Page - 86 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2016,9, 86 optimizing tourassignmentsofvehicles. This is knownas theVehicleRoutingProblem(VRP) [1]. Numerousvariants of this problemhavebeen addressedover the last years. Thebasic variant is the so-calledCapacitatedVRP (CVRP),where each customer has a given demand that has to be satisfiedwithoutexceedingamaximumvehiclecapacity. TheVRPwithTimeWindowsextends the CVRPbyaddingtimewindowsto thedepotandthecustomers. Toaccount foradditional realworld aspects, theclassicalVRPhasbeenredefinedinvariousmanners thatareoftencalledRichVRPs[2] orMulti-attributeVRPs [3]. Despite the extensive literature in theVRParea,most of the existing contributionshaveassumedthat thefleet tobemanagedcomprisesonly internalcombustionengine vehicles (ICEVs),which isnotexactly thecurrentpicture. A large percentage of the oil consumed in regions such as Europe or the USA is used in transport, while road transport accounts for an important percentage of CO2 emissions of the overall transport activity. Furthermore, thewhole transport sector causes about 28%of the total greenhousegas (GHG)emissions in countries suchas theUSA. Inorder tomitigate this situation, onepossibility is to incorporate emission costs as anobjective tobeminimized in routingmodels, thus tradingoff environmental andeconomicgoals [4,5]. Adifferent approach is theutilizationof lesspollutingmeansof transportsuchasplug-inhybridelectricvehiclesandelectricvehicles (EVs), whose specific characteristicshave tobe included inadequate routingmodels. Ineffect, aspartof the initiative to improve the local air quality,modern cities encourage fleets of vehicles to adopt alternative technologies, suchasEVs. Several factors arepromoting theuseof these technologies, including: (i) companies receive incentives to reduce their carbon footprint; (ii) highvariabilityof oil-basedproductsandlong-termcost riskassociatedwithdependenceonoil-basedenergysources; (iii) availabilityofgovernmentsubsidies toreduceacquisitioncost; and(iv)advances inalternative energy technologies (such as EVs), which have potential for amore environmentally sustainable solutions at a cost that is starting to be competitive. From both an environmental and energy standpoints, theuseofEVsshouldbeafirstpriority for thereductionofprimaryenergyconsumption. Althoughhigherconcernsare theadvantagesofEVs in termsofefficiencyandflexibility in theuse ofenergy, theEVtechnology is currently facingseveralweakpoints,whichcanbesummarizedas follows: (i) the lowenergydensityofbatteriescomparedto the fuelof ICEVs; (ii) the longrecharge timesofEVsbatteriescomparedto therelatively fastprocessof refuelinga tank in ICEVs;and(iii) the scarcityofpublicand/orprivatechargingstations forEVbatteries. Inearlieryears,EVsfailedbecause of excessive battery prices andvery short driving ranges. AsEVshave becomeone of themajor researchareas in theautomotivesector, themagnitudeof theseproblemshasbeennotablydiminished. Although the replacementof conventional ICEVswithEVs isnotprofitableundermostoperation scenariosgiven thecurrent cost conditions, theavailabilityof increasingly long-livedbatteries, the trends forrisingfuelcosts, andlowerEVpurchasecostsare likely tochangethepicture [6]. Figure1 showsthenoticeable increaseexperiencedduringthe lastyears in thenumberofEV-relatedarticles published inScopus-indexed journals,whichproves thegrowing interest that theuseofEVs isarising amongresearchersandpractitioners. Accordingly, thispaper identifiesandreviews, fromanOR/CSperspective, severalopenresearch challengesrelatedto the introductionofEVs inL&Tactivities, includingthefollowingdimensions: (a) environmental-related issues; (b) strategic andplanning challenges associatedwith “standard” EVsandwithhydrogen-basedEVs;and(c)emergingoperational issuesrelatedto theuseofEVs in VRPs. Table1 summarizes thedifferent researchchallenges thathavebeen identified inour study andthatwillbeconvenientlydescribedandreviewedindifferentsectionsof thismanuscript. Fora betterunderstanding, theseresearch issueshavebeenclassifiedin threedimensions: environmental, strategicandplanning,andoperational. Thepaperalsoanalyzes indetailhowthe introductionofEVs inL&TsystemsgeneratesnewVRPvariants, i.e., in thecontextof theGreenVRP, thisworkpointsout someof themostpromisingresearch linesyet tobe fullyexplored. Finally, thepaperalso includes adiscussiononwhichoptimizationapproaches canbetter contribute todealwith theseopenand difficult researchchallenges. 86