A water–water preheater and a steam–water heater network: Temperature dynamics

The dynamic simulation of an integrated, double pipe heat exchanger network was validated through experimentation. A steam–water, concentric tube, heat exchanger was coupled to a water–water preheater. When the preheater was configured for cocurrent flow with equal fluid velocities in its annulus and core, Lagrangian-based derivations yielded analytical solutions that accurately predicted observed temperature dynamics. When the preheater was configured for countercurrent flow with distinct fluid velocities in its annulus and core, analytical solutions for the heater and connecting tubing were coupled with Eulerian based numerical solutions for the preheater. Programming used Mathcad. Nonlinear regression analysis of steady state data was used to determine system parameters. The significance of time delays through the integration of unit operations is illustrated.     

Hydropower–water and renewable energy in Turkey: Sources and policy

Turkey's energy consumption has been growing much faster than its production. It forces Turkey to make a rapid action to supply energy demand. From the viewpoint of primary energy sources (petroleum and natural gas), Turkey is not a rich country, but it has an abundant hydropower potential to be used for generation of electricity. Hydropower is the most important kind of renewable, sustainable energy and a proven technology for electricity generation. The aim of this paper is to discuss sources and policy of hydropower, water and renewable energy in Turkey and compares the hydropower application with Europe.     

Isothermal and non-isothermal oil–water flow and viscous fingering in a porous medium

Immiscible flow of heavy oil in a porous formation by high temperature pressurized water has been numerically studied. The physical region is a square domain in the horizontal plane with low and high pressure points at the opposite corners along one of the diagonals. Water, the invading fluid, when introduced at high pressure displaces the in situ oil towards the low pressure production zone. The extent of displacement of oil by water through the porous medium in a given amount of time and the appearance of preferential flow paths ( fingers) is the subject of the present investigation. The resistance to water–oil movement arises from the viscous forces in the fluid phases and the capillary force at their interface. Based on their relative magnitudes, various forms of displacement mechanisms can be realized. As the viscosity ratio of heavy oil to water is large, viscous forces in the oil phase become dominant and constitute the major factor for controlling the flow distortions in the porous formation. A mathematical model that can treat the individual fluid pressures, capillary effects and heat transfer has been employed in the present work. A fully implicit, two-dimensional numerical model has been used to compute the pressure and temperature fields. The domain decomposition technique has been adopted in the numerical solution since the problem is computationally intensive. Naturally occurring oil-rich reservoirs to which the present study is applicable are inhomogeneous and layered. A qualitative study has been carried out to explore the effect of permeability variations on the flow patterns. Numerical calculations show that non-isothermal effects as well as layering promote the formation of viscous fingers and consequently the sweep efficiency of the high pressure water front.     

Noble fabrication of Ni–Mo cathode for alkaline water electrolysis and alkaline polymer electrolyte water electrolysis

Among the catalysts for hydrogen evolution reaction (HER) in alkaline media, Ni–Mo turns out to be the most active one. Conventional preparations of Ni–Mo electrode involve repeated spraying of dilute solutions of precursors onto the electrode substrate, which is time-consuming and usually results in cracking and brittle electrodes. Here we report a noble fabrication of Ni–Mo electrode for HER. NiMoO₄ powder was synthesized and used as the precursor. After reduction in H₂ at 500 °C, the NiMoO₄ powder layer was converted to a uniform and robust electrode containing metallic Ni and amorphous Mo(IV) oxides. The distribution of Ni and Mo components in this electrode is naturally uniform, which can maximize the interaction between Ni and Mo and benefit the electrocatalysis. The thus-obtained Ni–Mo electrode exhibits a very high catalytic activity toward the HER: the current density reaches 700 mA/cm² at 150 mV overpotential in 5 M KOH solution at 70 °C. This new fabrication method of Ni–Mo electrode is not only suitable for alkaline water electrolysis (AWE), but also applicable to the alkaline polymer electrolyte water electrolysis (APEWE), an emerging technique for efficient production of H₂.     

Study of hydrogen production and storage based on aluminum–water reaction

The rate and yield of hydrogen production from the reaction between activated aluminum and water has been investigated. The effect of different parameters such as water–aluminum ratio, water temperature and aluminum particle size and shape was studied experimentally. The aluminum activation method developed in-house involves 1%–2.5% of lithium-based activator which is diffused into the aluminum particles, enabling sustained reaction with tap water or sea water at room temperature. Hydrogen production rates in the range of 200–600 ml/min/g Al, at a yield of about 90%, depending on operating parameters, were demonstrated. The work further studied the application in proton exchange membrane (PEM) fuel cells in order to generate green electric energy, demonstrating theoretical specific electric energy storage that can exceed batteries by 10–20 folds.     

Simulation of ash–water interaction and its influence on ash characteristics

With ever-increasing demand for electricity, the production of ash, produced from the coal-fired thermal power plants, and its hazardous impact on the environment, is continuously increasing. This poses a very challenging task of safe handling, proper disposal and utilization of the ash. Several ash utilization schemes, developed and reported by earlier researchers, are being practiced frequently these days. However, these studies do not take into account the quality of ash produced and the changes it may undergo, before it can be used for different applications.A common method of disposing ash is its wet disposal, where the ash is mixed with water to make slurry and is disposed off in the ash ponds or lagoons. Such a disposal system causes ash, and the alkalis present in it, to interact with water over a period of time and may lead to the formation of ash zeolites. As such, it would be interesting to study the effect of this interaction (i.e. formation of zeolites also termed as zeolitization of the ash) on physical, chemical and mineralogical characteristics of the ash. As geotechnical properties of a material depend on these characteristics, the influence of zeolitization on these properties of the ash must also be investigated. Such investigations are essential for the bulk utilization of the lagoon ash, in particular as a fill material, where properties like compaction, consolidation, hydraulic conductivity and its shear strength are very important.In order to simulate such ash–water interaction, controlled laboratory experiments have been conducted on a typical Indian lagoon ash. The present study deals with the details of the effect of zeolitization on physical, chemical, mineralogical and geotechnical characteristics of this ash. Studies were also conducted to explore the possibility of utilization of the lagoon ash, and the zeolitized ash, for various environmental applications viz. retention and removal of heavy metals from the industrial sludge.     

Finite time thermodynamics study and exergetic analysis of ammonia–water absorption systems

This paper presents an optimization study of a single stage absorption machine operating with an ammonia–water mixture under steady state conditions. The power in the evaporator, the temperatures of the external fluids entering the four external heat exchangers as well as the effectiveness of these heat exchangers and the efficiency of the pump are assumed fixed. The results include the minimum value of the total thermal conductance UA_tot as well as the corresponding mean internal temperatures, overall irreversibility and exergetic efficiency for a range of values of the coefficient of performance (COP). They show the existence of three optimum values of the COP: the first minimises UA_tot, the second minimises the overall irreversibility and the third maximises the exergetic efficiency. They also show that these three COP values are lower than the maximum COP which corresponds to the convergence of the internal and external temperatures towards a common value. The influence of various parameters on the minimum thermal conductance of the heat exchangers and on the corresponding exergy efficiency has also been evaluated. From an exergetic viewpoint it is interesting to reduce the temperature at the desorber and at the evaporator and to raise the values of that parameter at the condenser and the absorber. However these changes must be accompanied by an important increase in the total UA if it is desired to conserve a constant COP. The internal heat exchangers between the working fluid and the solution improve both the overall exergy efficiency and the coefficient of performance of the absorption apparatus.     

Marangoni convection flow and heat transfer characteristics of water–CNT nanofluid droplets

ABSTRACT

The heat transfer characteristics of liquid droplets are influenced by the hydrophobicity of the surfaces. Fluid properties and surface energy play important roles in heat transfer assessment. In the present study, the influence of the contact angle on the flow field developed inside a nanofluid droplet consisting of a mixture of water and carbon nanotubes (CNT) is investigated. Flow field and heat transfer characteristics are simulated numerically in line with the experimental conditions. It is found that the flow velocity predicted numerically is in good agreement with the experimental data. Nusselt and Bond numbers increase at large contact angles and Marangoni force dominates over buoyancy force.     

Innovative concept for gasification for hydrogen based on the heat integration between water gas shift unit and coal–water–slurry gasification unit

In order to achieve the energy cascade utilization and improve the energy utilization efficiency of coal–water–slurry (CWS) gasification for hydrogen system, the heat integration scheme (HIS) between the water gas shift unit and the gasification unit is put forward. The effects of temperature change of CWS and oxygen on the gasification performance are investigated. Both the HIS and the non-heat integration scheme (NHIS) are analyzed by using gasification performance, energy conversion efficiency and exergy efficiency. The results show that the specific coal consumption and the specific oxygen consumption decrease by 2.7% and 6.5%, respectively, as the feedstock is preheated up to the temperature of 150 °C. The energy conversion efficiency of HIS and NHIS are nearly the same. The exergy efficiency of HIS (62.66%) is better than that of NHIS (62.02%). Therefore, the HIS is better than the NHIS by heat integration between the WGS unit and the gasification unit.     

Combustion of micro aluminum–water mixtures

An experimental investigation on the combustion behavior of micro-sized aluminum (μAl)–water mixtures was conducted. It was easily ignited and self-deflagrated on μAl and liquid water when using a paper shell tube. Linear burning rates of quasi-homogeneous mixtures of μAl and liquid water as a function of pressure, mixture composition, density and environment gas medium were measured. Steady-state burning rates were obtained at room temperature using a windowed vessel for a pressure range of 1–80 bar in a nitrogen atmosphere, particle size of 0.5 × 30 × 30 μm and overall mixture equivalence ratios from 0.67 to 2.0. The pressure exponent was obtained as 0.47 at room temperature and compared to the case of nano-sized aluminum (nAl) and liquid water. When a wire was inserted into the sample, for increasing local heat transfer, burning rates were found to be faster.     

Performance comparison of double-effect parallel-flow and series flow water–lithium bromide absorption systems

The double-effect parallel flow absorption refrigeration cycle with water–lithium bromide as working fluid is analysed based on the concept of equilibrium temperature at the low pressure generator. Coefficient of performance (COP) and its sensitivity to operating conditions are compared with those for series flow cycle. Maximum attainable COP for parallel flow cycle is greater than that for series flow cycle throughout the range of operating conditions considered here. Performance of parallel flow system is more sensitive to the effectiveness of low pressure heat exchanger than that of series flow system.     

Self-supported multidimensional Ni–Fe phosphide networks as novel and robust water splitting catalyst

The water splitting has become one of the most promising hydrogen production methods. The Ni–Fe–P materials were first synthesized and in situ grown on nickel foam by typical hydrothermal and phosphating methods. The Ni–Fe–P-300 catalyst shows excellent water splitting activity (cell voltage of 1.59 V @10 mA cm⁻²) and stability after phosphating. The results of density functional theory (DFT) demonstrate that the water molecules preferentially adsorbed on the Fe site and Fe might be the real catalytic active site. A series of characterization indicated that a small amount of phosphorus loss was probably caused on the catalyst surface, but the electrocatalytic activity was not affected by the small amount of oxide species formation. This study offers a promising way to design and optimize electrocatalysts for the water splitting in alkaline solution.     

Modeling and measurements of water–vapor flow and icing at low pressures with application to pharmaceutical freeze-drying

A modeling and computational framework for analysis of rarefied water vapor flow and icing in application to pharmaceutical freeze-drying is developed. The direct simulation Monte Carlo (DSMC) technique is applied to model the relevant gaseous transport processes in a low-pressure environment encountered in freeze-drying. The developing ice front on a supercooled surface is simulated based on the water vapor mass flux computed from DSMC. Verification of icing simulations has been done by comparison with the analytical solution for a free-molecular flow over a circular cylinder. To validate the vapor flow and icing simulations, measurements of ice accretion in a laboratory-scale freeze-dryer are conducted with the use of time-lapse photography. The simulations corresponding to the measured time-average water sublimation rate agree well with the observed patterns and rates of ice accretion. The developed computational framework has been applied to investigate factors underlying the observed non-uniformity of ice growth. It has been shown that two key factors impact the uniformity of ice accretion: (i) the direction of the vapor flow at the inlet to the condenser which is governed by the geometry of a duct connecting the product chamber to the condenser; and (ii) the pressure of non-condensable gases in the condenser reservoir. The DSMC simulations demonstrate that by tailoring the condensing surfaces topology to the flowfield structure of the water vapor jet expanding into a low-pressure reservoir, it is possible to significantly increase the total rate of vapor removal and improve the overall efficiency of the freeze-drying process.     

Oxygen-enhanced water gas shift over ceria-supported Au–Cu bimetallic catalysts prepared by wet impregnation and deposition–precipitation

Au–Cu/ceria bimetallic catalysts were prepared incorporating Au by incipient wetness impregnation (IWI) and deposition-precipitation (DP) methods (with loadings of 1 wt.% and 7 wt.% of Au and Cu, respectively). The as-prepared catalysts were characterized by techniques such as BET, XRD, Raman, XPS, H₂-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements. The results indicated a good dispersion of gold and copper for copper ceria catalyst and Au–Cu bimetallic catalysts. Addition of Au to CuO/CeO₂ increases highly the capacity to release lattice oxygen to oxidized CO at low temperatures compared to pure CuO/CeO₂. Au/CeO₂ and Au–CuO/CeO₂ catalyst prepared by DP show higher OSC value than counterparts prepared by IWI, either at 120 and 250 °C. Also, gold-containing catalysts prepared by DP show lower temperature of reduction that the samples prepared by IWI as a consequence of the higher dispersion of gold in the former samples. The presence of gold at different oxidation states was observed by XPS analysis. Preparation method strongly affects to the atom ratio of Au and Au + Cu with respect to surface ceria. The gold incorporation method was a key factor that enhances the redox properties and activity in both WGS and OWGS reactions. The present study shows the gas phase oxygen enhanced the activity of monometallic CuO/ceria and bimetallic Au–Cu/ceria prepared by IWI and DP methods in both WGS and OWGS reactions. AuCC catalyst prepared by DP shows higher hydrogen yield and also higher CO conversion than other prepared by IWI during OWGS reaction.     

Zinc–nickel alloy electrodeposits for water electrolysis

Electrodeposited zinc–nickel alloys of various compositions were prepared. A suitable electrolyte and conditions to produce alloys of various compositions were identified. Alloys produced on electroformed nickel foils were etched in caustic to leach out zinc and to produce the Raney type, porous electro catalytic surface for hydrogen evolution. The electrodes were examined by polarization measurements, to evaluate their Tafel parameters, cyclic voltammetry, to test the change in surface properties on repeated cycling, scanning electron microscopy to identify their microstructure and X-ray diffraction. The catalytic activity as well as the life of the electrode produced from 50% zinc alloy was found to be better than others.     

Thermodynamic models for H2O–CO2–H2 mixtures in near-critical and supercritical regions of water

The PVTx properties of the H₂O–CO₂–H₂ mixtures have significant applications in the technology of supercritical water gasification of coal. Here, we first carry out the molecular dynamics simulations of the PVTx properties of the H₂O–CO₂–H₂ mixtures in the near-critical and supercritical regions of water to generate 600 datasets at 750–1150 K and 4.0–443.5 MPa. The molar fraction of each composition in the ternary mixtures ranges from 10% to 80%. Later we investigate the applicability of a well-known thermodynamic model for the ternary mixtures, namely the Duan-Møller-Weare equation of state (DMW EOS). It is observed that the DMW EOS shows great potential in the prediction of the PVTx properties of the ternary mixtures. However, it is noted that the mixing parameters describing the binary interactions of H₂O–H₂ and CO₂–H₂ are still unknown in the DMW EOS. By determining the missing mixing parameters using the Levenberg-Marquardt algorithm, the accuracy of the original DMW EOS is improved for the ternary mixtures. Moreover, optimizing the coefficients in the DMW EOS further promotes the accuracy of the model for the H₂O–CO₂–H₂ mixtures. The results from this work may facilitate the development of supercritical water gasification of coal.     

Development and comparison of two-bed silica gel–water adsorption chillers driven by low-grade heat source

A novel silica gel–water adsorption chiller (driven by hot water of 60–90 °C) with three vacuum chambers has been built in Shanghai Jiao Tong University (SJTU). This chiller was an improvement of an earlier deigned chiller and it integrated two single-bed systems (basic system) with only one vacuum valve. The performance of the chiller was tested and compared with the former adsorption chiller. The results show that the cooling power and COP of the chiller are 8.70 kW and 0.39 for the heat source temperature of 82.5 °C, cooling water temperature of 30.4 °C and chilled water outlet temperature of 12 °C. For a higher chilled water outlet temperature of about 16 °C, the COP increases to 0.43 while the cooling power is about 11.0 kW. Compared with that of the former chiller, the COP of this chiller increases by 20%.     

Investigation into hydrolysis and alcoholysis of sodium borohydride in ethanol–water solutions in the presence of supported Co–Ce–B catalyst

The efficacies of attapulgite clay (ATC)-, titanium dioxide (TiO₂)- and silica gel (SG)-supported cobalt–cerium–boron (Co–Ce–B) substances as catalysts were investigated for the alcoholysis and hydrolysis of sodium borohydride (NaBH₄) in ethanol–water solutions. Ce served as a helpful co-catalyst among the prepared Co–Ce–B catalysts, and the catalytic activity decreased in the following sequence: TiO₂-supported > ATC-supported > SG-supported > unsupported. The effects of Ce/(Co+Ce) molar ratio, ethanol concentration, reaction temperature, NaBH₄ concentration and NaOH concentration on the hydrogen production rate were investigated. For the ATC-supported catalyst, when the Ce/(Co+Ce) molar ratio was 10%, the catalyst exhibited the best catalytic activity. Optimal NaBH₄ concentration, NaOH concentration and ethanol concentration to promote hydrogen generation rate was around 8 wt.%, 15 wt.% and 30 wt.%, respectively. It can be found that the addition of ATC greatly improved the recycle ability of the catalysts in the multi-cycle tests. The surface morphology of the catalysts before and after the recycle tests was studied from SEM images. The compositions of the catalysts were determined by XRD and EDS analyses. The occurrence of NaB(OH)₄ in the alcoholysis by-product provided pertinent indications of ethanol recovery after the tests. The value of activation energy in the hydrogen generation process in the presence of ATC-supported Co–Ce–B catalyst was calculated to be 29.51 kJ/mol. An overall kinetic equation was also proposed.     

Review of seasonal heat storage in large basins: Water tanks and gravel–water pits

In order to respond to climatic change, many efforts have been made to reduce harmful gas emissions. According to energy policies, an important goal is the implementation of renewable energy sources, as well as electrical and oil combustion savings through energy conservation. This paper focuses on an extensive review of the technologies developed, so far, for central solar heating systems employing seasonal sensible water storage in artificial large scale basins. Among technologies developed since the late 1970s, the use of underground spaces as an energy storage medium – Underground Thermal Energy Storage (UTES) – has been investigated and closely observed in experimental plants in many countries, most of them, as part of government programmes. These projects attempt to optimise technical and economic aspects within an international knowledge exchange; as a result, UTES is becoming a reliable option to save energy through energy conservation. Other alternatives to UTES include large water tanks and gravel–water pits, also called man-made or artificial aquifers. This implies developing this technology by construction and leaving natural aquifers untouched. The present article reviews most studies and results obtained in this particular area to show the technical and economical feasibility for each system and specifics problems occurred during construction and operation. Advantages and disadvantages are pointed out to compare both alternatives. The projects discussed have been carried out mainly in European states with some references to other countries.