The absorption factor of crystalline silicon PV cells: A numerical and experimental study

The absorption factor of a PV cell is defined as the fraction of incident solar irradiance that is absorbed by the cell. This absorption factor is one of the major parameters determining the cell temperature under operational conditions. Experimentally the absorption factor can be derived from reflection and transmission measurements. The spectral reflection and transmission factors were measured for a set of crystalline silicon (c-Si) samples with a gradually increasing complexity. The experimental results agree very well with the results from a 2D numerical model that was developed. It was found that the AM1.5 absorption factor of a typical encapsulated c-Si photovoltaic cell is as high as 90.5%. Insight was gained in the cell parameters that influence this absorption factor. The presence of texture at the front of the c-Si wafer of sufficient steepness is essential to achieve such a high absorption factor. Sub-bandgap solar irradiance is mainly absorbed in the very thin emitter by means of free-carrier absorption. By minimizing reflective losses over the entire solar spectrum, the AM1.5 absorption of c-Si cells can theoretically be increased to 93.0%. The effect on the annual yield of PV and PV/thermal systems is quantified.     

Functional requirements for component films in a solar thin-film photovoltaic/thermal panel

The functional requirements of the component films of a solar thin-film photovoltaic/thermal panel were considered. Particular emphasis was placed on the new functions, that each layer is required to perform, in addition to their pre-existing functions. The cut-off wavelength of the window layer, required for solar selectivity, can be achieved with charge carrier concentrations typical of photovoltaic devices, and thus does not compromise electrical efficiency. The upper (semiconductor) absorber layer has a sufficiently high thermal conductivity that there is negligible temperature difference across the film, and thus negligible loss in thermal performance. The lower (cermet) absorber layer can be fabricated with a high ceramic content, to maintain high solar selectivity, without significant increase in electrical resistance. A thin layer of molybdenum-based cermet at the top of this layer can provide an Ohmic contact to the upper absorber layer. A layer of aluminium nitride between the metal substrate and the back metal contact can provide electrical isolation to avoid short-circuiting of series-connected cells, while maintaining a thermal path to the metal substrate and heat extraction systems. Potential problems of differential contraction of heated films and substrates were identified, with a recommendation that fabrication processes, which avoid heating, are preferable.     

Research on the influence of PV cell to thermal characteristics of photovoltaic/thermal solar system

In the paper, we analyzed internal thermal transmission characteristics of water‐heating photovoltaic/thermal (PV/T) solar collector covered by photovoltaic (PV) cell, established photothermal conversion model of PV/T solar system, and analyzed the influence of PV cell coverage to photothermal characteristics of PV/T solar system. Results show that the thermal efficiency of PV/T solar system by optimizing PV cells coverage can reach 68%. In addition, by designing four water‐heating PV/T solar system prototypes with PV cell coverage of 0.4, 0.56, 0.7, and 0.82, respectively, we conducted experimental researches for the four prototypes and found that the four prototypes can achieve thermal efficiencies of 58%, 51%, 64%, and 67%, respectively, in heating 250 L of water to 50°C. The experiment results are consistent with theoretical analysis results, indicating that it is feasible to improve thermal characteristics of PV/T solar system by optimizing PV cell coverage. Copyright © 2017 John Wiley & Sons, Ltd.     

Optimized variable-focus-parabolic-trough reflector for solar thermal concentrator system

A solar thermal concentrator system is proposed comprising a cylindrical heat-pipe receiver and a variable-focus-parabolic-trough (VFPT) reflector in which the focal length varies as a function of the vertical displacement of the incidence point relative to the horizontal centerline of the receiver. The light ray paths within the concentrator system are analyzed using a skew-ray tracing approach. A method is then proposed for optimizing the geometry of the concentrator system in such a way as to optimize the uniformity of the irradiance distribution on the heat-pipe surface. The validity of the proposed optimization method is demonstrated by means of ZEMAX/SolidWorks-Flow simulations. It is shown that the optimized VFPT concentrator yields a significant improvement in both the irradiance uniformity and the heating efficiency compared to conventional cylindrical-trough and parabolic-trough concentrators.     

Experimental validation of an optical and thermal model of a linear Fresnel collector system

This paper describes the design and validation of a mathematical model for a solar Fresnel collector. The function of the model is to simulate the optical and thermal dynamics of a Fresnel system for heating water. The model is validated using real data gathered from a cooling plant with double effect absorption chiller located in the School of Engineering University of Seville, Spain (Experimental cooling plant is also described in the paper). Comparison of calculated and plant measured data shows that the error is lower than 3% in the optical model and within 7% in the thermal model.The model uses a new approach to include a solar tracking mirror mechanism in one axis. This tracking has been designed to maximise the reception of available solar radiation by the absorption pipe. The thermal model used is based around classical models for solar receivers and it is validated with real operating data gathered from a supervisor system.The Fresnel model has been designed with sufficient flexibility to consider different geometries and thermal parameters, and may be used to simulate the performance of a proposed Fresnel collector system at any location.     

Industrially feasible multi-crystalline metal wrap through (MWT) silicon solar cells exceeding 16% efficiency

On the way to higher efficiencies, back contact solar cells seem to be a promising alternative to the conventional screen-printed solar cells. Especially, the metal wrap through (MWT) solar cell concept with only two additional process steps is appropriate for a fast transfer to industry. Hence, an industrially feasible process based on a new contact design was developed and tested at the pilot-line of the Photovoltaic Technology Evaluations Center (PV-TEC). A maximum cell efficiency of 16% is achieved. Compared with conventionally processed cells made of the same mc Si-block, an efficiency gain of 0.5% absolute is observed. Due to a cell interconnection on the back the serial resistance losses in the tabs decrease. Therefore, a fill factor of almost 77% and an efficiency of 15% for a MWT module prototype (16 MWT cells) is reached.     

抛物面聚光太阳能PV/T系统的热电性能分析

建立了带有散热翅片的聚光太阳能PV/T热电联产系统内部传热过程的一维稳态数学模型,对传热过程进行了数值模拟,分析了空气质量流速、入射光强度、聚光比、环境温度、上部通道高度及翅片参数对系统的空气温度、电池板温度及系统热、电效率的影响.结果表明:随着入射光强、聚光比的增加,空气出口温度和电池板温度都会增加,系统热电总效率增加;通过增空气流量可以有效降低电池温度,提高电池的光电转换效率和系统的总能量利用效率;吸热板背面的翅片可以强化通道内空气的传热过程,降低电池板的温度,系统效率可增加约2%;在相同的光照条件下,人口空气温度越低,上部通道越窄,系统热效率越高.研究结果为聚光太阳能PV/T热电联产系统的设计和运行提供了理论依据.     

Optimized rapid thermal process for high efficiency silicon solar cells

Rapid thermal processing is opening new possibilities for a low-cost and environmentally safe silicon solar cell production, keeping the process time at high temperature in the order of 1 min, due to enhanced diffusion and oxidation mechanisms. Controlling the surface concentration of the junction is one of the major parameters, in order to obtain suitable front surface recombination velocities. Simultaneous diffusion of phosphorus and aluminum is used to realize emitter and back surface field in a single high-temperature step, with optimized gettering effect. Controlling the mentioned parameters on industrial 1 Ω cm Cz material lead in 17.5% efficient solar cells on a surface of 25 cm². All results are discussed in terms of process temperature, dopant source concentration and effective process time, below 1 min including high heating and cooling rates.     

Perspectives for solar thermal applications in Taiwan

Taiwan has long depended on imported fossil energy. The government is thus actively promoting the use of renewable energy. Since 2000, domestic installations of solar water heaters have increased substantially because of the long-term subsidies provided for such systems. However, data on the annual installation area of solar collectors in recent years indicated that the solar thermal industry in Taiwan has reached a bottleneck. The long-term policy providing subsidies must thus be revised. It is proposed that future thermal applications in Taiwan should focus on building-integrated solar thermal, photovoltaic/thermal, and industrial heating processes. Regarding building-integrated solar thermal systems, the current subsidy model can be continued (according to area of solar collectors); nevertheless, the application of photovoltaic/thermal and industrial heating systems must be determined according to the thermal output of such systems.     

A solar thermal cooling and heating system for a building: Experimental and model based performance analysis and design

A solar thermal cooling and heating system at Carnegie Mellon University was studied through its design, installation, modeling, and evaluation to deal with the question of how solar energy might most effectively be used in supplying energy for the operation of a building. This solar cooling and heating system incorporates 52 m² of linear parabolic trough solar collectors; a 16 kW double effect, water-lithium bromide (LiBr) absorption chiller, and a heat recovery heat exchanger with their circulation pumps and control valves. It generates chilled and heated water, dependent on the season, for space cooling and heating. This system is the smallest high temperature solar cooling system in the world. Till now, only this system of the kind has been successfully operated for more than one year. Performance of the system has been tested and the measured data were used to verify system performance models developed in the TRaNsient SYstem Simulation program (TRNSYS). On the basis of the installed solar system, base case performance models were programmed; and then they were modified and extended to investigate measures for improving system performance. The measures included changes in the area and orientation of the solar collectors, the inclusion of thermal storage in the system, changes in the pipe diameter and length, and various system operational control strategies. It was found that this solar thermal system could potentially supply 39% of cooling and 20% of heating energy for this building space in Pittsburgh, PA, if it included a properly sized storage tank and short, low diameter connecting pipes. Guidelines for the design and operation of an efficient and effective solar cooling and heating system for a given building space have been provided.     

Design and thermal performance evaluation of a novel solar air heater

In the present scenario, numerous applications perform on solar energy for cooking, heating and cooling, and power generation, globally. Solar air heaters are one of these applications purposely used for, drying, timber seasoning and space heating. In the present work, a solar air heater (SAH) has been designed to produce a good exhaust temperature for long hours especially in the case of poor ambient conditions or during off sunshine hours. A mixture of desert and granular carbon in the ratio of 4:6 has been used as thermal heat storage inside the SAH. Two halogen lights of 300 W are used to increase the exhaust temperature of the SAH by placing them in the inlet and outlet ducts. All the experiments have conducted on natural and forced convection for performance evaluation on two similar design solar air heaters (with and without heat storage). The comparisons are made with two similar design solar air heaters carrying desert and granular carbon, as an individual heat storing media, to find out an optimum design of a SAH with long term heating. The thermal efficiencies of the novel SAH range from 18.04% to 20.78% of natural convection and 52.21%–80.05% with forced convection.     

Solar thermal aided power generation

Fossil fuel based power generation is and will still be the back bone of our world economy, albeit such form of power generation significantly contributes to global CO₂ emissions. Solar energy is a clean, environmental friendly energy source for power generation, however solar photovoltaic electricity generation is not practical for large commercial scales due to its cost and high-tech nature. Solar thermal is another way to use solar energy to generate power. Many attempts to establish solar (solo) thermal power stations have been practiced all over the world. Although there are some advantages in solo solar thermal power systems, the efficiencies and costs of these systems are not so attractive. Alternately by modifying, if possible, the existing coal-fired power stations to generate green sustainable power, a much more efficient means of power generation can be reached. This paper presents the concept of solar aided power generation in conventional coal-fired power stations, i.e., integrating solar (thermal) energy into conventional fossil fuelled power generation cycles (termed as solar aided thermal power). The solar aided power generation (SAPG) concept has technically been derived to use the strong points of the two technologies (traditional regenerative Rankine cycle with relatively higher efficiency and solar heating at relatively low temperature range). The SAPG does not only contribute to increase the efficiencies of the conventional power station and reduce its emission of the greenhouse gases, but also provides a better way to use solar heat to generate the power. This paper presents the advantages of the SAPG at conceptual level.     

Light scattering coefficients for textured SnO

Transparent electrodes that are used in amorphous silicon solar cells are textured to provide light scattering. We studied the light scattering behavior in transmission and reflection and found that in order to describe the measured spectra thickness variations of 50–60 nm over several micrometers have to be assumed. This is in qualitative agreement with measured rms roughnesses as determined with atomic force microscopy (AFM). It is important to include these thickness variations in the modeling of amorphous silicon solar cells. The wavelength dependence of the light scattering in transmission at the TCO–air interface was found to be λ⁻³ for light incident from both sides. Scattering of the weakly absorbed long wavelength light at the back contact is therefore essential in order to obtain high solar cell efficiencies.     

Maximum light trapping for the random back-reflective silicon film solar cell with omnidirectional light transmission characteristics

The light trapping characteristics in the wavelength range of 0.5−1.2 μm for the random back-reflective silicon film with omnidirectional top anti-reflection are numerically analyzed based on the simplified probability method. The spectrum averaged maximum external quantum efficiency (EQE) for the 5 μm thick silicon film is evaluated with an increase of 10.6% compared with the best bulk planar silicon solar cell—suggesting that an efficiency higher than those of the best bulk planar cell can be obtained for thin film silicon solar cells several microns thick. The light absorption curves drop slowly with increased back absorption, exhibiting that the performance of the thin film silicon solar cell with light trapping is tolerant of back absorption.     

Water immersion cooling of PV cells in a high concentration system

Temperature control of solar cells at high concentrations is a key issue. Short-term efficiency drop and long-term degradation should be avoided by effective cooling methods. Liquid immersion cooling eliminates the contact thermal resistance of back cooling and should improve cell performance. A 250X dish concentrator with two-axis tracking was utilized to evaluate a new CPV system using de-ionized water for immersion cooling. Time-dependent temperature distributions of the PV module of high power back point-contact cells were measured, as well as the I-V curves. The cooling capacities of the liquid immersion approach are very favorable. The module temperature can be cooled to 45 °C at a 940 W/m² direct normal irradiance, 17 °C ambient temperature and 30 °C water inlet temperature. The temperature distribution of the module is quite uniform, but the electrical performance of the cell module degrades after a fairly long time immersion in the de-ionized water.     

Solar-driven high temperature hydrogen production via integrated spectrally split concentrated photovoltaics (SSCPV) and solar power tower

Hydrogen production can be achieved via combined concentrated photovoltaic (CPV) and concentrated solar power (CSP) in which concentrated radiation is spectrally split and then converted in a photovoltaic receiver and a thermal absorber. This study thus proposes an innovative solar process design integrating both thermal and quantum components of solar energy while providing a complete assessment of its global performance to demonstrate its practical interest. A stand-alone solar-to-hydrogen path was modeled and numerically simulated, which was both electrically and thermally supplied by a solar power generation unit to feed the electrolyzer power utilization unit with enhanced solar-to-hydrogen conversion efficiency. Following balance of plant (BoP), the heliostat field and cavity receiver were designed to match the entire system in which the receiver only intercepts a definite range of infrared wavelength while the rest is converted by separately insulated PV cells. Moreover, dichroic reflectors and optimum cutoff wavelength were applied to fulfill separate optimization and heat load reduction of each solar cell. Finally, the solid oxide electrolysis cell (SOEC) was designed to utilize the generated thermal and electrical power appropriately. In best case scenario, a solar-to-hydrogen conversion efficiency of 36.5% was achieved under 899 W/m² direct normal irradiance (DNI) and 1000 suns concentration. The solar plant outputs at this operating point were 850 g/h H₂ and 6754 g/h O₂. Further improvement in efficiency can be achieved through alignment in regard to the site location and annual insolation variation.     

中国低纬度地区的夏季太阳总辐照度研究

目前GB 50736—2012已给出中国北纬20°～50°地区的夏季太阳总辐照度,尚缺少北纬10°～20°之间低纬度地区的规范值。为解决该问题,对处于低纬度地区的永暑、西沙的太阳辐射数据进行统计整编,并根据中国太阳辐射基础数据的确定原则,在现行规范的基础数据上补充太阳高度角为3°、大气透明度等级为4时的太阳辐射基础数据;然后通过插值法与理论计算生成中国北纬10°和15°地区、大气透明度等级为4时的夏季太阳总辐照度表,为低纬度岛礁地区太阳辐照度研究和太阳能应用提供规范数据。     

Prediction of global solar irradiance based on time series analysis: Application to solar thermal power plants energy production planning

Due to strong increase of solar power generation, the predictions of incoming solar energy are acquiring more importance. Photovoltaic and solar thermal are the main sources of electricity generation from solar energy. In the case of solar thermal energy plants with storage energy system, its management and operation need reliable predictions of solar irradiance with the same temporal resolution as the temporal capacity of the back-up system. These plants can work like a conventional power plant and compete in the energy stock market avoiding intermittence in electricity production.This work presents a comparisons of statistical models based on time series applied to predict half daily values of global solar irradiance with a temporal horizon of 3 days. Half daily values consist of accumulated hourly global solar irradiance from solar raise to solar noon and from noon until dawn for each day. The dataset of ground solar radiation used belongs to stations of Spanish National Weather Service (AEMet). The models tested are autoregressive, neural networks and fuzzy logic models. Due to the fact that half daily solar irradiance time series is non-stationary, it has been necessary to transform it to two new stationary variables (clearness index and lost component) which are used as input of the predictive models. Improvement in terms of RMSD of the models essayed is compared against the model based on persistence. The validation process shows that all models essayed improve persistence. The best approach to forecast half daily values of solar irradiance is neural network models with lost component as input, except Lerida station where models based on clearness index have less uncertainty because this magnitude has a linear behaviour and it is easier to simulate by models.     

临近空间太阳电池模型修正与发电量预测研究

由于在飞行过程中,温度、辐照度和倾角变化都会对临近空间飞行器上太阳电池的输出功率及效率产生影响,该文利用太阳光模拟器及薄型晶体硅太阳电池,进行多组测量实验,得到在不同温度、辐照度和倾角条件下,太阳电池的短路电流、开路电压等参数,并通过与模型仿真结果进行对比,对已有太阳电池电模型进行修正,得到更接近真实飞行工况的临近空间飞行器用薄型晶体硅太阳电池的模型。最后,基于修正后的模型通过仿真对太阳电池阵列在临近空间的全天发电功率变化趋势进行预测,可为临近空间飞行器用太阳电池阵列设计与功率预测提供重要参考。