Although facility scale thermal energy storage of sensible heat in the range of 200-550°C has achieved a high maturity, state-of- the-art approaches are still not very cost effective. An innovative storage concept is thus proposed that avoids the two major cost- driving factors of the concrete storage and 2-tank molten salt systems. First, the storage volume is comprised of low-cost sensible storage material such as concrete, natural stone or clinker bricks. These materials are several times cheaper than eutectic salt mixtures used in the 2-tank-storage system. Secondly, the system uses an intermediate air cycle, allowing for direct contact with the storage material. The necessary heat exchanger for transferring the heat from the primary oil loop to the intermediate air cycle consists of significantly less steel compared to the tube register inside the concrete storage. Dynamic models of the storage system have been implemented in a Matlab/Simulink environment to analyze its performance theoretically. The investigations show, that the overall performance and profitability of the storage system are mainly linked to the thermal efficiency and pressure drop of the heat exchanger, as well as the operation strategy. To demonstrate the feasibility of the storage concept and to investigate its performance characteristic under realistic conditions, a pilot scale test facility is set up.

The paper presents recent developments of alternative anhydrous alkali metal and alkaline earth nitrate and nitrite mixtures with low melting temperatures and high thermal stabilities for concentrating solar power (CSP) applications. First, known minimum melting temperatures of the relevant binary and ternary systems are reported. These systems have typical liquidus temperatures above 100 °C. The paper summarizes minimum melting temperatures of subsystems of the quinary reciprocal system Ca, K, Li, Na // NO2, NO3. Own measurements on the quaternary reciprocal system K, Li, Na // NO2, NO3 with a differential scanning calorimeter (DSC) and an optical melting point apparatus (MPA) are reported. Phase diagram details of this system with a minimum liquidus temperature of around 80 °C are presented. Finally, the thermal stability of nitrates, nitrites and minimum melting temperature mixtures are examined by thermogravimetry with coupled gas analysis (mass spectrometer).

Currently the major supply of renewable electricity (apart from hydropower) comes from Wind (>200 GW installed worldwide) and Photovoltaic (PV, >100 GW installed worldwide). In some electric grid there annual share is already about 20%, and strong cost reductions have been achieved. However the supply of electricity from wind or PV plants is intermittent and not necessarily matching demand, it is not dispatchable. This increases the need for electricity storage technologies and growing R&D activities are dedicated to improve current electricity storage technologies, to increase total capacity and decrease cost. Concentrating solar power plants, that convert solar energy to electricity via thermal conversion offer the option to include low cost thermal storage systems to shift electricity generation to hours of higher demand and/or lack of sunshine. This dispatchability makes them a very important part in electric grids with high shares of fluctuating renewable energies. CSP thus can help enabling a higher share of fluctuating over renewable energies (see figure 1). This contribution will focus on the current state of art of thermal storage technologies for concentrating solar power (CSP) Plants and include an overview on other current electricity storage technologies.

Thermal energy storage is a key technology for the commercialization of solar thermal power plants. This paper gives an overview of a coupled system comprised of concrete regenerators and latent heat storages for direct steam generation, as developed by the German Aerospace Center. Methodologies for an effective transient numerical description of the heat conduction processes inside the single modules and in the whole storage system are presented and their validity is proven by experiments. The presented process has a nominal system pressure of 105.6/80.0 bar for charging/discharging; the corresponding boiling temperatures are 315/295 degrees C. As storage material of the latent heat storage, sodium nitrate with a melting point of 305 degrees C is applied. With the presented models, a prediction of the storage system's temperatures, capacities, and effectiveness is possible. As a result, the design of a 1000-MWh(th) storage system is presented.

For sensible thermal energy storage (TES) in liquids in the temperature range from 250 degrees C to 550 degrees C, a mixture of 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3), known as Solar Salt, is commonly utilized. At the time of writing, TES technology for concentrating solar power is the major application. Although commercial systems have been demonstrated, there are still several material aspects to be investigated. In this paper we address thermophysical properties and metallic corrosion, as well as thermal decomposition processes. The paper reviews temperature dependent thermophysical properties of Solar Salt. Deviations among the authors of these properties were small for the density (+/- 1.5%), medium for the heat capacity (+/- 7%) and large for thermal diffusivity and thermal conductivity values (+/- 15%). The paper gives an overview of the various aspects of steel corrosion in molten alkali nitrate salts. From literature data, four steel type categories mainly depending on the temperature range are defined. The paper presents thermal stability examinations of Solar Salt and NaNO3 by isothermal lab-scale tests and thermal analysis measurements. Salt analysis in the isothermal test showed a steadily increasing oxide level at a constant nitrite to nitrate ratio. The result shows that there are kinetic differences in the first decomposition process with nitrite formation and the second decomposition process with oxide formation. The impact of the partial oxygen pressure on the decomposition temperature was examined by thermogravimetric measurements. Measurements show an improved stability limit for higher partial oxygen pressures. (C) 2013 Elsevier Ltd. All rights reserved.

Solar thermal power plants are a key technology for electricity generation from renewable energy resources. Thermal energy storage (TES) systems correct the mismatch between the solar supply and the power demand. TES makes it possible to meet the intermediate load profile with dispatchable power, a benefit that has a high value to power utilities and that gives concentrating solar power (CSP) technology an edge over photovoltaic and wind power. Hence, TES is a key technology for solar thermal energy utilization with growing present and future importance. This chapter focuses on material aspects of alkali nitrate salts. They include thermal properties, thermal decomposition processes, steel corrosion issues, and phase diagrams of multicomponent salt systems. In addition, two CSP applications using molten nitrate salts as sensible and latent TES are discussed.

This presentation is about the development of a thermal energy storage system for CSP. It describes the selection of the salt system’s components by a combination of a theoretical and experimental approach based on economic, ecological and technical critera. Having found the most appropriate components, the next chalenge is to mix the components in a way that the system can be used in a most efficient way, by lowering the onset of the liquidus temperature. Concerning this, two approaches will be shown: One approach is a systematic approach using high-throughput experimental methods and phase diagram fitting. This approach will be compared to an alternative experimental method which significantly reduces the experimental time and which is being used for the lower temperature range. The advantages and disadvantages of both approaches are discussed.

High-temperature PCM is a key component for storing latent heat. This is necessary for efficient energy storage in connection with the two-phase heat transfer fluid water/steam. Due to the low thermal conductivity of the nitrate salts used for high-temperature PCM storage, it is necessary to increase the heat transfer rate to meet the necessary power density demands for specific applications. A design for heat transfer enhancement using radially finned tubes has been developed by DLR and applied in a 700 kW h PCM storage demonstration module.The 700 kW h PCM storage has been tested successfully in a combined storage system for DSG. The operation of this PCM storage module for evaporating water in constant and sliding pressure mode was succeeded. Three different operation modes regarding flow for discharging the PCM storage have been tested successfully, showing potential for future cost reductions for the complete storage system. Here, either only the recirculation pump or even the complete circulation cycle including the steam drum could be eliminated. Good system operability of the systems was achieved.Furthermore, a new design based on extruded aluminum profiles, aiming at further cost reduction, has been developed by the company F.W. Broekelmann Aluminiumwerk GmbH & Co. KG in cooperation with DLR. Various fin designs have been analyzed with the finite element analysis software ANSYS(R) and optimized for a better temperature/heat distribution. The final design has been analyzed in a lab-scale PCM storage module. Test results of different operation parameters and thermo-mechanical examinations are shown and compared to theoretical analyses of the new design. (C) 2012 Elsevier Ltd. All rights reserved.

Two tank storage systems using molten salt represent today's state of the art in energy storage for concentrating solar power (CSP) plants. This concept shows a limited potential for further cost reductions, since the capital costs are dominated by the expenses for the salt inventory. The application of solid storage materials represents a promising approach to reduce capital costs. While this approach avoids also the risk of freezing and lessens corrosion problems, the efficiency of the heat transfer between the heat transfer fluid (HTF) and the solid storage medium is crucial. This paper introduces the CellFlux concept, which uses an intermediate closed air loop to transfer energy between the HTF and the solid storage material. A modular concept is chosen to optimize the size of the air flow channels. An initial project will provide the fundamentals needed to design a CellFlux storage unit. The feasibility will be proven by a 100 kW/500 kWh pilot storage module.

An innovative storage concept is proposed here that uses an intermediate air cycle to transfer heat from a primary working fluid to a cost effective solid sensible storage material. The intermediate working fluid can be brought into direct contact with the storage material, allowing high heat transfer rates. The intermediate working fluid receives the energy from the primary working fluid through a heat exchanger. The proposed storage system has a different dynamic behavior than state of the art two-tank molten salt storage system. Hence, dynamic models of the storage system and a SEGS-like solar thermal power plant have been implemented in a Matlab/Simulink environment to analyze its performance theoretically. A 75 kW pilot scale experimental storage system has been set up to provide data for model validation. The poster describes the implementation of the component models and the comparison with experimental data.

Neben dem Ausbau des elektrischen Netzes, dem einfachen Abschalten von Windradern oder der Integration gut regelbarer (erneuerbarer) Kraftwerke, konnen Energiespeicher zur Integration erneuerbaren Stroms beitragen. Sie sind in der Lage, momentan zur Verfugung stehenden Strom bis zur seiner Nutzung zu speichern. Energie kann im Allgemeinen in Form von Elektrizitat (z. B. in Batterien) oder in Form von Warme und Kalte gespeichert werden. Letzteres bezeichnet man als thermische Energiespeicherung.