Energy Return On Investment Research Articles

Sustainable Valorization of Wood Residue for the Production of Biofuel Materials Via Continuous Flow Hydrothermal Liquefaction

When forests are extensively cleared for infrastructure and agricultural purposes, over half of the wood generated is regarded as waste. To minimize the negative consequences and successfully use renewable energy sources, further applications must be investigated. Biomass has the potential to be converted into biofuels using a process known as hydrothermal liquefaction (HTL). The conversion of wood residue under continuous flow, subcritical HTL, under both untreated and alkaline pretreated conditions was examined in this study. According to the results, pretreatment with 4% NaOH sped up recovery times compared to using raw wood and increased glucose production by 1.8 times (equivalent to 90 g/L) in comparison to using raw wood. Additionally, the fluid's pH was raised from 6.1 to 3.5 due to the alkaline pretreatment, which also switched the hydrolysis of arabinose and cellobiose to rhamnose at 0.2 MPa. The average Net Energy Ratio (NER) for glucose during liquefaction reached values as high as 63%, while the energy output from glucose during the pretreatment reached 246 kJ. The move to green energy transition toward net zero will be aided by these findings, which will usher in new waste conversion techniques to produce sustainable biofuels using continuous flow HTL.Graphical Abstract

Life cycle assessment of deep-eutectic-solvent-assisted hydrothermal disintegration of microalgae for biodiesel and biogas co-production

A life-cycle assessment was carried out to evaluate the industrial feasibility of deep eutectic solvents (DES) for microalgae pretreatment. The effects of DES on hydrothermal disintegration (HTD) of microalgae for biodiesel and biogas co-production was assessed in terms of net energy ratio (NER, Energy input/Energy output) and greenhouse gas (GHG) emissions. Although the addition of DES during HTD increased the energy output by 36.33 %, the NER increased from 0.65 to 0.83 due to the larger increase in energy input. The increase in energy input is because DES synthesis consumed a lot of energy, accounting for 45.93 % of the total energy input. The addition of DES also increased the net GHG emissions from −25.53 g CO2-eq MJ−1 to 26.04 g CO2-eq MJ−1. It is essential to reduce the GHG emissions and energy consumption via DES recovery for commercial applications. When DES was recovered by a combined method of membrane filtration and recrystallization, a low NER of 0.58 along with negative net GHG emissions were achieved for microalgal biofuel production via DES-HTD.

Hunting behavior of a solitary sailfish Istiophorus platypterus and estimated energy gain after prey capture

Foraging behavior and interaction with prey is an integral component of the ecological niche of predators but is inherently difficult to observe for highly mobile animals in the marine environment. Billfishes have been described as energy speculators, expending a large amount of energy foraging, expecting to offset high costs with periodic high energetic gain. Surface-based group feeding of sailfish, Istiophorus platypterus, is commonly observed, yet sailfish are believed to be largely solitary roaming predators with high metabolic requirements, suggesting that individual foraging also represents a major component of predator–prey interactions. Here, we use biologging data and video to examine daily activity levels and foraging behavior, estimate metabolic costs, and document a solitary predation event for a 40 kg sailfish. We estimate a median active metabolic rate of 218.9 ± 70.5 mgO2 kg−1 h−1 which increased to 518.8 ± 586.3 mgO2 kg−1 h−1 during prey pursuit. Assuming a successful predation, we estimate a daily net energy gain of 2.4 MJ (5.1 MJ acquired, 2.7 MJ expended), supporting the energy speculator model. While group hunting may be a common activity used by sailfish to acquire energy, our calculations indicate that opportunistic individual foraging events offer a net energy return that contributes to the fitness of these highly mobile predators.

Investigating the techno-economic and environmental performance of chemical looping technology for hydrogen production

There is limited research assessing the techno-economic feasibility and environmental footprint of direct partial oxidation of methane using chemical looping technology for hydrogen production. The available studies have not investigated essential aspects of the impact of hydrogen storage cost or economies of scale on hydrogen production cost. In this study, process models were developed based on fundamental science for hydrogen production using chemical looping technology. The results show that for a production plant capacity of 607 tonnes/day, the hydrogen cost is $3.66/kg. When the uncertainty in the input parameters is considered, the hydrogen cost ranges from 3.51 to 4.70 $/kg. The parameters that most influence hydrogen cost are discount rate and hydrogen storage. The hydrogen cost versus capacity profile shows that operating chemical looping at a higher capacity is financially beneficial because of economies of scale. The developed scale factor is 0.64. The net energy ratio is 0.84, indicating chemical looping is a better energy producer than the traditional method of hydrogen production, i.e., steam methane reforming. The greenhouse gas emissions are 2.86 kgCO2/kg-H2. The results show that chemical looping partial oxidation of methane provides better energy efficiency and a better environmental footprint than conventional steam methane reforming.

A combined assessment of the energy, economic and environmental performance of a photovoltaic system in the Italian context

Policymakers are increasingly moving towards greater investments in research in the renewable energy sector, in order to reduce costs, making private investment affordable, so as to accelerate the achievement of grid parity. This evidence boosts for investigating the way the convenience of investing in a solar photovoltaic (PV) system, in Italy, is unrelated to any form of public incentive. Under this perspective, this paper is focused upon designing a residential 3 kW PV system and providing a full set of indicators for the assessment of its multi-dimension performance in an holistic, integrated approach. Particularly, energy and environmental indicators, likewise the Energy Payback Time (EPBT), Energy Return on Investment (EROI) and Environmental Impact Mitigation potential (EIMP) allowed the authors to measure some of the relevant sustainability-related issues of a residential PV system. Those were found to be equal to 1.35 years, 7.05 and 23,215 kg CO2 eq, respectively. Whereas, the authors used the Levelized Cost of Energy (0.15064 €/kWh), the Net Present Value (€ 2881), and the Payback Period (8.26 years), to evaluate the economic and financial feasibility of the PV system modelized. The variations of EPBT and EROIEL with respect to solar radiation and the efficiency of the PV system and LCOE to discount rate and initial investment cost have been investigated through a sensitivity analysis.

Offshore floating photovoltaic system energy returns assessment—A life cycle energy analysis-based perspective

<abstract> <p>Researchers have long regarded photovoltaics (PV) as a poor energy return (ER) compared to fossil fuels. Although the latter's energy-return-on-investment (EROI), like oil, coal, and gas, are above 25:1 at the primary, they are about 6:1 at the final stage. Following the technology creation, it is essential to investigate whether the solar module technology innovation affects the ER. Much literature delivers the ERs of fossil fuels and PV. However, it does not address the life cycle analysis or life cycle energy analysis (LCEA) assessments. This paper, employing time-series and LCEA analyses, performs an ER evaluation of the 181-MWp global most extensive offshore floating PV (OFPV) in a 30-year life cycle at Changhua Coastal Industrial Park, Taiwan. The results show that the energy payback time (EPBT) is about one year. The EROI is about 29.8, which is superior or complies with the upper limits of previous studies under the same insolation. The approach proposed in this study should help future PV stations' ER analysis and clarify whether the innovation benefits from improving the system's performance. The results also assist in investors' decision-making regarding deploying PV projects in the future.</p> </abstract>

Technical, Environmental, and Process Safety Assessment of Acetone-Butanol-Ethanol Fermentation of Cassava Residues

The North-Colombian region has enormous potential for producing bioproducts and bioenergy from agricultural residues. Yet, scaling bioproducts and bioenergy to industrial practice requires further investigation, especially for environmental impact minimization and improved process safety. This work assesses two alternatives for valorizing cassava residues via acetone, butanol, and ethanol (ABE) fermentation. Two ABE fermentation routes are assessed. In Route 1, pretreatment and purification involve dilute-acid pretreatment and multi-effect distillation and decantation operations, while Route 2 includes steam explosion and reactive distillation. Hazard Identification and Risk Assessment (HIRA) and Waste Reduction Algorithm (WAR) were applied to assess ABE fermentation. Simulation results indicate butanol yields of 0.10–0.12 kg/kg feedstock and net energy ratio (NER) <1. Route 2 shows the highest total output of Potential Environmental Impacts (PEI) with 5.56 PEI/kg butanol. Both ABE fermentation routes obtained Fire and Explosion Damage Index (FEDI) values above 300 for acetone and ethanol recovery/purification stages. Both routes are classified as “hazardous” considering the flammability of handled substances, and their relative safety performance is remarkably similar. These results pave the way toward deploying both routes for adding value to the cassava residues in North Colombia by applying safe, efficient, and environmentally friendly transformation technologies.

Sustainability assessment of palm oil-based refinery systems for food, fuel, and chemicals

Palm-based biorefinery system has gained attention worldwide because of potentially high economic returns. However, environmental impacts also increase with the additional production. Therefore, this study aims to assess the sustainability of (1) current palm-based biorefinery system in Thailand, including cooking oil and biodiesel, and (2) palm-based biorefinery system with value-added products, i.e., succinic acid, lactic acid, bio-hydrogenated diesel (BHD), and epichlorohydrin (ECH) that represent biomaterial, biofuel, and biochemical products, respectively. Accordingly, seven palm-based biorefinery scenarios were designed, and their sustainability was assessed through life cycle assessment (LCA), net energy balance (NEB) and net energy ratio (NER), employment generation, and eco-efficiency. The results revealed that value-added production increased global warming impacts by around 3 – 79% compared with the current system. Although environmental impacts increased due to the additional processes related to the production of the value-added products, total product values also increased, especially for succinic acid, generally leading to higher eco-efficiency values. The current palm-based biorefinery system with succinic acid production had the highest eco-efficiency among all the scenarios considered. The BHD production scenario had the highest NEB and NER because the products were used for energy. Employment generation increased for all the scenarios between 2 – 86% compared with the current system.

Global warming impact and energy analysis of tofu product: A case study in ABC factory Merauke

Tofu is one of the staple foods that are in demand by the wider community due to its high nutritional content, especially protein, and has quite a lot of processed variants. Therefore, the production of tofu must be developed to obtain the best quality, and adequate quantity of production and at the same time, become a product with an environmentally friendly production process. There are still many conventional tofu factories found in Merauke. However, its production process is still a little concerned about environmental aspects. This study aims to identify inputs, outputs, and environmental impacts of the tofu product life cycle, determine potential sources of impact (hotspots) and recommend improvement scenarios. The Life Cycle Assessment (LCA) method is used to estimate and analyze the potential environmental impacts that occur due to the production of 1 kilogram of tofu products at ABC Factory in Merauke. The environmental impact produced by this tofu product is the potential for global warming, so the amount of greenhouse gas emissions that occur will be analyzed. In addition, energy efficiency will be calculated, and conclusions and interpretations will be drawn from the previously analyzed inventory results. The limitation of the system used in this study starts from the soybean cultivation process, transportation of raw materials, and the process of making tofu at ABC Factory. From the results obtained that tofu with local soybean generates GHG emissions is 0.337 kg CO2-eq per kg of tofu, while tofu with imported soybean is 0.801 kg CO2-eq per kg of product. Energy efficiency is indicated by the Net Energy Value (NEV) and the Net Energy Ratio (NER) of the tofu products. The Tofu with local soybean has NEV is 3.529 MJ, while tofu with imported soybean is 0.961 MJ. The value of NER>1 for both tofu products. From the results of inventory analysis, several alternatives can be recommended to reduce the environmental impact that occurs.

Fuzzy set scoring methodology for multi-objective sustainability assessment of algal biofuel pathways

This work implements a multi-objective scoring methodology based on fuzzy set theory to evaluate and compare the sustainability metrics of microalgal biorefining concepts. Fuzzy membership functions are developed for three key metrics: minimum fuel selling price, global warming potential, and net energy ratio. Membership functions yield satisfaction scores ( λ ) between 0 and 1 for each objective; achieving λ = 1 indicates a sustainability metric has fully met its target, while λ = 0 is unacceptable for the standards of that metric. The individual λ are combined into an overall satisfaction score, λ o , also ranging from 0 to 1, which is used here to simplify the comparison of sustainability metrics. In addition to baseline biorefining conditions, uncertainties across the algal biofuel production system are assessed through a Monte Carlo analysis of λ o . A case study is developed to compare Combined Algal Processing (a biochemical conversion pathway) against hydrothermal liquefaction (a thermochemical conversion pathway). Baseline results show that lipids are always heavily favored for fuel production, but no practical cultivation operations would be able to achieve such lipid-heavy biomass. The Monte Carlo analysis reveals that current algal fuel conversion technologies cannot simultaneously satisfy all key sustainability objectives. If biomass productivity is doubled and the feedstock is high in lipids, hydrothermal liquefaction can achieve a mean λ o = 0.53 ± 0.12 , still well below a desirable λ o of 1. Combined Algal Processing struggles to achieve high satisfaction scores even in optimistic scenarios and requires a high-lipid feedstock to achieve a mean λ o = 0.10 ± 0.03 . Hydrothermal liquefaction can be integrated downstream of Combined Algal Processing to increase fuel yield and optimistically can achieve a mean λ o = 0.48 ± 0.07 . A combination of system improvements, including biomass productivity, feed composition, fuel yield, and implementation of co-product pathways must work together to satisfy all the relevant sustainability metrics for algal biofuels. The scoring methodology developed here can be readily applied to other microalgal biorefinery concepts and, more broadly, to sustainable fuel development and production. • Holistic evaluation of algal based biofuel systems. • Sustainability assessed through economic and environmental metrics. • Fuzzy set scoring system is applied to holistically assess algae fuel pathways. • Overall satisfaction of sustainability targets depends on algae composition. • Thermochemical pathway outscores biochemical pathway when biofuels are sole product.

Environmental and economic optimization and prioritization tool-kit for residential building renovation strategies with life cycle approach

The most recent regulations, as well as the scientific studies, remark the importance of the evaluation of the entire life cycle on building renovations, relative to the environmental impact and economic feasibility, making the Life Cycle Assessment (LCA) the prioritizing analysis. The objective of the study is to develop a simplified methodology for the environmental and economic assessment of residential building renovations with life cycle approach. For this, a script-based tool-kit is developed: the first tool optimizes the thickness of envelope insulation based strategies; the second tool is for the prioritization of strategies by assessing their environmental performance and economic feasibility. In order to follow the objective, the development of the two tools is presented: both tools follow a parallel scheme where the input parameters are required by an excel file and the calculation script provides the results automatically by exporting the results excel file. The evaluation provides the quantification of the relative environmental improvement with the net energy ratio (NER), and the economic feasibility by the financial indicator of internal rate of return (IRR). The tool-kit is applied in a case study of a multifamily residential building. The results show, on the one hand, that the usability of the tool-kit can be determinant in the decision-making of stakeholders; and in the other hand, the importance of carrying out a dynamic assessment, taking into account the variation of the results caused by the uncertain parameters that differ in time. Moreover, the tool-kit can assist the development of cost-effective decarbonisation strategies.

Sustainability assessment of ethanol and biodiesel production in Argentina, Brazil, Colombia, and Guatemala

This study assessed the production, land use, environmental impacts, and energy balance associated with ethanol and biodiesel production in Argentina, Brazil, Colombia, and Guatemala. These countries are considered developing economies and produced 97% of biofuel in Latin America, achieving production of 46 billion liters in 2019. The implementation of public policies (ie. RenovaBio, LCFS frameworks) to stimulate the adoption of low-carbon fuels may encourage biofuel production. Hence, the official data for each country were used to quantify the biofuel production and land required for their production. To evaluate the environmental impacts, a cradle-to-gate attributional life cycle assessment (ALCA) was performed. The results revealed that transforming 5% of current pastures into arable land for raw materials could double biofuel production. The findings also showed that increases in raw material productivity could significantly reduce land demand, suggesting that efforts in this direction should be intensified. When ethanol and biodiesel production were compared to gasoline and diesel production, considerable reductions in global warming (up to 84%) and ozone layer depletion were observed; most importantly, the positive energy ratio (ER, 2.5–9.3 MJbioenergy/MJfossil) and net energy ratio (NER, 0.51–0.96 MJnet energy/MJbiofuel) indicated energy sustainability of biofuel production in these countries. In addition, water use and biodiversity impacts were presented based on previous studies. Finally, the impact of public policies such as the RenovaBio program, expected to incentivize farmers, biofuel producers, and policymakers to improve the biofuel supply chain, were evaluated in the context of these countries.

Designing Resource-Efficient and Environmentally Safe Cropping Systems for Sustainable Energy Use and Economic Returns in Indo-Gangetic Plains, India

Achieving an economically feasible and environmentally robust model in agriculture while satisfying the expanding population’s food demands is a global challenge. Hence, a three-year (2014–2017) study was conducted at Punjab Agricultural University, Ludhiana to design environmentally clean, energy-efficient, and profitable cropping systems. Twelve cropping systems viz., rice-wheat (CS1), basmati rice-hayola (transplanted)-mung bean (CS2), basmati rice-radish-maize (CS3), maize-potato-maize (CS4), maize + turmeric-barley + linseed (CS5), maize + turmeric-wheat + linseed (CS6), maize + radish-wheat + linseed-mung bean (CS7), groundnut + pigeon pea (5:1)-wheat + sarson (9:1) (CS8), maize + black gram-pea (bed) + celery (furrows) (CS9),: maize + pigeon pea-chickpea (bed) + gobhi sarson (furrows) (CS10), maize (green cobs) + vegetable cowpea + dhaincha (Sesbania spp.)-chickpea + gobhi sarson (CS11) and sorghum + cowpea (fodder)-wheat + gobhi sarson (9:1) (CS12) were tested in a four-times-replicated randomized block design. CS11 had the maximum system productivity (28.57 Mg ha−1), production efficiency (78.27 Kg Day−1 ha−1), irrigation water use efficiency (2.38 kg m−3), system net returns (4413.3 US$ ha−1), and benefit to cost (B:C) ratio (2.83) over others. In comparison to the CS1 system, this cropping system required ~78% less irrigation water for a unit economic production. However, the cultivation of CS12 registered the highest energy use efficiency (49.06%), net energy returns (6.46 × 103 MJ ha⁻¹), and global warming potential (GWP) (Mg CO2 e ha−1) at spatial scale. Among all the intensified systems, CS11 had the lowest GHGI (0.29 kg CO2 e kg−1). Furthermore, cultivation of CS6 resulted in the maximum bacterial and actinomycetes population in the soil, while CS5 yielded the highest fungal count (23.8 × 103 cfu g−1 dry soil) in soil. Our study suggests that the cultivation of CS11 is a resource-efficient, economically viable, and environmentally clean production system and could be a potential alternative to rice-wheat systems for developing a green economy policy for agricultural development in the Indo-Gangetic Plains (IGP) of India.

Quantitative impact assessment of temperature on the economics of a rooftop solar photovoltaic system.

This paper analyzes the economics of a grid-interactive rooftop solar photovoltaic (PV) system and the impact of the temperature on it. The analysis related to energy metrics, lifecycle costing, and environmental economics was performed considering the PV system's life as 30years. The system economics is also compared at different conditions like theoretical, temperature-corrected, and real electricity generation data. The parameters like energy payback time (EPBT), energy return on energy invested (EROI), and lifecycle conversion efficiency are determined as 5.95years, 5.04, and 0.078, respectively, based on actual generation. The unit electricity cost of the rooftop PV system was estimated as INR 5.37 at the 5% interest rate. The electricity cost varies with the interest rate variation and operation system life. The results show a reduction in overall economic performance on the increase in module temperature. The effect of temperature on the economics of the system is presented in terms of the per degree rise of module temperature. One degree increase of module temperature 8.5days in EPBT of the PV system increases, and INR 0.021 increases in the unit cost of electricity considering a reference temperature 25°C. A PV system has environmental benefits by reducing greenhouse gas emissions, which are also affected by the rise of module temperature. The system lost INR 355.93 in carbon credits at an increase of one-degree module temperature.

The Energy Return on Investment of Whole-Energy Systems: Application to Belgium.

Planning the defossilization of energy systems while maintaining access to abundant primary energy resources is a non-trivial multi-objective problem encompassing economic, technical, environmental, and social aspects. However, most long-term policies consider the cost of the system as the leading indicator in the energy system models to decrease the carbon footprint. This paper is the first to develop a novel approach by adding a surrogate indicator for the social and economic aspects, the energy return on investment (EROI), in a whole-energy system optimization model. In addition, we conducted a global sensitivity analysis to identify the main parameters driving the EROI uncertainty. This method is illustrated in the 2035 Belgian energy system for several greenhouse gas (GHG) emissions targets. Nevertheless, it can be applied to any worldwide or country energy system. The main results are threefold when the GHG emissions are reduced by 80%: (i) the EROI decreases from 8.9 to 3.9; (ii) the imported renewable gas (methane) represents 60 % of the system primary energy mix; (iii) the sensitivity analysis reveals this fuel drives 67% of the variation of the EROI. These results raise questions about meeting the climate targets without adverse socio-economic impact, demonstrating the importance of considering the EROI in energy system models.

Bioenergy recovery potential from upflow microaerobic sludge blanket reactor fed with swine wastewater

This paper reports the bioenergy recovery potential inherent to swine wastewater (SWW) when treated in an upflow microaerobic sludge blanket reactor (UMSB) operating with different oxygen loads. The UMSB reactor performance was compared with the one achieved with an upflow anaerobic sludge blanket (UASB) reactor that worked as a control. The biogas production was measured on a lab scale and used to estimate the UMSB reactor self-sufficiency energy. The highest methane yield achieved in the UMSB reactor was 228.6 NL kgCOD−1 d−1 at an organic loading rate (OLR) of 12.2 kgCOD m−3d−1 (HRT of 10 h). Both OLR and air dose increase generated methane dilution in the biogas, which reduced the possibility of electrical energy recovery. Nevertheless, the results showed that the UMSB reactor was self-sufficient and had energy efficiency at the 12 h HRT with doses of 0.17 and 0.25 LO2 Lfeed−1d−1 and at the 10 h HRT with a dose of 0.25 LO2 Lfeed−1d−1. The UMSB reactor achieved an energy return on investment (EROI) of 11.6–18.7, showing a potentially viable alternative for power generation.

Life cycle carbon, energy, water, and landuse footprints of agricultural residue-based biojet fuel in Bangladesh

Biojet fuel is a low carbon alternative to fossil jet fuel. There is limited literature on life cycle environmental performance of biojet fuel from 2nd generation biomass feedstocks. This work (i) assesses biojet fuel potential from rice, wheat, jute and potato residues in Bangladesh, and (ii) conducts farm to factory gate attributional Life Cycle Assessment of biojet fuel. Different scenarios suggest Bangladesh can produce 1074 to 2548 million liters of biojet fuel annually, with 1810 million liters per annum being the most optimistic potential. This is 4.5 times higher than the current jet fuel demand of the country. Carbon, energy, and landuse footprints of biojet fuel are lower than fossil jet fuel, however, water footprint is higher. Energy Return on Investment (EROI) of biojet fuel ranges from 1.60 to 1.78. The results will contribute to UN SDGs 7 & 13 and biojet fuel development in Bangladesh.

Evolution of the conceptualization of hydrogen through knowledge maps, energy return on investment (EROI) and national policy strategies

In order to address Climate Change and energy dependency challenges, hydrogen (H2) is emerging as a promising energy carrier. Studies related to its production have conceptualized it as green (GH2), clean, renewable (RH2), ecological, and sustainable (SH2). The aim of this research is to deepen the understanding of the GH2 concept and to state boundaries between different terms. To reach this objective, a bibliometric analysis of publications indexed in SCOPUS is launched. Also, in order to assess the potential of renewable energy sources (RES) for GH2 production, a review of the meta-analysis literature on the Energy Return on Energy Invested (EROI) ratio as regards these RES is performed. Additionally, an analysis of main national strategies on GH2 is launched. Results indicate that the GH2 concept is gaining remarkable relevance, while the keyword maps show no significant differences between SH2, RH2 and GH2. EROI reveals low average values for the different biomass energy production processes. For their part, GH2 national strategies focus mainly on solar and wind technologies, albeit leaving the door open to biomass, where EROI could become an adequate metric to guide these strategies towards a low carbon energy path. Although the role of biomass may become fundamental in this energy transition process, given its low EROI values and considering that it is not a totally clean RES, it should be indexed as RH2, but not always as GH2. Finally, a proposal that guides a more appropriate use of the term GH2 is made.Graphical abstract

Economic evaluation of production capacity for natural gas hydrate industrial exploitation in the South China Sea

Natural gas hydrate (NGH) is a worldwide strategic and prospecting commercial resource in the 21st century. The industrialization of NGH has great strategic significance for the achievement of peak carbon dioxide emissions and carbon neutrality. Prior to its industrialization, an economic evaluation of production capacity for each well per day should be conducted to determine whether it is profitable at different given gas prices. In this study, a new hybrid method based on the discounted cash flow (DCF) method and the energy return on investment (EROI) method is used to estimate the economic production rate of NGH exploitation at four different gas price scenarios. The results show that the lowest production rate to make NGH exploitation economic ranges from 1.96 to 29.60 × 104 m3/d/well. With the change in the number of wells, gas–water ratio, gas price, decreasing rate in production cost, and sensitivity analysis are carried out. It shows that all these key factors have a significantly negative effect on the economic production rate initially, and then the sensitivity to the economic production rate will become lower and lower with the rising value of each key factor.

Impact of Various Visible Spectra on Attached Microalgal Growth on Palm Decanter Cake in Triggering Protein, Carbohydrate, and Lipid to Biodiesel Production

Attached microalgal growth of Chlorella vulgaris on palm decanter cake (PDC) under irradiation with various visible monochromatic and polychromatic spectra to produce biodiesel was studied in this work. The results demonstrated that the white spectrum cultivation exhibited the highest microalgal density of 1.13 g/g along with 1.213 g/L day of microalgal productivity. Correspondingly, the biodiesel obtained was comprised mainly of C16 and C18 fatty acids, possessing a high cetane number and oxidation stability from the high saturated fatty acid content (70.38%), which was appealing in terms of most biodiesel production requirements. Nevertheless, the highest lipid content (14.341%) and lipid productivity (93.428 mg/L per day) were discovered with green spectrum cultivation. Blue and white spectra led to similar protein contents (34%) as well as carbohydrate contents (61%), corroborating PDC as a feasible carbon and nutrient source for growing microalgae. Lastly, the energy feasibilities of growing the attached microalgae under visible spectra were investigated, with the highest net energy ratio (NER) of 0.302 found for the yellow spectrum. This value outweighed that in many other works which have used suspended growth systems to produce microalgal fuel feedstock. The microalgal growth attached to PDC is deemed to be a suitable alternative cultivation mode for producing sustainable microalgal feedstock for the biofuel industry.