Sustainability Of The Energy Use In Transport Engineering Essay

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Sustainability of the energy use in transport is becoming increasing important due to the international concerns of energy security and climate change. Introducing fuels based on renewable energy to replace the oil use in the transport sector is one of the solutions to approach the sustainable transport development. Basically, three main options namely electricity, hydrogen and biofuel derived from biomass make it possible to introduce renewable energy into the transport sector.

Compared to liquid and gaseous fuels derived from biogas, electricity and hydrogen have the common strengths of a high level of flexibility in relation to primary energy sources and the possibility of selecting between several renewable energy sources [1]. The choice between the utilisation of either electricity or hydrogen is a complex process in terms of the maturity of technologies, diversified paths and massive investments in the correspondingly required infrastructure.

The review contributes to the evaluation process by analysing the characteristics of the technology options using electricity and hydrogen in the transport sector. Battery electric vehicle (BEV) and hydrogen fuel cell vehicle (HFCV) are two typical types of vehicles using electricity and hydrogen as on-board energy to power them. This review places the development of BEVs and HFCVs in the surroundings of energy systems and focuses on the well-to-wheels energy efficiency of those two types of vehicles.

The review starts with the overall working principle of BEVs and HFCVs, followed by the analyses of well-to-wheels energy efficiency and infrastructural requirements.

Overview of battery electric vehicles

Working principle

A battery electric vehicle (BEV) is the simplest type of EVs from a conceptual perspective, using electrical power from a single source, the electrochemical battery, to power one or more electric motors [2-5]. The source of power stems from the chemical energy stored in battery packs which are rechargeable from the electricity grid. The powertrain of BEVs is different from the one of traditional internal combustion engine vehicles (ICEVs).

Table 2. A comparison of major components of an ICEV and a BEV [6]

Functions of the components

Major components of a gasoline vehicle

Major components of a BEV

Stores the energy to run the vehicle

Gasoline tank

Battery

Replaces the energy to run the vehicle

Gasoline pump station

Charger

Provides the force to move the vehicle

Gasoline engine

Electric motor

Controls acceleration and speed

Fuel injection system

Controller

Provides power to accessories

Generator/Alternator

DC/DC converter

Converts DC to AC to power AC motor

Not needed

DC/AC converter

Lowers the toxicity of exhaust gases

Emission controls

Not needed

Table 2-1 shows the comparison between the key components of an ICEV and a BEV. In a BEV, batteries function similarly to the fuel tank in an ICEV, storing energy until it is needed. The flow of energy is regulated by a controller, which provides electrical energy to the motor at the required rate. In many BEVs, there is no transmission because the rotary motion of the motor can be applied directly to the differential gears [6]. BEVs can also reclaim or capture energy through regenerative braking, and most accessories are driven by batteries instead of the motor.

Hybrid electric vehicle (HEV) is characterized by having both electric motors and internal combustion engines in its power system [6]. Plug-in hybrid electric vehicle (PHEV) is a HEV which can be recharged from the grid and be seen as an intermediate technology between BEVs and HEVs. This review mainly focuses on BEVs, however the discussion on the infrastructure requirements of BEVs in Chapter 6 is also applicable to PHEVs.

Specifications of BEVs

A series of BEVs were introduced by various automakers, from General Motors’ EV-1, Toyota’s RAV4-EV to the current Tesla Roadster and BYD e6. In the early 1990s, the dominating battery technology was lead-acid, though investigations on other chemistries were also conducted, such as sodium-sulfur and zinc-bromine [6]. By the mid-1990s the nickel-metal hydride (NiMH) battery emerged as a more attractive option for many EV applications due to its higher battery density than lead-acid and good power characteristics. Li-ion technology is becoming the main battery technology in EVs because of its superior performance and energy storage function. Table 2-2 lists the main specifications of selected BEVs.

Table 2. Specifications of selected BEVs [7]

Vehicle

Battery

Engine

Power (kW)

Range

(km)

Top speed

(km/h)

0-60mph

Initial price ($)

Toyota RAV4

Ni-MH/27kWh

-

160-190

126

18 sec

-

Tesla Roadster

Lithium-ion/53kWh

215

>320

200

3.7 -3.9 sec

109,000

BYD e6

Lithium-ion/48kWh

75

300

140

<8 sec

35,000

Nissan Leaf

Lithium-ion/24 kWh

80

175

145

9.9 sec

32,780

Th!nk City

Lithium-ion/23 kWh

34

160

110

9.66

-

Overview of hydrogen fuel cell vehicles

Working principle

Hydrogen fuel cell vehicles (HFCVs) use hydrogen as their onboard fuel for the motive power [8]. The powertrain of HFCVs converts the chemical energy of hydrogen to mechanical energy through the reaction of hydrogen and oxygen in a fuel cell [9]. Hydrogen can either be stored as it is on the vehicle, or in the form of methanol which is then converted into hydrogen and CO2. However, this adds to the complexity and produces CO2 and other pollutants [10]. This review focuses on the former method that hydrogen is stored on the vehicle. HFCVs also have electric energy storage for accelerating and capturing the braking energy, such as super-capacitors in Honda FCX V3 and batteries in Honda FCX Clarity.

A hybrid HFCV on the other hand is the hybrid between BEVs and HFCVs, because its batteries can be charged from the electric grid, as well as the hydrogen tank can be refilled at a hydrogen station [11]. Depending on the battery capacity, the hybrid HFCV can run solely base on electricity for a certain distance. Ford Edge HySeries Drive is a representative of hybrid HFCVs. The discussions with regard to the energy efficiency mainly refer to HFCVs rather than hybrid HFCVs; however, the discussions of the infrastructural requirements of HFCVs are also applicable to hybrid HFCVs.

Specifications of HFCVs

HFCVs are not as popular as BEVs in the current market. Honda FCX Clarity is the most famous HFCV in the world. It is also acknowledged as the most efficient HFCV in the current automobile industry [11]. Table 3-1 lists the specifications of Honda FCX Clarity.

Table 3. Specifications of Honda FCX Clarity [11]

Vehicle

Type

Power of electric motor (kW)

Range

(km)

Energy efficiency (Wh/km)

Energy efficiency

(km/kilo hydrogen)

Honda FCX Clarity

HFCV

100

386

360

96

Vehicle-to-Grid

The basic concept of vehicle-to-grid (V2G) power is that electric vehicles (EVs) are treated as electricity storage device, either providing or receiving power to/from the grid while they are parked. An EV can be a BEV, an HFCV, a PHEV or a hybrid HFCV. BEVs can be charged during the low power demand time and discharged during electricity rush hours. While, HFCVs generate the power through the reaction of hydrogen and oxygen in fuel cells and PHEVs or hybrid HFCVs function in either mode [12,13]. The central of the V2G concept is the integration of EVs aggregation into the grid so that EVs can make beneficial contributions as both a controllable load and a generation/storage device [14].

Each vehicle with the V2G technology must have three required elements [13,15]: 1) a connection to the grid for the electrical energy flow, 2) a control or logical connection necessary for communication with the grid operator, and 3) controls and metering on-board the vehicle. The energy that EVs can feed into the grid is limited by three independent factors [12]: 1) the current carrying capacity of the wires and other circuitry connecting the vehicle through the building to the grid, 2) the stored energy in the vehicle, divided by the time it is used, and 3) the rated maximum power of the vehicle’s power electronics.

The main benefits of V2G technology are [12,15]:

Profitable grid management, especially for ancillary services (A/S). A/S support the stable operation of power system, where an on-line generator is synchronized to the gird to stabilize the frequency and voltage, assuring the grid is ready to be increased/decreased instantly via automatic generation control and spinning reserves.

V2G is also envisioned as a solution to the intermittency issue of renewable energy, holding both backup and storage functions which are typical ways to cope with intermittency.

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