Slag as an inventory material for heat storage in a concentrated solar tower power plant: Final project results of experimental studies on design and performance of the thermal energy storage

Abstract

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.