抛物面槽式太阳能集热器场热损失分析

在已有的计算集热器场吸收有用能量模型的基础上,加入影响集热器场效率的热学因素,优化了集热器场效率计算模型,并验证了优化模型的精确性。利用优化模型对抛物面槽式太阳能集热器场热损失机理进行了研究。结果表明,集热器集热元件热效率、入射角以及由入射角引起的端部损失是影响集热器场效率的主要因素。在太阳辐射强度一定的情况下,入射角越小、集热器热收集元件的热效率越高时,集热器场效率越高。     

典型太阳能集热器热性能研究

以当今市场保有量最高的两种典型太阳能集热器——全玻璃真空管型太阳能集热器和平板型太阳能集热器为研究对象,从能量传递角度,构建了太阳能集热器在理想状态下的热效率模型,并依据模型计算了极限热效率;最后对两种太阳能集热器做了热性能测试实验,并使用最小二乘法拟合出太阳能集热器瞬时热效率与归一化温差的关系,对比两种太阳能集热器在同一实验条件下的极限热效率和热损失系数,并验证了该模型的正确性。     

V型多通道平板太阳能集热器的热性能研究

提出一种V型多通道平板太阳能集热器,对其建立稳态传热模型,利用Matlab软件编程进行求解,并进行实验测试验证模型的准确性.利用已验证的传热模型,模拟分析V型多通道平板太阳能集热器的结构和物性参数对其热性能的影响,结果表明当透明盖板和吸热体发射率变大时,集热器的集热效率会呈下降趋势;当V型槽顶角、吸热体长度及空气夹层厚...     

大尺寸平板太阳集热器集热效率研究

平板型太阳集热器集热效率除了与内部结构参数、运行参数等因素有关外,还受集热器外形尺寸大小的影响。通过建立相同集热面积条件下单一大尺寸平板集热器、并联常规平板集热器的三维物理模型,分析了2类集热系统集热效率、集热量、热损失随尺寸规模的变化关系。发现相同集热面积条件下,相比并联常规平板集热器形式,大尺寸平板集热器集热效率有所增加,约增大4%;但对于常规并联平板集热器,随着并联面积增大其热损失所占比例均有所增大,集热效率也有一定的降低;而对于单一大尺寸平板集热器,随着外形尺寸增大,热损失所占比例降低,且集热效率有所升高。研究结论可对大型平板集热器的尺寸设计提供设计指导。     

不同气候条件下平板太阳能集热器动态热性能的解析

利用一种可结合环境气象数据等参数来计算平板太阳能集热器一段时间内集热效率动态分布的数学模型,对比研究了不同气候区典型城市年平均效率及逐月效率分布的差异,分析了入口温度与环境温度温差、太阳辐照度对集热器全年瞬时效率的影响,并将该模型与《太阳能供热采暖工程技术规范》中的集热效率计算方法进行了对比。通过分析说明了在研究太阳能集热器的集热效率时,应结合气象条件和运行参数动态进行研究分析,冬季的瞬时效率并非总是低于夏季。该模型可更精确地模拟实际工程情况来计算集热器平均效率,是一种更加适应不同气候地区的太阳能集热器热性能模型,为工程实例中平板太阳能集热器平均效率的选取提供了参考。     

无盖板渗透型集热器热性能的对比试验

利用太阳能空气加热系统实验台,对黑、深绿和深蓝3种颜色无盖板渗透型集热器的热性能进行了户外瞬态对比试验。试验结果表明:太阳辐射照度和风量是影响系统热性能的重要因素。在高档和低档两种风量下,黑色集热器的瞬时平均热效率分别为76.04%和67.50%,高于普通平板太阳能空气集热器;集热器表面颜色对其热性能有一定影响,在高档和低档两种风量下,深绿色和深蓝色集热器的瞬时平均热效率比黑色集热器低15%～22%,空气温升低3～4℃,但仍然优于普通平板空气集热器。从保持建筑立面美观考虑,无盖板渗透型集热器的集热板可以采用颜色较深的彩色,不会对系统热性能造成较大影响。     

一种槽式太阳能空气集热器热性能的试验研究

为了研究日光温室用槽式太阳能空气集热器的热性能,基于TracePro光学模拟软件设计了一种槽式太阳能空气集热器,对其进行试验研究,分析了不同因素对集热性能的影响规律。实验结果表明,管中空气流速的变化对集热器集热效率和集热量的影响规律是相同的,在不同的流速下,存在最佳空气流速约为4.4 m/s,使得集热器的集热量和集热效率最大,集热量达到373.2 W,集热效率约为25%,此时集热性能最好。对于不同太阳辐照度,正午时刻之前,太阳辐照度越大,集热器的集热效率越大,正午时刻之后,集热器的集热效率基本保持不变,15:40之后集热器集热效率逐渐减小。当太阳辐照度和管中流速相同时,室外温度越高,集热器集热效率越大,集热性能越好。集热管中空气温度沿着集热管出口方向不断增大,太阳辐照度越大,集热管相同位置空气温度越高。该研究结果可为此种槽式太阳能空气集热器在日光温室的应用中提供参考。     

基于平板微热管阵列的新型太阳能空气集热器热性能及阻力特性研究

以平板微热管阵列为核心部件设计一种新型的太阳能空气集热器,对其热性能和阻力特性进行详实的实验研究。探讨不同季节对空气集热器集热效率的影响;空气流量对集热效率的影响;空气集热器的流动阻力特性。实验结果表明:夏季稳定运行状态下集热器的集热效率可达73%,流动阻力小于25 Pa。     

内插式真空管太阳能热水器热特性理论及实验研究

设计了一种内插式真空管热水集热器,通过对其能量转换、传输特性研究,建立了内插式真空管集热器的热特性分析模型,并对不同设计参数和气候条件对内插式真空管热特性的影响进行了分析。结果表明,设计的集热器理论热效率最大可达79.5%,但其受环境温度、集热器工作温度等影响较大。此外,设计了由14只内插式真空管组成的太阳能热水器,采用迟滞算法将热水温度控制在50～55℃,实验结果表明,样机系统的日热效率可达55.0%,样机输出热水近120 kg,具有较好的实用价值。     

两种太阳集热器的热性能对比分析

介绍了平板型太阳集热器和热管式真空管太阳集热器,并对其热性能进行了测试分析对比.平板型太阳集热器所能达到的最高效率略高于热管式真空管太阳集热器,但是随着集热温度升高,其热损失增大,效率降低,热管式真空管太阳集热器的集热效率波动较小,稳定在较高的水平。     

真空玻璃与单层玻璃盖板PV/T集热器性能的对比研究

提出一种真空玻璃盖板平板式PV/T集热器,建立了真空玻璃和单层玻璃盖板PV/T集热器的传热模型,并分别搭建了两种PV/T热水系统的实验平台进行模型的实验验证。预测结果与实验测量结果的均方根偏差（RMSD）在0.71% ~ 11.17%之间。利用数学模型模拟了真空玻璃与单层玻璃盖板PV/T集热器在合肥冬季的热、电性能,并比较了两者性能的差异。模拟结果表明真空玻璃盖板PV/T集热器的顶部热损失平均为22 W,而单层玻璃盖板PV/T集热器热损失平均为107 W。使用真空玻璃盖板能显著减少PV/T集热器的顶部热损失。真空玻璃盖板PV/T集热器相对单层玻璃盖板PV/T集热器的全天热效率提高了5.68%,二者分别为41.76%和36.08%,全天电效率分别为11.76%和12.79%,相差1.03%。     

Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors

Unglazed transpired collectors or UTC (also known as perforated collectors) are a relatively new development in solar collector technology, introduced in the early nineties for ventilation air heating. These collectors are used in several large buildings in Canada, USA and Europe, effecting considerable savings in energy and heating costs. Transpired collectors are a potential replacement for glazed flat plate collectors. This paper presents the details of a mathematical model for UTC using heat transfer expressions for the collector components, and empirical relations for estimating the various heat transfer coefficients. It predicts the thermal performance of unglazed transpired solar collectors over a wide range of design and operating conditions. Results of the model were analysed to predict the effects of key parameters on the performance of a UTC for a delivery air temperature of 45–55 °C for drying applications. The parametric studies were carried out by varying the porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity, and finding their influence on collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered. Results indicate promising thermal performance of UTC in this temperature band, offering itself as an attractive alternate to glazed solar collectors for drying of food products.The results of the model have been used to develop nomograms, which can be a valuable tool for a collector designer in optimising the design and thermal performance of UTC. It also enables the prediction of the absolute thermal performance of a UTC under a given set of conditions.     

Multi-linear performance model for hybrid (C)PVT solar collectors

A performance model is presented that enables yield predictions of hybrid photovoltaic and thermal (PVT) collectors. It applies for both non-concentrating (PVT) and concentrating (CPVT) systems. The model is based on considerations on energy balance, heat transfer and the dependence of the photovoltaic efficiency on absorber temperature. Similar to the quasi dynamic model for thermal collectors, linear parameterizations of both electrical and thermal power outputs are derived. The respective linear coefficients are obtained from multi-linear regression on solely standard measurement data.The model is applied to measurement data of a CPVT collector to exemplify the procedure and to validate the model. Good agreement of empirically determined collector coefficients with expectations from known parameters is achieved.     

Theoretical and experimental study of efficiency factor,heat transfer and thermal heat loss coefficients in solar air collectors with selective and nonselective absorbers

The thermal heat performance of a solar air collector depends strongly on the thermal heat loss and the efficiency factor. In order to increase these performances, it is necessary to use a solar air collector which is well insulated and where the fluid flow is fully developed turbulent flow. It needs a high heat transfer between the absorber plate and the fluid to decrease the absorber‐plate temperature and hence the heat loss by radiation from the absorber to the ambient. This increases the efficiency factor. In the present paper, the heat loss and efficiency factor are treated for solar air collectors with selective and nonselective absorber plate. It is shown that the selectivity of the absorber plate cannot play an important role in a well‐insulated solar collector with a fanned system which permits a fully developed turbulent flow and, in consequence, high heat transfer. Copyright © 1999 John Wiley & Sons, Ltd.     

Performance study of a photovoltaic solar assisted heat pump with variable-frequency compressor – A case study in Tibet

The performance of a photovoltaic solar assisted heat pump (PV-SAHP) with variable-frequency compressor is reported in this paper. The system is a direct integration of photovoltaic/thermal solar collectors and heat pump. The solar collectors extract the required thermal energy from the heat pump and at the same time, the cooling effect of the refrigerant lowers the working temperature of the solar cells. So this combined system has a relatively high thermal performance with an improved photovoltaic efficiency. To adapt to the continuously changing solar radiation and ambient temperature conditions, the refrigerant mass flow rate should match the heat gain at the evaporator accordingly. A variable-frequency compressor and an electricity-operated expansion valve were used in the proposed system. Mathematical models were developed to evaluate the energy performance of the combined system based on the weather conditions of Tibet. The simulation results indicated that on a typical sunny winter day with light breeze, the average COP could reach 6.01, and the average electricity efficiency, thermal efficiency and overall efficiency were 0.135, 0.479 and 0.625 respectively.     

Experimental performance analysis and optimisation of medium temperature solar thermal collectors with silicon oil as a heat transfer fluid

Heat transfer fluids (HTFs) play an essential role in solar water heating systems by transferring collected energy from the collector, perhaps via a heat exchanger to the store. If the store is at a much higher temperature than the fluid, the store acts as a heat source, whereas the fluid acts as a coolant, thus reversing the collection process. This action must be avoided through good controls. Experimental performance analysis and comparison of three different types of solar collectors; a non‐concentrating evacuated tube heat pipe and two concentrating single‐sided and double‐sided coated evacuated tube heat pipes collectors are installed and tested using Dow‐corning 550® silicon oil as an HTF under the same operating in‐door control conditions, and results are presented in this paper. The performance of these solar collectors was determined from the overall increase in inlet and outlet fluid temperatures, overall fluid temperature differential, energy collection rate, optical efficiencies, and thermal performances. Temperature differential, energy, and collection efficiency diagrams plotted against time were used to represent and compare the solar collectors. Finally, a comparative analysis of these solar collectors using either pressurised water or Dow‐corning 550 silicon oil as HTF is presented. Copyright © 2011 John Wiley & Sons, Ltd.     

A new dynamic test method for thermal performance of all-glass evacuated solar air collectors

A new study on testing thermal performance of all-glass evacuated solar collectors with the air as heat transfer fluid under dynamic conditions outdoors has been developed. The model of this dynamic method was established with the energy balance analysis on solar collectors of this type. Compared with the first order model under steady-state conditions, this model can characterize thermal efficiency of solar collectors under more extensive conditions, reducing considerable operating time spent in waiting for the right test conditions. Through the derivation of the proposed model, it proved a strong relationship existed between this model and the first order model mentioned above. The dynamic model projection for the outlet temperatures was in good agreement with the measured result.     

Performance of a building integrated photovoltaic/thermal (BIPVT) solar collector

The idea of combining photovoltaic and solar thermal collectors (PVT collectors) to provide electrical and heat energy is an area that has, until recently, received only limited attention. Although PVTs are not as prevalent as solar thermal systems, the integration of photovoltaic and solar thermal collectors into the walls or roofing structure of a building could provide greater opportunity for the use of renewable solar energy technologies. In this study, the design of a novel building integrated photovoltaic/thermal (BIPVT) solar collector was theoretically analysed through the use of a modified Hottel-Whillier model and was validated with experimental data from testing on a prototype BIPVT collector.The results showed that key design parameters such as the fin efficiency, the thermal conductivity between the PV cells and their supporting structure, and the lamination method had a significant influence on both the electrical and thermal efficiency of the BIPVT. Furthermore, it was shown that the BIPVT could be made of lower cost materials, such as pre-coated colour steel, without significant decreases in efficiency.Finally, it was shown that by integrating the BIPVT into the building rather than onto the building could result in a lower cost system. This was illustrated by the finding that insulating the rear of the BIPVT may be unnecessary when it is integrated into a roof above an enclosed air filled attic, as this air space acts as a passive insulating barrier.     

Thermal enhancement of solar parabolic trough collectors by using nanofluids and converging-diverging absorber tube

Parabolic trough collectors are the most mature technology for utilizing the solar energy in high temperature applications. The objective of this study is the thermal efficiency enhancement of the commercial parabolic collector IST-PTC by increasing the convective heat transfer coefficient between the working fluid and the absorber. There are two main factors which influence on this parameter, the working fluid type and the absorber geometry. For this reason three working fluids are investigated, thermal oil, thermal oil with nanoparticles and pressurized water. Moreover, a dimpled absorber tube with sine geometry is tested because this shape increases the heat transfer surface and increases the turbulence in the flow. The final results show that these two techniques improve the heat transfer coefficient and the thermal efficiency of the collector. More specifically, the use of nanofluids increases the collector efficiency by 4.25% while the geometry improvement increases the efficiency by 4.55%. Furthermore, collector parameters such as the heat loss coefficient, the exergetic efficiency, the pressure losses and the absorber temperature are presented for all the examined cases. The model is designed with Solidworks and is simulated by its flow simulation studio.     

Solar multi-mode heating system based on latent heat thermal energy storage and its application

为克服太阳能间断性和不稳定性的缺点进而实现太阳能集热与采暖的能量供需调节和全天候连续供热,提出了基于相变储热的太阳能多模式采暖方法（太阳能集热直接采暖、太阳能集热采暖+相变储热、太阳能相变储热采暖）,并在西藏林芝市某建筑搭建了太阳能与相变储热相结合的采暖系统,该系统可根据太阳能集热温度和外界供热需求实现太阳能多模式采暖的自动控制和自动运行。实验研究表明：在西藏地区采用真空管太阳能集热器可以和中低温相变储热器很好地结合,白天储热器在储热过程中平均储热功率为10.63 kW,储热量达到92.67 kW·h,相变平台明显;晚上储热器在放热过程中供热量达85.23 kW·h,放热功率和放热温度平稳,储放热效率达92%,其储热密度是传统水箱的3.6倍,可连续供热时间长达10 h,从而实现了基于相变储热的太阳能全天候连续供热,相关研究结果对我国西藏地区实施太阳能采暖具有一定的指导作用。