Electromobility – in-car comfort versus vehicle range

Research in focus April 2013

Owners of electric vehicles have always been forced to make a choice: enjoy comfortable temperatures while driving? Or be able to drive further? The heightened energy demand stems from the fact that the vehicle is not warmed using waste heat from the engine, but by heat that has to be generated purely from electrical energy. A group led by computer scientist Sebastian Stratbücker at the Energy Efficiency and Indoor Climate department of the Fraunhofer Institute for Building Physics IBP is currently collaborating with Volkswagen AG in a joint project called E-Komfort. The project is developing climate control concepts for electric vehicles that will spare owners from having to choose between in-car comfort and vehicle range. The project team wants battery charge to be used more effectively to maximize the potential range of the vehicle. Their approach is to minimize the amount of electricity needed for climate control by heating and cooling the passenger cell in a targeted and selective manner.

The human body is constantly subjected to both warm and cold conditions that vary depending on a person’s surroundings, be it in the living room at home or in the passenger cell of a car. And yet the degree of homogeneity of the ambient temperature varies a great deal between these different environments. Where temperatures are fairly constant throughout a living room space, there are considerable differences in temperature inside a vehicle. People sat in the front do not experience the same climate as passengers sat in the rear, for example, and the temperatures in the footwells can also differ. In a bid to avoid unpleasant differences in temperature and to create an interior temperature that is as comfortable as possible, conventional climate control concepts usually heat or cool the entire passenger space as one. The aim has always been to create a constant temperature throughout the vehicle that is the same for all passengers. “This approach wastes a lot of energy by heating up all the components along with the interior air,” explains Stratbücker, group manager of the Simulation group. Taking this approach, much of an electric vehicle’s available battery charge is used to regulate the passenger cell climate, which significantly reduces the vehicle’s range. In winter, this reduction can be as much as 50 percent, he adds.

For the research team led by Stratbücker, the first step toward finding a solution was to establish the criteria that determine comfort in an electric vehicle. Scientists made use of the Indoor Environment department’s longstanding expertise in collecting and evaluating indoor climate parameters. For many years, the department has dealt with issues relating to comfort and well-being. To this end, it makes use of measurement facilities, carries out statistical analysis of comprehensive trials conducted on test subjects and, increasingly, uses simulation-based methods to design and optimize indoor climate systems. This expertise is also used to in building simulations, for instance, as a way of establishing the best design in terms of thermal comfort and energy efficiency during a building’s planning phase.
Unfortunately, existing evaluation parameters such as the predicted mean vote as used in the ISO 7730 standard, can’t simply be carried over as is from the building sector to the automotive sector. The PMV value represents the average sensation of comfort of a larger group of people and is based on a global statement of thermal comfort – that is to say, for the human body as a whole and only for steady, homogenous ambient temperatures close to the comfort zone. So research scientists from Fraunhofer IBP working on the E-Komfort project, which is funded by Germany’s Federal Ministry of Education and Research (BMBF), first developed the methods required to determine people’s thermal sensation with the help of suitable indicators. One such method is the newly developed climate-measuring system known as DressMAN (Dummy REpresenting Suit for Simulation of huMAN heat loss). The new system concept relies more heavily on the use of commercially available components in combination with system software developed completely in house at Fraunhofer IBP. As part of the E-Komfort research project, a new comfort sensor was developed that is able to measure what is known as the equivalent temperature. This climate index, defined in ISO 14505-2, incorporates measurements of air temperature, air velocity and thermal radiation. Thermal environmental conditions can thus be described using a single numerical value, which enables different climate scenarios to be compared and evaluated. Scientists working on the E-Komfort project then take an actual electric vehicle and work with partners Volkswagen AG and P+Z Engineering GmbH to investigate effective local heating and cooling measures.
In addition, simulation programs are used to investigate the effects of different temperatures on the human body using computational fluid dynamics (CFD). If realistic calculations are to be made, a variety of factors must all be taken into account, including warm inflow from air-conditioning systems, solar radiation, cold radiation from glazing, and people’s thermophysiological reaction to each of these factors. “Our simulation tools can consider local influences and people’s reactions by differentiating between individual body parts and areas of skin,” says Stratbücker. “The simulation program enables us to make a detailed reenactment of how human thermoregulation responds to variable, inhomogeneous stimuli.” When it gets cold, the blood vessels in human skin contract to prevent as much heat as possible from being lost to the environment. The body uses the opposite effect as a response to heat: blood vessels expand in order to increase the flow of blood and thus raise the temperature at the surface of the skin, before unpleasant sweating commences. Skin functions as an extremely sensitive membrane separating the body from its surroundings (German website).
These laborious analyses and simulations give scientists aiming to improve well-being in the passenger cell an idea of which body parts require special attention. “Thanks to models derived from test subject trials, it is known which areas of the body react especially sensitively to small changes in temperature and which have a greater range of tolerance,” explains Stratbücker. The research scientists hope that by implementing targeted local measures, such as using heated seats, heat radiators or small fans, they can ensure a person’s overall sensation of comfort is within the comfort zone. What these measures all have in common is that they require very low power levels since they work in the immediate vicinity of the subject, meaning a global sensation of well-being can be generated without placing too great a demand on the electric vehicle’s battery. The scientists hope that selectively cooling and heating electric vehicles in this way will resolve the conflict of objectives between range and in-car comfort.
The scientists’ main focus is on optimizing electric vehicles’ mileage. Given that air-conditioning systems consume significant amounts of electricity in electric vehicles, and temperature is one of the most important comfort factors, the scientists are concentrating above all on working to optimize the climate in the passenger cell. “This is where we can bring the entire breadth of our core thermal comfort competencies to bear,” says Stratbücker, who adds that his institute’s measuring techniques, its experience in conducting trials using test subjects, and its facilities for reenacting and simulating thermal boundary conditions and thermophysiological reactions make Fraunhofer IBP perfectly qualified to carry out the project.
Scientists are currently collating the data they have collected from the complex simulations they have carried out. They will use this data to develop simpler models for optimizing vehicle range and thermal management in electric cars. Novel vehicle concepts for the automotive industry will have to be designed and standardized if electromobility is to continue to grow in popularity. User acceptance of electromobility will depend not only on the costs involved, but above all on whether the increasing number of owners of electric vehicles still find themselves having to choose between in-car comfort and vehicle range in future.

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