Oxygen efficiency with regard to carbon capture

Carbon capture is often discussed in the literature with the sole focus on power processes, despite the fact that carbon dioxide emissions from other sources are just as relevant for the impact on the atmosphere. Furthermore, some carbon capture methods are relatively inefficient when applied to power production processes. Carbon capture should preferably be performed where the cost is as low as possible, i.e. not necessarily from power production processes. As an example, carbon capture using combustion with pure oxygen is far more energy efficient if it is used together with lime kilns or cement kilns than together with power production processes. A new concept termed “oxygen efficiency” is introduced in this paper. It describes the amount of carbon dioxide that can potentially be captured per unit of oxygen. As such, the oxygen efficiency quantifies the value of a certain unit of oxygen for carbon capture reasons. The base concept is that the energy penalty for the production of one part of oxygen is the same no matter where it is produced; hence, if this unit of oxygen can be used to capture more carbon dioxide, it is more efficient. Typically, the oxygen efficiency would be five times greater for carbon capture when utilising pure oxygen together with cement kilns rather than together with methane-fired power plants. Furthermore, the concept of oxygen efficiency illustrates the importance of considering how carbon capture methods can be utilised in the most efficient way, in addition to evaluating which carbon capture method is the most suitable for a particular technology.     

Exergetic evaluation of methods for improving power generation efficiency of a gas turbine cogeneration system

A cogeneration system generating both heat and power for district heating and cooling is required to be more efficient to improve its economy. In this paper, three typical methods for improving the power generation efficiency of a gas turbine cogeneration system are evaluated by examining exergy flow at various points of the system. The three methods investigated are: (a) to raise turbine inlet temperature, (b) to incorporate a regenerative cycle, and (c) to introduce a dual-fluid cycle. Exergy flows at various points of each cogeneration system have been evaluated. It has been shown through quantitve analyses of exergy flows (1) what kind of energy loss of the system can be reduced by introducing each efficiency-improving method, (2) that the method of incorporating a regenerative cycle is highly useful in improving exergy efficiency of the cogeneration system. © 1997 by John Wiley & Sons, Ltd.     

Strategies for improving Co/Ni-based bimetal-organic framework to water splitting

The preparation of high-efficiency, stable, and low-cost oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) electrocatalysts remains a challenge for new energy systems. In this study, three-dimensional (3D) cobalt-nickel bimetal MOFs were used as precursors to synthesize catalysts through thermal decomposition, carbonization, nitriding, oxidation, phosphating, sulfurizing, and selenization, respectively. In 1.0 M KOH electrolyte, the overpotential of Co/Ni-MOFs@Se for OER was 238 mV and the that of Co/Ni-MOFs@P for HER was 194 mV at a current density of 10 mA cm⁻². Based on the excellent OER and HER performances of Co/Ni-MOFs@Se and Co/Ni-MOFs@P, these two materials were further assembled into electrodes for overall water splitting. Results showed that a potential of only 1.59 V was required to provide a current density of 10 mA cm⁻². The electrodes also exhibited long-term durability in a 2000 min stability test without significant changes in the catalytic performances. According to the difference in the doped non-metal elements, an electrode pair with a suitable matching degree was constructed, thereby improving the overall water splitting performance. Thus, the controllable modification of the metal-organic frameworks (MOFs)-derived carbon materials (CMs) effectively improved the materials’ catalytic water splitting performance. It was possible to further develop an efficient, inexpensive, and low-cost assembled electrode pair.     

提高热电厂效率的几项措施

从热经济性角度提出提高热电厂效率的几项技术和措施:通过凝汽器补充软化水,将外供蒸汽过热度降低;使用喷射式混合加热器回收热力除氧器排汽,作为生水加热器;利用压力匹配器代替减压减温器;用两相流加热器代替面式高压加热器等。     

Enhanced efficiency of hematite photoanode for water splitting with the doping of Ge

Ge doped α-Fe₂O₃ nanowires are synthesized through a hydrothermal procedure with GeO₂ as a precursor and investigated as photoanodes for water splitting. The content of Ge in the photoanode rises with the increase of the amount of GeO₂ in the precursor solution. A proper amount of Ge facilities the preferred oriented growth of the (110) plane of α-Fe₂O₃, while excessive Ge hinders the growth of α-Fe₂O₃ crystals. The doping of Ge increases the absorption efficiency and decreases the recombining rate of the photogenerated electrons and holes. Ge also improves the density and transfer rate of the charge carriers in the photoanode. Ge doped α-Fe₂O₃ photoanode exhibits a highest photocurrent density of 0.92 mA cm^?2 at 1.23 V vs. reversible hydrogen electrode under AM 1.5 G simulated sunlight, which is nearly twice of that obtained by pure α-Fe₂O₃ under the same condition.     

New method for improving the bulk charge separation of hematite with enhanced water splitting

Due to its poor bulk charge separation efficiency, the photoelectrochemical (PEC) performance of pristine hematite prepared directly from an electrodeposited Fe film is limited. Au-modification of hematite via a simple immersion method improves the PEC performance two-fold to 0.31 mA cm⁻². The Au nanoparticles deposited from HAuCl₄ act as plasmonic photosensitizers and electron collectors to improve the light absorption and bulk charge separation efficiency of the photoanode. In addition, the increase in the (110) plane and specific surface area induced by HAuCl₄ enhances the bulk charge separation efficiency. After further modification with Ti, the photocurrent response of the resulting Ti/Au/α-Fe₂O₃ photoanode improves to 0.51 mA cm⁻²; this increase is attributed to its increased light absorption, bulk charge separation efficiency (η_bulk), and surface charge injection efficiency (η_surface). In this work, the effect of Au and Ti on the crystalline structure, morphology and PEC performance of the novel electrodeposited hematite photoanode are investigated by systematical characterization.     

Biomimetic molecular water splitting catalysts for hydrogen generation

During the last few decades, the scientific community has been striving hard to develop new and alternative sources for renewable energy and fuel. Hydrogen or carbon free fuels obtained from catalytic water splitting using sunlight offer an attractive solution for a cleaner and greener future. In this pursuit, to establish effective molecular catalytic systems for efficient water oxidation is considered to be a bottleneck, hampering the design, implementation and exploitation of electrochemical and photo-electrochemical modular devices for light driven energy conversion into hydrogen-based storable fuels. From metal oxides to composite materials, noble metal complexes to transition metal organometallics, multinuclear to mono-site catalysts, various water oxidation complexes (WOCs) have been investigated both in a homogeneous environment and on surfaces in photo- or electrochemical conditions. However, a truly biomimetic catalytic system that matches the performance of photosystem-II for efficient water splitting, operating with four consecutive proton coupled electron transfer (PCET) steps to generate oxygen and hydrogen for hundred thousands of cycles at high rate is yet to be achieved. We here present an overview of biomimetic molecular complexes that have been investigated recently for water oxidation catalysis in homogeneous solution using chemical oxidants, or as heterogeneous species for catalytic electrochemical systems.     

Hydrogen generation by splitting water with Al–Ca alloy

A new hydrogen generation material, Al-Ca alloy, is prepared by ball milling method. Results show the prepared Al-Ca alloy can react with to produce hydrogen, but its hydrogen yield is lower. NaCl addition can further greatly improve hydrogen generation of Al-Ca alloys. The amount of NaCl addition and ball milling time depends on the Ca contents of alloys. As the Ca contents of alloy increase, the amount of NaCl addition or ball milling time may be reduced accordingly. Increasing Ca contents, NaCl addition or ball milling time is beneficial to improve the hydrogen generation rate. Al-Ca alloys can react with water to produce hydrogen at the temperature ranging from 10 °C to 80 °C, and simultaneously a great amount of heat is released. With the increase of air exposure time, the dense Al₂O₃ and CaO layer formed on the surface of alloy particles will reduce the oxidation reaction rate. Chloride ions and sulfate ions can greatly decrease the induction period of hydrogen generation reaction and obviously improve hydrogen generation rate. Ca²⁺ ions and Mg²⁺ ions can affect the production of hydrogen due to their strong affinity to OH⁻, especially Mg²⁺ ions which greatly decrease the hydrogen yield to 20%.     

Hydrogen generation by splitting water with Al–Li alloys

In order to prevent the inert alumina film from forming on the surface of Al metal particles, Li is added into Al to form Al–Li alloy. It can improve the reactivity of Al with water. The prepared Al–Li alloy can rapidly split water to produce hydrogen. With increasing Li content of alloy, the hydrogen generation rate is promoted. The ultimate hydrogen yields of samples can reach 100%. The effect of initial water temperature on the hydrogen generation has been investigated. Even in the water at 0 °C, hydrogen can also be produced rapidly. Composition of solution has some effect on the hydrogen generation. Especially, Mg²⁺ or NO₃^? has negative influence on the hydrogen generation and can reduce the ultimate hydrogen yield of alloy. Longer air exposure time will also decrease the ultimate hydrogen yield. After reaction, Al and Li enter into the residue in the form of LiAl₂(OH)₇·2H₂O and LiAl₂(OH)₇·xH₂O or Al(OH)₃. After calcinations, these reaction by‐products can be easily recycled by existing metallurgical process. Copyright © 2012 John Wiley & Sons, Ltd.     

Using natural gas generation to improve power system efficiency in China

China's electricity sector faces the challenge of managing cost increases, improving reliability, and reducing its environmental footprint even as operating conditions become more complex due to increasing renewable penetration, growing peak demand, and falling system load factors. Addressing these challenges will require changes in how power generation is planned, priced, and dispatched in China. This is especially true for natural gas generation, which is likely to play an important role in power systems worldwide as a flexible generation resource. Although natural gas is commonly perceived to be economically uncompetitive with coal in China, these perceptions are based on analysis that fails to account for the different roles that natural gas generation plays in power systems—baseload, load following, and peaking generation. Our analysis shows that natural gas generation is already cost-effective for meeting peak demand in China, resulting in improved capacity factors and heat rates for coal-fired generators and lower system costs. We find that the largest barrier to using natural gas for peaking generation in China is generation pricing, which could be addressed through modest reforms to support low capacity factor generation.     

The exogenous factors affecting the cost efficiency of power generation

This paper employs a stochastic frontier analysis (SFA) to examine cost efficiency and scale economies in Taiwan Power Company (TPC) by using the panel data covering the period of 1995–2006. In most previous studies, the efficiency estimated by the Panel Data without testing the endogeneity may bring about a biased estimator resulting from the correlation between input and individual effect. A Hausman test is conducted in this paper to examine the endogeneity of input variables and thus an appropriate model is selected based on the test result. This study finds that the power generation executes an increasing return to scale across all the power plants based on the pooled data. We also use installed capacity, service years of the power plant, and type of fuel as explanatory variable for accounting for the estimated cost efficiency of each plant by a logistic regression model to examine the factor affecting the individual efficiency estimates. The results demonstrate that the variable of installed capacity keeps a positive relationship with cost efficiency while the factor of working years has a negative relationship.     

Influence of the morphology of ZnO nanowires on the photoelectrochemical water splitting efficiency

Defect-free ZnO nanowire arrays were synthesized and assessed as photoanodes in photoelectrochemical cells. Several tens of samples classified into five different average diameters in the range 40–260 nm were prepared to explore the role of morphology and polar surfaces. The photoelectrochemical performance of the NWs was studied in basic aqueous electrolytes (pH 12.7). A non-monotonic behavior of the performance was demonstrated, which maximizes for nanowires with diameter ～120 nm. The maximum applied bias photoconversion efficiency is ～6.3% upon irradiation with 11.5 mW at 365 nm. The photoanodes exhibit a rather stable performance for ～10 h, while they stabilize at ～60% of the initial photocurrent after 20 h of continuous bias illumination. The degradation of their performance was attributed to partial detachment of the NWs from the supporting conductive film. The enhanced stability is attributed to the decrease of the pH at the electrochemical interface to values that inhibit the dissolution of ZnO.     

Unassisted water splitting with 9.3% efficiency by a single quantum nanostructure photoelectrode

To split water and produce hydrogen by white light is an excellent solution for the storage and supply of clean and sustainable energy. Efficiency and stability are the key challenges for a successful exploitation. InGaN, evaluated against other semiconductors, metal oxides, carbon based - and organic materials has most suited intrinsic materials properties. Based on this optimum materials choice we report photoelectrochemical (PEC) hydrogen generation under white light illumination by an InGaN-based quantum nanostructure photoelectrode. No degradation occurs for operation over 10 h. Our novel concept, combining quantum nanostructure physics with electrochemistry and catalysis leads to almost 10% efficiency at zero external voltage. The efficiency rises above 25% at 0.2 V. This is unmatched for a single photoelectrode, representing the most advanced technology of low complexity.     

International comparison of energy efficiency of fossil power generation

The purpose of this study is to compare the energy efficiency of fossil-fired power generation for Australia, China, France, Germany, India, Japan, Nordic countries (Denmark, Finland, Sweden and Norway aggregated), South Korea, United Kingdom and Ireland, and United States. Together these countries generate 65% of worldwide fossil power generation. Separate benchmark indicators are calculated for the energy efficiency of natural gas, oil and coal-fired power generation, based on weighted-average energy efficiencies. These indicators are aggregated to an overall benchmark for fossil-fired power generation. The weighted average efficiencies are 35% for coal, 45% for natural gas and 38% for oil-fired power generation.     

Entropy production, efficiency, and economics in the thermochemical generation of synthetic fuels: II. The methanol water splitting cycle

The availability/heat penalty analysis described in our earlier paper [1] has been applied to the methanol-sulfuric acid water splitting cycle proposed by Schulten and Behr [2]. This four-reaction thermochemical process has been evaluated in terms of preliminary engineering design and economics by the Electric Power Research Institute (EPRI), the University of Kentucky and the CE-Lummus Co. [3]. The distribution of heat penalties induced by thermodynamic irreversibilities in the plant is shown, and the quantitative relationship among capital costs, production costs, heat penalties, and process efficiency is presented. The total thermal energy output of the nuclear reactor is the sum of the theoretical heat requirement and all the heat penalties in the plant. The costs, heat penalties and efficiency are also shown for the hybrid sulfuric acid process.     

Photocatalytic hydrogen generation by splitting of water from electrospun hybrid nanostructures

MWCNT-TiO₂ hybrid nanostructures are prepared using sol–gel and electrospinning followed by post annealing of as-spun nanofibers at 450 °C per 1 h in air. These hybrid nanostructures composed of MWCNTs varied from 0 to 20% (w/w) and are characterized by SEM, TEM, XRD, and FT-IR analysis. MWCNT-TiO₂ hybrid structures are utilized in commercially available Methylene blue (MB) dye degradation and found that 2% of MWCNT exhibit superior kinetic constant 6.379 × 10⁻³ min⁻¹ extracted. In addition, we demonstrate that the doping of MWCTs within TiO₂ leads to a significant enhancement of the UV–vis light assisted photocatalytic activity is optimized in comparison with higher (5, 10 and 20%) compositions. UV–vis assisted photocatalytic hydrogen is evolved by photoelectrolytic splitting of water by using MWCNT-TiO₂ hybrid nanostructures as electrode.     

Operational simulation of wind power plants for electrolytic hydrogen production connected to a distributed electricity generation grid

Two procedures are analyzed to control the flow of hydrogen produced by an electrolyzer in a plant connected to a distributed electricity grid. The general idea of both procedures is to approximate the consumption power of the electrolyzer to the tracked hourly mean useful power of a wind generation system. The first technique uses a perceptron to predict hourly wind-speed values as the basis for the power consumption of the electrolyzer. The second approximates the hourly consumption of the electrolyzer to the useful power of the wind generation system over the previous hour. Calculations have shown that the control procedure, using either one of these two techniques, leads to substantial improvements in the main parameters of the plant, compared to an installation in which electrolyzer consumption is constant. In particular, the number of batteries in the accumulation system may be reduced. Moreover, considering the possibility that the hydrogen production plant might supply electricity to the external electricity grid, various objectives for operational optimization of the installation are analyzed. A function that defines the joint exploitation of the wind energy by the electrolyzer and the external electricity grid is introduced and then, by using that function, an optimal operating regime for the plant is determined.     

rGO decorated semiconductor heterojunction of BiVO4/NiO to enhance PEC water splitting efficiency

Monoclinic phase of BiVO₄ is a promising photoanode material for photoelectrochemical (PEC) water splitting, but its sluggish water oxidation kinetics and frequent bulk charge recombination greatly reduce its efficiency of PEC water splitting. A novel BiVO₄/NiO/rGO photoanode was very simply prepared by electrodeposition, solution immersion and spin coating methods, in particular, the solution immersion method to loading NiO has never been reported in PEC research. Compared with BiVO₄, the photocurrent density of the ternary photoanode reaches 1.52 mA/cm² at 1.23 V vs RHE, which is 2.41 and 1.39 times higher than that of pure BiVO₄ and binary BiVO₄/NiO photoanode, respectively. The onset potential of the ternary photoanode shows a significant cathodic shift of 130 mV compared with the BiVO₄ photoanode. Moreover, the measured incident photon-to-current efficiency (IPCE) value reaches 50.52% at λ = 420 nm. The improvement is attributed to the type-II heterojunction formation that enhances the separation efficiency of electron/hole and the rGO decoration that accelerates the electron transfer and provides more active sites for gas adsorption.