Functional Comparison
Researchers, environmentalists, and vehicle owners, among others, are keen on knowing if EVs produce less GHG emissions or have smaller carbon footprint than ICEVs. Disclosure of relevant data by vehicle manufacturers to the public like the LCA models and standards adopted as well as methods of data collection and analysis is vital for comparison of the vehicles’ PCF. Many environmental reports, especially for those from manufacturers, emphasize the environmental impact of their products in the consumption phase, as they argue that the vast majority of emissions generated by vehicles today occur in the product-use phase, i.e. when consumers are driving their vehicles [28]. However, this does not reflect the full picture of environmental impact of such products from an end-to-end view, i.e. from cradle to grave. However, there may be exceptions in which Toyota is one of them. They focus on the emissions from manufacturing phase of products and future goals for energy consumption [29]. Toyota presents scenarios and figures about how much fossil resources are consumed for energy supply to produce the vehicle itself, and to produce the energy to propel the vehicle during the Use phase in unit of annual consumption/reserve in the manufacturing phase of products. Nevertheless, this is encouraging but not sufficient. As suggested by the GREET model, the environmental impact of EVs is determined by the lifetime impact of EVs versus ICE vehicles requires looking at the entire lifecycle - from raw materials to emissions to disposal but not just at the emissions resulting from vehicle usage. Therefore, examining the energy efficiency of how fuel is generated in the LCA is equally important. Table III below provides the comparison of Gasoline-driven internal combustion engine vehicles (ICEV), Lithium battery electric vehicles (BEV), H2 gas tank powered fuel vehicle (HFCV1), and Solid-H2 fuel cell vehicle (HFCV2) functionally.
Functional Comparison of ICEV, BEV, HFCV1 and HFCV2
System Boundary
LCA is a useful tool for quantifying the overall environmental impacts of a product, process, or service. The scientific boundary definition are important to ensure the accuracy of LCA results. The objective of this research is to present an insightful discussion on the emissions, i.e. carbon footprint of LDVs fueled by major fuel types including ICEV, BEV and more importantly HFCV with a meaningfully defined system boundary. By using the GREET modeling methods [3], we aim at providing a consistent LCA platform with reliable, widely accepted methods/protocols. A GREET model consists of two modules [4]: 1) GREET1 evaluates well-to-wheels (WTW) energy use and emissions of vehicle/fuel systems, and 2) GREET2 evaluates energy use and emissions of vehicle manufacturing cycle.
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Modeling of the various vehicle technologies evaluated in this study was conducted using publicly available data and models. Although [29] claimed that their report defines the scope of the assessment to include all energy, materials, substances and processes with necessary data to assess all life cycle phases for all new parts, the report actually provide inventory results in form of bar-charts showing the patterns under specific cases but not the detail figures which can be used for comparison. Only emission figures are provided in the use phase. Similar to the Tesla report [28], its system boundary focused on the use phase. Hence, our system boundary is defined as follows.
Fuel production phase represents an average emission produced from the four phases of GREET1 before vehicle operation in Fig. 2, from either one of the two hydrogen production pathways, i.e. natural gas (NG) and renewable electrolysis (RE).
The CO2 emission figures of fuel consumption phase (i.e. vehicle operation phase in GREET1)
Fuel Cycle (GREET1)
According to [29], the hydrogen pathways in the first launch countries; the UK, Germany and Denmark are used. One is central reforming from natural gas piped from North Sea and from Russia and the other the electrolysis using renewable energy from wind power. For CO2 emissions of both pathways, European Commission Joint Research Centre database are originated. The main upstream emission factors are adopted from [23]. For other fuels and renewables such as hydropower, geothermal, solar, wind and tidal power the upstream emission factors were considered equal to zero. A comparison of emissions among ICEV, BEV and three different models of HFCVs are summarized in Table IV.
The results indicate that the emissions of a HFCV using LCA based on the GREET model depends very much on how fuel production mechanisms are designed. Based on the manufacturers’ data, the CO2 emission in the fuel consumption phase is zero across all models of HFCV. In fact, the type of hydrogen production pathway adopted determines the size of emissions of fuel production. When NG (Natural gas) is used, the total CO2 emission to produce a tank of hydrogen for a HFCV is 70.7 kg, while the emission is reduced to 8.6 kg when electrolysis with wind powered renewable energy is used which is about 8.22 times smaller. A study conducted in Australia [27] comparing the technologies of conventional ICEVs, EVs and HFCVs using the grid supported by non-renewable energy sources. The results showed that HFCVs only slightly outperformed conventional ICEVs. They suggested that significant reductions in greenhouse gas (GHG) emissions with HFCVs appear only possible in Australia if their country makes a fundamental shift towards an almost 100% renewable energy system.
Mitigating Environmental Impacts of EVs
With reference to the GREET model, GREET1, i.e. the well-to-wheels (WTW) stage can be subdivided into the well-to-tank (WTT) stage and the tank-to-wheel (TTW) stage, where WTT stage concerns the delivery of energy from its source to the storage equipment in the vehicle, while the TTW stage focuses on conversion efficiency where the energy carrier is used to propel the vehicle during operation [36]. Since the WTT stage covers all operations along the hydrogen production pathway, we focus the analysis on this stage.
Renewable energy
The environmental burden of the WTT stage differs a lot, depending on how the energy carrier is produced [37]. As shown in Table IV, there is a big difference between electricity produced from NG and RE where the renewable energy used in electrolysis in the case was wind power. Owing to solar and wind power are affected by weather conditions, resulting in unstable power generation, which makes indefinite storage difficult [38], we further suggest an alternative fuel production method aiming at lower CO2 emissions.
Biomass Gasification
Biomass is an abundant domestic resource which can be converted into biohydrogen with biomass gasification, steam reforming, or biological conversion like biocatalysed electrolysis or fermentative hydrogen production [32]. Gasification is a highly exothermal process in which biomass is reacting with an oxygen-rich gas under the aid of steam. The biomass is converted to biogas using a gasifier and to hydrogen using a catalytic steam methane reformer with a water shift gas reaction. There are four delivery options for this pathway: (1) pipeline delivery, (2) gaseous truck delivery, (3) liquid truck delivery with gaseous dispensing, and (4) liquid truck delivery with cyro-compressed dispensing, where the 3rd option produces lowest CO2 emissions (Table V), which is suggested to be an alternative hydrogen production pathway to renewal electrolysis.
Conclusion
Drawing upon the data disclosed by the vehicle manufacturers, this study compares the PCF of light duty vehicles of different fuel types including ICEVs (vehicles powered by traditional fuel), Tesla Model 3 (BEV) and Toyota MIRAI together with three other major brands of HFCV including Hyundai ix35, Honda Clarity Fuel Cell, and Mercedes-Benz GLC F-CELL based on the LCA model of GREET. The results indicate that the fuel cycle of GREET1 contributes significantly to the vehicle’s PCF. The cleaner the production of hydrogen or electricity, the less emissions the vehicles will have on the environment. Besides, this study found that higher transparency in disclosure of relevant data in the PCF methodology adopted by vehicle manufacturers is needed for making comparison of their vehicles’ emissions possible. A comparative evaluation on the carbon footprint of technology for hydrogen production including distributed grid electricity, water electrolysis and biomass gasification by four different methodologies was provided. Future research needs to examine the best practices of PCF calculation and reporting for new energy vehicles including private cars and trucks.
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