The dynamics of hairpin vortices in a laminar boundary layer

+177hlm.;24c

De ontwikkeling van thermochemische warmteopslag

Voor het bewaren van warmte bestaan diverse commercieel beschikbare technologieën, waarbij warmte wordt opgeslagen als voelbare warmte of als smeltwarmte. Warmteverlies en beschikbaar volume leveren beperkingen op voor de hoeveelheid warmte die zo opgeslagen kan worden. Warmte is echter veel compacter op te slaan door opslag in een reversibele chemische reactie. Bovendien is de warmte na het opladen met deze techniek langdurig verliesvrij te bewaren. Dit biedt nieuwe mogelijkheden voor langetermijn warmteopslag; in het bijzonder voor seizoensopslag in de gebouwde omgeving. Om dergelijke toepassingen mogelijk te maken is het noodzakelijk dat stabiliteit en reactiesnelheid van deze thermochemische materialen geoptimaliseerd worden, evenals warmte en damptransport op zowel materiaal- als reactorniveau. In zijn rede gaat Zondag in op warmteopslag en thermochemische warmteopslag en geeft hij aan welk onderzoek de komende jaren bij de TU/e op dit gebied zal worden uitgevoerd

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

Design of a thermochemical heat storage system for tap water heating in the built environment

Replacing the use of fossil fuel by solar energy, as one of the most promising sustainable energy sources, is of high interest, because of climate change and depletion of fossil resources. However, to reach high solar fractions and to overcome the mismatch between supply and demand of solar heat, storage of solar energy is necessary. A reliable method for long term heat storage is to use thermochemical materials, TCMs. The heat storage process is based on a reversible adsorption-desorption reaction of water vapor on the TCM, which is exothermic in one direction and endothermic in the reverse direction. In this research, Zeolite 13X is used as TCM. The system is an open sorption heat storage system for providing hot tap water. In the experimental test setup, the humid air is provided in a bubble column by blowing air from bottom of the column. The exothermic hydration process starts with humid air entering into a packed bed reactor filled with zeolite 13X. The reactor is a vertical cylindrical tank which is made of steel; it has a layer of Teflon inside and has a layer of insulation outside. The temperature profile in the reactor is measured as a function of time both along the flow direction and perpendicular to the flow by thermocouples. In addition, input and output temperatures and humidity are measured. In the resulting adsorption reaction between water vapor and TCM, energy is released. This released energy heats up the air flow which passes through the reactor and the hot output air flow is used to heat up the water in a water tank. The water tank is also a vertical cylindrical tank which is made of steel and has a layer of insulation outside. The hot output air from the reactor passes through a coiled tubing inside the water tank to heat up water. The temperature of the water in the tank is measured at two different heights. A problem in open solid sorption systems using air as heat transport medium is the limited temperature step which can be achieved in the sorption bed. In the present study this problem is solved using a heat recovery system enabling higher output air temperatures. The residual heat in the exhaust air is used to preheat the reactor inflow, in an air-to-air heat exchanger. In the endothermic dehydration process, the hydrated zeolite is dried with hot air. In this study, a lab-scale prototype TCM based heat storage system is designed and optimized, which, by making use of a heat recovery loop, is able to provide hot tap water. Results of the experimental investigation on charge-discharge cycles will be presented.<br/

Quantum chemical analysis of the structures of MgSO4 hydrates

Magnesium sulfate salts can form hydrated compounds with up to seven degree of hydration with an energy exchange of the order of 2.8GJ/m3 [1]. In addition, this salt is abundant in nature and thus this material is a potential candidate for storing energy in seasonal heat storage systems. One of the main issues in using this material for seasonal heat storage system is its slow kinetics and low extent of water take-up under normal atmospheric conditions [2]. In addition, the salt undergoes considerable changes in its crystalline structure during hydration and dehydration, and often they encounter the formation of cracks and pores in the crystal structure [3]. This significantly affects the efficiency of the salt in storing energy and also reusability of the material. A molecular level investigation is necessary to understand the process of hydration and dehydration in detail. Presence of an extensive network of hydrogen bonds in MgSO4.7H2O crystal is identified by Allan Zalkin et al [4]. Significant delocalization of hydrogen atoms within the hydrogen bonds are reported in the study. The 7th water molecule in a hepta-hydrate crystal is captured in the interstitial space within the crystals due to coulombic forces and they are very easily removable. Thus modeling a stable molecule of magnesium sulfate hepta hydrate is difficult. So we undertake the hexa hydrated magnesium sulfate to study the equilibrium molecular structure. The hydrogen bonds present in the structure, which stabilizes the molecule, is a focus of attention in this study. In addition, we report Natural Bond Orbital (NBO) [5] charges of Mg and S as a function of degree of hydration in this study. The NBO analysis gives information about electronic occupations in the molecule. In addition, the variation of the natural charges give information about the nature of inters atomic interactions involved in the hydration process of magnesium sulfates. The hydration process is accompanied by a considerable amount of energy exchange with the surroundings. In addition, significant changes in the crystal structure are predicted to happen during hydration. The binding of a water molecule on a slab of magnesium sulfate will resemble the hydration phenomena of a real crystal. Maslyuk et al [6] have performed such an analysis on kieserite structures and found the influence of hydrogen bonds during hydration. A similar study has done towards the last part of this account, which gives important information about hydration process of magnesium sulfate crystal

Solar-Driven Continuous CO₂ Reduction to CO and CH₄ using Heterogeneous Photothermal Catalysts:Recent Progress and Remaining Challenges

The urgent need to reduce the carbon dioxide level in the atmosphere and keep the effects of climate change manageable has brought the concept of carbon capture and utilization to the forefront of scientific research. Amongst the promising pathways for this conversion, sunlight-powered photothermal processes, synergistically using both thermal and non-thermal effects of light, have gained significant attention. Research in this field focuses both on the development of catalysts and continuous-flow photoreactors, which offer significant advantages over batch reactors, particularly for scale-up. Here, we focus on sunlight-driven photothermal conversion of CO2 to chemical feedstock CO and CH4 as synthetic fuel. This review provides an overview of the recent progress in the development of photothermal catalysts and continuous-flow photoreactors and outlines the remaining challenges in these areas. Furthermore, it provides insight in additional components required to complete photothermal reaction systems for continuous production (e. g., solar concentrators, sensors and artificial light sources). In addition, our review emphasizes the necessity of integrated collaboration between different research areas, like chemistry, material science, chemical engineering, and optics, to establish optimized systems and reach the full potential of this technology.</p