Active coatings for heat storage
The young researcher group NEOTHERM constructs novel sorptive heat storage materials for better use of solar heat for domestic heating and the reuse of industrial waste heat. For this purpose the scale-up of suitable materials is investigated. The products could be implemented in solar thermal energy systems or in waste heat recovery devices. Special composites could also be used in heat pump applications for air conditioning.
|Project status||several composites are manufactured; active materials are modified to purpose; characterisation and testing is ongoing|
|Temperature type||Materials for use at lower temperatures (for MOFs about 60-150/200 °C) as well as higher temperatures (for zeolites and zeolite related materials about 250-400 °C) are investigated.|
|Storage/Loading||will be evaluated during the project|
|Storage duration||any time period (if humidity is hermetically excluded)|
|Number of cycles||depending on the application; aim is some hundred cycles|
|Charging and discharging temperature||will be investigated during the project|
|Storage capacity||will be investigated during the project|
|Storage density||will be investigated during the project|
|Project duration||June 2013 until May 2018|
Heating is a notable expense factor, both for industrial and private contexts. In principle the sun supplies enough cost free energy, but the periods of sufficient supply (during daytime and in summer) is not consistent with the periods of high demand (during night and in winter). Therefore efficient heat storage is desired. One way of heat storage is sorptive heat storage. The recovery of heat is achieved by immobilization/condensation of small molecules in microporous materials with high inner surfaces and micropore volumina. The storage of heat is performed by the reverse process, i.e. desorption of the molecules from the active materials. In order to mobilise and detach molecules from surfaces and remove them from micropores energy needs to be supplied. This process can be promoted by thermal energy – i. e. heat – from the environment. Like that heat is “stored within” the material. Such a process is sensible when water molecules are leaving human skin. Here too energy is required to separate the molecules from each other (evaporation) and from the skin (desorption). As this energy is taken from the skin in form of heat the skin appears cold. The stored heat is recovered during the process of adsorption, i.e. the uptake of small molecules by the microporous active material. The kinetic energy of the molecules is released and heat is provided. The material heats up and can be used as heat source for different purposes. Depending on the material a significant sensible temperature occurs. Zeolites for example can become more than 100 °C hot.
Effective energy storage by active coatings
The process of attachment (adsorption) and removal (desorption) of small molecules on surfaces can be used to store heat. The energy is stored during desorption of the molecules from a surface, during the adsorption heat is released. During the process the kinetic energy of the molecules is transferred to the storage material and provided as heat. To draw optimum advantage from such a process a very large surface area is needed, as could be found in microporous compounds. Zeolites and MOF materials (metal organic frameworks), for example, offer such huge inner surfaces. Such compounds should be used as coatings on supports with high thermal conductivity in order to effectively exploit their properties. Fine powders can not be handled in heat storage devices. They block the gas flow and hamper the transport of water molecules in and out of the active materials. Macroporous cellular supports enable simple and effective drying and wetting of the active coatings allowing a high mass transfer without significant pressure loss. In addition the “thermal management” is improved by the cellular supports. The heat deriving from adsorptive processes can be led off, which enables the use of the heat and also allows an optimal discharge of the stored heat due to avoidance of hot spots. Hot spots prevent further adsorption and therefore decrease the heat recovery per time. Only like that effective heat storage and recapture processes can be performed.
The focus of the scientific work lies on zeolites (and zeolite-like materials) as well as on MOFs. This allows the coverage of a wide temperature range from around some hundred °C to lower temperature about 40 °C. The coating of the microporous compounds on cellular supports (ceramics or metals) allows an effective application of the active materials, as the support guarantees a suitable mass and heat transfer. The tailor-made synthesis of such cellular supports allows an optimised attachment of the active coatings regarding heat transfer through their boundary surface. Furthermore, the porosities of the components can be adjusted hierarchically to each other leading to optimised energy densities coupled with sufficient mass flow.
Novel storage materials for industry and private households
Starting point for the project is the desired use of „free of cost“ solar heat and industrial waste heat. The projects goal is the provision of novel heat storage materials, which can be applied for the use of industrial waste heat and for private households, for example for storing heat from solar thermal energy systems. The storage is affected by sorption processes and novel micro-macro-porous materials are developed for that purpose. The SorTech company (situated in Halle/Saale) acts as advisory partner for the NEOTHERM young researcher group. The modules which are developed could be integrated into different applications, for example to increase the efficiency of solar thermal heat energy systems or to recover waste heat from combustion processes.
The novel materials will be optimised with respect to their energy storage properties concerning their chemical composition as well as the macro- and microscopical structural set-up of the pore system (pores for mass transfer, pores for heat storage). This will be done by systematic synthesis of the composites and their detailed characterisation, supplemented by simulation of their sorption properties.
Until 2018 a demonstator module will be developed
The project “Novel Composite Materials for Thermochemical Energy Storarge (NEOTHERM)” is in an advanced state. Several compounds which serve as active coatings for the composite materials are synthesised and characterised; in the field of zeolites as well as in the field of MOFs. The modification of these materials is pushed further and still under investigation. Macroporous support materials have been purchased (ob-SiC foams and metallic foams) and synthesised to purpose (Al2O3 ceramic foams). Such cellular ceramics are further improved by adjusting their synthesis procedures and by surface modification. Suitable characterisation methods are established within the young researcher group or provided by collaborations.
The third milestone was achieved end of 2015: Cellular supports have been coated with zeolites and MOFs. The main focus of research is now the comprehensive characterisation of the composites regarding relevant parameters for thermal heat storage, which will result in further optimisation of the materials. The most promising composite will be implemented in a demonstrator module showing the operating mode of the material. The young researcher group wants to attain this goal until mid of 2017. Then the scale-up of the composite materials is planned, targeting a size about 10 x 10 x 10 cm.