• Energy Research

Abstract An important option for increasing the efficiency in Concentrated Solarthermal Power (CSP) is the development of systems generating steam directly in the solar field. To produce dispatchable and affordable electrical power, the integration of an efficient and economical thermal energy storage is an attractive option. To (dis)charge a storage with steam, latent heat storages are particularly suitable for the use with two-phase heat transfer media. Due to low heat conductivities of the phase change materials, the heat exchanger design is crucial. Up to now no latent storage is commercially deployed in CSP. A new latent storage approach is being developed at Fraunhofer ISE; a Screw Heat Exchanger (SHE) is employed for heat transfer. With a SHE, material can be transported during the phase change. With this innovative concept, the size of the heat transfer area and the storage capacity can be decoupled and it seems possible to develop an economically viable latent storage. To understand the dynamics of the phase change and the heat transfer characteristics in the SHE, testing will be done with a laboratory prototype. The phase change material (PCM) used is an eutectic mixture of NaNO3 and KNO3 with a melting point at Tm = 221 °C. It is a salt mixture with low price and high availability, as is the requirement for the development of a low cost thermal energy storage. A proof of concept for the storage concept has been obtained. The problematic crystallization process in the SHE was successfully performed. A thermal power of Q = 11 kW was transferred to a salt mass flow of about 50 kg/h. It showed the feasibility of the transport of a PCM during the phase change due to the self-cleaning effect of the screws. The heat exchanger has been designed, constructed and equipped with measurement devices. Peripheral transport components have been selected to allow semi-automated storage operation. The storage system will be tested to gain experience for the design of process equipment necessary for the eventual use in large scale solarthermal power plants. Parallel to the commissioning, the heat exchanger design was assessed and improved. The heat transfer characteristics of the storage is dependent on parameters as the PCM mass flow, the rotational speed of the screw shafts, the temperature difference of PCM to the heat transfer media, the inclination of the heat exchanger and the granule size. The correlation between these factors will be examined in further experiments to assess their influence on operation strategies.

AbstractIn this article we study the integration of a solar plant to a copper recuperation process in Northern Chile. We analyze the integration of concentrating and non-concentrating solar energy technologies, identifying their main advantages and disadvantages. Given the energy demand of the process, solar field areas over 30.000 m2 are studied.

AbstractA small-scale Linear Fresnel Collector (LFC) for the generation of process heat has been tested by Fraunhofer ISE;its performance was evaluated by means of two different methods. The first is a quasi-dynamic testing method performed according to the testing standard ISO 9806:2013, with modificationsin the model to accurately describe LFCs. Due to the two-dimensional Incidence Angle Modifier (IAM) of an LFC, an iterative multi-linear regression (MLR)approach has been developed to be able to comprehensively evaluate the optical performance. The second method is a dynamic testing method based on a parameter identification incorporating a multi-node/plug-flow collector model without strict restraints on mass flow and inlet temperature stability.Both methods are briefly described in their conceptual design and theirbasic requirements, revealing their similarities and differences. Each method is then applied to real measurement datafrom an LFC, assessing practicability and identification accuracy. For both methods, the mean absolutedifferencebetween identifiedIAM values and results from ray tracing fell in a range of 0.013-0.017, leading toa similar accuracy inLFC performance evaluation. Differences in optical efficiency between the two methodsare smaller,with anaverage absolutedifferencebelow 0.0098, even whenusing different measurement data and simulation models. Thus the dynamic methodrepresents a good starting point for the further development of an alternative dynamic testing and evaluation method with more flexibility than the current testing standard. This will be significantwhenevaluating large-scale concentrating collectors and collectors with direct steam generation.

For design and component specification of a Linear Fresnel Collector (LFC) cavity receiver, the prediction of temperature distribution and heat loss is of great importance. In this paper we present a sensitivity analysis for a range of geometry and material parameters. For the LFC receiver analysis we use two models developed at Fraunhofer ISE. One is a detailed model, combining the spatial distribution of reflected radiation via ray tracing with detailed convective simulations through computational fluid dynamics. The second one is a fast algorithm based on a thermal resistance model. It is applying a similar methodology as the well-known model for vacuum absorber, enhancing an absorber tube model by parameters describing the influence of the secondary mirror and cover glass. The thermal resistance model is described in detail. Obtained results indicate a significant effect of the secondary mirror temperature on heat loss for specific geometries.

Abstract Elastic light scattering has been used to study structural properties of different transparent aerogels, which may be used as filling materials in super-windows. With a goniometer having an angular resolution better than 0.6° and a He-Ne laser as the light sourcewe investigated the angular distribution of scattered intensity from transparent silica aerogels and one xerogel. The densities ranged between 0.11 and 0.60 g cm -3 . An exponential correlation function for the density fluctuations of a random porous medium has been utilised to analyse the large-angle scattering, which is dominated by bulk scattering, for different polarisation of the incident light. The determination of correlation lengths in the nanometre range was possible, because the absolute scattering intensities were determined. For relative angular dependence measurements, this range would have been accessible only to small angle X-ray scattering (SAXS). The resulting mean pore sizes between 8 nm and 50 nm and specific surface areas between 500 and 700 m 2 /g agree well with nitrogen-porosimetry data from the literature. The data compare quite well with correlation lengths calculated from specular transmittance data from an ordinary spectrophotometer. This method, which is not sensitive to the angular distribution of superposed forward scattering with large correlation lengths, has also been applied to a series of base-catalysed TMOS aerogels with different catalystconcentrations. The forward scattering peak of the signal may be attributed to correlation lengths in the micrometre range. Experimental results for aerogel surfaces with evaporated aluminium indicate that this might be due to the surface properties. A quantitative analysis, however, is not possible yet.

Abstract In this paper the results of a research project to establish a fundamental understanding of the physical relationships between the material structure and the infrared optical properties of polymeric transparent insulation (TI) materials are described. First, a systematic characterization of potential materials for TI applications in respect to their relevant physical properties in the infrared radiation range was done. Second, based on the investigation of polymer films with different thicknesses model functions for the infrared optical properties were established. Third, the infrared optical properties were interpreted on base of polymerphysical relations. The characterization in the infrared range and the calculation of infrared optical film properties have revealed, that various functional groups of the macromolecular structure are highly absorbing. For polymers with service temperatures of about 100 °C the carbon–oxygen single bond was identified as highly effective. For 50 μm thick films a good correlation between the concentration of the functional carbon–oxygen group and the non-spectral infrared optical thickness as well as the hemispherical emittance was found. An outstanding performance profile for TI wall applications with black absorbers was obtained for cellulose based materials.

AbstractCharacterization is defined as the act of describing distinctive characteristics or essential features. In most solar thermal collecting systems the energy performance characterization is commonly used as the most important criteria by which the system (or component) is represented. Building Integrated Solar Thermal Systems (BISTS) however are typically classified across a range of operating parameters, system features and mounting configurations, and other criteria are similarly important to be considered. Therefore BISTS characterization should also account for the architectural and building physics integration based on structural, functional and aesthetical features. A comprehensive characterization of BISTS is necessary to give designers, installers and end users confidence that the final solution selected is appropriate to the comprehensive building requirements.