A parametric analysis of future ethanol use in the light-duty transportation sector: Can the US meet its Renewable Fuel Standard goals without an enforcement mechanism?

The modified Renewable Fuel Standard (RFS2) prescribes a volume of biofuels to be used in the United States transportation sector each year through 2022. As the dominant component of the transportation sector, we consider the feasibility of the light-duty vehicle (LDV) parc to provide enough demand for biofuels to satisfy RFS2. Sensitivity studies show that the fuel price differential between gasoline and ethanol blendstocks, such as E85, is the principal factor in LDV biofuel consumption. The numbers of flex fuel vehicles and biofuel refueling stations will grow given a favorable price differential. However, unless the feedstock price differential becomes extreme (biomass prices below $100 per dry ton and oil prices above $215 per barrel), which deviates from historical price trends, LDV parc biofuel consumption will fall short of the RFS2 mandate without an enforcement mechanism. Additionally, such commodity prices might increase biofuel consumption in the short-term, but discourage use of biofuels in the long-term as other technologies that do not rely on any gasoline blendstock may be preferable. Finally, the RFS2 program goals of reducing fossil fuel consumption and transportation greenhouse gas emissions could be achieved through other pathways, such as notable improvements in conventional vehicle efficiency.     

Assessing sustainable forest biomass potential and bioenergy implications for the northern Lake States region,USA

Forestlands in the United States have tremendous potential for providing feedstocks necessary to meet emerging renewable energy standards. The Lake States region is one area recognized for its high potential of supplying forest-derived biomass; however, the long-term availability of roundwood harvests and associated residues from this region has not been fully explored. Better distribution and temporal availability estimates are needed to formulate emerging state policies regarding renewable energy development. We used a novel predictive methodology to quantify sustainable biomass availability and likely harvest levels over a 100-year period in the Lake States region. USDA Forest Inventory and Analysis estimates of timberland were combined with published growth and yield models, and historic harvest data using the Forest Age Class Change Simulator (FACCS) to generate availability estimates. Monte-Carlo simulation was used to develop probability distributions of biomass harvests and to incorporate the uncertainty of future harvest levels. Our results indicate that 11.27–15.71 Mt y⁻¹ dry roundwood could be sustainably harvested from the Lake States. Assuming 65% collection rate, 1.87–2.62 Mt y⁻¹ residue could be removed, which if substituted for coal would generate 2.12–2.99 GW h of electricity on equivalent energy basis while reducing GHG (CO₂e) emission by 1.91–2.69 Mt annually. In addition to promoting energy security and reducing GHG emissions, forest residues for energy may create additional revenues and employment opportunities in a region historically dependent on forest-based industries.     

Modelling and optimization of the smart hybrid renewable energy for communities (SHREC)

The future energy system in community level should be more ‘smart’ to secure reliability, enhance market service, minimize environmental impact, reduce costs and improve the use of renewable energy source (RES). Therefore, this paper proposes an energy integration system – smart hybrid renewable energy for communities (SHREC). It considers both thermal (heating and cooling) and electricity market in a large community level and highlight the interactions between them through utilizing RES, combined heat and power (CHP) and energy storages. A planning model based on CHP modelling is developed for the SHREC system. A linear programming (LP) algorithm is developed to optimize the SHREC system in a weekly period and the results are compared with an existing energy optimization software. We also demonstrate the model in a sample SHREC system during three typical weeks with cold, warm and mid-season weather in the year 2011. The results indicate that the developed modelling and optimization method is more efficient and flexible for the smart hybrid renewable energy systems.     

Evaluation of ground energy storage assisted electric vehicle DC fast charger for demand charge reduction and providing demand response

In 2012 there was approximately 2400 electric vehicle DC Fast Charging stations sold globally. According to Pike Research (Jerram and Gartner, 2012), it is anticipated that by 2020 there will be approximately 460,000 of them installed worldwide. A typical public DC fast charger delivers a maximum power output of 50 kW which allows a typical passenger vehicle to be 80% charged in 10–15 min, compared with 6–8 h for a 6.6 kW AC level 2 charging unit. While DC fast chargers offer users the convenience of being able to rapidly charge their vehicle, the unit's high power demand has the potential to put sudden strain on the electricity network, and incur significant demand charges.Depending on the utility rate structure, a DC fast charger can experience annual demand charges of several thousand dollars. Therefore in these cases there is an opportunity to mitigate or even avoid the demand charges incurred by coupling the unit with an appropriately sized energy storage system and coordinating the way in which it integrates. This paper explores the technical and economical suitability of coupling a ground energy storage system with a DC fast charge unit for mitigation or avoidance of demand charges and lessening the impact on the local electricity network. This paper also discusses the concept of having the system participate in demand response programs in order to provide grid support and to further improve the economic suitability of an energy storage system.     

The 2013 reforms of the Flemish renewable electricity support: Missed opportunities

The Flemish renewable electricity support system has struggled to address a number of problematic issues in the past. These included excessive profit margins and general malfunctioning of the green certificate market, as well as a lack of qualification of various existing renewable energy technologies. The Flemish government responded to these issues by introducing major reforms in 2013, including “banding” to differentiate the support for various technologies. However, reliable methods for differentiating renewable electricity technologies and calculating support levels have not been sufficiently developed. The main objective of the 2013 reforms was to reduce support costs, but application of German feed-in tariffs on 18 reference technologies has shown that most projects in Flanders continue to receive high levels of support. The 2013 reforms did not succeed in addressing malfunctioning of the green certificate market. On the contrary, the confidence of investors in renewable electricity plants has decreased as the terms of support can be altered retroactively by adjusting remuneration levels and through political interventions. Future adaptations are likely to be made which will further decrease the overall stability and effectiveness of the system.     

Off-grid electricity generation with renewable energy technologies in India: An application of HOMER

Renewable energy-based off-grid or decentralised electricity supply has traditionally considered a single technology-based limited level of supply to meet the basic needs, without considering reliable energy provision to rural consumers. The purpose of this paper is to propose the best hybrid technology combination for electricity generation from a mix of renewable energy resources to satisfy the electrical needs in a reliable manner of an off-grid remote village, Palari in the state of Chhattisgarh, India. Four renewable resources, namely, small-scale hydropower, solar photovoltaic systems, wind turbines and bio-diesel generators are considered. The paper estimates the residential, institutional, commercial, agricultural and small-scale industrial demand in the pre-HOMER analysis. Using HOMER, the paper identifies the optimal off-grid option and compares this with conventional grid extension. The solution obtained shows that a hybrid combination of renewable energy generators at an off-grid location can be a cost-effective alternative to grid extension and it is sustainable, techno-economically viable and environmentally sound. The paper also presents a post-HOMER analysis and discusses issues that are likely to affect/influence the realisation of the optimal solution.     

The improved CO2 capture system with heat recovery based on absorption heat transformer and flash evaporator

Absorption heat transformer (AHT) and flash evaporator (FE) are used to reduce the heat consumption of CO₂ capture processes and an AHT–FE-aided capture system is proposed. Analyses are carried out to verify the effectiveness in reducing heat consumption. Compared with the base CO₂ capture system of 3000 t/d CO₂ capture capacity from a 660 MW coal-fired power unit, the AHT–FE-aided capture system reduces the heat consumption from 3.873 GJ/tCO₂ to 3.772 GJ/tCO₂, and the corresponding energy saving is 2.62%. The economic analysis shows that the annual profit would be 2.94 million RMB Yuan. The payback period of the AHT–FE-aided capture system is approximately 2.4 years. Therefore, the AHT–FE-aided capture system is both economically and technically feasible for improving the CO₂ capture energy performance.     

Cellulosic ethanol production: Landscape scale net carbon strongly affected by forest decision making

In producing cellulosic ethanol as a renewable biofuel from forest biomass, a tradeoff exists between the displacement of fossil fuel carbon (C) emissions by biofuels and the high rates of C storage in aggrading forest stands. To assess this tradeoff, the landscape area affected by feedstock harvest must be considered, which depends on numerous factors including forest productivity, the amount of forest in a fragmented landscape, and the willingness of forest landowners to sell timber as a bioenergy feedstock. We studied landscape scale net C balance by combining these considerations in a new, basic simulation model, CEBRAM, and applying it to a hypothetical landscape of short-rotation aspen forests in northern Michigan, USA. The model was parameterized for forest species, growth and ecosystem C storage, as well as landscape spatial patterns of forest cover in this region. To understand and parameterize forest owner decision making we surveyed 505 nonindustrial private forest (NIPF) owners in Michigan. Survey results indicated that 47% of these NIPF owners would willingly harvest forest biomass for bioenergy. Model results showed that at this rate the net C balance was 0.024 kg/m² for a cellulosic ethanol system without considering land use over a 40 year time horizon. When C storage in aggrading, nonparticipating NIPF land was included, net C balance was 1.09 kg/m² over 40 years. In this region, greater overall C gains can be realized through aspen forest aggradation than through the displacement of gasoline by cellulosic ethanol produced from forest biomass.     

Considering uncertainty in the optimal energy management of renewable micro-grids including storage devices

This paper proposes a new probabilistic framework based on 2m Point Estimate Method (2m PEM) to consider the uncertainties in the optimal energy management of the Micro Girds (MGs) including different renewable power sources like Photovoltaics (PVs), Wind Turbine (WT), Micro Turbine (MT), Fuel Cell (FC) as well as storage devices. The proposed probabilistic framework requires 2m runs of the deterministic framework to consider the uncertainty of m uncertain variables in the terms of the first three moments of the relevant probability density functions. Therefore, the uncertainty regarding the load demand forecasting error, grid bid changes and WT and PV output power variations are considered concurrently. Investigating the MG problem with uncertainty in a 24 h time interval with several equality and inequality constraints requires a powerful optimization technique which could escape from the local optima as well as premature convergence. Consequently, a novel self adaptive optimization algorithm based on θ-Particle Swarm Optimization (θ-PSO) algorithm is proposed to explore the total search space globally. The θ-PSO algorithm uses the phase angle vectors to update the velocity/position of particles such that faster and more stable convergence is achieved. In addition, the proposed self adaptive modification method consists of three sub-modification methods which will let the particles choosel the modification method which best fits their current situation. The feasibility and satisfying performance of the proposed method is tested on a typical grid-connected MG as the case study.     

Breakeven price of biomass from switchgrass,big bluestem,and Indiangrass in a dual-purpose production system in Tennessee

The objective was to determine the breakeven price for switchgrass (SG) (Panicum virgatum L.), a mix of big bluestem (Andropogon gerardii Vitman) and Indiangrass (BBIG) (Sorghastrum nutans L. Nash), and a combination of SG and BBIG (SG/BBIG) produced under three harvest treatments. Two-harvest treatments included a forage harvest at early boot (EB) and at early seedhead (ESH) plus a biomass harvest at fall dormancy (FD). The third harvest treatment was a single biomass harvest at FD. Mixed models were used to determine if there were differences in yield, crude protein, and nutrient removal for each of the native warm-season grass (NWSG) treatments at each harvest. The EB plus FD harvest system would be preferred over the ESH plus FD harvest system for all NWSG treatments. BBIG was the only NWSG treatment with a breakeven price for biomass that decreased with an EB harvest. For all three NWSG treatments, a producer would be better off harvesting once a year for biomass than twice for forage and biomass. The cost of harvesting and replacing the nutrients for the forage harvest was greater than the revenue received from selling the forage.     

Optimizing feedstock selection for biofuel production in Hawaii: CuO oxidative lignin products in C4 grasses

Global interest in renewable fuels is rapidly growing with particular emphasis on local energy growth and creating new energy feedstocks, specifically liquid fuel sources. However, the interactive effect of plant species/variety and growth environment on plant structural components, which may influence conversion efficiency and thus play an important role in optimizing the production of biofuels, is not fully understood. In this study cupric oxide (CuO) extractable lignin, which extracts and quantifies lignin-derived monomers, was determined for 25 cultivated and naturalized tropical perennial C₄ grass varieties of napiergrass and Guinea grass that were under assessment for suitability as feedstocks for liquid fuel generation. Principal component analysis of CuO extractable lignin-derived monomers showed differences in composition between napiergrass and Guinea grass, as well as environmental differences within many, but not all napiergrass varieties. Among the samples tested, the greatest differences in lignin composition occurred specifically in vanillyl lignin structures. A wide range (1–2.5) in the ratio of cinnamyl to vanillyl structures (C/V), which often relates to enzymatic degradability in natural systems, was also found. It is expected that the observed CuO lignin differences representing the structure and bonding of lignin will relate to ease of chemical and enzymatic conversion and the effectiveness of pretreatment and conversion in biofuel application. We hypothesize that the C/V ratio of feedstock will positively relate to conversion efficiency and if supported, then compositional lignin metrics such as the C/V ratio could be a predictor to select for more easily degradable biomass for biofuel production.     

Thermodynamic and exergoeconomic analysis of energy recovery system of biogas from a wastewater treatment plant and use in a Stirling engine

The aim of this research it is to show how the biogas biomethanisation from primary and secondary treatment of activated sludge from a wastewater treatment plant (WWTP), can be an alternative renewable energy option from fossil fuels, which offers competitive advantages and points out new horizons for the use of this fuel. This will allow to achieve some important priorities of energy plans in EU countries: to reduce the organic matter deposited in landfills and CO₂ emissions and to find viable solutions to minimize the environmental impact of sewage sludge (SS).This study analyses the biogas combustion and energy recovery processes from a thermodynamic, thermoeconomic and exergetic point of view.The results show that the boiler of the process is the main source of irreversibility and exergy destruction. Moreover, the energy and exergy economic value of exhaust gases from the combustion chamber, are significant and worthwhile to be exploited. For this reason, the present study explores the applicability and suitability of integrating a Stirling engine in such process. The study reveals that it is possible to create a small micro-cogeneration system which leads to sustainable waste management and energy savings in the treatment plant itself.     

Environmental assessment of electricity generation from an Italian anaerobic digestion plant

A standard ISO Life Cycle Assessment study was carried out to evaluate the environmental sustainability of electricity production from an anaerobic digestion (AD) plant using a mixture of dedicated energy crops, agricultural residues and livestock effluents as input materials. The functional unit was 1 MJ of electricity. System boundaries were from cradle to grave and covered all the phases from energy crops cultivation to the production of biogas and its use in a Combined Heat and Power plant to produce electricity. Liquid and solid digestate storage and spreading on agricultural land were included. Primary data were collected from the AD plant for all the above phases. Since heat produced is used only internally, no allocation was applied in the study. As regards digestate management, CH₄ emissions were calculated from literature, whereas four literature methods were applied for calculation of nitrogen emissions with the goal to perform a sensitivity analysis on LCA results. ILCD Handbook impact assessment methodologies were used. Results show that the main hotspots are energy crops cultivation and the management of digestate, mainly because of both nitrogen and methane emissions, affecting Global Warming, Acidification, Marine and Freshwater Eutrophication. Finally, a detailed Monte Carlo analysis, was carried out to evaluate the results uncertainty. The study represents the state of the art about the environmental performance of the AD plant with the use of sensitivity and uncertainty analysis, which both improve the reliability of results, and allows drawing general conclusions on how to mitigate the environmental impacts of AD process.     

Second-generation bioethanol of hydrothermally pretreated stover biomass from maize genotypes

Twelve maize genotypes, were agronomically evaluated and their stover hydrothermally pretreated in a temperature range of 210–225 °C to assess the effects of genotype and pretreatment severity on stover recalcitrance toward bioethanol conversion. Maize genotypes exhibited significant variation for biomass yield and all agronomic evaluated, while among all cell wall constituents measured in the unpretreated stover, only ash content showed differences among genotypes. The pretreatment severities assayed impacted most stover compositional traits, and the glucose recovered after enzymatic hydrolysis displayed a similar profile among genotypes with similar genetic background. Harsher pretreatment conditions maximized the potential cellulosic bioethanol production (208–239 L/t), while the mildest maximized the bioethanol from the hemicellulosic hydrolysates (137–175 L/t). Consequently, when both pentose and hexose sugars were considered, the total potential bioethanol produced at the lowest and highest pretreatment temperatures was similar in all genotypes (292–358 L/t), indicating that the lowest temperature (210 °C) was the optimal among all assayed. Importantly, the ranking of genotypes for bioethanol yield (L/ha) closely resembled the ranking for stover yield (t/ha), indicating that breeding for biomass yield would increase the bioethanol production per hectare regardless of the manufacturing process. Similarly, the genetic regulation of corn stover moisture is possible and relevant for efficient energy production as biomass moisture has a potential impact on stover transportation, storage and processing requirements. Overall, these results indicate that local landrace populations are important genetic resources to improve cultivated crops, and that simultaneous breeding for production of grain and stover bioethanol is possible in corn.     

Anaerobic digestion of maize and cellulose under thermophilic and mesophilic conditions – A comparative study

The aim of this study was to advance in understanding of digestion process of energy crops. Cellulose and maize silage were fermented in batch mode at mesophilic (38 °C) and thermophilic (55 °C) conditions and corresponding organic loads of 5.5 ± 0.2 kgVS/m³, 11.2 ± 0.3 kgVS/m³ and 16.7 ± 0.4 kgVS/m³.For both substrates more stable and faster digestion took place at 38 °C. Due to complex structure maize degradation was characterized by varying digestion rate and longer total digestion time resulting form breakdown of hard-degradable fractions. The digestion retard at increased OLRs of cellulose and lower degradation level obtained for all cellulose series confirm a higher overloading potential for systems dealing with single-component-substrates but also the enhanced sensitivity of such systems to any inconvenient digestion conditions.Based on observed patterns of volatile fatty acids and oxidation-reduction potential, different fermentation mechanisms can be concluded for cellulose and maize, but also for different temperature modes. Conversion of maize at highly reductive conditions with increased concentrations of butyric acid was accompanied by much higher activity of hydrogenotrophic methanogens than for cellulose digestion.Two factors showed a strong potential to influence test results: an insufficient VS content of inoculum, which caused reduced biogas yields, and a high natural biodiversity of maize silage, resulting in higher biogas yields than calculated based on the maize composition.     

Experimental study of the functionality of a semisubmersible wind turbine combined with flap-type Wave Energy Converters

In the present paper the functionality of the Semisubmersible wind energy and Flap-type wave energy Converter (SFC) is examined experimentally. In order to study the functionality of the SFC, the focus is on operational environmental conditions. SFC is a combined concept that utilizes offshore wind energy and ocean wave energy for power production. Details are presented as far as the physical modelling of the wind turbine with the use of a redesigned small-scale rotor and of the Power Take-Off mechanism of the Wave Energy Converters (WECs) with the use of a configuration that is based on a mechanical rotary damper. Tests with quasi-static excitation, motion decay, regular and irregular waves without and with wind that is uniform are conducted on an 1:50 scale physical model. The experimental data are compared with numerical predictions obtained by a fully coupled numerical model using Simo/Riflex tool. A good agreement is observed between experimental and numerical predictions. The combined operation of WECs doesn't affect the tension of mooring lines nor the acceleration of nacelle and the bending moment in tower's base. The produced power of the WECs of the SFC and consequently the functionality of the SFC is estimated.     

Growth,yield, fiber content and lodging resistance in eight varieties of Cenchrus purpureus (Schumach.) Morrone intended as energy crop

Growth, biomass yield, fiber content and lodging resistance were studied, during a six month growth period, for eight varieties of Cenchrus purpureus, intended as energy crop, in Veracruz, Mexico. Then, only yield at day 182 was assessed for two additional years. The varieties were: CT115 (CT), African Cane (AC), Taiwan (TAI), King Grass (KG), Vruckwona (VRU), Roxo (RX), OM22 (OM) and Cameroon (CAM). Local weather is warm and sub-humid, historical data for monthly average temperature and annual rainfall were 25.8 °C and 1142 mm, respectively. Height, diameter and light interception were measured monthly from day 65–185. At day 185, biomass yield and tiller density were measured. Number of lying tillers was counted to estimate lodging resistance. Cellulose and hemicellulose content were estimated in leaf and stem. No differences were found for dry matter yield or stem yield at day 185 in the first year. Regarding the next two years, TAI yielded above CT, OM or ROX. Average dry matter yield was higher in the second year than in the establishment cycle (38.6 vs 21.1 Mg ha⁻¹), but decreased in the third year (32.2 Mg ha⁻¹). In both stem and whole plant, AC and KG showed higher hemicellulose content than RX, OM or CT; while AC and VRU had higher cellulose than RX in stem, or than CT in the whole plant. Furthermore, varieties AC, KG, VRU and TAI were resistant to lodging and had a higher fiber content, so they are recommended as energetic crops.     

A new future for carbohydrate fuel cells

The development of renewable energy sources to reduce our dependence on limiting fossil fuel reserves continues to be a critical research initiative. Utilizing the abundant high energy content of carbohydrates contained in biomass (cellulose and hemicellulose) must be considered to be an important contribution to our overall energy budget. Carbohydrate-derived furan-based liquid fuels and especially ethanol are becoming important added components forming gasoline blends to lower overall fossil fuel use. Alternate renewable energy processes that more efficiently use the carbohydrate energy content are desirable and would lower the overall carbohydrate input requirement for energy production. Recently, new catalysts have shown the feasibility of efficiently transporting the 24 electrons in glucose to fuel cell electrodes making possible the direct conversion of the stored energy in carbohydrates into electricity with the benign formation of carbonate and water as products. The conversion of glycerol, a byproduct of biodiesel production, into three-carbon carbohydrates provides another opportunity to produce electricity from an abundant carbohydrate source. New developments in catalyst systems promise to make carbohydrate fuel cells an important part of future energy strategies.     

Can slurry biogas systems be cost effective without subsidy in Mexico?

Biogas from pig slurry in Mexico has potential to produce 21 PJ per year, equivalent to 3.5% of natural gas consumption in 2013. In this paper, three different scenarios are analysed: mono-digestion of pig slurry in a finisher farm (scenario 1); co-digestion of pig slurry and elephant grass in a finisher farm in situ (scenario 2) and co-digestion of pig slurry and elephant grass in centralised biogas plants (scenario 3). The digesters proposed are anaerobic high density polyurethane (HDPE) covered lagoons. HDPE centralised plants can have capital costs 5 times cheaper than European biogas plants. The economics of utilisation of biogas for electricity generation and as biomethane (a natural gas substitute) were investigated. Economic evaluations for on-site slurry digestion (Scenario 1) and on-site co-digestion of elephant grass and pig slurry (Scenario 2) showed potential for profitability with tariffs less than $US 0.12/kWh_e. For centralised systems (Scenario 3) tariffs of $US 0.161/kWh_e to $US 0.195/kWh_e are required. Slurry transportation, energy use and harvest and ensiling account for 65% of the operational costs in centralised plants (Scenario 3). Biomethane production could compete with natural gas if a subsidy of 4.5 c/L diesel (1 m³ of biomethane) equivalent was available.     

Changes in feedstock quality in willow chip piles created in winter from a commercial scale harvest

Storage and handling are important facets of biomass logistics because there are associated costs and biomass properties can change significantly as material proceeds through the supply chain. Thus, this aspect of biomass supply systems requires continued study. Shrub willow chips were harvested, and used to create six piles that each contained between 10 and 22 Mg (fresh biomass). Material was monitored for several months in temporary storage to assess changes in biomass quality (moisture, ash and energy content). Internal pile temperatures increased due to biological activity and conditions within a pile quickly differentiated based on location (shell, core, top, and side). Mean moisture content increased from 42 to 47% (mass fraction) between harvest and delivery of the chips, but ranged between 37 and over 60% over the next three months depending on pile location with the shell generally drier than the core. Mean ash content increased 1 to 2% points (mass fraction) between harvesting, reloading and delivery to the trial location, but became more variable during storage. Higher heating values (HHV) were stable between 18.6 and 19.0 MJ kg⁻¹ over the six months, but lower heating values (LHV) ranged between 8.6 and 11.7 MJ kg⁻¹ and mirrored changes in moisture content. There was minimal change in chip quality over two months, but quality became more variable over longer time periods. This period could be extended, and negative effects on chip quality could be mitigated, by improving storage methods, blending different types of chips, or employing pretreatments.