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The construction of flexible electrochemical devices for energy storage and generation is of utmost importance in modern society. In this article, we report on the synthesis of flexible MoS2-based composite paper by high-energy shear force milling and simple vacuum filtration. This composite material combines high flexibility, mechanical strength and good chemical stability. Chronopotentiometric charge–discharge measurements were used to determine the capacitance of our paper material. The highest capacitance achieved was 33 mF·cm−2 at a current density of 1 mA·cm−2, demonstrating potential application in supercapacitors. We further used the material as a cathode for the hydrogen evolution reaction (HER) with an onset potential of approximately −0.2 V vs RHE. The onset potential was even lower (approximately −0.1 V vs RHE) after treatment with n-butyllithium, suggesting the introduction of new active sites. Finally, a potential use in lithium ion batteries (LIB) was examined. Our material can be used directly without any binder, additive carbon or copper current collector and delivers specific capacity of 740 mA·h·g−1 at a current density of 0.1 A·g−1. After 40 cycles at this current density the material still reached a capacity retention of 91%. Our findings show that this composite material could find application in electrochemical energy storage and generation devices where high flexibility and mechanical strength are desired.Keywords: flexible composites; hydrogen evolution reaction (HER); lithium ion batteries (LIBs); molybdenum disulfide; nanoarchitectonics; supercapacitors

Power consumption is becoming more expensive by the day. This is not far from the fact that power generation is facing a lot of challenges coupled with increasing demand. Industrial sector power demand shares a significant proportion of this generated power. It is therefore a necessary task to manage well industrial energy use through appropriate energy efficiency methods practice so as to minimise energy losses. In this study energy efficiency opportunities in pulp and paper (P&P) mill are identified and the saving potential is then quantified. An energy flow model with the help of an energy audit are used to quantify energy supply, generation, conversion and end use thereby exposing areas of energy losses of the mill. This analysis will serve as an eye opener to future and current P&P making operations in South Africa, and as a base case for stimulating changes toward more efficient energy utilization in the pulp and paper industry. Quite a reasonable energy losses avenues were identified, audited and options of improvements suggested. The results proved that quite a sensible amount of energy is lost in the P&P case study and a near future look into the opportunities and barriers have been noted.

Purpose This paper aims to measure the state-level energy efficiency in Indian paper industry and simultaneously explain inter-state variation in efficiency by inefficiency effect model. Three variables, labor productivity, capital intensity and structure of paper industry, are included in inefficiency effect model to assess the likely impact on energy efficiency. Design/methodology/approach Sub-vector input distance function is derived through neo-classical production function which provides measures to estimate energy efficiency. Assuming a translog production function specification, energy efficiency is estimated by using Battese and Coelli (1995) stochastic frontier analysis (SFA). The authors also estimated four other SFA models, and energy efficiency from all the models is compared for robustness checking. Findings The results show the existence of a vast potential to improve energy efficiency. Inefficiency effect model reported a positive impact of structure of the industry and capital intensity on energy efficiency performance, while labor productivity does not have any significant impact on energy efficiency. There exists considerable energy efficiency variation among states. Uttarakhand, Punjab and Orissa are the best performing states while Rajasthan, Jharkhand and Goa have worst energy efficiency performance based on average efficiency. The ranks assigned to states according to inefficiency effects model are found contrary to the simple measure of energy efficiency, i.e. energy intensity. Thus, energy intensity may not always be a good proxy for underlying energy efficiency and need to be compared with a comprehensive possible measure. Originality/value To the best of the authors’ knowledge, this is the first study which measures energy efficiency of Indian paper industry through stochastic frontier model using region-level data. Instead of relying on traditional energy efficiency indicators (energy-output ratio), total-factor energy efficiency approach is used to conduct the empirical exercise. Deviations from the frontier because of factors beyond the scope of producers are also incorporated into analysis to portray the magnitude of random factors in influencing the efficiency performance.

Thanks to their versatile functionality, metal oxides (MOs) constitute one of the key family materials in a variety of current demands for sensor, catalysis, energy storage and conversion, optical electronics, and piezoelectric mechanics. Much effort has focused on engineering specific nanostructure and macroscopic morphology of MOs that aims to enhance their performances, but the design and controlled synthesis of ultrafine nanostructured MOs in a cost-effective and facile way remains a challenge. In this work, we have exploited the advantages of intrinsic structures of graphene oxide (GO) papers, serving as a sacrificial template, to design and synthesize two-dimensional (2D) layered and free-standing MO papers with ultrafine nanostructures. Physicochemical characterizations showed that these MO materials are nanostructured, porous, flexible, and ultralight. The as-synthesized materials were tested for their potential application in photoelectrochemical (PEC) energy conversion. In terms of PEC water splitting, copper oxide papers were used as an example and exhibited excellent performances with an extremely high photocurrent-to-weight ratio of 3 A cm-2 g-1. We have also shown that the synthesis method is generally valid for many earth-abundant transition metals including copper, nickel, iron, cobalt, and manganese.

Recently, radioactive wastes are inevitable by-product due to rapid development and use of nuclear energy. Especially, there are large amount of liquid radioactive wastes to control nuclear fission reaction at nuclear generating station. In order to reduce total volume of solution state radioactive wastes, people have used solar evaporation without any specific instrument and it takes long time to get liquid state concentrated radioactive waste which is inefficient process. Herein we propose Janus-type membrane with different surface hydrophilicity toward efficient solar steam generation to concentrate radioactive liquid wastes. We used vapor deposition polymerization method to introduce polypyrrole (Py) and polystyrene (PS) on the top and bottom side of a cellulose paper. Py coating broadens the absorption wavelength range for enhanced photo-thermal energy conversion. Hydrophobic PS coating not only makes membrane easy to floating on water but also prevents salt accumulation during evaporation. Using this membrane, we can efficiently concentrate polluted water including Cs ion without salt accumulation on membrane, and thus no specific decrement in evaporation efficiency by time.

This work deals with the mass, energy and exergy analyses of the energy conversion process in a 50 MWth thermal power plant, using residual forest biomass as fuel and integrating bubbling fluidized bed combustion technology. This is a representative case study of the energy production from eucalyptus residual biomass in the Portuguese pulp and paper industry. Analyses are made of the mass, energy and exergy flows, and how they change in the main components of the system, namely the combustion chamber, boiler, drum, superheater, economizer, and combustion air pre-heaters. Sankey and Grassmann diagrams highlight the mass, energy and exergy flows. Special emphasis is given to the energy and exergy efficiencies, which must be based on the useful produced superheated steam to include the influence of the biomass moisture content. The main goal is to find where and what potential exists for energy efficiency improvements. Results show that the main exergy destruction occurs in the combustion process, and that energy and exergy losses are minimal due to the system’s good thermal insulation. Results also show that the main potential for improvement is on the decrease of the residual biomass moisture content.

Being inspired by the “seismonastic reaction” of Mimosa pudica, an asymmetric swelling system was constructed to induce the controllable directional deformation of filter paper. In this work, multifunctional biomimetic Janus paper was facilely fabricated via depositing poly(vinylidene fluoride) (PVDF) on one side of Qualitative Filter Paper (QFP), the permeation of polymer solutions within filter paper was well controlled during fabrication. The wetting and swelling behavior of the prepared Janus paper were detected. The Janus paper showed controllable swelling-induced deformations in water. Both the degree and orientation of the deformation were fully investigated. On the one hand, the degree of deformation depends on the gradient wettability and hygroscopicity of the Janus paper, on the other hand, the orientation of deformation is related to the storage and release of stress. Additionally, the steady deformation during swelling endows the Janus paper with novel shape memory property both under idling and loading conditions. The Janus paper was also applied to achieve reversible energy conversion from the swelling potential energy to mechanical potential energy.

This Special Issue on “Selected papers from IEEE ICKII 2019” selected 13 excellent papers from 260 papers presented in IEEE ICKII 2019 on topics in energies. The fields include: energy fundamentals, energy sources and energy carriers, energy exploration, intermediate and final energy use, energy conversion systems, and energy research and development. The main goal of this Special Isue is to discover new scientific knowledge relevant to the topic of energies.

As promising fresh-water purification devices, solar steam generation systems have attracted significant attention recently. However, in practice, the approach often suffers from a poor solar energy conversion efficiency and a low water production rate due to poor material selection and inefficient microscopic structure design. Here, we fabricate an efficient solar steam generation system by "building" polyoxometalate "nano-walls" on rice paper-derived three-dimensional porous carbon paper. In this solar steam generation system, the height of the vertically aligned CoP4 Mo6 "nano-walls" range from 100 to 150 nm with thicknesses about 15 to 25 nm. Under 1 sun irradiation (1 sun = 1 kW m-2 ), the surface temperature increases from 29 to 50 °C in a short time with a solar thermal conversion efficiency achieving 92.8 %. The stability and durability of this solar steam generation system, which withstands fifteen cycle continuous tests, also offer good prospects. Its attractive solar energy conversion performance originates from the intense sunlight absorption and high conversion ability of the CoP4 Mo6 "nano-walls", as well as extremely promising heat localization and water transportation properties of the three-dimensional porous carbon paper. This solar steam generation system, which has produced some inspiring results, is employed for seawater desalination and for purification of water polluted with organic dyes.

Flexible electronics are developing rapidly due to promising applications in displays, sensors, and energy conversion fields. For biodegradable, lightweight, and flexible thin film electronics to be explored, O-(2,3-Dihydroxypropyl) cellulose (DHPC) was synthesized by homogeneous etherification of cellulose in 7 wt % NaOH/12 wt % urea aqueous solution without extra catalyst. DHPC exhibited a high level of transparency, outstanding ductility, and good adhesiveness but poor mechanical properties. Thus, stiff tunicate cellulose nanocrystals (TCNCs) were introduced to construct tough nanocomposite papers. The reinforcement of nanocomposite papers was well predicted by a percolating model, indicating the formation of the network of TCNCs. On the basis of the excellent interfacial compatibility between TCNCs and DHPC, supported by atomic force microscope mapping, the nanocomposite papers exhibited smooth surface, high transparency, as well as satisfactory mechanical properties, which were suitable for the const...

Harvesting energy from environmental moisture is a promising technique for sustainable electric power resource. Herein, a new scalable fabrication method for flexible moisture-electric energy transformation device by dispersing or direct printing carbon dots (CDs) on paper is reported. A facile strategy for self-establishing ionic gradient on both sides of the device by constructing an asymmetric structure (the side of device is encapsulated) is proposed, which avoids prior processing of the materials for building asymmetric functional groups. The carboxyl groups on CDs near the open side of the device ionize more protons than that near the closed side, resulting in a potential drop across the device due to the concentration difference of the released protons on the two sides. The device can produce a sustainable voltage of −40 mV for over 100 min at a fixed humidity value. In addition, the device responds to direct touching of a finger or human exhalation, thus indicating its great potential for applications in electronic skins and humidity sensors.

Nowadays, multi-functional systems driven by solar energy have been explored to alleviate the crisis of energy and freshwater scarcity. However, it remains a great challenge to integrate multi-functions into one hybrid system with a monolithic material, which can harvest, convert and store solar energy synergistically. Here, we firstly propose a multi-functional carbon nanotube paper, which starts from a material level to solve these problems. The carbon nanotube paper is fabricated by vacuum filtration. It exhibits high efficiency of solar-thermal conversion. Secondly, three types of energy conversion/storage systems (e.g., solar-thermal evaporation system, solar-thermoelectric generation system, and supercapacitor) are constructed based on the carbon nanotube paper, which extend the research to a device level. The solar-thermal evaporation system shows an evaporation rate of 1.28 kg m-2h−1 under simulated sunlight irradiation of 1 kW m−2. The solar-thermoelectric generation system demonstrates a stable electric power generation with an output voltage of 100 mV under light irradiation of 1 kW m−2. The supercapacitors perform the energy storage function with an areal specific capacitance of 19.6 mF cm−2. These devices and systems can be used in freshwater generation (0.84 kg m-2h−1, sunlight irradiation of 1 kW m−2), water purification/desalination (inorganic ions rejection larger than 99.9%) and portable solar charging system (maximum output voltage of 5 V). Finally, a hybrid energy system is innovatively proposed from the system level to simultaneously produce freshwater, generate and store electric power. Through the synergetic coupling of solar conversion and storage technologies, the hybrid energy system can supply sustainable freshwater and electricity for remote areas in daily life, highlighting the superiority of multi-energy (thermal energy, electrical energy, and electrochemical energy) complementary.

Herein, porous WP 2 nanosheet arrays integrated on graphite paper (P-WP 2 NSs/GP) as 3D flexible cathode for electrocatalytic hydrogen evolution reaction (HER) are prepared by in situ phosphidation via vacuum encapsulation assisted by pulsed laser deposition technique. Compared to the electrode without pre-deposition process, the enhanced catalytic activities are attributed to the increased effective catalyst loading and the reinforced charge transport kinetics. The results make the present P-WP 2 NSs/GP as a promising cathode for energy conversion and paves a new way for designing and fabricating efficient electrodes toward HER.

Lithium-Air batteries (LABs) are considered as one of the most promising energy conversion systems for delivering large specific energy. However, practical applications still face major challenges, including poor rate capability, short cycle life and low round-trip efficiencies. Here we report a novel strategy to improve the catalytic activity of MnO2 through the combination of three-dimensional (3D) MnO2 and carbon nanotubes/carbon fiber paper (CNTs/CFP) with binder-free structure. Side reactions related to the binder are precluded in this design. The presence of CNTs not only promotes the formation of 3D structure of air electrode, but directs the uniform deposition of MnO2. When applied as a cathode in LABs, the as-prepared MnO2/CNTs/CFP electrode achieves comparable specific capacity (with a discharge capacity of 8723.5 mAh∙g-1(CNTs+MnO2) at 100mA∙g-1). The encouraging electrochemical performance is found to benefit from free-standing nanoporous structures, which provide more active sites for enabling oxygen reduction. It is also attribute to the decreased side reactions.

It is our great pleasure and honor to present this special issue of Heat Transfer Engineering to highlight some of the research works presented at the AIGE 2016 Conference on Energy Conversion, Man...

Construction of flexible electrochemical devices for energy storage and generation is of utmost importance in the modern society. In this article, we report the synthesis of flexible MoS2 based composite paper by high-energy shear force milling and simple vacuum filtration. This composite material combines high flexibility, mechanical strength and good chemical stability. Chronopotentiometric charge-discharge measurements were used to determine the capacitance of our paper material. Highest capacitance of 33 mF cm-2 was achieved at current density of 1 mA cm-2 showing potential application in supercapacitors. We further used the material as a cathode for hydrogen evolution reaction (HER) with an onset potential of ca. -0.2 V vs RHE. The onset potential was even lower (ca. -0.1 V vs RHE) after treatment with n-butyllithium suggesting the introduction of new active sites. Finally, a potential use in Lithium ion batteries (LIB) was examined. Our material can be used directly without any binder, additive carbon or copper current collector and delivers specific capacity of 740 mA h g-1 at a current density of 0.1 A g-1. After 40 cycles at this current density the material still reached a capacity retention of 91%. Our findings show that this composite material could find application in electrochemical energy storage and generation devices where high flexibility and mechanical strength are desired.

This paper investigates the impact of combining CO2 capture and storage with alternative systems for biomass-based combined heat and power production (CHP) in Kraft pulp and paper mills. We compare heat, power, and CO2 balances of systems with alternative configurations of the CHP and CO2-capture systems. Because the captured CO2 comes from renewable biomass, the studied systems yield negative CO2 emissions. It is shown that pulp mills and integrated pulp and paper mills have the potential to become net exporters of biomass-based electricity while at the same time removing CO2 from the atmosphere on a net basis. The study shows that that the overall best CO2 abatement is achieved when CO2 capture is carried out within a biomass integrated gasifier with combined cycle where the syngas undergoes a CO-shift reaction. This configuration combines efficient energy conversion with a high CO2 capture efficiency. Furthermore, cost curves are constructed, which show how the cost of CO2 capture and storage in pulp and paper mills depends on system configuration and the CO2 transportation distance.

Solar photovoltaic technologies could fully deploy and impact the energy conversion systems in our society if mass-produced energy-storage solutions exist. A supercapacitor can regulate the fluctua ...

The International Conference on Polygeneration (ICP) have established as a highly reputed conference series for leading scholars in the field of energy conversion with more than one form and its re...

Graphene nanosheet devices have attracted significant interest in transistor and photodetector applications owing to photothermoelectric effects of graphene. However, their solar energy conversion ability has not been fully explored due to disadvantages of their transparency and zero gap semiconductor in this special field. Here, we propose the potential of graphene-carbon nanotube papers for energy conversion and storage under sunlight and heat. We show that macroscopic 3-dimensional structure of graphene-carbon nanotube papers provides advantages over graphene nanomaterials in converting large area power sources, namely solar and thermal energy into considerable current and voltage. We also report a graphene-carbon nanotube paper stack with p-n interfaces that incorporates a solar/thermal energy cell and a p–n junction capacitor, which can be simultaneously used for energy conversion and storage with a charge-storage capacity of 70.5 μC cm−2 at one sun intensity.

Electrohydrodynamic processing including electrospinning and electrospraying is suitable for depositing controllable structures in fiber or film form. In this work, the electrohydrodynamic approach was used to prepare poly(vinylidene fluoride)/polyaniline composite film. Scanning electron microscopic analysis was performed to show the morphology of the film on soft tissue paper. Self-poling of the film was found due to the high voltage application in the electrohydrodynamic process. The addition of polyaniline into PVDF improves the conductivity of the film significantly. Mechanoelectrical response of the film was demonstrated by measuring the open circuit voltage of the specimens under bending. The peak voltage generated is over 1.2 V due to the bending deformation of the film. The film has ultrafast response to the deformation. It is concluded that the film has multiple functions for mechanoelectrical energy conversion, vibration sensing, and structural integrity monitoring applications.

The penetration and diffusion of ink droplet in paper coating have significant influence on the image quality in the inkjet imaging. The flight physical phenomenon of the ink droplet was researched and analyzed according to the hydrodynamic equations of multiphase flow discrete phase in this paper. Base on studying on the surface structure of paper coating, the mechanism of impact, penetration and diffusion between ink droplet and paper coating also was researched in this paper from the perspective of energy conversion in the imaging process. The proposed mathematical model consisted of the factors of hydrodynamic characteristics for ink droplet and the physical characteristics of paper coating, we could get the maximum diameter of ink droplet spreading in paper coating through solving the model and obtain viscous loss and energy consumption in penetration of ink droplet by experiment, and then calculated the average penetration depth of ink droplet.

Energy generation using liquid movement over a graphene surface generally demands a very high rate of flow (e.g.∼100 ml min(-1)). In addition, a continuous flow of liquid is unable to generate a desired voltage, as it needs modification of the substrate such as development of nanopores and criss-cross network structures. Here, we report an ultra-low-cost yet highly efficient portable device for energy conversion, by exploiting the capillary flow of an electrolyte on a filter paper matrix in which a naturally deposited gradient of reduced graphene oxide is induced through chemical synthesis. In addition, the fibres and pores present in the paper offer a criss-cross network, acting as a natural splitter of a continuous flow into tiny droplets. Our methodology thus obviates the need for any elaborate procedure for pre-generation of droplets. Further, we fabricate the necessary electrodes on filter paper by simply scribing pencil tips on the same filter paper, which facilitates the necessary electrochemical reactions. Effectively, at the anode, electrons are released, which travel through the outer circuit for cation reduction at the cathode and deliver an electrical output (voltage/current), realizing the conversion of the chemical form of energy to the electrical form in the filter paper. An absorbent pad at the channel outlet ensures continuous flow of fresh solution in the device, resulting in an inexpensive platform for power generation over a prolonged period of time. A maximum current density of 325 mA cm(-2) and a maximum power density of 53 mW cm(-2) have been observed, which significantly outweigh the capabilities of other reported devices fabricated for similar purposes.

The performance of vacuum, slow and fast pyrolysis processes to transfer energy from the paper waste sludge (PWS) to liquid and solid products was compared. Paper waste sludges with low and high ash content (8.5 and 46.7 wt.%) were converted under optimised conditions for temperature and pellet size to maximise both product yields and energy content. Comparison of the gross energy conversions, as a combination of the bio-oil/tarry phase and char (ECsum), revealed that the fast pyrolysis performance was between 18.5% and 20.1% higher for the low ash PWS, and 18.4% and 36.5% higher for high ash PWS, when compared to the slow and vacuum pyrolysis processes respectively. For both PWSs, this finding was mainly attributed to higher production of condensable organic compounds and lower water yields during FP. The low ash PWS chars, fast pyrolysis bio-oils and vacuum pyrolysis tarry phase products had high calorific values (∼18-23 MJ kg-1) making them promising for energy applications. Considering the low calorific values of the chars from alternative pyrolysis processes (∼4-7 MJ kg-1), the high ash PWS should rather be converted to fast pyrolysis bio-oil to maximise the recovery of usable energy products.

The pulp and paper industry both uses and produces large amounts of energy and rising fuel prices bring along significant challenges to the sector. Several strategies can be applied in order to remain competitive e.g. an increase in energy efficiency, a switch in fuel and/or a novel energy conversion technology. In this study, we investigate if improved energy conversion strategies can reduce energy costs, primary energy use and CO 2 emissions of paper mills within different European countries (The Netherlands, Poland and Sweden). Our results show that differences in history and availability of resources has led to different conversion strategies in the three countries. We found that a natural gas combined cycle, which is largely implemented in The Netherlands due to long term domestic availability of natural gas, has the lowest primary energy demand. Due to a long history of low electricity prices, CHP has not been common in Sweden. Many Swedish mills use biomass as an energy source and due to low CO 2 emissions of the national grid the CO 2 emission profile of the Swedish paper industry is very low. Our results imply that future conversion routes for the Swedish paper and board industry are again wood based; in case of increasing electricity prices these would be biomass gasification based CHP solutions. With a reduction of 800 kg CO 2 /ton paper on average, a switch from coal to biomass would reduce CO 2 emissions in the Polish pulp and paper industry with approximately 0.6 Mton/year. A carbon price of 20–25 €/ton would, according to our results, be enough to provoke this switch. Due to the high share of natural gas, the CO 2 emission reduction potential of the Dutch paper industry (1.5 Mton/year) is even larger than in Poland. Due to high biomass prices and relative low CO 2 emission profile of natural gas, a carbon prices of more than 60 €/ton CO 2 would be needed to provoke a switch in The Netherlands. Provided with few alternatives, the most effective strategy in The Netherlands would be an increase in energy efficiency.

A simple and scalable method was proposed to fabricate graphene papers, and the graphene sheets were prepared using conventional chemical vapor deposition (CVD) method. The CVD graphene papers possess much higher electrical conductivity of 1097 S cm−1, compared with other reported carbon-related papers (graphene, carbon nanotube, etc.). The graphene papers have good flexibility with only

Recently, great interest has been aroused in flexible/bendable electronic equipment such as rollup displays and wearable devices. As flexible energy conversion and energy storage units with high energy and power density represent indispensable components of flexible electronics, they should be carefully considered. However, it is a great challenge to fabricate flexible/bendable power sources. This is mainly due to the lack of reliable materials that combine both electronically superior conductivity and mechanical flexibility, which also possess high stability in electrochemical environments. In this work, we report a new approach to flexible energy devices. We suggest the use of a flexible electrode based on free-standing graphene paper, to be applied in lithium rechargeable batteries. This is the first report in which graphene paper is adopted as a key element applied in a flexible lithium rechargeable battery. Moreover graphene paper is a functional material, which does not only act as a conducting agent, but also as a current collector. The unique combination of its outstanding properties such as high mechanical strength, large surface area, and superior electrical conductivity make graphene paper, a promising base material for flexible energy storage devices. In essence, we discover that the graphene based flexible electrode exhibits significantly improved performances in electrochemical properties, such as in energy density and power density. Moreover graphene paper has better life cycle compared to non-flexible conventional electrode architecture. Accordingly, we believe that our findings will contribute to the full realization of flexible lithium rechargeable batteries used in bendable electronic equipments.

A method for the analysis of process energy requirements has been used to identify in an early design stage the potential process retrofit measures in an integrated pulp and paper mill. The minimum energy requirements (MER) of the process were computed by means of a dual representation that segregates the thermodynamic requirement of the process from its technological implementation. Energy and exergy recovery opportunities have been examined to improve the integration of the utility system to the process. An MILP optimisation targeting method has been applied to identify the best energy conversion options and to optimise the production of combined heat and power (CHP). Replacing the steam injections to mixing tanks by heat exchangers would decrease the MER by 10%, and increase the combined production of heat and power by a factor 1.7. Improving the exergy efficiency of the paper drying technology would be more difficult to implement, but the results indicate that this could bring an additional 12% gain of electricity cogenerated with no change to the MER.

Maximum power point tracking (MPPT) is a very important necessity in a system of energy conversion from a renewable energy source. Every year a number of publications appear in various journals and conferences claiming to offer better and faster MPPT techniques for wind energy conversion system (WECS). This research paper provides a concise yet comprehensive critical analysis of these techniques with an in-depth review of their strengths and drawbacks. This review is unique as there has been no other research paper so far that offers such a complete and up-to-date investigation of MPPT techniques in WECS. Therefore this research paper can serve as a precise reference for the future research on MPPT for WECS.

The Next Generation Nuclear Plant (NGNP) Project was initiated at Idaho National Laboratory by the U.S. Department of Energy pursuant to the 2005 Energy Policy Act and based on research and development activities supported by the Generation IV Nuclear Energy Systems Initiative. The principal objective of the NGNP Project is to support commercialization of the high temperature gas-cooled reactor (HTGR) technology. The HTGR is helium cooled and graphite moderated and can operate at reactor outlet temperatures much higher than those of conventional light water reactor (LWR) technologies. Accordingly, it can be applied in many industrial applications as a substitute for burning fossil fuels, such as natural gas, in addition to producing electricity, which is the principal application of current LWRs. These varied industrial applications may involve a standard HTGR modular design using different Energy Conversion Systems. Additionally, some of these process heat applications will require process heat delivery systems to lie partially outside the HTGR operator’s facility.

Welcome to this special section of the Journal of Micromechanics and Microengineering (JMM). This section, co-edited by myself and by Professor Jeffrey Lang of the Massachusetts Institute of Technology, contains expanded versions of selected papers presented at the Power MEMS meeting held in Atlanta, GA, USA, in December of 2012. Professor Lang and I had the privilege of co-chairing Power MEMS 2012, the 12th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications. The scope of the PowerMEMS series of workshops ranges from basic principles, to materials and fabrication, to devices and systems, to applications. The many applications of power MEMS (microelectromehcanical systems) range from MEMS-enabled energy harvesting, storage, conversion and conditioning, to integrated systems that manage these processes. Why is the power MEMS field growing in importance? Smaller-scale power and power supplies (microwatts to tens of watts) are gaining in prominence due to many factors, including the ubiquity of low power portable electronic equipment and the proliferation of wireless sensor nodes that require extraction of energy from their embedding environment in order to function. MEMS manufacturing methods can be utilized to improve the performance of traditional power supply elements, such as allowing batteries to charge faster or shrinking the physical size of passive elements in small-scale power supplies. MEMS technologies can be used to fabricate energy harvesters that extract energy from an embedding environment to power wireless sensor nodes, in-body medical implants and other devices, in which the harvesters are on the small scales that are appropriately matched to the overall size of these microsystems. MEMS can enable the manufacturing of energy storage elements from nontraditional materials by bringing appropriate structure and surface morphology to these materials as well as fabricating the electrical interfaces required for their operation and interconnection. Clearly, the marriage of MEMS technologies and energy conversion is a vital application space; and we are pleased to bring you some of the latest results from that space in this special section. Approximately 130 papers were presented at the Power MEMS 2012 conference. From these, the 20 papers you have before you were selected based on paper quality and topical balance. As you can see, papers representing many of the important areas of power MEMS are included: energy harvesters using multiple transduction schemes; MEMS-based fabrication of compact passive elements (inductors, supercapacitors, transformers); MEMS-enabled power diagnostics; MEMS-based batteries; and low power circuitry adapted to interfacing MEMS-based harvesters to overall systems. All of the papers you will read in this special section comprise substantial expansion from the proceedings articles and were reviewed through JMM's normal reviewing process. Both Professor Lang and I hope that you will share our enthusiasm for the field of power MEMS and that you will find this special section of JMM exciting, interesting and useful. Sincerely, Mark G Allen

This special section of Journal of Micromechanics and Microengineering features papers selected from the 11th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2011), held at Sejong Hotel in Seoul, Korea during 15–18 November 2011. Since the first PowerMEMS workshop held in Sendai, Japan in 2000, the workshop has developed as the premier forum for reporting research results in micro and nanotechnology for power generation, energy conversion, harvesting and processing applications, including in-depth technical issues on nanostructures and materials for small-scale high-density energy and thermal management. Potential PowerMEMS applications cover not only portable power devices for consumer electronics and remote sensors, but also micro engines, impulsive thrusters and fuel cells for systems ranging from the nanometer to the millimeter scale. The 2011 technical program consists of 1 plenary talk, 4 invited talks and 118 contributed presentations. The 48 oral and 70 poster presentations, selected by 27 Technical Program Committee Members from 131 submitted abstracts, have stimulated lively discussion maximizing the interaction between participants. Among them, this special section includes 9 papers covering micro-scale power generators, energy converters, harvesters, thrusters and thermal coolers. Finally, we are grateful to the members of the International Steering Committee, the Technical Program Committee, and the Local Organizing Committee for their efforts and contributions to PowerMEMS 2011. We also thank the two companies Samsung Electro-Mechanics and LG Elite for technical tour arrangements. Special thanks go to Dr Ian Forbes, the editorial staff of the Journal of Micromechanics and Microengineering, as well as to the staff of IOP Publishing for making this special section possible.

Nanogenerators capable of harvesting energy from environmental mechanical energy are attractive for many applications. In this paper, we present a simple, low-cost approach to convert low-frequency mechanical energy into electric power using piezoelectric ZnO nanorods grown on a common paper substrate. This energy conversion device has ultrahigh flexibility and piezoelectric sensitivity and can produce an output voltage of up to 10 mV and an output current of about 10 nA. It is demonstrated that the device's electric output behavior can be optionally changed between four types of mode simply by controlling the straining rate. Furthermore, it is also shown that the electric output can be enhanced by scaling the size of the device. This energy-harvesting technology provides a simple and cost-effective platform to capture low-frequency mechanical energy, such as body movements, for practical applications.

This special section of Journal of Micromechanics and Microengineering features papers selected from the 10th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2010). The workshop was organized in Leuven, Belgium from 30 November to 3 December 2010 by Katholieke Universiteit Leuven and the imec/Holst Centre. This was a special PowerMEMS Workshop, for several reasons. First of all, we celebrated the 10th anniversary of the workshop: the first PowerMEMS meeting was organized in Sendai, Japan in 2000. None of the organizers or participants of this first meeting could have predicted the impact of the workshop over the next decade. The second reason was that, for the first time, the conference organization spanned two countries: Belgium and the Netherlands. Thanks to the advances in information technology, teams from Katholieke Universiteit Leuven (Belgium) and the imec/Holst Centre in Eindhoven (the Netherlands) have been able to work together seamlessly as one team. The objective of the PowerMEMS Workshop is to stimulate innovation in micro and nanotechnology for power generation and energy conversion applications. Its scope ranges from integrated microelectromechanical systems (MEMS) for power generation, dissipation, harvesting, and management, to novel nanostructures and materials for energy-related applications. True to the objective of the PowerMEMSWorkshop, the 2010 technical program covered a broad range of energy related research, ranging from the nanometer to the millimeter scale, discussed in 5 invited and 52 oral presentations, and 112 posters. This special section includes 14 papers covering vibration energy harvesters, thermal applications and micro power systems. Finally, we wish to express sincere appreciation to the members of the International Steering Committee, the Technical Program Committee and last but not least the Local Organizing Committee. This special issue was edited in collaboration with the staff of IOP Publishing. PowerMEMS 2010 contents Harvesting energy from airflow with a michromachined piezoelectric harvester inside a Helmholtz resonator S P Matova, R Elfrink, R J M Vullers and R van Schaijk Analysis and characterization of triangular electrode structures for electrostatic energy harvestingDaniel Hoffmann, Bernd Folkmer and Yiannos Manoli A smart and self-sufficient frequency tunable vibration energy harvesterC Eichhorn, R Tchagsim, N Wilhelm and P Woias Power output enhancement of a vibration-driven electret generator for wireless sensor applicationsTatsuakira Masaki, Kenji Sakurai, Toru Yokoyama, Masayo Ikuta, Hiroshi Sameshima, Masashi Doi, Tomonori Seki and Masatoshi Oba Harvesting traffic-induced vibrations for structural health monitoring of bridgesT V Galchev, J McCullagh, R L Peterson and K Najafi Dispenser-printed planar thick-film thermoelectric energy generatorsA Chen, D Madan, P K Wright and J W Evans Silicon nanowire arrays as thermoelectric material for a power microgeneratorD Davila, A Tarancon, M Fernandez-Regulez, C Calaza, M Salleras, A San Paulo and L Fonseca A micro thermal switch with a stiffness-enhanced thermal isolation structureTakashiro Tsukamoto, Masayoshi Esashi and Shuji Tanaka A dielectric liquid contact thermal switch with electrowetting actuationA R McLanahan, C D Richards and R F Richards A self-regulating valve for single-phase liquid cooling of microelectronicsRadu Donose, Michael De Volder, Jan Peirs and Dominiek Reynaerts A MEMS-enabled 3D zinc–air microbattery with improved discharge characteristics based on a multilayer metallic substructureA Armutlulu, Y Fang, S H Kim, C H Ji, S A Bidstrup Allen and M G Allen Design, fabrication and testing of an air-breathing micro direct methanol fuel cell with compound anode flow fieldLuwen Wang, Yufeng Zhang, Youran Zhao, Zijiang An, Zhiping Zhou and Xiaowei Liu A shadow-mask evaporated pyroMEMS igniterD A de Koninck, D Briand and N F de Rooij Aerodynamic journal bearing with a flexible, damped support operating at 7.2 million DNTobias Waumans, Jan Peirs, Farid Al-Bender and Dominiek Reynaerts Thermoelectric energy harvester on the heated human machineVladimir Leonov