1

SEF01: Energy Systems Technology

Whole-Chain Energy Efficiency Assessment for Three Different

Methods of Lighting an Automotive Headlight T. Hurst, S. Lioutas, D. Papadopoulos. November 2010.

1. Abstract The aim of this project was to assess the efficiencies of different supply chains in providing

illumination for an automotive low-beam headlight. Three energy supply methods were chosen; a

battery electric vehicle; a car running a traditional diesel internal combustion engine; a hydrogen fuel

cell vehicle. Comparison of the whole chains leads to the conclusion that a fuel cell vehicle using Xenon

HID bulbs has the best overall efficiency at 30%, with the IC vehicle coming in last with 7% for Xenon.

2. Introduction: setting up the system

To effectively normalise these different routes up to the point of power delivery to the headlight, they

are given an equal starting point by using natural gas as the base fuel source. The three routes will be

analysed separately up to the point of delivering the energy to light the bulb, thereafter three different

illumination methods will be assessed, before the efficiencies of the whole chains are quantified. A

schematic overview of the stages involved in getting power to the bulb is shown in Figure 1.

The aim is to compare the separate routes, so the energy losses in producing the base fuel are briefly

quantified according to the extensive research of Hekkert et al (2005) as 96%, 96% and 99% (total

92%) for extraction, production and distribution respectively. It is assumed that the natural gas has

been delivered to a seaport somewhere 300 km from the city where the vehicles are to be used. Sited

at this fictional port are also the power station and refineries necessary to convert the gas into the

necessary energy carrier for each chain.

To avoid the results becoming trivial, it must be recognised that powering the lights is not considered

to be the main purpose of the engines. Had this been the case, then the energy needed to move the

vehicles would be considered as a loss, leaving the efficiencies of the power-to-bulb stages at around

5% or less. This implies, for instance, that the internal combustion energy is only generating the power

demanded by the systems involved in generating light.

Electricity

Generation

The Internal Combustion Engine Energy Vector

The Hydrogen Fuel Cell Energy Vector

Natural

Gas

Hydrogen

Production

Hydrogen

Compression

Hydrogen

Distribution

Hydrogen

Delivery

Energy

Conversion

Conditioning

Electronics

Running the

Fuel Cell

Battery Roundtrip

Efficiency

Power

Available at

Headlight

Diesel

Production

Diesel

Distribution

IC Engine

Efficiency

Alternator

Efficiency

Battery Efficiency

The Battery Electric Vehicle Energy Vector

Electricity

Transmission

Electricity

Distribution

Battery

Efficiency

Battery

Charger

Efficiency

Figure 1. Three different routes to powering an automobile headlight from natural gas.

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3. The Battery Electric Vehicle Chain

The first set of whole-chain efficiency calculations are based on a battery electric vehicle as the end

user. The assumed battery electric vehicle in this case runs on a Li-ion battery pack. For this scenario

the Tesla Roadster battery electric car is assumed which runs on a Li-ion battery of nominal voltage

375V.

3.1 Electricity Generation

The efficiency with which natural gas is converted to electricity is the first to be considered in

the chain.

The electricity needed to supply an EV-battery car, is generated by a typical Combined Cycle Gas

Turbine power station, which operates in the UK. Specifically The Rye House, which is

considered for this work is a 715MW power station (Smith & Sharpe 1995) where electricity is

generated both from gas and low pressure steam turbines. The total efficiency of the station

comes as a result of the energy losses that occur in the energy flow through each process.

The most important losses occur in the steam condensing system of the plant, where the low

pressure turbine’s exhaust steam is cooled to liquid. More precisely, the losses for the plant

occur in (i) the heat recovery boilers module because of radiation and mechanical losses (0.4%)

and stack losses (10.9%), (ii) the Steam turboset module because of mechanical and electrical

losses (0.5%), auxiliary power (0.9%) and mainly condenser losses (35.9%) and finally (iii) in

the Gas turboset where mechanical and electrical (0.5%) and auxiliary losses (0.1%) occur. The

total losses described above contribute to a total electricity generation efficiency of 50.8%

(Smith & Sharpe 1995).

3.2 Transmission

The energy produced by the power station enters the transmission grid for delivery. The GB

transmission grid operates at three different voltage levels: 400kV, 275kV and 132kV. Most

power stations are directly connected to the system which consists of 25,000 circuit kilometers

and more than 1000 transformers. This is the case for the CCGT plant considered for the

electricity production. The generated voltage is 11kV from the gas turbine and 15.75kV from the

steam turbine (Smith & Sharpe 1995). These voltages are upconverted to 400kV by the

generator transformers, in order to adapt to the grid’s high voltage levels. The generated output

is supplied to the grid via underground cable to the grid at a point 0.5 km away.

The efficiency of the transmission system is determined by the losses that occur between the

power plant and the grid supply point GSP and are made up of (i) Fixed losses (0.49%); Corona

losses on outdoor transmission equipment and iron losses in transformers, (ii) Variable (I2R)

losses because of transmission heating losses in the overhead lines, underground cables etc

(1.8%), heating losses in grid supply transformers to low voltage level (0.19%), generator

transformer losses (0.22%). The total efficiency as a result of the losses across the transmission

grid is 97.23%. The losses presented above are calculated from the data available for the power

losses (in MW) over the peak demand estimation for the year 2010-2011 (National Grid, 2010,

nationalgrid.com)

3.3 Distribution

After the electricity has been transmitted to the Grid’s Supply Point and downconverted to

132kV, it enters the distribution network. The distribution network delivers electricity to the

end user at lower voltage levels and is the final stage of the electricity transfer from the plant to

the final application. The distribution network in UK operates at 33kV, 11kV and 400V, with

Na

tura

l

Ga

s

Electricity Gen

eratio

n

50

.8%

Electricity Tran

smissio

n

97

.23%

Electricity Distrib

utio

n

94

.8%

Ba

ttery Ch

arg

er

80%

3

conversions between voltage levels introducing additional losses. More precisely as a result of

line losses and transformer load and non-load losses in 33-132kV and 6.6-11kV and line losses

in 400V level, the total losses are 4.47%. This calculation was carried out by ATEN &FERRIS

(2009) for E.on Central Networks, a distributor network operator. Mazza & Hammerschlag

(2005) include a 6% losses in their Wind to Wheel Energy Assesment. For this work an average

loss of 5.2% is used. According to this, the total efficiency of the distribution network is 94.8%

3.4 Battery Charger

After the energy in the form of electricity has been transmitted through the grid and been

distributed for domestic use, it has to be stored to the battery of the Electric Car. The Tesla

roadster is charged for this case with the “High Power Wall Connector” produced by Tesla

Motors. The use of this instrument introduces losses, part as a result of the conditioning (AC-DC)

electronics included. The car can draw 80% of the charger’s amperage (Tesla, 2010,

teslamotors.com).

3.5 Battery efficiency

Tesla Roadster runs on a 375V Li-ion battery which can store 53kWh of energy and deliver a power of

200kW. The battery pack is made of 6800 Li-ion battery cells (Berdichevsky, Ketly, Straubel & Toomre

2006)

The total efficiency of the battery is analyzed in two specific efficiencies. Coulombic efficiency and

Energy efficiency. Coulombic efficiency is a result of very small side reaction currents and is near

100%, so no losses are introduced. Energy efficiency is defined as the total stored energy in the battery

that can be measured as electrical energy. Because of losses as a result of heat production through the

impedances, energy efficiency is always under 100% (Valoen & Shoesmith 2007). The energy

efficiency depends on the rate of the current flow during charge and discharge procedure.

The charging and discharging efficiency for a Li-ion battery is according to Valoen & Shoesmith’s

(2007) figures 92% and 93% respectively. Mazza & Hammerschlag (2005) assume 5% charging losses

and 7% self discharging losses for a Li-ion battery. Kennedy, Patterson & Camilleri (2000) give a 95%

for charge-discharge efficiency while Helms, Pehnt, Lambrecht & Liebich assume 90% charging

efficiency and 95% battery losses. An average battery energy efficiency of 87% is assumed for the

battery pack used in this scenario.

The total efficiency of the BEV to the point of delivery is ηtot = ηCCGT * ηTRANS * ηDIST* ηCHRG * ηBAT

Process stage Efficiency (%)

Electricity Generation from CCGT plant 51

Transmission 97

Distribution 95

Battery Charger 80

Battery roundtrip efficiency 87

Total efficiency with intermediate battery 33

Table 1. Process and total efficiencies for the BEV chain up to the point of delivery

Ba

ttery Efficiency

87

%

Po

wer

Ava

ilab

le

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4. The ICE vehicle

The internal combustion engine (ICE) is a mature and reliable technology that has been developed over the course of 100 years and still stands as the most common option for the propulsion of a vehicle worldwide. The need for more efficient use of fossil fuels in transportation has led to a turn from gasoline engines, whose thermal efficiency rarely surpasses 25%, to diesel-engine vehicles which have thermal efficiencies higher than 35% (Crouse & Anglin, 1994, p.111). According to the European Automobile Manufacturers Association (ACEA, 2008), diesel-powered cars accounted for 53.3% of total new car registrations in Western Europe in 2007, up from 13.8% in 1990. In our analysis we assume that the vehicle is powered by a diesel engine which burns diesel that is derived from natural gas.

4.1 Diesel production

The basic technology used for diesel production from natural gas is the Fischer - Tropsch process. The process, which was first developed by Franz Fischer and Hans Tropsch in the 1920’s, includes a set of chemical reactions in the presence of a catalyst (Gill et al, 2010).

According to Stodolsky et al (1999) as cited by Hekkert et al (2005) the most probable overall efficiency of the F-T process is 65%. The F-T process gives a diesel fuel with higher cetane number and lower aromatic level than conventional diesel, almost zero sulfur content which can reduce particulate matter emissions from the engine and about 94% of the volumetric energy content of conventional diesel, so the diesel engine does not need to be modified to operate on it (Y.Huang., L.Zhou & K.Pan , 2007).

4.2 Diesel distribution

Distribution of F-T diesel is equal to distribution of oil-derived diesel (Hekkert et al, 2005), so we assume an efficiency of 99% (Wang, 1999).

4.3 Internal Combustion Engine (Diesel)

Most of the available energy in the fuel is converted to heat losses and other mechanical losses rather than work in an internal combustion engine. The best diesel IC engines now have an overall efficiency of 40% to 45% (IMechE, 2009) compared to only 30%, 15 years ago as a result of many different advanced techniques applied by engineers like turbocharging and common rail direct injection. It is worth mentioning that the engine’s efficiency does not have a constant value but highly depends on several factors such as load, engine speed and torque. It is a fact that during driving cycles where the engine operates mainly in part loads, the efficiency is lower. According to Achten et al (2008), a vehicle (Volkswagen Passat sedan) equipped with a 100kW diesel engine during the NEDC (New European Driving Cycle), which is used for defining the specific fuel consumption (in litres per 100 km) and emissions of light – duty vehicles (Dieselnet, 2010), runs in poor efficiency areas most of the time. During 80% of the time the engine needs to deliver less than 10 kW resulting in an average engine efficiency of 18% (INNAS, 2010).

The mechanical power that is produced from the engine splits up in two directions: one part goes to the mechanical driveline for vehicle propulsion, whereas the other part goes to the alternator. At steady state conditions, an average of 5 to 10 percent of the total energy in the fuel is consumed by the drivetrain and accessories like power steering, air conditioning and the alternator (Howey, North & Martinez-Botas, 2010). As explained in the introduction, we will not include this efficiency in the chain.

4.4 Electrical Systems

Electrical power supply systems of present vehicles are equipped with an alternator in combination with a 14V regulator, to maintain a constant voltage at the power net when the engine is running. In this way, the alternator supplies power continuously to all electric loads as well as the 12V battery, with the battery providing energy to the loads only during peak-power demands or when the engine is turned off (Nuijten et al, 2003).

Na

tura

l

Ga

s

Diesel P

rod

uctio

n (F-T)

65

%

Diesel D

istribu

tion

99

%

IC En

gin

e

18%

5

4.4.1 Alternator

The alternator or generator is used to power the vehicle’s electric system and to charge the vehicle’s battery, ensuring that the battery’s state of charge is maintained at an adequate level. The generation of power has an effect on fuel consumption of the vehicle and depends on the alternator’s efficiency. The maximum efficiency of a modern, air-cooled alternator is around 70% at full load. In automotive applications, the alternator mainly operates in the part-load range, in which an efficiency of up to 75% is achieved (Meyer, 2007).

Bosch’s 14 Volt Efficiency Line generator has an efficiency range from 70 to 77%, according to the VDA cycle, which is a German standard used to evaluate the performances of a generator in its operation area (A.Gimeno, G.Friedrich, 2008). Because the alternator operates most of the time in part load, we assume the lowest efficiency, thus 70%.

There are strong interrelationships between the battery, alternator, engine speed and engine load that constantly vary during a driving cycle so the efficiencies are just a subcategory of the system’s performance used to provide an indicative case for our study.

4.4.2 Battery (Lead – Acid)

Flooded Lead – Acid batteries are being widely used in car industry for many years. When the engine is running at very low speed or idling, the load demanded by the electrical equipment is greater than the current supplied by the alternator so the battery is discharged, providing the extra current. There is always an amount of energy lost during battery charging and discharging which is highly dependent on the battery’s state of charge (SOC), with higher charge efficiencies at low SOC (Stevens & Corey, 2002). For our study we assume a charging efficiency of the battery of 60% which is approximately the efficiency in the 85% and 90 % SOC range (Stevens & Corey, 2002). This number is just an approach, as no data for battery efficiency during driving cycle could be found which takes into serious account that in a vehicle the battery spends most of its working life in a SOC greater than 90% (Reasbeck & Smith, 1997).

The total efficiency of ICE vehicle to the point of delivery is ηtot = ηF-T * ηDIST * ηICE * ηALT ( * ηbat)

Process stage Efficiency (%)

F-T Diesel Production 65

Diesel Distribution 99

Internal Combustion Engine 18

Alternator Efficiency 70

Battery Efficiency 60

Total efficiency for direct powering of light via

the alternator

8

Total efficiency with intermediate battery 5

Table 2. Process and total efficiencies for the diesel fuel-cell chain up to the point of delivery

5. The Fuel Cell Electric Vehicle Chain

The third set of whole-chain efficiency calculations are based on a fuel cell electric vehicle as the end

user. There are a number of different fuel cells in production and development, but the assumed

vehicle here is the Ballard Mark 902 68kW PEM fuel cell stack that runs on high-quality (over 99%

purity) hydrogen, and is able to deliver electricity either straight to the headlight, or via intermediate

storage through a 15 kW Lithium-ion battery.

Altern

ato

r

70

%

Po

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Ava

ilab

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Ba

ttery Efficiency

60

%

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5.1 Hydrogen production

The first efficiency that needs to be considered is that covering the production of hydrogen from

the base energy source of natural gas.

In industry, the method of choice is via steam reforming (Spath & Mann, 2001). This can

increasingly be achieved at a range of scales, from cars with integrated reformers (HFCN, 2010)

up to full industrial systems. For this system, a mid to large, central hydrogen production facility

is assumed for its higher efficiencies (Hekkert et al, 2005).

When steam reforming hydrogen, a number of processes are performed on the natural gas

feedstock (NYSERDA, 2006): initial syngas production; water gas shift; feedstock and product

purification. While it is difficult to quantify the efficiencies of the individual steps, a number of

different sources (Haryanto et al, 2005; Hekkert et al, 2005; NYSERDA, 2006) agree on a range of

efficiencies for the overall hydrogen production process. The range of efficiencies is 65% - 78%,

so for the purpose of this exercise an average efficiency of 71% is assumed for the production of

hydrogen.

5.2 Hydrogen compression

For hydrogen to be efficiently transported, there are two available options: compression or

liquefaction (Hekkert et al, 2005). Analysis of available literature (Bossel, Eliasson & Taylor,

2005; Hekkert et al, 2005) shows that liquefaction (whilst it does of course produce a more

energy-dense fuel) is more energy intensive than compression, and suffers from such problems

as gas loss through boil-off during distribution and storage. For this reason, the more practical

option of compression is chosen. Bossel (2003), Hekkert et al (2005) and Wang (1999) agree on

a representative range of 88% – 94% for the compression process, so here an average efficiency

of 91% is taken.

5.3 Hydrogen distribution

Once compressed, the hydrogen must be distributed and delivered to the tank of the fuel cell

vehicle. It is assumed that the hydrogen is delivered by pipeline to local delivery points, and will

require some intermediate compression. Based on the calculations of Bossel, Eliasson and Taylor

(2005), which give losses of 0.77% per 100m of pipeline, the total loss incurred is 2.31%. This

results in an efficiency for distribution of 97% (conservatively rounding down).

5.4 Hydrogen delivery

Whilst it is straightforward to transfer liquids between two containers by gravity, compressed

hydrogen is still gaseous, which brings complications, particularly temperature changes due to

decompression. The fuel must be transferred from a large storage tank to the vehicle’s smaller,

but higher pressure tank, so will require some compression work from a pump. According to

Bossel, Eliasson and Taylor (2005), losses incurred when transferring hydrogen from a large

tank at 10 MPa to a vehicle tank at 35 MPa (realistic, as this is the pressure used by the

Mitsubishi FCV (Mitsubishi 2010)) are at least 3%, implying a process efficiency of 97%.

5.5 Energy Conversion

It is now necessary to convert the stored hydrogen in the tank into electricity. The conversion of

gaseous hydrogen and oxygen into water is the opposite reaction to the electrolysis of water,

and according to Harrison et al (2010) will yield an available voltage of 1.229 V (calculated

based on Gibbs free energy changed into a thermodynamic voltage). The voltage efficiency of a

fuel cell can be calculated as:

Voltage Efficiency = Operating Voltage / Thermodynamic Voltage (Harrison et al, 2010)

Na

tura

l

Ga

s

Hyd

rog

en P

rod

uctio

n

71

%

Hyd

rog

en C

om

pressio

n

91

%

Hyd

rog

en D

istribu

tion

97

%

Hyd

rog

en D

elivery

97%

Energ

y Co

nversio

n

61%

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According to Bossel (2003), polarization losses and ohmic resistances mean that a fuel cell with

a common design voltage of 0.7 Volts may experience a range of voltages depending on load

conditions, so a mean voltage of 0.75 Volts is a good value to work with. Using the above

equation gives a voltage efficiency of ~61%. This is the efficiency for an individual cell; a stack

will likely have a slightly lower overall efficiency due to imperfections, small parasitic losses and

unwanted heat generation.

5.6 Conditioning electronics

According to Kreutz & Ogden (2000), a fuel cell stack able to produce a voltage in the range of

350 – 600 Volts will not require DC-AC or DC-DC voltage up-conversion. The Ballard Mark 902

automotive fuel cell stack has found use in a number of fuel cell vehicles (Fuell Cell Today, 2002;

Mitsubishi 2010; HFCN 2002), and is able to supply between 250 and 450 Volts. It is therefore

assumed to require no or minimal voltage / power conditioning, so the efficiency of this stage is

100%.

5.7 Running the fuel cell

Not all power produced by the fuel cell is used to deliver the user’s desired functions. Some

power must be used just to keep the fuel cell running, by powering pumps and heaters, for

example. Bossel (2003) puts a figure of 10% on the losses caused by these systems, resulting in

90% of the power output by the fuel cell being available for use.

5.8 Battery charging / discharging

As mentioned previously, it is assumed that depending on the stage of operation, the headlight is

either powered directly by the fuel cell, or by electricity from the battery’s intermediate storage.

A value is therefore needed for the efficiency of charging and discharging the battery. The

investigations into duty-cycle eccentricity of HEV battery packs carried out by Valøen and

Shoesmith (2007) give a roundtrip efficiency range of approximately 80% - 92% for a Lithium-

ion cell. The average value of 86% will be used for this stage. It should be noted that a Li-ion

cell’s charge and discharge efficiency is much less dependent on its state of charge than, say, a

NiMH or NiCd cell (Valøen & Shoesmith, 2007), so the use of a single number to represent round-

trip efficiency is a fairer representation.

5.9 Power available to the headlight

After all these intermediate stages, power is finally available to be delivered to the car headlight.

Table 3 summarises the efficiencies and shows the calculated total efficiency of the fuel cell

chain up to the point of delivery to the headlight.

The efficiency of FCEV to the point of delivery is ηtot = ηPRD * ηCOM * ηDST* ηDLV * ηCNV * ηCND * ηRUN * (ηBAT)

Process stage Efficiency (%)

Hydrogen production from NG 71

Hydrogen compression 91

Hydrogen distribution 97

Hydrogen delivery 97

Energy conversion 61

Conditioning electronics 100

Co

nd

ition

ing

Electron

ics

10

0%

Ru

nn

ing

the Fu

el Cell

90

%

Ba

ttery Ro

un

dtrip

Efficiency

86

%

Po

wer

Ava

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8

Running the fuel cell 90

Battery roundtrip efficiency 86

Total efficiency for direct powering of light 33

Total efficiency with intermediate battery 29

Table 3. Process and total efficiencies for the fuel-cell chain up to the point of delivery

6. Illumination

There is no set unit of absolute efficiency for illumination delivered by a lighting element, so for this

project, a relative value must be defined. An efficiency value is defined here normalised against the

headlight (not just bulb) able to emit 1000 lumens onto the road (high-end delivered output of dipped-

beam headlights (Ackerman, 2007)) with the lowest power input.

6.1 The LED headlight

Audi recently introduced the full-LED headlight in its A8 and R8 models, and for a few years the Lexus

LS600h has been using an LED low-beam headlight. Previously LED’s were only available as indicators

and daytime running lights, advances by Audi, Philips Lighting, Automotive Lighting and Koito

(COOLON, 2010) have led to systems that are competitive in terms of output and consumption with

current HID lighting.

LED headlight technology had suffered from overheating limiting luminous output, but these issues

have been addressed, and R8 headlights now boast a luminous output of 70 lumens per watt (L/w)

(Ackerman, 2007). A contributor to this improved rating is the higher optical efficiency of an LED

relative to an incandescent bulb; Ackerman (2007) claims an LED light source has an optical efficiency

of 45% compared to 33%, so a lower total output is required for the headlight to deliver a given

illumination on the road. This will go some way to offset the heat losses. The Audi lighting solution

(Ackerman, 2007) uses fans to cool the chips, and their power consumption is not available. Koito,

however, use heat fins and heat pipes (Yagi et al, 2006) to regulate temperature without extra energy

input, and their 5 LEDs draw 700 mA each. Based on a nominal 12V battery supply, this implies a

power consumption of 42W for each headlight. An optical efficiency of 45% means a 2222 lumen

output will yield 1000 lumens for the road, giving Koito low-beam LED headlights an efficiency of

(2222 / 42) 53 L/w.

6.2 The HID Xenon headlight

Xenon-based headlights are currently the lighting source of choice for the majority of car

manufacturers. A representative light is chosen as the Philips Xenstart D1S Xenon bulb, suitable for

use in low-beams, which has an output of 3200 lumens (at an optical efficiency of 33% this yields 1060

lumens) at 35 watts (XenonLighting, 2010). This translates to an efficiency of (3200 / 35) 91 L/w.

6.3 The halogen headlight

Halogen lights were for a long time the prevalent light source, until recently being overtaken by Xenon

sources. A representative bulb is chosen as the Sylvania 9006LL halogen bulb, suitable for low beam

applications, which has an output of 1000 lumens (Sylvania, 2010). Given the 33% optical efficiency, a

headlight comprising three of these bulbs will be required to deliver the target luminance of 1000

lumens to the road. Each bulb requires 55W, so an efficiency of (3000 / 165) 18 L/w is calculated.

9

Process Efficiency (%) Process Process

Electricity Generation 51 Diesel Production H Production

Transmission 97 Diesel Distribution H Compression

Distribution 95 Engine H Distribution

Battery Charger 80 Alternator H Delivery

Battery Roundtrip 87 Battery Conversion

Conditioning

Running

Battery

RoundtripLight Source Light Source Without Battery With Battery Light Source Without Battery With Battery

Xenon 29 Xenon 7 5 Xenon 30 26

LED 17 LED 5 3 LED 18 15

Halogen 5 Halogen 2 1 Halogen 6 5

TOTA

L

EFFI

CIE

NC

Y

(%)

Efficiency of Natural Gas Extraction, Production and Distribution: 92%

18

70

60

Battery Electric Vehicle Internal Combustion Powered Vehicle

86

90

100

61

97

97

91

71

Efficiency (%)

Fuel Cell Electric Vehicle

Efficiency (%)

65

99

6.4 Efficiencies

The Xenon HID light source has the best output at 91 L/w, so this value is normalised to an efficiency

of 100%. Based on this, efficiencies for the LED and halogen bulbs are calculated as 58% and 20%

respectively.

7. Whole-chain comparisons

Summarised in Table 4 are the calculated efficiencies for each stage of all three energy delivery

methods in their entirety. All three lighting methods are compared for each route, delivering three

final efficiencies. In the cases of the internal combustion vehicle and the fuel-cell electric vehicle an

extra three efficiencies are calculated to reflect the different operating modes, i.e. power directly from

the source or indirectly via the battery. Values are rounded to the nearest whole number to reflect the

uncertainties involved in generating them.

Table 4. Whole-chain efficiencies for all three energy vectors

Table 4 shows that the most efficient method for delivering light onto the road is via a fuel cell electric

vehicle utilising a Xenon HID headlight, at 30%. However, for drive cycles where power is being

delivered via the battery, the efficiency of the FCEV drops and the battery electric vehicle becomes the

more efficient option, with 29%.

Figure 2 allows for the comparison of the relative losses for each of the energy delivery routes,

excluding the efficiencies of natural gas extraction and delivery, and the final light sources, which are

common for all three cases.

It can be seen that for the battery electric vehicle, the largest source of losses is in electricity

generation; for the internal combustion engine the largest losses come from the engine itself; for the

FCEV most of the losses are incurred in generating electricity in the fuel cell.

Figure 2. Relative losses for each route

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8. Analysis and discussion

One of the key points to take note of is that in all three cases, the major source of loss is in the

conversion between two energy forms: converting the chemical energy of the fuel into either

mechanical or electrical energy.

Despite choosing some of the most efficient conversion methods and technologies available (i.e. CCGT

power stations and diesel IC engines), the overall efficiencies of the individual chains are disarmingly

low; when taking into account the production of the fuel and the actual lighting they are even more so,

for instance a total efficiency of 1% for an IC engine running halogen headlights.

The areas where the greatest impacts can be made from improving efficiency (i.e. the energy

conversion stages) are also the areas that are already at or close to the limits of practical efficiency.

Improving on these would likely require significant investment in materials with higher heat

tolerances, better durability and reduced weight.

In all three cases, the introduction of a battery charge and discharge stage reduces efficiency

(obviously this is unavoidable for the battery electric vehicle), accounting for between 13% and 21%

of total losses. This is quite significant and there are certainly some savings to be made by utilising

technologies such as regenerative breaking to reduce overall consumption. In the case of the fuel cell

vehicle, improving cold-start and power-ramping technology and reducing the time that a battery is

required has good potential for savings. The use of IC stop-start technology, whilst it would require

more power from the battery due to the engine being off more often, would have a considerable

impact on fuel economy and so overall process chain efficiency by eliminating idling losses (Achten et

al, 2008).

An obvious solution to improve the overall efficiency of the IC engine chain is to change the base fuel

source from natural gas to crude oil, as the efficiency of refining this into diesel is significantly higher

(Corbett & Winebrake, 2008). As mentioned in the introduction, however, natural gas was chosen to

allow for an even-footed comparison of all three chains. Despite the production of diesel deriving from

natural gas being less efficient than conventional sources, it could be a viable alternative fuel for

transportation taking into account the increased availability of natural gas.

It should be noted that all numbers derived are the best approximations possible with the data

available. Values should be taken as indicative, as it is difficult to represent the efficiencies for every

possible transient state during a driving cycle in a single number.

9. Conclusions

Based on the initial conditions defined in the introduction, it has been shown that the most efficient

way of powering an automotive headlight from a natural gas fuel source is via a hydrogen fuel cell

vehicle using a Xenon HID light source. The least efficient method is a diesel IC vehicle using a halogen

light source. A battery electric vehicle yields similar overall efficiencies to the FCEV, and overtakes

when the FCEV is using battery power.

While there is scope for improving the reliability of these conclusions, the values derived are a good

starting point, giving a fair overview of the losses and true inefficiencies involved in achieving a

desired energy output.

A particular benefit of this investigation is the ability to analyse the relative losses of the stages in the

different energy chains, so allowing for identification of areas where savings and improvements could

be implemented to maximum effect.

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9. References:

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