Renewable Energy Global Innovations featured article: Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures
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Bio-oils are becoming alternatives to fossil fuels in view of the ever rising global energy demands, greenhouse emissions, and crude oil shortage. Bio-oils extracted from lignocellulosic biomass are composed of multicomponent molecules extracted from lignin, cellulose, and hemicellulose. The lignin component of lignocellulose is a promising renewable feedstock for the production of a number of chemicals and fuels. Lignin has an advantage over its counterparts hemicellulose and cellulose considering that bio-oils extracted from lignin can be upgraded to higher quality transportation fuels. This is in view of the high stability of the lignin aromatic structures with reference to resonance stabilization.
Fast pyrolysis has been an extensively applied method for converting lignin into bio-oil. Unfortunately, the potential of the resulting bio-oil is limited by the presence of a number of oxygenated functional groups, which in consequence lead to undesirable physicochemical attributes including low thermal and chemical stability, low heating values, easy corrosiveness and high density. In addition, lignin-extracted bio-oils are incompatible with either direct use or in combination with a petroleum fraction. This has forced researchers to look for alternative strategies which can enhance the commercial viability of lignin-extracted bio-fuels for use as a transportation fuel.
One method to remove chemically bonded oxygen from lignin-extracted oil and therefore enable the use of lignin-extracted bio-fuels is through catalytic hydrodeoxygenation processes. However, the direct application of catalysts in such processes is associated with numerous challenges owing to the complex nature of its oxygenated aromatic elements such as anisole, furan, benzofuran, and phenols. In order to overcome these issues, heterogeneous catalysts are attracting much attention, containing both a metal centre for the hydrogenation of the acidic support and an aromatic ring for the deoxygenation. While noble metals such as platinum, palladium, and ruthenium have been used in this process, their application is limited by the high cost and scarcity of these noble metals. Therefore, researchers have focused their attention on transition metal based catalysts including cobalt, nickel, iron, and copper-nickel.
Putla Sudarsanam and Suresh Bhargava at the Centre for Advanced Materials and Industrial Chemistry, School of Science, RMIT University in Australia in collaboration with Murtala Ambursa, Lee Hwei Voon, and Sharifah Bee Abd Hamid at the University of Malaya investigated the preparation, characterization and catalytic application of bimetallic Cu-Ni catalysts supported on Ti-MCM-41 for the hydrodeoxygenation of guaiacol. Their research work is published in the journal Fuel Processing Technology.
The authors prepared Cu-Ni catalysts supported on either Ti-MCM-41 or pure MCM-41 and compared their catalytic efficiencies for the hydrodeoxygenation of guaiacol. They performed the catalytic experiments in an autoclave reactor and investigated the effect of hydrogen pressure on guaiacol conversion as well as product selectivity.
Through the catalytic activity investigation, the authors observed that the CuNi/Ti-MCM-41 catalysts had a higher catalytic performance in guaiacol conversion as well as cyclohexane selectivity as opposed to a CuNi/MCM-41 catalyst. The high catalytic activity of the CuNi/Ti-MCM-41 catalyst was as a result of the cooperative function of the larger surface area, hexagonal pore geometry, medium-sized mesopores, adequate acidic sites, and excellent redox attributes. For this reason, Ti-MCM-41 is an efficient catalyst support for the hydrodeoxygenation of oxygenated elements to saturated hydrocarbons.
The research team also found that increased hydrogen pressures improved the guaiacol conversion and selectivity of the oxygenated compounds to hydrocarbons over the Ti-MCM-41 supported CuNi catalyst, increasing the impact of these catalysts within this important field of research.
Murtala M. Ambursa, Putla Sudarsanam, Lee Hwei Voon, Sharifah Bee Abd Hamid, Suresh K. Bhargava. Bimetallic Cu-Ni catalysts supported on MCM-41 and Ti-MCM-41 porous materials for hydro-deoxygenation of lignin model compound into transportation fuels. Fuel Processing Technology, volume 162 (2017), pages 87–97.
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Phase change materials (PCMs) are important for thermal energy storage applications, especially in buildings and in solar thermal systems. The use of PCMs can increase energy efficiency by storage of solar thermal energy, and reducing heating and cooling demands. Suitable phase change materials need to be inexpensive and reliable, with high latent heats and a phase change at a temperature appropriate to the application. Organic phase change materials can satisfy many of these criteria and are suitable for latent-heat thermal energy storage systems operating in the ambient-to-moderate temperature range. Non-paraffin organic PCMs such as esters, fatty acids and fatty alcohols are particularly attractive because these materials are non-toxic and can be extracted from renewable sources, such as from plant and animal fat. They are also abundant and inexpensive, with a commodity cost of around $1/kg for fatty acids, for example. Their economic and embodied energy payback times can be very favorable.
The optimal design of an energy storage system requires accurate data regarding the thermophysical properties of PCMs. Furthermore, for phase change materials to be useful and reliable in their applications, they must be chemically and thermally stable after many melt-freeze cycles, and must be chemically inert with the materials with which they are in contact. For the very promising fatty acid PCMs, there were significant knowledge gaps and even inconsistencies. For example, there are considerable discrepancies between the values reported by different researchers for the latent heat of fusion and the phase change temperature of the same fatty acid PCM. In addition, thermal properties, such as the thermal conductivity and heat capacity, which are especially necessary for numerical studies for optimization, were largely unknown.
Researchers led by Professor Mary Anne White at Dalhousie University in Canada presented a comprehensive analysis of the thermophysical properties, thermal stability and chemical compatibility of six important organic PCMs: dodecanoic acid, decanoic acid, hexadecenoic acid, tetradecanoic acid, 1-octadecanol, and octadecanoic acid. Their research work is published in the journal Solar Energy Materials and Solar Cells. “We focused on fatty acid PCMs with melting temperatures between 30 and 70 °C because these materials span a wide range of thermal energy storage applications, from integration in building materials and residential solar water heating systems to cooling of portable electronic devices. They are cheap and can be sustainably sourced. They are also useful for preparing new PCMs by forming eutectic mixtures with lower melting temperatures”.
The research team implemented consistent procedures and experimental methods for all six phase change materials to provide direct comparisons. They accurately measured the thermal properties of the PCMs in the solid and liquid phases and related physical properties. In addition to thermophysical characterization, the authors determined the thermal stability of phase change materials over thousands of freeze-thaw cycles, as well as their chemical compatibility with 16 different materials. “Chemical compatibility of PCMs with materials is very important information but had been significantly overlooked in this field. For instance, many studies consider the addition of metallic fillers to enhance the thermal conductivity of fatty acid PCMs, but it is not known whether the fatty acids will react with these fillers over time”.
From the long-term cycling results, the authors observed that all the six phase change materials studied were thermally stable over 3000 melt-freeze cycles. In fact, there were no significant changes in the heats of fusion and melting temperatures, even for a low-purity sample of hexadecenoic acid. For this reason, it was determined that these PCMs are thermally reliable for long-term thermal energy storage applications.
The authors also investigated chemical compatibilities of the phase change materials with nine metal alloys and seven plastic materials. “We chose a wide range of materials which are found in most of the thermal energy storage applications that these PCMs were considered for in previous studies, or will likely be used for in the future. For example, copper and aluminum are typical filler materials, whereas the magnesium alloy, Mg AZ91D, and polycarbonate are commonly used in today’s portable electronic devices”. They found that two copper alloys, namely Cu110 and Cu101, and one magnesium alloy, Mg AZ91D, were incompatible with the fatty acids, while the nickel alloy Ni C7521 was compatible only with octadecanoic and hexadecenoic acids. They observed that octadecanol was compatible with all the alloys. Polycarbonate was the only plastic material that did not react significantly with any of the phase change materials investigated.
The in-depth information provided in the study conducted at Dalhousie University regarding the thermophysical properties, thermal stability and chemical compatibility of organic non-paraffin phase change materials provides all the required data to design and optimize thermal energy storage systems in the temperature range 30 to 70 °C, for applications from the built environment to compact electronic devices.
Samer Kahwaji, Michel B. Johnson, Ali C. Kheirabadi, Dominic Groulx, Mary Anne White. Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures. Solar Energy Materials and Solar Cells, volume 167 (2017), pages 109–120.
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Climate change mitigation and carbon emission reduction policies have been taking center stage in Australia in the last decade. Australians are broadly in favor of developing clean sources of energy but there’s a political divide about how to approach climate change. To prove its commitment and determination in championing against global warming, the Australian government committed to reduce emissions by 26-28% on 2005 levels by 2030. However, the achievability of such stringent target with the current subsidized emissions abatement policy has been put into question. Regardless, the Emissions Trading Scheme has been demonstrated as a good policy for Australia owing to its small unfavorable effects on the economy.
Dr. Duy Nong at Colorado State University with Dr. Sam Meng and Professor Mahinda Siriwardana at University of New England in Australia evaluated the effects of the proposed Emissions Trading Schemes on the Australian economy as well as the emissions level using MONASH-Green model, and a database containing detailed energy sectors. In their study they applied the stylized BOTE macro model developed by Adams and Parmenter (2013) in order to inform the macroeconomic results generated by the full model. Their work is now published in the research journal, Energy Policy.
The researchers commenced by replacing the output production function in MONASH Green with the ORANI-G model production system as their study did not contain composite commodity outputs. They then altered the input demand structure in MONASH-Green. The team then collected and compiled database from a variety of sources. After policy design, they developed a simulation where they considered microeconomic effects, effects on households, emissions trading among sectors and effects on sectoral outputs and employment.
From the simulation results, the authors demonstrated that the costs of the proposed emissions trading scheme were as low as of those of related studies conducted earlier by Adams et al. For example, it was seen that the permit price would have to increase from A$4.1 in 2015 through A$13.1 in 2020 to A$41.3 in 2030 in order to enable Australia to achieve the 2020 and 2030 emissions targets. More so, the operation of the proposed Emissions Trading Scheme in Australia would cause the economy to contract progressively over the lifetime of the Emissions Trading Scheme. This is due to the fact that the energy sectors will tend to substitute high emission-intensive energy commodities such as brown coal with black coal. Additionally, employment at sectoral level will fluctuate in line with variations in their outputs.
Thus the MONASH-Green model has been successfully used in their study to assess the effects of a proposed Emissions Trading Schemes on the Australian economy, particularly on the energy sectors and multi household groups. The simulation results indicate that the current price of carbon permits cannot suffice to meet the set carbon reduction targets by 2030 without prior adjustments and reviews. These results lend strong support towards the transition to renewable energy. This policy study is therefore likely to be of continuing relevance to Australia and could form future climate policy.
Duy Nonga, Sam Meng, Mahinda Siriwardana. An assessment of a proposed ETS in Australia by using the MONASH-Green model. Energy Policy 108 (2017) 281–291
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Solar energy is the most abundant renewable energy that has the capability to meet the world’s growing demand. However, it requires good solar concentrators to increase the trap efficiency. The efficient mean to utilize solar energy is to convert solar energy into heat stored in water by solar thermal collectors. Techniques such as high efficiency heat transfer absorber and solar radiation concentration are the main methods to improve the performance of solar thermal collector.
Pulsating heat pipe is one of the highly efficient absorber with simple stricture and low cost. The pulsating heat pipe has three working states namely: start-up, steady state and dry-out as the heat input increases. Altogether, the pulsating heat pipe exhibits an excellent potential for application as heat collector credit due to its high heat transfer capacity. However, the heat flux of the of the evaporation section of pulsating heat pipe should be sufficiently high to meet the demand of its steady and high-efficiency work, which has a significant effect on the thermal performance of pulsating heat pipe. Therefore, a solar concentrator is necessary in order to increase the heat flux of the pulsating heat pipe absorber to ensure that efficient heat transfer capacity of pulsating heat pipe can be fully utilized.
Researchers led by Professor Rong Ji Xu from Beijing University of Civil Engineering and Architecture and in collaboration with Dr. Hua Sheng Wang at Queen Mary University of London proposed a study on a novel solar collector that integrates a closed-end pulsating heat pipe and a compound parabolic concentrator. Their main objective was to test the operating characteristics and thermal performance of the detailed designed collector, under different weather conditions. Their work is now published in the research journal, Energy Conversion and Management.
Briefly, the research team initiated their empirical procedure by developing a prototype of the solar collector. Secondly, they analyzed the operating characteristics of the pulsating heat pipe absorber. The team then assessed the thermal efficiency of the solar collector under different weather conditions.
The authors observed that the collector showed start-up, operational and shutdown stages at the starting and ending temperatures of 75 0C. More so, they noted that the solar collector operated stably even in cloudy days. Additionally, the thermal resistance of the pulsating heat pipe absorber was seen to decrease with the increase in ambient temperature, solar intensity, and evaporation temperature which was found to be the main factor that affects the thermal efficiency of the collector.
Rong Ji Xu and colleagues successfully presented a novel solar collector that integrates a closed-end pulsating heat pipe and a compound parabolic concentrator. In their study, they have assessed the effects of operating parameters on the operating characteristics of the pulsating heat pipe and the performance of the solar collector under varying weather conditions. The experimental results suggest that the heat flux of the pulsating heat pipe absorber’s evaporation section concentrated by compound parabolic concentrator with a concentration ratio of 3.4 is appropriate and the use of compound parabolic concentrator is reasonable. Their proposed design offers a promising efficiency of 50% when compared with conventional solar collectors and pulsating heat pipe solar collectors.
According to Rong Ji Xu, the mathematical model of the solar collector has been built. The effects of the solar density, ambient temperature, weed speed, glass thickness and collecting temperature on the thermal performance were simulated. A theoretical efficiency of 70% can be realized which is more promising than experimental results.
Rong Ji Xu, Xiao Hui Zhang, Rui Xiang Wang, Shu Hui Xu, Hua Sheng Wang. Experimental investigation of a solar collector integrated with a pulsating heat pipe and a compound parabolic concentrator. Energy Conversion and Management 148 (2017) 68–77
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In the extraction of oil and gas, fracturing fluids are used to create and widen a fracture for easier production. The fluid should be compatible with the rocks. Liquid CO2 has been used as a fracturing fluid because it is highly soluble in most oil extracts therefore less damaging to the oil extracted as compared to water-based fluids. The high solubility in oil helps in lowering the viscosity of oil and improves the extraction of oil from the ground.
Liquid CO2 is, however, difficult to control in porous media due to its low viscosity and lack of filter cake formation properties for controlled filtration. This may affect the shape of the desired fracture. Researchers have tried to improve on the viscosity of liquid CO2 by adding thickening agents, however, improvement of viscosity of liquid CO2 by enhancement does not guarantee a change in its filtration performance and liquid CO2 dissolves poorly with thickening agents making the insoluble residues potential pollutants.
Filtration control performance of liquid CO2 was improved without pollution by mixing it with Nitrogen (N2). Gupta et al. stabilized the N2 and CO2 mixture by introducing fluorochemical stabilizers into the liquid CO2 and then bubbling N2 into the liquid to form the N2/liquid CO2 foam. The liquid CO2 was the external phase while the N2 was the inner face and the fluorochemical stabilizers separated the two phases.
Qichao Lv, under the guidance of Professor Zhaomin Li, at China University of Petroleum investigated the dynamic filtration control performance of N2/liquid CO2 foam with a fluorochemical (HFE) as a stabilizer.
The filtration behavior of N2/liquid CO2 foam is uncertain as the external phase of the foam is unstable. Temperature and pressure can affect its density, viscosity and phase and changing the flow properties of the foam. Moreover, the behavior of waterless foam with the unique fluorochemical interface is uncertain. The interface may also be a potential pollutant. The experiment seeks to study the factors that may affect the dynamic filtration performance of N2/liquid CO2 foam including viscosity, foam quality, temperature, pressure, permeability and the damaging effects of the foam on porous media after filtration.
The setup for the experiment was done as shown in their paper where the preparation for the foaming solution was done and the viscosity measurement and dynamic filtration tests were done. The viscosity measurement results showed that the use of foam enhanced the viscosity of liquid CO2. The apparent viscosity is related to temperature, pressure and foam quality. The viscosity increased as the foam quality was increased from 31 % to 71 %. The viscosity of high quality foams were 1 order of magnitude larger than that of liquid CO2 at the same conditions. The apparent viscosity is at a maximum at a foam quality of about 80 % before it starts decreasing as it becomes fragile and sensitive to disturbances such as interactions and pressure fluctuations.
The filtration control performance of the N2/liquid CO2 foam was compared to that of liquid CO2 and a N2/liquid CO2 mixture. The results showed that the filtration control properties of the N2/liquid CO2 foam was better than the others. The leak off coefficient lowered with an increase in foam quality up to 80 % where it increased. Foams of 50 – 80 % quality had a high filtration performance with permeability change. Low initial foam quality foams had better filtration performance at high pressure difference. As the foam enters the porous media, the liquid part would evaporate hence increasing the foam quality with depth. Damage by the foam on the porous media depends upon the pressure difference between the two sides of the porous media. Damage is small under low pressure difference as the CO2 turns to gas under a high pressure media damaging the porous media.
In their study the research team were able to prove that by mixing N2, liquid CO2 and HFE, properties such as viscosity and filtration control performance of the resultant N2/liquid CO2 foam, increased substantially without damage to porous media.
Lv Q, Li Z, Li B, Zhang C, Shi D, Zheng C, Zhou T. Experimental study on the dynamic filtration control performance of N2/liquid CO2 foam in porous media. Fuel. 2017 Aug 15; 202:435-45.
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Layered cathode active materials partially-substituted cobalt with transition metals and spinel active materials have been received more attention due to their cost effectiveness as compared to lithium cobaltate. Scholars have realized with time that the spinel cathode active materials are safe with limited specific capacities while layered cathode active materials possess high specific capacity with safety issues. Presently, layered cathode active materials are more preferred as cathode materials for lithium ion batteries, where nickel-rich layered cathode active materials are front runners except for safety issues, due to their high capacity. To overcome the safety issue challenge, measures, such as the substitution of the transition metal ions with other metal counter ions and the modification of the surface by means of coating with metal oxides, have been put in place.
Nanoparticle coatings on layered cathode active materials have been known to suppress the thermal reaction between the electrode and electrolyte. It has thus been seen necessary to coat or decorate the surface of the cathode since probable exothermic reaction starts from the cathode surface when the electrolyte is decomposed. Extensive studies have been performed on coating materials that inhibit this reaction but little exists about nanofibers-based metal oxides decorated on lithium nickel cobalt aluminum oxide cathode active materials.
Professor Chang Woo Lee and colleagues from the Department of Chemical Engineering, College of Engineering, Kyung Hee University in South Korea proposed a study to modify the surface of lithium nickel cobalt aluminum oxide particles by decorating them with titania nanofibers. They aimed at comparatively studying, with the novel lithium nickel cobalt aluminum oxide, the various quantities of titania nanofibers decorated over lithium nickel cobalt aluminum oxide (LNCA). Their research work is now published in the peer-reviewed journal, Journal of Industrial and Engineering Chemistry.
The researchers commenced their empirical procedure by obtaining titania nanofibers precursor through electrospinning a sol-gel polymeric solution. They then obtained the LNCA precursor. The titania nanofibers precursors were split at 0.5 wt%, 1 wt% and 1.5 wt% before addition of the LNCA precursor powders. The precursor powder mixture was then sintered at 465oC for three hours and then calcined at 850oC for five hours in air. The team then conducted an X-ray photoelectron spectroscopic analysis to investigate the chemical composition of the cycled electrode surface. The team eventually obtained cathode samples and used them to conduct differential scanning calorimetry scans.
The authors also observed that the increase of titania nanofibers decoration over 1wt% ratio showed negative effect during the electrochemical process, as observed using electrochemical impedance spectra for the 1.5wt% titania nanofibers-decorated LNCA. Hence usage of titania nanofibers more than 1wt% was excluded from detailed investigation. The surface modification of LNCA electrodes by 1wt% titania nanofibers decoration greatly increased the cycleability, capacity, and thermal stability of lithium ion batteries at room temperature as well as at elevated temperature. Among titania nanofibers decorated LNCAs, the 1wt% titania nanofibers -decorated LNCA cathode had shown better capacity retention of 89.2% and 81.9% at room and elevated temperature, respectively.
The results of their study second the suggestion of the applicability of titania nanofibers as surface modifiers in order to enhance the electrochemical and thermal properties of lithium ion batteries. Moreover, it has been seen that the capability of the titania nanofibers-decorated LNCA was enhanced compared to that of the pristine LNCA. The onset temperature of thermal decomposition is also shifted towards higher temperature for titania nanofibers-decorated LNCA electrodes than pristine LNCA electrodes.
Subburaj, Yong Nam Jo, K. Prasanna, Ki Jae Kim, Chang Woo Lee. Titanium oxide nanofibers decorated nickel-rich cathodes as high performance electrodes in lithium ion batteries. Journal of Industrial and Engineering Chemistry, volume 51 (2017) pages 223–228.
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