Thin film angular selective glazing

Angular selective glazing based on special obliquely deposited thin films, has considerable potential for optimizing the overall performance of a window in temperate to hot climates. In contrast to most currently used glazings for these environments it has the ability to control solar heat gain and glare while providing a clear view through the window and considerable but not excessive light gain. Various deposition techniques and film materials will be discussed and the spectral and integrated optical performance of single and double-layered systems presented for various directions of incidence. Full optical characterisation requires measurements for incidence directions over half a hemisphere. Some of this is presented but most data is confined to planes of incidence where angular selectivity is greatest as determined by deposition geometry. Two layer systems are designed to provide a low emittance and better solar control than the single layer systems while retaining good visible angular selectivity. The single layers are primarily to provide angular selective control of the visual radiation since they have moderately high transmittance in the near infrared and a high emittance. Theoretical models used to extract structural information and model the complex optical properties are introduced and their predictions assessed. Preliminary results on simulation of the performance of full size angular selective windows in comparison to other types of solar control glazing such as low e heavily tinted windows are discussed in terms of their relative impact on cooling load, lighting contribution, glare control and total energy savings.     

A study on the performance of a two-absorber reverse flat-plate collector

The reverse flat-plate collector is a non-concentrating collector. It can collect solar heat at high temperatures which cannot be achieved by conventional non-concentrating collectors. In this paper, the authors have proposed a number of modified versions of the originally proposed reverse flat-plate collector. The new designs are of single, as well as double, absorber type. The thermal performance of these modified reverse flat-plate collectors is compared with that of a single absorber reverse flat-plate collector, as well as with the corresponding normal flat-plate collector. It is found that the new design having two absorbers gives the best thermal performance as compared with other configurations. The analytical models presented in this paper very well describe the experimental results.     

Cr-N-O型选择性吸收涂层制备及热稳定性研究

采用反应磁控溅射方法制备SiO_x/Cr-N-O/Al选择性吸收涂层,该涂层太阳吸收比为95.9%、发射比3.8%、吸收发射比25.2。结合光学显微镜微观形貌分析、X射线衍射结构分析(XRD)、X射线光电子能谱成分分析(XPS)探讨涂层在250和400℃大气环境下热稳定性机理:250℃大气热处理后,涂层保持较高光谱选择性,表面形貌与物相结构未出现明显变化,SiO_x层氧化程度增大导致太阳吸收比升高,金属Cr和金属Al相互扩散导致发射比升高;400℃大气热处理后,涂层光谱选择性降低,表面出现微米级孔洞,XRD及XPS结果表明Cr-N-O吸收层被氧化,导致太阳吸收比降低,金属Cr和金属Al相互扩散导致发射比明显升高。     

复合抛物面聚光太阳能PV/T系统的实验研究

设计并搭建了CPC低倍聚光太阳能PV/T单通道空气系统实验台,对不同工作环境下聚光PV/T系统的热电性能进行了实验研究。实验研究结果显示:在聚光条件下,系统的各表面温度随光照强度的增加而升高,随下部通道入口空气流速的增加而降低。聚光PV/T系统的最大输出功率可达到60W,比对应相同电池面积平板系统最大输出功率高20W。聚光PV/T系统的各效率随光照强度增加而增大,系统的最大电效率为11%,最大热效率为70%,最大火用效率为16%,比单纯发电时最大火用效率提高约5%。实验获得了一批新的有价值的实验数据,为聚光太阳能光伏光热系统的进一步研究提供了依据。     

The real utility ranges of the solar selective coatings

The efficiency of the solar selective coatings (η_sel) with various combinations of the optical properties (solar absorptance and thermal emittance), and their impact on the performance of solar thermal systems of different concentration ratios (CRs) have been analyzed. The stagnation temperatures of the selective coatings have been measured using stagnation temperature measurement (STM) chamber. From the results of the simulation study, it is recommended that selective solar absorber coatings should be used only in systems with CR=1 (solar flat plate collectors) and its use in systems with high CR values (parabolic collector) is an additional expenditure with reduced efficiency. The dependence of the performance of the system with CR=1 on the low emittance of the absorber coating has also been experimentally confirmed by the stagnation temperatures recorded for different selective coatings measured in an STM chamber, a system with unit CR. For systems with CR=1, a simple and new parameter, specific area ratio (SAR) has been proposed which further pinpoints the exact solar system wherein the selective coatings are to be used.     

Power losses in an asymmetric compound parabolic photovoltaic concentrator

An asymmetric compound parabolic photovoltaic concentrator (ACPPVC) of geometrical concentration ratio 2 was designed, developed and evaluated for building façade integration. Despite the two times theoretical concentration, the maximum output power achieved was only 1.62 times that of a similar non-concentrating system for a wide range of solar radiation intensities due to a combination of optical and electrical resistance losses. In this paper, the power loss of the ACPPVC system is explained by a comparative power loss analysis of the non-concentrating photovoltaic system for long- and short-tabbed solar cell strings that showed an average of 3.4% electrical power loss due to resistance in the interconnections between each individual solar cell. The optical losses of the ACPPVC were 15% caused by the combined effect of the number of reflections at the reflectors and the misalignment of the imperfection in the reflector geometry solar cells in addition to the power loss due to increased temperature of 0.6%. Good agreement was found between the measured and calculated optical efficiency of the system.     

Non-concentrating and asymmetric compound parabolic concentrating building façade integrated photovoltaics: An experimental comparison

Concentration of solar energy increases the illuminated flux on the photovoltaic (PV) surface thus less PV material is required. A novel asymmetric compound parabolic photovoltaic concentrator has been characterised experimentally with a similar non-concentrating system. Different numbers of PV strings connected within the system have been analysed and a power ratio of 1.62 measured compared to a similar non-concentrating PV panel with the same cell area. The solar to electrical conversion efficiency of 8.6% and 6.8% was achieved for the non-concentrating panel the concentrating system, respectively. The measured average solar cell temperature of the PV in the concentrator system was only 12 °C higher than that of the similar non-concentrating system with same cell area.     

Integrated collector storage solar systems with asymmetric CPC reflectors

New types of ICS solar systems were designed and outdoor tests of experimental models were performed. The systems consist of single cylindrical horizontal water storage tanks placed inside stationary truncated asymmetric CPC reflector troughs of different design. We used high emittance absorber surface, low cost curved reflectors, iron oxide glazing and thermal insulation at the non illuminated tank surfaces, aiming towards cost effective ICS systems with satisfactory heat preservation during the night. Four experimental models of different designs were constructed and tested to determine their performance regarding their mean daily efficiency and thermal losses during the night. The new ICS systems were compared to an ICS system with symmetric CPC reflectors of similar construction and dimensions and also to a typical Flat Plate Thermosiphonic Unit (FPTU). Test results showed that the ICS systems with asymmetric CPC reflectors present almost the same mean daily efficiency and better preservation of hot water temperature during the night, compared to the ICS system with the symmetric CPC reflectors. The comparison with the FPTU system confirmed the satisfied daily operation of all ICS systems and their moderate storage heat preservation during the night. Theoretical results showed acceptable thermal performance of all ICS systems regarding annual operation.     

Computations of the optical properties of metal/insulator-composites for solar selective absorbers

Solar selective absorbers are very useful for photo thermal energy conversion. The absorbers normally consist of thin films (mostly composite), sandwiched between the antireflection layer and (base layer on) a metallic substrate, selectively absorbing in the solar spectrum and reflecting in the thermal spectrum. The optical performance of the absorbers depends on the thin film design, thickness, surface roughness and optical constants of the constituents. The reflectivity of the underlying metal and porosity of the antireflection coating plays important roles in the selectivity behavior of the coatings. Computer simulations, applying effective medium theories, have been used to investigate the simplest possible design for composite solar selective coatings. A very high solar absorption is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the air interface. The destructive interference created in the visible spectrum has increased the solar absorption to 98%. This paper also addresses the optical performance of several metals/dielectric composites like Sm, Ru, Tm, Ti, Re, W, V, Tb, Er in alumina or quartz on the basis of their refractive indices. The antireflection coating porosity and surface roughness has been analyzed to achieve maximum solar absorption without increasing the thermal emittance. Antireflection layer porosity is a function of dielectric refractive index and has nominal effect on the performance of the coating. While, up to the roughness of 1×10⁻⁷ m RMS, the solar absorption increases and for higher roughness, the thermal emittance increases only.     

High temperature; stable,spectrally selective solar absorbers for thermochemical hydrogen production

The thermochemical reduction of water by reactions similar to the Mark I process requires a processing temperature of 730°C. The efficient utilization of solar photothermal energy conversion, in distributed collector systems, to attain this temperature will require the use of suitable spectrally selective surfaces which are stable at the operating temperature.A coating system with demonstrated high temperature capability has been developed at the Optical Sciences Center under NSF/RANN sponsorship. A silicon thin film absorber is deposited by chemical vapor deposition (CVD) on a silver thin film reflector. This optical stack is fabricated at temperatures in excess of 800°C, and the CVD technology is amenable to large scale production in a flow-through system.At 500°C the present Si-Ag system has typical solar absorptance and total normal emittance values of 0.75 and 0.06, respectively. Samples were fabricated which maintained their high spectral selectivity after 2000 thermal cycles between 150°C and 450°C, and after 100 h at 600°C. Further process studies now underway indicate that the solar absorptance can be improved to better than 0.85 by employing a Si-Ge multilayer absorber and that the operating range of the stacks can potentially be raised to the 800–900°C range by employing a refractory metal thin film reflector.     

Solar still versus solar evaporator: A comparative study between their thermal behaviors

This paper presents a theoretical comparison between the thermal behavior of a basin type solar still and that from a solar evaporator. The analyses are twofold: (a) comparing the system temperatures and the heat and mass transfer rates in the transient mode; (b) obtaining parametric representations from both systems using their heat and mass transfer equations. Such comparisons had never been done before. It is shown, among other things, that the evaporation in solar stills is much less than that in open evaporation despite the higher water temperatures in the former system. This is also true even when the water temperature of both systems is the same. It is also observed that the distillation and evaporation rates increase with the increasing water temperature and temperature difference. For relatively high water temperatures of each system the evaporative fraction is equivalent to more than 50% of the corresponding total heat transfer rate.     

Short communication partly selective black paint coatings

Partial selectivity has been obtained in commercially available black paint coatings by spray and dip techniques. It has been shown that the dip technique is superior to the conventional spray technique in giving low thickness, continuous and abrasion resistant coatings, having lower thermal emittance (ε) with comparable solar absorptance (α). Coatings without a primer underlayer have been found to be quite adherent. an ε value of 0.39 corresponding to an acceptable à value of 0.87 has been achieved in this technique by a suitable combination of thinner concentration and pulling rates.     

Analysis of the thermal properties of air-conditioning-type building materials

This work develops air-conditioning-type building materials suitable for different climate regions. Results show that using traditional building materials alone is unsuitable for any climate region, and that building materials of various thermal properties are needed to satisfy the requirements specific to each location. Low solar absorptance and high emittance materials are appropriate for hot climates, whereas high solar absorptance and low emittance materials are suitable for cold climates. Materials with variable solar absorptance and emittance properties are applicable to regions characterized by cold winters and hot summers. In addition, cement that is reversibly thermochromic at normal temperature and materials with variable transmittance properties can warm buildings in winter and prevent overheating in summer, thereby satisfying the need to create a thermally comfortable building environment.     

Measurement of Total Hemispherical Emittance of Spacecraft Thermal Control Coatings at Low Temperatures

Spacecraft payloads that operate at cryogenic temperatures often use passive thermal control systems that incorporate a variety of thermal control coatings to reject heat to space. The thermal performance of these systems depends significantly on the emittance of their thermal control coatings. This paper presents total hemispherical emittance measurements carried out at the ISRO Satellite Centre (ISAC) for thermal control coatings: Aeroglaze Z307 absorptive conductive polyurethane black coating (from 8 0K to 150 K) and PUC conductive black polyurethane coating (from 70 K to 200 K). The experiments were conducted in vacuum using a steady-state calorimetric method. This paper highlights the importance of obtaining a steady state for accurately estimating the emittance at cryogenic temperatures and the criterion for achieving this. The results indicate that the emittance in the cryogenic temperature range increases with temperature for the coatings considered in this paper. Determination of the uncertainty in the experimental results is also presented. Further, the sensitivity of emittance estimates to chamber wall emittance is discussed.     

Smart solar tanks for small solar domestic hot water systems

Investigation of small SDHW systems based on smart solar tanks are presented. The domestic water in a smart solar tank can be heated both by solar collectors and by means of an auxiliary energy supply system. The auxiliary energy supply system––in this study electric heating elements––heats up the hot-water tank from the top and the water volume heated by the auxiliary energy supply system is fitted to the hot-water consumption and consumption pattern. In periods with a large hot-water demand, the volume is large; in periods with a small hot-water demand, the volume is small.Two small SDHW systems, based on differently designed smart solar tanks and a traditional SDHW system were investigated by means of laboratory experiments and theoretical calculations. The investigations showed that the yearly thermal performance of SDHW systems with smart solar tanks is 5–35% higher than the thermal performance of traditional SDHW systems. Estimates indicate that the performance/cost ratio can be improved by up to 25% by using a smart solar tank instead of a traditional tank when the backup energy system is electric heating elements. Further, smart solar tanks are suitable for unknown, variable, large or small hot-water consumption and the risk of oversized solar heating systems and oversized tank volumes is reduced by using smart solar tanks. Based on the investigations it is recommended to start development of smart solar tank units with an oil-fired boiler or a natural gas burner as auxiliary energy supply system.     

Optical properties and thermal stability of TiAlN/AlON tandem absorber prepared by reactive DC/RF magnetron sputtering

Spectrally selective TiAlN/AlON tandem absorbers were deposited on copper and stainless steel substrates using a reactive DC/RF magnetron sputtering system. The compositions and thicknesses of the individual component layers were optimized to achieve high absorptance (α=0.931-0.942) and low emittance (ε=0.05-0.06) on copper substrate. The experimental spectroscopic ellipsometric data have been fitted with the theoretical models to derive the dispersion of the optical constants (n and k). In order to study the thermal stability of the tandem absorbers, they were subjected to heat treatment (in air and vacuum) for different durations and temperatures. The tandem absorber deposited on Cu substrates exhibited high solar selectivity (α/ε) of 0.946/0.07 even after heat treatment in air up to 600 °C for 2 h. At 625 °C, the solar selectivity decreased significantly on Cu substrates (e.g., α/ε=0.924/0.30). The tandem absorber on Cu substrates was also stable in air up to 100 h at 400 °C with a solar selectivity of 0.919/0.06. Studies on the accelerated aging tests indicated that the activation energy for the degradation of the tandem absorber is of the order of 100 kJ/mol.     

State of the art on high-temperature thermal energy storage for power generation. Part 2—Case studies

Power generation systems are attracting a lot of interest from researchers and companies. Storage is becoming a component with high importance to ensure system reliability and economic profitability. A few experiences of storage components have taken place until the moment in solar power plants, most of them as research initiatives. In this paper, real experiences with active storage systems and passive storage systems are compiled, giving detailed information of advantages and disadvantages of each one. Also, a summary of different technologies and materials used in solar power plants with thermal storage systems existing in the world is presented.     

Parametric study of a reverse flat plate absorber cabinet dryer: A new concept

In this communication, the concept of reverse flat plate collector has been used as a heating medium of air for the drying of agricultural products in a cabinet dryer. The reverse flat plate absorber is a non-concentrating collector which can collect solar heat at high temperature unlike conventional nonconcentrating collectors. The thermal performance of the proposed dryer is analyzed by solving the various energy balance equations. An attempt has been made to optimize the vent area of the dryer for speedy flow of humid air from the drying chamber to the atmosphere. In order to have parametric studies, numerical computations have been carried out for a typical day in June for Delhi climatic conditions. The performance of this system is compared with that of conventional cabinet dryers. It is found that the reverse flat plate absorber dryer gives the better performance.     

A new look at the long-term performance of general solar thermal systems

This work addresses the problem of evaluating the long-term performance of solar thermal systems, which is quantified through the monthly or seasonal/annual solar fraction. It is shown that for a general solar system it may be expressed as a function of monthly utilizabilities, calculated for two different temperature (radiation) levels, which correspond to minimum and maximum operating temperatures. Both systems without storage and with storage are considered. Examples for solar cooling and solar cogeneration systems are shown.     

The effective size of the solar cone for solar concentrating systems

In this paper we define a virtual solar cone, whose principle axis is aligned with the solar vector, having a radial angular displacement containing a pre-defined proportion of the terrestrial solar radiation. By simulating various sunshape profiles, the angular extent of the energy distribution is established to give the effective size of the solar cone for a range of atmospheric conditions. Then, by simulating the reflection of these solar distributions off a set of non-ideal mirrored surfaces, accounting for non-specular reflection and mirror shape errors, the combined effect of sunshape and mirror properties on the solar image is obtained. Clear trends are presented that show the dependence of the effective size of the solar image on the accuracy of a mirrored surface for different sunshapes. We then identify the effective size of the solar image at the absorber plane that must be accommodated in the design and optimisation of solar concentrating systems.