Natural convection in high temperature flat plate latent heat thermal energy storage systems

The impact of natural convection on melting in high temperature flat plate latent heat thermal energy storage systems is studied with an experimentally validated numerical model in a parameter study with various widths and heights of enclosure dimensions. The storage material is the eutectic mixture of sodium nitrate and potassium nitrate (KNO3-NaNO3). The investigated half widths of the rectangular enclosures between two heated vertical flat plates are 5, 10 and 25 mm; their heights are 25, 50, 100, 200, 500 and 1000 mm. These parameters result in low to very high aspect ratios between 0.5 and 40 and Rayleigh numbers between 1.2 · 10^4 and 1.6 · 10^6. The results are evaluated by dimensional analysis to find general dependencies between enclosure dimensions and natural convection occurrence and strength. To assess the influence of natural convection on the heat transfer enhancement, the convective enhancement factor is introduced. This non-dimensional number is defined as the ratio of actual heat flux by natural convection to a hypothetical heat flux by conduction only. The central findings of the present work are correlations for the mean convective enhancement factor and the critical liquid phase fraction for natural convection onset that are valid for a wide parameter range. The results indicate that heat transfer enhancement due to natural convection increases with greater widths and smaller heights of storage material enclosures. Hence, the vertical segmentation of high enclosures into smaller ones should be considered to enhance heat transfer during charging

Natural convection melting in a high temperature flat plate latent heat storage system: Parameter study of enclosure dimensions

The impact of natural convection melting in a high temperature flat plate latent heat thermal energy storage system is studied numerically. The storage material is the eutectic mixture of sodium nitrate and potassium nitrate (KNO3-NaNO3). A parameter study on storage material enclosure dimensions is conducted for various widths and heights. Low to very high aspect ratios between 0.5 and 40 and Rayleigh numbers from 1.2·10^4 to 1.6·10^6 are obtained. The liquid phase fraction evolution with time is scaled to non-dimensional form and the impact of natural convection on the heat flux is shown with a convective enhancement factor that is the ratio of heat flux to a hypothetical heat flux only by conduction. The results can be used for future design optimizations considering the effect of natural convection

Development of a high temperature and high power PCM storage for standby operation

Thermal energy storages integrated in industrial processes allow for higher degrees of energy flexibility and reduction of fossil fuel use. The combination of latent heat thermal energy storage systems with water/steam processes can lead to optimized thermal gradients and therefore good system efficiencies. Storage units and systems have been proven at pilot scale. The integration in industrial processes remains a challenge, due to the size of the systems as well as the hurdles in design, permitting and build. This thesis encompasses the parametrization, design, build, integration and initial operation of a megawatt-scale latent heat thermal energy storage unit, producing superheated steam for an operating cogeneration plant and industrial process customers. The storage unit can produce superheated steam at more than 300 °C and 25 bar at a mass flow rate of 8 t/h for at least 15 minutes. The data of the dispatchable production of superheated steam for more than 20 minutes show the thermal power and capacity at 5.5 MW and 1.9 MWh. In order to develop this storage system and show the feasibility of the novel aspects of producing superheated steam in once-through operation, providing megawatt-scale thermal power and capacity, and integrating it into an operating system, various steps are involved. These steps are a combination of upscaling the thermal power and capacity, designing for a feasible build and for given system requirements, and system integration development. The upscaling in thermal power results in the development of a very dense fin structure and tight tube-spacing. The upscaling in capacity requires the development of a design model with capabilities for analysis of thermal losses and possible non-ideal flow through the headers, as well as more banal aspects such as transportability and weight considerations as well as physical filling capability with the pelleted salt during commissioning, and accessibility for permitting bodies. System integration in an operating system considers charging and discharging with the available components and maximization of benefits to the plant. This integration, requiring not just a design and optimization of the storage technology itself but of the whole system, is a novel point of view for the development of latent heat storage systems. It is not critical that the storage itself provide all of the parameters, but that the system integration makes this feasible

Superheated steam production from a large-scale latent heat storage system within a cogeneration plant

During phase change, phase change materials absorb or release latent heat at a nearly constant temperature. Latent heat thus can be stored and integrated with evaporation/condensation systems such as steam generators within a relatively narrow range of operating temperature. Storage units and systems have been proven at pilot scale but none to-date have been integrated in industrial processes. This remains a challenge, due to the size of the systems and to hurdles in design, permission and build. Here we integrate a megawatt-scale latent heat storage into a cogeneration power plant in Wellesweiler-Neunkirchen, Saarland, Germany. The storage produced superheated steam for at least 15 min at more than 300 °C at a mass flow rate of 8 tonnes per hour. This provided thermal power at 5.46 MW and results in 1.9 MWh thermal capacity. Our study demonstrates the feasibility of using latent heat storage in the industrial production of superheated steam

Process integration of thermal energy storage systems – Evaluation methodology and case studies

As a key tool for decarbonization, thermal energy storage systems integrated into processes can address issues related to energy efficiency and process flexibility, improve utilization of renewable energy resources and thus reduce greenhouse gas emissions. However, integration of these systems is dominated by the variety of potential processes in which the storage technologies can be deployed as well as the various benefits they deliver. Therefore, the requirements for thermal energy storage systems vary greatly depending on the chosen application, just as the systems themselves have different capabilities depending on their technical principles. This paper addresses this issue by developing a systematic methodology that approaches the challenge of characterizing and evaluating thermal energy storage systems in different applications in three concrete steps. To begin, a set of guidelines for process analysis has been created to disclose process requirements for storage integration. The methodology continues by explicitly defining the system boundary of a thermal energy storage system, as well as addressing technical and economic parameters. Finally, the approach concludes by determining the benefit of an integrated thermal energy storage system to an application and examines how key performance indicators vary based on the perspectives of different stakeholders. Within this work, the methodology is then applied to two case studies of high-temperature storage in concentrating solar power and cogeneration plants. Also introduced are the concepts of retrofit and greenfield applications, which are used to clarify differences between integrated storage systems. The paper shows how such a systematic approach can be used to consistently analyse processes for storage integration, facilitate comparison between thermal energy storage systems integrated into processes across applications and finally grasp how different interests perceive the benefits of the integrated storage system. This type of systematic methodology for technology integration has not been previously developed and as such, is a novel and important contribution to the thermal energy storage community. In the long term, this work builds the basis for a discussion on benefits of thermal energy storage system integration with diverse stakeholders including storage system designers, process owners and policy makers.This work has been partially funded by the German Federal Ministry of Economic Affairs and Energy in the framework of the TESIN project (03ESP011) and the THESAN project (03ET1297). This work has also been partially funded by the Ministerio de Economía y Competitividad de España (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors at the University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. Jaume Gasia would like to thank the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2018 FI_B2 00100). The authors are responsible for the content of this publication

Design, build and operation of the CHESTER system

Within the framework of the European Union's H2020 project CHESTER, a first laboratory scale Carnot Batterie prototype has been developed and built. This so-called CHESTER-system expands the functionality of a Carnot Batterie by coupling it with a renewable district heating supply and a seasonal thermal energy storage. The CHESTER-system built at DLR Stuttgart is the first of its kind to use a Rankine-based heat pump cycle for the transformation of electricity into heat and an organic Rankine cycle for the re-transformation of heat into electricity, which are connected to each other as an exergetically optimized overall system via a cascade of high-temperature latent heat storage and 2-tank pressurized water storage. The technological design of the plant towards the combination with a seasonal heat storage is unique. It allows the plant to operate in a completely decoupled mode in terms of time and a partially decoupled mode in terms of energy. In sum, this results in a storage and energy management system that is able to absorb electricity and heat independently of each other in terms of time and quantity during charging, store both - heat even seasonally - convert both into each other as needed, and provide both again independently of each other in terms of time and quantity in a completely demand-oriented manner. This contribution will provide insight into the design process, the technical challenges faced in building the system, and will show for the first time the operational results obtained during experimental operation

Commissioning of high temperature thermal energy storage for high power levels

A high power and capacity PCM storage unit has been integrated into a heat- and power cogeneration plant in Saarland, Germany. This storage will act as an intermediate back-up to a heat recovery steam generator and gas turbine and is therefore situated in parallel to this unit, also between the feedwater pumps and the steam main. The steam required is superheated at 300°C and 26 bar, resulting in a maximum thermal power of 6 MW. The storage needs to provide a minimum capacity of 1.5 MWh. Operation of this storage unit will increase efficiency and decrease fossil fuel use by reducing the use of a conventional back-up boiler. The filling of the unit with ~32 t of storage material in conjunction with the commissioning of the storage unit is in process and initial data has been acquired. These data will be analyzed and presented. This is part of the project TESIN, funded by the German Ministry of Economic Affairs and Energy

Design and build of a novel dual-tube PCM storage unit

To allow for a de-coupling of the charging and discharging parameters, a dual-tube PCM storage unit using extruded aluminium fins was designed and built. This concept allows on the one hand for completely different media and flow parameters to be used, it also allows for temporally independent charging and discharging, simplifying system integration and control. This is a benefit for industrial systems, in which differing systems can be coupled through a storage unit, allowing for serial or parallel charging and discharging, with differences in power or capacity balanced by the storage unit. A 160 kWh storage unit has been analysed, designed, built and will be experimentally tested by the conference

TESIN Schlussbericht

Der vorliegende Schlussbericht dokumentiert die Weiterentwicklung von Hochtemperaturwärmespeichern für Industrieprozesse. Das Projekt umfasste die Systemanalyse von Elektrostahlwerken und Heiz(kraft)werken und die Auslegung von Wärmespeichern hierfür, sowie die Entwicklung, Auslegung, Bau, Integration und Inbetriebnahme eines Hochtemperatur-Latentwärmespeichers für ein laufendes Heizkraftwerk. Hierbei wurden Erkenntnisse und Bauweisen aus vorhergehenden Projekten wie dem ITES Projekts (FKZ 16UM0064), und dem zeitweise parallel laufenden DSGSTORE Projekt (FKZ 0325333) angewendet und weiter entwickelt. Die Ergebnisse zeigen, dass durch die Weiterentwicklung die Produktion von überhitztem Dampf bei hohen Temperaturen durch Latentwärmespeicher sowie die Integration und Aufbau nach aktuellen Richtlinien möglich ist. Zudem wurde gezeigt, dass eine Integration in verschiedene Prozesse eine Energieeinsparung ermöglichen würde, die aber bei einer nachträglichen Integration deutlich geringer ausfällt als bei einer Integration während der Prozessentwicklung. Die beim Bau des Speichers gewonnenen Daten und Erkenntnisse sollen zukünftig die Integration von thermischen Energiespeichern in Industrieprozesse erleichtern