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Capacited Vehicle Routing Problem with CO2 Emission Minimization Considering Path Slopes

ABSTRACT

This work presents the application of a CO2 emission estimation function for cargo vehicles on a Capacited Vehicle Routing Problems (CVRP) setting, considering route’s slopes variation. Comparisons were established with functions minimizing fuel consumption and route length in a case study about selective collection of recyclable waste at Sorocaba, state of São Paulo, Brazil. Routes with lower emissions have been achieved without significantly increasing fuel consumption or distance traveled.

Keywords:
CO2 emission; vehicle routing problem; path slope

1 INTRODUCTION

Carbon dioxide (CO2) emissions from fossil fuel powered vehicles are an environmental problem because they contribute directly to the greenhouse gases (GHG) effect. Thus, minimization of such emissions can result in an overall decrease of pollution levels.

Green Vehicle Routing Problems (G-VRP) are VRP aimed for environmental problems such as fossil fuel emissions and consumption reduction or traffic jam prevention, to name but a few.

Specifically about GHG emissions,1010 J. Hickman, D. Hassel, R. Joumard, Z. Samaras & S. Sorenson. Methodology for calculating transport emissions and energy consumption. Technical report, Transportation Research Laboratory (1999). compiles data about the influence of driving patterns over emission factors for heavy duty vehicles. This methodology has been applied for VRPs in1111 M. Çimen & M. Soysal. Time-dependent green vehicle routing problem with stochastic vehicle speeds: An approximate dynamic programming algorithm. Transportation Research Part D: Transport and Environment, 54(2017), 82-98. doi: https://doi.org/10.1016/j.trd.2017.04.016. URL http://www.sciencedirect.com/science/article/pii/S1361920916305284.
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), (88 M.A. Figliozzi. The impacts of congestion on time-definitive urban freight distribution networks CO2 emission levels: Results from a case study in Portland, Oregon. Transportation Research Part C: Emerging Technologies, 19(5) (2011), 766-778. doi: https://doi.org/10.1016/j.trc.2010.11.002. URL http://www.sciencedirect.com/science/article/pii/S0968090X10001634. Freight Transportation and Logistics (selected papers from ODYSSEUS 2009 - the 4th International Workshop on Freight Transportation and Logistics).
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), (77 M. Figliozzi. Vehicle Routing Problem for Emissions Minimization. Transportation Research Record, 2197(1) (2010), 1-7. doi: 10.3141/2197-01. URL https://doi.org/10.3141/2197-01.
https://doi.org/10.3141/2197-01...
and1313 O. Jabali, T. Van Woensel & A. De Kok. Analysis of travel times and CO2 emissions in time-dependent vehicle routing. Production and Operations Management, 21(6) (2012), 1060-1074..

The aforementioned driving patterns are relaxed in the form of a Pollution-Routing Problem (PRP) as explored by22 T. Bektaş & G. Laporte. The Pollution-Routing Problem. Transportation Research Part B: Methodological, 45(8) (2011), 1232-1250. doi: https://doi.org/10.1016/j.trb.2011.02.004. URL http://www.sciencedirect.com/science/article/pii/S019126151100018X. Supply chain disruption and risk management.
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), (55 E. Demir, T. Bektaş & G. Laporte. The bi-objective Pollution-Routing Problem. European Journal of Operational Research, 232(3) (2014), 464-478. doi: https://doi.org/10.1016/j.ejor.2013.08.002. URL http://www.sciencedirect.com/science/article/pii/S0377221713006486.
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and44 E. Demir, T. Bektaş & G. Laporte. An adaptive large neighborhood search heuristic for the Pollution-Routing Problem. European Journal of Operational Research, 223(2) (2012), 346-359. doi: https://doi.org/10.1016/j.ejor.2012.06.044. URL http://www.sciencedirect.com/science/article/pii/S0377221712004997.
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. They assume that the instantaneous emission rate E(g/s) at the exhaust is directly related to fuel consumption rate F(g/s) through the relation E=δ1F+δ2 where δ1 and δ2 are GHC specific parameters. The exact same formulation is used by1717 L. Pradenas, B. Oportus & V. Parada. Mitigation of greenhouse gas emissions in vehicle routing problems with backhauling. Expert Systems with Applications, 40(8) (2013), 2985-2991. doi: https://doi.org/10.1016/j.eswa.2012.12.014. URL http://www.sciencedirect.com/science/article/pii/S0957417412012559.
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on a VRP with backhauling.

In2222 Y. Xiao & A. Konak. A simulating annealing algorithm to solve the green vehicle routing and scheduling problem with hierarchical objectives and weighted tardiness. Applied Soft Computing, 34(2015), 372-388. doi: https://doi.org/10.1016/j.asoc.2015.04.054. URL http://www.sciencedirect.com/science/article/pii/S1568494615002811.
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a bi-objective Green Vehicle Routing and Scheduling Problem (G-VRSP) is presented. The main objective is the minimization of CO2 emissions, depending on the types both of the vehicle and the fuel. The secondary objective is a penalty function accounting for delays at clients.

COPERT - Computer programme to calculate emissions from road transport1515 L. Ntziachristos & Z. Samaras. COPERT III Computer programme to calculate emissions from road transport - metodology and emission factors (version 2.1) (2000). URL URL https://www.eea.europa.eu/publications/Technical_report_No_49/at_download/file . Acesso em 31/07/2019.
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- is a software financed by the European Environment Agency, designed to estimate pollutant emission from road transport and urban traffic. The estimates are based on vehicle model, type of fuel, circulation area and speed limits. The fifth version of COPERT for Windows can be freely downloaded from https://copert.emisia.com/installing/.

In1919 G. Tavares, Z. Zsigraiova, V. Semiao & M. Carvalho. Optimisation of MSW collection routes for minimum fuel consumption using 3D GIS modelling. Waste Management, 29(3) (2009), 1176-1185. doi: https://doi.org/10.1016/j.wasman.2008.07.013.
https://doi.org/10.1016/j.wasman.2008.07...
and2020 G. Tavares, Z. Zsigraiova, V. Semiao & M.d.G. Carvalho. A case study of fuel savings through optimisation of MSW transportation routes. Management of Environmental Quality: An International Journal, 19(4) (2008), 444-454. a geoprocessing tool, ArcGIS 3D, was used to map geographical information along the routes, taking into account elevation. COPERT1515 L. Ntziachristos & Z. Samaras. COPERT III Computer programme to calculate emissions from road transport - metodology and emission factors (version 2.1) (2000). URL URL https://www.eea.europa.eu/publications/Technical_report_No_49/at_download/file . Acesso em 31/07/2019.
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was used to fit fuel consumption parameters. Both papers have shown that the use of geographical information (mainly elevation) can bring significant improvements in the solution when compared with “flat” models, where the effects of inclination is disregarded. Amongst all the works presented here, these two are the only ones making use of geographical information in their models. Unfortunately, ArcGIS 3D and its optimization extension ArcGIS Network Analyst are closed source commercial software, thus hiding the details about the VRP used.

The present work builds upon2121 E.M. Toro, J.F. Franco, M.G. Echeverri & F.G. Guimarães. A multi-objective model for the green capacitated location-routing problem considering environmental impact. Computers and Industrial Engineering, 110(2017), 114-125. doi: https://doi.org/10.1016/j.cie.2017.05.013. URL http://www.sciencedirect.com/science/article/pii/S0360835217302176.
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where the authors propose to minimize CO2 emissions associating them with the physical work resulting from the forces the vehicle is subjected to. Even though the authors include the slope in the equations, all results were obtained disregarding inclination (null slope). So, our contribution consists in effectively incorporating the slopes - and their corresponding effect onto CO2 emissions - into the objective function.

A case study regarding selective collection of recyclable waste in the city of Sorocaba, state of São Paulo, Brazil, provides the common ground to compare the proposed objective function with the ones presented by2323 Y. Xiao, Q. Zhao, I. Kaku & Y. Xu. Development of a fuel consumption optimization model for the capacitated vehicle routing problem. Computers and Operations Research, 39(7) (2012), 1419-1431. doi: https://doi.org/10.1016/j.cor.2011.08.013. URL http://www.sciencedirect.com/science/article/pii/S0305054811002450.
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and applied by1818 G.T. Santos, L.A.P. Cantão & R.F. Cantão. An ant colony system metaheuristics applied to a cooperative of recyclable materials of Sorocaba: a case of study. In V.N. Coelho, I.M. Coelho, T.A. de Oliveira & L.S. Ochi (editors), “Smart and Digital Cities”. Springer International Publishing (2019), p. 79-97., in addition to the classical distance (symmetrical distances from the origin to depot).

The mathematical model for the Capacited Vehicle Routing Problem (CVRP) is given by (1.1), as described in2323 Y. Xiao, Q. Zhao, I. Kaku & Y. Xu. Development of a fuel consumption optimization model for the capacitated vehicle routing problem. Computers and Operations Research, 39(7) (2012), 1419-1431. doi: https://doi.org/10.1016/j.cor.2011.08.013. URL http://www.sciencedirect.com/science/article/pii/S0305054811002450.
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and1212 S. Irnich, P. Toth & D. Vigo. The family of vehicle routing problems. In P. Toth & D. Vigo (editors), “Vehicle Routing: problems methods, and applications”. SIAM, second edition ed. (2014)., which in turn is an Integer Linear Programming Problem (ILPP).

Different options for the objective function (1.1a) will be examined in Section 2, so we opted to introduce the model with a generic one.

min i = 1 n j = 1 n c i j x i j (1.1a)

Subject to j = 0 i j n x i j = 1 i N * (1.1b)

j = 0 i j n x i j - j = 0 i j n x j i = 0 i N (1.1c)

j = 0 i j n y j i - j = 0 i j n y i j = D i i N * (1.1d)

y i j Q x i j , i , j N (1.1e)

j = 0 n x 0 j = | K | (1.1f)

x i j { 0,1 } i , j N * , (1.1g)

where

  • n is the number of nodes other than node 0, that represents the depot.

  • N={0,1,2,...,n} and N={1,2,...,n}.

  • xij is a binary variable evaluating to 1 if the vehicle goes from i to j, 0 otherwise.

  • yij is the truck load from i to j.

  • cij is the cost to go from i to j.

  • Di is the demand associated with node i.

  • Q is the maximum vehicle capacity.

  • ||K|| is the fleet size and also the number of routes.

Meaning of the equations in the model:

  • (1.1a) Minimization of the cost to carry out the routes.

  • (1.1b) Each customer can only be visited once.

  • (1.1c) Vehicles must enter and leave a node right away.

  • (1.1d) Represents the load increase after visiting a node. The added weight is equal to the demand of the visited node.

  • (1.1e) Loads must not exceed the vehicle capacity Q. Vehicles must return to the depot when they reach or are close to its maximum capacity.

  • (1.1f) Each vehicle in the fleet must follow one of the |K| routes, ensuring that no cycles are formed1212 S. Irnich, P. Toth & D. Vigo. The family of vehicle routing problems. In P. Toth & D. Vigo (editors), “Vehicle Routing: problems methods, and applications”. SIAM, second edition ed. (2014)..

  • (1.1g)xij is a binary variable, evaluating to 1 if the vehicle goes from i to j, 0 otherwise. Different objective functions will be plugged to the Problem (1.1), as presented below.

2 THE CVRP COST FUNCTIONS

Three cost functions for the CVRP were used, each one modeling a separate feature, namely the amount of CO2 emission considering the slope of the streets, fuel consumption (with no slope) and distance traveled by each vehicle.

2.1 Calculation of fuel consumption and total emission

Toro et al.2121 E.M. Toro, J.F. Franco, M.G. Echeverri & F.G. Guimarães. A multi-objective model for the green capacitated location-routing problem considering environmental impact. Computers and Industrial Engineering, 110(2017), 114-125. doi: https://doi.org/10.1016/j.cie.2017.05.013. URL http://www.sciencedirect.com/science/article/pii/S0360835217302176.
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present a Green Capacitated Location-Routing Problem (G-CLRP), where fuel consumption between nodes i and j is obtained based on the forces acting on the vehicle, as shown in Figure 1. Note that in the description of forces it is assumed that the vehicle is going up.

Figure 1
Forces acting on a truck moving upwards.

Defining

  • βij: slope of the path between i and j.

  • FM: force generated by the engine and transmitted to the tires of the vehicle.

  • mg: vehicle weight (mass×gravity).

  • N: normal force of the inclined plane on the vehicle.

  • vij: vehicle speed between i and j.

  • dij: distance between nodes i and j.

  • FR: forces opposing to the vehicle movement (friction, wind and internal).

Force balancing equations

Assuming the same constant speed along all sections of all routes (vij=v=constant,i,jN), the balance of forces is given as follows:

Σ F x = m a x F M - F R - m g sin β i j = 0 Σ F y = m a y N - m g cos β i j = 0

where

F R = F R , tires + F R , w + F R , i + m v 2 2 d i j

  • FR,tires represents the frictional force between tires and terrain that opposes the movement of the vehicle.

  • FR,w is the air resistance.

  • FR,i represents the equivalent force of the internal forces that oppose the movement of the vehicle.

  • mvij22dij is the force necessary for the vehicle to reach the permanent kinetic energy regime (constant speed).

  • m is the mass of the vehicle which is given by the mass of the empty vehicle m 0 plus the load t carried between nodes i and j(t ij ), that is, m=m0+tij.

By definition FR,tires=Nb, where b is a terrain-dependent constant1 1 According to https://www.ctborracha.com/borracha-sintese-historica/propriedades-das-borrachas-vulcanizadas/propriedades-tribologicas/ the coefficient of kinetic friction between the tire and the asphalt is b=0.72N. Accessed in 08/21/2019. . So,

F M = F R + m g sin β i j = F R , tires + F R , w + F R , i + m v 2 2 d i j + m g sin β i j = N b + F R , w + F R , i + m v 2 2 d i j + m g sin β i j = ( m g cos β i j ) b + F R , w + F R , i + m v 2 2 d i j + m g sin β i j .

Taking the magnitude of FM, we have

F M = ( m g cos β i j ) b + F R , w + F R , i + m v 2 2 d i j + m g sin β i j . (2.1)

The work Uij=FMdij from i to j is then given by:

U i j = [ ( m g cos β i j ) b + F R , w + F R , i + m v 2 2 d i j + m g sin β i j ] d i j = [ ( m 0 + t i j ) g b cos β i j + F R , w + F R , i + ( m 0 + t i j ) v 2 2 d i j + ( m 0 + t i j ) g sin β i j ] d i j = [ m 0 g ( b cos β i j + v i j 2 2 g d i j + sin β i j ) + F R , w + F R , i ] d i j + [ g ( b cos β i j + v i j 2 2 g d i j + sin β i j ) ] t i j d i j . (2.2)

Downward force balancing equation

What if the vehicle is going down? The forces FM and FR will be reversed. Consequently:

Σ F x = m a x F M - F R + m g sin β i j = 0 Σ F y = m a y N - m g cos β i j = 0

Similarly, we have:

U i j = [ m 0 g ( b cos β i j + v i j 2 2 g d i j - sin β i j ) + F R , w + F R , i ] d i j + [ g ( b cos β i j + v i j 2 2 g d i j - sin β i j ) ] t i j d i j .

General Case

Assuming constant speed, we get

U i j = α i j d i j + γ i j t i j d i j , (2.3)

where the constants αij depend on the mean slope between i and j, the unloaded vehicle weight, the energy to achieve the steady state speed, the resistance on the tires, the air resistance and the internal vehicle losses. Some of these quantities, in turn, depend on the speed of the vehicle. Constants γij depend on the slope of the path between i and j and the resistance of the tires. The work required between i and j has a component that is related to the unloaded vehicle, αij d ij and another component that is related to the carried load, that is, γij t ij d ij .

The work required for the vehicle to complete a route is given by the sum of the work of each arc. Associating it to a binary variable x ij , we have:

i , j V U i j = i , j V α i j d i j x i j + i , j V γ i j d i j t i j , (2.4)

where x ij is 1 if the arc between i and j is used, 0 otherwise. Note that it is possible to obtain the average slope of (i, j) thus allowing the estimation of αij and γij for each arc, and so the objective function is linear.

The amount of fuel used to perform the total work Σi,jVUij is obtained with a conversion factor E 1 (gallons/J). The emitted amount per unit of fuel is given by another conversion factor, E 2 (kg of CO2/gallon). These factors depend on the type of vehicle and the fuel used, see33 CETESB. Emissões veiculares no estado de São Paulo 2017. https://cetesb.sp.gov.br/veicular/relatorios-e-publicacoes/ (2017). Acesso em 21/08/2019.
https://cetesb.sp.gov.br/veicular/relato...
and66 M. do Meio Ambiente. Primeiro Inventário Nacional de Emissões Atmosféricas por Veículos Automotores Rodoviários - Relatório Final. http://antigo.antt.gov.br/index.php/content/view/5632/1Inventario_Nacional_de_Emissoes_Atmosfericas_por_Veiculos_Automotores_Rodoviarios.html. Acesso em 21/08/2019.
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. Finally, the total emission can be calculated as:

E 1 × E 2 × i , j V U i j = E × i , j V U i j . (2.5)

Some dimensional analysis shows that

gallon J E 1 × C O 2 gallon E 2 × J Σ U i j = C O 2 .

2.2 Fuel consumption rate-FCR

In2323 Y. Xiao, Q. Zhao, I. Kaku & Y. Xu. Development of a fuel consumption optimization model for the capacitated vehicle routing problem. Computers and Operations Research, 39(7) (2012), 1419-1431. doi: https://doi.org/10.1016/j.cor.2011.08.013. URL http://www.sciencedirect.com/science/article/pii/S0305054811002450.
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, an attempt is made to solve a capacitated vehicle routing problem using a function that estimates the fuel consumption rate (FCR), where the distance traveled per unit volume of fuel is inversely proportional to the vehicle weight. Since Q 0 corresponds to the weight of the empty vehicle and Q 1 to the transported load, FCR is formulated as a linear function depending on Q 1:

ρ ( Q 1 ) = α ( Q 0 + Q 1 ) + b , (2.6)

with b being constant. Let Q be the maximum capacity of the vehicle, ρ* and ρ0 as the fuel consumption rate for the full-loaded and empty truck, respectively. It can be seen from (2.6) that ρ(Q)=α(Q0+Q)+b and ρ0=αQ0+b. Then:

ρ * - ρ 0 = α ( Q 0 + Q ) + b - ( α Q 0 + b ) = α Q 0 + α Q + b - α Q 0 - b = α Q .

Isolating α, we have:

α = ρ * - ρ 0 Q (2.7)

which is the slope of the line given by Equation (2.6). Rewriting it:

ρ ( Q 1 ) = α Q 0 + b + α Q 1 = ρ 0 + α Q 1 = ρ 0 + ρ * - ρ 0 Q Q 1 (2.8)

Where ρ0=αQ0+bρ0=ρ*-ρ0QQ0+b is the empty truck weight.

For any arc between i and j, where j is the next customer to be served after leaving i, fuel cost is given by:

C f u e l i j = c 0 ρ i j d i j , (2.9)

where

  • c0 is the unit cost of fuel.

  • ρij FCR along the route from i to j.

  • dij distance traveled between i and j.

Denoting n as the number of customers on the route, we have:

C f u e l = i = 1 n j = 1 n C f u e l i j = i = 1 n j = 1 n c 0 ρ i j d i j x i j , (2.10)

where x ij is binary, assuming value 1 if the vehicle goes from node i to j, and 0 otherwise.

Denoting y ij as the load weight carried between i and j, Equation (2.8) becomes

ρ i j = ρ 0 + ρ * - ρ 0 Q y i j = ρ 0 + α y i j . (2.11)

Considering that vehicles have a fixed operational cost F and 0 represents the depot, the objective function becomes:

min j = 1 n F x 0 j + i = 0 n j = 0 n c 0 d i j x i j ( ρ 0 + α y i j ) , (2.12)

which is nonlinear. Constraint (1.1e), however, guarantees that yij=0 when xij=0 2323 Y. Xiao, Q. Zhao, I. Kaku & Y. Xu. Development of a fuel consumption optimization model for the capacitated vehicle routing problem. Computers and Operations Research, 39(7) (2012), 1419-1431. doi: https://doi.org/10.1016/j.cor.2011.08.013. URL http://www.sciencedirect.com/science/article/pii/S0305054811002450.
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, thus allowing us to rewrite (2.12) as

min j = 1 n F x 0 j + i = 0 n j = 0 n c 0 d i j ( ρ 0 x i j + α y i j ) . (2.13)

This function was used with a CVRP by1818 G.T. Santos, L.A.P. Cantão & R.F. Cantão. An ant colony system metaheuristics applied to a cooperative of recyclable materials of Sorocaba: a case of study. In V.N. Coelho, I.M. Coelho, T.A. de Oliveira & L.S. Ochi (editors), “Smart and Digital Cities”. Springer International Publishing (2019), p. 79-97. in a case study applied to selective recyclable waste collection.

3 COMPUTATIONAL TESTS

Tests were performed on an Intel Core i7-2600 desktop, with 3.4 GHz, 8.0 GB of RAM and Microsoft Windows 7 Home Premium operating system. Instances were solved with CPLEX version 23.7.

Data were taken from1818 G.T. Santos, L.A.P. Cantão & R.F. Cantão. An ant colony system metaheuristics applied to a cooperative of recyclable materials of Sorocaba: a case of study. In V.N. Coelho, I.M. Coelho, T.A. de Oliveira & L.S. Ochi (editors), “Smart and Digital Cities”. Springer International Publishing (2019), p. 79-97. which presents a CVRP for the garbage collection of a cooperative in the city of Sorocaba, state of São Paulo, Brazil. At each node, in addition to its geographical coordinates (latitude and longitude), we add its altitude obtained with Google Earth Pro software, version 7.3.2.5776, for Debian GNU/Linux operating system. Distances between locations were calculated using the Haversine distance

d i j = 2 6371 arcsin [ sin ( L a t j - L a t i 2 ) 2 + cos ( L a t i ) cos ( L a t j ) sin ( L o n j - L o n i 2 ) 2 ] 1 / 2 (3.1)

where Lat and Lon represent latitude and longitude, respectively11 M. Basyir, M. Nasir, Suryati & W. Mellyssa. Determination of Nearest Emergency Service Office using Haversine Formula Based on Android Platform. EMITTER International Journal of Engineering Technology, 5(2) (2017)..

Each instance was tested with the constraints presented in (1.1) and three different objective functions:

  • 1. Distance Σd ij x ij .

  • 2. FCR (2.13) without the first part of the function (fixed cost of the vehicle), with the same data as modeled in1818 G.T. Santos, L.A.P. Cantão & R.F. Cantão. An ant colony system metaheuristics applied to a cooperative of recyclable materials of Sorocaba: a case of study. In V.N. Coelho, I.M. Coelho, T.A. de Oliveira & L.S. Ochi (editors), “Smart and Digital Cities”. Springer International Publishing (2019), p. 79-97., that is,

ρ ( Q ) = 1 × 10 5 Q + 0.1111,

with an estimated fuel cost of R$ 2,999 per liter (R$: Brazilian Real).

Function (2.4)

In order to use (2.4), we need to determine the parameters of (2.2). We have chosen a IVECO Tector truck, 4×2, of 9 tonnes2 2 https://www.iveco.com/brasil/produtos/pages/tector\_carac\_bene.aspx .

As the F R, i value is not available in the literature and we are not able to properly define a value for it, we are assuming FR,i=0. For F R, w (aerodynamic drag coefficient), we have:

F R , w = 1 2 ρ C x A v 2

being:

  • ρ=1.184kg/m3, air density with a temperature around 25ºC.

  • Cx=0.9, aerodynamic coefficient for a truck.

  • A=2.491×1.890=4.70799m2, estimated crosssectional area for a 9-ton Tector truck.

  • v=20×(1000/3600)m/s, constant speed between nodes.

Other function parameters:

  • m0=3025kg, empty truck weight.

  • g=9.81m/s2, gravitational constant.

  • b=0.72N, friction of the tire with the asphalt.

The angle βij can be obtained with the help of Figure 2.

Figure 2
Right triangle used to calculate βij .

Given the distances we estimate the difference in height between two nodes:

Δ A l t i j = A l t j - A l t i ,

where Alt i and Alt j are the heights at nodes i and j, respectively, on the opposite side to βij . The adjacent side to βij is given by:

x i j = d i j 2 - Δ A l t i j 2 .

Finally,

β i j = arctan ( Δ A l t i j x i j ) .

For the Emission Factor, we assume E=694CO2(g/KWh) 3 3 https://cetesb.sp.gov.br/veicular/relatorios-e-publicacoes/ , for medium-sized trucks.

3.1 Validation problem

For model validation, five points were chosen in the city of Sorocaba, state of São Paulo, Brazil, corresponding to recyclable waste generators of reasonable size (three universities, one shopping mall and one residential building playing depot). All points have different heights, as can be seen in Table 1.

Table 1
Test data.

Inclinations can be seen on (3.2), where we assign index 0 (origin/depot) to the first row and first column.

[ 0 0.0122 0.0020 0.0066 0.0155 - 0.0122 0 0.0007 0.0055 0.0106 - 0.0020 - 0.0007 0 0.0034 0.0043 - 0.0066 - 0.0055 - 0.0034 0 - 0.0026 - 0.0155 - 0.0106 - 0.0043 0.0026 0 ] (3.2)

Results are shown in Tables 2, 3 and 4. For each objective function (minimization of CO2, FCR and minimum distance), the solution is evaluated in the other two.

Table 2
Result when minimizing CO2.

Table 3
Result when minimizing FCR.

Table 4
Results when minimizing distance.

Looking at Tables 2, 3 and 4, we note that two different routes were found for the three objective functions. Problems minimizing CO2 and fuel consumption (FCR) provided the same solution, while for the minimum distance problem we have a different route, with the same optimal value in distance for all cases. To better understand these results, Tables 5 and 6 show the cost of each arc for both routes, rounded to three decimals.

Table 5
Route 0-3-4-2-1-0.

Table 6
Route 0-1-2-4-3-0.

The distance traveled on both routes is equal because the arcs traversed are symmetrical, for example (0-3) and (3-0), on routes 1 and 2 respectively. However, when we calculate the arc cost (1-2) and (2-1) for the other functions, these have different costs, and consequently, the route on Table 5 minimizes the costs of the other functions discussed here. If we only consider distance tough, both routes provide a minimal cost.

Looking at Table 6 it can be seen that routes start with smaller distances and end with greater ones, making both the FCR cost and CO2 emissions higher, since they are proportional to truck weight. The route on Table 5 reverses the case: the longest path is traveled with less load.

In this small validation example, solutions for the FCR function and emission of CO2 resulted in the same path, due to the size of the problem; in other cases, as seen next, different solutions were obtained.

3.2 Case Study: CORESO Cooperative

CORESO cooperative collects recyclable materials from some selected neighborhoods in the city of Sorocaba, state of São Paulo, Brazil, predominantly on the eastern part of the city. In 2016, it covered 230 streets (nodes) from Monday to Friday, comprising around 9000 collection points (door-to-door collection and collective generators). The collection points are illustrated4 4 Figures 3 to 18 were made with a Python9 script created by the authors. The script uses the GeoPandas library14 with maps provided by OpenStreetMap16. in Figure 3.

Figure 3
Collection points for recyclable materials at Sorocaba, state of São Paulo, Brazil. In the upper picture the points and the city boundaries can be seen. In the lower picture, there is a zoom comprising all points.

Routes were separated by weekdays, as requested by the cooperative administrative board, and demand requires two daily routes, for trucks with a capacity of 4000 kg. Tables 7, 8 and 9 present in bold the results obtained with the three objective functions previously described (CO2, FCR and distance) respectively, and the number of nodes of each route, including the depot. The remaining columns show the values of the other two objective functions when calculated on the optimal route. The number of nodes sums up to 235 because some streets are visited more than once a week.

Table 7
Results for the cooperative when minimizing emissions of CO2. For the sake of comparison, numbers in italics represent the solution when all slopes are considered zero. Although seemingly small, there are differences in four of five days.

Table 8
Results for the cooperative when minimizing consumption of fuel (FCR).

Table 9
Results for the cooperative when minimizing distance.

Table 7 brings, for comparison purposes, the solution for the CO2 emissions minimization problem, when all slopes are made zero (numbers in italics). For Monday, Wednesday and Thursday results considering the inclination are smaller, and thus better, than the zero slope ones. Although counterintuitive, that is one of the main results of this work. Clearly, larger slopes must result in larger CO2 emissions, when sections of the route are taken individually. Equations (2.2) and (2.4) show that, when considering the whole route, inclination and weight (truck plus cargo) play together, resulting in sections with larger slopes coming first in the route, when the truck is almost empty, thus effectively reducing the emission. Tuesday reaches a match and Friday is a little worse.

Table 7 (minimization of emissions) shows that the net reduction on the CO2 emission is 4.793 kg per week, totaling almost 250 kg a year when slopes are taken into account. In the case of the fuel minimization function (results in Table 8), the decrease on the CO2 emission is smaller (4.502 kg per week, 234.104 kg a year) when compared with the minimization of emissions with zero slope, as expected.

Minimization of emissions (Table 7) with slopes in play and minimization of fuel consumption (Table 8) have very close weekly emissions - a difference of 0.291 kg per week - with an added bonus of an economy of roughly R$ 5.30 a year. Of course the decision is up to the managerial board, but an extra reduction of emissions seems to be worth this extra cost.

Distance minimization (Table 9) can short the routes in 419 m per week or 21 km a year, with an expressive increase in CO2 emissions (about 1350 kg a year). One can argue that shorter travels can extend the lifespan of the trucks and save time, but these savings are negligible: 21 km corresponds to only 0.67 % of the total distance traveled during a year.

To summarize, the minimization of CO2 emissions can prevent 250 kg of pollutant reaching the atmosphere a year, while increasing fuel costs and distance traveled by a negligible amount of R$ 5.30 and 21 km, also per year respectively, a very small price to pay for the reduction of greenhouse gases and thus the overall improvement of environmental conditions.

3.3 Results for Monday

Table 10 and Figures 4, 5 and 6 illustrate for Monday, results and the routes minimizing CO2 emissions, FCR cost and distance, respectively. Table A.1 presents the streets and their respective indices.

Figure 4
Routes minimizing CO2 emissions for Monday. 60 collection points are covered, totaling a distance of 17932.364 m, 104.993 kg of emitted CO2 and a FCR cost of R$ 6.694. Numbers in routes refer to Table A.1.

Figure 5
Routes minimizing FCR for Monday. 60 collection points are covered, totaling a distance of 17932.364 m, 104.993 kg of emitted CO2 and a FCR cost of R$ 6.644. Numbers in nodes refer to Table A.1.

Figure 6
Routes minimizing distance for Monday. 60 collection points are covered, totaling a distance of 17932.364 m, 108.493 kg of emitted CO2 and a FCR cost of R$ 6.721. Numbers in nodes refer to Table A.1.

Table 10
Summary of results for Monday. Numbers in bold are the minimum for that objective function.

As it can be seen, routes for the minimization of CO2 emissions and FCR cost are exactly the same, showing that inclinations probably were not big enough to make a difference. For distance minimization we have the same collection points, but in opposite directions, hence the same distance in all cases, but with an increase of 3.5 kg in emissions due to heavier loads close to the end of the route.

3.4 Results for Tuesday

Table 11 and Figures 7, 8 and 9 illustrate for Tuesday, results and routes minimizing CO2 emissions, FCR cost and distance, respectively. Table A.2 presents the streets and their respective indices.

Figure 7
Routes minimizing CO2 emissions for Tuesday. 49 collection points are covered, totaling a distance of 10576.397 m, 61.394 kg of emitted CO2 and a FCR cost of R$ 3.903. Numbers in nodes refer to Table A.2.

Figure 8
Routes minimizing FCR for Tuesday. 49 collection points are covered, totaling a distance of 10572.502 m, 61.407 kg of emitted CO2 and a FCR cost of R$ 3.903. Numbers in nodes refer to Table A.2.

Figure 9
Routes minimizing distance for Tuesday. 49 collection points are covered, totaling a distance of 10560.528 m, 69.564 kg of emitted CO2 and a FCR cost of R$ 4.069. Numbers in nodes refer to Table A.2.

Table 11
Summary of results for Tuesday. Numbers in bold are the minimum for that objective function.

For Tuesday results for emission and FCR minimization are very close, as well as their respective routes (only a few streets in different places along the route). On the other hand, distance minimization was able to cut off only 15.869 m with a corresponding increase of 8.17 kg of CO2, clearly showing that a few tweaks in the routes can have a big impact on the amount of emission, with minimal effect on the distance.

It is interesting to notice that on the distance minimization problem, there is a clear unbalance on the route size, as if the solution is built by trying to travel as much as possible in one route (possibly through shorter sections), leaving a few streets for the second.

3.5 Results for Wednesday

Table 12 and Figures 10, 11 and 12 illustrate for Wednesday, results and routes minimizing CO2 emissions, FCR cost and distance, respectively. Table A.3 presents the streets and their respective indices.

Figure 10
Routes minimizing CO2 emissions for Wednesday. 46 collection points are covered, totaling a distance of 9606.025 m, 54.400 kg of emitted CO2 and a FCR cost of R$ 3.516. Numbers in nodes refer to Table A.3.

Figure 11
Routes minimizing FCR for Wednesday. 46 collection points are covered, totaling a distance of 9568.617 m, 54.544 kg of emitted CO2 and a FCR cost of R$ 3.510. Numbers in nodes refer to Table A.3.

Figure 12
Routes minimizing distance for Wednesday. 46 collection points are covered, totaling a distance of 9490.884 m, 66.614 kg of emitted CO2 and a FCR cost of R$ 3.755. Numbers in nodes refer to Table A.3.

Table 12
Summary of results for Wednesday. Numbers in bold are the minimum for that objective function.

Once more minimization of emissions and FCR have very similar routes, with a negligible difference on the CO2 emission: only 0.144 kg. On the other hand, Wednesday has the biggest difference in emission when comparing with the distance minimization function, 12.214 kg, for a route 115.141 m shorter. Environmental-wise, it makes much more sense to choose the minimum emission route.

3.6 Results for Thursday

Table 13 and Figures 13, 14 and 15 illustrate for Thursday, results and routes minimizing CO2 emissions, FCR cost and distance, respectively. Table A.4 presents the streets and their respective indices.

Figure 13
Routes minimizing CO2 emissions for Thursday. 46 collection points are covered, totaling a distance of 9606.025 m, 54.400 kg of emitted CO2 and a FCR cost of R$ 3.516. Numbers in nodes refer to Table A.4.

Figure 14
Routes minimizing FCR for Thursday. 46 collection points are covered, totaling a distance of 9568.617 m, 54.544 kg of emitted CO2 and a FCR cost of R$ 3.510. Numbers in nodes refer to Table A.4.

Figure 15
Routes minimizing distance for Thursday. 46 collection points are covered, totaling a distance of 9490.884 m, 66.614 kg of emitted CO2 and a FCR cost of R$ 3.755. Numbers in nodes refer to Table A.4.

Table 13
Summary of results for Thursday. Numbers in bold are the minimum for that objective function.

Thursday is perhaps the most uniform day, with very close results for all objective functions, maybe because it has the smallest number of streets in the week. Despite that, emissions and FCR minimization have very different routes when compared to the previous days. Emissions and distance are not that different either, with a reduction of 1.981 kg of CO2 for an extra 163.344 m.

3.7 Results for Friday

Table 14 and Figures 16, 17 and 18 illustrate for Friday, the routes minimizing CO2 emissions, FCR cost and distance, respectively. Table A.5 presents the streets and their respective indices.

Figure 16
Routes minimizing CO2 emissions for Friday. 46 collection points are covered, totaling a distance of 9606.025 m, 54.400 kg of emitted CO2 and a FCR cost of R$ 3.516. Numbers in nodes refer to Table A.5.

Figure 17
Routes minimizing FCR for Friday. 46 collection points are covered, totaling a distance of 9568.617 m, 54.544 kg of emitted CO2 and a FCR cost of R$ 3.510. Numbers in nodes refer to Table A.5.

Figure 18
Routes minimizing distance for Friday. 46 collection points are covered, totaling a distance of 9490.884 m, 66.614 kg of emitted CO2 and a FCR cost of R$ 3.755. Numbers in nodes refer to Table A.5.

Table 14
Summary of results for Friday. Numbers in bold are the minimum for that objective function.

Finally, Friday also has very uniform results, with FCR and distance minimization giving the same routes, with little gain when compared to CO2 emission minimization (only 0.110 kg, the smallest amongst all days).

The biggest challenge was to implement and test Function (2.4). The inclusion of Function (2.13) and distance was used to compare results. We note that, with the routes obtained, the results are quite satisfactory in the sense that, even running paths with greater distances, still we have gained in the reduction of emission CO2 and fuel consumption, which benefits the environment in addition to generating a smaller workforce for the vehicle, increasing its useful life.

4 FINAL CONSIDERATIONS

In this work we have presented the application of a Capacited Vehicle Routing Problem (CVRP) for a recycling cooperative at Sorocaba, state of São Paulo, Brazil, comparing three different and independent objectives: minimization of either distance, or fuel consumption rate or CO2 emissions. For the latter the inclination of each section of the route is considered, which affects the emission rate.

The streets were previously chosen by the cooperative managerial board and grouped by business days. Two trucks perform the collection during the week simultaneously, meaning that for each objective two routes are obtained.

Results have shown that is possible, with minor changes in the routes already in practice by the cooperative, emit a considerable smaller amount of CO2 in the atmosphere in the course of a year, with negligible increase either in traveled distance or fuel cost. It should be stressed that these minor adjustments are more easily adopted by both the board and the workers because they are associated with smaller business logic distress.

Giving the competing nature of the objectives here addressed, CO2 emission, distance and fuel consumption rate, further research points to multiobjective formulations. A bi-objective problem - minimization of both CO2 and distance - is being prospectively addressed by the authors.

Investment in green routes is necessary since environmental problems caused by pollutant emissions from motor vehicles is one of the biggest factors of air pollution and consequently of climate change. With the increase in the fleet of vehicles, alternative fuels are increasingly becoming the focus of research. However, they cannot replace fossil fuels yet as they are energetically less efficient, thus proactively seeking better routes while keeping costs in check presents itself as a very viable option.

Acknowledgments

To the Department of Systems and Energy (DSE) of the Faculty of Electric Engineering and Computing (FEEC) of the State University of Campinas (UNICAMP) for the opportunity to carry out a Post-Doctorate internship and complete the present work.

To the reviewers, whose invaluable remarks and suggestions helped a lot to improve this paper.

References

APPENDIX A: COLLECTION POINTS TABLES

Table A.1
Collection points for Monday.

Table A.2
Collection points for Tuesday.

Table A.3
Collection points for Wednesday.

Table A.4
Collection points for Thursday.

Table A.5
Collection points for Friday.

Publication Dates

  • Publication in this collection
    05 Sept 2022
  • Date of issue
    2022

History

  • Received
    12 Nov 2020
  • Accepted
    07 Feb 2022
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