Special Issue "Renewable Energy"

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A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Applied Energy".

Deadline for manuscript submissions: closed (31 January 2013)

Special Issue Editor

Guest Editor
Prof. Dr. Tomonobu Senjyu (Website)

Department of Electrical and Electronics Engineering, Faculty of Engineering, University of the Ryukyus, 1 Senbaru Nihihara-cho, Okinawa 903-0213, Japan
Phone: +81-98-895-8686
Fax: +81 895 8686
Interests: renewable energy; power system analysis; power system optimization; distribution system control and optimization; power system control; power electronics in power systems

Special Issue Information

Dear Colleagues,

Renewable energy is a promising energy source in order not to have emission of carbon dioxide at the time of power generation. Although renewable energy is mainly exploited photovoltaic energy or wind energy, however it changes by climate conditions, stabilization of electric power is needed. The smart grid attracts attention as research on stabilization and effective use of renewable energy in recent years, and research on a smart grid is advanced in the world. As core technology of smart grid, power electronics, optimization technology, demand response, smart meter, large storage battery, cooperative control, etc. are needed, and it is technology indispensable to large-scale introduction of renewable energy. In this special issue, the contribution of the paper about the circumference technology for large-scale introduction of renewable energy is expected broadly.

Prof. Dr. Tomonobu Senjyu
Guest Editor

Submission

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Keywords

  • photovoltaic generation
  • wind-turbine generation
  • smart grid
  • demand response
  • battery
  • unit commitment
  • optimal operation
  • coordinated control
  • distributed generators

Published Papers (14 papers)

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Research

Jump to: Review

Open AccessArticle A Novel MPPT Control Method of Thermoelectric Power Generation with Single Sensor
Appl. Sci. 2013, 3(2), 545-558; doi:10.3390/app3020545
Received: 31 December 2012 / Revised: 7 March 2013 / Accepted: 10 April 2013 / Published: 29 April 2013
Cited by 5 | PDF Full-text (344 KB) | HTML Full-text | XML Full-text
Abstract
This paper proposes a novel Maximum Power Point Tracking (MPPT) control method of thermoelectric power generation for the constant load. This paper reveals the characteristics and the internal resistance of thermoelectric power module (TM). Analyzing the thermoelectric power generation system with boost [...] Read more.
This paper proposes a novel Maximum Power Point Tracking (MPPT) control method of thermoelectric power generation for the constant load. This paper reveals the characteristics and the internal resistance of thermoelectric power module (TM). Analyzing the thermoelectric power generation system with boost chopper by state space averaging method, the output voltage and current of TM are estimated by with only single current sensor. The proposed method can seek without calculating the output power of TM in this proposed method. The basic principle of the proposed MPPT control method is discussed, and then confirmed by digital computer simulation using PSIM. Simulation results demonstrate that the output voltage can track the maximum power point voltage by the proposed MPPT control method. The generated power of the TM is 0.36 W when the temperature difference is 35 °C. This is well accorded with the V-P characteristics. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Control of a Stand-Alone Variable Speed Wind Energy Supply System
Appl. Sci. 2013, 3(2), 437-456; doi:10.3390/app3020437
Received: 28 December 2012 / Revised: 13 March 2013 / Accepted: 20 March 2013 / Published: 11 April 2013
Cited by 7 | PDF Full-text (511 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a simple control strategy for the operation of a variable speed stand-alone wind turbine with a permanent magnet synchronous generator (PMSG). The PMSG is connected to a three phase resistive load through a switch mode rectifier and a voltage [...] Read more.
This paper presents a simple control strategy for the operation of a variable speed stand-alone wind turbine with a permanent magnet synchronous generator (PMSG). The PMSG is connected to a three phase resistive load through a switch mode rectifier and a voltage source inverter. Control of the generator side converter is used to achieve maximum power extraction from the available wind power. Control of the DC-DC bidirectional buck-boost converter, which is connected between batteries bank and DC-link voltage, is used to maintain the DC-link voltage at a constant value. It is also used to make the batteries bank stores the surplus of wind energy and supplies this energy to the load during a wind power shortage. The load side voltage source inverter uses a relatively complex vector control scheme to control the output load voltage in terms of amplitude and frequency. The control strategy works under wind speed variation as well as with variable load. Extensive simulation results have been performed using MATLAB/SIMULINK. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Power Collection from Wave Energy Farms
Appl. Sci. 2013, 3(2), 420-436; doi:10.3390/app3020420
Received: 7 January 2013 / Revised: 12 March 2013 / Accepted: 13 March 2013 / Published: 2 April 2013
Cited by 10 | PDF Full-text (3053 KB) | HTML Full-text | XML Full-text
Abstract
Most Wave Energy Converters (WECs) produce highly distorted power due to thereciprocal motion induced by ocean waves. Some WEC systems have integrated energystorage that overcomes this limitation, but add significant expenses to an already costlysystem. As an alternative approach, this article investigates [...] Read more.
Most Wave Energy Converters (WECs) produce highly distorted power due to thereciprocal motion induced by ocean waves. Some WEC systems have integrated energystorage that overcomes this limitation, but add significant expenses to an already costlysystem. As an alternative approach, this article investigates the direct export option thatrelies on aggregate smoothing among several WECs. By optimizing the positioning of theWEC devices with respect to the incoming waves, fluctuations may be mutually canceledout between the devices. This work is based on Fred. Olsen’s WEC system Lifesaver, anda WEC farm consisting of 48 devices is designed in detail and simulated. The major costdriver for the electrical export system is the required oversize factor necessary for transferof the average power output. Due to the low power quality, this number can be as high as20 at the entry point of the electrical system, and it is thus crucial to quickly improve thepower quality so that the downstream power system is efficiently utilized. The simulationsundertaken in this work indicate that a high quality power output can be achieved at the farmlevel, but that a significant oversize factor will be required in the intermediate power systemwithin the farm. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Modelling the Contribution of Domestic Heat Pumps to Delivering UK Energy Policy Objectives
Appl. Sci. 2013, 3(2), 338-354; doi:10.3390/app3020338
Received: 25 December 2012 / Revised: 8 February 2013 / Accepted: 17 February 2013 / Published: 28 March 2013
Cited by 1 | PDF Full-text (488 KB) | HTML Full-text | XML Full-text
Abstract
The UK Government has ambitious targets for CO2 emissions reductions, particularly for the domestic housing stock. One technology that is expected to contribute significantly is heat pumps, both air and ground source. However, recent field trial results suggest that heat pumps [...] Read more.
The UK Government has ambitious targets for CO2 emissions reductions, particularly for the domestic housing stock. One technology that is expected to contribute significantly is heat pumps, both air and ground source. However, recent field trial results suggest that heat pumps in the UK are not delivering to performance expectations. This paper looks at the implications of these results for the UK housing stock’s future CO2 emissions. The English Housing Condition Survey dataset is used as the basis for a Monte Carlo simulation in order to model CO2 emissions and energy consumption for the whole of English housing stock out to 2050. The results suggest that, given the current UK electricity grid CO2 emission factor, in the short term poor heat pump performance could lead to a rise in emissions where natural gas boilers are displaced. In the longer term, heat pumps can realise emissions reductions when installed at high penetration levels when combined with a grid decarbonisation strategy. Until grid decarbonisation occurs, an alternative phased strategy is proposed that includes phased replacement of resistive electric heating, first in households in fuel poverty and then the remainder of properties with this heating type. Following this phased strategy, real emissions savings are possible along with a potential reduction in fuel poverty. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle A Comparative Analysis of Energy Costs of Photovoltaic, Solar Thermal, and Wind Electricity Generation Technologies
Appl. Sci. 2013, 3(2), 325-337; doi:10.3390/app3020325
Received: 31 December 2012 / Revised: 13 February 2013 / Accepted: 5 March 2013 / Published: 25 March 2013
Cited by 11 | PDF Full-text (257 KB) | HTML Full-text | XML Full-text
Abstract
Global installed capacity of renewable energy technologies is growing rapidly. The ability of renewable technologies to enable a rapid transition to a low carbon energy system is highly dependent on the energy that must be “consumed” during their life-cycle. This paper presents [...] Read more.
Global installed capacity of renewable energy technologies is growing rapidly. The ability of renewable technologies to enable a rapid transition to a low carbon energy system is highly dependent on the energy that must be “consumed” during their life-cycle. This paper presents the results of meta-analyses of life-cycle assessments (LCA) of energy costs of three renewable technologies: solar photovoltaic (PV), concentrating solar power (CSP), and wind. The paper presents these findings as energetic analogies with financial cost parameters for assessing energy technologies: overnight capital cost, operating costs and levelized cost of electricity (LCOE). The findings suggest that wind energy has the lowest energy costs, followed by CSP and then PV. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle An Appropriate Wind Model for Wind Integrated Power Systems Reliability Evaluation Considering Wind Speed Correlations
Appl. Sci. 2013, 3(1), 107-121; doi:10.3390/app3010107
Received: 31 December 2012 / Revised: 29 January 2013 / Accepted: 29 January 2013 / Published: 6 February 2013
Cited by 11 | PDF Full-text (319 KB) | HTML Full-text | XML Full-text
Abstract
Adverse environmental impacts of carbon emissions are causing increasing concerns to the general public throughout the world. Electric energy generation from conventional energy sources is considered to be a major contributor to these harmful emissions. High emphasis is therefore being given to [...] Read more.
Adverse environmental impacts of carbon emissions are causing increasing concerns to the general public throughout the world. Electric energy generation from conventional energy sources is considered to be a major contributor to these harmful emissions. High emphasis is therefore being given to green alternatives of energy, such as wind and solar. Wind energy is being perceived as a promising alternative. This source of energy technology and its applications have undergone significant research and development over the past decade. As a result, many modern power systems include a significant portion of power generation from wind energy sources. The impact of wind generation on the overall system performance increases substantially as wind penetration in power systems continues to increase to relatively high levels. It becomes increasingly important to accurately model the wind behavior, the interaction with other wind sources and conventional sources, and incorporate the characteristics of the energy demand in order to carry out a realistic evaluation of system reliability. Power systems with high wind penetrations are often connected to multiple wind farms at different geographic locations. Wind speed correlations between the different wind farms largely affect the total wind power generation characteristics of such systems, and therefore should be an important parameter in the wind modeling process. This paper evaluates the effect of the correlation between multiple wind farms on the adequacy indices of wind-integrated systems. The paper also proposes a simple and appropriate probabilistic analytical model that incorporates wind correlations, and can be used for adequacy evaluation of multiple wind-integrated systems. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Biofuels and Land Use Change: Applying Recent Evidence to Model Estimates
Appl. Sci. 2013, 3(1), 14-38; doi:10.3390/app3010014
Received: 10 October 2012 / Revised: 4 December 2012 / Accepted: 6 January 2013 / Published: 11 January 2013
Cited by 8 | PDF Full-text (1690 KB) | HTML Full-text | XML Full-text
Abstract
Biofuels impact on global land use has been a controversial yet important topic. Up until recently, there has not been enough biofuels to have caused major land use change, so the evidence from actual global land use data has been scant. However, [...] Read more.
Biofuels impact on global land use has been a controversial yet important topic. Up until recently, there has not been enough biofuels to have caused major land use change, so the evidence from actual global land use data has been scant. However, in the past decade, there have been 72 million hectares added to global crop cover. In this research we take advantage of this new data to calibrate the Global Trade Analysis Project (GTAP) model and parameters. We make two major changes. First, we calibrate the land transformation parameters (called constant elasticity of transformation, CET) to global regions so that the parameters better reflect the actual land cover change that has occurred. Second, we alter the land cover nesting structure. In the old GTAP model, cropland, pasture, and forest were all in the same nest suggesting, everything else being equal, that pasture or forest convert to cropland with equal ease and cost. However, we now take advantage of the fact that pasture converts to cropland at lower cost than forest. The paper provides the theoretical and empirical justification for these two model improvements. Then it re-evaluates the global land use impacts due to the USA ethanol program using the improved model tuned with actual observations. Finally, it shows that compared to the old model, the new model projects: (1) less expansion in global cropland due to ethanol expansion; (2) lower U.S. share in global cropland expansion; (3) and lower forest share in global cropland expansion. Full article
(This article belongs to the Special Issue Renewable Energy)
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Open AccessArticle Power Management System for Load Banks Supplied by Pitch Controlled Wind Turbine System
Appl. Sci. 2012, 2(4), 801-815; doi:10.3390/app2040801
Received: 28 September 2012 / Revised: 15 November 2012 / Accepted: 15 November 2012 / Published: 28 November 2012
Cited by 1 | PDF Full-text (496 KB) | HTML Full-text | XML Full-text
Abstract
An automatic power management system, to monitor the distribution of power to a set of load banks, is presented in this paper. The required power is generated from a laboratory-size pitch-controlled wind turbine experimental workstation. The management system is a sequence of [...] Read more.
An automatic power management system, to monitor the distribution of power to a set of load banks, is presented in this paper. The required power is generated from a laboratory-size pitch-controlled wind turbine experimental workstation. The management system is a sequence of logic expressions, based on the generated power, a profile of the banks’ states provided by the supervisor and the banks’ priorities, to obtain an optimal behavior of the system and to ensure the load requirement. A modified PI pitch angle control is proposed to regulate the generated power for tracking the power reference in order to maintain a consistent brilliance of the load’s LEDs, to reduce the activity of the pitch actuator and to deal with fluctuation problems. Experimental results are provided to show the effectiveness of the proposed automatic power management system for load banks supplied by a pitch controlled wind turbine. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle A Design Fuzzy Logic Controller for a Permanent Magnet Wind Generator to Enhance the Dynamic Stability of Wind Farms
Appl. Sci. 2012, 2(4), 780-800; doi:10.3390/app2040780
Received: 21 August 2012 / Revised: 29 October 2012 / Accepted: 9 November 2012 / Published: 22 November 2012
Cited by 11 | PDF Full-text (2595 KB) | HTML Full-text | XML Full-text
Abstract
In this paper, a design fuzzy logic controller for a variable speed permanent magnet wind generator connected to a grid system through a LC-filter is proposed. A new current control method of grid side conversion is developed by integrating the fuzzy controller, [...] Read more.
In this paper, a design fuzzy logic controller for a variable speed permanent magnet wind generator connected to a grid system through a LC-filter is proposed. A new current control method of grid side conversion is developed by integrating the fuzzy controller, in which both active and reactive power, delivered to a power grid system, is controlled effectively. The fuzzy logic controller is designed to adjust the gain parameters of the PI controllers under any operating conditions, so that the dynamic stability is enhanced. A new simple method, based on frequency response of the bode diagram, is proposed in the design of the fuzzy logic controller. To evaluate the controller system capabilities, simulation analyses are performed on a small wind farm model system including an induction wind generator connected to an infinite bus. The simulations have been performed using PSCAD/EMTDC. Simulation results show that the proposed control scheme is more effective for enhancing the stability of wind farms during temporary and permanent network disturbances and randomly fluctuating wind speed, compared with that of a conventional PI controller. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Ethanol Production from Waste Potato Mash by Using Saccharomyces Cerevisiae
Appl. Sci. 2012, 2(4), 738-753; doi:10.3390/app2040738
Received: 16 July 2012 / Revised: 19 September 2012 / Accepted: 10 October 2012 / Published: 22 October 2012
Cited by 13 | PDF Full-text (301 KB) | HTML Full-text | XML Full-text
Abstract
Bio-ethanol is one of the energy sources that can be produced by renewable sources. Waste potato mash was chosen as a renewable carbon source for ethanol fermentation because it is relatively inexpensive compared with other feedstock considered as food sources. However, a [...] Read more.
Bio-ethanol is one of the energy sources that can be produced by renewable sources. Waste potato mash was chosen as a renewable carbon source for ethanol fermentation because it is relatively inexpensive compared with other feedstock considered as food sources. However, a pretreatment process is needed: specifically, liquefaction and saccharification processes are needed to convert starch of potato into fermentable sugars before ethanol fermentation. In this study, hydrolysis of waste potato mash and growth parameters of the ethanol fermentation were optimized to obtain maximum ethanol production. In order to obtain maximum glucose conversions, the relationship among parameters of the liquefaction and saccharification process was investigated by a response surface method. The optimum combination of temperature, dose of enzyme (α-amylase) and amount of waste potato mash was 95 °C, 1 mL of enzyme (18.8 mg protein/mL) and 4.04 g dry-weight/100 mL DI water, with a 68.86% loss in dry weight for liquefaction. For saccharification, temperature, dose of enzyme and saccharification time were optimized and optimum condition was determined as 60 °C-72 h-0.8 mL (300 Unit/mL) of amyloglucosidase combination, yielded 34.9 g/L glucose. After optimization of hydrolysis of the waste potato mash, ethanol fermentation was studied. Effects of pH and inoculum size were evaluated to obtain maximum ethanol. Results showed that pH of 5.5 and 3% inolculum size were optimum pH and inoculum size, respectively for maximum ethanol concentration and production rate. The maximum bio-ethanol production rate was obtained at the optimum conditions of 30.99 g/L ethanol. Since yeast extract is not the most economical nitrogen source, four animal-based substitutes (poultry meal, hull and fines mix, feather meal, and meat and bone meal) were evaluated to determine an economical alternative nitrogen source to yeast extract. Poultry meal and feather meal were able to produce 35 g/L and 32.9 g/L ethanol, respectively, which is higher than yeast extract (30.8 g/L). In conclusion, waste potato mash was found as a promising carbon source for ethanol fermentation with alternate nitrogen sources. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Feasibility Study of Energy Storage Systems in Wind/Diesel Applications Using the HOMER Model
Appl. Sci. 2012, 2(4), 726-737; doi:10.3390/app2040726
Received: 27 August 2012 / Revised: 24 September 2012 / Accepted: 25 September 2012 / Published: 18 October 2012
Cited by 7 | PDF Full-text (225 KB) | HTML Full-text | XML Full-text
Abstract
With an increased focus on solutions to the ensuing “climate crisis”, the need for energy storage systems is becoming increasingly important as a means to increase the penetration of renewable technologies such as wind energy. The Vanadium Redox Battery is one such [...] Read more.
With an increased focus on solutions to the ensuing “climate crisis”, the need for energy storage systems is becoming increasingly important as a means to increase the penetration of renewable technologies such as wind energy. The Vanadium Redox Battery is one such energy storage system showing considerable potential owing to its flexibility in power output and capacity, high efficiency and long operating life. This study models the use of the Vanadium Redox Battery as an integration technology in realistic large-scale remote wind/diesel power systems using the HOMER Micropower Optimization Model computer program developed by the US National Renewable Energy Laboratory. Results from this modelling demonstrate the significant financial and environmental benefits to be gained in installing energy storage in a wind farm. The storage system considered here was a Vanadium Redox Battery. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Performance Evaluation of a Lithium-Chloride Absorption Refrigeration and an Assessment of Its Suitability for Biomass Waste Heat
Appl. Sci. 2012, 2(4), 709-725; doi:10.3390/app2040709
Received: 1 August 2012 / Revised: 19 September 2012 / Accepted: 3 October 2012 / Published: 10 October 2012
Cited by 3 | PDF Full-text (1878 KB) | HTML Full-text | XML Full-text
Abstract
This paper presents a computer model that will evaluate the performance of a thermo-chemical accumulator. The model is based on operational data such as temperatures and flow rates. The ultimate goal for this model is to estimate the coefficient of performance (COP) [...] Read more.
This paper presents a computer model that will evaluate the performance of a thermo-chemical accumulator. The model is based on operational data such as temperatures and flow rates. The ultimate goal for this model is to estimate the coefficient of performance (COP) of this unit when run on hot water from biomass combustion as the heat source. The outputs of the model are verified by comparing the simulation of the actual machine with published experimental data. The computed results for cooling COP are within 10% of the measured data. The simulations are all run for heat load temperatures varying between 80 °C and 110 °C. As expected, simulation results showed an increase in COP with increased heat source temperatures. The results demonstrate that the potential of combined solar and biomass combustion as a heat source for absorption cooling/heating in climates with low solar radiation can be coupled with biomass waste. Full article
(This article belongs to the Special Issue Renewable Energy)
Open AccessArticle Continuous Process for Biodiesel Production in Packed Bed Reactor from Waste Frying Oil Using Potassium Hydroxide Supported on Jatropha curcas Fruit Shell as Solid Catalyst
Appl. Sci. 2012, 2(3), 641-653; doi:10.3390/app2030641
Received: 31 July 2012 / Revised: 24 August 2012 / Accepted: 24 August 2012 / Published: 29 August 2012
Cited by 8 | PDF Full-text (1481 KB) | HTML Full-text | XML Full-text
Abstract
The transesterification of waste frying oil (WFO) with methanol in the presence of potassium hydroxide catalyst supported on Jatropha curcas fruit shell activated carbon (KOH/JS) was studied. The catalyst systems were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and the [...] Read more.
The transesterification of waste frying oil (WFO) with methanol in the presence of potassium hydroxide catalyst supported on Jatropha curcas fruit shell activated carbon (KOH/JS) was studied. The catalyst systems were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and the Brunauer–Emmett–Teller (BET) method. The effects of reaction variables such as residence time, reaction temperature, methanol/oil molar ratio and catalyst bed height in packed bed reactor (PBR) on the yield of biodiesel were investigated. SEM images showed that KOH was well distributed on the catalyst support. The optimum conditions for achieving the conversion yield of 86.7% consisted of a residence time of 2 h, reaction temperature of 60 °C, methanol/oil molar ratio of 16 and catalyst bed height of 250 mm. KOH/JS could be used repeatedly five times without any activation treatment, and no significant activity loss was observed. The results confirmed that KOH/JS catalyst had a great potential to be used for industrial application in the transesterification of WFO. The fuel properties of biodiesel were also determined. Full article
(This article belongs to the Special Issue Renewable Energy)
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Review

Jump to: Research

Open AccessReview A Review of the Role of Amphiphiles in Biomass to Ethanol Conversion
Appl. Sci. 2013, 3(2), 396-419; doi:10.3390/app3020396
Received: 31 December 2012 / Revised: 18 February 2013 / Accepted: 25 February 2013 / Published: 2 April 2013
Cited by 6 | PDF Full-text (528 KB) | HTML Full-text | XML Full-text
Abstract
One of the concerns for economical production of ethanol from biomass is the large volume and high cost of the cellulolytic enzymes used to convert biomass into fermentable sugars. The presence of acetyl groups in hemicellulose and lignin in plant cell walls [...] Read more.
One of the concerns for economical production of ethanol from biomass is the large volume and high cost of the cellulolytic enzymes used to convert biomass into fermentable sugars. The presence of acetyl groups in hemicellulose and lignin in plant cell walls reduces accessibility of biomass to the enzymes and makes conversion a slow process. In addition to low enzyme accessibility, a rapid deactivation of cellulases during biomass hydrolysis can be another factor contributing to the low sugar recovery. As of now, the economical reduction in lignin content of the biomass is considered a bottleneck, and raises issues for several reasons. The presence of lignin in biomass reduces the swelling of cellulose fibrils and accessibility of enzyme to carbohydrate polymers. It also causes an irreversible adsorption of the cellulolytic enzymes that prevents effective enzyme activity and recycling. Amphiphiles, such as surfactants and proteins have been found to improve enzyme activity by several mechanisms of action that are not yet fully understood. Reduction in irreversible adsorption of enzyme to non-specific sites, reduction in viscosity of liquid and surface tension and consequently reduced contact of enzyme with air-liquid interface, and modifications in biomass chemical structure are some of the benefits derived from surface active molecules. Application of some of these amphiphiles could potentially reduce the capital and operating costs of bioethanol production by reducing fermentation time and the amount of enzyme used for saccharification of biomass. In this review article, the benefit of applying amphiphiles at various stages of ethanol production (i.e., pretreatment, hydrolysis and hydrolysis-fermentation) is reviewed and the proposed mechanisms of actions are described. Full article
(This article belongs to the Special Issue Renewable Energy)

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