Energy demand is growing, affordable and secure energy supply are fundamental to global economic growth and human development. The scenario described by “World Energy Outlook 2013” [1] (WEO 2013) together with the forecasts by “2014 World Energy Issues Monitor” [2] (2014 WEIM) presents large uncertainness about future and a dramatic increase in terms of energy demand, driven by non-OECD economic growth. Figure 1 shows historical data by WEO 2013 and Fig. 2 present provisional data by 2014 WEIM.
Future energy perspectives present diffused uncertainness related to the high volatility of energy prices, the lack of global agreement on climate change mitigation, the necessary demand for new energy infrastructures, too slow development of Carbon capture technologies, and the necessity of increasing energy efficiency.
It is evident that the provisions for the future are out of the sustainability of the planet, both in terms of destruction of resources and in terms of climate change, which directly related to the emission in terms of GHG.
Transport sector overview
Even if it is not the main contributor to the energy consumption, the transport sector will play a fundamental role for the future wellness of the humanity. In particular, energy use in the transportation sector includes energy consumed in moving people and goods by road, rail, air, water, and pipeline. Those transportation systems are essential in an increasingly globalized world, as well as for enhancing standards of living.
Trade and economic activity seem the most significant factors increasing demand for freight transportation. The factors that will affect the demand of passenger transportations appear much more complex and include uncertain parameters such as travel behavior, land use patterns, and urbanization. This increased complexity presents a larger uncertainness about—the effects of passenger transportation in terms of macroeconomic and fuel market impacts.
Any possible analysis of energetic impact of transport modes must necessary consider different modes and their energy efficiency to allow the definition of effective strategies to reduce the energy consumption, by adopting the two main decisional elements for the future. In particular, they are a short-term strategy based on a better planning of transport modes and on a long-term strategy based on substantial improvements of vehicles.
Any analysis about transport modes must considers two fundamental parameters they are speed and energy intensity. Increasing speed increases social efficiency and allows reducing costs for both public and private institutions and for citizens. On the other side, energy consumption causes economic, environmental and social costs.
An overview of scientific literature
The first fundamental attempt to analyze the relations between speed and energy consumption of different transport modes has been produced by Gabrielli and von Karman [3]. This analysis introduces a physical parameter, named specific resistance of vehicle ε, which is defined as the ratio of motor output power P
max divided by the product of total vehicle weight W by maximum speed V
max.
$$ \varepsilon = \frac{{P_{\hbox{max} } }}{{W \cdot V_{\hbox{max} } }} $$
(1)
It is fundamental to notice that Gabrielli and Von Karman consider the gross weight of the vehicle, because “exact information about the useful load of vehicles was not available to the authors.” They have clearly demonstrated that specific resistance has a minimum value, which applies to all the examined transport modes, which appears as a physical limit of all transport modes. It corresponds to the line of equation
$$ \varepsilon_{\hbox{min} } = A \cdot V_{\hbox{max} } , $$
(2)
where A = 0.000175 h/mile. The Gabrielli–von Karman limit line of vehicular performances depicts this relationship. It is the diagonal line indicated in Fig. 3.
Stamper [4] reconsidered Gabrielli–von Karman results in terms of ratio between payload weight and fuel consumption, introducing one of the future trends of transport energy efficiency in terms of payload of the different vehicles, without considering the vehicle as a part of the transported weight. Stamper has defined “useful transport work” by multiplying payload weight and distance traveled and “transport efficiency” as the ratio of useful transport work to thermal energy expended. This model is useful on a logistic point of view but losses any physical connection to the real nature of transport which is composed by two fundamental elements, the vehicle and the payload.
In a subsequent analysis, Teitler and Proodian [5] have categorized military vehicles and have considered a new characteristic dimension, which has named “specific fuel expenditure”, which can be defined as
$$ \varepsilon_{\text{F}} = \frac{\zeta }{{\eta \cdot W_{\text{P}} }} $$
(3)
where ζ is the energy per unit volume of fuel η is the distance traveled per unit volume of fuel, and W
P is the weight of the payload. A new variable has been introduced it is the reciprocal of ε
F has been defined as “fuel transport effectiveness”, which relates directly to the cruising speed of the vehicle V
C by a factor of proportionality C
F :
$$ \frac{1}{{\varepsilon_{\text{F}} }} = \frac{1}{{C_{\text{F}} \cdot V_{\text{C}} }} $$
(4)
This definition allows defining the factor of proportionality C
F as the “the next level of fuel transport effectiveness to be used as a future standard”, which is represented in Fig. 4, with the dashed diagonal line [6].
Referencing Gabrielli–von Karman [1] and Teitler and Proodian [3], Minetti [7], Young [8] and Hobson [9] have considered A or C
F as a factor describing an experiential performance limit and ε
−1F
or ε
F as a general performance parameter.
Radtke [10] has produced a further development of the above models. He observed that by combining speed and energy expenditure it could be obtained a novel performance parameter ε
F, which considers payload and energy needs under cruising conditions. Those considerations allow obtaining a new performance parameter Q
C obtained by treating the payload as a mass (denoted M
P) rather than a weight yields a performance parameter Q with units of time. For cruise conditions, Q
C has been obtained:
$$ Q_{\text{C}} = \frac{{g_{\text{o}} }}{{C_{\text{F}} }} = V_{\text{C}} \cdot M_{\text{P}} \cdot \frac{\eta }{\zeta } $$
(5)
Radtke has used certified data such as the EPA fuel economy ratings to represent how vehicles are actually used. In particular, he adopted the highway rating which is used to describe free flow traffic at highway speeds [11]. He has the produced an energetic analysis of different vehicles including aircrafts and electric vehicles.
Dewulf and Van Langenhove [12] have adopted a completely different approach based on an elementary exergetic analysis. They present an effective assessment of the sustainability of transport technologies in terms of resource productivity, based on the concept of material input per unit of service (MIPS). If MIPS evaluation is quantified in terms of the second law of thermodynamics, it is possible to calculate both resource input and service output in exergetic terms. It leads to the concept of EMIPS (acronym of Exergetic Material Input per Unit of Service) specifically defined for transport technology. It takes into account the total mass to be transported and the total distance, but also the mass per single transport and the speed, allowing an effective comparison between railway, truck, and passenger car transport.
Transport modes and vehicles has been then evaluated in terms of exergetic material input pro unit of service (EMIPS):
$$ R/S = \frac{{{\text{Ex}}_{\text{resources}} }}{{{\text{Ex}}_{\text{service}} }} = {\text{EMIPS}} $$
(6)
The amount of resources extracted from the ecosystem to provide the transport service has quantified defining an inventory of all exergetic resources in the whole life cycle.
The method allows evaluating cumulative exergy consumption also introducing an effective differentiation between non-renewable and renewable resource inputs according to Gong and Wall [13].
Dewulf has evaluated the exergy associated with the transport to overcome aerodynamic resistance, inertia effects and friction to bring a total mass (TM) in a number of transports (N) with a mass per single transport (MPST) within a delivery time (DT) over a total distance (TD). The physical requirement is the exergy to accelerate and to overcome friction. If one is able to define the exergy associated to this service, being a function of TM, MPST, DT, and TD, then the exergetic efficiency of transport technology can be determined:
$$ R/S = {\text{EMIPS}} = \frac{{{\text{Ex}}_{\text{resources}} }}{{{\text{Ex}}_{\text{service}} ( {\text{TM,MPST,DT,TD)}}}} $$
(7)
Dewulf takes into account two types of dissipations: E
kin, kinetic energy, and E
D to overcome the aerodynamic drag.:
$$ E_{\text{d}} = E_{\text{kin}} + E_{\text{D}} $$
where the kinetic energy depends on the maximum speed v
max during the trajectory:v = v
max if v ≠ 0 and dv/dt = 0.
$$ E_{\text{kin}} = \frac{1}{2}m \cdot v_{ \hbox{max} }^{ 2} $$
On the other hand, for a given shape vehicle aerodynamic resistance causes an energetic loss
$$ E_{\text{D}} = \int\limits_{ 0}^{{t_{\text{tot}} }} {\left( {\frac{1}{2} \cdot C_{\text{D}} \cdot \rho \cdot A \cdot v^{ 2} } \right)} \cdot v \cdot dt $$
where C
D is the drag coefficient, A is the cross section, and ρ is the density of air. It can be observed that high speed is very unfavorable, because the energy losses due to aerodynamic resistance relates to v
3. Wind direction has been reasonably neglected assuming that it varies casually with an almost uniform distribution and that the number of transports inwind is the same as the ones upwind.
The final expression of the exergy service has been expressed as:
$$ {\text{Ex}}_{\text{service}} {\,=\,}\frac{\text{TM}}{\text{MPST}}\left( {\frac{ 1}{ 2}{\text{MPST}}\frac{{{\text{TD}}^{ 2} }}{{{\text{DT}}^{ 2} }} + \frac{1}{2}C_{\text{D}} \rho A\frac{{{\text{TD}}^{ 3} }}{{{\text{DT}}^{ 2} }}} \right) $$
(8)
Chester and others [13–15] have studied the environmental life cycle assessment (LCA) of transportation systems. They create a framework for assessing the energy use and resulting environmental impacts of passenger and freight mobility, comparing the equivalent energy or environmental effects of different technologies or fuels. They have produced an effective LCA framework for the assessment of transportation systems, which includes vehicle technologies, engine technologies, fuel/energy pathways, infrastructure, and supply chains. This research has been focused on developing a suitable LCA framework for policies and decisions. In particular, different energetic consumption has been evaluated all over the whole product lifecycle. Figure 5 shows a sample of the analysis, which can be produced by applying Chester methodology [15].
Objectives
This research, aims to produce a robust model, which can allow comparing different transport modes and overcome the limits of preceding research.
It aims to define an effective model with a set of fundamental goals. In particular, it aims defining an effective and robust model, which takes into account the complexity of the energetic factors related to transport.
Referring to preceding literature, it aims to overcome the generality of the Gabrielli–von Karman analysis [3], but it aims to consider the vehicle as a whole, such as they do. They miss an effective evaluation of the energy necessary for moving the vehicle itself and the energy necessary for moving the payload.
The proposed analysis is fundamental for understanding future directions of vehicle improvement. It aims to overcome the analysis by the author, influenced by logistical issues, which refers the energy consumption to the payload [4–10]. It also aims improving both Dewulf exergetic analyses by considering a more analytical differentiation of energy dissipations during service. It appears clear that Dewulf model misses an evaluation of rolling dissipation, which are not negligible and could not be merged with aerodynamic drag, because of a completely different nature and physical law.
Even if it moves in the direction traced by Chester [13–15], it aims to consider also the necessary amount of energy for dismantling and recycling the materials of the vehicle, opening the road to a better LCA management.
Comparing Dewulf and Chester results, which are completely compatible it appears evident the differences between exergetic and energetic analysis, even if both evidences the dominant contributions to energy consumption and GHG emissions for on-road and air modes are from components that relate directly to transport operations.