Cooling

The cooling system in general

The thermal management of the GLC F-CELL Type 253 includes the cooling of the fuel cell, the climate control of the vehicle interior and the cooling of the components of the high-voltage on-board electrical system. The fuel cell system and the components of the high-voltage on-board electrical system are cooled by three closed, separate coolant circuits:

 High-temperature circuit

 Low-temperature circuit 1

 Low-temperature circuit 2

The high-temperature cooling circuit takes over the cooling of all components of the fuel cell system that are involved in cold combustion as well as the climate control of the vehicle interior. The flow temperature of the fuel cell must be 68 °C. The flow temperature lies in a range from 50 °C to 80 °C.

Low-temperature circuit 1 cools the electric machine and the DC/DC converter, the PDU (Power Distribution Unit) and the exhaust air turbocharger of the fuel cell system..

Low-temperature circuit 2 cools the high-voltage battery, the DC/DC converter and the alternating current charger for high-voltage batteries.

Each of the low-temperature circuits has one and the high-temperature cooling circuit two variable speed coolant pumps as well as various regulation valves.

The cooler for the high-temperature circuit, the two coolers of the low-temperature circuits and the condenser are integrated in one cooling module with side coolers. A fan motor located centrally behind it (M4/7) ventilates the cooling module. The fan motor and all coolant pumps are activated by the powertrain control unit (N127) via LIN. In the high-temperature cooling circuit, a particulate filter protects the fuel cell from pollutants (> 100 µm). An ion exchanger ensures that the coolant maintains a low degree of conductivity (< 5 µS/cm).

Different coolants are used in the coolant circuits. While a special coolant is provided in the high temperature circuit, a standard coolant is used in the low-temperature circuits.

 Only coolant that is specially suitable for fuel cell vehicles may be used. Refilling with commercially available coolants or water causes irreparable damage to the fuel cell.
Please read the notes about the fuel cell coolant: AH06.50-P-0001-01RFW

 Extensive information on the fuel cell system and warnings when the fill level falls below that required are shown on the instrument cluster (A1).

High temperature cooling circuit

The fuel cell operates in a temperature range of approx. 80° C and is therefore cooled by a separate high-temperature cooling circuit. The PreCon function (pre-entry climate control) enables preheating of the fuel cell. A cold start is possible down to -25° C.

The high-temperature cooling circuit is regulated by the powertrain control unit (N127). The coolant pressure in the cooling circuit is temperature-dependent and also depends on the speed of the fuel cell coolant pumps.

The temperature of the high-temperature cooling circuit is recorded via the following temperature sensors:

 High temperature coolant circuit temperature sensor 1 (B94/28)

 Cathode outlet temperature sensor (B94/26)

 Cathode inlet temperature sensor (B94/27)

The coolant level in the expansion reservoir of the high-temperature cooling circuit is recorded by the high-temperature circuit coolant fill level sensor (S41/1).

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Low-temperature circuit 1

Low-temperature circuit 1 cools the electric machine and the DC/DC converter, the PDU (Power Distribution Unit) and the exhaust air turbocharger of the fuel cell system..

Low-temperature circuit 1 is regulated by the powertrain control unit. It evaluates the data from the low-temperature coolant circuit 2 temperature sensor and actuates the rear electric machine coolant pump where necessary.

 The power electronics are also flowed through for low outside temperature, always with a minimum coolant volume flow (dependent on the coolant temperature).

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Low-temperature circuit 2

Low-temperature circuit 2 cools the high-voltage battery, the DC/DC converter and the alternating current charger for high-voltage batteries.

Low-temperature circuit 2 is regulated by the powertrain control unit. It evaluates the data from the low-temperature coolant circuit 2 temperature sensor and actuates the high-voltage battery coolant pump where necessary. According to the ambient temperature waste heat from the high-voltage battery is led away over the low-temperature circuit 2 cooler or over the heat exchanger attached to the refrigerant circuit. Regulation of low-temperature circuit 2 takes place over actuation of the high-voltage battery cooling switchover valve.

The heat exchanger cools the coolant by means of the refrigerant injected into the heat exchanger and evaporated. The cooled down coolant is subsequently available to low-temperature circuit 2. When the high-voltage battery is being charged, low-temperature circuit switchover valve 2 is switched in the direction of the alternating current charger for the high-voltage battery and DC/DC converter control unit in moderate temperatures. The waste heat from the electronics is dissipated via the low-temperature circuit 2 cooler.

At low temperatures of the high-voltage battery, the coolant is led over the heat exchanger blocked off from the high-voltage battery cooling expansion valve. In this case, the heat capacity of the high-voltage battery is used to cool the electronics of the alternating current charger for high-voltage batteries and of the DC/DC converter control unit.

 The energy management module in the powertrain control unit controls the energy flow of the fuel cell system. It is, in relation to the electric variables, the interface to the battery management system control units, power electronics and to the electric refrigerant compressor.

If the enable is awarded by the energy management the powertrain control unit makes the request to the climate control control unit (N22/1) via the CAN network for actuation of the electric refrigerant compressor. The air conditioning control unit then actuates the electric refrigerant compressor via the climate control LIN 2 (LIN B8­2). The high-voltage battery cooling expansion valve is opened by the battery management system control unit and the refrigerant flows through the heat exchanger. In this way, thermal energy is extracted from the low-temperature circuit 2.

The cooling output is primarily dependent on the level of actuation to the electric refrigerant compressor. In idle and during an automatic engine stop, the output of the electric refrigerant compressor is limited to a maximum of approx. 2 kW. If the high-voltage battery's state of charge is very low, the output of the electric refrigerant compressor is regulated down to 0 kW.

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Overpressure protection function

Air or hydrogen can enter the cooling circuit through leaks in the fuel cell stack, the charge air cooler or the hydrogen preheater. The gas input is dissipated to the high-temperature cooling circuit expansion reservoir via a bleed line and discharged into the atmosphere in a defined way by means of an overpressure valve. The valve opens at an overpressure > 0.8 bar. The escaping gas/air mix is diverted to the engine compartment.