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.     

Thermosyphon heat-pipe evacuated tube solar water heaters for northern maritime climates

Under transient climatic conditions previous research has reported that evacuated tube solar water heaters (ETSWHs) with heat-pipe absorbers are the most effective solution for collection of solar energy. The cost of such systems is greater than the mass produced “water in glass” evacuated tube solar water heater mainly manufactured in China. Previous studies have reported that the costs of solar water heating can be reduced through the adoption of thermosyphon fluid circulation. Well designed thermosyphon systems are as effective as pumped systems but with lower capital and running costs. To investigate if costs could be reduced and performance levels maintained, outdoor testing of three thermosyphon heat-pipe ETSWHs primarily designed for pumped fluid circulation was carried out under a northern maritime climate. Experimental data from a year’s side by side monitoring of two thermosyphon ETSWHs (both with the same area of 2 m²) was collected and used to validate a correlation based on a modified version of the f-chart design tool between the observed and expected performance for both systems. The R² value between measured and predicted monthly solar fractions was greater than 0.99 for both systems. The R² value between measured and predicted diurnal solar fractions was calculated as greater than 0.95 for both systems. The only difference between the two was that one utilised internal heat-pipe condensers whilst the other used external ones. The system with internal condensers was found to be 17% more efficient. A simplifying assumption of a constant temperature rise across the collectors reduced the calculations required to predict the performance of thermosyphon heat-pipe ETSWHs and was also statistically significant. To determine if the assumption was valid for other thermosyphon heat-pipe ETSWHs with different collector parameters a third system with internal condensers an area of 3 m², a heat removal factor (F_R) of 0.816 based on the absorber area and a collector loss coefficient (F_RU_L) of 2.25 W m^?2 K^?1 was assembled and its performance monitored, when the same assumption was applied the R² value between the measured and predicted daily solar fractions was calculated as 0.96 experimentally demonstrating that this relationship was still statistically significant for another heat-pipe thermosyphon ETSWH with different collector parameters.     

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.     

Water-in-glass manifolds for heat extraction from evacuated solar collector tubes

Measurements are reported on three novel manifolds of the water-in-glass type for evacuated all-glasssingle-ended tubular collectors. The manifolds provide for series connection of tubes, but because there is virtually no partitioning of the inner volume of the collector tubes, the manifolds are extremely simple and exhibit low impedance to fluid flow. The efficiency of heat extraction from the tubes has been determined by measuring temperatures at various points on the surface of glass tubes in a panel of area 1.2 m² while heating the tubes electrically to simulate solar energy input. Measurements have been made for a range of tube inclinations (0–80°), water flow rates (0.5–5 lmin⁻¹, water inlet temperatures (13–70°C), and effective solar fluxes (100–1000 W/m²) for two absorber tube diameters. The results show that for a wide range of operating conditions buoyancy effects alone result in efficient heat transfer to the tops of the tubes. The manifold designs described offer a possible low cost solution to the problem of manifolding evacuated collectors for sub-100°C heat extraction for domestic and industrial applications.     

加装导流板真空管热水器传热特性的研究

通过数值模拟方法分析了导流板位置对真空管热水器传热特性的影响,导流板加装在真空管中间或中间偏下1/3位置的效果较明显。综合考虑加工工艺和温度升高幅度等因素,对管内中间位置导流板结构和原结构进行了室内及室外热性能对比试验。室内对比试验表明,在管内中间位置安装导流板结构的水箱内平均水温变化与其数值模拟结果一致,比无导流板结构的水箱水温高0.5℃。室外热性能测试结果显示,水箱内平均水温、得热量和升温效果均优于未加导流板的原结构。     

Long term performance of evacuated tubular solar water heaters in Sydney, Australia

The performance of a domestic hot water system employing evacuated tubular collectors is compared with two others employing flat plate collectors over a period of one year. The efficiency of the evacuated tubular collector system was about 1.8 times that of a non selective flat plate system and about 1.3 times that of a high quality selective flat plate system. The superior performance of the evacuated system is explained in terms of the distribution of incident energy as a function of (ΔT/G) for domestic systems and the normal incidence collector efficiency curves.     

全玻璃真空管太阳空气集热器

简要介绍了全玻璃真空管太阳空气集热器结构、工作原理和对其进行测试的结果，在空气流量为60m^3／h和90m^3／h时，瞬时效率曲线分别为η=0．73-5．54(Tm-Ta)／I和η=0．78-5．45(Tm-Ta)／I。实验结果表明：内插管式全玻璃真空管太阳空气集热器具有较好的集热性能。     

真空管热水器倾角的数值模拟分析

利用fluent软件中的太阳载荷模型对真空管家用太阳热水器进行了三维数值模拟计算,分析了30°、45°、60°倾角下真空管热水器内的流场和温度场随加热时间的变化及传热和流动过程。结果表明,真空管热水器在加热期间,在真空管管口的上壁面出现温度最大值,水箱内真空管管口以下水的温度相对于管口以上的温度分层很明显,水箱内真空管管口以上的热水温度几乎相同,说明水箱内管口以上的水进行了充分混合;对于30°和45°倾角的系统,随着加热时间的增加,水箱内管口以下的水温与管口以上的水温从10 K增加到30 K,以45°倾角为例,当考虑了水箱内管口以下的冷水区时,随着加热时间的增加,真空管与水箱内的温差从0.73 K增大到1.13 K,仅考虑水箱内均匀分布的三点时,随着加热时间的增加,真空管与水箱内的温差从0.15 K减小到了0.03 K;随着系统倾角从30°增加到60°,水箱内管口以下的冷水区域在逐渐减小。因此,为了减小水箱底部的冷水区,插入水箱内的真空管应尽可能短。     

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。     

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.     

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.     

导流板结构对真空管热水器流动与换热的影响

针对真空管与水箱连接处出现随机涡流,不利于换热的问题,建立了真空管内加装导流板结构模型,运用Ansys Fluent软件对加装不同长度、厚度导流板结构的热水器进行三维数值模拟研究。导流板的存在减小了管内及连接处冷热流体的混合,使速度场均匀,流动稳定无扰动,保证了冷热流体的有序流动;特别是在真空管的下半部分,流体由水箱流入真空管处的速度提高,且流动稳定无涡流;加装不同长度导流板后,真空管底部、中部以及接近水箱入口处的水流平均速度提高了20%～313%,大大地增加了真空管下半部,特别是底部的热交换效率。     

Optimal tilt-angles of all-glass evacuated tube solar collectors

In this paper, a detailed mathematical procedure is developed to estimate daily collectible radiation on single tube of all-glass evacuated tube solar collectors based on solar geometry, knowledge of two-dimensional radiation transfer. Results shows that the annual collectible radiation on a tube is affected by many factors such as collector type, central distance between tubes, size of solar tubes, tilt and azimuth angles, use of diffuse flat reflector (DFR, in short); For collectors with identical parameters, T-type collectors (collectors with solar tubes tilt-arranged) annually collect slightly more radiation than H-type collectors (those with solar tubes horizontally arranged) do. The use of DFR can significantly improve the energy collection of collectors. Unlike the flat-plate collectors, all-glass evacuated tube solar collectors should be generally mounted with a tilt-angle less than the site latitude in order to maximize the annual energy collection. For most areas with the site latitude larger than 30° in China, T-type collectors should be installed with a tilt-angle about 10° less than the site latitude, whereas for H-type collectors without DFR, the reasonable tilt-angle should be about 20° less than the site latitude. Effects of some parameters on the annual collectible radiation on the collectors are also presented.     

Integrated collector storage solar water heaters

The Integrated Collector Storage Solar Water Heater (ICSSWH) developed from early systems comprised simply of a simple black tank placed in the sun. The ICSSWH, by its combined collection and storage function suffers substantial heat losses to ambient, especially at night-time and non-collection periods. To be viable economically, the system has evolved to incorporate new and novel methods of maximising solar radiation collection whilst minimising thermal loss. Advances in ICS vessel design have included glazing system, methods of insulation, reflector configurations, use of evacuation, internal and external baffles and phase change materials.     

Measurement and simulation of flow rate in a water-in-glass evacuated tube solar water heater

This paper evaluates the characteristics of water-in-glass evacuated tube solar water heaters including assessment of the circulation rate through single ended tubes. A numerical model of the heat transfer and fluid flow inside a single-ended evacuated tube has been developed assuming no interaction between adjacent tubes in the collector array. Flow measurement using Particle Image Velocimetry (PIV) has been undertaken to validate the numerical model. The experimental rig consists of a single full-scale tube coupled to a storage tank. A non-dimensional correlation has been developed of the circulation rate through a single evacuated tube mounted at 45° inclination over a diffuse reflector. Simulation results show that the natural convection flow rate in the tube is high enough to disturb the tank’s stratification and that the tank temperature strongly affects the circulation flow rate through the tubes. Circumferential heat distribution was found to be an important parameter influencing the flow structure and circulation rate through the tube, hence a separate correlation needs to be developed if a concentrating reflector is incorporated into the collector.     

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。     

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.     

Gas heat conduction in an evacuated tube solar collector

We investigated experimentally the pressure dependency of the gas heat conduction in an evacuated plate-in-tube solar collector. A stationary heat loss experiment was built up with an electrically heated real-size collector model. The gas pressure was varied from 10⁻³ to 10⁴ Pa, the temperatures of the absorber and the casing were held at 150°C (electrical heaters) and 30°C (water cooling), respectively. Losses by radiation and solid conduction were determined experimentally at pressures below 0.1 Pa. At higher pressures these background losses were subtracted from the total heat losses, to receive the heat losses by gas heat conduction. The experimental results were compared with approximative theoretical models. The onset of convection is in agreement with the usual theories for parallel plates, taking the largest distance between the absorber and the glass tube as the plate distance. As a first approximation the pressure dependency of the gas heat conduction is described by the usual theory for parallel plates, taking the smallest distance between the absorber and the glass tube as the plate distance.