Alternative Sources of Energy in Transport: Comparison
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The concentration of CO2 in the atmosphere has shown a significant increase as compared to the preindustrial era. The CO2 concentration level stood at 402.31 ppm, approximately 42% higher than the mid-1800 levels (around 280 ppm). Additionally, there have been notable increases in CH4 and N2O levels. The transport sector is considered one of the most polluting sectors, which is why alternative technologies to traditional combustion engines and fuels are being explored, such as battery electric (BEV), hybrid electric (HEV), plug-in hybrid (PHEV), compressed natural gas (CNG), biogas (BG), and more.
  • alternative energy sources
  • transport
  • LCA
  • hydrogen
  • electricity
  • batteries

1. Biodiesel from Various Oils

Renewable fuels derived from vegetable oils are seen as an appealing substitute to fossil fuels, with the European Union primarily focusing on biodiesel made from oil waste and fats, soybean, rapeseed, and palm oil. Some vegetable oils, such as palm, sunflower, jatropha, and rapeseed, share similar characteristics with diesel, although some have issues with low volatility and high viscosity. Blending higher chain alcohols with biodiesel leads to better performance than blending lower chain alcohols, and hydrogen performs similarly to standard diesel engines, with a notable reduction in NOx.
While there are nearly different types of esters of edible oils, non-edible oil can be used as an alternative fuel with standard diesel. However, alcohols and ethers are the most suitable additives for improving performance, combustion, and emission characteristics. Studies have shown that the use of biodiesel results in reduced emissions of HC and CO and smoke and increased emissions of NOx [13,14][1][2].

2. Biogas

Upgrading biogas to biomethane is a promising option to make the automotive energy sector more sustainable, especially for transportation. Bio-CNG and bio-LNG fuels are regarded as viable alternatives for freight, and the European biomethane industry demonstrates significant production capabilities, driven by rising demand. However, promoting biomethane to reduce GHG emissions must be done with caution since it can cause fugitive emissions, such as those resulting from gas transportation and combustion, which can offset the benefits. Furthermore, the current high costs of biomethane can limit its wider adoption as an alternative fuel. Despite these challenges, the European biomethane sector holds promising potential to meet the future energy demands of the European transportation sector [15][3].

3. Dimethyl Ether

Life cycle assessments conducted on dimethyl ether (DME) have demonstrated that bio-DME fuel produced using a CO2-enhanced process can reduce its impact on climate change, toxicity, and ecotoxicity by at least 20%. Using pure DME can result in a 72% reduction in GHG emissions, and its impact on human health and ecosystems can be reduced by 55% and 68%, respectively. The use of DME fuel can also limit the emission of carcinogenic particulates, such as diesel soot, and decrease the toxicity of traffic emissions. However, when DME is mixed with diesel (15% DME by weight in diesel), the environmental impact is only slightly reduced by up to 7% compared to pure diesel. A CO2-enhanced process for producing bio-DME can significantly reduce the environmental impact of the production process by recycling waste CO2 as a gasifying agent. Bio-DME is considered a cleaner automotive fuel compared to pure diesel [16][4].

4. Electricity

The adoption of electric vehicles (EVs) is a significant step towards reducing environmental impact. Unlike vehicles with internal combustion engines, battery electric vehicles (BEVs) do not emit direct air pollutants during operation. However, the environmental performance of BEVs, as well as fuel cell vehicles (FCVs) and plug-in hybrid electric vehicles (PHEVs), may be worse than modern fossil fuel vehicles due to emissions from vehicle and fuel production chains. The performance of EVs also depends on regional driving patterns and the sources of electricity used, as demonstrated by the varying air pollution levels observed in different countries. Despite the potential drawbacks, EVs coupled with low-carbon electricity sources such as biofuels and natural gas are more sustainable from a life cycle perspective. To better understand the environmental impacts of future battery electric vehicles, life-cycle background databases must include data on the production phase and energy sources used. Recent research has indicated that the environmental impact of BEVs is strongly affected by the battery size and energy requirements. Alternative fuels, such as bioethanol, biodiesel, biomethanol, biogas, and solar energy, may be viable options for the future. As such, it is crucial to include future developments in the electricity sector in life-cycle background databases to gain a better understanding of the environmental impacts of BEVs. Additionally, the production phase and the energy sources used to manufacture these vehicles should also be taken into account, as BEVs are more sensitive to changes in the energy sector than traditional combustion vehicles [2,3,5,8,17,18][5][6][7][8][9][10].
One of the problems with battery vehicles is the question of having enough renewable energy to power electric fleets and whether there will be enough lithium and cobalt reserves to meet the demand for battery construction. As of 2022, lithium resources were estimated at approximately 98 million tons worldwide, with reserves in various countries such as the US, Bolivia, Argentina, Chile, Australia, China, and Germany. Additionally, if electric vehicles continue to be charged using fossil-fuel-generated electricity, the reduction in greenhouse gas emissions may not be significant. For instance, replacing 100% of the Brazilian fleet with electric vehicles would increase daily demand by 533 GWh/day or 194.55 TWh/year, equivalent to a 19.40% increase in electricity consumption. However, this could still be positive for the environment, as Brazil’s electricity matrix was composed of 84.5% renewable energy sources, specifically 71% hydro, 23% thermal (8% natural gas, 7% biomass, 4% fuel oil, 2% coal, 1% nuclear, 1% other), and 6% wind in 2016, compared to the global average of only 23.65% in the same year [3,12,15,19,20,21][3][6][11][12][13][14].
Currently, the most commonly used electricity storage units in BEVs are lithium-ion batteries, specifically lithium manganese oxide and lithium iron phosphate. This is due to their favorable specific energy density and power characteristics. However, the overall environmental impact of these batteries is influenced significantly by the electricity used to charge them during their use phase, impacting categories such as climate change, particulate matter formation, human toxicity, and material depletion. To improve the environmental performance of these batteries, charging them with renewable electricity is crucial. Moreover, the recycling of materials is a major concern for both human toxicity and material depletion. Recycling has been shown to have a significant impact on the overall efficiency of equipment life cycles, and there is a need for more efficient large-scale recycling industries to achieve better results. Currently, only lead acid batteries have highly efficient recycling processes. Thus, to achieve better environmental outcomes, more efficient recycling processes are needed for the latest battery technologies [19][12].
The electricity mix is a critical factor in determining the life-cycle emissions of electric vehicles, but it can vary greatly from hour to hour. In Belgium, studies of BEV usage patterns have shown that drivers tend to charge their vehicles during the day when energy demand and costs are high. However, charging during off-peak hours (midnight to 8 AM) can result in the lowest emissions for all analyzed emissions. This is because off-peak hours are mainly supplied by base load nuclear plants, renewable energy sources, and some flexible natural gas plants with better efficiency. Charging during off-peak hours could reduce WTT CO2, PM, NOX, and SO2 emissions per km by 12%, 15%, 13%, and 12%, respectively. Additionally, the charging mode (fast or normal) could impact the performance and durability of lithium batteries, with more cycles leading to greater durability. Finally, it should be noted that the efficient large-scale recycling of lithium-ion batteries is crucial to improving their environmental performance over their life cycle [12][11].

5. Hydrogen

The reason for the growing interest in hydrogen as an energy carrier and the drive to replace fossil fuels is due to its favorable characteristics—it releases a large amount of energy when reacting with oxygen, and the only byproduct is water. With advancements in technology for production, storage, and use, it has the potential to be a clean, safe, and sustainable energy source. However, currently, the majority of hydrogen production relies on fossil fuels, particularly natural gas, which accounts for about three-quarters of the world’s production. Coal is also a significant raw material for hydrogen production. Various methods are used for producing hydrogen, including natural gas and light hydrocarbon conversion, water electrolysis, biomass gasification, and coal gasification. Biomass-based hydrogen production has similar efficiency as water-based methods, but offers lower operating costs and higher energy efficiency. To achieve a green hydrogen economy, it is crucial to shift to renewable feedstocks for hydrogen production instead of fossil fuels. Currently, a highly efficient pulverized coal boiler is being explored for industrial use, with thermal efficiencies above 90% and emissions comparable to natural gas boilers. While hydrogen poses challenges in storage, transport, and use, it remains an important fuel. Improving engine optimization and onboard storage systems can enhance the driving range of hydrogen vehicles. In the short-term, vehicles using hydrogen mixed with fossil fuels can serve as an initial push towards a hydrogen economy, while pure hydrogen vehicles remain a more preferable decarbonization solution. However, producing hydrogen with a high renewable content can be challenging due to the intermittency of renewable energy. Policy, technology, and natural resources can work together to overcome this challenge [1,3,8,19,22,23,24,25,26][6][8][12][15][16][17][18][19][20].
There are various types of hydrogen that differ based on their production method. Grey hydrogen is the most prevalent type of hydrogen at present. It is produced through steam methane reformation of natural gas or methane, but without capturing the greenhouse gases produced during the process.
Blue hydrogen is similar to grey hydrogen in that it is primarily produced using steam reformation of natural gas or coal, but with the added step of carbon capture and storage of CO2, which sets it apart.
Turquoise hydrogen is generated through methane pyrolysis and solid carbon, and its cost is significantly impacted by the value of solid carbon and natural gas prices. The production of turquoise hydrogen using renewable natural gas will have a crucial role in achieving a clean energy transition in comparison to grey and blue hydrogen. Currently, it is still in the experimental stage of development.
Purple hydrogen, also known as pink or red hydrogen, is generated through a nuclear thermochemical cycle. Nonetheless, further technological advancements are required to enhance its efficiency and decrease costs on a worldwide level.
Green hydrogen, which is produced through water electrolysis using renewable resources, is a zero-carbon form of hydrogen and is crucial for deep decarbonization of the transportation sector. However, the production cost of green hydrogen is presently higher than that of blue hydrogen, primarily due to the cost of the electrolyzer, its capacity factor, and the cost of renewable electricity [19][12].

6. Waste

As the amount of waste increases, so does the demand for its processing. Land-filling is not a solution in the long term, so studies on the use of different types of waste as a source of alternative fuels are multiplying. In this chapter, several studies on the topic of processing different types of waste into fuel are presented.
One such method is converting tires into tire oil and then into fuel via pyrolysis. This process generates liquid oil, char, and pyro gas, which can be useful products. Through vacuum distillation, the resulting tire oil is purer than crude tire pyrolysis oil (TPO) and has similar properties to diesel fuel. Additional modification steps, such as moisture removal, desulfurization, and vacuum distillation, can further enhance the properties of the TPO. Overall, tire oil pyrolysis is a sustainable and effective method that can produce a high-quality fuel, is economically feasible, and offers an optimal solution for waste tire management and petroleum product replacement. To improve combustion quality and reduce emissions, some studies have suggested adding biodiesel and nanoparticles to the TPO-DF blend, which increases oxygen by weight. Ultimately, distilled TPO can serve as a viable alternative to diesel fuel [27][21].
An alternative choice is to utilize waste PET bottles to generate pyrolytic plastic oil. However, it is not advised to directly employ it in diesel engines. Instead, it is recommended to use it as an additive to diminish the quantity of diesel fuel utilized. Furthermore, it is suggested not to surpass a volume of 20% to maintain similar engine characteristics and emission rates to diesel fuel [28][22].
In the aftermath of the COVID-19 pandemic, there have been studies on the use of medical plastic waste generated during this period. The World Health Organization (WHO) declared the COVID-19 pandemic a public health emergency on 30 January 2020. As a result, many governments around the world recommended several preventive measures to reduce the risk of COVID-19 transmission, including the use of face masks, face shields, personal protective equipment kits, and gloves. The surge in waste, particularly plastics, in both the medical and general sectors was evident and posed a significant threat to the environment. In response to the increasing demand for PPE among healthcare workers, service employees, and individuals, China’s production of single-use face masks increased to 116 million per day in February 2020, more than twelve times the average amount. Globally, supplements have seen a 40% increase in use for food packaging and a 17% increase for other purposes. Plastics are an essential component of modern society due to their durability and resistance to degradation from chemicals, physical forces, and biological factors. Medical plastic waste and infected plastic trash require sterilization before reuse. Among various sterilization techniques, microwave sterilization is a suitable method for medical plastic waste. Pyrolysis oil production is a promising method for managing COVID-19 medical waste. The pyrolysis technique yields mainly pyrolysis oil (70–80%) and a small amount of solid char (10–15%). Due to tests of physicochemical properties (calorific value, density, API gravity, kinematic viscosity, ash content, cetane number, pour point, flash point, and fire point) that were carried out by the Institute of Petroleum and the American Society for Testing and Materials, it was found that fuel obtained from various plastics has the potential to be a fossil fuel for internal combustion engines. However, pyrolysis requires optimal conditions, appropriate catalysts, and a gas cleaning system to mitigate concerns. Nonetheless, the high cost of collecting and recycling medical plastic waste remains the primary obstacle to widespread deployment of the technology. Improving the equipment, process design, scalability, building more facilities, and enhancing product properties could alleviate this challenge [29,30][23][24].
A different strategy is the utilization of Waste-to-Hydrogen, which provides a two-fold solution by simultaneously producing non-fossil-fuel-based hydrogen and promoting sustainable waste management. Gasification and fermentation are two primary WtH technologies that have been shown to reduce CO2-eq emissions per kg of H2 by 50–69% compared to the traditional steam methane reforming hydrogen production method used to fuel vehicles. In addition, gasification of municipal solid and wood waste exhibits lower global warming potentials than dark fermentation of wet waste and combined dark and photo fermentation. Gasification technology has been established since the 1970s, and numerous industrial or large-scale gasification processes for plastic waste have been developed as a result [31,32,33][25][26][27].

7. Microalgae

Microalgae are tiny aquatic photosynthetic organisms that require CO2 for growth. They do not compete with food crops for resources, and their ability to absorb harmful CO2 emissions makes them a promising biofuel source. Due to their high photon conversion efficiency, microalgae convert solar energy to chemical energy more efficiently than other crops used for biodiesel. Additionally, microalgae have a high oil content and a fast growth rate, which makes them suitable for biodiesel production. They can be cultivated on non-arid, non-productive land, including coastal land, brackish water, and wastewater, and their production is non-seasonal. Furthermore, microalgae are effective in bioremediation of wastewater, removing nitrogen and phosphorus, which are two of the most challenging elements to remove in wastewater treatment. The advantages of microalgae biodiesel production have led researchers to focus on improving cultivation and production processes. To produce biodiesel from algae, four consecutive stages must be undergone, including cultivation, harvesting, lipid extraction, and transesterification and fermentation. Research has demonstrated that the use of algae biodiesel blend in engines can improve performance beyond that of pure diesel, exhibiting a decrease in brake specific fuel consumption, an increase in brake thermal efficiency, a rise in heat release rate, exhaust temperature, peak pressure, and more, surpassing other biodiesel blends. A two-step process has also been tested, revealing that the calorific value of the produced biodiesel was lower than that of gasoline or diesel but higher than popular biodiesels such as palm and Jatropha. The viscosity and density of the algal biodiesel were almost identical to petro-diesel, and all properties fell within American Society for Testing and Materials standards. Moreover, the feedstock to oil conversion ratio obtained with algae is approximately 70% [34,35,36,37][28][29][30][31].

8. Alcohol Blends

Alcohol fuels are another promising category of renewable fuels for internal combustion engines. Methanol, a low-carbon-intensity electro-fuel, and ethanol, a low-carbon-intensity biofuel, show great potential. In fact, these two fuels can be produced synergistically. In a fully renewable scenario, gas fermentation and CO2 to hydrocarbon catalysis can be combined with conventional ethanol production to co-produce ethanol and methanol at prices competitive with current gasoline costs. This co-production can significantly increase the yield of alcohol fuel per hectare of crop, all while remaining carbon-neutral and cost-competitive with conventional fossil fuels. Wet ethanol 80, for example, can be produced with lower carbon intensity than conventional fossil fuels at a cost comparable to the availability of other renewable fuels. Wet ethanol 80 emits slightly lower engine NOx emissions, with a net fuel conversion efficiency penalty compared to methanol due to the effect of water dilution. A combination of wet ethanol 80 and methanol was found to behave similarly to pure wet ethanol 80 and pure methanol, suggesting that these fuels are interchangeable and can be blended to match the output of coproduction plants. An engine can be designed to operate on high-cooling potential alcohol fuels, such as wet ethanol, methanol, or a blend of the two, based on local availability. In Brazil, flexible fuel vehicles (FFVs) can also use 100% ethanol (E100) as fuel, which has led to FFVs accounting for over 90% of new car sales and around half of the country’s light vehicle fleet. In 2014, the transportation sector was responsible for 32.5% of energy consumption and 46.3% of greenhouse gas emissions in Brazil, and it has experienced the highest growth rate in energy consumption (4.42% per year between 2002 and 2012) [3,38][6][32].

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