Improvement in Performance of a Thermochemical Heat Storage System by Implementing an Internal Heat Recovery System

A lab-scale prototype of a thermochemical heat storage system, employing a water-zeolite 13X as the working pair, is designed and optimized for providing hot tap water. During the hydration process, humid air is introduced to the packed bed reactor filled with dehydrated zeolite 13X, and the released heat of adsorption heats up the air passing through the reactor. The hot outflow air is led to an air-to-water heat exchanger integrated in a water tank and heats up the water. The residual heat in the exhaust air is used to preheat the reactor inflow in an air-to-air heat exchanger. The temperatures of all system components are measured, and the thermal powers and heat losses are calculated. Experiments are performed in the system with and without using the heat recovery, and improvement in performance of the heat storage system is investigated

OPTIMIZATION OF FLOW DIRECTION TO MINIMIZE PARTICULATE FOULING OF HEAT EXCHANGERS

The influence of flow direction with respect to gravity on particulate fouling of heat exchangers is investigated experimentally to determine the optimal flow direction to minimize fouling. Four orientations of flow have been investigated, horizontal flow, upward flow, downward flow and a flow under an angle of 45°. It is observed that fouling starts at the point of stagnation irrespective of the flow direction, and also at the top of the heat exchanger tubes. Particulate fouling grows from these two points till they meet and the fouling layer covers the whole surface of the heat exchanger tube. Fouling at the upper half of the tubes is much faster than the lower half of the tubes, and the fouling rate is faster at the bottom tubes of the heat exchanger section than at the upper tubes. The best orientation for lingering particulate fouling is the downward flow, where the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer only starts from one point

Molecular dynamics study on thermal dehydration process of epsomite (MgSO4.7H2O)

Water vapour sorption in salt hydrates is one of the most promising means of compact, low loss and long-term solar heat storage in the built environment. Among all, epsomite (MgSO4·7H2O) excels for its high-energy storage density and vast availability. However, in practical applications, the slow kinetics and evident structural changes during hydration and dehydration significantly jeopardise the heat storage/recovery rate. A molecular dynamics (MD) study is carried out to investigate the thermal properties and structural changes in the thermal dehydration process of the epsomite. The MD simulation is carried out at 450 K and a vapour pressure of 20 mbar, in accordance with experimental heat storage conditions. The study identifies the dehydration as multiple stages from the initial quick water loss and collapse of the crystal framework to the adsorption of water molecules, which inhibits complete dehydration. Further, the anisotropic diffusion behaviour supports the important role of the porous matrix structure in the heat and mass transfer process. The enthalpy changes, partial densities, mass diffusion coefficients of water and radial distribution functions are calculated and compared with corresponding experimental data to support the conclusions

Characterization of sugar alcohols as seasonal heat storage media - experimental and theoretical investigations

Sugar alcohols are under investigation as phase change materials for long term heat storage applications. The thermal performance in such systems is strongly dominated by the nucleation and crystal growth kinetics, which is further linked to the crystal-melt interfacial free energy (surface tension), the latent heat, and the viscosity. We carry out a comprehensive study of sugar alcohols to examine their thermodynamic and kinetic properties, from both experiments and theoretical calculations. The theoretical study follows a bottom-up approach. A generalized AMBER force field obtained from first principle calculations is selected to construct the molecular models. Heat capacity, self-diffusion constant, viscosity, latent heat, and interfacial free energy of selected model materials are calculated through molecular dynamic simulations. In the experimental study, differential scanning calorimetry and viscosity measurements are performed. Also, the kinetics of the crystal growth is examined using a microscope. The experimental results are integrated with the Rozmanov model, and a strong dependence of growth speed on the degree of subcooling is identified. All the experimental measurements are compared with our theoretical work, and the results showed good agreement. The methodologies used in the calculation are proved effective and reliable for future prediction of unknown systems. In this study, the high viscosity and the high interfacial free energy are both found responsible for the sluggish kinetics of nucleation and crystal growth in sugar alcohols

Effect of condensable species on particulate fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprises particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger like superheater, evaporator etc. If the temperature of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles which are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a simple modeling approach for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with the detailed impaction experiments