In order to increase the attractiveness of the electric mobility, neither the comfort nor the driving range of the vehicle may suffer under extreme ambient temperatures. Optimized thermal management helps to meet customer comfort requirements, while at the same time reduces energy demand . In this article, the structure and the validation of a new developed simulation model are explained. Based on the model, measures to reduce the energy demand with regard to cabin temperature control are investigated. Finally, the results, as well as a summary of the energy saving potentials are presented.
The generated vehicle simulation model consists of several modular components, as depicted in Figure 1. Each of these objects represents a subsystem of the electric vehicle. With this model, it is possible to simulate different driving profiles, as well as ambient conditions. Thus, various thermal management measures can be assessed regarding their influence on system efficiency [2, 3]. For the validation of the simulation model, measurements were carried out on both, the component and system level.
Various cabin measures are investigated regarding their energy saving potentials with the created simulation model. All simulations were conducted during the WLTP driving cycle for 30 minutes at an ambient temperature range from -20 to +20 °C. At the same time, the cabin is heated from the respective ambient temperature to an average cabin temperature of 20 °C and the total energy demand is determined. In addition, to the energy savings for a particular ambient temperature, the energy saving potential in annual means is calculated. Therefore, climatic data from Europe is used . The results, which are referred to as Reference in the following, show the energy consumption of the electric vehicle when the cabin is heated up only with the PTC-air heater. Under these conditions, the vehicle has an energy demand of 31.4 kWh/100 km at an ambient temperature of -20 °C. This result is also used as a reference point for the calculation of the energy savings. Furthermore, the energy consumption of the vehicle without heating the cabin is shown in all diagrams. It is 22.2 kWh/100 km at an ambient temperature of -20 °C and is therefore, significantly below that of the heated vehicle, as seen in Figure 2.
>> AS A FURTHER MEASURE TO REDUCE HEATING ENERGY DEMAND DURING WINTER OPERATION, RADIATION SURFACES ARE CONSIDERED
A promising approach for saving heat energy is a reduction of the fresh air rate in the air conditioning of the cabin. In contrast to fresh-air operation, a large proportion of the required heating energy can be saved in recirculation mode. At an ambient temperature of -20 °C, the total energy demand is reduced from 31.4 kWh/ 100 km to 27.9 kWh/100 km, which corresponds to a saving of 11.1 percent, as seen in Figure 2. Averaged over the year, this results in a saving of 3.1 percent. As a further measure to reduce heating energy demand during winter operation, radiation surfaces are considered, as seen in Figure 3.
Through the use of radiation heat surfaces, the air temperature inside the cabin can be lowered with a comparable sensation of comfort of the occupants. For the corresponding comfort tests, six electric radiation surfaces were installed in the driver and passenger foot wells (three on each side). In the simulation, the result of the previous measurements were confirmed. The energy demand at -20 °C can be reduced from 31.4 kWh/100 km to 30.1 kWh/100 km by the use of radiation surfaces. This corresponds to an improvement of 4.1 percent, as seen in Figure 2. Averaged over the year, an energy savings potential of 3.8 percent can be realized. The generated heat is sufficient to heat the cabin completely without the PTC-air heater from an ambient temperature of 10 °C or higher and without reducing the comfort for the occupants. In order to reduce the heating energy demand even further, targeted heating of the driver is chosen as another measure. The total energy demand at -20 °C can be reduced to 30.8 kWh/100 km or by 1.9 percent from 31.4 kWh/100 km, as noted in Figure 2. Averaged over the entire year, a savings of 0.5 percent is achieved. By integrating a heat pump into the vehicle, the energy demand can be reduced additionally. With the heat pump, heat can be absorbed at lower temperature levels and discharged at higher temperature levels. The amount of heat emitted is thereby much higher than the work expended . For an ambient temperature of -10 °C, the energy requirement can be reduced from 27.5 kWh/100 km to 23.7 kWh/100 km or by 13.8 percent, as depicted in Figure 2. The energy savings potential is reduced for rising ambient temperatures. The heat generated is not sufficient to heat the cabin to 20 °C at an ambient temperature of -20 °C due to the integrated compressor. For this reason, Figure 2 shows only the energy demand at ambient temperatures from -10 to +20 °C.
>> THROUGH THE USE OF RADIATION HEAT SURFACES, THE AIR TEMPERATURE INSIDE THE CABIN CAN BE LOWERED WITH A COMPARABLE SENSATION OF COMFORT OF THE OCCUPANTS
In addition, combinations of the individual measures were also considered, as noted in Figure 4. By combining the individual measures, the energy saving potential can be increased even further. The starting point for the comparison of the different combinations is the energy consumption of the vehicle in recirculation mode. Combination 1 describes the expansion of the recirculation mode with the use of radiation surfaces. The energy demand at an ambient temperature of -20 °C can be reduced from 31.4 to 27.1 kWh/100 km. This corresponds to a saving of 13.7 percent. The average energy consumption over the year can be reduced by 5.6 percent. If the system is extended by targeted heating of the driver (combination 2), 2.3 percent of the energy can additionally be saved at an ambient temperature of -20 °C. This results in an energy consumption of 26.4.kWh/100 km and thus a saving of 15.9 percent. For the targeted heating of the driver synergy, effects with other individual measures can be seen. Over the entire year, 6.4 percent of the required energy can be saved. Finally, combination 3, an expansion of combination 2 by a heat pump operation, is considered. The simulation shows that a total of 22.9 percent of the energy can be saved at an ambient temperature of -20 °C compared to reference operation. The energy demand is reduced from 31.4 kWh/100 km to 24.2 kWh/100 km. Averaged over the year, an energy savings of 7.2 percent can be determined. Using combination 3, the energy demand at -20 °C of 24.2 kWh/100 km is only 9.0 percent higher than that of the vehicle without cabin heating. Without any measures, the energy consumption with cabin heating was 41.4 percent higher than without, as seen in Figure 4.
>> A TOTAL OF 22.9 PERCENT OF THE ENERGY CAN BE SAVED AT AN AMBIENT TEMPERATURE OF -20 °C COMPARED TO REFERENCE OPERATION
Summary and Conclusion
In this article, various measures for the cabin air-conditioning have been introduced using a vehicle simulation model that reduces the energy demand of the electric vehicle at low ambient temperatures. By combining the individual measures, the shown energy savings potential can be increased even further. Throughout the year, 7.2 percent of the total energy can be saved. This saved energy can be used to increase the range and thus directly contribute to an improvement in the acceptance of electric vehicles. Intelligent operating strategies for the individual components of the thermal management system could result in a significantly higher saving potential. These operating strategies and the use of the heat pump as a refrigerating machine at high ambient temperatures will therefore be the subject of further research.
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