Characterization and modelling of K2CO3 cycles for thermochemical energy storage applications

Thermochemical heat storage in salt hydrates is a promising concept to bridge the gap between supply and demand of solar thermal energy in the built environment. Using a suitable thermochemical material (TCM), a heat battery can be created to supply low-temperature thermal energy during colder time periods. The principle is based on a reversible hydration-dehydration reaction with water vapour. The TCM can be charged (dehydrated) at a temperature of 120°C by using solar thermal collectors. Conversely, the discharge (hydration) occurs at room temperature using a constant water vapour pressure of 12 mbar. Previous studies have indicated that potassium carbonate (K2CO3) is a good candidate to fulfil the role of TCM in built environment applications. To generate adequate power from a heat battery for hot tap water or space heating, the kinetics of the TCM need to be sufficiently fast. It is hypothesized that the kinetics of the material improve over multiple charge and discharge cycles due to crack formation and volume increase of the grains. The aim of this work is to evaluate the kinetics of 500-700 µm K2CO3 grains using thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), and to quantify the improvement in kinetics over multiple charge and discharge cycles. The kinetics serve as input for an existing nucleation and growth model, simulating the fractional conversion at grain level. In the TGA/DSC experiments, the material was charged and discharged numerous times under a constant water vapour pressure of 12 mbar. The cycling temperature varies from room temperature to a maximum temperature of 120°C. The conversion time of each cycle was monitored. Additionally, using an optical microscope, cycling experiments of K2CO3 were performed in a micro climate chamber with the same conditions as in the TGA/DSC experiments. This allows tracking of the apparent surface area of the grains and the observation of crack formation for each cycle. The existing nucleation and growth model is enhanced by incorporating grain growth and crack formation observed from the optical experiments. Thermal characterization by means of TGA/DSC has indicated that indeed the kinetics of the material improve over multiple cycles. Typical conversion rates are increased by a factor 10 comparing the first and the 12th cycle. Preliminary optical microscope experiments show an increase of the apparent grain surface area of approximately 55%. Additionally, crack formation is observed over multiple hydration and dehydration cycles leading to increased inter-particle porosity, likely adding to the improved kinetics

A reduced-order model for dynamic simulation of district heating networks

This study concerns the development of a data-based compact model for the prediction of the fluid temperature evolution in district heating (DH) pipeline networks. This so-called "reduced-order model" (ROM) is obtained from reduction of the conservation law for energy for each pipe segment to a semi-analytical input-output relation between the pipe outlet temperature and the pipe inlet and ground temperatures that can be identified from training data. The ROM basically is valid for generic pipe configurations involving 3D unsteady heat transfer and 3D steady flow as long as heat-transfer mechanisms are linearly dependent on the temperature field. Moreover, the training data can be generated by physics-based computational "full-order" models (FOMs) yet also by (calibration) experiments or field measurements. Performance tests using computational training data for a single 1D pipe configuration demonstrate that the ROM (i) can be successfully identified and (ii) can accurately describe the response of the outlet temperature to arbitrary input profiles for inlet and ground temperatures. Application of the ROM to two case studies, i.e. fast simulation of a small DH network and design of a controller for user-defined temperature regulation of a DH system, demonstrate its predictive ability and efficiency also for realistic systems. Dedicated cost analyses further reveal that the ROM may significantly reduce the computational costs compared to FOMs by (up to) orders of magnitude for higher-dimensional pipe configurations. These findings advance the proposed ROM as a robust and efficient simulation tool for practical DH systems with a far greater predictive ability than existing compact models.Comment: 30 pages, 19 figure

Enhanced Hydrogen Storage in Gold-doped Carbon Nanotubes: A first-principles study

Sorbent materials are a promising alternative to advance hydrogen storage technologies. The general disadvantage is the relatively weak solid-gas interaction and adsorption energy, providing low gravimetric and volumetric capacities and extreme operational conditions. Here we propose Au-doped carbon nanotubes (CNTs) as an efficient alternative for reversible hydrogen capture at high temperatures. This work investigates the properties of several modified CNTs using density functional theory. We analyze the binding and formation energies of the uniformed Au-doped CNTs and assess their adsorption capability. The hydrogen storage mechanisms of the nanostructures are studied in depth using partial density of states and charge transfer analysis showing that the increase of diameter has a positive effect on the outcome. Our findings show that the modified structures are able to capture from six to nine hydrogen molecules per gold atom, achieving volumetric capacities ranging from 154 to 330 g/l, surpassing the DOE target. In addition, the calculated desorption temperatures indicate high performance of Au-doped CNTs, obtaining hydrogen capture-release working conditions above 200 K

A new volumetric strain-based method for determining the crack initiation threshold of rocks under compression

The crack initiation stress threshold ( ci) is an essential parameter in the brittle failure process of rocks. In this paper, a volumetric strain response method (VSRM) is proposed to determine the σci based on two new concepts, i.e., the dilatancy resistance state index ( ci) and the maximum value of the dilatancy resistance state index difference (| ci|), which represent the state of dilatancy resistance of the rock and the shear sliding resistance capacity of the crack-like pores during the compressive period, respectively. The deviatoric stress corresponding to the maximum | ci| is taken as the ci . We then examine the feasibility and validity of the VSRM using the experimental results. The results from the VSRM are also compared with those calculated by other strain-based methods, including the volumetric strain method (VSM), crack volumetric strain method (CVSM), lateral strain method (LSM) and lateral strain response method (LSRM). Compared with the other methods, the VSRM is effective and reduces subjectivity when determining the ci . Finally, with the help of the proposed VSRM, influences from chemical corrosion and confining stress on the ci and ci of the carbonate rock are analyzed. This study provides a subjective and practical method for determining σci . Moreover, it sheds light on the effects of confinement and chemical corrosion on σci

A data-based reduced-order model for dynamic simulation and control of district-heating networks

This study concerns the development of a data-based compact model for the prediction of the fluid temperature evolution in district heating (DH) pipeline networks. This so-called “reduced-order model” (ROM) is obtained from reduction of the conservation law for energy for each pipe segment to a semi-analytical input–output relation between the pipe outlet temperature and the pipe inlet and ground temperatures that can be identified from training data. The ROM basically is valid for generic pipe configurations involving 3D unsteady heat transfer and 3D steady flow as long as heat-transfer mechanisms are linearly dependent on the temperature field. Moreover, the training data can be generated by physics-based computational “full-order” models (FOMs) yet also by (calibration) experiments or field measurements. Performance tests using computational training data for a single-pipe configuration demonstrate that the ROM (i) can be successfully identified and (ii) can accurately describe the response of the outlet temperature to arbitrary input profiles for inlet and ground temperatures. Application of the ROM to two case studies, i.e. fast simulation of a small DH network and design of a controller for user-defined temperature regulation of a DH system, demonstrate its predictive ability and efficiency also for realistic systems. Dedicated cost analyses further reveal that the ROM may significantly reduce the computational costs compared to FOMs by (up to) orders of magnitude for higher-dimensional pipe configurations. These findings advance the proposed ROM as a robust and efficient simulation tool for practical DH systems with a far greater predictive ability than existing compact models

Theory and experiment of differential acoustic resonance spectroscopy

Abstract Recent advances in Differential Acoustic Resonance Spectroscopy (DARS) techniques have given rise to applications in the field of poromechanics. We report on the experimental demonstration of bulk modulus measurements on poroelastic samples at sonic frequencies (1 kHz) with DARS. Normal mode perturbation is due to scattering of a foreign object (i.e., a rock sample) within an otherwise fluid-filled resonator. The perturbation theory on an elastic object determines its bulk modulus (inverse compressibility). The experimental bulk modulus of medium-to high-permeability (>10 mD) poroelastic samples is in agreement with predictions from quasi-static loading of a porous sphere using the Biot theory. This result demonstrates that pore fluid flow governs the dominant relaxation process of the rock during compression. For low-permeability samples (<10 mD), pressure equilibration via slow wave diffusion is limited, and only qualitative agreement is found between the upper bound (Gassmann undrained modulus) and the lower bound (volume-weighted compressibilities of the two constituents). DARS experiments, in conjunction with the poroelastic theory presented here, allow one to infer such rock physical properties as the effective bulk modulus at sonic frequencies