The argument for the transition toward decarbonization of all energy sectors has hit top gear due to the spread of the COVID-19 pandemic. While it is often assumed that full electrification of these sectors will lead to their full decarbonization, little thought on how electricity is currently generated, stored, transmitted, and consumed has been considered. The ideal scenario of 100 percent renewable power generation is widely accepted to occur in the near future and is considered as an enabler for full electrification, and, hence, full decarbonization, without a clear path for achieving it. Circular arguments are constantly being made on this topic, a common one being that renewable energy infrastructure such as wind turbines and solar panels will eventually be manufactured using renewable energy produced by several such renewable energy installations. A good possibility exists for aggressive renewable energy penetration installations to come with a penalty of increased carbon emissions in the interim by increasing our dependence on fossil energy, a ubiquitously and economically available resource. This is similar to Jevons paradox and the energy rebound effect. Once we facilitate enough renewable energy installations, our dependence on fossil energy would gradually lessen, while simultaneously promoting a shift toward renewable energy and renewable fuels, including electrofuels. However, we are still decades away from such a scenario.
Hence, the purpose of this paper is to position propane as a solution for accelerating decarbonization of the medium- and heavy-duty transportation sector and several other energy sectors.
To reinforce this premise, a cursory life-cycle analysis of equivalent carbon dioxide (CO2eq) emissions was conducted between a medium-duty (Class 6-7) electric vehicle and a propane-fueled vehicle. The intent here is to evaluate the U.S. state-level difference in CO2eq emissions between the two vehicles and provide an alternate hypothesis for decarbonization using propane and its blends.
Several qualitative and quantitative assumptions of the lifecycle analysis were made. The CO2eq emissions from the vehicle body, doors, chassis, tires, tire replacement, wheels, wheel replacement, final assembly, interior and exterior, and lead-acid battery were all assumed to be similar between the two vehicles such that the difference between them is negligible. EVs are heavier than internal combustion engine vehicles (predominantly due to battery mass) and hence may need additional material “padding” but incremental emissions attributed to those are considered negligible. Furthermore, no credit was assumed for the Lithium-ion battery second life, i.e. for its use in utility scale grid applications after its end-of-life for transportation applications. In addition, no credits for recycling the components of the Lithium-ion battery were assumed as this is still an active topic of research. Similarly, CO2eq emissions from end-of-life were assumed to be similar for the two vehicles such that the difference between the two is negligible (even though the EV is heavier than the conventional vehicle). Finally, electricity that is generated in a state is assumed to be used for charging the EV even though electricity is imported from neighboring states (and sometimes countries outside the U.S.) in several states i.e. the net carbon intensity could be computed based on emissions attributed to electricity generation or electricity consumption. It is also assumed that the medium-duty vehicle is charged only in the state to which it belongs to or operates most of the time. It is reiterated here that the purpose of this analysis is to evaluate the difference between total-life cycle CO2eq emissions between a medium-duty propane vehicle and EV but not to accurately quantify their individual carbon footprint.
The State of the U.S. Electrical Grid
The state-level energy mix for electricity generation for all fifty U.S. states and Washington, D.C., uses coal, natural gas, petroleum, biomass and other, nuclear, geothermal, solar-photovoltaic (PV), wind, and hydroelectricity. It is very clear the magnitude of effort needed for enabling a 100 percent renewable energy electrical grid in the U.S. Currently, the U.S. renewable energy penetration stands at approximately 18 percent.
Five scenarios were simulated in this study. It is to be noted that DME and renewable DME are similar to propane in terms of their physical properties. DME has been long considered as a replacement for diesel fuel for medium- and heavy-duty transportation sectors due to its high cetane rating and low soot formation tendency. Renewable DME can be blended with conventional or renewable propane further reducing the blended fuel’s carbon footprint. In this study, a 20 to 80 percent (by mass) renewable DME propane (or renewable propane) blend was assumed (Note: the Propane Education & Research Council is collaborating with Oberon Fuels to study the impact of this fuel blend on ICEV performance and emissions).
Furthermore, renewable fuel and vehicle component production carbon intensities were assumed the same as status-quo even under a decarbonized electric grid scenario (Case V). The carbon intensity of renewable fuels and component production carbon intensities will be much lower due to cleaner electricity generation. Projections are out-of-scope for the current analysis. In addition, propane vehicle fuel economy has been assumed the same as status-quo even for Case V. In reality, the fuel economy will improve significantly due to the evolution of propane engine technologies over the next 20 to 30 years.
Well-to-Wheels Carbon Intensity of “Fuel”
As mentioned before, the CARB methodology was adopted to calculate the well-to-wheels carbon intensity of propane for each state in a specific PADD region. Since the fifty U.S. states and the district are divided into five different PADD regions, five different values of propane are obtained. Differences in the propane carbon intensities predominantly arise from the percentage of propane obtained from natural gas processing and that obtained from oil refining, which varies for each PADD region. Based on the state average energy mix for electricity generation and assuming a 10 percent charging loss, the carbon intensity of the electricity used for charging the EV was obtained. For validating the carbon intensity of the grid electricity, consider the state of California where the current analysis provides a carbon intensity of 87.5 gCO2eq/MJ of consumed electricity. This value includes 10 percent charging loss and hence the actual electrical grid carbon intensity would be 78.75 gCO2eq/MJ, including the electrical transmission loss. Earlier this year, CARB published a carbon intensity value of 82.92 gCO2eq/MJ for average grid electricity used for charging EVs3 . This value is 5 percent higher than the value deduced from the current analysis. Hence, this analysis may indeed underestimate the carbon footprint of EVs. Nonetheless, no further correction to electrical grid carbon intensities was considered in this analysis.
However, the carbon intensity of the energy by itself does not provide the entire picture of the total carbon footprint of vehicles as the EV powertrain efficiency is far superior (>70 percent efficient) than that of the internal combustion engine vehicle (~35-40 percent efficient) i.e. converting electrical energy to mechanical work is more efficient as compared to converting fuel chemical or internal energy to mechanical work due to higher second-law of thermodynamic irreversibility and energy losses. Hence, the powertrain efficiency should be considered for a rational comparison of carbon footprint between the two vehicles, the results of which are discussed in the next section.
Some key observations:
- Currently, propane fueled medium- and heavy-duty vehicles provide a lower carbon footprint solution in 38 U.S. states and the district when compared to medium- and heavy-duty EVs that are charged using the electrical grid.
- Currently, renewable propane-fueled vehicles provide a lower carbon footprint solution in all 50 U.S. states except Vermont when compared to medium-duty EVs that are charged using the electrical grid.
- Currently, vehicles with a fuel blend of propane and renewable DME (80 percent-20 percent by mass) can enable a lower carbon footprint solution in every state except Vermont when compared to medium-duty EVs that are charged using the electrical grid.
- Currently, vehicles with a fuel blend of renewable propane and renewable DME (80 percent-20 percent by mass) can enable a lower carbon footprint solution in all 50 states when compared to medium-duty EVs that are charged using the electrical grid.
- Finally, even in an ideal scenario of a decarbonized grid with 95 percent carbon intensity reduction and with Lithium-ion batteries manufactured with zero-carbon energy sources (61 kgCO2eq/kWh) and lasting for a million miles (5,000 cycles), vehicles fueled with a blend of renewable propane and renewable DME enable a lower carbon footprint solution in all 50 states compared to medium-duty EVs.
It is noted here that the current supply of renewable fuels does not meet the fuel demand. However, the Western Propane Gas Association is targeting a 100 percent replacement of conventional propane with renewable propane by the year 2030 in California, while the entire U.S. propane industry is targeting at least a 50 percent replacement of conventional propane with renewable propane by 2050. In addition, investments into renewable diesel facilities by companies such as Marathon Petroleum and Phillips 66 should help address the fuel supply issue as renewable propane is a byproduct of renewable diesel and sustainable aviation fuel (~5-10 percent of off-gas is renewable propane).
Furthermore, recent advancements in carbon capture technology and tapping captured CO2 from power plants (plus industrial facilities and marine sector) for synthetic DME production are also very encouraging.
To read the full study, conclusion, and recommendations, download the full report below. Connect with Dr. Vishwanathan on LinkedIn to get the latest updates on propane research.