Maike Johnson

Detailed Partial Load Investigation of a Thermal Energy Storage Concept for Solar Thermal Power Plants with Direct Steam Generation

Direct steam generation enables the implementation of a higher steam temperature for parabolic trough concentrated solar power plants. This leads to much better cycle efficiencies and lower electricity generating costs. For a flexible and more economic operation of such a power plant, it is necessary to develop thermal energy storage systems for the extension of the production time of the power plant. In the case of steam as the heat transfer fluid, it is important to use a storage material that uses latent heat for the storage process. This leads to a minimum of exergy losses during the storage process. In the case of a concentrating solar power plant, superheated steam is needed during the discharging process. This steam cannot be superheated by the latent heat storage system. Therefore, a sensible molten salt storage system is used for this task. In contrast to the state-of-the-art thermal energy storages within the concentrating solar power area of application, a storage system for a direct steam gene...

Experimental analysis of the performance of optimized fin structures in a latent heat energy storage test rig

Energy storage systems are a key technology for developing a more sustainable energy supply system and lowering overall CO2 emissions. Among the variety of storage technologies, high temperature phase change material (PCM) storage is a promising option with a wide range of applications. PCM storages using an extended finned tube storage concept have been designed and techno-economically optimized for solar thermal power plant operations. These finned tube components were experimentally tested in order to validate the optimized design and simulation models used. Analysis of the charging and discharging characteristics of the storage at the pilot scale gives insight into the heat distribution both axially as well as radially in the storage material, thereby allowing for a realistic validation of the design. The design was optimized for discharging of the storage, as this is the more critical operation mode in power plant applications. The data show good agreement between the model and the experiments for discharging.

Experimental testing of various heat transfer structures in a flat plate thermal energy storage unit

For solar process heat applications with steam as the working fluid and varying application parameters, a novel latent heat storage concept has been developed using an adaptation of a flat plate heat exchanger as the storage concept. Since the pressure level in these applications usually does not exceed 30 bar, an adaptation with storage material chambers arranged between heat transfer medium chambers is possible. Phase change materials are used as the storage medium, so that the isothermal evaporation of steam during discharging of the storage is paired with the isothermal solidification of the storage material. Heat transfer structures can be inserted into the chambers to adjust the power level for a given application. By combining the required number of flat plate heat exchanger compartments and inserting the appropriate heat transfer structure, the design can easily be adjusted for the required power level and capacity for a specific application. Within this work, the technical feasibility of this concept is proven. The dependence of the operating characteristics on the geometry of the heat exchanger is identified. A focus is on varying the power density by integrating conductive heat structures in the PCM.

Integration eines Latentwärmespeichers im Heizkraftwerk Wellesweiler

In dem Heizkraftwerk Wellesweiler von Steag New Energies, gezeigt in Abbild-ung 1, befindet sich unter den Abnehm-ern des Dampfes ein Folienwerk, wel-ches hohe Anspruche an die Qualitat und die Verfugbarkeit des Dampfes stellt. Deshalb wird immer, parallel zu einer mit einem Abhitzekessel kombi-nierten Gasturbine, einer von zwei unter anderem zur Besicherung dort aufge-stellten Heizkessel betrieben. Dieser lauft rund um die Uhr mindestens auf Minimallast, da er im Falle einer Storung der Turbine, die Sicherstellung der Dampfver-sorgung innerhalb von kurzer Zeit ubernehmen muss. Durch den Minimallastbetrieb ent-fallen die sonst notwendigen Anfahrzeiten. Deshalb erzeugt das Kraftwerk vor allem in den Sommermonaten uberschussige Warme. Durch den Einsatz eines Warmespeichers entfallt der Minimallastbetrieb eines zusatz-lichen Heizkessels zur Sicherstellung der Dampfversorgung, da der Speicher die kurz-zeitige Dampfversorgung im Falle einer Storung der Turbine ubernimmt, bis ein Dampf-kessel hochgefahren ist. Hierfur muss ein Latentwarmespeicher fur eine kurze Zeit (circa 15 Minuten) den erfor-derlichen uberhitzten Dampf produzieren. Bisherige Speicher wurden fur langere Ent-ladezeiten im Bereich 1-2 oder 6-8 Stunden, und mit entsprechend geringeren Leistung-en ausgelegt. Zudem wird in dieser Anwendung uberhitzter Dampf benotigt, der eben-falls im Latentwarmespeicher erzeugt werden soll. Bisherige Erfahrungen beschrankten sich auf die Erzeugung von Sattdampf.

PCM-Graphite Latent Heat Storage Systems for Industrial Process Heat Recovery

Increasing energy prices and shortage of fossil fuels lead to a growing interest in alternative energy sources. In combination with energy storage systems the generation of solar process heat can be provided independent from the weather leading for example to a cost efficient stabilization of power output. For this application latent heat storage units with phase change materials (PCMs) can be designed to store solar process heat within a narrow temperature interval utilizing the high storage density of the different PCMs. This is achieved using the latent heat of melting in the melting / solidification process, or the latent heat of re-crystallization in a solid / solid phase transition. However, this advantage can only be used in technical applications if the heat transfer in the PCM is sufficiently high. As most pure PCMs exhibit a low thermal conductivity (about 1 W/(m•K) or less), methods to improve heat transfer in PCMs have been under investigation for decades. The heat transfer in a PCM can be increased by addition of highly thermal conductive materials. Due to its superior properties - high thermal conductivity, good processability, and chemical inertness - graphite has distinct advantages for this purpose. Depending on the requirements of the respective application, various routes to combine PCM and graphite are used. For example, besides the fabrication of PCM/graphite composite materials, the increase of heat exchanger surface by highly thermal conductive graphite plates is a favorable method for large scale applications, in particular. Effective thermal conductivities up to 30 W/(m•K) have been realized. This paper gives an overview of actual and potential applications of PCM/graphite heat storage systems focusing on storage of solar heat for high temperature applications such as process heat generation and solar thermal power plants.