Characterization of potassium carbonate salt hydrate for thermochemical energy storage in buildings

Thermochemical heat storage in salt hydrates is a promising method to improve the solar fraction in the built environment. One of the most promising salt hydrates to be used as thermochemical material is potassium carbonate. In this study, the use of potassium carbonate in heat storage applications is investigated experimentally. The most important objective is to form a kinetic model for the de/re-hydration reaction of the material. In order to do so, it is crucial to understand the behavior of the salt when it reacts with water vapor. Reaction kinetics and mechanism are investigated for K2CO3, as one of the most promising materials. Characterization of the materials is carried out with combined Thermo-Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) methods. By employing the experimental results, kinetics models are developed for the hydration and dehydration reactions of the material. The kinetics model can be further used to predict the performance of a heat storage system working with K2CO3. In addition, cyclability and reaction enthalpy are investigated

Development of a validated 2D model for flow, moisture and heat transport in a packed bed reactor using MRI experiment and a lab-scale reactor setup

Sorption materials such as zeolite are intensively investigated for thermochemical heat storage applications. The heat storage process is based on a reversible adsorption-desorption reaction, which is exothermic in one direction (hydration) and endothermic in the reverse direction (dehydration). For evaluating the transport phenomena occurring in a heat storage reactor, a detailed model is needed, considering also the transversal terms. In a cylindrical reactor, these terms appear as radial effects that disturb the plug flow assumption in the packed bed, and hence, a model with only axial terms is insufficient to simulate the bed. The radial effects in a porous medium, created by presence of the wall surrounding it, can be caused by: (i) heat losses to the ambient through the wall, (ii) a higher bed void fraction in the wall region, resulting in flow channelling, and (iii) non-uniform initial state of charge near the wall for the subsequent re/de-hydration (e.g. due to heat loss during dehydration). A 2D model is developed for transport phenomena in a packed bed by doing a literature survey on representative models. The model is validated by experimental results measured in a lab-scale setup by comparing the pressure drop over the bed, velocity profile below the bed and temperature profile inside the bed. In addition, the concentration of adsorbed water is compared with experimental results from MRI (Magnetic Resonance Imaging) experiments. The validated numerical model is employed to understand the significance of the above mentioned effects on the thermal performance of the reactor. An accurate model for the thermal dynamics of an adsorption bed on reactor scale is obtained, which is used to present suggestions to optimize the charging and discharging process times, hence, to improve the performance of the reactor

Hot tap water production by a 4 kW sorption segmented reactor in household scale for seasonal heat storage

\u3cp\u3eReplacing fossil fuel by solar energy as a promising sustainable energy source, is of high interest, for both electricity and heat generation. However, to reach high solar thermal fractions and to overcome the mismatch between supply and demand of solar heat, long term heat storage is necessary. A promising method for long term heat storage is to use thermochemical materials, TCMs. The reversible adsorption–desorption reactions, which are exothermic in the hydration direction and endothermic in the reverse dehydration direction, can be used to store heat. A 250 L setup based on a gas–solid reaction between water–zeolite 13X is designed and tested. Humid air is introduced into a packed bed reactor filled with dehydrated material, and due to the adsorption of water vapour on TCM, heat is released. The reactor consists of four segments of 62.5 L each, which can be operated in different modes. The temperature is measured at several locations to gain insight into the effect of segmentation. Experiments are performeignore.txtd for hydration–dehydration cycles in different modes. Using the temperatures measured at different locations in the system, a complete thermal picture of the system is calculated, including thermal powers of the segments. A maximum power of around 4 kW is obtained by running the segments in parallel mode. Compactness and robustness are two important factors for the successful introduction of heat storage systems in the built environment, and both can be met by reactor segmentation. With the segmented reactor concept, a high flexibility can be achieved in the performance of a heat storage system, while still being compact. The system is also able to produce domestic hot tap water with the required temperature of 60 °C. This can be done by implementing a recuperating unit to preheat the inflow by recovering the residual heat in the outflow. In this work, the recuperator is simulated by a heater, and applicability of the system for domestic purposes is assessed. An energy density of 198 kWh/m\u3csup\u3e3\u3c/sup\u3e is calculated on material level, and the energy density calculated on reactor level is around 108 kWh/m\u3csup\u3e3\u3c/sup\u3e and 61 kWh/m\u3csup\u3e3\u3c/sup\u3e for experiment without and with preheating, respectively.\u3c/p\u3

Realization of a 4kW thermochemical segmented reactor in household scale for seasonal heat storage

Replacing fossil fuel by solar energy as a promising sustainable energy source, is of high interest, for both electricity and heat generation. However, to reach high thermal solar fractions and to overcome the mismatch between supply and demand of solar heat, long term heat storage is necessary. A promising method for long term heat storage is to use thermochemical materials, TCMs. The reversible adsorption-desorption reactions, which are exothermic in the hydration direction and endothermic in the reverse dehydration direction, can be used to store heat. A 250L setup based on a gas-solid reaction between water-zeolite 13X is designed and tested. Humid air is introduced to a packed bed reactor filled with dehydrated material and by the resulting adsorption of water vapour on TCM, heat is released. The reactor consists of four segments of 62.5L each, which can be operated in different modes. The temperature is measured at several locations to gain insight into the effect of segmentation. Experiments are performed for hydration-dehydration cycles in different modes. Using the temperatures measured at different locations in the system, a complete thermal picture of the system is calculated, including thermal powers of the segments. A maximum power of around 4kW is obtained by running the segments in parallel mode. Compactness and robustness are two important factors for the successful introduction of heat storage systems in the built environment, and both can be met by reactor segmentation. With the segmented reactor concept, a high flexibility can be achieved in the performance of a heat storage system, while still being compact

Direct numerical simulation of the thermal dehydration reaction in a TGA experiment

This work presents a detailed mathematical model of the coupled mass and heat transfer processes in salt hydrate grains in a TGA experiment. The purpose of developing this numerical model is to get a more fundamental understanding of the influence of parameters like particle size, nucleation rate and vapor pressure on the dehydration reaction in a TGA experiment. Such a model needs a detailed description of the fluid flow and water vapor distribution between the particles. The dehydration reaction of grains of TCMs is described by the nucleation and nuclei growth model presented in our earlier work. The flow around grains is solved by means of the finite volume method using OpenFOAM including heat and mass transfer. Direct numerical simulations of TGA-experiments under various conditions are performed. Such simulations provide direct insight into the physics of mass and heat transport processes coupled with detailed reaction kinetics at grain scale. The numerical results are compared to the experimental results. The developed CFD model can be a promising tool to calculate the overall kinetics for dehydration reactions under realistic heat storage conditions. To that end, the effect of buoyancy should also be included in the model to get a more accurate description of convection within the sample

Experimental study of the bubble size distribution in a pseudo-2D bubble column

This work presents an experimental study of the bubble size distribution of a bubbly flow using digital image analysis (DIA). In order to facilitate the image measurement technique a pseudo-2D bubble column is chosen for the experiments. To obtain well-defined inlet conditions a gas sparger, consisting of 20 needles, is used. By employing DIA, the bubble size distribution (BSD) has been measured for a range of superficial gas velocities. The resulting BSD's are expressed in terms of a probability density function (PDF). For low superficial gas velocities of 5 and 10mm/s the PDF has a unimodal shape, while for higher superficial gas velocities of 15 and 20mm/s the PDF has a bimodal shape. The effects of coalescence and break-up of bubbles are visible by evaluating the changes of the resulting BSDs for increasing superficial gas velocity. A comparison of gas hold-ups is made between the calculated BSD and the liquid expansion height. This comparison shows how well the BSD obtained with DIA describes the actual gas hold-up in the column

Investigation of a household-scale open sorption energy storage system based on the Zeolite 13X/water reacting pair

\u3cp\u3eSorption thermal energy storage is a promising concept for seasonal heat storage. Advantages of sorption heat storage are high energy storage density (compared to sensible and phase change heat storage) and negligible energy losses during storage over long time periods. In order to investigate the potential of sorption thermal energy storage, a high power open sorption heat storage system has been designed and built for household space heating applications. In this paper, the characteristics of the open zeolite 13X/water sorption energy storage system will be presented. The setup consists of four segments with a total capacity of 250 L of zeolite. A segmented reactor has been designed to reduce the pressure drop over the system, which results in less required fan power. This configuration also decreases the response time and makes the system scalable. Dehydration of the reactor is performed by supplying hot air to the zeolite bed. Hydration is performed by supplying humidified air to the bed. In all the segments, the pressure drop, temperature, and humidity are monitored. Furthermore, inside one of the reactor segments, the temperature is monitored at different locations in the zeolite bed. Several tests, using different mass flow rates, have been performed. During the tests, a maximum temperature step of 24 °C was realized. The maximum delivered power was 4.4 kW and the obtained storage capacity was 52 kWh. The reactor efficiency was 76% taking into consideration the conductive heat losses through the reactor wall and the sensible heat taken up by the thermal mass of the solids. Furthermore, it has been noticed that the flow through the bed was not completely uniform. This has a negative influence on the performance of the system.\u3c/p\u3