Concentrating solar thermal (CST) technologies are a sustainable way to produce high‐temperature heat. Four concepts of integrating photovoltaics (PV) into CST plants, namely Rear‐PV, PV‐Mirror, bifacial PV‐Mirror and Spillage‐concentrating PV (CPV), are compared and the technological and economic outcome is discussed. The concepts are presented for the use with solar tower systems, but can also be applied to other configurations. In this work, parameters for each concept to quantify annual energy production and investment costs are derived. It is determined that implementing Rear‐PV, PV‐Mirror, bifacial PV‐Mirror, and Spillage‐CPV in a concentrating solar power tower plant leads to an additional energy yield as high as 23%, 29%, 40%, and 36%, respectively, on the same mirror aperture size. For the concepts of the Rear‐PV, PV‐Mirror, and bifacial PV‐Mirror, maximum allowable cost per aperture area can be 3.0, 4.8, and 5.7 times the cost of conventional mirrors, to reach a break‐even of the specific investment cost per annually produced energy. Such values are considered to be achievable for PV‐Mirror and bifacial PV‐Mirror, but not for Rear‐PV. For Spillage‐CPV, a break‐even of investment cost can be achieved if installed in areas with spillage radiation flux exceeding ≈350 kWm−2 at peak.

Solar reflectors for concentrating solar power (CSP) concentrators require a high specular reflectance over the whole solar spectrum. A preliminary procedure to measure the reflectance of solar mirror materials has been proposed in the SolarPaces reflectance measurement guideline [1]. However, the guideline clearly states that the currently available measurement instruments need further improvement in order to be able to fully characterize the reflectance properties of solar reflectors. In this work a high precision spectral reflectometer has been developed that permits the measurement of the solar weighted specular reflectance at different incidence and acceptance angles and thus provides all relevant reflectance data of solar mirrors. Four typical reflector materials have been measured and the solar weighted reflectance as a function of the acceptance angle has been modeled.

Durability analysis of solar mirrors is attracting attention from Concentrating Solar Power (CSP) developers because guaranties of a suitable optical behavior during the whole service life time are required. To give realistic life time estimations in affordable time, research institutes are investing a lot of effort in developing appropriate accelerated aging tests. In this paper, an extensive test campaign of accelerated aging tests has been performed with silvered-glass mirrors from several manufacturers. Three commercial outdoor-proven products, a not outdoor-proven mirror prototype and two mirrors which showed degradation after only three years of outdoor operation in Spain and Australia have been tested. The accelerated agingtests were conducted for an extended period of time in order to determine the testing time at which the two mirror materials that showed degradation outdoors start to fail under the accelerated conditions. The degradation of the outdoor exposed mirrors is analyzed microscopically and compared to the degradation appearing under accelerated aging. The presented data on accelerated testing represents useful information for standardization of durability testing of solar mirrors.

During the last years, the importance of correctly measuring the reflectance properties of solar reflector materials has increased due to the fact that more and more different kinds of materials compete on the market. Customers and manufacturers likewise need reliable tools to ensure the quality of the reflector and evaluate its performance in the desired concentrated solar power (CSP) application. The SolarPACES Reflectance Measurement Guideline [1] defines the parameters that are necessary for a thorough reflector qualification. The work on this guideline made it evident, that the state of the art of measurement technology is not sufficient for measuring all of these parameters in the ideal way. For some reflector materials, this can lead to improper estimations of their reflectance characteristics and their performance. Until now it was not possible to measure the most important parameter, the specular reflectance, ρs(λ,θ,φ), in the ideal format as a function of wavelength, λ, acceptance angle, φ, and incidence angle, θ. The currently most common method is to measure ρs(λ,θ,φ) at few selected φ and λ and to estimate the solar weighted specular reflectance, ρs(SW,θ,φ). One solution is the measurement of the bidirectional reflectance distribution function (BRDF) and use it to calculate ρs(λ,θ,φ) as a function of φ and θ. At the OPAC laboratory a new instrument MIRA (Mirror Reflectance Function Analyzer) was developed that offers the possibility to measure the bidirectional reflectance distribution of the light reflected by a mirror into the hemisphere at variable incidence angles [2]. In advantage to traditional goniometers or gonioreflectometers of which only few reach the necessary angular measurement resolution, MIRA uses a time saving technique and occupies comparably little space that is available in any laboratory. The instrument allows measurements at different selected wavelengths for the approximation of the solar weighted specular reflectance. This paper gives a detailed description of the instrument prototype and first preliminary measurement results are compared with a reference measurement. The instrument design and data evaluation method has a German patent [3].

Accelerated aging testing is used in several industries to estimate the lifetime of products and components. Manufacturers of solar reflector materials usually test the durability of their products following standards from automotive and photovoltaic industry. The testing time and the "pass" or "fail" criteria differ from manufacturer to manufacturer which makes it hard for customers to compare the available mirrors on the market and to select the most durable product based on the data sheets.This paper gives an extensive review of the state of the art of currently applied accelerated aging tests for solar reflectors. The testing conditions of relevant tests are summarized. Based on the experience gained over the last years in the aging laboratory of CIEMAT and DLR, a typical testing program to qualify the durability of reflector materials is being proposed. The appearing degradation of glass and aluminum mirrors under accelerated aging is analyzed and compared to outdoor exposure results. (C) 2013 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

The challenge of improving component quality and reducing cost has focused the attention of the solar thermal power industry on reliable component characterization methods. Since the reflector plays a key role in the energy conversion chain, the analysis of its reflectance properties has become a lively discussed issue in recent years. State of the art measurement instruments for specular reflectance do not give satisfying results, because they do not resolve sufficiently the near specular scatter of possible low cost mirror material candidates. The measurement of the BRDF offers a better solution than the traditional approach of placing a detector in the specular reflected beam path. However, due to the requirement of high angular resolution in the range of 1 mrad (0.057°) or better and the challenge of measuring high dynamic differences between the specular peak and the scatter signal, typical commercial scanning goniophotometers capable of this are rare. These instruments also face the disadvantages of impractically long acquisition times and, to reach the high angular resolution, occupy a large space (several meters side length). We have taken on the appealing idea of a parallel imaging goniophotometer and designed a prototype based on this principle. A mirrored ellipsoid is used to redirect the reflected light coming from a sample towards a camera with a fisheye lens. This way the complete light distribution is captured simultaneously. A key feature allows the distinction of the high intensity specular peak and the low intensity scatter. In this article we explain the prototype design and demonstrate its functionality based on comparison measurements done with a commercial scanning goniophotometer. We identify limitations related in part to the concept and in part to the specific prototype and suggest improvements. Finally we conclude that the concept is well suitable for the analysis of near specular scatter of mirror materials, although less adequate for the analysis of rough surfaces that require a full 180° view angle. Results obtained with this instrument are useful to evaluate the performance of a reflector material for a specific concentrating solar collector design and also serve in other applications that require near specular scatter analysis like degradation and soiling research.

Solar thermal technologies use concentration collectors to provide high temperature heat to a power cycle to produce electricity. After some early commercial implementation already in the 1980s, a technology take-off has started at the beginning of the Millennium. This paper summarizes that general concept and technical, economic and environmental characteristics and reviews the status of the technology for parabolic trough, linear Fresnel, central receiver and thermal storage technology. It discusses the actual research directions the cost reduction potential.

Solar reflectors for Concentrating Solar Power (CSP) concentrators require a high reflectance and high specularity over the whole solar spectrum. During their lifetime of at least 20 years, the reflectors must withstand harsh outdoor conditions without loosing their reflective properties. Currently, there are not many devices available to measure the specular reflectance. In this work a prototype of a specular reflectometer with spatial resolution has been developed. The major advantage of the prototype compared to other reflectometers is the possibility of measuring the specular reflectance on an extended measuring spot of more than 5 cm in diameter with a spatial resolution of 37 pixel/mm. Additionally, measurements can be taken at three different acceptance half angles (phi = 3.5,6.0, and 12.5 mrad) and at three different wavelengths (lambda = 410 nm, 500 nm, and 656 nm). This lab scale instrument can be employed to monitor degradation effects, such as corrosion spots, and evaluate their influence on the specular reflectance of solar mirror materials. (C) 2013 Elsevier B.V. All rights reserved.

In order to reduce electricity generation costs of concentrating solar power (CSP) technologies, new low-cost reflector materials are being developed. These materials need to withstand harsh outdoor conditions without a significant loss in specular reflectance. In this work, samples of enhanced anodized aluminum reflectors protected by a sol–gel coating that have been exposed at different weathering sites were analyzed with an innovative specular reflectometer in order to monitor corrosion and scattering caused by surface roughness. A model to estimate the specular reflectance as a function of exposure conditions at different weathering sites has been developed.

A rotary test bench for parabolic trough collector components was erected at Plataforma Solar de Almeria, Spain. The test bench allows the qualification of all collector components of complete moudles of up to 20 m length, i.e. structures, reflectors, recievers and flexible joints. It is equipped with high precision instrumentation and controls to allow for precise, quick and automated measurements. An active temperature control allows the operation at constant conditions. The overall accuracy for performance measurements of collector modules was expedted to be 2-3%. First measurements are presented that verify the oustanding precision of the test bench.

A solar-hybrid microturbine system using a tube receiver and a Turbec T100 was tested at the Plataforma Solar de Almeria. The design receiver outlet temperature of 800°C was achieved with an open receiver aperture and also using a pressureless quartz window. Measured receiver efficiencies of 39.7% for the open receiver at 782°C outlet temperature and 44% at 803°C outlet temperature for the configuration with the window are much lower than the design values due to design flaws in the cavity insulation and a too low mass flow from the turbine. Nevertheless, the encountered problems are solvable and further lessons learned are given. For a commercial system design using small heliostats of 1m2-mirror area a peak receiver efficiency of >85% was simulated using a face-down receiver configuration with a much smaller aperture. The solar-hybrid system can also be combined with a pressurized pebble-bed heat storage. For an off-grid application a solar share of 82% was simulated.

In solar tower plants absorber tubes are the main components of various solar receivers, e.g. steam receivers, salt receivers, pressurized air receivers. The solar radiation on the absorber tube causes an inhomogeneous temperature distribution because most of the solar radiation along the tube circumference is one-sided. The resulting internal stresses of the tube and the maximum fluid temperature decrease the lifetime significantly. Within the projects SOLHYCO and FUTUR a profiled multilayer tube (PML) is currently under development in order to reduce this problem. It consists of three metallic layers: a high temperature nickel-based alloy at the outer side, a copper layer as intermediate layer and another high temperature nickel-based alloy at the inner side of the tube. The outer layer provides the structural strength while the copper is used to conduct the heat from the irradiated side to the opposite side. The inner layer protects the copper from corrosion and oxidation at high temperatures. In addition, a wire coil is inserted (profiled) to increase the heat transfer on the inside. The PML is manufactured in a hydro-forming process by deforming the tube composite with water under high pressure. To demonstrate the performance and to determine the heat transfer, the pressure loss and the temperature distribution, a test loop was built to simulate the different loads under laboratory conditions. The thermo hydraulic measurements and finite element calculations show that the temperature gradient and the maximum temperature can be reduced significantly. Based on these studies the advantages of the PML in comparison to common tubes will be presented as well as several possibilities for future improvements.

Zusammenfassung Das DLR entwickelt gemeinsam mit nationalen wie internationalen Partnern solar-hybride Gasturbinensysteme. In verschiedenen Projekten wurden bzw. werden alle wichtigen Komponenten dieser Systeme entwickelt und getestet. Es konnte gezeigt werden, dass die Solarstrahlung bei hohen Temperaturen von bis zu 1000°C mit hohem Receiver-Wirkungsgrad von 80% als thermische Energie in Gasturbinensysteme eingekoppelt werden kann und die kontinuierliche Stromproduktion durch die solar-hybride Regelung problemlos moglich ist. Neben der Entwicklung eines Mikrogasturbinensystems (100 kWe) fur Kraft-Warmekopplung wird derzeit mit der Planung fur ein erstes Demonstrationskraftwerk fur Kombiprozesse im 5 MW Masstab begonnen, das 2010 in Sevilla ans Netz gehen soll.