High temperature latent heat storages are being developed for both concentrating solar thermal power applications as well as integration in industrial processes. One of the concepts being developed is an extended finned-tube in a shell-and-tube assembly. This concept can be used at high pressures for steam applications and be built at a large scale. The design of the extended fins allowing for independent thermal expansion of the steel tubes and the aluminum fins with a physically possible assembly has not thus far been optimized. Due to the large fin surfaces necessary for storing large amounts of heat, conventional finned-tube assemblies have to date not been applicable for thermal energy storage systems. Designs using spring steel clips on axial fins have been proven, using conservatively high numbers of clips. In this paper, various fin and tube diameters with spring steel clips as well as other mounting methods are compared. Experiments were conducted to analyze the mechanical strength of the assembly; these are described and the results discussed. In addition, two assembly methods were tested using the same fin geometry and testing environment, allowing for a thermodynamic comparison of the assemblies. The tests have shown that while the steel clips allow for the best heat transfer, the crimping method has a higher bond strength. These results can be used for reducing costs and optimizing design of high temperature latent heat storages.

This paper gives an overview on the SolarPACES guideline for Bankable STE Yield Assessment which was published in January 2017 by SolarPACES. After summarizing the history of the standardization activity behind this document, on overview on the content and its key elements is given. The paper ends by addressing the various players involved in STE yield assessment and how they can make benefit from applying this guideline in their day-to-day work. The document is available for free download from SolarPACES at http://www.solarpaces.org/yield-analysis-guideline.

This paper provides an overview on an important step towards a SolarPACES guideline for CSP yield calculation. With the increasing number of CSP installations, standardization becomes more and more important for further reduction of costs and increase in quality. Yield calculation is a key issue throughout all phases of project development and throughout most of the involved players. Due to the need for more complex process models for CSP compared to other renewables like PV or wind, the yield calculation procedure is more demanding. Uncertainties in the process are covered by additional but partially unnecessary risk surcharges since systematic approaches for avoiding expensive redundancies in risk buffers are not available. It is the main motivation of the mentioned CSP yield calculation guideline to overcome this situation by providing a detailed methodology for yield calculation. A first comprehensive draft version has been compiled and will be subject of discussion in the SolarPACES working group guiSmo. The following sections illustrate the motivation for the guideline, the contents at a glance, as well as expected benefits for selected players.

Concentrated solar power plants with integrated storage systems are key technologies for sustainable energy supply systems and reduced anthropogenic CO2-emissions. Developing technologies include direct steam generation in parabolic trough systems, which offer benefits due to higher steam temperatures and, thus, higher electrical efficiencies. However, no large scale energy storage technology is available yet. A promising option is a combined system consisting of a state-of-the art sensible molten salt storage system and a high temperature latent heat thermal energy storage system (LHTESS). This paper discusses the systematic development and optimization of heat transfer structures in LHTESS from a technological and economic point of view. Two evaluation parameters are developed in order to minimize the specific investment costs. First, the specific product costs determine the optimum equipment of the latent heat storage module, i.e. the finned tube. The second parameter reflects the interacting behavior of the LHTESS and the steam turbine during discharge. This behavior is described with a simplified power block model that couples both components. (C) 2015 Elsevier Ltd. All rights reserved.

Thermal energy storage (TES) is capable to reduce the demand of conventional energy sources for two reasons: First, they prevent the mismatch between the energy supply and the power demand when generating electricity from renewable energy sources. Second, utilization of waste heat in industrial processes by thermal energy storage reduces the final energy consumption. This review focuses mainly on material aspects of alkali nitrate salts. They include thermal properties, thermal decomposition processes as well as a new method to develop optimized salt systems.

Packed beds using air at atmospheric pressure as heat transferringmedium are the most cost effective systems for sensible heat storage. The basic idea of the CellFlux concept is to apply this concept also for liquid and/or pressurized primary HTFs by the introduction of an intermediate working fluid cycle. A heat exchanger is used for transferring energy between the primary HTF and the intermediate air cycle which eventually transfers the energy to a packed bed. The CellFlux concept can be implemented by using standard components. Essential is the minimization of efficiency losses resulting from the circulation of the air as well as the heat transfer processes within the heat exchanger and the storage volume. Some example cost estimations for the heat exchanger are given. The feasibility of the CellFlux concept has been proven by a pilot scale test facility operated at a maximum temperature of 380°C and 100kW. A novel approach promising further cost reductions has been applied by realizing a horizontal flow direction. Results from the theoretical and experimental analysis of the CellFlux concept will be presented. Distinctive for the CellFlux concept is the flexibility regarding working fluid (thermal oil, molten salt, pressurized water, CO2), temperature range (0-800°C), power (kW-multi MW) and storage medium (rocks, clinker bricks, concrete). This allows a wide range of applications. An example for application in combined heat and power will be given.

The state-of-the-art latent heat energy storage system is equipped with aluminum fins at the heat exchanger pipes in order to compensate the low thermal conductivity of the phase change material (PCM). The necessary amount of fins is directly coupled to the capacity of the storage system, what makes larger systems expensive. The PCMflux concept is developed in order to realize both a controllable and a possibly more cost effectivelatent heat storage system. These aims are addressed by separating the storage material from the heat exchanger. As a result, the PCM can be moved over the heat exchanger. The PCM thereby is macroencapsulated into containers. These pass the heat exchanger while the PCM inside changes phase. In order to improve thermal contact between containers and heat exchanger, Hitec® as an intermediate fluid is used to avoid poor dry contact. In this article,an experimental setup is described to examine the heat transfer through thin layers of molten Hitec®. The results of the experiments show an increased heat transfer by a factor of 9.9. This improves feasibility of the PCMflux concept significantly.

The CellFlux concept is a new sensible storage system where thermal energy from a primary working fluid (HTF) is transferred to an intermediate working fluid (IWF) which flows in direct contact through a cost effective packed bed solid sensible storage material. The IWF is kept in a closed loop, conveyed by a fan. It is possible to combine multiple storage modules to a large system. The paper presents the results of the analysis of a single storage cell. Various options for the combination of storage cells in a storage unit are described; the importance of the concept selected for the integration of the storage unit into the power plant is shown. The combination of CellFlux with molten salt HTFs is introduced and results from cost estimations are given.

Thermal energy storage is essential to increase the efficiency of industrial processes by waste heat utilization and to enable future solar electricity generation by solar thermal power plants. Taking into account the need to reduce the emission of CO2 and to save conventional energy both processes are indispensable.However with the (direct) storage media used so far the range of applications is limited. Currently organic media are used which are not appropriate at high temperatures due to high vapor pressures, low flash points and low thermal stabilities. Those challenges can be tackled by using inorganic media. Yet applications are limited so far because state of the art inorganic storage materials cannot be used below 100 degrees C requiring expensive anti-freeze systems.This paper presents a novel storage medium combining advantages of both organic media and state of the art inorganic media. This storage medium was developed by a novel method [1] and was assessed by the investigation of the most essential thermal properties, such as the heat capacity, viscosity, phase diagram around the eutectic point as well as the thermal stability. To get an idea about the thermal stability, state of the art TGA measurements were performed which suggested a higher thermal stability as compared to organic media. (C) 2015 The Authors. Published by Elsevier Ltd.

Solar thermal power plants are a promising option for future solar electricity generation. Their main advantage is the possibility to utilize integrated thermal storage capacities, allowing electricity generation on demand. In state of the art solar thermal power plants, two-tank molten-salt thermal energy storages are used. Significant cost reductions are expected by using thermocline thermal energy storage by storing the liquid storage material inside a single tank when compared to a two tank storage system. By embedding a low cost solid filler material inside the storage tank further cost reductions can be achieved. In earlier studies [1, 2] several potential filler materials have been investigated. In these study quartzite turned out to be a promising candidate due to its satisfying thermal stability and availability. At a temperature of approx. 573°C the crystal structure of quartzite changes from trigonal α-quartz phase to the hexagonal β-quartz phase [3]. This quartz conversion results in a volume change [4] that may cause cracking of the quartzite crystals due to weight loads in a packed bed. Since these thermal tests of the study mentioned were limited to 500°C this dunting was not considered. Thus, despite of the published studies there is a need for further, more detailed analysis. One trend in today’s development of solar thermal power plants is to use molten salt as storage material and heat transfer fluid at operating temperatures of 560°C and above. Accordingly, the quartz inversion might limit the applicability of quartzite as a filler material at elevated operating temperatures. Due to this concern, an investigation has been started to investigate the utilizability of natural rocks as low cost filler materials. In the first phase of this investigation a comprehensive literature survey was conducted. Based on this study, magmatic and sedimentary rocks turned out to the most promising rock classes for this application. For the further investigation, basalt was chosen as a suited representative for magmatic and quartzite for sedimentary rocks. In lab-scale tests, these candidate materials were investigated with respect to their: • Calcite content • Thermal stability up to 900°C in air • Thermal stability up to 560°C in molten salt • Cyclic stability between 290°C and 560°C in molten salt • Specific heat capacity up to 600°C In this paper the results of these investigations are presented and future activities are outlined.

The PCMflux concept as a dynamic latent heat storage system avoids a decrease of heat flux while discharging. This drop is typical for state-of-the-art latent heat storage units with locally fixed storage material. In the PCMflux concept, the storage material is physically separated from fixed heat exchanger pipes and moved towards them. The forward velocity of the storage material is directly related to the resulting heat flux of the storage system. Modifying this velocity, the heat flux can be controlled accurately. The correlation of these two factors is described by the dimensionless number . This number depends on different influence factors such as design parameters and properties of the involved materials. is developed and discussed. It turns out to be a crucial parameter for both designing and controlling the PCMflux system. This paper focusses on the description of the PCMflux concept and the derivation of describing the correlation between forward velocity and heat flux.

Thermal storage technologies are a key component for increasing energy efficiency and assisting in the integration of regenerative energy sources in the energy market. In latent heat energy storage, the storage material changes phase as energy is charged into the storage. This makes use of the large amount of enthalpy that is absorbed or released during phase change of a material. Since a storage unit is a link in the chain between supply and demand of heat, it has to be adapted for each particular application. The aspect that is most difficult to adapt is the required power level. The inherent heat transfer in phase change materials (PCMs) is typically low and a limiting factor. Therefore, heat transfer structures have been developed that increase the surface area between the PCM and the heat transfer fluid. In proven design concepts for latent heat storage, a tube bundle is immersed in PCM as a heat exchanger between the storage medium and the heat transfer fluid. In order to increase the power level, the heat transfer surface is increased with exterior fins on the tubes.A new concept that was designed, tested and analyzed at DLR is a storage adaptation of a flat plate heat exchanger. The design tested replaces the secondary heat exchanger medium with isolated chambers of storage material. The primary heat transfer fluid flows through the one side of the flat plate thermal storage, transferring heat across the steel encasements to the storage medium. A flat plate lab storage unit was built to prove this design concept. The testing of this storage unit, including various temperature gradients, flow rates and the insertion of heat transfer structures, is reported in this paper. (c) 2014 The Authors. Published by Elsevier Ltd.

Systems using air at ambient pressure as a working fluid and solid media like natural stones or bricks as storage materials represent the most cost effective option to store sensible heat at medium and high temperatures. The application of this storage concept also for liquid working fluids like thermal oil or molten salts is the basic idea of the CellFlux storage concept. Here, an intermediate air cycle transfers thermal energy between a heat exchanger and a solid medium storage volume. The development of the CellFlux concept comprises various stages: in the initial phase, options for the main subcomponents are identified and evaluated. The integration concept of the CellFlux storage unit into the power plant is essential for the success of the concept. The size of the heat exchanger strongly depends on the average temperature difference between air and working fluid. The costs for the heat exchanger are power dependent and dominate the total capital costs of the CellFlux storage unit. In order to demonstrate the feasibility of the CellFlux, a storage unit with a storage volume of 30m3 for operation with thermal oil at up to 400̊C was designed.

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.

One possibility to increase the efficiency and thus economic viability of solar thermal power plants is to increase their operating temperature. This approach demands the substitution of the state-of-the-art heat transfer fluid (HTF) that limits the operating temperature to roughly 400°C. Promising heat transfer fluids for future applications are molten salts or water/steam. If water/steam is used as HTF, the feed-water from the power block is fed to the solar field (SF) and directly evaporated and superheated. This process is called direct steam generation (DSG). A recent study [1] has pointed out that the economic potential of the DSG process is utilized only, if the SF design is simplified and a competitive thermal storage is available. Thus, an R&D project was launched in Germany to develop a complete storage system covering the energy of the evaporation as well as of the pre- and superheating section. It consists of a phase change material (PCM) storage for evaporation and a molten salt storage for pre- and superheating. One specific feature of superheated steam is its changing specific heat capacity with temperature. Using molten salt as storage medium with a nearly constant specific heat and the application of an obvious simple heat exchange would lead to an inefficient process. A significantly reduced live steam temperature and thus power block efficiency during discharge would be the results. Furthermore, the specific storage density of the molten salt system would be reduced too. In this paper this effect will be discussed in more detail. The consequences for the storage system will be discussed and solutions of the developed processes for the integration of such storage into a DSG power plant will be presented that reduce or overcome the mentioned restrictions.

Yield analysis is a crucial task during project deployment of solar thermal power plants. Currently, many different modeling approaches and computer tools for yield analysis are used. Within the SolarPACES project guiSmo, aiming at the development of guidelines for the annual yield prediction of solar thermal power plants, relevant effects for the steady-state modeling of plant sub-systems have been identified [1].The target of the ongoing project phase is the development of general definitions for all relevant effects. Furthermore, suitable modeling approaches have to be identified and described unambiguously. For every effect, the impact on the predicted electricity yield is estimated in order to assess the significance of the considered effect. Since the most significant effects have to be considered in a subsequent uncertainty analysis, reasonable model and parameter uncertainties have to be defined, too. Finally, default model parameter for state-of-the-art components or subsystems will be edited.Beside the present status of the project, this paper presents investigations on the effects of the optical losses of the sub-system collector field on the annual yield. These effects are namely the peak optical efficiency, incidence angle modifier (IAM), shading and end losses. For each effect, a precise definition is presented and relevant modeling approaches are identified and their pros and cons are discussed. To investigate the influence of these effects, a reference solar thermal power plant with parabolic troughs is defined. The annual yield is simulated for this reference system investigating the identified modeling approaches and assessing their significance. (C) 2013 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/)

With the success of CSP technology in the last years more players are active in the market, inducing the need for harmonization of technical terms and methodologies. The mission of the SolarPACES "guiSmo" project which was started in 2010 is to develop a guideline for CSP yield analysis [1]. Activities carried out so far have shown that people have different understandings of many terms used in daily CSP practice. Especially for the development of guidelines, the essential terms need to be clearly defined in order to avoid inconsistencies within the same project. A first version of a nomenclature has been compiled by the "guiSmo" team and will undergo final discussion. The aim is to come to a harmonized version by Summer 2013 which will then be presented at the ASME Energy Sustainability conference. The compilation so far includes essential definitions of terms like direct normal irradiance, incident angles, heat flows, and efficiencies on a system level. The definitions presented will be discussed together with existing standards like the ISO 80000 (physical quantities and units of measurement), the ISO 9488 (Solar energy-vocabulary) and other relevant sources. Although the list of terms is primarily put together for the work in the "guiSmo" project, it might serve as a basis for standardization in the official councils. An international group of solar experts is involved in the preparation of the document in order to ensure high quality and international support for the results.