could pose low energy consumption to meet thermal comfort requirements. A computational fluid dynamics (CFD) simulation study on heat transfer of chilled water flow in the capillary of ceiling radiant cooling panel was performed to attain surface temperature distributions and cooling capacities. Six influencing factors included chilled water inlet parameters, conditions of gypsum plaster and capillary mats structural parameters were considered to obtain the complicated relationships between capillary radiant panel conditions and heat transfer performance. The index of temperature non-uniformity coefficient was proposed to evaluate temperature profiles of ceiling panel surface. The results of the simulation were compared with the values depicted in ASHRAE Handbook and good agreement had been achieved. The average difference between simulation results and the values reported by ASHRAE handbook was within the region of 15%. The research results showed that temperature non-uniformity coefficient was negatively correlated with temperature of chilled inlet water (linear correlation), water velocity (correlation coefficient R = −0.85), and pipe diameter (correlation coefficient R = −0.93), but positively and linearly correlated with tube spacing. Cooling capacity was found to have negative linear correlation with temperature of chilled inlet water, covering thickness and tube spacing.     

Parametric analysis of space cooling systems based on night ceiling cooling with PCM‐embedded piping

A parametric analysis is conducted for space cooling systems based on cold water flowing, during the night, within regularly arranged pipes embedded in a layer of phase change material (PCM), located among the structural layers of the ceiling. The introduced PCM layer in conjunction with night cooling add to the usual ceiling cooling systems offers the advantages of low energy consumption, high cool storage capacity, operation under reduced night electricity price, smoothing of electricity consumption by eliminating daily peak loads, improved thermal comfort and elimination of ceiling dripping. Our parametric analysis is based on a transient three‐dimensional finite‐difference solution of the related heat‐transfer problem for various values of all the main system parameters. PCM phase change process is simulated by using the effective thermal capacity function, which is determined experimentally for PCM suitable for air‐conditioning applications. Our tests showed that the main parameters of the system are pipe spacing, PCM layer thickness, pipe depth within the ceiling, cooling water inlet temperature, night cooling duration and PCM properties (thermal conductivity, phase change heat and ends of phase change temperature range). The effect of all the above parameters is analysed and suggestions are made for selecting the proper combinations of their values in order to obtain the lowest energy consumption in conjunction with the highest level of thermal comfort. Copyright © 2010 John Wiley & Sons, Ltd.     

Heat gain reduction by means of thermoelectric roof solar collector

This paper presents a numerical investigation on attic heat gain reduction by using thermoelectric modules integrated in a conventional roof solar collector (RSC). This system, called thermoelectric roof solar collector (TE-RSC), is composed of a transparent glass, air gap, a copper plate, thermoelectric modules (TE) and rectangular fin heat sink. Due to the incident solar radiation, a temperature difference is created between the hot and cold sides of TE modules that generates a direct current. This current is used to drive a ventilating fan for cooling the TE-RSC and enhancing attic ventilation that reduces ceiling heat gain. The system performance was simulated using TRNSYS program with new TE and DC fan components developed by our team and compared to a common house.Simulation results using real house configuration showed that a TE-RSC unit of 0.0525 m² surface area can generate about 9 W under 972 W/m² global solar radiation and 35 °C ambient temperature. The induced air change varied between 20 and 40 and the corresponding ceiling heat transfer rate reduction is about 3–5 W/m². The annual electrical energy saving was about 362 kWh. Finally, economical calculations indicated that the payback period of the TE-RSC is 4.36 years and the internal rate of return is 22.05%.     

Numerical study on nonuniform segmented enhancement method for thermoelectric power generator

Thermoelectric power generators (TEGs) have received considerable attention in the vehicular waste heat recovery. Large temperature difference between hot-side end and cold-side end of TE modules should be maintained to keep large power generation. However, the scaling-up problem of inevitably temperature drop of exhaust gas along the flow direction of vehicles is introduced when multiple TE modules are assembled. In this work, a three-dimensional fluid–thermal–electric multiphysics model with equivalent properties of TE modules is established. The influencing mechanism of longitudinal vortex generator (LVG) pair number and distribution in the hot-side heat exchanger on the local power generation performance of the TEGs is examined. It is found that the extent of the local power generation enhancement gets smaller or even deteriorates at the downstream TE modules by uniformly increasing the LVG pair number along the flow direction. Hence, to make the heat flux efficiently utilized, a nonuniform segmented enhancement method is used for the hot-side heat exchanger. The results show that compared with the TEG with a uniform heat exchanger or a downstream-denser heat exchanger, the total Seebeck voltage generated by the TEG with an upstream-denser heat exchanger is 28% higher, and its output power is 64.4% higher at the matched load resistance. The TEG with the upstream-denser heat exchanger significantly improves the overall power generation performance without additional pumping power.     

Thermoelectric conversion of waste heat from IC engine‐driven vehicles: A review of its application,issues, and solutions

Thermoelectric generator (TEG) is a promising thermoelectric (TE) conversion technology to effectively recover and convert waste heat from vehicle exhaust into useful energy, ie, electricity. Exhaust TEG (ETEG) is a system that is incorporated into the exhaust manifold of a vehicle. Exhaust TEG comprises of a heat exchanger, TEG modules, heat sink, and power conditioning unit. The present work reviews different vehicular ETEGs based on engine type, engine‐rated power, type and number of TEG module, efficiency of ETEG and TEG, exhaust and coolant temperature, and power output of ETEG . In addition to these, the technical issues faced in these ETEGs are addressed under 2 categories, viz., primary (TEG with low ZT TE material and inefficient heat exchanger and heat sink) and secondary issues (low operating temperature TEG modules and installation position of ETEG). In addition to it, effects of vibration and thermal cycling of exhaust system on TEG modules that may arise in ETEG are also discussed. A review of preventive solutions to the issues is also presented. Finally, the economic aspects of an ETEG are also discussed. The review highlights the need of commercialization of TE materials with ZT > 2, high‐temperature operating range, and segmented TEG modules in large volumes so that their practice can be extended in vehicular applications. Heat exchanger modeling using computational fluid dynamics and interfacing with heat transfer theory is essential to maintain temperature uniformity across the TEG modules. Installation of ETEG in the exhaust pipe should be such that it does not affect the performance of the engine. It is also realized that sturdy TEG modules should be developed for long‐term operation to prevent degradation due to mechanical vibration and thermal cycling of the vehicle. Further, ETEG is economically beneficial in vehicles such as trucks owing to availability of high thermal energy in their exhaust stream.     

Experimental examination of large capacity liFePO4 battery pack at high temperature and rapid discharge using novel liquid cooling strategy

To overcome the significant amounts of heat generated by large‐capacity battery modules under high‐temperature and rapid‐discharge conditions, a new liquid cooling strategy based on thermal silica plates was designed and developed. The superior thermal conductivity of the thermal silica plate combined with the excellent cooling effect of water led to a feasible and effective composite liquid cooling system during long cycle testing. The experimental results showed that the addition of thermal silica plates can greatly improve the cooling capacity that can allow the maximum temperature difference to be controlled at 6.1°C and reduce the maximum temperature of the battery module by 11.3°C, but still outside the optimum operating temperature range. The water flow significantly enhanced the cooling performance/stability, and slight temperature fluctuations were observed during cycling. The cooling performance obviously improved as the flow rate rose. When the velocity reached a critical value, further increase in water flow rate induced a slight influence on the cooling capacity due to the limitation of the materials. The maximum temperature (T_max ) could be reduced to 48.7°C, and temperature difference (?T ) could be maintained within 5°C when the water flow velocity increased to 4 mL/s, which was determined as the best value. The energy consumed by the water pump is only 1.37% of the total energy of the battery module. Overall, these findings should provide novel strategies for the design and optimization of battery thermal management system.     

Coupling of thermoelectric modules with a photovoltaic panel for air pre‐heating and pre‐cooling application; an annual simulation

Thermoelectric (TE) modules are possible reversible pre‐cooling and pre‐heating devices for ventilation air in buildings. In this study, the opportunity of direct coupling of TE modules with photovoltaic (PV) cells is considered. This coupling is evaluated through a numerical simulation depending on the meteorological conditions of Chambéry, Alpine region in France, and on the cooling or heating use of the TE modules, through annual energy and exergy efficiencies. For the considered conditions, TE module performances are of the same order as the ones of the vapour compression heat pumps, with a TE coefficient of performance higher than 2 for low values of input DC current. The PV–TE coupling efficiency varies between 0.096 and 0.23 over the year, with an average value of 0.157. Evolutions of the exergy effectiveness of PV and TE elements follow the same trends as the corresponding energy efficiencies but with steeper variations for the coupling exergy yield that varies between 0.004 and 0.014, with an annual average value of 0.010. The direct PV–TE coupling does not seem to be a sustainable option for the summer cooling purpose particularly. A case study with indirect coupling under a warm climate is considered and shows that the use of TE devices could be efficient in housing to ensure summer thermal comfort, but the corresponding necessary PV area would induce a high investment. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance evaluation of photovoltaic/thermal–HDH desalination system

The efficiency of photovoltaic (PV) panel drops with increase in cell temperature. The temperature of the PV panel can be controlled with various cooling techniques. In the proposed work the PV panel is cooled by circulating water and the recovered heat energy is used to run a humidification and dehumidification desalination to produce distilled water from sea water (or) brackish water. This work deals with a detailed analysis of performance of combined power and desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system. A mathematical model of PV/thermal–humidification dehumidification plant was developed and simulations were carried out in MATLAB environment. The performance of photovoltaic/ thermal desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system was investigated under various solar radiation levels (800–1000 W/m²). For each solar radiation level the effect of mass flow rate of coolant water (30–110 kg/h) on water outlet temperature, PV efficiency, PVT thermal efficiency, distilled water production, and plant efficiency was studied. Results show that under each solar radiation level increasing coolant flow rate increases efficiency of PV panel and reduces the plant efficiency. The highest PV efficiency (16.598%) was reached under 800 W/m² at mass flow rate of 110 kg/h and the highest plant efficiency (43.15%) was reached under 800 W/m² at a mass flow rate of 30 kg/h. The maximum amount of distilled water production rate (0.82 L/h) was reached under 1000 W/m² at water mass flow rate of 30 kg/h.     

Theory and experimental validation of cross-flow micro-channel heat exchanger module with reference to high Mach aircraft gas turbine engines

This study explores the design, analysis, and performance assessment of a new class of heat exchangers intended for high Mach aircraft gas turbine engines. Because the compressor air that is used to cool turbine blades and other components in a high Mach engine is itself too hot, aircraft fuel is needed to precool the compressor air, cooling is achieved with a new heat exchanger. The heat exchanger consists of a large number of miniature, closely-spaced modules. Within each module, the fuel flows through a series of parallel micro-channels, while the air flows externally over rows of short, straight fins perpendicular to the direction of fuel flow. A theoretical model was developed to predict the thermal performance of the module for various operating conditions. To confirm the accuracy of the model, a single module was constructed and tested using water to simulate the aircraft fuel. The theoretical model was used to predict the air temperature drop, water temperature rise, and heat transfer rate for each fluid stream. Comparisons between theory and experiment show good overall agreement in exit temperatures and heat transfer rates. This study shows the theoretical model is a reliable tool for predicting the performance of heat exchanger modules under actual fuel and air turbine engine conditions and for the design of aircraft heat exchangers of different sizes and design envelopes.     

Advanced Micro-Heat Exchangers for High Heat Flux

Three micro-heat exchangers for use in a liquid cooling system with a long offset strip, short offset strip, and chevron flow path based on the traditional heat transfer enhancement concepts were designed and tested. A straight channel heat exchanger was also made for comparison. The liquid crystal thermography method described by Lin and Yang (2005) was used to observe the flow and temperature distributions in the micro-heat exchangers. The test results show that the chevron channel heat exchanger provides the lowest thermal resistance. However, its pressure drop is also the highest, approximately five times higher than that for other three heat exchangers. The offset strip heat exchangers provide better thermal performance than does the straight channel heat exchanger. The performance of the heat exchanger with the shorter strip is better than that of heat exchanger with longer strip. From the above results, all of the three micro-heat exchangers with conventional heat transfer enhancement showed less thermal resistance than the straight channel heat exchanger. The conventional heat transfer techniques may be effectively applied in the high-flux micro-heat exchanger design.     

An analysis of the heat transfer rate and efficiency of TE (thermoelectric) cooling systems

The heat transfer rate and efficiency of TE (thermoelectric) cooling systems were investigated. The emphasis of the present study is focused on the use of large-scale TE refrigerators for air conditioning applications. A one-dimensional heat transfer analysis was performed to determine the cooling power and electricity consumption of the TE elements. The constant-property results are in good agreement with the variable-property solutions for TE materials and temperatures typical for air conditioning applications. A heat transfer analysis was also carried out for TE refrigerators equipped with a heat exchanger. Both parallel- and counter-flow heat exchangers were considered. Fluid temperature variations of these two flow arrangements were found to be quite different, but the efficiencies and cold fluid exit temperatures differed only slightly when a uniform current was used for all TE elements. If the length of the heat exchanger exceeds an optimal value, the cold fluid temperature begins to rise and the efficiency drops for both parallel- and counter-flow arrangements. The second law of thermodynamics was applied to the optimization of TE refrigerators operating between two constant-temperature reservoirs and between two flowing fluids. It was found that if a TE cooling system incorporates a heat exchanger, a nonuniform current distribution should be used to achieve the maximum efficiency and the lowest cold fluid temperature. The optimization results for TE refrigerators operating between two constant-temperature reservoirs are not applicable to TE cooling systems between two flowing fluids. The most energy-efficient current distribution for the parallel-flow arrangement is the one which increase in the direction of the cold fluid.     

基于辐射供冷的太阳能吸收式空调试验研究

对基于辐射供冷的太阳能吸收式空调系统进行了试验。该系统采用96 m2的U型管式真空管太阳能集热器驱动额定制冷量为8 kW的吸收式制冷机组,吸收式制冷机产生的冷冻水被输送到辐射吊顶中,为50 m2的实验室提供夏季空调。吸收式制冷机运行在夏季晴朗天气时,平均制冷量为4.5 kW。辅助独立除湿机组与辐射吊顶联合运行。试验房间的热舒适指标PMV为-0.29~0.32,可满足热舒适要求。     

Liquid metal based thermoelectric generation system for waste heat recovery

A new type of thermoelectric generator (TEG) system based on liquid metal which serves to harvest and transport waste heat, is proposed in this paper. To demonstrate the feasibility of the new TEG system, an experimental prototype which combined commercially available thermoelectric (TE) modules with the electromagnetic pump was set up. Output voltage from TE modules and temperature changes of the main parts (waste heat source, liquid metal heating plate, water-cooling plates I and II) of the liquid metal based TEG system were experimentally measured, as well as the flow rate of cooling water and the load resistance. It was shown that the maximum open-circuit voltage of 34.7 V was obtained when the temperature of the waste heat source was 195.9 °C and the temperature gap between liquid metal heating plate and cooling-water plates was nearly 100 °C. These experimental results obviously verify that using liquid metal based TEG system for waste heat recovery is highly feasible. In addition, the TEG system performance is discussed and a calculated efficiency of 2% in the whole TEG system is obtained. Possible suggestions to further improve this type of generator in the future are given.     

基于虚拟仪器技术的多功能换热器综合性能测试系统的研究

介绍了融合虚拟仪器技术的换热器综合性能测试系统的工作原理和软硬件组成。换热器综合性能测试系统主要由油-水换热系统和(水)乙二醇水溶液-水换热系统组成,适用于多股流体换热器的传热特性和流动阻力的综合性能测试。测试系统以LabVIEW软件为平台,由硬件设备和精密测试仪器组成,通过计算机调节控制现场各设备的运行状态,自动采集测试数据,对测试数据进行处理、显示并储存在数据文件中,同时还具有自动报错和实时联锁保护功能,保证了整套测试系统的安全稳定运行。在实验测试过程中能保证冷热两侧热平衡计算精度在[-5%,+5%]区间内。该系统可为新型换热器的科学研究、工程设计和工程应用提供可靠的数据依据和实验保障。     

Numerical simulation of closed wet cooling towers for chilled ceiling systems

A numerical technique for evaluating the performance of a closed wet cooling tower for chilled ceiling systems is presented. The technique is based on computational flow dynamics (CFD) for the two-phase flow of gas and water droplets. The eulerian approach is used for the gas phase flow and the lagrangian approach for the water droplet phase flow, with two-way coupling between two phases. Numerical simulation indicates that CFD can be used to predict the performance of a closed wet cooling tower, given the appropriate rate of heat generation from the heat exchanger. The technique is suitable for optimization of the design and operation of the cooling tower for chilled ceilings.     

Performance analysis of a double-pass thermoelectric solar air collector

The thermoelectric (TE) solar air collector, sometimes known as the hybrid solar collector, generates both thermal and electrical energies simultaneously. A double-pass TE solar air collector has been developed and tested. The TE solar collector was composed of transparent glass, air gap, an absorber plate, thermoelectric modules and rectangular fin heat sink. The incident solar radiation heats up the absorber plate so that a temperature difference is created between the thermoelectric modules that generates a direct current. Only a small part of the absorbed solar radiation is converted to electricity, while the rest increases the temperature of the absorber plate. The ambient air flows through the heat sink located in the lower channel to gain heat. The heated air then flows to the upper channel where it receives additional heating from the absorber plate. Improvements to the thermal and overall efficiencies of the system can be achieved by the use of the double-pass collector system and TE technology. Results show that the thermal efficiency increases as the air flow rate increases. Meanwhile, the electrical power output and the conversion efficiency depend on the temperature difference between the hot and cold side of the TE modules. At a temperature difference of 22.8 °C, the unit achieved a power output of 2.13 W and the conversion efficiency of 6.17%. Therefore, the proposed TE solar collector concept is anticipated to contribute to wider applications of the TE hybrid systems due to the increased overall efficiency.     

Investigation on thermal management performance of wedge‐shaped microchannels for rectangular Li‐ion batteries

The performance of power battery is a significant factor affecting the overall quality of electric vehicles. To optimize the thermal management effect of battery pack, cold plate with wedge‐shaped microchannels was proposed in this paper. On the basis of the models of the independent cold plate and the battery‐cooling module, the effects of outlet aspect ratio, flow rate, and branching structure on the heat dissipation performance of the cold plate were studied at first. Afterwards, the effects of cooling surface, flow rate, and branching structure on the temperature distribution of the battery module were simulated. The results showed that the wedge‐shaped channels provided a good cooling efficiency and surface temperature uniformity. When the wedge‐shaped channel was used in thermal management of the battery module, the side‐cooling method reduced the temperature difference of batteries by more than 35.71% compared with front cooling under the mass flow rate of 2 × 10^?5 kg/s. At a discharge rate of 3.5 C, the flow rate of 1 × 10^?4 kg/s controlled the battery temperature to within 45°C, and the branching structure designed for the module successfully decreased the maximum temperature difference from 7.27°C to 4.67°C, which has been reduced by approximately 35.78%.     

Integrated analysis of cooling water systems: Modeling and experimental validation

Cooling towers are widely used in many industrial and utility plants as a cooling medium, whose thermal performance is of vital importance. Despite the wide interest in cooling tower design, rating and its importance in energy conservation, there are few investigations concerning the integrated analysis of cooling systems. This work presents an approach for the systemic performance analysis of a cooling water system. The approach combines experimental design with mathematical modeling. An experimental investigation was carried out to characterize the mass transfer in the packing of the cooling tower as a function of the liquid and gas flow rates, whose results were within the range of the measurement accuracy. Then, an integrated model was developed that relies on the mass and heat transfer of the cooling tower, as well as on the hydraulic and thermal interactions with a heat exchanger network. The integrated model for the cooling water system was simulated and the temperature results agree with the experimental data of the real operation of the pilot plant. A case study illustrates the interaction in the system and the need for a systemic analysis of cooling water system. The proposed mathematical and experimental analysis should be useful for performance analysis of real-world cooling water systems.     

Numerical and experimental assessment of thermal performance of vertical energy piles: An application

A district space heating and cooling system using geothermal energy from bearing piles was designed in Shanghai and will be installed in two years before 2010. This paper describes the pile-foundation heat exchangers applied in an energy pile system for an actual architectural complex in Shanghai, 30% of whose cooling/heating load was designed to be provided by a ground-source heat pump (GSHP) system using the energy piles. In situ performance tests of heat transfer are carried out to figure out the most efficient type of energy pile and to specify the design of energy pile system. Numerical investigation is also performed to confirm the test results and to demonstrate the medium temperature variations along the pipes. The averaged heat resistance and heat injection rate of different types of energy piles are calculated from the test and numerical results. The effect of pile type, medium flow rate and inlet temperature on thermal performance is separately discussed. From the viewpoint of energy efficiency and adjustability, the W-shaped underground heat exchanger with moderate medium flow rate is finally adopted for the energy pile system.     

Performance of the novel design thermoelectric cooling system

Thermoelectric coolers (TECs) have been used for cooling electrical elements of high dissipating energy to provide a fixed temperature, with the advantages of being reliable, noiseless, tightly packed, with no moving parts as well as no harmful gases. As high performance of TECs is required, developed emerging thermoelectric (TE) materials can be employed to enhance their performance. Hence, this study provides an experimental investigation to study the effect of different operational parameters on the performance of the TE cooling system including the system coefficient of performance (COP) and the rate of heat transfer. The parameters investigated are the applied input power, inlet working fluid velocity, and the arrangement of utilized TECs modules. The used test rig consists of two attached horizontal ducts, and the interface surface between them contains three TE modules, each of power 48 W. The applied input power and the working fluid flow rate range from 16 to 48 W and from 0.0025 to 0.01 L/min, respectively. Results show that the maximum COP occurs at lower values of applied input power. A noticeable enhancement in the COP is found when all TEC modules are in use. A new correlation of the COP of the TEC system is obtained.