Stefan Zunft

Jülich Solar Power Tower—Experimental Evaluation of the Storage Subsystem and Performance Calculation

Background. Storage technology based on solid media heated in direct contact – so called regenerators – are well suited to promote the market introduction of solar central receiver plants with air receivers. However, starting from existing technologies, several design issues need to be addressed. Method of Approach. A test campaign was performed at the Solar Power Tower Julich, an experimental central receiver plant, to experimentally verify the functionality and to quantify the performance of the plant’s storage subsystem. To this end, a gas burner used during commissioning of the plant, was reactivated and used to run a series of operation sequences. Computer simulations have been set up and applied to retrace the storage behaviour to confirm the validity of the underlying models and to gain further insight into the relevant phenomena. Results. The test results confirm the full functionality of the storage subsystem, the ability to perform cycling at high discharge heat rates and relatively low heat losses, supporting the view that the technology represents a promising basis for up-scaled implementations. Conclusions. Measurement data and simulation results are in good agreement, confirming the maturity of existing design tools.

Moving Bed Heat Exchangers for Use With Heat Storage in Concentrating Solar Plants: A Multiphase Model

Heat storage based on particulate materials is a promising option to provide a demand-oriented electricity production with utility-scale solar power plants. For energy storage discharge, a moving bed heat exchanger is considered and its design is investigated. As a basis for a flexible design tool, a multiphase model based on the Eulerian continuum approach was set up to describe the bulk flow and the thermal performance. The model was applied to an example heat exchanger layout, and the simulation results were compared with an empirical model, confirming the validity of the approach. Initial parameter variations identify the key parameters and their effect on the thermal performance.

Theoretical and experimental investigation of a Moving Bed Heat Exchanger for Solar Central Receiver Power Plants

A Moving Bed Heat Exchanger for heat extraction from solar heated granular materials is investigated with respect to flow behaviour. To overcome limitations of existing empirical models, a numerical CFD model is established and parametrised with the help of experiments. Parametric studies are performed to quantify the effect of inlet velocities on the velocity field. A good agreement with an empirical model is found. Also, a comparison with PIV measurements confirms its validity, making it a solid basis for future design work.Sensible solid media storage based on particulate materials is a promising concept for solar central receiver systems. The bulk material can be used as heat transfer and storage medium at high temperatures. A new heat exchanger concept based on the principles of a moving bed has been developed deploying a compact design, a good part-load behaviour and low parasitic loads. Numerical investigations applying a multiphase CFD approach are conducted in order to identify the determining factors on heat transfer. In a subsequent design study, the tube shape as well as the tube arrangement is varied in order to identify a design improving the thermal performance of the device. A circular tube geometry is found to be the most economic one compared to rhombs or ovals. A narrow tube arrangement taking into account the flow properties of the bulk offers a potentially higher thermal efficiency and an even more compact design of the heat exchanger.

Using concrete and other solid storage media in thermal energy storage (TES) systems

Storing sensible heat in solids allows the highest storage temperature levels and avoids the problem of high vapour pressure of liquid media. A wide choice of materials is usable and can deliver economically attractive solutions. Commercially available today are regenerator-type storages, where a gaseous heat transfer fluid, such as flue gas or air, is in direct contact with a solid storage medium and exchanges heat as it flows along a flow path through the storage medium. Different advanced concepts, like concrete storage, advanced solutions for regenerators and solid media storage based on particulate materials are described, including possible fields of application.

Solar tower power plant using a particle-heated steam generator: Modeling and parametric study

Within the framework of the project HiTExStor II, a system model for the entire power plant consisting of volumetric air receiver, air-sand heat exchanger, sand storage system, steam generator and water-steam cycle was implemented in software “Ebsilon Professional”. As a steam generator, the two technologies fluidized bed cooler and moving bed heat exchangers were considered. Physical models for the non-conventional power plant components as air- sand heat exchanger, fluidized bed coolers and moving bed heat exchanger had to be created and implemented in the simulation environment. Using the simulation model for the power plant, the individual components and subassemblies have been designed and the operating parameters were optimized in extensive parametric studies in terms of the essential degrees of freedom. The annual net electricity output for different systems was determined in annual performance calculations at a selected location (Huelva, Spain) using the optimized values for the studied parameters...

Packed bed heat storage: Continuum mechanics model and validation

Thermal energy storage (TES) systems are key elements for various types of new power plant concepts. As possible cost-effective storage inventory option, packed beds of miscellaneous material come into consideration. However, high technical risks arise due to thermal expansion and shrinking of the packed bed’s particles during cyclic thermal operation, possibly leading to material failure. Therefore, suitable tools for designing the heat storage system are mandatory. While particle discrete models offer detailed simulation results, the computing time for large scale applications is inefficient. In contrast, continuous models offer time-efficient simulation results but are in need of effective packed bed parameters. This work focuses on providing insight into some basic methods and tools on how to obtain such parameters and on how they are implemented into a continuum model. In this context, a particle discrete model as well as a test rig for carrying out uniaxial compression tests (UCT) is introduced. Per...