• Energy Research

AbstractEnergy storage systems are essential to secure a reliable electricity and heat supply in an energy system with high shares of fluctuating renewable energy sources. Thermal energy storage systems offer the possibility to store energy in the form of heat relatively simply and at low cost. In concentrating solar power systems, for instance, molten salt‐based thermal storage systems already enable a 24/7 electricity generation. The use of liquid metals as heat transfer fluids in thermal energy storage systems enables high heat transfer rates and a large operating temperature range (100°C to >700°C, depending on the liquid metal). Hence, different heat storage solutions have been proposed in the literature, which are summarized in this perspective. Based on these, future technical advances are suggested such as reducing the liquid metal share in the heat storage, using waste material as storage medium or using liquid metal as heat transfer fluids not only in sensible, but in latent or thermochemical energy storage systems. The use of waste material in a packed bed configuration with 5% porosity, for example, can bring down the storage material costs to 2%–3% of the costs of a liquid metal‐only storage system. Furthermore, future application fields beyond concentrating solar power are proposed: supply of industrial process heat, waste heat integration and power‐to‐heat‐to‐power processes.

Abstract Liquid metals are promising candidates for highly efficient thermal receivers in concentrating solar power plants due to their excellent thermal conductivity. In the SOMMER (SOlar furnace with a Molten MEtal-cooled Receiver) facility at the Karlsruhe Institute of Technology (KIT), the cooling of a 10-kW thermal receiver by a lead–bismuth eutectic flow has been successfully demonstrated at high heat flux conditions in a solar furnace without any receiver damage. The experimental results show that peak heat fluxes of up to 4 MW/m2 can be achieved in the SOMMER facility and efficiently cooled with a liquid metal flow. In this study, the experimentally determined heat flux densities in the focal point of the solar furnace and the power input by the receiver are presented. In addition, the estimation of the thermal losses of the receiver are described and the results are discussed. The test results from three different shutter blinds positions as well as three different inlet temperatures (200 °C, 250 °C, 300 °C) and different mass flows are presented. During the measurement campaign in the summer of 2019 the Direct Normal Irradiance (DNI) values ranged from 670 W/m2 to 960 W/m2. Under these conditions maximum wall temperatures of 670 °C were measured. All in all, the results prove the excellent cooling ability of liquid metals under high heat flux conditions of up to 4 MW/m2.

Within the thermal energy storage (TES) initiative NAtional Demonstrator for IseNtropic Energy storage (NADINE), three projects have been conducted, each focusing on TES at different temperature levels. Herein, technical concepts for using liquid metal technology in innovative high‐temperature TES systems are dealt with. This approach implies some challenges; first, the unit costs are relatively large which makes a reduction of the mass inventory necessary. Second, the high thermal diffusivity, which is beneficial in any heat exchanger unit, reduces the efficiency in a single‐tank TES due to the fast degradation of the thermocline. These limitations can be overcome using a nonexpensive solid filler material, and, if properly designed, similar performance as in state‐of‐the‐art molten salt systems can be obtained, while maintaining the advantage of operating at temperatures well beyond their upper limit. Optimization strategies are presented for a reference case including transient behavior of the whole system. The sensitivity of multiple parameters, e.g., porosity, particle size, and influence of storage capacity regarding the discharge efficiency, is investigated.

Abstract Sodium has proven to be an efficient heat transport fluid in a central receiver system in the IEA-SSPS facility in Almeria in the 1980s with a 5 MWh th direct two tank storage system. In recent years the interest in liquid metals, particularly sodium, as heat transfer fluids for concentrating solar power has reawakened. However, an assessment of thermal energy storage options in a liquid metal-based concentrating solar power system has not been performed yet. In this paper sensible, latent and thermochemical systems, described in the literature and potentially suitable for a solar power plant using sodium, are investigated. As sensible systems, direct sodium two tank and one tank thermocline storage systems with filler are considered, as well as indirect molten salt systems. Latent systems include configurations with finned tubes, packed beds of phase change material capsules and active screw type systems. In addition, metal hydride dehydrogenation, ammonia dissociation and hydroxide dehydration are considered as thermochemical storage systems. The presented storage systems are discussed and compared on the basis of the following criteria: Storage medium cost, storage density, cycling behaviour, maturity level and suitability for sodium as heat transfer fluid. As a result, the direct sodium thermocline one tank storage with filler material represents a promising direct storage option. It could be further enhanced by a cascaded arrangement of phase change capsules. Moreover, the thermochemical storage systems based on ammonia dissociation or hydroxide dehydration are identified as best indirect storage options.

Abstract Concentrating solar power plants are currently working with Solar Salt and conventional Rankine steam power cycles with upper temperatures of 565 ° C. To achieve higher efficiencies, advanced power cycles are currently investigated (500–700 ° C). As heat transfer fluids, both molten sodium and three types of molten salt are considered in this study. For power tower plants, the heat transfer fluid is typically also the storage medium. This is the case for state-of-the-art commercial plants using molten salt, and past and present pilot plants using sodium. However, this work shows for both cases that a packed bed arrangement, where the heat transfer fluid is replaced by a filler material, may be a technically feasible and economically viable alternative. Furthermore, for sodium there are additional safety concerns related to having a large sodium inventory, which the packed bed arrangement can help alleviate. In this study, a 40 MWhth storage system with quartzite as filler material is numerically investigated with a one-dimensional model. The results are evaluated in terms of discharge efficiency, pumping power, storage cost and thermocline degradation during standby to assess the potential of this storage solution for future scientific investigations. The packed bed system with sodium shows slightly higher discharge efficiencies (96.8%) than with molten salt (95.2–95.7%) and also lower required pumping power. However, the thermocline region expands faster during standby due to the high thermal conductivity of sodium. The influence of porosity, tank diameter-to-height ratio and filler particle diameter is analysed in a parametric study. Highest discharge efficiencies are achieved for both sodium and molten salts with small tank diameter-to-height ratios and small filler particles. For sodium, low porosities are preferable, while for molten salts, high porosities lead to better discharge efficiencies.

Thermal energy storage systems for high temperatures >600 °C are currently mainly based on solid storage materials that are thermally charged and discharged by a gaseous heat transfer fluid. Usually, these systems benefit from low storage material costs but suffer from moderate heat transfer rates from the gas to the storage medium. Therefore, at the Karlsruhe Liquid Metal Laboratory, liquid metals are investigated as alternative heat transfer fluids for such heat storage systems, making use of the broad temperature range, in which the metals are in a liquid state, and their efficient heat transport capabilities. In this work, the design and construction of a high-temperature test rig using liquid lead is presented. The goal of the experiments is to demonstrate the operability of a pump, valves and measurement equipment at 700 °C in a challenging corrosive environment. Based on material pre-tests in stagnant lead at 700 °C, which are also shown in this study, aluminizing and pre-oxidation of the pipes and components are applied for enhanced corrosion protection.