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The photoelectrochromic (PEC) device, a combination of a dye solar cell and an electrochromic film, can be used for the dynamic solar control of buildings under illumination or with an external voltage. Typically, titanium oxide (TiO 2 ) and tungsten oxide (WO 3 ) films are being prepared from colloids or sol-gel chemistry with high porosity and high surface area as functional layers for a photoelectrochromic device in lab scale. We have, for the first time, successfully developed a PEC device with conventionally sputter deposited TiO 2 and WO 3 films. This coating technique is attractive due to the well documented upscaling capability and industrial viability for window applications. The functional layers WO 3 and TiO 2 were deposited onto fluorine doped oxide (F:SnO 2 ) coated glass . Characterization of WO 3 and TiO 2 was performed in an electrochromic cell and in a dye solar cell, respectively. Three types of PEC devices with different layer configuration were constructed and their performance based on visual transmissions was compared. In best case, the visual transmission could be switched from 61 down to 15% in 30 min under 1.5 A.M. illumination.

Accurate and complete performance evaluation is playing a major role in the further development of concentrating solar collectors. To ensure dependable test results, an appropriate testing and evaluation procedure is required. Moreover, the selection and installation of suitable measurement instrumentation are essential for obtaining reliable data for the performance evaluation. The quality of the measurement instrumentation greatly influences the representativeness of the test results. Details on the measurement instrumentation recommended for the testing of low-temperature solar collectors have already been provided in the testing standard EN ISO 9806:2013. Due to the larger dimensions of concentrating collectors and thus different working temperatures and mass flow rates, these recommendations cannot be directly applied for the testing of concentrating solar collectors. A good selection of measurement instrumentation will always be a trade-off between feasibility, cost of the instrumentation and its associated uncertainties. For this reason, it is crucial for every testing and certification institution to assess the quality of measurement data during the instrumentation selection process. Until now, this aspect has been sparsely addressed in the relevant literature concerning collector testing procedures. However, uncertainty examinations have become particularly relevant for in situ testing, in which the choice of measurement instrumentation has to be adapted to the specific measurement situation on-site. In situ testing is considered to be particularly beneficial (if not even indispensable) for concentrating collectors in terms of cost effectiveness and feasibility. With the objective of simplifying the selection of measurement instrumentation, we present an elaborate methodology and comprehensive case study concerning the uncertainty calculation of line-concentrating solar collectors. The assessment of the suitability of measurement instrumentation is conducted based on two operational reference cases. These cases adequately cover the complete range of collector types and operating conditions typically involved in the field of line-concentrating solar collectors. The analysis is designed such that the results are also transferable to other testing situations, which are not specifically studied within this publication. The presented systematic uncertainty case study thus serves as a guideline for the selection of appropriate measurement instrumentation, providing useful indications for every testing and certification entity dedicated to the planning and execution of significant and reliable collector performance testing. The associated risk decrease of performance testing is essential for the further development and economic aspects of concentrating solar technology.

Iodide/triiodide ( I − / I 3 − ) redox electrolyte in hybrid electrochromic (EC) devices provides high charge capacity and thereby a high optical modulation can be achieved. However, its corrosive nature and absorption in UV and in blue range of light forming an unpleasant yellowish coloration limits smart window application. In this regard, tetramethylthiourea/tetramethylformaminium disulphide (TMTU/TMFDS2+), 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO)/TEMPO+ and potassium ferrocyanide/potassium ferricyanide (K4[Fe(CN)6]/K3[Fe(CN)6]) are studied as alternative redox couples. Their characterisation is performed considering electrochemical and optical properties. The redox potential is evaluated by cyclic voltammetry. The charge transfer resistance (Rct) is determined by electrochemical impedance spectroscopy on sputtered and sol–gel tungsten trioxide (WO3), sol–gel titanium oxide (TiO2), fluorine doped tin oxide (F:SnO2) as well as on sputtered platinum (Pt). TMTU/TMFDS2+ has demonstrated an appropriate redox potential as well as a high charge transfer resistance on oxides and low on Pt layers for high bias potentials. Furthermore, the kinetics of the change in transmission of the EC devices is compared considering the loss current. The EC device comprising TMTU/TMFDS2+ exhibits excellent optical modulation in visual transmission from 71 down to 14% at −795 mV within 5 min.

Optical assessment of Central Receiver Solar Tower systems should be based on annual simulations due to the high variability of solar radiation throughout the year. The most basic approach is using a discretization of the temporal domain which requires a large number of computationally costly optical simulations. This study proposes an alternative approach based on a discretization of the sky which reduces the required number of simulations and therefore the computational effort significantly. For the subsequent annual performance assessment data three different interpolation methods are presented, compared and their respective advantages and drawbacks are discussed. The methodology is demonstrated by means of three representative heliostat field setups: the PS10 field, the Gemasolar field and a small, generic field. In this context, the dependency of the interpolation accuracy with respect to the resolution of the sky grid is investigated. For this purpose, annual optical efficiency weighted with direct normal irradiation is used as an integral figure of merit, while the Root-mean-square deviation is calculated as a benchmark value for single time step assessment. Eventually, it is shown that the proposed method decreases the required time for the annual optical assessment by several orders of magnitude while maintaining the original accuracy.

For the further development and dissemination of solar thermal technology, a continuous demonstration of its reliability is required. For this purpose, meaningful performance and acceptance testing is indispensable. The reliable determination of optical and thermal collector performance parameters involves a suitable testing and evaluation procedure. It additionally requires a dependable quality assessment of the test results. Sophisticated statistical inference calculations, however, are not commonly available in solar thermal collector testing. If applied at all, mostly standardly implemented (linear) confidence interval computations are used. The present publication proposes an advanced approach of confidence level computation, the so-called bootstrapping technique. It represents a common method in the area of economics and is suited to cope with the complexity of confidence calculations within the context of dynamic performance testing. The basic methodology and specific implementation of the bootstrapping approach are introduced in detail. Since this approach is new in performance evaluation procedures, it is validated with confidence results obtained from an extensive evaluation of a large measurement data basis of a linear Fresnel process heat collector. However, the procedure is equally suited for other collector types as parabolic trough, flat plate, and others. The validation with measurement data reveals the valuable capabilities of the bootstrap procedure. It moreover proves the standard confidence methods to fail, because these provide unrealistically narrow confidence intervals. Comparative results between the different methods are thoroughly discussed. They demonstrate the introduced bootstrapping approach to be a powerful tool, generating considerably more representative and therefore reliable confidence intervals than the customary methods. Consequently, bootstrapping is considered a key feature of an enhanced performance evaluation method, since it may provide improved information concerning parameter distribution, confidence levels, and hence the validity of corresponding test results. Meaningful performance testing represents an essential aspect to further increase the viability and reliability of the solar thermal technology in order to facilitate its easier commissioning and wide acceptance.

In the process of improving the performance of concentrating solar collectors a variety of reflector materials are undergoing development. To allow comparable performance predictions, precise and replicable measuring methods are needed. In this paper we propose a measurement method and set-up (VLABS: very-low-angle beam spread) to determine the specular reflectance for variable angular acceptance. Our measurement set-up and method VLABS is advanced in measurement accuracy and speed, it gives as a result the specular reflectance for variable acceptance angles with an angular resolution of 0.03 mrad. We show exemplary results for a glass and an aluminum reflector and discuss the impact of acceptance angles up to 33 mrad on measured reflectance values at different wavelengths.

In the German national research and development project “HeizSolar” the building concept of Solar-Active-Housesis analyzed. The concept allows for a high fraction of solar thermal energyof more than 50% of the total heat supply. Within the project nine existing buildings have been monitored since the beginning of the heating period of 2011. The technology applied for this kind of supply concept has been introduced in [1,2]. Based on the data obtained from the in-situ monitoring, typical operation conditions have been identified and analyzed in [3]. The measured datawas also used to derive parameters to calibratea dynamic system simulation model for Solar-Active-Houses[4]. Theelaborated model focuses on the performance of the solar thermal collector as well as the solar circuit. What is more, the dynamic interaction between the solar thermal heat generator and the auxiliary heating systems as well as the heat distribution system are incorporated in detail. Finally the diffuse heat flux resulting of the integration of large hot water storage tanks into the building envelope is implemented to the model. This model is applied in the following to analyze the general thermal behavior of the supply concept and to derive optimization potentials. In detail the systems performance in regard to different types of heat transferal systems for space heating is demonstrated. Also the dimensioning in regard to the key parameters of collector area and the thermal capacity of the storage system consisting of a hot water storage tankare stated.

The operation of pre-industrial membrane distillation (MD) units is of great importance in order to implement this new technology. The systematic development from module to plant design for MD is conducted in an industrial research-cooperation. The consequent up-scaling from a lab plant at Hamburg University of Technology (TUHH) for MD module evaluation to a pre-industrial plant is presented. Permeate gap MD (PGMD) and Direct Contact MD (DCMD) spiral wound modules with different channel lengths have been used in this study to evaluate the most suitable module configuration and design. It has been decided to go along with DCMD modules with a relatively short channel length. A pilot plant for on land testing has been built and operated for 9 months before constructing the pre-industrial unit. In the pre-industrial plant waste heat was used and a heat recovery and recycle concept was implemented and tested successfully. It has been proven that long-term operation with real seawater is possible without significant performance decline. Besides operational data also MD plant design specifics will be discussed. The importance of ambient pressure conditions for the MD modules was determined. In addition, the possibility of piping system deaeration is discussed.

The application of a redox electrolyte in hybrid type electrochromic devices allows a high charge capacity, thereby a high optical contrast can be achieved. However, the current between the redox electrolyte and the electrochromic layer (tungsten oxide, WO3) interface in equilibrium at colored state of device is critical for a window with large area. This loss current should be maintained below 10 μA/cm2 to avoid an inhomogeneous coloration of a large window caused by an ohmic voltage drop in transparent conductive oxide (TCO). The power consumption of such a loss current can be neglected. In this regard, we have investigated silicon nitride (Si3N4) and silicon oxide (SiO2) films as an intermediate electronic barrier layer on the top of the WO3 with thicknesses of 12, 35, 80 and 180 nm. These films were coated by direct current reactive magnetron sputtering technique. The electron barrier properties were studied for the iodide/triiodide ( I − / I 3 − ) and tetramethylthiourea/tetramethylformaminium disulfide dication (TMTU/TMFDS2+) redox couples for application in electrochromic devices. For both redox couples, Si3N4 showed an effective electronic barrier layer. The thickness of 80 nm of Si3N4 reduced the loss current from 240 down to 20 μA/cm2 for the I − / I 3 − redox electrolyte with the visual (solar) transmission from 45% (33%)down to 0.7% (0.5%)at 1 V.Similarly, the loss current was reduced from 70 down to 7 μA/cm2with the visual (solar) transmission from 71% (54.5%) down to 3% (2%) for TMTU/TMFDS2+ redox electrolyte. However no significant reduction of loss circuit current was achieved with SiO2 barrier layer for both redox couples.

A new multi-channel spiral-wound membrane distillation (MD) module with a membrane surface area of 27.5 m 2 was developed, constructed and characterized. For comparison, a set of different conventional single-channel direct contact membrane distillation (DCMD) module prototypes is introduced and experimentally evaluated, as well. Furthermore, a comprehensive assessment on DCMD module design and operation as well as methods for comparative thermodynamic MD module evaluation has been conducted. The methodical core of this work specifically addresses an integrative method for the derivation of the thermal energy demand of DCMD systems, comprising specifications of the external heat recovery system. A new strategy for the optimization of energy demand in DCMD is identified, which aims on the formation of parallel temperature profiles by adjusting the flow streams on the evaporator and the condenser channel. Suitable methods for a comparative analysis of different MD performance results are suggested and fundamental MD specifications are derived and applied to evaluate a set of different DCMD prototypes. The methods involve the analysis of the permeate output rate, flux and thermal energy consumption with respect to feed flow rate conditions. The optimization of flux and thermal energy consumption is identified to rely on conflicting requirements, which cannot be satisfied at the same time. The optimal combination of flux and thermal energy consumption depends on the projected scenario and may not be identified by a technical assessment alone. Economic considerations must be included. As final outcome, a method of specifying MD results by a representation of flux versus thermal energy consumption is suggested in order to allow a meaningful evaluation and to achieve global comparability among different MD configurations and other thermal separation technologies.

In the case of membrane distillation (MD), the membrane is the core component, but in a full-scale module a spacer is also required in the flow channel. It is increasing mixing and thus the heat and mass transport to the membrane. In this work commercially available membranes and spacers for full-scale module design are evaluated. Special attention is paid to the pressure drop and the heat transfer properties of different spacers. It is shown that MD modules need to be operated in a different flow regime than other membrane processes due to pressure drop limitations. Especially spacers with a high mean mesh size as well as spacers with a high mean mesh size to thickness ratio show good results. Pressure drop and heat transfer characteristics are combined in order to assess the most suitable spacer. For the evaluation of the membranes, Liquid Entry Pressure (LEP) and flux are measured. It is confirmed that membranes with a low pore size show high LEP values. Furthermore, it can be shown that membranes with a high pore size dM to thickness dM ratio showed the highest flux.

The paper presents investigations on the process performance of different membranes with and without backing structure in direct contact membrane distillation. Influences of backing structures and their orientation are identified. An integrated membrane and backing model was developed. Laminates with different backing designs were examined over a wide range of operating conditions and compared to results for the same membranes without backing. Experimental results and model predictions for flux and thermal efficiency are compared. The model predictions agree well with the experimental results. An influence on the effective area for diffusion, an increase in effective diffusion path length in the membrane and the formation of a complex network of thermal resistances are considered to be the main effects, leading to a significant reduction in process performance due to backing structures. The backing was identified to be one of the key components for further improvement in MD applications. An integrated assessment of membrane properties in combination with backing structures was carried out by analysing membranes with different thicknesses and different pore sizes. The experimental observations show good agreement with theoretical considerations. A combined analysis of specific flux and thermal efficiency leads to comprehensive understanding of the process and its potential for optimisation.

Membrane distillation (MD) is considered a promising technology for desalination applications. In this paper, the main transport phenomena in membrane distillation are analyzed theoretically. One of the main limiting factors is the molecular diffusion resistance caused by stagnant air in the membrane pore volume. A great potential for flux enhancement has been identified to be the removal of air in the membrane pores. Preliminary experiments on water deaeration using a commercial membrane contactor were carried out. The dynamics of membrane and air gap deaeration using deaerated feed water were studied experimentally. A module test facility was equipped with a feed water deaeration setup. Full-scale spiral-wound modules with a membrane area of 5 and 10 m 2 have been used for comprehensive experimental studies on the use of deaerated feed water. The impact of deaeration at different pressures, feed flow rates, feed water salinities and temperature levels is quantified and discussed. A comparison of module operation with and without preceding feed water deaeration is given. The experimental results include module output rate, thermal energy demand, thermal efficiency and an estimation of the electrical energy demand. In all experiments, the beneficial effect of feed water deaeration can clearly be shown for total flux and thermal energy demand simultaneously. For selected operation points, the relative effect was identified to be more than 50 % and the specific thermal energy demand dropped to values of less than 100 kWh th m − 3 .

The electrical potential between the cells and ground can cause leakage currents that are a necessary but not sufficient indicator for potential induced degradation that is because of the migration of sodium ions into the silicon cells. Outdoor exposure of modules with high-voltage bias in two different climate regions with monitoring of the climatic data, the module temperatures as well as the leakage currents was used to investigate the effect of rain-induced and soiling-induced increase of the surface conductivity, the leakage current and the decrease of the module performance.

Photovoltaic–thermal (PVT) collectors are hybrid collectors which make, in principle, optimal use of the solar resource by co-generating electricity and heat in a single module. Spectrally selective but transparent low-emissivity (low-e) coatings are a suitable measure to reduce thermal losses. In this paper the development of PVT collectors with low-e coatings by means of experiment and simulation is presented. A highly transparent low-e coating based on silver, specifically optimized for the application in PVT collectors, is developed. By combining layers of low-refractive SiO2 and high-refractive TiO2 a low emissivity of ɛ373K = 0.13 is achieved while maintaining high solar transmittance of τAM1.5 = 0.79. This coating is applied in a glazed, flat plate, liquid-type PVT collector. Thermal and electrical performance were tested and compared to a PVT collector with the same design yet without low-e coating. A numerical model of the PVT collector is presented showing good agreement of simulation and experiment. With this model the impact of low-e coatings on thermal and electrical performance is analysed and discussed. The newly developed low-e coating reduces the collector heat losses by 82% while the electrical efficiency drops by only 3%.

Hybrid photovoltaic-thermal (PVT) collectors co-generate solar electricity and heat in a single component with an optimum utilization of space. Spectrally selective but transparent low-emissivity (low-e) coatings are a proven method to reduce thermal losses. However, overheating and stagnation are critical issues for these collectors due to material degradation, thermal stress, and low electrical efficiency. This paper presents a PVT collector concept with variable film insulation as overheating protection. An inflatable glass-film cushion regulates thermal losses. Performance and stagnation tests were carried out with a prototype. During normal operation the collector achieves a high thermal efficiency. In periods of standstill, the deflated cushion has a high heat dissipation rate by deactivating the low-e coating. Stagnation temperatures are thus limited to 95 C. To conclude, the PVT collector combines the advantages of glazed and unglazed PVT collectors which are a high thermal efficiency and low stagnation temperatures.

The authors demonstrate the feasibility of quantifying cell-level performance heterogeneity from module-level I-V curves by determining conditions of bypass diode turn-on. Analysis of these curves falls outside of typical diode-based models of photovoltaic (PV) performance. The authors show that this approach can leverage statistical and machine learning techniques for broad application to massive datasets, and combine those insights with simulations and laboratory-based experiments to provide useful information into the metastability of the interfaces of a PV cell. The authors find good agreement between the experimentally determined curves and the simulated curves, which guide the variable selection in the massive dataset collected from sites in Cleveland, OH, USA, the Negev Desert, Israel, Isla Gran Canaria, Spain, and Mount Zugspitze, Germany.

The Linear Fresnel Collector (LFC) technology is currently being commercialised by several companies for the application in solar thermal power plants. This study compares the electricity generation costs for LFC and Parabolic Trough Collector (PTC). PTC is the most commercial CSP technology to date and is therefore regarded as the benchmark. For reasons of comparability, direct steam generation is assumed for both LFC and PTC. For the LFC, cost data comparable to typical CSP plant sizes are hardly available. Therefore, the break even cost – referring to aperture-specific collector investment – is determined, where cost-parity of the electricity generation with a PTC reference plant is reached. This study varies the assumptions on collector performance and operation and maintenance costs to reflect different designs of LFC technologies. The calculations were carried out using cost and hourly simulation performance models. Depending on the assumptions, the costs for a linear Fresnel collector solar field should range between 78 and 216 €/m 2 to reach cost-parity at assumed reference solar field costs of 275 €/m 2 for the PTC. The LFC principle of arranging the mirrors horizontally leads to lower aperture-related optical efficiency which must be compensated by lower cost per m 2 of aperture compared to PTC. The LFC is a collector with significant cost reduction potential, mainly due to cheaper mirrors and structural advantages.The presented cost and performance targets shown in this study must be met by LFC technology developers to reach the PTC benchmark.

A matrix-based approach efficiently simulates ray- and wave-optical phenomena together within multi-scale structured sheets, for increased efficiency in textured silicon solar cells.

Due to its high resolution and applicability for large area patterning, nanoimprint lithography (NIL) is a promising technology for photovoltaic (PV) applications. However, a successful industrial application of NIL processes is only possible if large-area processing on thin, brittle, and potentially rough substrates can be achieved in a high-throughput process. The development of NIL processes using the SmartNIL technology from EV Group with a focus on PV applications is described. The authors applied this tooling to realize a honeycomb texture (8 mm period) on the front side of multicrystalline silicon solar cells, leading to an improvement in optical efficiency of 7% relative and a total efficiency gain of 0.5% absolute compared to the industrial standard texture (isotexture). On the rear side of monocrystalline silicon solar cells, the authors realized diffraction gratings to make use of light trapping effects. An absorption enhancement of up to 35% absolute at a wavelength of 1100 nm is demonstrated. Furthermore, photolithography was combined with NIL processes to introduce features for metal contacts into honeycomb master structures, which were initially realized using interference lithography. As a final application, the authors investigated the realization of very fine contact fingers with prismatic shape in order to minimize reflection losses.

Research concerning the field of Liquid Heating Collectors (SLHCs) cannot be directly transferred to the large area of applications for Solar Air Heating Collectors (SAHC). Larger cross sections for transporting the air are necessary. This results in rather complicated fluid distributions within the products. In this paper we combine simulation and experimental techniques providing a high level of detail. Highly resolved three dimensional fluid dynamic simulations contain all the relevant heat transfer mechanisms in a plate SAHC: Heat conduction, convection patterns and radiation are modelled in the two air gaps. In all solid materials of the collector only heat conduction has to be accounted for. For the exchange of heat between the two air zones the heat transfer through the absorber is modelled. Nevertheless careful simplification is needed to be able to concentrate on the important details. Furthermore we shed light on numerical instabilities observed in the air gap between absorber and glass in the simulated example. A consistent numerical description is given, concluding, that Computational Fluid Dynamics (CFD) is an appropriate tool for design and optimisation of SAHC concepts. The effects of collector tilting, insulation level and heat transfer surface increase have been assessed as a prove of applicability. The performance of these models will finally be compared to the experimental data obtained using a high-precision SAHC testing facility. For the experimental data base refined local measurement techniques, which meet highest accuracy requirements, are developed and employed.

In standard solar cells, light impinges under a very small angular range, whereas the solar cell emits light into the whole half space. Due to this expansion of etendue, entropy is generated, which limits the maximal efficiency of solar cells. This limit can be overcome by either increasing the angle of incidence by concentration or by decreasing the angle of emission by an angularly confining element or by a combination of both. In an ideal solar cell with radiative recombination as the only loss mechanism, angular confinement and concentration are thermodynamically equivalent. It is shown that concentration in a device, where non-radiative losses such as Shockley-Read-Hall and Auger recombination are considered, is not equivalent to angular confinement. As soon as non-radiative losses are considered, the gain in efficiency due to angular confinement drops significantly in contrast to the gain caused by concentration. With the help of detailed balance calculations, it is furthermore shown that angular confinement can help to increase the efficiency of solar cells under concentrated sunlight even if no measurable gain is expected for the solar cell under 1-sun-illumination. Our analysis predicts a relative gain of 3.14% relative in efficiency for a realistic solar cell with a concentration factor of 500.

We describe a procedure to estimate the calibration function for an experimental setup that contains a transmitting diffuser whose bidirectional transmittance distribution function for light transmitted in normal direction is nearly independent of the incidence angle. This type of diffusing screen may be used in experimental setups to measure the irradiance distribution in the focal plane of concentrator optics that have large angles of incidence in the focal plane. It is shown that the influence of this screen and of the remaining components on the irradiance distribution may be described empirically by a Gaussian function and thus may be corrected for. Furthermore, it is demonstrated that the correction is necessary to avoid an underestimation of the concentrator optics’ ability to concentrate the incoming radiation.

Fresnel lenses are widely used in concentrating photovoltaic (CPV) systems as primary optical elements focusing sunlight onto small solar cells or onto entrance apertures of secondary optical elements attached to the solar cells. Calculations using the Young-Maggi-Rubinowicz theory of diffraction yield analytical expressions for the amount of light spilling outside these target areas due to diffraction at the edges of the concentrator Fresnel lenses. Explicit equations are given for the diffraction loss due to planar Fresnel lenses with small prisms and due to arbitrarily shaped Fresnel lenses. Furthermore, the cases of illumination by monochromatic, polychromatic, totally spatially coherent and partially spatially coherent light (e.g. from the solar disc) are treated, resulting in analytical formulae. Examples using realistic values show losses due to diffraction of up to several percent.

We demonstrate an innovative process to fabricate uniformly shaped plasmonic nanoparticles. Laser interference lithography, nano-imprint lithography and a lift-off process are employed for the controlled production of periodically arranged nanoparticles on large areas. Round and elliptic silver particles with diameters of about 200 nm on an area of 5×5cm2 are investigated. Measurements of resonant absorption by the metal particles are in agreement with data computer-simulated by rigorous coupled wave analysis. We observe that the plasmonic resonance of elliptic particles depends on the polarization of incident light and that porosity of the metal influences the plasmonic band.

Angularly selective filters can increase the efficiency of radiatively limited solar cells. A restriction of the acceptance angle is linked to the kind of utilizable solar spectrum (global or direct radiation). This has to be considered when calculating the potential enhancement of both the efficiency and the power output. In this paper, different concepts to realize angularly selective filters are compared regarding their limits for efficiency and power output per unit area. First experimental results of a promising system based on a thin-film filter as the angularly selective element are given to demonstrate the practical relevance of such systems.

International audience; Photovoltaic-thermal (PVT) hybrid collectors are co-generation components that convert solar energy into both electricity and heat and represent in principle one of the most efficient ways to use solar energy. The aim of the study presented in this paper is to assess the performance of this type of collector as part of a solar thermal system. First of all, the development and the test results of an experimental flat plate PVT collector are described. Then, in the second part, the performance of this hybrid collector being part of a solar thermal system in a building is determined and compared to that of systems operating with standard solar devices (i.e. solar thermal collector and PV panel), through simulations using TRNSYS. Comparisons according to various evaluation criteria were made assuming the same surface area and under the same climatic conditions. The results show that in configuration of limited available space for solar collector area, the use of efficient PVT collectors in the building envelop can be more advantageous than standard PV and solar thermal components, not only from an energetic point of view, but also considering the exergy and the primary energy saving.

Light trapping is of very high importance for silicon photovoltaics (PV) and especially for thin-film silicon solar cells. In this paper we investigate and compare theoretically the light trapping properties of periodic and stochastic structures having similar geometrical features. The theoretical investigations are based on the actual surface geometry of a scattering structure, characterized by an atomic force microscope. This structure is used for light trapping in thin-film microcrystalline silicon solar cells. Very good agreement is found in a first comparison between simulation and experimental results. The geometrical parameters of the stochastic structure are varied and it is found that the light trapping mainly depends on the aspect ratio (length/height). Furthermore, the maximum possible light trapping with this kind of stochastic structure geometry is investigated. In a second step, the stochastic structure is analysed and typical geometrical features are extracted, which are then arranged in a periodic structure. Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature. From the obtained results we conclude that a potential advantage of periodic structures for PV applications will very likely not be found in the absorption enhancement in the solar cell material. However, uniformity and higher definition in production of these structures can lead to potential improvements concerning electrical characteristics and parasitic absorption, e.g. in a back reflector. (C) 2012 Optical Society of America

The development of small to medium size, autonomous and robust desalination units is needed to establish an independent water supply in remote areas. This is the motivation for research on alternative desalination processes. Membrane distillation (MD) seems to meet the specific requirements very well. This work is focused on experimental studies on full scale demonstration systems, utilizing a parallel multi MD-module setup. Three different plant concepts are introduced, one of them is waste heat driven and two of them are powered by solar thermal collectors. Design parameters and system design are presented. After the analysis of plant operation a comparison among the plants as well as a comparison with laboratory experiments is carried out and discussed. Impact of different feed flow rates, salinities, operating hours and process temperatures are taken into consideration and put into relation. GOR values and specific thermal heat demand are derived and compared. Energy balances of all three plants are given, uncovering heat losses and identifying room for improvement.

Hybrid photovoltaic-thermal solar collectors (PVT collectors) convert solar radiation into both electrical power and useable heat. The goal of combining these two forms of energy conversion in one product is to increase overall efficiency by accessing a higher technical energy potential. Combining the two types of energy transformation in a hybrid product is an innovative approach that is currently entering the market in the form of several products from several producers. As a result of boundary conditions (the early stage of market diffusion and the absence of standards, norms, and certifications), there is an enormous deficit of technical information for PVT collectors. This leads to restrained policy implementation from government entities, fewer incentives for producers and more wariness on the part of the end consumer; the combination of these factors constitutes a strong market barrier. In addition, with respect to product quality labels and product certification, PVT collectors must be discussed in a sophisticated way and, therefore, require an appropriate scientific description. In this paper, possible changes and steps with regards to standards, regulations and certification procedures are suggested to provide solutions over the short, medium and long terms. In addition, an extended hybrid collector model (in analogy to the quasi-dynamic thermal performance model) is presented and proposed for implementation into the existing certification.

Light trapping is becoming of increasing importance in crystalline silicon solar cells as thinner wafers are used to reduce costs. In this work, we report on light trapping by rear-side diffraction gratings produced by nano-imprint lithography using interference lithography as the mastering technology. Gratings fabricated on crystalline silicon wafers are shown to provide significant absorption enhancements. Through a combination of optical measurement and simulation, it is shown that the crossed grating provides better absorption enhancement than the linear grating, and that the parasitic reflector absorption is reduced by planarizing the rear reflector, leading to an increase in the useful absorption in the silicon. Finally, electrooptical simulations are performed of solar cells employing the fabricated grating structures to estimate efficiency enhancement potential.

Molds are used to dictate their shape to other materials in embossing or filling processes. In optics fabrication especially, the exact surface slope of the polymer replica is of high relevance. The quality control of molds is challenging: non-invasive, optical metrologies struggle with shiny surfaces that minimize the scattering of light. In addition, the inspection of complex shaped molds with a stepped optical surface can be difficult. In response, the authors show a backward ray-tracing approach combined with fringe-reflection technique to determine the slopes of a Fresnel-shaped mold surface with topography features in the magnitude order of a quarter millimeter. The error is kept small by stitching together several measurements with different sample rotations.

In this paper, we present guidelines for the design of backside gratings for crystalline silicon solar cells. We use a specially developed method based on a combination of rigorous 3D wave optical simulations and detailed semiconductor device modeling. We also present experimental results of fabricated structures. Simulation-based optimizations of grating period and depth d of a binary grating and calculations of the optical and electrical characteristics of solar cells with optimized gratings are shown. The investigated solar cell setup features a thickness of dbulk=40m and a flat front surface. For this setup, we show a maximum increase in short-circuit current density of jSC=1.8mA/cm2 corresponding to an efficiency enhancement of 1% absolute. Furthermore, we investigate different loss mechanisms: (i) an increased rear surface recombination velocity S0,b because of an altered surface caused by the introduction of the grating and (ii) absorption in the aluminum backsi de reflector. We analyze the trade-off point between gain due to improved optical properties and loss due to corrupted electrical properties. We find that, increasing the efficiency by 1% absolute due to improved light trapping, the maximum tolerable recombination velocity is S0,b(max)=5.2×103cm/s. From simulations and measurements, we conclude that structuring of the aluminum backside reflector should be avoided because of parasitic absorption. Adding a dielectric buffer layer between silicon and the structured aluminum, absorption losses can be tuned. We find that for a planar reflector, the thickness of a SiO2 buffer layer should exceed dSiO2=120nm.

Many markets require large area surface relief micro- and nanostructures. Important examples are light management structures for display applications or the radiation power management in solar systems. Structuring techniques with both up-scaling and mass production potential are an essential precondition for their realization. Interference lithography is a promising technique to originate fine-tailored structures on areas of up to 1.2 × 1.2 m2. Subsequent replication techniques have the potential for enabling an industrial fabrication. After a description of the basic technologies, we present application examples of large area structures used as template for polarization optical films. Furthermore, light trapping structures for crystalline silicon solar cells are shown. For this application, etching masks were fabricated by nanoimprint lithography. A subsequent etching step was applied to transfer the structures into the silicon. Finally, 3-dimensional photonic structures with distinguished optical properties are presented.

A novel texturing method to realize a honeycomb texture on multicrystalline silicon solar cells is presented in this paper. The demonstrated process chain is based on nanoimprint lithography (NIL), where an ultra-violet (UV)-curable polymer layer is structured by mechanical embossing in a high-throughput process. This patterned polymer layer can then be used as an etching mask for plasma or wet chemical etching processes to transfer the defined pattern into the silicon substrates. Within this study, the whole process chain of this novel texturing scheme, using interference lithography, cast moulding, NIL, and plasma etching, is described with a focus on the NIL process. The textured substrates are characterized by reflection measurements, which are compared with standard solar cell textures. Besides optical measurements, first results of honeycomb textured solar cells are presented. Short-circuit current densities above 40 mA/cm2 were achieved on high-quality float zone material. To increase the feasibility of an industrial realization of this process chain, we are developing a roller-NIL tool to structure an etching mask in a continuous in-line process. First results of this novel tool are also shown in this study.

Recent investigations of Fresnel lens concentrator optics have shown that optical efficiency and module performance depend on lens temperature. In former works we have succeeded in modeling these effects, achieving very good agreement with measurements of silicone on glass (SOG) Fresnel lenses. In the work presented here, we combine this lens model with irradiation spectra and ambient temperature data at typical sites for concentrator photovoltaic power plants for a whole year of operation. The solar cell is simulated with a model based upon the detailed balance limit introduced by Shockley and Queisser. This concentrator photovoltaic system consisting of a triple junction solar cell and a Fresnel lens concentrator is simulated including changes in temperature and spectrum over daytime and during all seasons. Comparing two simulations, one with the Fresnel lens kept at constant temperature and one with a realistic temperature variation of the lens, allows us to analyze the impact of Fresnel lens temperature on system efficiency under realistic environmental conditions. In our analysis poly(methylmethacrylate) (PMMA) Fresnel lenses and SOG Fresnel lenses show comparable energy harvesting efficiency at close to market locations.

Restricting the angular range in which a photovoltaic system emits light, is a promising but rather unexplored approach to enhance conversion efficiency. In this paper we analyze and discuss the effect of a directionally selective filter on the absorption of light and the generation of charge carriers in a germanium solar cell. A directionally selective filter transmits photons of perpendicular incidence and reflects photons under oblique incidence in a given spectral range. To investigate its effect on light trapping, we perform reflection and quantum efficiency measurements. The reflection measurements show that a wavelength dependent absorption enhancement is induced by the application of the directionally selective filter. We calculate a maximum absorption enhancement of 45% at lambda approximate to 1900 nm. We show that the absorption enhancement is caused by light trapping of non-absorbed and scattered light and is not due to a suppression of radiative processes. A trapping of photons generated by radiative recombination could not be detected. Measurements of the quantum efficiency confirm the results of the reflection measurements. The generation of charge carriers is increased by up to 33% at lambda approximate to 1900 nm. A comparison of path length enhancement factors calculated from reflection and quantum efficiency measurements indicates a low parasitic absorption in the solar cell device.

Since micro- and nanostructures for photon management are of increasing importance in novel high-efficiency solar cell concepts, structuring techniques with up-scaling potential play a key role in their realization. Interference lithography and Nanoimprint processes are presented as technologies for origination and replication of fine-tailored photonic structures on large areas. The combination of these processes is presented as feasible route to generate high efficiency honeycomb textures on multicrystalline silicon.

This paper deals with the design of a single glazed flat plate Photovoltaic–Thermal (PV–T) solar collector. First, the thermal and electrical performances of several single glazed flat plate PV–T concepts based on water circulation are investigated, using a simple 2D thermal model, then different ways of improvement are presented. It mainly consists in focusing on the heat transfer between PV cells and fluid, and also on the optical properties of materials. Thus, the most appropriate concept configuration has been identified and suitable material properties have been selected. A prototype collector has been designed, built and tested. A high thermal efficiency was reached at zero reduced temperature. For this level of thermal efficiency, the corresponding electrical efficiency has is lower than efficiency of a standard PV panel using the same technology. However, this solar PV–T collector is reaching, in these standard conditions, the highest efficiency level reported in the literature.

Restricting the angular emission characteristics of a solar cell results in a reduction of entropy generation and allows for higher solar cell efficiencies. In this paper, we present the concept of a photonic solar cell which exhibits the combination of a solar cell and a photonic crystal. The basic idea is to structure a solar cell in such a way that the applied structure is a photonic crystal consisting of the solar cell material. The aim of having a solar cell in the form of a photonic crystal is to restrict the emission of the solar cell into certain angles This reduction reduces the radiative losses and results in a higher VOC and, consequently, a higher efficiency of the cell. To calculate the efficiency of a photonic solar cell we introduce an adaptation of the detailed balance approach. This adaptation takes into account the reduced radiative losses due to the presence of a photonic band structure. In the second part of the paper we use this approach to calculat e the efficiency for an exemplary system. The exemplary system is an idealized edge filter, similar to a Bragg stack. The idealization concerns the reflection characteristic of this filter and its dependence on the polarization. For this idealized system we calculate an efficiency that exceeds that of a conventional solar cell by up to 2.5% absolute.

Dünne Schichten aus Silber werden bereits großflächig in Wärmeschutzverglasungen eingesetzt und zeigen gute Leitfähigkeiten bei hohen Transmissionen vor allem im sichtbaren Spektralbereich. Im nahen Infraroten fällt die Transmission ab, was jedoch durch eine Ausnutzung von Interferenzen wieder ausgeglichen werden kann. Dünne Schichten aus Silber können kostengünstig hergestellt und insbesondere auch gut auf Foliensubstraten abgeschieden werden. Durch zusätzliche dielektrische Schichten lässt sich das Schichtsystem auf weitere Anforderungen anpassen, ohne die Leitfähigkeit zu beeinträchtigen. Beispiele sind die Vermeidung von Barrieren, die Anpassung der Austrittsarbeit oder die Stabilisierung gegen folgende Prozessschritte. Dadurch sind Silberelektroden für organische Leuchtdioden oder Solarzellen von besonderem Interesse.

In many places world wide drinkable water is already a scarce good and its lack will rise dramatically in the future. Missing energy sources and no grid connections complicates the use of standard desalination techniques in these places. Fraunhofer ISE develops solar thermally driven compact desalination systems based on membrane distillation (MD) for capacity range between 100 and 500 L/day and larger systems for the capacity range up to 10 m(3)/day. All systems can be operated energy self sufficient and almost maintenance free. Membrane distillation is a technique which is operated with thermal energy but also uses a membrane for the separation of pure water from the concentrated solution. With regard to the implementation in solar driven stand-alone desalination systems it holds important advantages. Altogether eight pilot plants were installed in five different countries.

In this paper we introduce the three-dimensional formulation of the OPTOS formalism, a matrix-based method that allows for the efficient simulation of non-coherent light propagation and absorption in thick textured sheets. As application examples, we calculate the absorptance of solar cells featuring textures on front and rear side with different feature sizes operating in different optical regimes. A discretization of polar and azimuth angle enables a three-dimensional description of systems with arbitrary surface textures. We present redistribution matrices for 3D surface textures, including pyramidal textures, binary crossed gratings and a Lambertian scatterer. The results of the OPTOS simulations for silicon sheets with different combinations of these surfaces are in accordance with both optical measurements and results based on established simulation methods like ray tracing. Using OPTOS, we show that the integration of a diffractive grating at the rear side of a silicon solar cell featuring a pyramidal front side results in absorption close to the Yablonovitch Limit enhancing the photocurrent density by 0.6 mA/cmsup2/supfor a 200 µm thick cell.

We discuss the influence of angularly selective filters on thin film gallium arsenide solar cells. For this reason, the detailed balance model was refined to fit our needs with respect to Auger recombination, reflection, transmission, and realistic absorption. For calculating real systems, an approach was made to include optical effects of angularly selective filters into electron-hole dynamic equations implemented in PC1D, a one dimensional solar cell calculation tool. With this approach, we find a relative Voc increase of 5% for an idealized 100nm GaAs cell, including Auger recombination.

A directionally selective multilayer filter is applied to a hydrogenated amorphous silicon solar cell to improve the light trapping. The filter prevents non-absorbed long-wavelength photons from leaving the cell under oblique angles leading to an enhancement of the total optical path length for weakly absorbed light within the device by a factor of kappa(r) = 3.5. Parasitic absorption in the contact layers limits the effective path length improvement for the photovoltaic quantum efficiency to a factor of kappa(EQE) = 1.5. The total short-circuit current density increases by DeltaJ(sc) = 0.2 mAcm(-2) due to the directional selectivity of the Bragg-like filter.

For decreasing thicknesses of wafer based silicon solar cells, photon management structures to maintain high quantum efficiencies will gain importance. Diffractive gratings on the wafer back side can be designed to achieve very high path length enhancements, especially for weakly absorbed infrared radiation. This technologically demanding concept has to be realised using processes with upscaling potential. Therefore, we present a fabrication process for producing photonic structures in silicon based on interference lithography and nanoimprint lithography (NIL). We realised linear as well as crossed gratings of different depths, which were etched into the wafer back side. Polarisation dependent reflection measurements were made to get information about potential absorption enhancement as well as the occurrence of parasitic absorption in the metal reflector. This is conducted for a PECVD silicon oxide buffer layer between grating and reflector as well as a spin coated silicon oxide layer. Besides these optical characterisations, we further investigated the electrical properties of the back surface, where we applied a concept in which electrical and optical properties are decoupled. This is realised by a layer stack on the wafer back side, consisting of a thin Al2O3 passivation and a doped amorphous silicon layer.

Effective light trapping concepts are increasingly relevant for thinner crystalline silicon solar cells. Diffractive rear side structures can be designed to enhance optical light path lengths in the weakly absorbed infrared wavelength range. However, because of the rather difficult passivation of these nano-scale structures and possible plasma induced damages, the optical gain has so far not been shown on final device level for wafer-based crystalline silicon solar cells in combination with a high open circuit voltage. In this work, we fabricated and characterized silicon solar cells with an optically active layer at the rear side which is electrically decoupled from the silicon bulk by a thin, planar passivation layer. A binary diffraction grating was realized on the rear side via Nanoimprint Lithography (NIL) and plasma etching. For the rear side metallization a laser based foil process and photolithographically opened point contacts with evaporated aluminum were used as two alternative process routes. The best cell shows an EQE increase of up to 29 % absolute at 1100 nm compared to planar reference cells. This corresponds to a total Jsc gain of 1.2 mA/cm2 and a best cell efficiency of 19.4 % (Voc: 683.1 mV, Jsc: 35.4 mA/cm2, FF: 81.0 %) with planar front side and SiO2 antireflection coating. Optical simulations, which fit well to the experimental data, show that cell efficiencies of 20.8 % and 21.3 % can be reached with a planar front by using an optimized single layer (SiNxOy) or double layer (MgF, SiNx) ARC, respectively. By calculating Jsc for different cell thicknesses it is shown that a 100 μm thick solar cell incorporating the fabricated diffractive grating is expected to exhibit a higher Jsc than a planar cell with 250 μm thickness.

Desert areas are noteworthy for solar system installation due to high yearly global irradiance. These arid regions bear extreme conditions for surfaces, like glazing and mirrors, of solar thermal systems. Surfaces are targets for soiling because of wind and high dust loads. Soiling is causing a decrease in transmittance that limits the overall performance. At a certain level depending on surface properties, like functionality and their reliability, is surface material soiling-prone. For surface qualification is a reproducible soiling test needed. An in-house developed artificial soiling device achieves this purpose. Realistic dust deposition is used to test the functionality and properties of surfaces for application in arid regions. The parameter study points out the connection between properties of surfaces and the adhesion behavior of real dust. Soiled surfaces are analyzed by by spectroscopic (FT-IR) and gravimetric measurements.