Numerical investigation on the flow behavior of a novel fluidization based particle thermal energy storage (FP-TES)
Introduction
Currently, our response to the challenges of climatic change is one of the most pressing concerns facing public and governments across the whole world. Therefore, a reduction of CO2-emission by reinforcing the efforts to increase the share of renewable energy on the electricity market as well as to control total energy consumption are required. Especially, the share of so-called new renewables in energy conversion to electricity, such as wind and solar power, more than quintupled between 2005 and 2018. Due to high growth rates in most industrial countries, a further increase is expected [1]. However, the integration of these volatile sources of energy demands flexible storage technologies and the enhancement of grid infrastructure to ensure power grid stability, [2,3]. Nowadays, the majority of grid stabilisation services is provided by hydropower pumped storages. Those are known for very good round trip efficiencies and long lifetimes but are also subject to topological restrictions and can cause serious ecological impact. Another solution can be the transformation of the excessive electrical power to heat and further using a thermal energy storage (TES) system or relieving the power grid by the implementation of TES at solar power plants and conventional thermal power plants [4]. To offer a viable alternative to pumped hydropower storages a TES system must meet various requirements. It should enable flexible and easy operation with high round trip efficiency at low costs and low thermal losses. Therefore, cost-effective and widely available storage materials with high energy density are needed. Economical bulk materials, such as quartz sand or even corundum powder, silicon carbide or bauxite, could be utilized as a sensible storage medium over a wide temperature range. Compared to other thermal energy storage systems like latent [5] or packed bed storages [6] the advantage of bulk material is that it can be used in active storage cycles being flexibly adapted to a lot of different applications, fluctuating charging and discharging power. Moreover, the heat conductivity of a packed bed of bulk material is rather low, considering that the insulation value of quartz sand is roughly half as high as for common insulation material [7]. As a result, the low conductivity leads to a reduction of thermal losses, regarding hot bulk material in a storage device. The reason therefore lies in the fact that the outer layers of bulk material provides an additional isolation to the inner ones.
Various concepts of sensible thermal energy storage systems based on quartz sand for the application in concentrating solar power plants and in the industry for heat recovery were already presented in Ref. [[8], [9], [10], [11], [12], [13]]. All those concepts are working with fluidized bed technology and relay on mechanical transportation devices, such as screw conveyors and/or bucket elevators, which are required to move particles out off and into the storages, respectively through a heat exchanger. Mechanical transportation devices, which are necessary for particle transport in the so far presented systems, are causing need for frequent maintenance and are sometimes even limiting the temperature.
In response to these issues, a novel TES concept the Fluidization Based Particle TES (FP-TES) has been developed at the TU Wien, Institute for Energy Systems and Thermodynamics. The FP-TES is a sensible TES working with bulk material as a storage medium based on advanced fluidization technology, managing particle transport without any mechanical transport devices. The FP-TES makes use of the advantages provided by the utilisation of bulk material as storage medium. Further it offers a higher possible operating temperature and higher reliability with lower effort for maintenance and auxiliary energy than other previously presented storage systems working with bulk material [[8], [9], [10], [11], [12], [13]].
Section snippets
Concept of FP-TES
The underlying concept of the FP-TES is very flexible in its application and can be utilized as a power-to-heat-to-power system as well as for heat recovery and storage in general. As already mentioned the FP-TES is working with bulk materials transported by an advanced fluidization technology. Detailed information regarding fluidization technology can be found in Ref. [14]. A conceptual sketch of the FP-TES approach is depicted in Fig. 1.
In terms of charging respectively discharging heat, the
Numerical investigation
In order to develop a procedural, fluidic and constructional layout of the FP-TES sufficiently accurate estimates of necessary fluidization grades, pressure gradients and manageable particle mass fluxes, as well as the continuous pressure variations above the distributor floors are needed. Processes taking place in the fluidized domain inside FP-TES could not be addressed with an analytic or known empirical approach, because of the atypical character of the communicating fluidized beds and the
Results of the numerical investigations
The numerical investigations showed, that fluidization of a sand filled hopper, as it is presented in Fig. 2, is not possible. With applied fluidization grades of more than 20 sufficient fluidization of the hopper (meaning fluidized bed in the middle of the hopper and fixed bed at outer diameters) cannot be achieved. To prevent fluidization from collapse due to bulk material continuously falling from the rim of the fixed bed into the fluidized area, internals in the hoppers are necessary. In
Experimental investigations
Based on the geometry and the results of the simulations regarding needed fluidization air mass flow and pressure drops as well as occurring pressure fluctuations a FP-TES test rig was designed and built, see Fig. 9. To ensure comparability of simulation and experimental results in the test rig quartz sand with a narrow particle size distribution and a mean Sauter diameter of 87 μm is used. The sphericity of the sand was estimated at 0.8 by the means of microscope images and literature data [23
Energetic considerations of an industrial application
Based on the results of the numerical and experimental investigations with the cold test rig a basic layout of the FP-TES for an industrial application can be designed. The storage capacity of FP-TES can be calculated by Equation (2) with the volume of the hopper, the bulk density and the specific heat capacity of the storage material and the temperature difference between the hot and the cold hopper.
To achieve relevant storage capacities the hoppers for an industrial application
Conclusions and future work
The initial goal was to investigate the behaviour of a TES system, which is handling particle transport by means of fluidized bed technology. To prove this concept preliminary numerical simulations are performed and a first experimental study on a cold test rig is done. In order to deal with changing BCs a transient CFPD approach is developed. In the experimental part of the work the behaviour of the FP-TES is investigated by a hopper level measurement and multiple static pressure sensors.
CRediT authorship contribution statement
Verena Sulzgruber: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Visualization. David Wünsch: Conceptualization, Methodology, Software, Resources. Markus Haider: Project administration, Funding acquisition. Heimo Walter: Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the FFG - Austrian Research Promotion Agency (FFG project number 858916) and the Austrian Wirtschaftsservice (prototype funding project number P1621702) for the financial support and funding of the conducted research work.
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