• Energy Research

Abstract A selection of granular natural and ceramic materials has been experimentally characterized with regard to their application as heat transfer and storage media in concentrating solar power plants. Thermophysical, thermomechanical, tribological and rheological measurements have been conducted in order to identify the most suitable candidates for this dynamic high temperature operation. Ceramic materials are found to comprise some advantages, but natural products offer a considerably more economic solution. Thermal bulk conductivity is found to be only marginally affected by the solid's thermal conductivity, while specific heat is the same for all solids. Ceramics entirely withstand thermal cycling, while quartz-containing materials are prone to severe degradation. Most materials are found to attain a saturated state of attrition while being sheared under load, wherein quartz sand offers the lowest mass fraction of debris at saturation level. In the investigated grain size range, all materials show good flowability. The generation of debris requires consideration in the design of the CSP storage components.

Abstract The storage system investigated in this work, namely the CellFlux system, consists of a regenerator type thermal energy storage volume which is coupled to a heat exchanger by a circulating intermediate working fluid. The numerical simulations in this work are based on experiments conducted with a large scale pilot plant having a bed length of more than 10 m. The storage volume is of a novel design with horizontal flow direction and is filled with hollow bricks as sensible heat storage material. So far, most publications focus on packed bed storage systems, often with molten salt or oil, only few consider regularly shaped channels with gaseous flow. For the investigation in this part of the publication, a one-dimensional dispersion concentric model for channel flow is implemented in MATLAB/Simulink. The analysis in part I of this publication has revealed a radial flow maldistribution, which can be predicted by a correction function correlated beforehand. The commonly used assumption of plug flow, however, will be afflicted with an error. Thus, the aim of this work is to determine the accuracy of the model both under the plug flow assumption and the application of aforementioned correction function. Three single blow experiments, where the storage is charged or discharged from a uniform initial temperature, are compared to the numerical model. The average temperature deviation between experiment and simulation reaches up to 10% without the correction function, but improves significantly through its application and then ranges between 0.4% and 4.8%. Moreover, studies based on larger experiments have either not observed or generally not addressed the effect of a flow maldistribution. This appears to be an issue for small scale experiments where typically more sophisticated models are applied. If it was possible to model the entire storage volume with the present 1D model, the flow maldistribution effects could average out. Also, computing effort would be low and allow simulations on system level. This approach was taken only by few authors so far, particularly for packed bed systems and was also not experimentally verified. In order to correctly depict the experimental set up, the model considers the different composition of the interior, such as flow distributors and inlet/outlet cones as well as the thermal heat capacity of the walls and losses to the environment. As a result the model shows good agreement with the experiments: the mean temperature difference of the exit temperature between experiment and simulation during cyclic operation remained below 5%.

Abstract Granular material offer many advantages which qualify them for use as heat transfer medium and heat storage material in solar thermal power plants and industrial processes. Moving bed heat exchangers (MBHE) with horizontal tubes are favorable to extract thermal energy from hot granular materials. However, due to the complex flow phenomena and heat transport mechanisms in granular materials precise design tools for MBHE are lacking. In previous studies we introduced a discrete particle model (DEM) to calculate the granular flow in MBHE and validated it by experiments. In this study we use this precise but computationally expensive DEM model to qualify a more efficient continuum model (CFD) which is being introduced in this work. We compare the two models with regard to the granular flow speed along the tube surface and find deviations of |Δu/uref|

1. Introduction One challenge for concentrated solar power (CSP) plants is to guarantee continuous energy output when the sun sets or is blocked by clouds. Thermal energy storage (TES) systems are key components for facing this issue. The state of the art storage system is the molten salt double-tank TES. This heat storage medium implies some disadvantages. The heat transfer fluid (HTF) is on the one hand expensive and on the other hand it demands continuous technical attention to avoid undesired malfunction, such as freezing and degradation. Furthermore, either a special salt receiver or a secondary cycle to be heated by the HTF using a heat exchanger is required, which causes a reduction in thermal efficiency. A cost-effective, high temperature and efficient performance, single-tank alternative could be provided by the regenerator-type storage based on directly heated solid media. It has a simple setup, is applicable to highest temperatures (> 1000°C) and has best prospects for a deployment in large installations. A solid media packed bed inventory is a straightforward design option, offering cost-effective solutions. Possible choices for packed bed materials are ceramic pebbles, ceramic saddles or broken natural stone. Due to its classification as waste, a high potential for further cost reduction can be achieved by using slag from steel industry. The main objective of the European project REslag is the experimental validation of the steel slag as TES material for packed bed systems. In this regard, the final results and conclusions obtained in experimental and simulation studies of a packed bed prototype that implements slag particles as TES inventory and air as HTF will be presented. The design chosen was a vertically installed and axially flowed through TES, as this was identified as the best design in an extensive evaluation phase at the beginning of the project. The work presented here includes a detailed study of the system performance under different charge, discharge and idle operation conditions. Furthermore, the simulation model for design and up scaled studies is validated by using the obtained experimental results. 2. Methodology and results In order to achieve the aforementioned objectives different experiments with two test rigs were conducted. The thermomechanical stability of the used materials was investigated by using a uniaxial compression test (UCT) bench. Here, possible inventory and insulation damages occurring during cyclic operation of the TES were determined. The thermal behavior of the slag-based TES was investigated at the HOTREG pilot plant at DLR in Stuttgart. The experimental campaign consisted of performing charge, discharge and idle operations under different temperatures up to 700°C, mass flow rates up to 650 kg h-1 and cycle times. The aim of the study was the experimental validation of the simulation model, and the construction of insulation and slag pebbles. The comparison of the experimental results to the simulation results revealed deviations below 10 %. This indicates that the simulation model is qualified for scale up studies. Overall the investigations carried out in the project REslag clearly indicates the thermal und mechanical suitability of a slag-based packed bed for the use as inventory material for regenerator-type TES in CSP plants with air as HTF. In the oral presentation, all results will be presented and summarized to a total conclusion.

Abstract In this paper an investigation of a temperature control with mass flow manipulation for a distributed field of parabolic trough collectors is described. The dynamics of a collector loop are modelled by a set of nonlinear first-order hyperbolic partial differential equations. A computer model is set up to calculate the closed-loop response of the control. Simulations reveal that a feedforward controller acting as a load compensator can effectively reduce the effect of variations of the irradiance on the collector outlet temperature. A preliminary design of the PI-controller can be done with Ljapunow's “direct method”. The refinements of the controller settings via simulation have to work from an assumption on the accuracy of the model used for the feedforward controller.