## 4 Discussion  
Research on heating in vehicles is of major importance, especially in cold weather zones, but it is not wide spread over all types of vehicles. Most studies are in the area of automobiles, with few on buses, due to the fact that analyzing and simulating a bus (which is significantly larger and has an intricate structure) is a more complex and time-consuming process. This article aimed to respond to some of the thermal problems related to electric buses. Estimation of the necessary heat to assure thermal comfort during cold winter days is quite important due to the limited power of these types of electric vehicles. Additionally,  
Figure 6: Schema cross-section through the bus structure for an open door. along with the limits in energy stored onboard and the vehicles complexity, their particular operating regime results in a complex study due to the various numbers of passengers, doors frequently opening, and the necessity to operate throughout the day. This study estimates the eight main heat load components on electric buses. Two of them were assumed to be negligible for our thermal model. Firstly, the heat load due to the electric traction motor, and secondly that due to the traction battery pack, as the actual elements are highly efficient. Therefore, six heat loads were considered for our model: metabolic heat load, Sun heat load, ventilation heat load, ambient heat load, heat flow through the open doors, and the heat load generated by the heating system in the bus. These heat loads were calculated for a 12 m electric bus with the following characteristics: 100 passengers, 150 kW electric traction motor, 3 doors, and 33 kW installed power for the heating system. The calculation was estimated for a harsh winter cold day of December in Romania, with a median temperature of \(-10\)\({}^{\circ}\)C. The thermal comfort of the passengers was calculated for a 20 \({}^{\circ}\)C temperature inside the bus, resulting a gradient of temperature of 30 \({}^{\circ}\)C.  
The value of the metabolic heat load depends on the passenger numbers in the vehicle, and the Sun heat load depends strongly on the weather conditions. These heat loads are as a contribution (a heat gain) to the inside bus heating (considered as positive in the thermal model of the vehicle), and, for a harsh day in winter, these represent about 22% and less than 15%, respectively, of the necessary heat of the bus. The heat load from ventilation is an energy-consuming heat load (with energy supplied from the onboard storage system) and it represents less than 10% of the necessary heat for the bus. Being a mandatory system in the vehicle, its contribution to the vehicle heating can be considered to be a heat gain. Thus, these three heat loads assure an input of heat of about 23% of the necessary heat inside the bus. Heat loss on the bus is composed of the ambient heat loss through the bus structure and the heat loss through the open doors at the stops, with an average value of energy demand of 12-14 kWh. Of these, through the bus structure the heat loss is the most significant, with its four components--the windows, the walls, the floor, and the roof--representing about 78% of the total heat loss. The main heat loss is through the windows (about 26%), while the other three heat loss components have almost the same contribution (about 8.1-8.3% each). Heat loss through the open doors depends on the route of the bus, the number of stops, and the number of passengers getting on and off the vehicle, with an estimation of 22% of the total heat loss. The heat load balance led to the conclusion that the heating system must supply a minimum heat load of 18.5 kW. This value is useful for more successfully designing the heating system and managing its operation.