“But since the fuel cell stack generates waste heat, why do we need to purchase an additional cabin heater?” This question comes up at the very first meeting of nearly every fuel cell bus project we’ve been involved in. It’s certainly a valid question. However, this line of thinking usually lasts only until the first “cold start”—if even that long.
Our answer has always been the same: From the very beginning of the project, a 20-kilowatt (kW) PTC (positive temperature coefficient) coolant heater should be explicitly included in the configuration; the waste heat from the fuel cell stack should be viewed as a “supplement” rather than the core of the heating system. Fleets operating hydrogen-powered buses in northern regions during winter reached this consensus several years ago. The fleet of hydrogen-powered buses deployed during the Beijing Winter Olympics serves as a clear example: every vehicle was equipped with a waste heat recovery system and an electric heater.
Let’s start with temperature. A diesel engine is like a “stove” that both drives the wheels and generates heat, resulting in very high coolant temperatures. In contrast, the advantage of proton exchange membrane (PEM) fuel cells lies in their high efficiency, so their coolant operates at a relatively low temperature. Now consider a “cold start”: the fuel cell stack is at a low temperature when first started and must first “warm up” to function normally and efficiently. In the first few minutes after startup, rather than being a heat source, it actually becomes a “load” that requires additional heating.
The bus has been parked outdoors all night. The driver performs routine checks before dawn, when the windshield is fogged over and the first passengers are already waiting. The fuel cell stack needs heat, the windshield needs to be defrosted and defogged, yet the temperature inside the passenger compartment hasn’t risen at all. This is precisely why the heating system was installed in the first place—not for midday, nor for when the vehicle is traveling at a constant speed, but to handle moments like this.
Research on fuel cell buses operating in winter conditions (around -7°C) reveals a general pattern: when the vehicle is traveling at a constant speed, waste heat from the fuel cell can indeed cover part of the cabin’s heating needs; however, once the vehicle enters a low-load condition, the remaining heating demand—which often accounts for the vast majority—must be met by auxiliary electric heaters.
In urban bus operations, the vehicle spends most of its time in low-load conditions: idling at terminal stations, traveling slowly between stops, and frequently stopping to open and close doors… During these times, the fuel cell consumes only a small amount of hydrogen and generates negligible heat, while more than twenty passengers inside the cabin expect a warm environment. If the heating system were configured solely for steady-speed driving conditions, the vehicle would “let passengers down” every day at the terminal—right in front of the very passengers it is supposed to serve. A realistic configuration would inevitably include PTC heaters.
In these projects, one detail is always mentioned last: the coolant (thermal management) system. Fuel cell coolant is not ordinary coolant. The fuel cell stack circuit must maintain low electrical conductivity, so deionized water and an ion exchanger are required; if any substance releases ions into the circuit, it will trigger warranty disputes. Therefore, PTC heaters are rarely placed directly within the fuel cell stack circuit. Typically, heaters have a separate circuit or transfer heat via a heat exchanger.
This raises an issue that teams often consider too late: Which circuit should the heater be connected to? Who is responsible for controlling it? If the fuel cell stack fails, how should the passenger compartment heating be handled? These issues must be planned out on paper in advance. If they are not addressed until the winter validation phase, the entire testing season could be wasted.
Take a 12-meter city bus operating in northern regions as an example, and calculate its load item by item: First is continuous heat loss—heat dissipates through the body panels and windows, and this load is always present. Second is the opening and closing of doors—stopping every few hundred meters causes heat to escape from the cabin while cold air rushes in. Third is defrosting—this is a mandatory requirement that must be prioritized; after all, the vehicle cannot operate if the driver cannot see the road clearly. Adding these three items together, the total load falls within the range of 15–25 kW.
If excess heat is available, it can offset part of the load. However, on the coldest mornings (rather than under average operating conditions), approximately 20 kW of heating power is still needed to bridge the gap. This is why the figure of 20 kW keeps coming up—it’s not a trend, but rather determined by the load gap that the fuel cell cannot cover. Our E Series products are designed for this purpose, with power ratings ranging from 20 to 24 kW and a voltage range of 400 to 800 V; if the passenger compartment is larger or the operating route is in a colder climate, the higher-tier Q Series products should be selected.
For 12-meter fuel cell city buses operating in cold climates, we recommend: installing a 20-kilowatt (kW) PTC coolant heater with an independent control loop; integrating the waste heat recovery system into this loop to share the heating load; implementing CAN bus control so that the energy management system can optimally allocate thermal energy based on hydrogen consumption and the route; and ensuring that defrosting takes priority immediately after the vehicle starts. Furthermore, field validation is required on the coldest routes, during the coldest periods, and under full-load conditions. After all, average data cannot address actual heating requirements.
On mild days, the heat generated by the fuel cell stack is generally sufficient, and the heater may require little to no operation. However, it cannot cover certain specific operating conditions: for example, during cold starts (when the stack is in its own preheating phase) and during low-load operation (when very little waste heat can be recovered). It is precisely under these conditions that the need for cabin heating becomes most acute. Therefore, a heater remains a necessary component—although it remains largely idle during good weather, it must be installed and connected to the ductwork.
Early fleets tried this approach, and from a thermal management perspective, it worked quite well. But the issue lies in the “narrative logic”: if a bus sold to a city on the basis of “zero emissions” is burning diesel on the side of the road, this is clearly unacceptable; moreover, some regions even explicitly prohibit such behavior. Using electric heating ensures that the “zero-emission” promise lives up to its name. For most projects of this kind, it is precisely this “zero-emission” promise that gives the vehicle its raison d’être.
It does consume energy—whether from hydrogen or the buffer battery. We won’t gloss over that fact. But consider the alternative: if a bus cannot even clear the frost from its windshield, it simply cannot be put into service. We recommend preheating in the parking lot (or garage); making full use of the waste heat generated by the fuel cell stack; and having the energy management system allocate the remaining heat requirements appropriately. This ensures both transparency in cost accounting and the vehicle’s proper operation.
Are you planning to deploy fuel cell buses in cold regions? Please provide the vehicle length, the most severe operating conditions, defrosting requirements, and the fuel cell stack’s cooling circuit architecture. We will determine the heater specifications for you prior to winter testing and address any technical issues related to the cooling circuit in advance.
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