*3.2. Charging Stations, EV Variability and Campus Profit*

The time of day, price of electricity and net profit for all involved in this V2G method are analysed and explained. If the campus buys at off-peak prices, not giving the EV owners an incentive, they make 43.56 p/day using a 3.63 kW/h charger (Table 1).

**Table 1.** Energy sale management based on time and price between EV owner and the university campus.


Figure 4 demonstrates how the profit can vary throughout the day; if the peak-time, and off-peak prices are paid for the electricity, there will be no profit. The '*x*' axis represents the hours of charge and the '*y*' axis represents net profit in pence. There are more discharge hours than charge hours for the EVs, meaning the building must purchase the electricity at 15 p/h and must sell it back to the EV at 12 p/h to break even. The main operating hours are 09:00–17:00, with 12 p showing the peak times and −15 p showing off-peak times for the building for an average day.

A 3.63 kW/h charger's values were calculated using the EVs' batteries at peak times and re-charging at off-peak prices. It also shows that if the University charges the EV batteries at off-peak rates instead of peak rates, the EV owner's and the campus can profit, as is shown in columns 4 and 5 in Table 1. If the university buys and sells at off-peak prices, it makes money throughout the day. While the campus pays both peak, and off-peak prices with a 1 kW/h charger, it breaks even, as the last payment of the day takes net profit to zero (Figure 4).

The tariff rate changes throughout the day. The low tariff rate is 12 p/h, and the high tariff rate is 15 p/h. When these prices are paid, the net profit ends at zero at 17:00. If the campus pays off-peak prices to the EV owner for any hour of the day, 12 p/day is earned as the last payment of the day takes net profit to 12 p. Figure 5 presents the outcome of net profit.

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 9 of 25

**Figure 4.** Campus net profit while peak-time and off-peak prices are paid. **Figure 4.** Campus net profit while peak-time and off-peak prices are paid. net profit.

A 3.63 kW/h charger's values were calculated using the EVs' batteries at peak times

**Figure 5.** Campus net profit while exclusively pays off-peak prices. **Figure 5.** Campus net profit while exclusively pays off-peak prices.

Table 2 presents a 6.66 kW/h charger that brings in a net profit of 79.92 p/day when the campus pays off-peak prices at all times of the day.

**Figure 5.** Campus net profit while exclusively pays off-peak prices. The battery capacity needed to match 10 charging stations costs £27,432.90 depending on the supplier, whereas the 10 charging stations purchase and installation costs £8790, giving the same amount of energy storage at one time, assuming maximum capacity of EVs. This provides a saving of £18,642.90. Lithium-ion batteries have a lifespan of 2–3 years or 300–500 charge cycles meaning this cost will build up over time. The charging stations are designed to last at least 10 years with parts that are easily replaceable, and most of them have a warranty of 3 years. The charging and discharging rate of a 6.66 kW/h charger throughout the day has been presented in Figure 6. The outcome of charging and discharging the EV battery by 6.66 kW/h using a 6.66 kW charger if the car leaves with the same capacity as it was once plugged in, the EV owner could earn 79.92 p/day. If the EVs' owners return to their vehicle and it is out of charge, they are unlikely to use the V2G method. Taking this into account, for this simulation, the campus must have the car fully charged by 17:00 when most classes are finished.


*Sustainability* **2021**, *13*, x FOR PEER REVIEW 10 of 25

the campus pays off-peak prices at all times of the day.

**Time Price (Pence) Charge/Discharge EV Owner (Pence) Campus (Pence)**

**Table 2.** Net profit per day while the campus pays off-peak prices.

08:00–09:00 79.92 Charge −79.92 +79.92 +79.92 09:00–10:00 99.99 Discharge +99.9 −99.9 −79.92 10:00–11:00 99.99 Discharge +99.9 −99.9 −79.92 11:00–12:00 79.92 Charge −79.92 +79.92 +79.92 12:00–13:00 99.99 Discharge +99.9 −99.9 −79.92 13:00–14:00 99.99 Discharge +99.9 −99.9 −79.92 14:00–15:00 79.92 Charge −79.92 +79.92 +79.92 15:00–16:00 79.92 Charge −79.92 +79.92 +79.92

Table 2 presents a 6.66 kW/h charger that brings in a net profit of 79.92 p/day when

Net profit 0 0 79.92

**Campus at off-Peak Prices (Pence)**

**Figure 6.** The profit earned by the campus per day. **Figure 6.** The profit earned by the campus per day.

The EV owner could earn 79.92 p per day, but the campus would lose that too. The campus, however, would not have to pay for large battery installation for the energy storage. In the case of 10 charging stations, the approximate loss becomes £7.99 a day for the university. The loss may accumulate up to the total cost of the battery capacity of 10 charging stations after 6.4 years. The battery must be changed every 2–3 years depending on the frequency of use. Overall, if the university campus installed ten 6.66 kW/h EV charging stations instead of buying the lithium-ion battery capacity to cover the capacity of the charging stations, the university would save £79,856.29 every 10 years. It is assumed that the batteries had to be replaced every two years and the charging stations are every 10 years. The day was simulated and mapped out, assuming the car was plugged in at 08:00 at 80% and unplugged at 17:00 at 100%, as is shown in Figure 7.

This method charges the battery as much as possible, which is 6.66 kW/h for 5 h in the day and thus, discharges it, so it sums to 100% by 17:00. This was calculated through:

$$D\mathbf{c} = \mathbf{Oc} + \mathbf{C}r - \mathbf{Fc} = \mathbf{32}\,\text{kW} + \mathbf{33.3}\,\text{kW} - 40\,\text{kW} = \mathbf{25.3}\,\text{kW} \tag{11}$$

In Equation (11), the '*Dc*' is the discharge/day, the '*Oc*' is the charge before the EV is plugged in, the '*Cr*' is how much the EV is charged during a cycle, and the '*Fc*' is the EV's capacity at 100% charge. This allows the EV to leave with 100% charge, while still discharging as much as possible at peak times.

**Figure 7.** The times of peak and off-peak while the battery is being charged or discharged.
