Air-Conditioning Systems for City Buses

Where city buses are concerned, the general attempts at increasing our energy efficiency and reducing our CO₂ emissions call for the optimization and further development of all energy consuming components. Due to their share of up to 43 % of the total energy consumption [1], the auxiliary components require our special attention. One of the most important energy consumers among the auxiliaries is the heating and air-conditioning system (HVAC).

The reduction of emissions and operating costs is one of the great development goals in the field of HVAC Systems. However, the trend towards the complete electrification of city buses raises the following additional challenges:

• Fully electric city buses do not have a combustion engine, which usually is the central heat source for passenger space heating.
• Keeping the aim of zero emissions on both a local and a global level in mind, the operation of fuel-driven auxiliary heating units must be viewed as critical.
• Because the amount of energy that can be taken out of the electric storage units is limited, HVAC systems need to fulfill higher demands.
• Both the electric energy storage units and the power electronics are only operational within certain given temperature limits, which might make the additional heating or cooling of these components necessary.

Against this backdrop, the Fraunhofer IVI conducts diverse research in the field of HVAC systems for electric vehicles.

One example of this research is the formulation and testing of concepts for the efficient heating of the passenger space in electric buses using heat losses, heat pumps and heat storage systems. In addition, the Fraunhofer IVI develops and implements different innovative concepts for the cooling of power electronics and traction batteries specifically for the application in electric city buses. Another area of work is the long-term thermal simulation of buses in order to determine their heating and cooling demands for a chosen period up to one year.  

The solutions developed undergo a certification process within the overall system. Afterwards they are integrated into the vehicles as individual components and demonstrated in the scope of application examples.

[1] Hausberger, S.; Kies, A.; u. a.: Reduction and Testing of Greenhouse Gas Emissions from Heavy Duty Vehicles - LOT 2, 2012

• Comparison and evaluation of different HVAC systems for buses (city and intercity buses, travel coaches) with regard to their energy demand and performance
• Estimation of the annual operating costs for different HVAC solutions in vehicle fleets based on customer-specific route and climate data

• Lastsynchrones Thermomanagement (Load-synchronous thermal management),
• SEB-EDDA: Cooling concept for power electronics and batteries,
• HVACCO2SIM: Simulation tool for determining the CO₂ emissions of HVAC systems
• AutoTram^® Extra Grand: Battery cooling system
• Balance of the heating system of a city bus during operation