Guiding the Selection and Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities (2010)

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Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 28 of 61 6 Economic Measures of Benefit In addition to providing voltage support as demonstrated in the simulation results, an ESD installation can also help capture more regenerative braking energy that may be otherwise wasted. A direct benefit of recovering braking energy is the energy cost saving. This section demonstrates the energy cost savings achievable for the light rail and the metro rail systems. 6.1 Electricity cost saving analysis – light rail It has been shown that an ESD can effectively mitigate problems associated with low train voltages for this system. With the installation of the ESD in location A4X, the system’s receptivity (the ratio of utilized regenerative energy over the total available regenerative energy) will also be improved, resulting in a greater potential to recover regenerative braking energy. As a result, the energy saving ratio due to regenerative braking (the ratio of energy consumption with regenerative braking over the energy consumption without regenerative braking) can be improved. When all substations are in service and we do not experience a voltage sag problem, we can take a different look at the effect of energy storage, principally to save energy. The control voltages of the ESD may be adjusted to best optimize energy saving rather than providing voltage support, thus maximizing the capture of regenerative energy. So, when voltage support is not needed, an adjustment in the voltage settings of the ESD can be made for optimal energy saving. Table 6–1 compares system-wide energy saving potential resulting from variations in ESD control voltages. This table also demonstrates the corresponding energy and power rating requirements for the ESD. An electricity cost saving analysis utilizing this strategy is undertaken in the next section. Table 6-1 System-wide Energy Summary (light rail) Case # A4X Type Receptivity (%) Energy Savings (%) ESD Energy (kWh) 60 CBH 83.6 29.9 n/a 70a ESD (Vc=720V) 84.1 30.1 0.20 70b ESD (Vc=760V) 84.5 30.2 1.30 70c ESD (Vc=793V) 88.0 31.5 2.30 80 TPSS 83.3 29.8 n/a Note - All TPSS in normal operation

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 29 of 61 This table also illustrates that an ESD installation at the A4X location can help improve the system-wide energy saving ratio, expressed in per cent. Without the ESD, the energy saving ratio is 29.9%. With the ESD, the ratio improves to 30.1% or 31.5%, depending on ESD voltage setting. If the same location is installed with a rectifier substation, the ratio is decreased to 29.8%. Remember that this is a system-wide energy comparison that indicates the effect of a single ESD installation on the entire system, and that the projected change in energy saving ratio percentage must be viewed in this context. To examine the potential energy and cost saving benefits of energy storage, a simple electricity cost analysis can be performed using the options noted above. We start with a comparison of the 15-minute power averages across all substations in the system under normal operation with and without an ESD, as shown in Table 6–2. The 15-minute averages are used because of the correlation with utility peak power charging average time periods, which are in most cases 15-minutes. Table 6-2 15-Minute Average Power Values by Substation With A4X- TPSS With A4X- ESD With A4X- TPSS With A4X- ESD A1 403 406 162 165 A2 449 458 178 185 A3 388 408 152 156 A4 382 410 153 159 A4X 208 0 81 0 A5 366 395 137 144 A6 412 425 153 162 A7 361 368 134 138 Sum 2,969 2,870 1,150 1,110 15-Minute Average Power (kW) 5-Minute Headway 15-Minute Headway Substation From the above table it can be seen that the 15-minute average power in each substation for 5-minute headways represents the peak power demand, while 15-minute headways are less when measured over the same 15-minute period. This table also shows the utility supplied power to each substation along the alignment. Higher power requirements are shown near the ESD because there is no supply at that point and neighboring substations would need to provide the additional power to charge the ESD, although only marginally. Both the 5-minute and 15-minute headway conditions are used to compute the overall energy saving in a 24 hour period. More

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 30 of 61 specifically, returning to Table 6–2, the following 24-hour train operation schedules are assumed: • 5 minutes in peak hours (6-10AM and 4-8PM) • 15 minutes in off-peak hours and weekends • No train service between 1AM and 4AM on any day Using actual published electricity tariffs from a USA utility company[3] , the annual cost for each substation is calculated based on individual traction substation billing arrangements. These are shown in Table 6–3. This table indicates that the ESD installation will have an annual electricity cost saving benefit of $45,169 based on the current tariffs compared against a full rectifier installation in the A4X location. Assuming that the energy cost increases at 5% per year, the annual cost savings over 10 years’ time are shown in Table 6–4. Table 6-3 Summary of Annual Electricity Cost Saving due to ESD (all figures in US $) Substation With A4X-TPSS With A4X- WESS Savings with WESS A1 $182,342 $184,763 -$2,422 A2 $202,057 $207,687 -$5,630 A3 $173,973 $181,808 -$7,835 A4 $172,945 $183,581 -$10,637 A4X $93,875 $0 $93,875 A5 $161,894 $172,907 -$11,012 A6 $181,397 $189,110 -$7,713 A7 $159,353 $162,810 -$3,456 Sum $1,327,835 $1,282,666 $45,169 Annual Electricity Cost Summary

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 31 of 61 Table 6-4 Summary of Cost Savings in 10 Years Total Energy Cost Saving Over 10 Years (Assuming 5% Annual Increase) Year Yearly Cost Savings (US$) 1 45,169 2 47,428 3 49,799 4 52,289 5 54,904 6 57,649 7 60,531 8 63,558 9 66,736 10 70,072 Total 568,135 6.2 Energy cost savings – metro rail Similarly as in the light rail system calculations, the estimated annual cost for the metro rail track section and cost savings [3] under different options are shown in Table 6–5. Table 6-5 Summary of Annual Electricity Cost Savings (All figures in US $) Substation 3MW Sub 3MW ESD 4MW ESD Total annual cost $4,831,516 $4,776,365 $4,760,599 Savings over 3MW Rectifier Sub Option $0 $55,152 $70,917 Assuming that the energy cost increases at 5% per year, the annual cost savings over 10 years’ time are shown in Table 6–6. These annual saving data are derived from daily savings in which weekdays are constructed from combining time periods for off-peak and peak hours associated with different percentages of energy savings due primarily to train headway differences. It should be pointed out that the above calculations are based on ESD installations at locations where voltage supports are required. If the motivation of an ESD installation is primarily on energy saving, the consideration for the location of the installation for optimum energy saving may be different. In addition, considerations for the candidate systems for such installations are different in order to maximize the amount of energy recovery.