Energy–exergy optimization of comminution

The comminution process has exceedingly low efficiency because it is highly irreversible. An outline of energy analysis for comminution is presented. The application refers to an ore-processing plant, which consists of a series of crushers feeding a traditional ball mill that delivers products to a downstream metallurgical process. For optimization, the design characteristics are fixed, i.e. decision variables can only be operational parameters. The chosen decision variable is the size of the feed (F) to the mill. In practice, the mill operator may control the feed granulometry and keep the product size constant by using a constant ball charge. The objective cost function is the sum of energy costs at the crusher and mill, which depend only on F. The exergy consumption of the crusher and mill are evaluated using the Bond correlation, including pertinent correction factors. Optimization leads to a 10% saving in overall energy costs.     

Evaluation of thermal efficiency of double-pass solar collector with porous–nonporous media

The double-pass solar collector with porous media in the lower channel provides a higher outlet temperature compared to the conventional single-pass collector. Therefore, the thermal efficiency of the solar collector is higher. A theoretical model has been developed for the double-pass solar collector. An experimental setup has been designed and constructed. The porous media has been arranged in different porosities to increase heat transfer, area density and the total heat transfer rate. Comparisons of the theoretical and the experimental results have been conducted. Such comparisons include the outlet temperatures and thermal efficiencies of the solar collector for various design and operating conditions. The relationships include the effect of changes in upper and lower channel depth on the thermal efficiency with and without porous media. Moreover, the effects of mass flow rate, solar radiation, and temperature rises on the thermal efficiency of the double-pass solar collector have been studied. In addition, heat transfer and pressure drop relationships have been developed for airflow through the porous media. Close agreement has been obtained between the theoretical and experimental results. The study concluded that the presence of porous media in the second channel increases the outlet temperature, therefore increases the thermal efficiency of the systems.     

Multigeneration system exergy analysis and thermal management of an industrial glassmaking process linked with a Cu–Cl cycle for hydrogen production

A multigeneration system for hydrogen production linked with a glassmaking process via thermal management is examined in this study. The exhaust gas is interconnected with a Rankine cycle and the copper-chlorine (Cu–Cl) cycle for hydrogen production. The present system consists of a steam Rankine cycle, Cu–Cl cycle with multistage compression, double-stage organic Rankine cycle, and multi-effect desalination system. A Cu–Cl cycle based on the four-step model is employed with the proposed system. The useful system outputs are electricity, hydrogen, and fresh water. The simulation software packages utilized in the analysis and modeling are Engineering Equation Solver and Aspen Plus. The energy efficiency of the overall system is 36.5% while 38.1% is the exergy efficiency. The parametric studies are conducted to investigate the system performance. In addition, the effects of exhaust gas variables, such as flow rate, temperature, and pressure are examined to investigate the system performance.     

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.     

Cost–benefit analysis of a photovoltaic power plant

This work evaluates the energy generated by photovoltaic generators with different mounting angles to the horizontal plane, and the optimum angle is estimated. Other aspects considered are the costs and legal framework associated with installing a photovoltaic power plant in Santa Fe, Argentina. After having done a cost–benefit analysis under different scenarios, results showing the feasibility of building a photovoltaic power plant were obtained. However, the assessment of costs shows that the rates set by Act 26190 need to be modified in order to increase its feasibility in the location studied.     

Numerical analysis of convective heat loss from a cylindrical–hemispherical receiver using a glass cover and an air curtain

It is imperative to mitigate the convective heat loss from the receiver to improve the overall efficiency of the parabolic dish concentrator. In this study, the reductions of convective heat loss from the cylindrical-hemispherical receiver are numerically analyzed and the model was validated by the experimental data from literature. In the first case, the impact of the glass cover on convective heat loss is examined under conditions of both natural and forced convections at various receiver orientations (γ = 0°, 30°, 60°, and 90°). Numerical results clearly demonstrate that the use of a glass cover significantly reduces the intrusion of surrounding air into the receiver cavity which leads to an enhancement of the stagnation zone inside the cavity and, as a consequence, a noticeable reduction in convective heat loss is observed. To perform analysis of the receiver with glass cover under forced convective condition, the wind velocities over the receiver are considered in the range of 1–6 m/s. The maximum reduction of convective heat loss using the glass cover is achieved to be 58.44% with wind velocity of 5 m/s at γ = 60°. In the second case, the influence of air curtain at the receiver aperture under natural convective heat loss conditions is analyzed. The analysis incorporates three variables: receiver orientation (γ = 0°–60°), nozzle width ( $L_{\mathrm{noz}}$ = 0.002–0.004 m), and nozzle outlet velocity ( $V_{\mathrm{noz}}$ = 0.5–3.5 m/s). The results show that the air curtain minimizes the outflow of receiver inside air and results in an improvement in the stagnation zone inside the cavity. The maximum effectiveness of the air curtain is found to be 43.2% at nozzle width of $L_{\mathrm{noz}}$ = 0.004 m and nozzle velocity of $V_{\mathrm{noz}}$ = 1.5 m/s at receiver orientation of 60°. It is also noteworthy that the optimal nozzle velocity decreases with the increase of nozzle widths.     

Energy and exergy analyses of a new four-step copper–chlorine cycle for geothermal-based hydrogen production

In this paper, energy and exergy analyses of the geothermal-based hydrogen production via thermochemical water decomposition using a new, four-step copper–chlorine (Cu–Cl) cycle are conducted, and the respective cycle energy and exergy efficiencies are examined. Also, a parametric study is performed to investigate how each step of the cycle and its overall cycle performance are affected by reference environment temperatures, reaction temperatures, as well as energy efficiency of the geothermal power plant itself. As a result, overall energy and exergy efficiencies of the cycle are found to be 21.67% and 19.35%, respectively, for a reference case.     

Energy and exergy analyses of a novel sulfur–iodine cycle assembled with HI–I2–H2O electrolysis for hydrogen production

A novel sulfur–iodine (SI or IS) cycle integrated with HI–I₂–H₂O electrolysis for hydrogen production was developed and thermodynamically analyzed in this work. HI–I₂–H₂O electrolysis was used to replace the conventional concentration, distillation, and decomposition processes of HI, so as to simplify the flowsheet of SI cycle. And then the new cycle was divided into Bunsen reaction, H₂SO₄ decomposition and HI–I₂–H₂O electrolysis sections. Through incorporating the user-defined module of HI–I₂–H₂O electrolysis with Aspen Plus, the cycle was simulated and 0.448 mol/h (10 L/h) of H₂ was produced. The overall energy and exergy efficiencies of the novel SI system were estimated to be 15.3–31.0% and 32.8%, respectively. Most exergy destruction occurred in the H₂SO₄ decomposer and condenser for H₂SO₄ decomposition and Bunsen reaction sections, which accounted for 93.0% and 63.4%, respectively. A high exergy efficiency of 92.4% for HI–I₂–H₂O electrolysis section with less exergy destruction was determined, mostly due to the transformation of the overall electricity in electrolytic cell to exergy. Appropriate internal heat exchange and waste heat recovery will favor improving the energy and exergy efficiencies.     

Energy and exergy analyses of a solar driven Mg–Cl hybrid thermochemical cycle for co-production of power and hydrogen

Analysis and performance assessment of a solar driven hydrogen production plant running on an Mg–Cl cycle, are conducted through energy and exergy methods. The proposed system consists of (a) a concentrating solar power cycle with thermal energy storage, (b) a steam power plant with reheating and regeneration, and (c) a hybrid thermochemical Mg–Cl hydrogen production cycle. The results show that higher steam to magnesium molar ratios are required for full yield of reactants at the hydrolysis step. This ratio even increases at low temperatures, although lowering the highest temperatures appears to be more favorable for linking such a cycle to lower temperature energy sources. Reducing the maximum cycle temperature decreases the plant energy and exergy efficiencies and may cause some undesirable reactions and effects. The overall system energy and exergy efficiencies are found to be 18.8% and 19.9%, respectively, by considering a solar heat input. These efficiencies are improved to 26.9% and 40.7% when the heat absorbed by the molten salt is considered and used as a main energy input to the system. The highest exergy destruction rate occurs in the solar field which accounts for 79% of total exergy destruction of the integrated system.     

Solar cookers with and without thermal storage—A review

The continuous increase in the level of green house gas emissions and the increase in fuel prices are the main driving forces behind efforts to more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. Among the different energy end uses, energy for cooking is one of the basic and dominant end uses in developing countries. Energy requirement for cooking accounts for 36% of total primary energy consumption in India. Hence, there is a critical need for the development of alternative, appropriate, affordable mode of cooking for use in developing countries. However, the large scale utilization of this form of energy is possible only if the effective technology for its storage can be developed with acceptable capital and running costs. Thermal energy storage is essential whenever there is a mismatch between the supply and consumption of energy. Latent heat storage in a phase change material is very attractive because of its high storage density with small temperature swing. The choice of PCM plays an important role in addition to heat transfer mechanism in the PCM. In this present work a review has been made to study all the research and development work carried out in the field of solar cooker in particular the storage type solar cookers. A novel concept of PCM-based storage type solar cooker is also presented which is under experimental investigation.     

Experimental thermal–hydraulic evaluation of CuO nanofluids in microchannels at various concentrations with and without suspension enhancers

This experimental study focuses on the effect of two important factors on the heat transfer and flow properties of copper oxide (CuO)/water nanofluids in a parallel microchannel flow configuration. The first factor considered is the solid media (CuO) concentration. In this investigation, concentration values of 0.005%, 0.01%, and 0.1% by volume were tested. The second factor is the use of a surfactant, cetyltrimethylammonium bromide (CTAB), as a suspension enhancer. All together, these two factors led to a total of six types of nanofluids, which were tested in addition to pure water, the reference fluid. The experimental setup allowed for the determination of the hydrodynamic and thermal performance of each nanofluid. In addition, a selection of the nanofluids were characterized by the use of scanning transmission electron microscopy (STEM) and Dynamic Light Scattering (DLS) techniques. The DLS transient settling measurements showed that for a nanofluid with a concentration of 0.1% by volume, the nanoparticle dispersion and suspension is negatively affected unless a surfactant is used. Hydrodynamic losses, which were evaluated by comparing the effect of the imposed pressure drop on the mass flow rate, were not meaningfully affected by the composition of the nanofluids tested. The measurements also showed that nanofluids containing a surfactant generally provided a modest increase in heat transfer rate when compared with tests performed using pure water. The largest increase was about 17% for a fluid with a concentration of 0.01% by volume. Consequently, the gains in heat transfer do not appear to be accompanied by a significant pumping power penalty. The results of this study suggest that the use of a surfactant is essential in maintaining a proper suspension of nanoparticles in the fluid, especially at higher concentrations.     

Numerical analysis and optimization of a spectrum splitting concentration photovoltaic–thermoelectric hybrid system

This paper presents the numerical modeling and optimization of a spectrum splitting photovoltaic–thermoelectric (PV–TE) hybrid system. In this work, a simulation model is established in consideration of solar concentration levels and several heat dissipation rates. Exemplarily, the performance of a hybrid system composed of a GaAs solar cell and a skutterudites CoSb₃ solar thermoelectric generator (TEG) is simulated. Analysis under different conditions has been carried out to evaluate the electrical and thermal performance of the hybrid system. Results show that the cutoff-wavelength of the GaAs–CoSb₃ hybrid system is mainly determined by the band gap of solar cell, when the solar concentration ratio is ranged between 550 to 770 and heat transfer coefficient h = 3000–4500 W/m² K, the hybrid system has good electrical performance and low operating temperatures. Based on the analysis of the GaAs–CoSb₃ hybrid system, guidelines for the PV–TE system design are proposed. It is also compared with a PV-only system working under the same cooling condition; results show that the PV–TE hybrid system is more suitable for working under high concentrations.     

Effects of polysiloxane doping on transmittance and durability of sol–gel derived antireflective coatings for photovoltaic glass

Antireflective coatings with high transmittance and excellent durability were prepared by sol–gel process using tetraethyl orthosilicate as precursor. The base-catalyzed sol was modified by acid-catalyzed polysiloxane, and the effects of different doping ratio on the performance of films were studied. The properties including distribution of sol particles, UV–Vis absorbance, refractive index, film thickness and morphology were characterized by nano-particle size analyzer, spectrophotometer, ellipsometer, atomic force microscope (AFM), scanning electron microscopy (SEM) respectively. The results showed that the particle size in sols grew bigger, porosity of films was reduced, refractive index increased and undesirable decrease of the transmittance appeared with the increase of doped polysiloxane. Refractive index could be adjusted from 1.23 to 1.42 continuously by changing the doping ratio. Although the visible light transmittance of the film declined from 97.81% to 94.24% quickly when the doping ratio increased from 0% to 16%, the transmittance attenuation after outdoor exposure for 90 days reduced from 2.70% to 0.22% successfully, and the abrasive-resistance properties of the silica films improved remarkably. Take all factors into consideration, modified base catalyzed sol with 5 vol% doping ratio exhibits excellent properties including high transmittance, good durability and high abrasion-resistance.     

A critical review of photovoltaic–thermal solar collectors for air heating

Integrated photovoltaic–thermal solar collectors have become of great interest in the solar thermal and photovoltaic (PV) research communities. Solar thermal systems and solar PV systems have each advanced markedly, and combining the two technologies provides the opportunity for increased efficiency and expanded utilization of solar energy. In this article, the authors critically review photovoltaic–thermal solar collectors for air heating. Included is a review of photovoltaic thermal technology and recent advances, particularly as applied to air heaters. It is determined that the photovoltaic–thermal (PV/T) air heater is or may in the future be practicable for preheating air for many applications, including space heating and drying, and that integrated PV/T collectors deliver more useful energy per unit collector area than separate PV and thermal systems. Although PV/T collectors are promising, it is evident that further research is required to improve efficiency, reduce costs and resolve several technical design issues related to the collectors.     

Residential solar air conditioning: Energy and exergy analyses of an ammonia–water absorption cooling system

Large scale heat-driven absorption cooling systems are available in the marketplace for industrial applications but the concept of a solar driven absorption chiller for air-conditioning applications is relatively new. Absorption chillers have a lower efficiency than compression refrigeration systems, when used for small scale applications and this restrains the absorption cooling system from air conditioning applications in residential buildings. The potential of a solar driven ammonia–water absorption chiller for residential air conditioning application is discussed and analyzed in this paper. A thermodynamic model has been developed based on a 10 kW air cooled ammonia–water absorption chiller driven by solar thermal energy. Both energy and exergy analyses have been conducted to evaluate the performance of this residential scale cooling system. The analyses uncovered that the absorber is where the most exergy loss occurs (63%) followed by the generator (13%) and the condenser (11%). Furthermore, the exergy loss of the condenser and absorber greatly increase with temperature, the generator less so, and the exergy loss in the evaporator is the least sensitive to increasing temperature.     

Thermodynamic analysis of energy intensive systems based on exergy–topological models

Improving the efficiency of energy intensive systems is a major challenge. The primary conventional techniques for tackling such problems are thermodynamic analysis and optimization. This paper describes an innovative general approach called the structural exergy analysis method for thermodynamic analysis of systems with arbitrary structures. The innovative method is based on the construction and analysis of a particular exergy–topological model. An example of its application to a gas-turbine installation is provided.     

High glass forming ability correlated with microstructure and hydrogen storage properties of a Mg–Cu–Ag–Y glass

Thermal characterization of an as-cast Mg₅₄Cu₂₈Ag₇Y₁₁ bulk metallic glass revealed that this alloy exhibits excellent glass forming ability. High-resolution X-ray diffraction study and transmission electron microscopy show that heating and isothermal annealing treatment results in the nucleation of nanocrystals of several phases. The average size of these nanocrystals (∼15–20 nm) only slightly varies with prolonged annealing, only their volume fraction increases. High-pressure calorimetry experiments indicate that the as-cast fully amorphous alloy exhibits the largest enthalpy of hydrogen desorption, compared to partially and fully crystallized states. Since the fully crystallized alloy does not desorb hydrogen, it is assumed that hydrogen storage capacity correlates only with the crystalline volume fraction of the partially crystallized Mg₅₄Cu₂₈Ag₇Y₁₁ BMG and additional parameters (crystalline phase selection, crystallite size, average matrix concentration) do not play a significant role.     

Comparison of solar hydrogen storage systems with and without power-electronic DC–DC-converters

For long-time-storage problems of electrical energy, hydrogen-storage-systems have some significant advantages. In photovoltaic systems, especially in island-applications, they can help to solve the seasonal storage problem with solar energy and a not correlated energy demand. In summer the surplus of photovoltaic energy is used to produce hydrogen-gas by water electrolysis. In winter the hydrogen is converted back to electricity with a fuel-cell at high efficiency. This usually needs extensive system technology and therefore it is only useful for complex photovoltaic systems.One problem in such hydrogen plants is efficiency and cost of the power electronic devices. Therefore, there exist some ideas and concepts in order to eliminate most of the converter-technology by a direct coupling of the electrochemical devices on the same DC-bus-bar without equalising the different operating-points by converters. The train of thought are backed up by simulations especially with regards to efficiency and the results which are shown seem to be a glimmer of hope. But what about the costs and the possibility for realisation in real hydrogen plants? The aim of this paper is to consider the mentioned aspects by surveying especially those hydrogen plants, which, from our point of view, may become technological maturity at first. The results of these investigations are then corroborated through a data analysis of the existing power-plant PHOEBUS of the AG-Solar in Jülich, Germany.     

Environmental,health and safety issues associated with the manufacture and use of II–VI photovoltaic devices

Federal and state agencies have classified cadmium and selenium compounds as hazardous. Consequently, facilities using these materials are subject to various regulations and guidelines developed by these agencies. The intent of these guidelines is to protect worker and public health from accidental and routine chemical exposures. In this context, the agencies provide specific limits on public and occupational exposures, and generalized guidance on methods or approaches for attaining such limits. This paper gives background information on the toxicology and pharmacology of cadmium and selenium compounds, and reviews several newly proposed or adopted Federal and state regulations which can affect photovoltaic manufacturing facility operations using these and other similar chemicals.