The charging choice and its dimensions

The charging behaviour literature analysed in section 3.2 has highlighted the following:  Electric vehicles (in real world trials) are not charged with a daily frequency;  Battery levels triggers the decision to initiate a charging operation.

These two observations suggest that the level of available energy plays a role in the determination of the charging behaviour. Therefore this can be considered as an attribute characaterising alternative charging options in a charging choice. In fact the objective of the charging operation is to increase the level of available energy by a certain amount.

90 Given the current charging infrastructure scenario the amount of energy charged during the charging operation is determined simply by knowing the charging duration (i.e. start time and end time of the charging operation) because the charging power is usually fixed. In technological scenarios when charging services are provided at different costs depending on how fast the battery is being charged, however, electric vehicle users may trade off available energy, charging duration and costs.

In fact even in scenarios where the infrastructure itself does not provide the capability to control the charging power, electric vehicle users can in a way control how long it will take to recharge their EV, by delaying the start charging time, for example to take advantage of off peak electricity price (Schey et al., 2012). Delaying the charging start time means increasing the overall time elapsed from the arrival time at the charging facility to the time the battery has reached the desired level. Users may decide, for example either

a) to delay the charging start time in order to charge at lower prices, but possibly to lower energy levels (if they need to depart before the battery has reached the desired level); or

b) to delay the charging start time in order to charge at lower prices but charge to a higher level, with a charging duration that may induce a later departure with respect to their preferred departure time; or

c) to avoid delaying the charging operation and pay more for a higher battery level so as to ensure a swifter vehicle availability in order to leave at the preferred departure time.

In more advanced scenarios (“smart charging scenarios”) communications between the electric vehicle charger and the electricity supplier (the charging service provider, CSP) may be allowed. In this case EV users may simply choose the target battery level they want to achieve and the time by when this should be achieved, based on prices communicated by the CSP. In turn the CSP will satisfy this request by delivering the energy according to a schedule that facilitates its operations and contributes to minimising its costs (Sundstrom and Binding, 2011). Clearly, tariffs for the charging service will tend to favour charging settings that allow greater flexibility for the CSP in the definition of charging schedules as the CSP objective is the optimisation (cost minimisation) of its operations. It is evident that the longer the time the vehicle is connected to the grid, the more flexible the charging operation can be, because there is more leeway in defining the charging schedule, within the limit of the maximum charging power. On the other hand, if the EV electric vehicle user wants to have the vehicle charged as fast as possible, the CSP must deliver the energy continuously at the maximum charging power.

91 Considering the potential settings for the charging operation described above, we can describe the charging choice at a given charging opportunity in terms of:

 charging start time preference;  charging end time preference;

 and preference in available energy at the end of the charging operation.

The charging operation start time, can only be coincident with or delayed with respect to the arrival time at the charging facility. Therefore, from the electric car driver perspective, the charging operation could be considered as starting at the vehicle arrival time, regardless when the actual energy transfer may take place. In this perspective both smart charging and conventional charging could be represented only as a two dimensional choice, where the two dimensions are: the final battery level and the time it takes to achieve it, since the arrival at the charging facility. This time indeed represent the “effective” charging duration. Hereafter we simply use the term charging duration (or charging time, CT) to refer to this quantity. Figure 8a shows the concept of charging choice as proposed in this study. At a given charging opportunity, EV users choose the energy they want available at the end of the charging operation as well as the charging duration. Note that in the figure, the available energy is presented in as percentage of the total battery capacity, state of charge. The charging choice space is constrained by the characteristics of the electric vehicle and the charger, which determine the (maximum) charging power. The other (obvious) constraint is the maximum battery capacity (SOC=100%). A particular charging alternative is represented by a point in the feasible charging space, thus characterized by the following attributes:

 available energy after charging,  the charging duration

An additional attribute is the cost of charging, which will depend on the electricity tariff: Individuals facing a charging choice will trade-off between these attributes depending on the electricity tariff structure. For instance, a user may decide to allow a longer charging duration to take advantage of lower electricity price periods according to a static time of use tariff (Figure 8b). On the other hand, a user may choose CT and SOC according to an offer that is signalled by his/her charging service provider, exploiting the communication capabilities of smart grid systems. The charging service provider, within the technological limits and the constraints posed by the user request (Figure 8c), will establish the actual charging schedule (Figure 8d).

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(a) (c)

(b) (d)

Figure 8 Conceptual view of a charging choice at a given charging opportunity. (a) shows the dimensions it entails; (b) shows a possible charging schedule underlying charging choice, where an EV driver delays the charging start time to take advantage of lower electricity prices; (c) shows the space leeway available to a charging service

provider underlying the charging choice; (d) is an example of charging schedule resulting from accepting external control of the charging operation by a charging service provider SOC initial SOC time SOC=100% driver’s choice (SOCj,CTj) SOC initial SOC time SOC=100% driver’s choice (SOCj,CTj)

provider’s options space

SOC initial SOC time SOC=100% driver’s choice (SOCj,CTj) SOC initial SOC time SOC=100% driver’s choice (SOCj,CTj) charging schedule

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