The effect of condenser heat transfer on the energy performance of a plate heat pipe solar collector

For a novel prototype solar collector, using a plate heat pipe, condenser heat transfer was analysed in detail. The condenser has the shape of a rectangular channel. Flow and heat transfer of water in the rectangular channel was modelled and the heat transfer coefficient assessed, using the Fluent code. Under typical operating conditions a mixed convection situation occurs. The channel is inclined and heating is through one wall only (upper channel surface). The range of temperature differences considered was similar to the one verified under real operating conditions, covering a wide range of Grashof numbers. Results showed that the Nusselt number is significantly higher than the one for forced convection in a rectangular channel with fully developed boundary layers. In order to enhance heat transfer, a modification to the rectangular channel was analysed, using baffles to improve flow distribution and increase velocity. The effect of this modification on collector energy performance (efficiency) was assessed. Copyright © 2005 John Wiley & Sons, Ltd.     

热管太阳能集热器系统的研究

建筑物大面积集中供热已成为节能降耗的新,也是太阳能热利用发展的趋势。文章介绍了在原有铜铝复合板的基础上,试制的一种结合多层建筑的集中供热热管式太阳能供热系统,克服了原有的平板式和真空管式集热系统易炸管,易冻裂及结垢,单管损坏后整个系统失效等问题。     

An experimental study on thermal management of concentrated photovoltaic cell using loop heat pipe and heat sink

One of the most critical innovations in the solar energy conversion is the use of concentrators for generating power from a smaller area of the cell. The thermal management has an exceptional role in the concentrated photovoltaic (CPV) cell, without which the operating temperature will increase owing to the thermal degradation. In the present study, a prototype of low CPV with single‐cell configuration using a Fresnel lens and a manual tracker with geometrical concentration ratio of up to 25 Suns is made. The performance of the CPV with passive cooling arrangements, such as heat sink and loop heat pipes (LHPs), is analyzed under real‐time outdoor conditions. The results obtained infer that the LHP‐based cooling system has brought down the average temperature rise above ambient to 37.8°C from 54.16°C and 72.6°C in the heat sink and bare CPV systems, respectively. Also, the LHP managed to reject the heat to the surrounding with an average thermal resistance of 1.005°C/W, which is the least when compared with the heat sink. Apart from the instabilities caused by the interference of clouds, the CPV with the LHP cooling system could generate 10% more power output than the one with a heat sink.     

Flower-type pulsating heat pipe for a solar collector

Inspired by the sunflower, we report a new structure of a solar collector that integrates a pulsating heat pipe (PHP) into a flat-plate collector. The proposed flower-type PHP solar collector is designed after a sunflower with petals that absorb sunlight and transfer nutrients to the stem after photosynthesis. The evaporator section adopts the shape of a flower to absorb sunlight fully, and the condenser section is rolled into a cylinder and placed in the lower part of the structure. A systematic experimental study is conducted upon start-up, and the performance characteristics, with acetone as the working fluid, are evaluated. We also did a heat loss analysis, which has a deviation of 8%. The effects of the mass flow rate of cooling water, filling ratio, length of the condenser section, and solar intensity are assessed. As the temperature of the heat absorber plate increases, the thermal resistance of the PHP can decrease to a minimum of 0.14°C/W. Under sunny weather conditions, the instantaneous thermal efficiency of the system with a filling ratio of 50% reaches 50%. Besides, we discussed the unstable operation conditions and possible dryout phenomenon that happened inside the PHP.     

Optimization of a wickless heat pipe flat plate solar collector

The transient thermal behaviour of wickless heat pipe flat plate solar collectors has been analyzed with regard to various parameters. These parameters include global solar radiation intensity, inlet cooling water temperature, absorber plate material and thickness, ratio of pitch distance to wickless heat pipe diameter and ratio of condenser section length to total wickless heat pipe length. The results show that the selection of an absorber plate having a high value of conduction heat transfer rate per unit temperature difference is limited by the pitch distance.     

A comprehensive investigation on thermal management of large‐capacity pouch cell using micro heat pipe array

A proper and effective battery thermal management system (BTMS) is critical for large‐capacity pouch cells to guarantee a suitable operating temperature and temperature difference. Hence, in this paper, a micro heat pipe array (MHPA) is utilized to build the thermal management system for large‐capacity pouch cells. In order to study the property of BTMS in depth, experimental and numerical investigation are carried out by considering the C‐rate, working medium, air velocity and duty. The experimental results present that the T_max can be maintained below 43.7°C and the ΔT is below 4.9°C at the discharge rate of 3C in the battery module with MHPA‐liquid. Moreover, the T_max of the battery module with MHPA‐liquid falls as the air velocity increases. The simulation results show that the variation and distribution of temperature matched well with experimental results. It demonstrates that the MHPA‐based BTMS is viable and effective for large‐capacity pouch cell battery, even at high C‐rate and cycle duty.     

Experimental investigation of a flat plate heat pipe performance using IR thermal imaging camera

This paper presents results and analysis of an experimental investigation into determining the thermal performance of a flat plate heat pipe using infra red (IR) thermal imaging camera. Steady state and transient temperature distribution of the evaporator surface of the flat plate heat pipe were measured using a single heat source with varied heat flux inputs. For performance comparison, the experimental measurements were also carried out on an identical flat plate heat pipe with a defect and on a solid copper block of similar dimensions. It was shown that temperature excursion on the surface of the fully functioning flat plate heat pipe is less than 3 °C for operating temperatures up to 90 °C and heat flux inputs ranging from 4 to 40 W/cm². Furthermore, the thermal spreading resistance of the flat plate heat pipe was found to be about 40 times smaller than that of the solid copper block and flat plate heat pipe with a defect.     

Water generation through desiccant using novel-designed solar still coupled with heat pipe vacuum tube collector: An experimental observation

In this article, an experiment has been carried out with heat pipe vacuum or evacuated tube collector to produce water from atmospheric air. In this experiment, the regeneration and adsorption method has been adopted, that is, water has been produced through the adsorption and regeneration of desiccants. The desiccant is heated through a hot surface to facilitate its regeneration. Limited experiments have been conducted to obtain water through the regeneration of desiccant using a hot surface. For the condensation of water vapor, a novel box has been designed, named the “novel-designed acrylic box.” The water is collected in a measuring flask or beaker to determine its quantity. Silica gel desiccant has been used for the adsorption and regeneration of water vapors. In this experiment, the adsorption process for silica gel was carried out in two different ways. In the first method, 1 kg of silica gel was scattered on the copper tray, that is, inside the system, while in the second method, 1 kg of silica gel was scattered on the paper, that is, outside of the system. In the first case silica gel adsorbed 137 g water vapor, and in the second case, it adsorbed 232 g water vapor. In the first case of adsorption, 70 mL water was produced while in the second case of adsorption, 175 mL water was produced from ambient air. The system's maximum efficiency was found to be 4.9%. Effects of various parameters, such as solar intensity, ambient temperature, wind speed, and so forth, have been studied.     

Development of a simplified heat pipe numerical model and case study/experimental validation using a long ‘wicked’ heat pipe

This paper describes existing numerical techniques used for simulating heat pipe operation, and the development of a simplified numerical model for normal wicked/wickless heat pipes based on the analysis of current modelling methods. Vapour flow was treated as a two‐dimensional flow. Heat transfers through the liquid–wick region and wall region were computed by solving a one‐dimensional heat conduction equation. Flow in the liquid–wick region was treated as a one‐dimensional problem. The liquid and vapour flows were coupled using a set of governing equations, incorporating thermal compressibility, hydro‐dynamical and capillary relationship, as well as geometrical correlation. The finite‐difference method was employed to carry out the numerical analysis, and FORTRAN language was used to develop a computer program. The model was used to investigate the operating characteristics of a long ‘wicked’ heat pipe, including variation of cross‐sectional area, axial/radial velocity, pressure and temperature of liquid/vapour flows with height position above the liquid level. To validate the modelling predictions, a test rig was constructed to carry out experimental testing. This included measurement of surface temperatures and heat flow associated with heat‐pipe heat transfer. The results from tests were found to be in general agreement with the numerical predictions when the test conditions were close to the simulation assumptions. Copyright © 2004 John Wiley & Sons, Ltd.     

An Experimental Study on the Performance of a Stainless Steel-Water Loop Heat Pipe under Natural Cooling Condition简

Aiming to improve the thermal characteristics of modern electronics, we experimentally study the performance of a stainless steel/water loop heat pipe （LHP） under natural cooling condition. The LHP heat transfer performance, including start-up performance, temperature oscillation and total thermal resistance at different heat loads and with different incline angles have been investigated systematically. Experimental results show that at an optimal heat load （i.e. 60 W） and with the LHP being inclined 60~ to the horizontal plane, the total thermal resistance is lowered to be -0.24 K/W, and the temperature of evaporator could be controlled steadily at around 90~C.     

Thermal deflection and thermal stresses in a thin circular plate under an axisymmetric heat source

Motivated by the need to investigate thermal effects on the deflection and stresses in a thin-wall workpiece during machining, the thermal problem is modeled with an axisymmetric input to emulate the heat generated at the tool-workpiece interface in a turning process. Using a compressor disk as an illustrative example, the boundary value problem is formulated with a plate model where the perimetric edge is clamped and insulated, and the upper and lower surfaces are subjected to heat convection. The closed form solution of temperature distribution is obtained via Green’s function method, based on which the thermal deflection/stresses are obtained in serial forms from the plate constitutive relations. The obtained solutions have been numerically verified with finite-element analysis (FEA), where simulations have been performed for three different materials with discrepant thermomechanical properties to study the thermal effects on the induced deflection and stresses. The analytical result is justified by its good agreement with FEA and its time efficiency in computation offers advantages in potential real-time application to manufacturing process monitoring.     

The experimental study of energy features for solar air heaters with different turbulator configurations

To assess the thermal performance in the climate conditions of western and central Iraq, the advantages of using a solar air collector with various turbulator absorber plates are experimentally explored. Four distinct kinds of absorber plates are provided flat plate (F), triangular (T), rectangular (R), and circular (C) turbulators at different air mass flow rates. The collector's economic properties and overall thermal performance are compared to the conventional flat plate turbulator heating systems. The main findings suggest that delta turbulators improve collector economics and overall thermal performance by generating vortex and dampening the formation of the thermal boundary layer in the direction of airflow. Furthermore, when the mass flow rate increases, the thermal performance improves, and the efficiency increases for all mass flow rates, resulting in good thermal performance for the rectangular plate collector when compared to other collectors. When compared to other types of configurations, the daily average efficiency of solar air collectors for flat plate (F), triangular (T), rectangular (R), and circular (C) turbulators are 28%, 67%, 39%, and 48%, respectively, at 50° tilt angle while at 90° tilt angle they are 44%, 76%, 54%, and 63%, respectively, as ṁ = 0.0377 kg/s. The maximum daily average efficiency fitted with rectangular turbulators have about 86% at the largest ṁ = 0.1 kg/s. This study will also give a unique direction to the work trend in the western and central parts of Iraq throughout the winter months.     

Thermal management system of lithium‐ion battery module based on micro heat pipe array

Temperature affects the performance of electric vehicle battery. To solve this problem, micro heat pipe arrays are utilized in a thermal management system that cools and heats battery modules. In the present study, the heat generation of a battery module during a charge‐discharge cycle under a constant current of 36 A (2C) was computed. Then, the cooling area of the condenser was calculated and experimentally validated. At rates of 1C and 2C, the thermal management system effectively reduced the temperature of the module to less than 40°C, and the temperature difference was controlled less than 5°C between battery surfaces of the module. A heating plate with 30‐W power effectively improved charge performance at low temperature within a short heating time and with uniform temperature distribution. Charge capacity obviously increased after heating when battery temperature was below 0°C. This study presents a new way to enhance the stability and safety of a battery module during the continuous charge‐discharge cycle at high temperatures and low temperatures accordingly.     

Theoretical and experimental investigation of a heat pipe heat exchanger for energy recovery of exhaust air

Heat pipes and two-phase thermosyphon systems are passive heat transfer systems that employ a two-phase cycle of a working fluid within a completely sealed system. Consequently, heat exchangers based on heat pipes have low thermal resistance and high effective thermal conductivity, which can reach up to the order of (10⁵ W/(m K)). In energy recovery systems where the two streams should be unmixed, such as air-conditioning systems of biological laboratories and operating rooms in hospitals, heat pipe heat exchangers (HPHEs) are recommended. In this study, an experimental and theoretical study was carried out on the thermal performance of an air-to-air HPHE filled with two refrigerants as working fluids, R22 and R407c. The heat pipe heat exchanger used was composed of two rows of copper heat pipes in a staggered manner, with 11 pipes per row. Tests were conducted at different airflow rates of 0.14, 0.18, and 0.22 m³/h, evaporator inlet-air temperatures of 40, 44, and 50°C, filling ratios of 45%, 70%, and 100%, and ratios of heat capacity rate of the evaporator to condenser sections (C_e/C_c) of 1 and 1.5. For HPHE's steady-state operation, a mathematical model for heat-transfer performance was set and solved using MATLAB. Results illustrated that the heat transfer rate was in direct proportion with the evaporator inlet-air temperature and flow rate. The highest HPHE's effectiveness was obtained at a 100% filling ratio and (C_e/C_c) of 1.5. The predicted and experimental values of condenser outlet-air temperature were in good agreement, with a maximum difference of 3%. HPHE's effectiveness was found to increase with the increase in evaporator inlet-air temperature and number of transfer units (NTU) and with the decrease in airflow rate, up to 33% and 20% for refrigerants R22 and R407c, respectively. Refrigerant R22 was the superior of the two refrigerants investigated.     

Performance analysis on a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation

In China, because of the emergence of a large number of high‐rise buildings, the solar hot water heater system often uses the balcony wall‐mounted method for installation. The thermoelectric energy converter is proposed as one of the possible technologies to incorporate solar water heater to produce electricity for building application. In this paper, the conceptual development and theoretical analysis of a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation are all proposed. The new system takes into account many advantages, including the high heat transfer, low convective heat loss, and low contact thermal resistance. The exergy analysis method based on the second law of thermodynamics is also introduced to evaluate the performance of this system. The results show that a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation has a high thermal performance with addition of electricity production. Copyright © 2016 John Wiley & Sons, Ltd.     

Plate flat heat pipe and liquid‐cooled coupled multistage heat dissipation system of Li‐ion battery

A thermal management system with the capability of achieving excellent heat dissipation is essential to the development of battery pack for transportation devices. To meet the temperature uniformity requirements of the battery pack, the plate flat heat pipe and liquid‐cooled coupled multistage heat dissipation system had been introduced. In this article, the research status of thermal management systems in battery pack was introduced. And the heat generation and heating power of the Li‐ion cell were studied. Then, the structure model of plate flat heat pipe system was proposed. Finally, the enhanced heat conduction effect of the thermal management system proposed in this article was comprehensively analyzed. Through the analysis of the results, in high discharge rates, the thermal management system proposed in this article could meet the temperature uniformity requirements of battery pack; also, the internal difference would reduce by 30.20%.     

High‐temperature latent heat storage technology to utilize exergy of solar heat and industrial exhaust heat

Latent heat storage (LHS) using phase change materials is quite attractive for utilization of the exergy of solar energy and industrial exhaust heat because of its high‐heat storage capacity, heat storage and supply at constant temperature, and repeatable utilization without degradation. In this article, general LHS technology is outlined, and then recent advances in the uses of LHS for high‐temperature applications (over 100 °C) are discussed, with respect to each type of phase change material (e.g., sugar alcohol, molten salt, and alloy). The prospects of future LHS systems are discussed from a principle of exergy recuperation. In addition, the technologies to minimize exergy loss in the future LHS system are discussed on the basis of the thermodynamic analysis by ‘thermodynamic compass’. Copyright © 2016 John Wiley & Sons, Ltd.     

Combined effects of the filling ratio and the vapour space thickness on the performance of a flat plate heat pipe

An experimental study of a flat plate heat pipe (FPHP) is presented. Temperature fields in the FPHP are measured for different filling ratios, heat fluxes and vapour space thicknesses. The system is hermetically sealed with a transparent plate for meniscus curvature radius observations by confocal microscopy. Experimental results show that the liquid distribution in the FPHP - and thus its thermal performance - depends strongly on both the filling ratio and the vapour space thickness. A small vapour space thickness induces liquid retention and thus reduces the thermal resistance of the system. Nevertheless, the vapour space thickness influences the level of the meniscus curvature radii in the grooves and hence reduces the maximum capillary pressure. As a result, it has to be carefully optimised to improve the performance of the FPHP. In all the cases, the optimum filling is in the range one to two times the total volume of the grooves. A theoretical approach, in non-working conditions, has been developed to model the distribution of the liquid inside the FPHP in function of the filling ratio and the vapour space thickness.     

Startup characteristics of a vertically placed high‐temperature heat pipe fin

In order to observe startup characteristics, a vertically installed high‐temperature heat pipe fin was tested. The temperature curves during the startup process are given. It was found that the evaporator bottom temperature in the high‐temperature heat pipe fin with a constant heat input increased very quickly over time. The temperature at the evaporator top and the condenser temperature lagged behind the temperature of the evaporator bottom. The evaporator outlet temperature coincided with the condenser middle temperature. The temperature at the end of the condenser exhibited a phenomenon of temperature pulsation. If the high‐temperature heat pipe fin was placed horizontally for a certain period of time and then tested in its vertical position, the temperature pulsation phenomenon at the condenser disappeared and a good isothermal condition emerged. Further analysis showed that larger heat inputs yielded faster startups and weaker pulsation during the startup period. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 411–416, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20022     

Comparative performance investigation of integrated collector‐storage solar water heaters with various heat loss reduction strategies

A detailed comparative assessment is reported on the thermal performance of integrated collector‐storage (ICS) solar water heaters with various strategies for reducing top heat losses. The objective of this investigation is to assess and compare heat loss reduction strategies. The shape of ICS solar water heater considered in present investigation is rectangular. The thermal performance of the solar water heater is evaluated and analyzed for the following cases: (1) single glass cover without night insulation; (2) single glass cover with night insulation; (3) double glass cover without night insulation; (4) transparent insulation with single glass cover; and (5) insulating baffle plate with single glass cover. Energy balances are developed for each case and solved using a finite difference technique. The numerical assessment of the system performance is performed for a typical July day in Toronto. Each strategy is observed to be beneficial, reducing top heat losses, and improving system performance. The greatest performance enhancements are observed for the water heater with a single glass cover and night insulation and for the system with a double glass cover and without night insulation. Copyright © 2010 John Wiley & Sons, Ltd.