Increasing the energy density of thermal storages
Scientists optimise materials and components for soprtion-based thermal
energy storages of high energy density. Sorption reservoirs for
buildings currently achieve an energy density of about 130 kWh / m³. In
the project MAKSORE the researchers want to achieve an effective usable
volumetric energy density of at least 180 kWh / m³.
|Project status||Search for a material suitable to use with an adsorption heat storage concept|
|Temperature type||Low temperature|
|Storage time||short (hours to days)|
|Number of cycles||Different storage concepts are investigated, number of cycles between ~1/a and ~300/a|
|Charging temperature||up to 250 °C|
|Discharge temperature||at least 40 °C or 30 K Hub|
|Energy density||650 MJ/m³|
|Project duration||September 2014 until March 2018|
Sorption heat storages allow for high energy densities compared to thermal storages for sensible heat (e. g. hot water storages). Furthermore, loss-free storage of a significant part of the heat over long periods of time is theoretically possible. Additional benefits can be gained from generation of useful cooling while discharging the storage. Fields of application are especially in buildings for the storage of solar heat, heat from cogeneration plants (CHP with or without cold production), but also in connection with surplus electrical power from renewable sources as heat.
An important objective in the context of the development of this storage technology is to further increase the effecitvely usable energy density in the application and to develop compact storages, that can be integrated in the building stock efficiently. To achieve this, leading research institutions of applied sciences that are active in the field of thermal storages, have joined with excellent working groups from fundamental sciences, in the effort to solve interdisciplinary development needs, regarding issues ranging from material science, heat and mass transfer processes through components to achieve breakthroughs regarding technical applications.
Three concepts to increase the energy density
In this project, materials science and process engineering are brought together with applied sciences in the field of sorption-based thermal energy storage. This storage type can potentially achieve a very high energy density. Three promising storage concepts have already been identified in the run-up to this project, for different applications in buildings. Those concepts allow for the deduction of application-specific requirements on sorption pairs and components of the storage. The three concepts are (A) a cascadingly operated, closed adsorption heat storage, which allows for an increase of the usable amount of extracted heat by means of an adsorption heat pump. Concept (B) is based on an open adsorption heat storage, which can be integrated into the ventilation system of a low energy building, and can be used to store solar heat over long periods of time. In concept (C), an adsorption heat storage is investigated that enables for increased energy density due to additionally used cristallization heat. For each of the storage concepts, the scientifically most relevant issues for a successful realization of the concept are advanced, when taking into account the best available composites or components, respectively. To achieve that, material properties, the coupling between heat and mass transfer and characteristics of the overall system are investigated. A main goal of the project is to extend the fundamental knowledge about sorption storages, with respect to materials and components, as well as regarding the technical realization. Starting out from three storage concepts, the maximum available energy density is to be increased significantly compared to state-of-the-art technology.
Temperature lift around 30 Kelvin
In heat storage applications (e.g. in buildings), sorption heat storages behave fundamentally different regarding operating techniques compared to sensible heat storages, e. g. hot water storages, but also different from latent heat storages that feature a storage material undergoing a phase change (mostly solid/liquid) when charging or discharging. Sorption heat storages work similar to sorption heat pumps, i. e. when heat is stored at a high temperature level (desorption of the storage), heat is simultaneously released at a lower temperature level from the condensation of the working fluid. When discharging heat from the storage (from the adsorption process, at high temperature), heat has got to be supplied to the storage at lower temperature for the evaporation of the working fluid, simultaneously. The temperature difference between the low temperature heat supplied to the evaporator and the useful heat released from the storage is called temperature lift. The achievable temperature lift is a very relevant figure regarding technical applicability of sorption heat storages. Experiences from earlier research projects show that e.g. adsorption heat storages based on silica gel as sorption material featured insufficient temperature lifts to be coupled efficiently to a large enough variety of heating systems.
In this project, a minimum of 30 K has been defined as target temperature lift (e. g. referring to useful heat at 35 °C and heat supply at 5 °C from a low temperature source). The achievable temperature lift depends on the sorption pair and also on the state of charge of the storage. Thus, the effective usable amount of heat (and therefore the effective energy density) depends on material properties as well as on boundary conditions of the application and the integration scheme of the storage into a supply system.
In turn, it is rational to start out from specific system concepts and to integrate research activities in a matrix of scientific objectives and challenges on the one hand, and into specific storage concepts on the other hand. A structure like this serves as a working scheme in this project.
A comprehensive focus besides increased temperature lifts of 30 K or more is an effectively usable volumetric energy density of 180 kWh/m³ or more (compared to approx. 130 kWh/m³ for the best available sorption heat storages for buildings).
Fundamentally investigating heat and mass transfer processes
Starting points for optimizing sorptive heat storage systems can be found on the one hand in utilizing materials with improved adsorption equilibria, i.e. a larger heat turnover within a process window defined by charging and discharging temperatures and pressures. On the other hand, optimization can be achieved by improving the sorption kinetics (process dynamics) by reducing heat and mass transport resistances. In both areas, there is still a large potential for improvement.
Which sorption materials are best suitable for a specific type of heat storage, strongly depends on the detailed temperature and pressure conditions of the application. The requirement regarding the target increase of the temperature lift is based mainly on the application potential of sorption heat storages in stock buildings. In cases of energetic retrofitting, the extisting heat distribution system (e.g. radiators) is often preserverd, leading to higher requirements regarding the temperature lift compared to new buildings.
In this project, the applicability of sorption pairs for heat storage applications is to be optimized by means of fundamental investigation of the relevant heat and mass transfer processes. Research and deveiopment activities can then target identified limitations specifically to improve the power characteristics of the storages. A key role (with adsorptive storages) play composite structures from sorption material and carrier material, that improve the thermal performance of the coupling between sorption materials and heat sources and sinks significantly.
To succeed in achieving comprehensive fundamental knowledge about relevant heat and mass transfer processes, those processes will be modelled at high level of detail. The simulation models are validated and calibrated with data from laboratory measurements.
Economic viability of sorption heat storages
To improve the economic viability of sorption heat storages, a key leverage is to increase the number of storage cycles over which the initial investment can be recovered. This approach is in line with the aim of this project to achieve a process intensification by improving heat and mass transfer properties of the materials and composites (and thus enabling a higher charging and discharging power of the storage).
The project is currently in the start-up phase. The next milestones are:
Month 15: A material suitable to use with a cascading adsorption heat storage concept has been identified and characterized (milestone A.1). For an absorption heat storage, a minimum of one sorption pair has been identified and characterized, that can be expected to reach higher energy density and better process dynamics in the applicable temperature range than available materials for latent heat storage (milestone C.1).
Month 18: For an open-cycle adsorption heat storage, a production process for extruded composite materials has been developed and tested. Honeycomb structures required for subsequent experiments have been produced by extrusion in sufficient quantity (milestone B.1).
Month 24: For a cascading, closed-cycle adsorption heat storage, the reproducibility of the coating process has been analyzed. At least one model of adsorption dynamics has been validated through pressure jump experiments (milestone A.2).
Month 27: For an open-cycle adsorption heat storage, the thermal properties of the composite materials have been characterized and their cycle stability has been determined. A process model is available that enables to predict thermal performance data of the system.
Month 27: For a cascading, closed-cycle adsorption heat storage, a system model is available enabling thermodynamically consistent cycle simulations. The model has been calibrated with experimental data on adsorption dynamics. Results on the achievable effective energy density of the storage and further figures of merit for the complete systeme have been obtained from that model (milestone A.3). For an open-cycle adsorption storage, the respective model and system simulation results are available at month 30 (milestone B.3).
Month 30: For the absorption storage concept developed within this project, a system model has been set up that enables a prediction of the heat fluxes as functions of time and operating parameters within an accuracy of 10% relative to experimental results (milestone C.2).
Sub-project KIT: At the institute of Fluid Machinery of KIT, the concept of a cascading adsorption heat storage is developed further and is subjected to a thermodynamic analysis, with special emphasis on the sensitivities regarding adsorbent materials and heat exchanger concepts. At the instittute of Thermal Process Engineering / Thin Film Technology of KIT, coating of sorption materials onto heat exchanger surfaces is systematically analyzed. Special emphasis is placed on the influence of drying parameters on the morphology of the coating and its influence on the sorption kinetics within the storage cycle.
Sub-project Fraunhofer ISE: The working group "Sorption technology - materials development and characterization" at Fraunhofer ISE has a twofold task within this project: To work on cross-sectional topics regarding materials characterization and analysis for the benefit of the whole project consortium and to work specifically on an improved thermal coupling between sorption materials and heat exchangers for the concept of a cascading adsorption heat storage.
Sub-project Univ. Stuttgart: Building on the experience from previous research projects at the institute of thermodynamics and heat engineering, the concept of an open-cycle adsorptive heat storage based on extruded honeycomb structures shall be developed further and transferred to new materials / composites. Air permeability and volumetric fraction of solid adsorbent are to be optimized.
Sub-project ZAE Bayern: At ZAE Bayern, the concept of a novel type of absorption heat storage is analyzed and further developed, which can utilize the heat of solution and/or the heat of crystallization of a sorption pair. In this system, a storage tank is employed in which there is a stratification of differently concentrated solutions at different temperatures according to their densities. Besides gaining a deeper understanding of the properties of suitable materials for this concept, the aim of this work is to identify advantageous process schemes for this new storage type.
Sub-project TH Wildau: At TH Wildau, the focus is on cross-cutting issues regarding the characterization and (post-synthetic) modification of sorption materials. Different thermodynamic models are evaluated with respect to the goal of creating thermodynamically consistent datasets for each material, encompassing data e.g. on sorption equilibria and on enthalpies of adsorption. For the concept of the cascading adsorption storage, it is analyzed how the adsorption properties of certain zeolites can be modified e.g. through ion exchange, and how beneficial this can be for the application performance of this type of storage.