Natural circulation flow through water-in-glass evacuated tube solar collectors

Experimental and numerical investigations were undertaken to develop a correlation for natural circulation flow rate through single-ended water-in-glass evacuated tubes mounted over a diffuse reflector. The circulation flow rate was correlated in terms of solar input, tank temperature, collector inclination and tube aspect ratio. The sensitivity of the flow rate correlation to the variation in circumferential heat flux distribution was also investigated.     

Performance of water-in-glass evacuated tube solar water heaters

The performance of water-in-glass evacuated tube solar water heaters is evaluated using experimental measurements of optical and heat loss characteristics and a simulation model of the thermosyphon circulation in single-ended tubes. The performance of water-in-glass evacuated tube solar collector systems are compared with flat plate solar collectors in a range of locations. The performance of a typical 30 tube evacuated tube array was found to be lower than a typical 2 panel flat plate array for domestic water heating in Sydney.     

全玻璃真空管太阳热水器数值模拟研究

基于FLUENT软件及TECPLOT软件,通过对均匀加热条件下的全玻璃真空管太阳热水器内流场及温度场的数值模拟,研究了热水器的对流换热与传质过程;分析了集热管倾角、真空管双面受热对热水器内流场及温度场的影响和真空管与水箱连接处、垂直等温面上的流体速度、温度分布。结果表明,在真空管与水箱连接处出现随机的涡流和真空管内出现分段的小环流,对传热传质过程不利,特别是对于带反射板的双面受热的集热器,应采取措施确保管内对流换热流动的有序性。建议加装导流板,并初步探讨了导流板的长度,确立了最佳板长模拟结构,为后续的理论研究与试验打下基础。     

Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles

To performance comparative studies, two sets of water-in-glass evacuated tube solar water heater (SWH, in short) were constructed and tested. Both SWHs were identical in all aspects but had different collector tilt-angle from the horizon with the one inclined at 22° (SWH-22) and the other at 46° (SWH-46). Experimental results revealed that the collector tilt-angle of SWHs had no significant influence on the heat removal from solar tubes to the water storage tank, both systems had almost the same daily solar thermal conversion efficiency but different daily solar and heat gains, and climatic conditions had a negligible effect on the daily thermal efficiency of systems due to less heat loss of the collector to the ambient air. These findings indicated that, to maximize the annual heat gain of such solar water heaters, the collector should be inclined at a tilt-angle for maximizing its annual collection of solar radiation. Experiments also showed that, for the SWH-22, the cold water from the storage tank circulated down to the sealed end of tubes along the lower wall of tubes and then returned to the storage tank along the upper wall of solar tubes with a clear water circulation loop; whereas for the SWH-46, the situation in the morning was the same as the SWH-22, but in the afternoon, the cold water from the storage tank on the way to the sealed end was partially or fully mixed with the hot water returning to the storage tank without a clear water circulation loop, furthermore, such mixing became more intense with the increase in the inlet water temperature of solar tubes. This indicated that increasing the collector tilt-angle of SWHs had no positive effect on the thermosiphon circulation of the water inside tubes. No noticeable inactive region near the sealed end of solar tubes for both systems was observed in experiments.     

Experimental investigation of natural convection heat exchange within a physical model of the manifold chamber of a thermosyphon heat-pipe evacuated tube solar water heater

The high capital costs associated with heat-pipe evacuated tube solar water heating systems can be reduced by replacing forced circulation with thermosyphon circulation. Currently research on thermosyphon heat-pipe evacuated tube solar water heaters is limited. An experimental investigation of the natural convective heat exchange regime that exists within the manifold chamber of a proprietary heat-pipe evacuated tube solar water was undertaken. This paper presents experimental data from a heat-pipe Evacuated Tube Solar Water Heater (ETSWH) subjected to the Northern Maritime Climate at the University of Ulster’s outdoor solar testing facility located at the Jordanstown campus. The thermal performance of this across solar noon (±30 min) was experimentally determined to be comparable to two physical laboratory 10 pin-fin model manifolds constructed to the same dimensions and geometry as the manifold chamber of the heat-pipe ETSWH when operated under steady laboratory conditions. When the surface temperatures of the pin-fins (simulated condensers) in the model manifold were normalised with respect to the lowest most pin-fin in the array the influence of buoyant flow was observed. Similarly to related studies in this field it was found that normalised surface temperatures on downstream pin-fins do not increase monotonically as would be expected if no interactions occur. It was found that at the pin-fin diameter to pitch used in the model manifold that normalised surface temperatures decrease at certain points in the array due to the action of buoyant flow generated from upstream pin-fins which increased heat transfer. Two-dimensional Particle Imaging Velocimetry (2D-PIV) was used to visualise the thermosyphon fluid flow regime. It was observed that the fluid flow regime varied across the model due to interactions between the fluid, chamber walls and pin-fins.     

Model-based design and analysis of heat pipe working fluid for optimal performance in a concentric evacuated tube solar water heater

In this study, a semi-dynamic model of a concentric evacuated tube solar water heater is developed to investigate the effect of working fluid design on technical and economic performance of a typical solar water heater in a household located at Sydney, Australia. The model is validated against experimental data. The effects of using water, ammonia, acetone, methanol, and pentane as working fluids of the built-in heat pipe are discussed comparatively during a typical day of operation. Water is identified as the best working fluid amongst the others. The variation of thermal resistance and critical heat flux of the heat pipes due to change in weather condition is presented and discussed. Three hypothetical working fluids are then proposed for further analysis which led to a working fluid design superior to water in performance. It is shown that the performance of the solar water heater can be significantly enhanced up to 28% and 50% from economical and technical points of view, respectively.     

全玻璃真空管太阳能热水器影响因素的数值模拟研究

利用热能工程专业基础知识和场协同原理,结合计算流体力学配套商业软件,对反光板、倾斜角度、辐照强度、集热管尺寸和水箱大小等因素,影响全玻璃真空管太阳能热水器的传热传质特性进行了数值模拟研究和可视化分析,通过数值模拟研究给出了全玻璃真空管太阳能热水器的的最佳条件。     

真空管式太阳热水器得热量的实验研究

对某型号真空管式太阳热水器进行多天的热性能测试,采用最小二乘法拟合出太阳热水系统能量方程系数,求出该型号太阳热水器的能量方程,同时进行误差和敏感性分析,指导试验设计.     

Investigation of evacuated tube heated by solar trough concentrating system

Two types of solar evacuated tube have been used to measure their heating efficiency and temperature with fluids of water and N₂ respectively with a parabolic trough concentrator. Experiments demonstrate that both evacuated tubes present a good heat transfer with the fluid of water, the heating efficiency is about 70–80%, and the water is easy to boil when liquid rate is less than 0.0046 kg/s. However, the efficiency of solar concentrating system with evacuated tube for heating N₂ gas is less than 40% when the temperature of N₂ gas reaches 320–460 °C. A model for evacuated tube heated by solar trough concentrating system has been built in order to further analyze the characteristics of fluid which flow evacuated tube. It is found that the model agrees with the experiments to within 5.2% accuracy. The characteristics of fluid via evacuated tube heated by solar concentrated system are analyzed under the varying conditions of solar radiation and trough aperture area. This study supports research work on using a solar trough concentrating system to perform ammonia thermo-chemical energy storage for 24 h power generation. The current research work also has application to solar refrigeration.     

Theoretical and experimental investigation of the filled-type evacuated tube solar collector with U tube

The filled-type evacuated tube with U-tube, in which the filled layer is used to transfer energy absorbed by the working fluid flowing in the U-tube, is proposed to eliminate the influence of thermal resistance between the absorber tube and the copper fin of the conventional evacuated solar collector. In this paper, the thermal performance of the filled-type evacuated tube with U-tube was researched by means of theoretical analysis and experimental study. The temperature of the working fluid in the flow direction was obtained, and the efficiency of the evacuated tube was also calculated, based on the energy balance equations for the working fluid in the U-tube. The effects of the heat loss coefficient and the thermal conductivity of the filled layer on the thermal performance of the evacuated tube were studied. In addition, the test setup of the thermal performance of the filled-type evacuated tube with U-tube was established. The evacuated tube considered in this study was a two-layered glass evacuated tube, and the absorber film was deposited in the outer surface of the absorber tube. The results show that the filled-type evacuated tube with U-tube has a favourable thermal performance. When the thermal conductivity of the heat transmission component is λ_c = 100, the efficiency of the filled-type evacuated tube with U-tube is 12% higher than that of the U-tube evacuated tube with a copper fin. The modelling predictions were validated using experimental data which show that there is a good concurrence between the measured and predicted results.     

加装导流板的真空管热水器流动与换热的数值模拟

针对真空管与水箱连接处冷热水混流、出现随机涡流而影响换热等问题,分别建立了两种加装导流板结构模型.用FLUENT软件对两种导流结构热水器进行三维数值模拟研究,并与原结构进行比较分析,研究了真空管内的对流换热过程以及水箱内流体的分层情况.结果表明:加装导流板后,管口和水箱内的冷、热水混合换热情况得到了改善,增强了管内流体...     

Assessment of electricity and hydrogen production performance of evacuated tube solar collectors

In this work, a unified renewable energy system has designed to assess the electricity and hydrogen production. This system consists of the evacuated tube solar collectors (ETSCs) which have the total surface area of 300 m², a salt gradient solar pond (SGSP) which has the surface area of 217 m², an Organic Rankine Cycle (ORC) and an electrolysis system. The stored heat in the heat storage zone (HSZ) transferred to the input water of the ETSCs by means of an exchanger and thereby ETSCs increase the temperature of preheated water to higher level as much as possible that primarily affects the performance of the ORC. The balance equations of the designed system were written and analyzed by utilizing the Engineering Equations Solver (EES) software. Hence, the energy and exergy efficiencies of the overall system were calculated as to be 5.92% and 18.21%, respectively. It was also found that hydrogen generation of the system can reach up to ratio 3204 g/day.     

Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage unit

The thermal performance of a prototype solar cooker based on an evacuated tube solar collector with phase change material (PCM) storage unit is investigated. The design has separate parts for energy collection and cooking coupled by a PCM storage unit. Solar energy is stored in the PCM storage unit during sunshine hours and is utilized for cooking in late evening/night time. Commercial grade erythritol was used as a latent heat storage material. Noon and evening cooking experiments were conducted with different loads and loading times. Cooking experiments and PCM storage processes were carried out simultaneously. It was observed that noon cooking did not affect the evening cooking, and evening cooking using PCM heat storage was found to be faster than noon cooking. The cooker performance under a variety of operating and climatic conditions was studied at Mie, Japan.     

Experimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions

New comparative tests on two different types of solar collectors are presented in this paper. A standard glazed flat plate collector and an evacuated tube collector are installed in parallel and tested at the same working conditions; the evacuated collector is a direct flow through type with external compound parabolic concentrator (CPC) reflectors.Efficiency in steady-state and quasi-dynamic conditions is measured following the standard EN 12975-2 and it is compared with the input/output curves measured for the whole day.The first purpose of the present work is the comparison of results in steady-state and quasi-dynamic test methods both for flat plate and evacuated tube collectors. Beside this, the objective is to characterize and to compare the daily energy performance of these two types of collectors. An effective mean for describing and analyzing the daily performance is the so called input/output diagram, in which the collected solar energy is plotted against the daily incident solar radiation. Test runs have been performed in several conditions to reproduce different conventional uses (hot water, space heating, solar cooling).Results are also presented in terms of daily efficiency versus daily average reduced temperature difference: this allows to represent the comparative characteristics of the two collectors when operating under variable conditions, especially with wide range of incidence angles.     

Optical evaluation and analysis of an internal low-concentrated evacuated tube heat pipe solar collector for powering solar air-conditioning systems

An optical evaluation and analysis of an internal low-concentrating evacuated tube heat pipe solar collector designed to enhance the collection of solar radiation for medium temperature applications is presented in this paper. The internal low-concentrating evacuated tube heat pipe solar collector was designed with an acceptance angle of 20° given a geometrical concentration ratio of 2.92. The truncation of the upper part of the reflector giving a geometrical concentration ratio of 1.95 was carried and enabled the internal low-concentrating evacuated tube heat pipe collector to be enclosed by a borosilicate glass tube with 100 mm and 93 mm outer and inner diameters, respectively. Ray trace analysis at different transverse angles determines optical efficiencies, related optical losses and flux distribution on the absorber of the internal low-concentrating evacuated tube heat pipe solar collector. A detailed two dimensional ray trace techniques considering only the direct insolation component predicated overall ray’s acceptance of 93.72% and optical efficiency of 79.13% from transverse angles of 0° to 20°.     

Energy consumption minimization in an innovative hybrid power production station by employing PV and evacuated tube collector solar thermal systems

Combination of a grid connected photovoltaic (PV) plant with a compressed air energy storage system (CAES) and a city gate station (CGS) has been proposed and investigated recently, leading to satisfactory performance results. The only deficiency of this system is the huge amount of fuel required to provide its heating demand. In this work, feasibility of employing evacuated tube solar thermal systems to supply the heating demand of the hybrid power plant is studied. After presenting detailed mathematical modeling, the solar heating units and other components of the power plant are properly sized. The results of simulations demonstrate that a total of 7000 evacuated tube collectors are required in the system, leading to elimination of the air heater from the CAES system completely and 17.2% fuel saving at the CGS. The total annual solar heat of 17.5 GWh is supplied for the system, 214 GWh power could directly be sold to the grid, 9.7 GWh power slumps is recovered and 53.5 GWh power is produced at nights. In the end, internal rate of return (IRR) method is used to compare economically the proposed system with similar systems proposed previously, outperforming all of the other candidates with an IRR of 11.1%.     

Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold

A study is reported of thermosiphon circulation in solar water heaters incorporating glass tubular evacuated collectors and a water-in-glass manifold of extremely simple design. The manifold is characterised by the absence of partitioning of the inner volumes of the absorber tubes into inlet/outlet channels and buoyancy effects are utilised to convey heat to a header pipe at the open end of the tubes. Solar energy input to the tubes has been simulated by electric heating. The thermosiphon system design is unusual in that there are no risers within the collector tubes, thus, the pressure head responsible for thermosiphon flow originates entirely from relatively short pipe runs between manifold and storage tank. Thermosiphon flow has been measured for a number of system designs and a wide range of operating conditions. The relative impedances of the system components has been evaluated allowing optimization of the system design. An investigation of the effect of withdrawal of hot water from the storage tank, with associated injection of cold water to the bottom of the tank, has illustrated that the self-regulating nature of the thermosiphoning system results in a large proportion of heat stored in the wate filled collector tubes being effieciently transferred to the storage tank, providing some water is drawn off intermittently.     

真空管集热的太阳能低温多效海水淡化系统变负荷性能研究

建立了真空管集热的太阳能低温多效蒸发海水淡化系统的数学模型,利用MATLAB对其进行了求解。从造水量和耗能量两方面分析了定负荷和变负荷系统的性能。分析结果表明:定负荷时的年造水量稍大于变负荷时的年造水量;定负荷时,在1、2、6、11、12月份需要电辅子系统加热,耗电量大,在其他月份需要冷却子系统排走太阳能供热系统提供的多余热量,增加了单位造水总能耗。变负荷时系统造水量在40%~110%范围内变化,与定负荷系统相比耗电少,对热能利用较充分。     

Simulation and economic analysis of a solar-powered adsorption refrigerator using an evacuated tube for thermal insulation

A detailed analysis and simulation of heat transfer of an evacuated tube for a solar-powered adsorption refrigerator, including inhomogeneous radiation heat transfer between its inner and outer tubes, two adjacent tubes in the group, the tube and back plate, sunlight reflection on back plate, etc. has been carried out. Also, a new cost–effectiveness parameter is provided to assess the comprehensive performance of the system, and consequently optimums for the diameter of tubes and the distance between two adjacent tubes are investigated.     

The use of ground heat storages and evacuated tube solar collectors for meeting the annual heating demand of family-sized houses

The use of storages for sensible heat is limited because parts of the input thermal energy end up as unavoidable heat losses. In order to minimize this loss, it is necessary to keep the surface area to volume ratio (S/V) as low as possible. This occurs when the volume of a body with a certain shape increases. In addition to a large volume it is important to use materials with a high volumetric thermal capacity, as long as sensible heat is being used for storage. This condition is best met by water or a combination of substances with water. In the field of interseasonal storages, for solar heat to cover the heating demands of small residential buildings, the general belief is that the relative small volume needed, results in too much heat loss and therefore individual seasonal storages seem to be of no useful solution.However, the theoretical considerations and simulations in this paper show that this is a prejudice. It is possible to supply a great deal of the thermal energy needed for small residential homes with interseasonal ground storage for solar heat. The loss of heat is acceptable if the storage is designed in the correct way.The ground heat storage should be of cuboidal shape, using the local soil as storage material, if possible. The storage containment must be heat-insulated and damp-proof. The placement of the storage could be within the heated building, adjacent to it or nearby. As such systems may be useful as retrofit for existing houses this study assumes that the storage system has no contact with the heated house. The heat is supplied by evacuated tube solar collectors and their feature to produce effective heat with high temperature (above 100 °C) is used.