Concentrated solar thermal power generation is becoming a very attractive renewable energy production system among all the different renewable options, as it has have a better potential for dispatchability. This dispatchability is inevitably linked with an efficient and cost-effective thermal storage system. Thus, of all components, thermal storage is a key one. However, it is also one of the less developed. Only a few plants in the world have tested high temperature thermal energy storage systems. In this paper, the different storage concepts are reviewed and classified. All materials considered in literature or plants are listed. And finally, modellization of such systems is reviewed.

State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization

Metasurfaces: From microwaves to visible

Thermal energy storage technologies and systems for concentrating solar power plants

The development of energy saving technologies is very actual issue of present day. One of perspective directions in developing these technologies is the thermal energy storage in various industry branches. The review considers the modern state of art in investigations and developments of high-temperature phase change materials perspective for storage thermal and a solar energy in the range of temperatures from 120 to 1000 °C. The considerable quantity of mixes and compositions on the basis of fluorides, chlorides, hydroxides, nitrates, carbonates, vanadates, molybdates and other salts, and also metal alloys is given. Thermophysical properties of potential heat storage salt compositions and metal alloys are presented. Compatibility of heat storage materials (HSM) and constructional materials have found its reflection in the present work. Data on long-term characteristics of some HSMs in the course of repeated cycles of fusion and solidification are analyzed. Article considers also other problems which should be solved for creation of commercial high-temperature heat storage devices with use of phase change materials.

High-temperature phase change materials for thermal energy storage

Recent developments in phase change materials for energy storage applications: A review

For future parabolic trough plants direct steam generation in the absorber pipes is a promising option for reducing the costs of solar thermal power generation. These new solar thermal power plants require innovative storage concepts, where the two-phase heat transfer fluid poses a major challenge. A three-part storage system is proposed where a phase change material (PCM) storage will be deployed for the two-phase evaporation, while concrete storage will be used for storing sensible heat, i.e., for preheating of water and superheating of steam. A pinch analysis helps to recognize interface constraints imposed by the solar field and the power block and describes a way to dimension the latent and sensible components. Laboratory test results of a PCM test module with ∼140 kg NaNO 3 , applying the sandwich concept for enhancement of heat transfer, are presented, proving the expected capacity and power density. The concrete storage material for sensible heat was improved to allow the operation up to 500°C for direct steam generation. A storage system with a total storage capacity of ∼1 MWh is described, combining a PCM module and a concrete module, which will be tested in 2009 under real steam conditions around 100 bars.

Development of a Thermal Energy Storage System for Parabolic Trough Power Plants With Direct Steam Generation

Molten chlorides are promising alternative thermal energy storage (TES) materials to be applied in concentrated solar power (CSP) plants owing to their higher thermal stability (stable at >800 °C) than the commercial TES materials – nitrate salt mixtures (decomposed at ∼550 °C). Higher operating temperatures of TES can increase efficiencies of thermal into electrical energy conversion for CSP power plants, but cause additional challenges, particularly increase corrosiveness of metal containers and structural materials. Corrosion rates significantly depend on concentration of corrosive hydroxide impurity in the molten chlorides. This paper presents an electrochemical method based on cyclic voltammetry (CV) to in-situ measure the concentration of the hydroxide impurity in molten MgCl2/KCl/NaCl (60/20/20 mol%) salts at 500–700 °C. Before each CV experiment, the concentration of the hydroxide impurity in the molten salts was measured via acid consumption (AC) measurements on the simultaneously collected salt sample. The results of CV and AC experiments show that the peak current densities obtained from CV are proportional to the concentrations of the hydroxide impurity obtained from AC in the studied system. The slopes of peak current densities vs. concentrations of the hydroxide impurity are determined, which compare well with literature data. This electrochemical method for monitoring the corrosive hydroxide impurity is expected to assist the corrosion control on containers and structural materials in the molten chlorides.

Electrochemical measurement of corrosive impurities in molten chlorides for thermal energy storage

Molten chloride salts are promising high-temperature thermal energy storage (TES) and heat transfer fluid (HTF) materials in concentrated solar power (CSP) plants due to their high stability limits (>800°C) but low prices, compared to the commercial nitrate salt mixtures. However, severe corrosion of structural materials in contact with chloride salts is one of the most critical challenges to limit their applications at elevated temperatures. First, this work presents the properties (melting point, vapor pressure, heat capacity, hygroscopy) and large-scale prices of single chlorides and chloride mixtures for selecting the suitable molten chloride salt mixtures in next generation CSP. Then, based on our previous and current work, it discusses hot corrosion behaviors and mechanisms of metallic alloys in molten chloride salts, as well as corresponding corrosion mitigation strategies to develop the technology towards commercial applications of molten chlorides in next generation CSP.

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Molten chloride salts for next generation CSP plants: Selection of promising chloride salts & study on corrosion of alloys in molten chloride salts

Latent thermal energy storage systems use the heat absorbed during melting and released during solidification of phase change materials (PCMs) From an energy efficiency point of view, PCM storage systems have the advantage that they operate with a small temperature difference between charging and discharging of the storage Also, these storage systems have high energy densities compared to sensible heat storages assuming such energy efficient operation PCMs considered for temperatures between 100 and 300°C are mainly anhydrous salts Applications include industrial process heat utilisation and solar power generation using direct steam technology However, there are heat transfer limitations on the storage design due to the low thermal conductivity of the salts Major approaches to overcome this inadequate heat transfer are the enlargement of the heat exchanger surface or the use of composite latent heat storage materials (CLHSM) This work selects the latter, where the properties of the high latent heat of the PCM and the good thermal conductivity of graphite are combined As a PCM, this work uses the equimolar composition of potassium nitrate (KNO3) and sodium nitrate (NaNO3) with a melting temperature of about 220°C There is a variety of potential preparation routes for CLHSM and these processes have a decisive impact on the interconnectivity of the graphite Generally a highly interconnected graphite matrix is desirable in order to achieve a high effective thermal conductivity We present in this paper the preparation of CLHSM from graphite and the eutectic KNO3-NaNO3 by the compression and the infiltration route using natural graphite flakes, ground expanded graphite particles and compressed expanded graphite plates This results in composites with a different level of interconnectivity of the graphite The composites are characterized in terms of their effective thermal conductivity as a function of temperature using the laser-flash method The impact of thermal cycling on the effective thermal conductivity, segregation, form stability and expansion of the CLHSM is also discussed The CLHSM show a considerable enhancement of the thermal conductivity compared with those of the single PCM

PCM-Graphite Composites for High Temperature Thermal Energy Storage

Storage of electrical energy is a key technology for a future climate-neutral energy supply with volatile photovoltaic and
wind generation. Besides the well-known technologies of pumped hydro, power-to-gas-to-power and batteries, the contribution of thermal energy storage is rather unknown. At the end of 2019 the worldwide power generation capacity from
molten salt storage in concentrating solar power (CSP) plants was 21 GWhel. This article gives an overview of molten salt
storage in CSP and new potential fields for decarbonization such as industrial processes, conventional power plants and
electrical energy storage.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cite.202000137

Thomas Bauer

Papers

Development of a Thermal Energy Storage System for Parabolic Trough Power Plants With Direct Steam Generation

Electrochemical measurement of corrosive impurities in molten chlorides for thermal energy storage

Molten chloride salts for next generation CSP plants: Selection of promising chloride salts & study on corrosion of alloys in molten chloride salts

PCM-Graphite Composites for High Temperature Thermal Energy Storage

Molten Salt Storage for Power Generation