U. Mich study: natural-gas-based ICE, BEV and FCV all show promise for environmental benefits relative to conventional ICE
Results of a lifecycle analysis by a team at the University of Michigan suggest that multiple types of natural gas-powered vehicles—i.e., natural-gas burning ICE vehicles; battery-electric vehicles (BEVs) recharged with gas-generated electricity; and fuel cell vehicles (FCVs) using hydrogen produced from natural gas—all show promise for reducing environmental impacts, energy demand and climate change impacts relative to conventional petroleum-fueled internal combustion engined vehicles for personal mobility.
Qiang Dai and Christian Lastoskie found that BEVs and FCVs in particular offer significant reductions in greenhouse gas emissions, especially if carbon capture and sequestration (CCS) technologies are implemented at the fuel conversion facilities. Their study appears in the ACS journal Energy & Fuels.
… in this study the environmental impacts of delivering driven vehicle miles are compared for three different vehicle fleets, each powered either directly or indirectly by natural gas. In addition to ensuring the same energy source, the analysis was conducted such that the same functional unit and system boundary are assumed for all of the three mobility options, to facilitate a direct comparison of their respective environmental footprints. The fleets studied are BEVs, using electricity generated from natural gas combined cycle (NGCC) power plants; CNGVs, which are internal combustion engine vehicles modified to burn compressed natural gas; and FCVs powered by hydrogen produced from steam methane reforming (SMR) of natural gas. The overarching goal of the LCA study is to assess which mode of natural gas-based personal passenger mobility offers the most compelling future environmental benefits, as both the civil power infrastructure and the transportation sector undergo a greening transition from coal and petroleum respectively to natural gas.—Dai and Lastoskie
|The system boundary for the LCA. The functional unit was defined to be 120,000 miles (193,000 km) evenly delivered over a 10-year period. A mid-sized ICE vehicle driven in the United States was also investigated as a basis for comparison. Credit: ACS, Dai and Lastoskie. Click to enlarge.|
Dai and Lastoskie assumed their three fleets would have the same glider and similar power train configuration, excluding power source parts. In all three cases, power generation methods were chosen based on best-in-class or near best-in-class components that are nonetheless widely commercially available.
The BEV Li-ion battery pack uses a LiNi⅓Co⅓Mn⅓O2 (NCM) chemistry for its cathode active material. NCM was chosen for its demonstrated specific energy of 135 kWh/kg and cycle life of 1300.
The CNGV is powered by a four-stroke Otto cycle spark-ignited engine—the same used in the conventional vehicles (PICVs). They assumed that the CNGV is fueled by natural gas contained within a 100 kg chrome steel storage tank.
The FCV is powered by an 80 kW PEMFC and a 20 kW Li-ion battery, also based on the NCM cathode chemistry.
The CCS processes for the study were based on the use of monoethanol-amine (MEA) as the absorbent; the lifecycle inventories for the carbon capture processes for the respective fuel conversion plants were adapted from two US DOE national laboratory reports.
Dai and Lastoskie used eight environmental indicators:
- human toxicity (HT)
- particulate matter formation (PMF)
- photochemical oxidant formation (POF)
- terrestrial acidification (TA)
- terrestrial toxicity (TET)
- natural land transformation (NLT)
- water depletion potential (WDP)
- mineral depletion potential (MDP)
|Comparison of environmental indicators, normalized to the vehicle with the highest impact for each indicator. Blue bars represent contributions from the glider and drivetrain; red bars, from the power source; and green bars from the use phase (w/R = with recycle; CCS = with CO2 capture). Credit: ACS, Dai and Lastoskie. Click to enlarge.|
They also reported climate change and primary energy use through the respective measures of GWP (in tonnes of CO2 equivalents) and CED (in GJ eq).
Among some of their findings were:
CNGVs have the highest impact for NLT because of the requirement for additional high-pressure natural gas pipelines. The fuel cell vehicle option meanwhile has the highest environmental impact in the PMF, TA, and MDP categories, mostly on account of the platinum content of the PEMFC. The environmental impact score for the BEV is the highest for WDP and TET.
The large assembly phase contribution to the BEV water depletion potential score is for sulfuric acid and direct water consumption for the refining of cobalt and nickel used in the battery pack, whereas evaporative cooling water losses account for the use-phase WDP footprint.
For the CED, GWP, and HT indicators, the projected impacts for all three modes of natural gas-powered mobility are lower than those conventional petroleum-fueled mobility, unless very pessimistic assumptions are made for the CNGV fuel economy; the NCM battery cycle life for the BEV; the platinum loading in the PEMFC for the FCV; or the SMR plant hydrogen conversion efficiency for the FCV.
For CED and GWP, the use-phase is the largest contributor to the environmental footprint across all vehicle configurations: 52−86% of total life cycle impact. The BEV has the smallest use phase environmental footprint, followed by FCV and CNGV.
The BEV has a well-to-wheel efficiency of about 40% when a 10% charging loss is assumed. The FCV has an overall energy conversion efficiency about 30%. CNGVs, particularly those retrofitted from gasoline engines, trail in efficiency with a fuel conversion efficiency of approximately 20%.
The assembly-phase impacts for the BEV and FCV power sources both contribute significantly to the human health impacts of mobility using these vehicles. For example, the Li-ion pack in the BEV accounts for 63% of the life cycle HT and PMF scores, but just 11% of the life cycle CED and GWP.
Further improvements in BEV pack cycle life and energy density and minimization of the precious metal content of LIBs and FEMFCs will extend the environmental merits of electric drive vehicles over their internal combustion engine counterparts.—Dai and Lastoskie
Qiang Dai and Christian M. Lastoskie (2014) “Life Cycle Assessment of Natural Gas-Powered Personal Mobility Options” Energy & Fuels doi: