This report is designed to compare the advantages and disadvantages of hybrid electric vehicles (HEVs) to those of conventional internal combustion vehicles (ICEVs) and battery electric vehicles (BEVs) over a range of metrics. The metrics considered are performance (fuel economy and vehicle design), environmental impacts (emissions associated with vehicle/battery manufacturing and vehicle operation) and realities of available infrastructure for BEVs. In this report, HEVs are considered exclusively as battery electric vehicles with an internal combustion engine; plug-in hybrid vehicles are not considered so as to draw a clear distinction between HEVs and BEVs.
2. Performance considerations
HEVs offer significant benefits for fuel economy when compared to conventional ICEVs [1]; this can be attributed to their unique vehicle design. Specifically, they employ regenerative braking [2], electric motor/drive assist and automatic start/shutoff [3]. These features are particularly useful in frequent stop/start situations and periods characterized by vehicular idling, such as urban driving situations [4; 3]. Regenerative braking can decrease fuel consumption by 25% by allocating more electricity for start-up mode [5].
The advantages of HEVs with respect to fuel economy extend beyond urban driving situations: in a designed study by the International Energy Agency [6], "commercially available HEVs hybrids deliver fuel efficiency improvements of around 30% compared to a conventional ICEV on a mixed urban/highway drive cycle." In addition, over a 45-55% highway-to-city driving ratio, HEVs have longer driving ranges than ICEVs or BEVs (see Table 1 below).
Table 1: Comparison of ICEV, HEV and BEV. [7]
In spite of these advantages the benefits of HEVs are inexorably tied to the performance of the nickel-metal hydride (NiMeH) battery in the engine and its available storage capacity. Increasing the storage capacity of the battery increases both material costs and vehicle weight [2; 5].
BEVs, on the other hand, offer significantly increased storage capacity by using lithium-ion (Li-ion) batteries and a wholly electric design. When compared with ICEVs, the reduction in energy intensity can be up to 80% [6]; this increase can be associated with the consumption of significantly smaller amounts of fuel than both ICEVs and HEVs (see Table 1). While these factors make BEVs ideal for urban driving, they are limited by their short driving range [7].
Conventional ICEVs are able to offer flexibility in operating fuel: petrol, diesel or bio-fuels. Petrol ICEVs offer long driving ranges [7] and are competitive with hybrid vehicles for fuel consumption at speeds above 95 km hr-1 [4], while diesel ICEVs have lower fuel consumption (compared to petrol ICEVs). In addition, bio-fuels can be incorporated at blends of 5-10% with petrol. A disadvantage of the use of diesel fuel or bio-fuels (in mixes greater than 10%) in ICEVs is that they require alternative engine designs [8; 9]. However, the opportunity to use either diesel or bio-fuels provides the benefit of diversifying fuel supply and reducing dependence on petrol for conventional ICEVs.
3. Environmental Considerations
Figure 1 clearly represents levels of emissions for an efficient petrol and diesel ICEV (VW Golf), a HEV and a BEV.
Figure 1: Emissions of ICEVs (petrol and diesel), a HEV and a BEV. [10]
As shown in Fig 1, HEVs produce fewer greenhouse gas emissions in carbon dioxide (CO2) equivalent than either petrol or diesel ICEVs in aggregate [4], but are significantly higher than BEVs. In emissions associated with operation of the vehicle (tank-to-well, TTW) [11], HEVs perform better than petrol and diesel ICEVs, although they produce more emissions at the manufacturing stage. BEVs have a significant advantage over both ICEVs and HEVs in that they produce no TTW emissions [12].
The production of batteries for HEVs (NiMeH) and BEVs (Li-ion) constitutes a decided disadvantage for these vehicle types in terms of environmental impact. A cradle-to-grave life cycle analysis of manufacturing and disposal of each battery presented by Sullivan and Gaines [13] points out that both batteries are significantly higher in terms of environmental impact than lead-acid batteries used in conventional ICEVs ( NiMeH batteries =13.6 kg CO2/kg; Li-ion batteries =12.5 kg CO2/kg; while lead-acid batteries = 3.2 kg CO2/kg). Rantik, as cited by Granovskii et al. [14], asserts that the "production of 1 kg of NiMeH battery requires 1.96 MJ electricity and 8.35 MJ of liquid petroleum gas", highlighting the energy-intense production cost of such batteries for HEVs. In addition, the inadequate energy density and safety concerns for Li-ion batteries [15; 16] make the commercial availability of these unlikely. For both battery types, however, recycling measures exist to return valuable metals for later use and ease the burden of production [13; 17; 10].
The production of the electricity needed to charge batteries for BEVs is an emission issue (see Figure 1 [10]). These emissions are well-to-tank (WTT) emissions, defined by Unnasch and Pont [11] as emissions associated with "feedstock extraction, transport, storage, processing, distribution, transport and storage" of a fuel. For example if this electricity comes from the UK grid (see Table 2), BEVs produce more than twice the magnitude of CO2 emissions compared to petrol and diesel ICEVs [1]. For electricity generated from coal, BEVs would not perform as well as grid-independent HEVs [18]. If, however, the electricity is generated from a combined cycle gas plant or from renewable energies, BEVs produce emissions that are orders of magnitude cleaner than ICEVs and HEVs [19].
Table 2: Comparison of CO2 emissions for petrol and diesel ICEVs and grid-powered EVs. [1]
Petrol ICEVs produce significant amounts of GHGs such as CO2, CH4, NO2 and SF6 while also emitting partially vaporized volatile organic compounds and air pollutants like COX and NOx [20]. Although diesel ICEVs can offer reductions of up to 24-33% in CO2 emissions [21], as pointed out by Walker [8], the emissions contain significant amounts of "carbonaceous soot and a volatile organic fraction of hydrocarbons condensed on the soot" which have acute respiratory effects. Bio-fuels, while offering moderate CO2 savings in comparison to petrol, face competition in land-use for agricultural purposes [9].
4. Infrastructure
The impact of BEVs on infrastructure should also be considered. BEVs must recharge their batteries upon complete discharge of their energy from the capacitor, and if that energy is taken from the grid, serious hurdles to large-scale integration can occur. As Offer [22] illustrates, if the entire private fleet of vehicles in the UK were switched to BEVs, the equivalent of "roughly 100 TWh of electricity, or 35 GW average continuous during an 8 hour over charging period" would be required. Because of their low driving range [7], BEVs would likely require fast-charging stations in cities and along popular routes to prevent the vehicle from shutting down [6]. Solutions, such as the optimization of the efficient use of generation capacity and the initiation of 'smart' (that is, controlled) charging processes to allow for domestic charging (often at a slower rate) have been proposed [23,24].
5. Conclusion
In general, each vehicle technology offers advantages and disadvantages, although the most promising future lies with BEVs due to their lack of required petrol or diesel as fuels and concomitant absence of TTW emissions. However, for BEVs to proliferate, increased research needs to be done on how to commercially produce Li-ion batteries that are suitable for BEVs, as well as developing the necessary electricity infrastructure without overburdening the grid and increasing emissions associated with non-renewable, non-nuclear based electricity.
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