Crystallization fouling of CaCO3 – Analysis of experimental thermal resistance and its uncertainty

Crystallization fouling occurs when dissolved salts precipitate from an aqueous solution. In the case of inversely soluble salts, like calcium carbonate (CaCO₃), this may lead to crystal growth on heated walls. Crystallization may also take place in the bulk solution either via homogeneous nucleation or heterogeneous nucleation on suspended material.In this paper, surface crystallization of CaCO₃ and crystallization in the bulk fluid and its effect on the fouling rate on a heated wall are studied. The fouling experiments are done in a laboratory scale set-up of a flat plate heat exchanger. Accuracy of the results is analyzed by uncertainty analysis. SEM and XRD are used to determine the morphology and the composition of the deposited material.The uncertainty analysis shows that the bias and precision uncertainties in the measured wall temperature are the largest source of uncertainty in the experiments. The total uncertainty in the fouling resistance in the studied case was found to be ±13.5% at the 95% confidence level, which is considered to be acceptable.Surface crystallization rate is found to be controlled by the wall temperature indicating that the surface integration dominates the fouling process. The flow velocity affects the fouling rate especially at high wall temperature by decreasing the fouling rate with increasing flow velocity. Crystallization to the bulk fluid is found to enhance significantly the fouling rate on the surface when compared to a case in which fouling is due to crystal growth on the surface.     

Miniature heat-pipe thermal performance prediction tool – software development

The software for flat miniature heat-pipe parameters (Q_max, R_hp, temperature field along the pipe surface, heat transfer coefficients in the evaporator and condenser zones h_e, h_c, etc.) prediction and numerical modeling was developed. The experimental data received for the flat miniature heat pipe (2.5–4 mm thickness, 50–250 mm length, 8–11 mm width) with a copper sintered powder wick saturated with water were compared with the data of numerical analysis and results showed that experimental verification testifies the validity of the software application.     

Interface thermal characteristics of flip chip packages – A numerical study

Flip chip ball grid array (FC-BGA) packages are commonly used for high inputs/outputs (I/O) ICs; they have been proven to provide good solutions for a variety of applications to maximize thermal and electrical performance. A fundamental limitation to such devices is the thermal resistance at the top of the package, which is characterized θ_JC parameter. The die-to-lid interface thermal resistance is identified as a critical issue for the thermal management of electronic packages. This paper focuses on the effect of the interface material property changes on the interface thermal resistance. The effect of package’s junction to case (Theta-JC or θ_JC) thermal performance is investigated for bare die, flat lid and cup lid packages using a validated thermal model. Thermal performance of a cup or flat lid attached and bare die packages were investigated for different interface materials. Improved Theta-JC performance was observed for the large die as compared to the smaller die. Several parametric studies were carried out to understand the effects of interface bond line thickness (BLT), different die sizes, the average void size during assembly and thermal conductivity of interface materials on package thermal resistance.     

Prediction of the thermal conductivity of metal hydrides – The inverse problem

With sustainability as an important and driving theme, not merely of research, but that of our existence itself, the effort in developing sustainable systems takes many directions. One of these directions is in the transport sector, particularly personal transport using hydrogen as fuel, which logically leads on to the problem of hydrogen storage. This paper deals with the prediction of the effective conductivity of beds of metal hydride for hydrogen storage. To enable modeling of the effective thermal conductivity of these systems, it is necessary to arrive at the functional dependence of the thermal conductivity of the solid hydride on its hydrogen concentration or content. This is the inverse problem in thermal conductivity of multiphase materials. Inverse methods in general are those where we start from known consequences in order to find unknown causes. Using published and known data of the effective thermal conductivity of the hydride–hydrogen assemblage, we arrive at the unknown hydride conductivity by analysis. Among the models available in the literature for determination of the effective conductivity of the bed from the properties of the constituent phases, the model of Raghavan and Martin is chosen for the analysis as it combines simplicity and physical rigor. The result is expected to be useful for predicting the thermal conductivity of hydride particles and determining the optimum heat transfer rates governing the absorption and desorption rates of hydrogen in the storage system.     

Optimization of hybrid – ground coupled and air source – heat pump systems in combination with thermal storage

Ground coupled heat pumps are attractive solutions for cooling and heating commercial buildings due to their high efficiency and their reduced environmental impact. Two possible ideas to improve the efficiency of these systems are decoupling energy generation from energy distribution and combining different HVAC systems. Based on these two ideas, we present several HVAC configurations which combine the following equipments: a ground coupled heat pump, an air to water heat pump and a thermal storage device. These HVAC configurations are linked to an office building in a cooling dominated area in order to evaluate in these conditions the total electrical consumption of each configuration to obtain which one satisfy the thermal demand more efficiently. The results of our simulations show that the electrical energy consumption obtained when the system employs a suitable configuration is of around the 60% compared with an HVAC system driven by an air to water heat pump and around the 82% compared with an HVAC system driven by a ground coupled heat pump.     

Packed-bed thermal storage for concentrated solar power – Pilot-scale demonstration and industrial-scale design

A thermal energy storage system, consisting of a packed bed of rocks as storing material and air as high-temperature heat transfer fluid, is analyzed for concentrated solar power (CSP) applications. A 6.5 MWh_th pilot-scale thermal storage unit immersed in the ground and of truncated conical shape is fabricated and experimentally demonstrated to generate thermoclines. A dynamic numerical heat transfer model is formulated for separate fluid and solid phases and variable thermo-physical properties in the range of 20–650 °C, and validated with experimental results. The validated model is further applied to design and simulate an array of two industrial-scale thermal storage units, each of 7.2 GWh_th capacity, for a 26 MW_el round-the-clock concentrated solar power plant during multiple 8 h-charging/16 h-discharging cycles, yielding 95% overall thermal efficiency.     

Efficient STEP (solar thermal electrochemical photo) production of hydrogen – an economic assessment

A consideration of the economic viability of hydrogen fuel production is important in the STEP (Solar Thermal Electrochemical Photo) production of hydrogen fuel. STEP is an innovative way to decrease costs and increase the efficiency of hydrogen fuel production, which is a synergistic process that can use concentrating photovoltaics (CPV) and solar thermal energy to drive a high temperature, low voltage, electrolysis (water-splitting), resulting in H₂ at decreased energy and higher solar efficiency. This study provides evidence that the STEP system is an economically viable solution for the production of hydrogen. STEP occurs at both higher electrolysis and solar conversion efficiencies than conventional room temperature photovoltaic (PV) generation of hydrogen. This paper probes the economic viability of this process, by comparing four different systems: (1) 10% or (2) 14% flat plate PV driven aqueous alkaline electrolysis H₂ production, (3) 25% CPV driven molten electrolysis H₂ production, and (4) 35% CPV driven solid oxide electrolysis H₂ production. The molten and solid oxide electrolysers are high temperature systems that can make use of light, normally discarded, for heating. This significantly increases system efficiency. Using levelized cost analysis, this study shows significant cost reduction using the STEP system. The total price per kg of hydrogen is shown to decrease from $5.74 to $4.96 to $3.01 to $2.61 with the four alternative systems. The advanced STEP plant requires less than one seventh of the land area of the 10% flat cell plant. To generate the 216 million kg H₂/year required by 1 million fuel cell vehicles, the 35% CPV driven solid oxide electrolysis requires a plant only 9.6 mi² in area. While PV and electrolysis components dominate the cost of conventional PV generated hydrogen, they do not dominate the cost of the STEP-generated hydrogen. The lower cost of STEP hydrogen is driven by residual distribution and gate costs.     

SusDesign – An approach for a sustainable process system design and its application to a thermal power plant

This paper presents a structured process design approach, SusDesign, for the sustainable development of process systems. At each level of process design, design alternatives are generated using a number of thermodynamic tools and applying pollution prevention strategies followed by analysis, evaluation and screening processes for the selection of potential design options. The evaluation and optimization are carried out based on an integrated environmental and cost potential (IECP) index, which has been estimated with the IECP tool. The present paper also describes a flowsheet optimization technique developed using different thermodynamic tools such as exergy/energy analysis, heat and mass integration, and cogeneration/trigeneration in a systematic manner.The proposed SusDesign approach has been successfully implemented in designing a 30 MW thermal power plant. In the case study, the IECP tool has been set up in Aspen HYSYS process simulator to carry out the analysis, evaluation and screening of design alternatives.The application of this approach has developed an efficient, cost effective and environmentally friendly thermal system design with an overall thermal efficiency of 70% and CO₂ and NO emissions of 0.28 kg/kW h and 0.2 g/kW h respectively. The cost of power generation is estimated as 4 ¢/kW h. These achievements are significant compared to the conventional thermal power plant, which demonstrates the potential of the SusDesign approach for the sustainable development of process systems.     

Equilibrium of NH4SCNNH3 system for thermal energy storage

A solid-gas reaction of ammonium thiocyanate (NH₄SCN) and ammonia produces liquid ammoniate (NH₄SCN·nNH₃). The region of the liquid phase and the equilibrium properties of the ammoniate have been determined. Crystalization of the ammoniate was not observed, though the liquid was cooled to − 10°C. The enthalpy changes in the liquid phase were also estimated. Accordingly, this system has a wide range of liquid phase and offers a medium for thermal energy storage or a chemical heat pump system.     

Auto-ignition model based on tabulated detailed kinetics and presumed temperature PDF – Application to internal combustion engine controlled by thermal stratifications

The prediction of the auto-ignition sensitivity to the temperature in new engine combustors is a challenge in the community of numerical combustion. This paper is devoted to the modeling of temperature fluctuations for the simulation of reactive flows in real internal engine configurations. It aims at validating the temperature fluctuation equation model Truffin and Benkenida and its coupling with the ECFM3Z combustion model of Colin and Benkenida. Especially, this study focuses on the auto-ignition process which is described by the TKI (Tabulated Kinetics for Ignition) model of Knop and Jay. TKI is based on a tabulation method for reaction rates and its coupling with the temperature fluctuation is achieved through a presumed PDF approach. The integral limits for the PDF integration are determined locally through transport equations with appropriate closures on isothermal walls. The resulting model, called TKI–υ_T, is applied on Homogeneous Charge Compression Ignition (HCCI) combustion mode engine for which the only source of thermal stratification is wall heat loss. Comparisons with experiments demonstrate the impact of temperature fluctuations and the ability of the model to improve the prediction of the auto-ignition model.     

Retrieval of thermal properties in a transient conduction–radiation problem with variable thermal conductivity

This article reports an inverse analysis of a transient conduction–radiation problem with variable thermal conductivity. Simultaneous retrieval of parameters is accomplished by minimizing the objective function represented by the square of the difference between the measured and the assumed temperature fields. The measured temperature field is calculated from the direct method involving the lattice Boltzmann method (LBM) and the finite volume method (FVM). In the direct method, the FVM is used to obtain the radiative information and the LBM is used to solve the energy equation. With perturbations imposed on the measured temperature data, minimization of the objective function is achieved with the help of the genetic algorithm (GA). The accuracies of the retrieved parameters have been studied for the effects of the genetic parameters such as the crossover and the mutation rates, the population size, the number of generations and the effect of noise on the measured temperature data. A good estimation of parameters has been obtained.     

Experimental investigation and model development for thermal conductivity of α-Al2O3-glycerol nanofluids

In order to investigate the effect of nanoparticle volume fraction, nanoparticle size and temperature on the thermal conductivity of glycerol based alumina (α-Al₂O₃) nanofluids, a set of experiments were carried out for temperature ranging from 20 °C to 45 °C. The nanofluids contained α-Al₂O₃ nanoparticles of three different sizes (31 nm, 55 nm and 134 nm) were prepared by two-step method at volume fractions ranging from 0.5% to 4%. The experimental results show that α-Al₂O₃-glycerol nanofluids have substantially higher thermal conductivity than the base fluid and the maximum enhancement of the relative thermal conductivity was 19.5% for the case of 31 nm at 4% volume fraction. The data analyses indicated that the volume fraction and size of the nanoparticles have significant effects on the thermal conductivity ratio (TCR) of Al₂O₃-glycerol nanofluids, while the temperature has almost no significant effect on the data for range of this study. At room temperature, the effective thermal conductivity remains almost constant for 50 h at 4% volume fractions. The comparison of the obtained experimental data and predictions from some existing theoretical and empirical models reveals that the thermal conductivity ratio and its trend could not be accurately explained by the models in open literature. Consequently, a new empirical correlation based on the experimental data has been developed in this study.     

Cu substituted ZnAl2O4 ex-LDH catalysts for medium-temperature WGS – effect of Cu/Zn ratio and thermal treatment on catalyst efficiency

The effect of Cu/Zn ratio of ex-LDH oxide-based catalysts for medium–temperature water–gas shift reaction (MT–WGS) was investigated. A series of CuZnAl–LDH precursors with different Cu/Zn molar ratio were synthesized by co-precipitation and oxide (Zn,Cu)_xAl₂O₄ catalysts were prepared via subsequent calcinations at 380 °C or 700 °C. The prepared materials were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetry with evolved gas analysis (TG/DTG/EGA), N₂ adsorption, N₂O chemisorption and temperature-programmed reduction (H₂-TPR). MT-WGS activity evaluation was carried out on the basis of measurements made in a differential reactor in kinetic regime. Catalysts’ properties were investigated and effect of composition (Cu/Zn molar ratio) and the calcination temperature of CuZnAl-LDH precursors on structural transformation, active surface area and MT-WGS rate constant was shown. The highest activity of (Zn,Cu)_xAl₂O₄ catalyst with Cu/Zn molar ratio of 1.5 calcined at mild conditions was attributed to easy reducible and accessible Cu surface.     

Modeling of a thermal energy storage system based on coupled metal hydrides (magnesium iron – sodium alanate) for concentrating solar power plants

Concentrating solar power plants represent low cost and efficient solutions for renewable electricity production only if adequate thermal energy storage systems are included. Metal hydride thermal energy storage systems have demonstrated the potential to achieve very high volumetric energy densities, high exergetic efficiencies, and low costs. The current work analyzes the technical feasibility and the performance of a storage system based on the high temperature Mg₂FeH₆ hydride coupled with the low temperature Na₃AlH₆ hydride. To accomplish this, a detailed transport model has been set up and the coupled metal hydride system has been simulated based on a laboratory scale experimental configuration. Proper kinetics expressions have been developed and included in the model to replicate the absorption and desorption process in the high temperature and low temperature hydride materials. The system showed adequate hydrogen transfer between the two metal hydrides, with almost complete charging and discharging, during both thermal energy storage and thermal energy release. The system operating temperatures varied from 450 °C to 500 °C, with hydrogen pressures between 30 bar and 70 bar. This makes the thermal energy storage system a suitable candidate for pairing with a solar driven steam power plant. The model results, obtained for the selected experimental configuration, showed an actual thermal energy storage system volumetric energy density of about 132 kWh/m³, which is more than 5 times the U.S. Department of Energy SunShot target (25 kWh/m³).     

A global transient,one-dimensional,two-phase model for direct methanol fuel cells (DMFCs) – Part II: Analysis of the time-dependent thermal behavior of DMFCs

A transient-thermal model based on a lumped system is newly developed and implemented in a one-dimensional (1D), two-phase rigorous direct methanol fuel cell (DMFC) model presented in Part I. In this model, the main focus lies on the investigation of the transient thermal behavior of DMFCs and its influence on methanol crossover, cell performance, and efficiency. 1D simulations are carried out and the time-dependent thermal behaviors of DMFCs are analyzed for various methanol-feed concentrations and external heat-transfer conditions. Predicting the close interactions between the evolution of the transient temperature, methanol crossover, cell voltage, and efficiency during DMFC operations, the simulations of transient behavior indicate that the insufficient cooling of DMFCs finally lead to thermal runaway, particularly under high methanol-feed concentrations. Therefore, it is concluded that an efficient cooling system is greatly needed to safeguard DMFC operations and enhance the performance of DMFCs. The present 1D DMFC model is a useful tool for attaining a better understanding of complicated physical phenomena in DMFCs, which assists in optimizing the operating conditions of such cells and material/design parameters.     

Performance evaluation of photovoltaic/thermal–HDH desalination system

The efficiency of photovoltaic (PV) panel drops with increase in cell temperature. The temperature of the PV panel can be controlled with various cooling techniques. In the proposed work the PV panel is cooled by circulating water and the recovered heat energy is used to run a humidification and dehumidification desalination to produce distilled water from sea water (or) brackish water. This work deals with a detailed analysis of performance of combined power and desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system. A mathematical model of PV/thermal–humidification dehumidification plant was developed and simulations were carried out in MATLAB environment. The performance of photovoltaic/ thermal desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system was investigated under various solar radiation levels (800–1000 W/m²). For each solar radiation level the effect of mass flow rate of coolant water (30–110 kg/h) on water outlet temperature, PV efficiency, PVT thermal efficiency, distilled water production, and plant efficiency was studied. Results show that under each solar radiation level increasing coolant flow rate increases efficiency of PV panel and reduces the plant efficiency. The highest PV efficiency (16.598%) was reached under 800 W/m² at mass flow rate of 110 kg/h and the highest plant efficiency (43.15%) was reached under 800 W/m² at a mass flow rate of 30 kg/h. The maximum amount of distilled water production rate (0.82 L/h) was reached under 1000 W/m² at water mass flow rate of 30 kg/h.     

Temperature-explicit formulation of energy equation for thermal–hydraulic analyses

A temperature equation which is derived from an enthalpy transport equation by using an assumption of a constant specific heat is very attractive for analyses of heat and fluid flows. It can be used for an analysis of a solid–fluid conjugate heat transfer, and it does not need a numerical method to obtain temperature from a temperature–enthalpy relation. But its application is limited because of the assumption. A new method is derived in this study, which is a temperature-explicit formulation of the energy equation. The enthalpy form of the energy equation is used in the method. But the final discrete form of the equation is expressed with temperature. The discretized equation from the temperature-explicit formulation can be used for a heat transfer analysis in a solid–fluid coupled region without any special treatment at the solid–fluid interface. And it can be applied for multiphase flows with a real gas effect. It is found by numerical tests in this study that the proposed method is very efficient and as accurate as the standard enthalpy formulation.     

Forced oscillation in diffusion flames near diffusive–thermal resonance

In this work, we carry out a systematic analysis of forced oscillation in planar diffusion flames under weak external forcing. The external forcing is introduced by independently imposing a flow field with small amplitude fluctuations. Employing the asymptotic theory of Cheatham and Matalon, the linear response is first examined. It is shown that when the Damköhler number Da is close to the critical value Da¹ corresponding to the marginal state of diffusive–thermal pulsating instability, the imposed velocity fluctuation may induce very large amplitude of flame oscillation as the frequency of velocity fluctuation c approaches c₀, the flame oscillation frequency at the onset of instability. This is a resonance phenomenon between the imposed flow oscillations and the intrinsic flame oscillations that are driven by the diffusive–thermal instability, and hence we refer to this as the diffusive–thermal resonance. The nonlinear near-resonant response is then examined with the Damköhler number Da chosen to be very close to the critical Damköhler number Da¹, and we derive an evolution equation for the amplitude of forced oscillation. Examination of the evolution equation reveals that in most situations, flames with larger Lewis number of fuel, smaller initial mixture strength, and smaller temperature difference between the oxidant and fuel stream are more responsive to the external forcing.     

Synthesis and thermal properties of poly(styrene-co-ally alcohol)-graft-stearic acid copolymers as novel solid–solid PCMs for thermal energy storage

A series of poly(styrene-co-allyalcohol)-graft-stearic acid copolymers were synthesized as novel polymeric solid–solid phase change materials (SSPCMs). The graft copolymerization reactions between poly(styrene-co-allyalcohol) and stearoyl chloride were verified by Fourier transform infrared (FT-IR) and Proton Nuclear Magnetic Resonance (¹H NMR) spectroscopy techniques. The crystal morphology of the SSPCMs was investigated using polarized optical microscopy (POM) technique. Thermal energy storage properties of the synthesized SSPCMs were measured using differential scanning calorimetry (DSC) analysis. The POM results showed that the crystalline phase of the copolymers transformed to amorphous phase above their phase transition temperatures. Thermal energy storage properties of the synthesized SSPCMs were investigated by differential scanning calorimetry (DSC) and found that they had typical solid–solid phase transition temperatures in the range of 27–30 °C and high latent heat enthalpy between 34 and 74 J/g. Especially, the copolymer with the mole ratio of 1/1 (poly(styrene-co-allyalcohol)/stearoyl chloride) is the most attractive one due to the highest latent heat storage capacity among them. The results of DSC and FT-IR analysis indicated that the synthesized SSPCMs had good thermal reliability and chemical stability after 5000 thermal cycles. Thermogravimetric (TG) analysis results suggested that the synthesized SSPCMs had high thermal resistance. In addition, thermal conductivity measurements signified that the synthesized PCMs had higher thermal conductivity compared to that of poly(styrene-co-allyalcohol). The synthesized copolymers as novel SSPCMs have considerable potential for thermal energy storage applications such as solar space heating and cooling in buildings and greenhouses.