REVIEW 2 major objections 4 minor 13 references
Optimized electric bus depot charging can cut total electricity costs by 25–60%, reduce the grid capacity needed for feasible operation by over 40%, and guard against 10% trip-energy uncertainty at a cost increase below 0.1%.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-03 09:33 UTC pith:HH3KGYGL
load-bearing objection A competent sensitivity study of depot charging, but the headline robustness cost (<0.1%) is not supported because the worst-case SOE is not cycled; add Ewc[N]>=Ewc[0] and rerun before believing it. the 2 major comments →
Optimal Electric Bus Depot Charging: Cost Savings, Grid Limits, and Robustness Trade-Offs
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper argues that a single convex robust formulation of overnight depot charging — minimizing energy cost plus a demand charge under worst-case state-of-energy constraints — outperforms charge-on-arrival across all studied depots. In four realistic Swiss depot scenarios, total electricity cost reductions range from roughly 25% under flat prices to nearly 60% under high price volatility, with smaller depots benefiting most because optimization removes charging peaks that dominate their bills. The formulation also cuts the minimum grid utilization factor needed for feasible operation by over 40% for larger depots, meaning a smaller grid connection can support the same fleet, while tight gr
What carries the argument
The central object is a convex optimal control problem over each bus's charging power, battery state of energy, and a single peak-power variable. Bounded demand uncertainty (each trip's energy demand within ±ρ of nominal) is enforced by propagating a worst-case state-of-energy trajectory that must stay nonnegative, guaranteeing feasibility for every admissible realization. Two nonconvexities are relaxed into convex constraints: the quadratic charging-loss equality becomes an inequality, and the maximum over time in the demand charge becomes a slack variable with an upper bound; the paper argues these inequalities are tight at the optimum, making the problem tractable as a finite-dimensional
Load-bearing premise
The load-bearing premise is that two mathematical shortcuts used to make the problem convex — replacing the battery-loss equality with an inequality and replacing the peak-power term with a slack variable — are always exactly tight at the optimal solution; the paper states this is checked after solving but reports no such check, and if either shortcut is not exact the quoted savings and feasibility numbers do not describe a physically realizable charging plan.
What would settle it
Solve the same depot instances without the two convex relaxations — for example with a nonlinear solver on the original equality-constrained problem — and compare the exact optimum's cost and peak power with the relaxed solution. Any gap in the battery-loss equality or in the peak-power variable at the reported optimum would falsify the central numbers; alternatively, report the promised a-posteriori equality check by computing actual charging power from the relaxed solution and comparing it with the relaxed variables.
If this is right
- For depots of 7–35 buses, optimized charging reduces total electricity cost by roughly 25% under flat prices and up to nearly 60% under high price volatility, compared with charge-on-arrival.
- Smaller depots achieve the largest relative savings because optimization eliminates charging peaks that otherwise dominate their electricity bills.
- Optimization lowers the minimum grid utilization factor required for feasible operation by more than 40% in larger depots, so a smaller grid connection can support the same fleet.
- A worst-case 10% energy-demand uncertainty adds less than 0.1% to total electricity cost, making robust feasibility nearly free in the studied scenarios.
- The optimized charging policy has an interpretable structure — idling above an implicit price threshold and drawing an approximately flat power profile below it — which can guide practical rule-based charging heuristics.
Where Pith is reading between the lines
- If the savings grow with price volatility, the value of smart depot charging should rise as electricity markets incorporate more renewables and intraday price swings widen; operators in such markets should expect savings near the upper end of the 25–60% range.
- The negligible cost of robustness suggests that adopting a conservative uncertainty set (e.g., 10% above nominal demand) is almost always preferable to ignoring uncertainty, because it buys feasibility at essentially no cost — though this conclusion depends on the box uncertainty model and on how well nominal demand is known.
- The grid-capacity result points toward a testable design rule: when planning a new depot, one could size the grid connection using the optimizer's minimum feasible capacity rather than charge-on-arrival peaks, potentially cutting infrastructure investment; the paper raises this joint fleet-and-grid sizing question as future work.
- The near-rectangular profile below a price threshold hints that a simple bang-bang heuristic might recover most of the optimization value; a natural test is to run such a threshold rule against the full optimizer on the same depot data.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a convex robust optimization model for overnight depot charging of electric bus fleets under bounded trip-energy demand uncertainty. The model minimizes energy cost plus a demand charge subject to charger, grid, and state-of-energy constraints, with robustness enforced through a worst-case state trajectory. The formulation is tested on four Swiss depots (7–35 buses) against a charge-on-arrival baseline. The paper reports total electricity cost reductions of 25–60% as electricity price volatility increases, a reduction of the minimum feasible grid connection capacity of up to 43%, and an additional robustness cost of less than 0.1% for a 10% demand uncertainty. It also claims that the resulting charging profiles have a simple price-threshold structure.
Significance. If the results are correct, the paper would provide a practical, computationally tractable tool for depot charging with explicit robustness guarantees, supported by realistic GTFS-based case studies and a systematic sensitivity analysis over price volatility and grid capacity. The worst-case reasoning for the lower SOE bound is sound: only the upper demand bound matters for the lower SOE constraint, and the upper SOE bound can be handled by reducing charging power. The convex reformulations are standard, and the sensitivity experiments are clearly presented. However, the central robustness claim has a load-bearing modeling flaw: the periodic terminal constraint is imposed only on the nominal state, not on the worst-case state. This flaw directly affects the reported <0.1% cost of robustness and therefore the paper's headline conclusion.
major comments (2)
- [§3.2, Eqs. (20c), (20f), (20k), (20l)] The formulation enforces a periodic terminal condition only on the nominal state Eu, through Eq. (20l), while the worst-case state Ewc is only required to be nonnegative in Eq. (20f). Combining (20b) and (20c) gives, for each bus, Ewc_u[N] = Eu[N] - ρ·Δt·Σ_k Pdem_u[k]. With Eq. (20l) stating Eu[N] ≥ Eu[0] = Ewc_u[0], the worst-case state can end at Ewc_u[0] - ρ·E_total,u, which may be far below the starting value as long as it is nonnegative. In a periodic operating cycle, this is not a sustainable robustness guarantee: if the next day is also worst-case, the bus starts lower and the guarantee erodes. The correct periodic robust constraint would be Ewc_u[N] ≥ Ewc_u[0]. Without it, the optimizer can meet the worst-case demand by drawing the extra energy ρ·E_total from the initial battery capacity instead of purchasing it, which explains why Fig. 3 reports a <0.1% robustness cost for ρ=10%
- [§3.1, Eq. (14)] The convex relaxation of the battery-loss equality, Eq. (14), is asserted to be tight at the optimum and this is said to be 'checked a posteriori,' but no such check is reported anywhere in Section 4 or the figures. All computed cost savings, SOE trajectories, and robustness figures are produced by solving the relaxed problem. If the optimum has strict inequality for some k and u, then Pchg,i is not the physical battery input power, and the computed SOC dynamics and the resulting costs are not physically realizable. The authors should report the maximum slack of Eq. (14) over all buses and time steps for every scenario, or prove that strict inequality is suboptimal. This is a load-bearing verification, not a cosmetic detail.
minor comments (4)
- [Abstract and §4.5, Table 2] The abstract states the optimization 'lowered the grid capacity required for feasible operation by over 40%,' but Table 2 reports reductions of 0.0%, 17%, 43%, and 38%. The claim is true only for one depot; please report the per-depot values and avoid the impression that the result holds broadly.
- [§1.2.2] There is a grammatical error: 'No prior work was identified that quantifies the “price of robustness”, in the sense of Bertsimas and Sim [12], has not been quantified' — this should be rephrased.
- [§2.3] The demand-charge rate is given as a monthly 12.1 $/kW and said to be 'normalized to the optimization horizon.' Please specify the normalization (e.g., whether the monthly charge is divided by 30 or by the number of days in the billing cycle), since this directly affects the reported total cost and the relative savings.
- [§4.4 and Fig. 4] The y-axis label 'Saving potential [%]' should be defined precisely: is it total electricity cost savings of the optimized strategy relative to charge-on-arrival? Also clarify that the flat price profile uses the same mean as the other profiles, as stated in the text.
Circularity Check
No significant circularity: results are computed optima, not fits; the flagged robustness-constraint caveat is a correctness risk, not a circular reduction.
full rationale
The paper's central claims are generated by solving well-posed convex programs and comparing them with a deterministic charge-on-arrival baseline; no parameter is fitted to the reported savings, and the volatility and grid-capacity sensitivities are recomputed from the same model under systematically varied tariffs and power limits. The robustness trade-off is the difference between two optimization runs sharing the same objective, with the uncertainty set as an input; that is the standard, non-circular meaning of the price of robustness. All references are external; there is no load-bearing self-citation chain or imported uniqueness theorem. Two caveats are noted for correctness rather than circularity: the paper states that the convex relaxations (14) and (16) are tight at the optimum 'which we check a posteriori' but reports no such check, and the robust formulation (20f) imposes no periodic terminal constraint on the worst-case SOE, so the reported <0.1% robustness cost may understate the cost of a sustainable multi-day guarantee. These issues affect the validity of specific numerical claims, but they are not cases where a 'prediction' is equivalent to the model input by construction.
Axiom & Free-Parameter Ledger
free parameters (3)
- Demand-charge normalization factor =
not stated
- Discretization time step Δt =
not stated
- Battery/charger loss parameters =
η=0.95, α=3.51×10⁻⁷ W⁻¹
axioms (6)
- domain assumption The convex relaxations in eqs. (14) and (16) hold with equality at the optimum
- domain assumption The demand-uncertainty set is a symmetric multiplicative box (1±ρ)P_dem applied at every time step
- domain assumption The 24-hour horizon is approximately periodic, so the terminal constraint E[N]≥E[0] with free initial SOE is appropriate
- domain assumption Each bus has a dedicated 150 kW charger and unidirectional power flow
- standard math Standard convex optimization and forward-Euler discretization are valid
- domain assumption Nominal demand profiles derived from GTFS schedules and elevation-based estimates are accurate
read the original abstract
Depot charging of electric bus fleets must minimize electricity costs, respect grid limits, and remain feasible despite uncertain trip energy demand. While cost-optimal charging is well studied, its value under different electricity prices and grid connection capacities, as well as the economic cost of robustness, remain poorly quantified. We address these gaps with a convex robust formulation in which bounded demand uncertainty is enforced through worst-case state-of-energy constraints. The formulation is evaluated against charge-on-arrival using realistic service schedules for four Swiss depots containing 7-35 buses. In the depots studied, smaller depots achieve the greatest relative benefit from optimization, with total electricity cost reductions exceeding 50%, because optimization mitigates charging peaks that strongly affect their costs. For all depot sizes, the savings from optimization increase with electricity price volatility. Optimization can also lower the grid capacity required for feasible operation by over 40%, although tight limits reduce peak shaving potential. Protection against energy-demand deviations of 10% increases total electricity cost by less than 0.1%. The resulting charging power profiles exhibit interpretable price-threshold and peak-shaping behavior, providing practical guidance for real-world implementations.
Figures
Reference graph
Works this paper leans on
-
[1]
Real-time charging scheduling and optimization of electric buses in a depot,
B. Verbrugge, A. M. Rauf, H. Rasool, M. Abdel- Monem, T. Geury, M. El Baghdadi, and O. Hegazy, “Real-time charging scheduling and optimization of electric buses in a depot,”Energies, vol. 15, no. 14, 2022
2022
-
[2]
A quadratic program- ming based optimisation to manage electric bus fleet charging,
A. Houbbadi, R. Trigui, S. Pelissier, E. Redondo- Iglesias, and T. Bouton, “A quadratic program- ming based optimisation to manage electric bus fleet charging,”International Journal of Electric and Hy- brid V ehicles, vol. 11, no. 4, pp. 289–307, 2019
2019
-
[3]
Smart charg- ing system in a bus depot: Cost-effective strategy,
D. Martini, M. Longo, and L. Daniel, “Smart charg- ing system in a bus depot: Cost-effective strategy,” IEEE Access, vol. 13, pp. 155 883–155 897, 2025
2025
-
[4]
Optimal charging schedule planning and economic analysis for elec- tric bus charging stations,
R.-C. Leou and J.-J. Hung, “Optimal charging schedule planning and economic analysis for elec- tric bus charging stations,”Energies, vol. 10, no. 4, 2017
2017
-
[5]
Plug-in electric bus depot charging with PV and ESS and their impact on LV feeder,
S. M. Arif, T. T. Lie, B. C. Seet, S. M. Ahsan, and H. A. Khan, “Plug-in electric bus depot charging with PV and ESS and their impact on LV feeder,” Energies, vol. 13, no. 9, 2020
2020
-
[6]
Economic and ecological optimization of electric bus charging considering variable elec- tricity prices and CO2eq intensities,
M. Rupp, C. Rieke, N. Handschuh, and I. Ku- perjans, “Economic and ecological optimization of electric bus charging considering variable elec- tricity prices and CO2eq intensities,”Transporta- tion Research Part D: Transport and Environment, vol. 81, p. 102293, 2020
2020
-
[7]
Energy uncertainty analysis of electric buses,
J. Veps ¨al¨ainen, A. Ritari, A. Lajunen, K. Kivek ¨as, and K. Tammi, “Energy uncertainty analysis of electric buses,”Energies, vol. 11, no. 12, 2018
2018
-
[8]
Robust optimization for integrated planning of electric- bus charger deployment and charging scheduling,
Y . Zhou, H. Wang, Y . Wang, and R. Li, “Robust optimization for integrated planning of electric- bus charger deployment and charging scheduling,” Transportation Research Part D: Transport and En- vironment, vol. 110, p. 103410, 2022
2022
-
[9]
Designing a robust and cost-efficient elec- trified bus network with sparse energy consumption data,
S. Momen, Y . Maknoon, B. van Arem, and S. S. Azadeh, “Designing a robust and cost-efficient elec- trified bus network with sparse energy consumption data,”Transportation Research Part C: Emerging Technologies, vol. 171, p. 105020, 2025
2025
-
[10]
En-route charge scheduling for an electric bus network: Stochas- ticity and real-world practice,
Z. Zeng, T. Wang, and X. Qu, “En-route charge scheduling for an electric bus network: Stochas- ticity and real-world practice,”Transportation Re- search Part E: Logistics and Transportation Review, vol. 185, p. 103498, 2024
2024
-
[11]
A robust optimiza- tion approach for E-bus charging and discharging scheduling with vehicle-to-grid integration,
M. Kang, B. Lee, and Y . Lee, “A robust optimiza- tion approach for E-bus charging and discharging scheduling with vehicle-to-grid integration,”Math- ematics, vol. 13, no. 9, 2025
2025
-
[12]
The price of robust- ness,
D. Bertsimas and M. Sim, “The price of robust- ness,”Operations research, vol. 52, no. 1, pp. 35– 53, 2004
2004
-
[13]
Electricity spot price Switzer- land,
Beneficial Apps AS, “Electricity spot price Switzer- land,” https://www.energyprices.eu/electricity/ switzerland, 2025, accessed: Jun. 10, 2025
2025
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