tirsdag 21. oktober 2014

Li-ion Batteries: On the verge of electrification of the car traffic


Group work at NorRen Summer School 2014
By Ahmet Oguz Tezel et al.


Given the rising concerns about the green-house-gas evolution and global warming, the shift from coal power to renewable energy production systems, such as wind and solar, and the shift from internal combustion engines (ICE) to electric vehicles (EV) should be considered necessity (Figure 1). Realization of these transitions, perhaps not widely known by public, relies on the development of advance energy storage systems. Li-ion batteries, which store the energy by mean of electrochemical activity, represent the forefront technology that responds to the demand on energy storage systems. Although receives competition from fuel cells, particularly in relation to EV applications, Li-ion batteries are predicted to dominate the market in the near future since fuel cells are not mature enough a technology and subject to further improvements to go with.


The battery technology has been continuously developed over the last decades, nearing to meeting the technological and practical requirements, although many challenges are yet to be overcome. In this project, we aim at evaluating the feasibility of the application of state of the art Li-ion batteries to EVs, exclusively to cars as the principle mode of passenger transport, by their practical performances in meeting the public needs. We prefer to avoid evaluating the car use as a whole and instead choose to speak in terms of its divisions ( i.e. CO2/km, driving range/day and average speed ). This approach enables us to identify the prevailing mode of car use and suggest a battery solution to the point. 

The assessment of EVs involves first the concerns as to the source of electricity that will be used to charge the battery followed by considerations of the battery performance in a specific application. For the latter, we find out how much CO2 is generated to produce per kWh of electricity from various power plants (Table 1). Then we propose two scenarios; first we assume that the batteries are charged solely with the energy provided by coal plants (the worst case scenario), second scenario is one in which the energy to charge the batteries is extracted from natural gas. Thereafter, we estimate the amount of electricity it will take to drive an average EV and ICE a kilometer (Table 2) and finally calculate the CO2 production/km from these different scenarios (Table 3). 

Table 1. CO2 emission (2011) per kWh of energy [2,3].

Power source
gCO2/kWh
Coal
880
Petroleum Diesel
266
Natural gas
390






Table 2. Energy consumption (a) and efficiency (b) of commercially available cars run on different engines [4, 5].

Model
kWh/km
Nissan Leaf
0.21
Chevy Volt
0.17
Average Diesel
0.6
Engine
% energy efficiency
Gasoline
20
Diesel
30
EV
70





An intriguing and encouraging finding is that the lower CO2 production/km with EVs is noticeable even if all the electricity used to charge the batteries was produced by natural gas plants (Table 3). It can be predicted that this will, possibly, be further improved with the renewables increasing their share in power grids.

Table 3. CO2 emission per km

Model
gCO2/km
Average EV (coal plant sourced)
238
  Average EV (Natural gas sourced)
105
Average Diesel
133




 
A second source of concern relates to the performance of EVs. This describes the most crucial problem when it comes to the ability of EVs to stand out in the market. In order to convince the consumer in favor of EVs, it should provide as long a driving range as well as high speed as the ICEs do today. These functionalities are defined by energy and power densities respectively. There is unfortunately a tradeoff between these quantities that leads us to narrow down our attention to most practiced driving mode. 

80% of the car traffic in EU is composed of cars carrying one passenger with driving ranges of less than 60 km/day, mainly at slow traffic [6]. We, thereby, collected the relevant data from the EV manufacturers and built a performance chart that demonstrates driving range, maximum speed and the cost of the state of art EVs available in the market (Table 4).  

Table 4. Performance chart of commercially available EVs (Li-ion)

Model
100 km/h  acceleration  time (s)
Max speed (km/h)
Range / charge (km)
Price  ($)
BMW-i3
7.2
150
160
41350
Nissan Leaf
11.5
140
160
28800
BYD e6
10
140
316
52000
Chevy spark EV
8
151
132
27495
Mitsubishi i
15
130
160
22995
Tesla Model S
5.6
200
502
71070

Being aware that there is no definite rule about the selection of one type of vehicle, our work is an attempt to illustrate to what extent the EVs are ready to meet the market demand for specific applications. We show that, with respect to major concerns such as CO2 generation and performance, the Li-ion battery technology is beyond where the public thinks it is. However, it should be noted that our study omits more advanced estimations including cost of installation of charge stations, cycle life of batteries and the impact of the elimination of tax revenues from gas on the national economies, which are very central to our topic. With these components included, the values tabulated in Table 3 may change to favor ICEs. 

Moreover, one should note that the performances given in Table 4 is subject to changes due to variations in the mode of driving. For instance, one should expect a decrease in the maximum range a car can run as the driving speed increases. The values in the table are provided by the supplier after prolonged tests conducted within standardized parameters.

References

IEA CO2 Emissions from Fuel Combustion Statistics, 2012.
IEA CO2 Emissions from Fuel Combustion Statistics, ISSN :1683-4291 (online), 2011.
Well-to-Wheels analysis of future automotive fuels and powertrains in the European context, European Commission Joint research Centre, Report, Version 2c, March 2007.
Newbery, D., `Green Emotion- The economics of electric vehicles`, E&E seminar, Cambridge, January, 2013.
U.K. Department of Transport, Statistics, Table ENV0103, December 2013.
Selliers, J.D., ‘European strategy on clean and energy-efficient vehicles’, European Association for Battery Electric Vehicles, European Commission public hearing, 2010.

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