• Energy Research
• Restricted close
• other engineering and technologies close
• DLR publication server close

Abstract Thermochemical energy storage (TCS) based on gas-solid reactions constitutes a promising concept to exploit reaction enthalpies for thermal energy storage. This concept facilitates the development of efficient storage solutions with higher energy densities compared to widely investigated sensible and latent thermal energy storage systems. Multivalent metal oxides are capable of undergoing a reversible redox reaction at high temperatures, which is why those storage materials are considered particularly suitable for the operating temperature range of concentrated solar power plants with central receiver systems to increase the total plant efficiency and ensure dispatchability of electricity. In the scope of this work a granular manganese-iron oxide with a Fe/Mn molar ratio of 1:3 has been selected as a potentially suitable storage material, which is non-toxic, abundant and economical. For this reason a preparation route from technical grade raw materials has been chosen. The reversible redox reaction is investigated with respect to the thermodynamic and kinetic characteristics by means of simultaneous thermal analysis in dynamic and isothermal series of measurements. Those revealed that the observed presence of a strong divergence of the reactive temperature range from the actual thermodynamic equilibrium can mainly be attributed to kinetic limitations. Expressions for the effective reaction rates are deduced from experimental data for the reduction and oxidation step, describing the dependence of the reaction rate on temperature and oxygen partial pressure, respectively. The expressions are valid for the temperature ranges in proximity to the equilibrium, which are relevant for the targeted operating conditions of the storage reactor in air. The storage material provides good cycling stability in terms of reversibility and widely maintained reactivity throughout 100 redox cycles in air. Future work comprises material modifications, which are expected to further enhance the mechanical stability of the particles. Overall, the manganese-iron oxide of the chosen composition exhibits a redox reactivity practical for regenerator-type storage systems combining a high temperature TCS zone and a lower temperature non-reactive zone merely used for sensible thermal energy storage.

In this paper we present a study on the use of remote sensing data combined to the 3D modeling of radiative transfer (RT) and energy balance in urban canopies in the aim to improve our knowledge on anthropogenic heat fluxes in several European cities (London, Basel, Heraklion, and Toulouse). The approach is based on the forcing by the use of LandSAT8 data of a coupled radiative transfer model DART (Direct Anisotropic Radiative Transfer) (www.cesbio.upstlse.fr/dart) with an energy balance module. LandSAT8 visible remote sensing data is used to better parametrize the albedo of the urban canopy and thermal remote sensing data is used to enhance the anthropogenic component in the coupled model. This work is conducted in the frame of the H2020 project URBANFLUXES, which aim is to improve the efficiency of remote-sensing data usage for the determination of the anthropogenic heat fluxes in urban canopies [5].

A rotating cooling system with a 180 deg turn is investigated experimentally using the 2C PIV technique to measure the flow inside. This cooling configuration consists of two ducts of arbitrary cross-sections representing a two-pass front part of an idealized but nevertheless engine relevant turbine blade cooling design. The system has been investigated with ribbed walls in both passages for cooling enhancement as well as with smooth walls as a reference version in order to identify the effects induced by ribs. The rib orientation on the walls is 45 deg. With a rib height of 0.1 of hydraulic duct diameter and a pitch of 10 times rib height, a representative well-established rib lay-out was selected. This paper presents measurements of the axial flow during rotation of this two-pass system for rotation numbers up to 0.1. Together with previously obtained stationary results [1], this data completes the investigation of the secondary flow field with rotational results acquired with a two-component PIV measuring technique with improved sequencer technique [2]. The Two-Pass Cooling System was analyzed on the rotating test rig using two-component Particle Image Velocimetry (2C PIV) a non-intrusive optical planar measurement technique. PIV is capable of obtaining complete flow maps of the instantaneous as well as averaged flow field even at high turbulence levels, which are typical for the narrow serpentine-shaped ribbed cooling systems. An in-house developed synchronization device enables very accurate control of the laser flashes and image acquisition with regard to the angular position of the measurement plane (light sheet) and thereby very accurately stabilizes the position of the channel within the image during PIV recording which then leads to very accurate mean velocities. The presented investigations were conducted in stationary and rotating mode. The results demonstrate the combined interaction of different vortices induced by several effects such as the inclination of ribs, Coriolis forces due to rotation and inertial forces within the bend. Additionally, a flow separation was observed at the divider wall downstream of the bend (in the second pass) that has a strong impact on the flow field depending on the rotational speed. The axial flow maps presented in this paper in combination with the secondary flow maps published previously are of sufficient high quality and spatial resolution to serve as a benchmark test case for the validation of flow solvers. The turbulent channel flow was investigated at a Reynolds number of 50,000 and at rotation numbers of 0.0 and 0.1.

The aim of this publication is a comparison of a wide selection of range extender technologies. Thereby a large amount of criteria are looked into. This assures that the comparison is feasible for a diversity of performance specifications. Hence an information level is achieved, that allows for a justified choice of technology according to the desired range extended electric vehicle concept.

In this paper an optimization based approach for stability analysis of electrical networks is addressed. The stability problem is transferred to an optimization problem for finding worst performance regarding stability criteria for a parameters range. If a violation of the criteria is found, instability of the system is proven. For design, the maximal possible design parameters can be scaled until a boundary is reached. In contrast to other present methods of stability analysis - modal analysis and mu analysis which are only suitable for linearized models - nonlinear models may be used directly by the optimization approach without the necessity of any averaging technique and linearization. This is necessary for systems like power electrical components with usually high nonlinear and wide ranging dynamics due to the very fast switching and a good solution for the stability analysis with industrial specified standards. The application of the proposed approach is illustrated via a regulated buck converter as a typical critical nonlinear component of the electrical network in the "More Electric Aircraft" architecture (MEA).

Abstract Reflectors are a vital part of a concentrating solar thermal power plant. One of their most important characteristics is their durability, which entails the maintenance of their optical properties throughout their service lifetime, aimed at 10–30 years or more. The assessment of their optical durability involves the design of two types of aging tests, outdoor exposure testing under real ambient conditions and accelerated exposure testing in weathering chambers under simulated conditions. After exposure to different stress factors for certain periods of time, the optical performance of reflectors is evaluated mainly in terms of reflectance, but also regarding qualitative parameters such as their visual appearance and degradation patterns. The ultimate goal of a durability study is to conceive meaningful accelerated testing procedures that simulate real outdoor degradation in a short time and provide service lifetime estimates for a certain type of reflector at a specific site. To achieve this, more research on service lifetime prediction should be conducted and the standardization of accelerated testing procedures and reflectance evaluation methods should become widespread, to obtain comparable representative results. In this article, the most significant durability studies performed on the three main types of solar reflectors (glass-based, aluminum and silvered-polymer) and prospective approaches for improving future endeavors are discussed.

Abstract The present study focuses on a thermal model describing a rotary kiln reactor. Several applications can be foreseen for this reactor, for example high temperature heat storage for thermal solar power plants. The energy is provided by concentrated solar radiation that heats up the cavity walls. A thermal model, describing the reactor behavior, is developed and validated. Particular attention is given to the radiation model, which constitutes the most important heat transfer. An innovative way of modeling the reactor aperture through a fictive surface at an imposed equivalent temperature leads to a significant decrease of the simulation time, without decreasing the precision of the solution. The model is validated by comparison first with other models, which make different assumptions and second with experimental results. After the validation, the model can be used for simulating the behavior under different operating condition or to define the possible improvements by a change of the reactor geometry such as the insulation’s thermal conductivity or thickness.

The transient thermal behavior of two solar receiver-reactors for hydrogen production has been modeled using Modelica/Dymola. The simulated reactors are dedicated to carry out the same chemical reactions but represent two different development stages of the project HYDROSOL and two different orders of magnitude concerning reactor size and hydrogen production capacity. The process itself is a two step thermochemical cycle, which uses mixed iron-oxides as a redox-system. The iron-oxide is coated on a ceramic substrate, which is placed inside the receiver-reactor and serves on the one hand as an absorber for solar radiation and on the other hand as the reaction zone for the chemical reaction. The process consists of a water splitting step in which hydrogen is produced and a regeneration step during which the used redox-material is being reduced. The reactor is operated between these two reaction conditions in regular intervals with alternating temperature levels of about 800 °C for the water splitting step and 1200 °C for the regeneration step. Because of this highly dynamic process and because of fluctuating solar radiation during the day, a mathematical tool was necessary to model the transient behavior of the reactor for theoretical studies. Two models have been developed for two existing receiver-reactors. One model has been set up to simulate the behavior of a small scale test reactor, which has been built and tested at the solar furnace of DLR in Cologne. Results are very promising and show that the model is able to reflect the thermal behavior of the reactor. Another model has been developed for a 100 kWth pilot reactor which was set up at the Plataforma Solar de Almeri´a in Spain. This model is based on the first model but special geometrical features had to be adapted. With this model temperatures and hydrogen production rates could be predicted.

Abstract Achieving the global and European climate targets requires a green transformation of the transport sector. Electrification may become a viable option in light road transportation, but the use of liquid fuels with high energy density will likely prevail in aviation, shipping and heavy-duty mobility. For these applications, electrofuels, produced from electricity and C O 2 , may be a promising option, especially in countries with high wind and solar energy potentials. In the present case study, the prospects of electrofuels in Denmark are investigated. At first, an in-depth analysis of the available raw material and energy resources was conducted. The design of a first Power-to-Liquid (PtL) plant was then developed and implemented in Aspen Plus® for a selected site in Denmark. The plant is subsequently analyzed in a techno-economic and ecological assessment. The results indicate that the unexploited Danish wind power potential is theoretically sufficient to entirely cover the expected future demand for alternative fuels through electrofuel production. Greenhouse gas reductions of 95% compared to fossil fuels can be achieved if electricity from renewable power sources is used. However, fuel production costs are significantly higher than crude oil market prices, resulting in very high GHG abatement costs compared to other carbon mitigation technologies.