Driven by the global consensus on decarbonization, the penetration rate of new energy vehicles has surged significantly. Hydrogen fuel cell vehicles, as a pivotal category, are celebrated for their cleanliness, high efficiency, and extended range, positioning them as a key pathway towards a low-carbon future. In my research, the focus has been on advancing the thermal management systems for these vehicles, particularly for heavy-duty applications like city buses. The performance, safety, and longevity of a fuel cell bus are critically dependent on its ability to manage heat—not just from the fuel cell stack itself, but from the entire powertrain, including the traction motor and, most importantly, the high-voltage battery management system (BMS). An efficient battery management system is inseparable from effective thermal control, as battery temperature directly impacts its state of charge estimation, power capability, and cycle life.
Traditional thermal management designs often treat components in isolation, leading to system redundancy, complex control, and suboptimal energy efficiency. My work addresses this by developing an integrated vehicle thermal management system (VTMS) for a fuel cell bus, with a core philosophy of synergistic heat exchange and waste heat recovery. The primary objectives are twofold: first, to ensure all critical components, especially the lithium-ion battery pack managed by the BMS, operate within their optimal temperature ranges under diverse climatic conditions; second, to minimize the parasitic energy consumption of the thermal system itself, thereby extending the vehicle’s driving range. This paper details the architecture, modeling, and simulation analysis of this proposed system, with particular emphasis on the role of the battery management system within the integrated thermal framework.
The complexity arises from the disparate thermal requirements of the key subsystems. The proton exchange membrane fuel cell (PEMFC) stack operates optimally between 65–80°C and requires precise cooling. The lithium-ion battery pack, governed by its battery management system, performs best within a much narrower band of 28–32°C, necessitating both cooling and heating. The permanent magnet synchronous motor (PMSM) can tolerate a wider range but is ideally kept around 80°C for efficiency. Finally, the passenger cabin demands the most stringent comfort control, typically between 23–27°C in summer and 18–22°C in winter. Managing these concurrently with minimal energy penalty is the central challenge.
System Architecture and Operational Modes
The proposed integrated VTMS, conceptualized from a first-person design perspective, breaks down the traditional barriers between subsystems. It comprises four interconnected loops: the Fuel Cell & Motor Loop, the Power Battery Loop, the Cabin Loop, and the Air-Conditioning (A/C) Refrigerant Loop. The innovation lies in the strategic coupling of these loops via heat exchangers and control valves, allowing heat to be transferred from where it is in excess to where it is needed.
A pivotal design decision was to parallelize the fuel cell and motor cooling circuits on a shared primary loop. While the fuel cell typically requires deionized coolant, we assume the efficacy of a deionizer in maintaining appropriate conductivity, simplifying the hydraulic architecture. A proportional valve on the motor branch dynamically regulates coolant flow to maintain the motor’s temperature setpoint. The power battery loop is fluidically coupled to both the cabin heater core and a battery chiller connected to the A/C system. This allows the battery to be cooled by cabin waste heat (in winter via the heater core) or by active A/C refrigeration (in summer). The cabin can be heated either by a Positive Temperature Coefficient (PTC) electric heater or by scavenging waste heat from the fuel cell and motor circuits through a separate heater core. The system’s schematic embodies this integrated philosophy, where the battery management system’s thermal requests are central to directing the flow of energy.
Based on component temperature states and ambient conditions, I defined nine distinct operational modes for the VTMS controller. These modes are governed by a rule-based logic with temperature thresholds. The core states for the battery management system (BMS), fuel cell stack, cabin, and motor are continuously monitored to trigger transitions. The modes are summarized in the table below:
| Mode | Condition (Typical Logic) | Primary Action |
|---|---|---|
| 1 | Tstack < 10°C, Tbat < 20°C | Cold start: PTC heats stack and battery. |
| 2 | Tbat < 20°C, Tcab < 20°C, Tstack ≥ 10°C | Heat battery and cabin via PTC. |
| 3 | Tcab < 20°C, Tbat ≥ 20°C, Tstack ≥ 10°C | Heat cabin via PTC. |
| 4 | Tamb < 10°C, Tstack > 70°C, Tmot > 80°C, Tcab < 20°C | Waste Heat Recovery (WHR): Heat cabin using stack/motor waste heat. |
| 5 | All temps within normal ranges. | Normal operation, minimal thermal activity. |
| 6 | Tstack > 65°C, Tmot > 80°C, Tbat ≥ 32°C, Tcab normal | Cool stack, motor, and battery (liquid cooling). |
| 7 | Tstack > 65°C, Tmot > 80°C, Tbat < 32°C, Tcab normal | Cool stack and motor only. |
| 8 | Tbat ≥ 32°C, Tstack & Tmot normal, Tcab normal | Cool battery only (liquid cooling). |
| 9 | Tstack > 65°C, Tmot > 80°C, Tbat ≥ 32°C, Tcab > 30°C | Cool all (stack, motor, battery via A/C) and cabin A/C. |
This mode-based strategy ensures that the integrated battery management system and thermal controller always prioritizes component safety while seeking opportunities for energy savings, such as delaying A/C activation or enabling waste heat recovery.
The Central Role of the Battery Management System (BMS)
In this integrated framework, the BMS is far more than a monitor of voltage and current. It acts as a critical thermal client and a source of thermal load. The battery management system must accurately estimate the battery’s internal temperature, often using models coupled with sparse sensor measurements, to request heating or cooling from the VTMS. The heat generation within the battery pack, a key input for the thermal model, is fundamentally governed by its electrochemistry and operational state monitored by the BMS.
The heat generation rate per unit volume in a battery cell can be described by Bernardi’s equation, which the BMS can approximate in real-time using measured parameters:
$$q = \frac{I}{V_{bat}} \left[ (U_{0} – U_{bat}) + T \frac{dU_{bat}}{dT} \right]$$
Where \(I\) is the current (positive for discharge, negative for charge), \(V_{bat}\) is the cell volume, \(U_0\) is the open-circuit voltage, \(U_{bat}\) is the operating voltage, \(T\) is the absolute temperature, and \(\frac{dU_{bat}}{dT}\) is the entropy coefficient or temperature dependency of the voltage. The first term \((U_{0} – U_{bat})I\) represents the irreversible Joule heating, and the second term \(T I \frac{dU_{bat}}{dT}\) represents the reversible entropic heat. An advanced BMS uses a coupled electrical-thermal model to predict this heat generation, informing the VTMS controller of the impending cooling load. Conversely, when heating is required, the BMS communicates the target temperature and may coordinate with the VTMS to utilize waste heat, minimizing the use of the energy-intensive PTC heater. Thus, the fidelity of the battery management system’s thermal model directly impacts the efficiency of the entire vehicle’s energy use.
Modeling and Simulation Framework
To analyze the performance of this integrated system, a detailed simulation model was built using AMESim. The model incorporates component libraries for the vehicle dynamics, fuel cell stack, electric motor, and cabin. For this study, the power battery pack was modeled using an equivalent circuit model (ECM) coupled with a thermal mass, representing the core functionality managed by the battery management system. The ECM parameters (open-circuit voltage, internal resistance) were made temperature-dependent based on typical lithium iron phosphate (LFP) chemistry characteristics. The key vehicle parameters used in the simulation are listed below:
| Parameter | Value / Type |
|---|---|
| Vehicle Dimensions (L×W×H) | 12000 × 2550 × 3450 mm |
| Curb / Gross Weight | 12600 kg / 18000 kg |
| Fuel Cell Stack Power | 80 kW (PEMFC) |
| Battery Capacity | 105 kWh (LFP) |
| Motor Rated Power/Torque | 100 kW / 1525 Nm |
The driving cycle selected was the China Heavy-duty Commercial Vehicle Test Cycle for Bus (CHTC-B), which includes both low-speed and high-speed segments, totaling 1310 seconds. The simulation was run for two consecutive cycles (2620s) to observe steady-state thermal behavior.
The heat generation models for the primary components were implemented as follows. For the fuel cell stack, the total waste heat is:
$$Q_{gen, stack} = N I_{stack} \left( U_{equal} – \frac{U_{stack}}{N} \right)$$
where \(N\) is the number of cells, \(I_{stack}\) is stack current, \(U_{stack}\) is stack voltage, and \(U_{equal}\) is the single-cell equilibrium voltage. For the motor, the heat generation is calculated from its output power and efficiency \(\eta_{mot}\):
$$Q_{mot} = (1 – \eta_{mot}) P_{mot}$$
The heat exchange across radiators, heaters, and chiller is governed by the fundamental equation:
$$Q = h A \Delta T_{lm}$$
where \(h\) is the overall heat transfer coefficient, \(A\) is the heat transfer area, and \(\Delta T_{lm}\) is the log-mean temperature difference. The controller logic described in Table 1 was implemented using state machines and PID controllers to regulate pump speeds, fan speeds, and valve positions.
Simulation Results and Discussion
High-Temperature Environment Performance (Cooling)
The system was evaluated under ambient temperatures of 34°C, 37°C, and 40°C. In all cases, the integrated VTMS successfully maintained component temperatures. The fuel cell stack temperature stabilized within the 70-75°C range. The motor temperature was controlled at its setpoint of 80°C via the proportional valve. Crucially, the battery management system’s thermal target was met: when the battery temperature reached 32°C, the system activated cooling. At 34°C ambient, the battery was cooled by the liquid circuit alone. At 37°C and 40°C, the A/C system was engaged (Mode 9) to provide enhanced cooling via the battery chiller, bringing the pack temperature down to 28°C effectively. The cabin temperature was also cooled to 25°C by the A/C system. The total vehicle equivalent hydrogen consumption, which includes the energy cost of operating the A/C compressor, pumps, and fans, increased with ambient temperature, as shown below:
| Ambient Temperature (°C) | Equivalent H₂ Consumption (g) | Increase vs. Previous |
|---|---|---|
| 34 | 988.59 | – |
| 37 | 1051.46 | +6.36% |
| 40 | 1145.03 | +8.90% |
The growing increment highlights the significant parasitic load imposed by the thermal management system, particularly the A/C, under extreme heat. This underscores the need for efficient component cooling to reduce this penalty, a task where the battery management system’s accurate thermal load prediction is valuable.
Low-Temperature Environment Performance (Heating & WHR)
For low-temperature conditions (-10°C, -5°C, 0°C, 5°C), the system executed cold-start strategies. The PTC heaters warmed the fuel cell to 10°C and the battery pack to 20°C, with energy drawn from the battery itself. Once the stack reached approximately 70°C, the system switched to Waste Heat Recovery (WHR) Mode 4 to heat the cabin, significantly reducing PTC usage. The effectiveness of this integrated strategy is evident in the cabin temperature rise and the overall energy savings.
| Ambient Temp. (°C) | Time to Cabin 20°C (s) – WHR Mode | H₂ Consumption – WHR (g) | H₂ Consumption – Pure PTC (g) | Energy Saving |
|---|---|---|---|---|
| 5 | 122 | 890.12 | 941.89 | 5.50% |
| 0 | 176 | 922.42 | 992.40 | 7.05% |
| -5 | 246 | 952.71 | 1045.17 | 8.85% |
| -10 | 342 | 984.17 | 1098.93 | 10.44% |
The results demonstrate that waste heat from the powertrain is sufficient to meet cabin heating demands down to at least -10°C ambient, yielding substantial energy savings. The battery management system also benefits indirectly, as the reduced PTC load for cabin heating preserves more electrical energy for propulsion or battery heating, improving overall range.
Extreme Cold Environment Analysis
To probe the limits of waste heat recovery, the system was simulated under extreme cold (-15°C, -20°C, -25°C, -30°C), assuming the stack and motor were already at operating temperature. The findings were critical for defining the control boundaries of the integrated battery management system and thermal controller.
- At -15°C, the waste heat alone was just adequate to bring the cabin to 20°C and maintain it.
- At -20°C and below, waste heat was insufficient. The cabin temperature plateaued below the comfort zone.
Therefore, for ambient temperatures below -15°C, a hybrid heating strategy (WHR + PTC assist) is necessary. The PTC provides the supplemental heat needed to reach and maintain the cabin setpoint. The additional energy cost of this PTC assist, expressed in equivalent hydrogen consumption, increases sharply with decreasing temperature:
| Ambient Temperature (°C) | Additional Equivalent H₂ for PTC Assist (g) |
|---|---|
| -20 | 33.50 |
| -25 | 36.89 (+10.1% vs. -20°C) |
| -30 | 44.10 (+19.5% vs. -25°C) |
This analysis provides a clear threshold for the supervisory controller: prioritize pure WHR down to -15°C, and activate the hybrid WHR+PTC strategy for colder conditions, with the battery management system being informed of the increased electrical load.
Conclusion
This first-person investigation into an integrated thermal and battery management system for a fuel cell bus demonstrates a viable path toward enhanced energy efficiency and operational reliability. By architecting a system where waste heat is a transferable resource and component loops are strategically coupled, the parasitic loads associated with thermal management can be significantly reduced. The proposed nine-mode control strategy effectively manages the diverse thermal needs of the fuel cell stack, motor, cabin, and—centrally—the lithium-ion battery pack. The role of an intelligent battery management system (BMS) is highlighted as fundamental, not only for managing the battery’s electrical state but also for providing accurate thermal demands and enabling synergistic heat exchange within the vehicle.
Simulation results confirm the system’s capability under a wide range of environmental stresses. In high-temperature conditions, all components are maintained within safe limits, albeit with an increasing energy cost. In low-temperature conditions, the waste heat recovery strategy proves highly effective, saving over 10% in equivalent hydrogen consumption compared to conventional PTC-only heating at -10°C ambient. The study also quantifies the limits of waste heat, defining -15°C as a threshold for requiring auxiliary PTC heating in extreme cold.
Future work will involve hardware-in-the-loop validation of the control strategy and further optimization of the component sizing and control algorithms. Additionally, integrating more advanced predictive functionalities into the battery management system, such as anticipating thermal loads based on route topography and weather forecasts, could unlock further efficiency gains. This integrated approach to thermal and battery management is a crucial step in developing the next generation of high-performance, long-range fuel cell commercial vehicles.