Definition of an equivalent contact temperature to evaluate the ergonomic quality of contact-based climatization systems in vehicles (FAT3)

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Two consecutive studies in the field of vehicle air conditioning (FAT publication series no. 261, 2013; FAT publication series no. 272, 2014) confirmed that an intelligent combination of heating devices in close proximity to the body (seat heating, steering wheel heating, and surface radiation heaters) in combination with a lowered operative temperature in the vehicle interior contribute significantly to energy savings. At the same time, the local heating devices were able to compensate for the thermal sensation of people sitting in the cabin although the overall temperature was lowered.

The FAT 3 project builds upon these preliminary studies and takes a much more detailed look at the seat contact area. The latter represents the interface between the clothed body surface and the corresponding seat surface at people’s back and buttocks, and is consequently in thermal exchange with approximately 25% of the human body surface. The associated physical processes are essentially characterized by the transport of thermal energy through clothing layers, sensible heat (conduction), and latent heat (evaporation) mechanisms.

Currently available thermal comfort models do not adequately represent this crucial contact area. However, studies at the Chair of Energy Efficiency and Sustainable Building (E3D) clearly show that this area has a significant influence on both the local and global thermal comfort sensation of people, and thus cannot be neglected as in the ISO 14505-2. The latter is commonly used in the automotive sector to assess local thermal comfort sensation by applying the holistic measure of equivalent temperature (Teq). However, only the dry heat transfer processes by convection and radiation between humans and the environment at the exposed body surfaces are considered with this approach. A normative evaluation parameter for the seat contact area is not available in this international standard, which means that the quality of currently available and locally effective air conditioning systems at contact points cannot be measured, compared, or evaluated properly.

In the scope of this project, this gap is to be closed and an evaluation scheme for quantifying the quality of local air-conditioning systems in the seat contact area is to be developed. The evaluation scheme is based on the standardized, empirical methods of ISO 14505-2 for the evaluation of the local comfort sensation with the help of the known climatic sum variable equivalent temperature (Teq).

Important subtasks are the introduction of a definition of terms and the development of a calculation formula for the determination of the uniform reference quantity “equivalent contact temperature” (ECT) under consideration of heat conduction, latent and sensible heat transport. Corresponding physical heat transfer processes at the human-seat interface are subjected to detailed metrological considerations for this purpose and require the specification of a corresponding measurement methodology, which includes in particular the recording of the heat flow through the seat, the relative (contact) humidity and temperatures of the seat surface or seat underside and seat back as well as the human skin surface in the contact area.

The robustness of the resulting computational approach is tested with experimental studies in mockup car cabins. Participants also answer comfort queries, which allow a correlation between the ECT and local/global hygro-thermal comfort perception of individuals. In order to ensure the comparability of the developed system with results from preliminary studies, appropriately defined climate scenarios are used. The consideration of elevated cabin air temperatures represents an extension of already developed findings, and enables the evaluation of the influence of seat ventilation on the thermal comfort perception of persons in vehicles.