FEV evaluates cabin heating system

Increased range and improved comfort for electric vehicles

1. June 2017 | Engineering Service

In conventionally powered vehicles, a portion of the excess radiated heat energy form the internal combustion engine is used to heat the passenger compartment. In electric vehicles, this heat source is not available due to their higher tank-to-wheel efficiency. Instead, an additional heater is necessary that is powered by the battery. The cruising range of electric cars is reduced by 50% when the heater is turned on as a result. Increasing the efficiency of the vehicle or the passenger compartment air conditioning system is a major challenge on the road to making electric cars competitive for everyday use.
In a recent study, FEV focused on the potential for energy savings from heating surfaces in the immediate vicinity of the driver. The heating system was installed in an electrical test vehicle and consists of an electric PTC air heater and electrically operated, radiating surfaces in the passenger compartment. Extensive measurements were conducted on the test vehicle in a climatic chamber and the energy consumption of the heating systems was recorded. By comparing the energy consumption in both heating processes, an energy savings potential of about 9% was identified.

Operating Temperature

The heating system uses both convection and heat radiation to heat the cabin. Therefore, measuring the air temperature alone is insufficient. In order to adequately consider the interaction of both of the heat transfer mechanisms, another method for evaluation of passenger compartment heating must be used.
One good possibility is the so-called operating temperature. The operating temperature weights the heat transfer coefficients of convective and radiative heat transfer. For this measurement, a dedicated sensor for measuring the radiant heat was developed as a first prototype. Experience from the construction sector shows that using the operating temperature instead of the air temperature in the control of a room’s heating system reduces energy consumption and improves thermal comfort.

Temperature Measurement

In order to assess passenger compartment heating, various temperature measurement points were determined inside the passenger compartment. Since the aim of the new measurement method was not to evaluate the heating performance but to compare the two heating methods, obtaining the temperatures at the front seats was sufficient. Temperatures were taken at head and foot levels. 8 temperature measuring points were set up in the test vehicle; 4 measuring points were installed in the driver and the passenger sides, respectively. 2 of the 4 measuring points were attached to the headrests on each side.
A self-developed flat sensor with a 3 cm edge length was used for detecting the operating temperature at foot level. This is because the heat-radiating surfaces were installed in the foot wells of the driver and the passenger sides. The average cabin temperature can be calculated by averaging the air temperatures at head level and the operating temperatures at the foot level.

Heating System

The conventional heating system, consisting of a pure electric air heater and a fan, was extended by electrically operated, heat-radiating surfaces in the vehicle’s foot wells.
A literature study on radiant heating for buildings revealed that radiant heaters can demonstrate energy consumption that is about 10% lower than convective heating systems. According to the study, the air temperature can be reduced by 5 K without adverse effects on the perception of thermal comfort. Additionally, heat losses due to air exchange are reduced. The proportion of the heating energy bound to a transport medium (air flow) is smaller when applying additional radiant heating systems.
Under cold ambient conditions in a vehicle, the feet and legs should be warmer than other parts of the body – which is why heat-radiating surfaces were installed near the feet and legs. The foot wells of the test vehicle were suitable because their walls offered sufficient space for installation. In both the original and the modified heating systems, cold ambient air enters the HVAC-system. The temperature of the air is increased by the PTC heater.
In the modified vehicle with the additional radiant heating system, the heater power was reduced, resulting in a lower increase in the air temperature. The heating power of the two heating systems was determined by measuring the temperature at both the head and foot levels.

Measurement Procedure

A series of measurements was conducted in a climatic chamber at an ambient temperature of -10 °C. The outlet of the air flow in all measurements was set to feet only. The test setup was not influenced by either wind speed or solar radiation. The test vehicle was heated for about 25 minutes. To compare the thermal output of all test configurations, three different temperature measurement locations were considered – at foot level, at head height, and the average temperature of the passenger compartment.

Graphic - heating systems for electric vehicles

Schematic view of the conventional heating system

Graphic - heating systems for electric vehicles

Schematic view of the new heating system

Average Passenger Compartment Temperatures and Energy-saving Potential

At the average passenger compartment temperature, the results showed that the PTC heating output was either too high (measurement combination 1) or too low (measurement combination 2). Therefore, the resulting temperature profiles from measurements Combination 1 and Combination 2 were interpolated, thus providing an estimate of the temperature profile for a measurement with the corresponding PTC heating power. The power consumption of the interpolated measurement was calculated accordingly, averaging the power consumption of measurements Comb. I and Comb. II. Using radiative heating, only 90.7% of the heating energy is necessary to achieve heating performance equal to that of a conventional system. Since the results are based on an interpolation, the potential for energy savings is not justified physically. It can be used as a prediction of energy-saving potential.

Subjective Impression of the Radiative Heating System

During winter in Alsdorf, Germany, subjective tests were conducted in order to adjust the settings of the various heating systems and to check the temperature sensors. During these tests, the radiant heating system was subjectively assessed. The ambient temperature during the test was 3°C, providing realistic conditions for the vehicle heating. A human subject was placed in the front seat of the equipped vehicle while the heating system was operated. The test subject’s clothing was appropriate for the winter conditions, consisting of jeans, a winter jacket and low shoes. Approximately two minutes after the start of the heating process, the radiant heat was sensed by the subject. The air temperature was lower than that of the heat radiating surfaces. In general, the radiant heat was perceived as pleasant. The test subject deliberately touched the radiating surfaces with his legs during the test, thereby improving thermal comfort. Due to the proximity of the radiant surfaces to the legs, this posture could be taken by the driver while driving the car. The test subject reported a positive overall impression of the radiant heating. Objective measurements, even those employing a thermal mannequin, do not consider behavioral responses such as those described above.

Graphic - heating system for electric vehicles

Table - heating system for electric vehicles

Measurements taken:
– Ref. I: Maximum PTC power of 3.2 kW and 0.1 kW of fan power
– Comb. I: Same as Ref. I with an additional 0.2 kW radiation heating power
– Comb. II: PTC power of 2.4 kW, 0.1 kW fan power and 0.2 kW radiation heating power