Read time: 8 minutes
Target audience: EV Thermal Engineers / HVAC and Cabin Comfort Engineers / Thermal-Fluid Industry / Automotive Researchers
Written by: Dr. Tabish Wahidi
Winter is hard on electric vehicles. With no engine to throw off free heat, an EV has to draw cabin warmth from the same battery that moves the car, which makes climate control one of the biggest non-propulsion loads on a cold day. Each watt the heater takes is a watt that cannot be spent on distance, so any way of keeping passengers comfortable with less heating power translates straight into more range.
Most vehicles still rely on forced-air HVAC: a central unit heats air and blows it around the cabin through vents. It works, but it has to warm a large volume of air, so it takes a while before passengers actually feel better. The moving air can also feel draughty enough to undo some of the benefit. Radiant heating takes a different route. Instead of conditioning the whole cabin, it delivers heat directly to the people sitting in it.
Judging a strategy like this before hardware exists calls for simulation that handles radiation, airflow, and the human body’s response at the same time. This article walks through a simulation study that pits standard HVAC heating against a radiant, heated-surfaces layout, and explains the connected toolchain behind the analysis.
The advantage of radiant heating makes sense once you look at how heat reaches a person. Three mechanisms are at work in a cabin:
In a radiant system, heated panels are built into the interior where they can exchange heat with occupants most effectively. Typical locations are the roof, the centre console, and the footwells, which together surround passengers with warm surfaces. Because people judge comfort by mean radiant temperature (MRT) in addition to the air temperature, those surfaces allow the air setpoint to be lowered without anyone feeling colder. The system no longer has to heat all the cabin air first.
Getting such a system right involves radiation, airflow, human physiology, and a large space of design choices, so no single program covers it. The study therefore links four tools into one workflow:
Each tool in the chain contributes physics that the one before it cannot provide, starting with the thermal model of the cabin.
The cabin is modelled in TAITherm, which solves how the temperatures of interior components change over time. Radiation between surfaces depends on geometry, so TAITherm’s automatic view factor calculation is what makes the radiant heat exchange trustworthy in an irregular cabin shape. The same model can also account for warm-up transients, solar load, and interactions between components. The heating hardware in this study was a multi-zone arrangement: radiant panels in the roof and foot wells, one on the console, plus a heated seat and steering wheel.
RapidFlow brings in the air side of the problem. It shows how the HVAC mode, air speed, and air temperature combine with the radiant surfaces to define the thermal conditions around each occupant, so convection and radiation are analysed together and not as separate effects.
Surface and air temperatures alone do not say whether someone is comfortable. The Human Thermal Extension links a physiological model of the body to the TAITherm solver and estimates sensation and comfort for individual body segments across the drive cycle. Its outputs include Berkeley Comfort ratings, equivalent homogeneous temperature (EHT), MRT, and PPD/PMV.
CoTherm ties the tools together and runs the process automatically. It supports DOE sweeps and optimization loops across many panel and system configurations, searching for the best compromise between comfort and energy use.
Simulations under cold-weather conditions compared three ways of heating the cabin:
The gap in power consumption was large. Reaching comfortable conditions for the driver took:
That is a cut of roughly 72% in heating power for an equivalent comfort result. For a battery electric vehicle, the energy not spent on heating stays available for driving, so winter range improves.
The heated surfaces case also ran with a lower cabin air temperature while still hitting the same comfort targets. That supports the physical explanation: radiant panels raise MRT and local surface temperatures directly, so there is no need to bring the whole air volume up to temperature before occupants feel warm.
Lower energy use is a strong engineering result, but passengers care about how the car feels, most of all in the opening minutes of a cold trip. A conventional HVAC system creates a built-in lag. Warm air has to fill a big cabin volume before anyone notices, and until then occupants are sitting on cold surfaces, exposed to a low MRT, and feeling air rush from the vents.
Radiant panels do not have that lag, because they start heating people and nearby surfaces as soon as they switch on. Scored with the Berkeley Thermal Comfort Model, the heated surfaces case pulled ahead of both HVAC modes at about 1 minute 30 seconds and stayed ahead for the rest of the drive cycle. It was the only case that reached a comfortable state during warm-up and held it.
This result fits how comfort works. It is shaped by many factors together: MRT, local skin temperatures, air speed, humidity, clothing, and activity level. Radiant heating acts directly on MRT and local surface temperatures, so it affects more of that picture than an air temperature change can. In practice, the benefits include:
Radiant heating offers a practical way to gain both efficiency and comfort in electric vehicles. In this study, warming occupants directly cut the heating demand from around 3,370 W to about 940 W, and it made passengers comfortable sooner and kept them that way longer than forced air did. Combining TAITherm, RapidFlow, the Human Thermal Extension, and CoTherm into one workflow lets engineers:
As EV programs keep pushing for more range and better efficiency, targeted heating that has been tested through integrated multi-physics simulation is likely to become a standard part of designing cabins that use less energy and feel better to sit in.
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