Electric Radiant Heat for EV Cabin Comfort
New HVAC tech helps Toyota tackle the EV heat-vs.-range conundrum.
Vehicle OEMs have been steadily increasing the range of their electric vehicles (EV) and plug-in hybrids (PHEV), but the ranges they claim publicly are in ideal thermal conditions. Now comes the hard part: Deliver those same ratings in real-world weather, in the kind of thermal extremes that challenge the performance of lithium-ion battery packs.
Although air conditioning use in hot summer weather typically causes a modest loss of EV range, it’s the 30-40% drop of range in cold winter weather when the cabin heater is deployed that continues to challenge EVs’ ability to fully replace ICE-powered vehicles. To date, most efforts to improve EV heating performance have focused on electric seat heaters, some electric steering wheel heaters and for the most part, heat pumps built into the HVAC refrigeration system.
But the heat pump performance is limited by the fact that refrigerant flow slows with low winter temperatures, so heat pumps must switch to less-efficient resistance or PTC (positive temperature coefficient) heaters when ambient temperatures drop below 0°C (32°F). The Toyota Prius Prime PHEV’s A/C refrigeration system incorporates a liquid-gas separator and injection of refrigerant gas, a development adapted from static-mount commercial heat pumps. This technology enables heat pump operation down to -10°C (14°F).
First use on bZ4X
In a presentation at the 2022 SAE WCX, Toyota detailed its test installation of an electric radiant heater in a Prius Prime, describing the system as a method for providing supplementary cabin heat quickly and with greater efficiency than other methods. This effort may indicate the Prime, which also has seat and steering wheel heaters, is being used as a testbed for solving the EV cabin heat-vs.-range conundrum. The Toyota testing showed use of the radiant heater resulted in a 5.3% reduction in fuel consumption, and much improved driver comfort within two minutes of cabin entry.
Radiant heating employs one of the three forms of heat transfer. When turned on, a radiant heater emits invisible infrared waves a relatively short distance in a narrow band. The infrared wave band quickly contacts objects in its path and warms them. Aimed at a driver in the Prius Prime’s cabin, the system warms some of the front part of the driver’s body, working in conjunction with a seat heater that covers the back of the body from the shoulders down to the knees. A steering-wheel heater warms the hands and the floor outlet from the HVAC eventually adds warmth to the driver’s knees down to the feet and provides some heat for the rest of the body.
Seat and steering wheel heaters represent another type of heat transfer – conductive heating, which is direct contact with a warm surface. The HVAC produces the third type of heat transfer: convective. This is via the movement of liquids, such as coolant from the engine or the liquid cooling system of an EV battery pack and vehicle drive system, to a heater core.
In a conventional ICE-powered vehicle, HVAC often is designed to warm the entire cabin, a gradual process that slows the heating effect, with the time to achieve cabin warmth extended. In the Prime test vehicle, HVAC is required to provide only some warmth for the feet and lower legs up to the knees. It still takes time to reach operating temperature, however, and Toyota relies on the other (electric) heaters to fill the early time gaps.
Because the Prime has a combustion engine, there also is some heat from its cooling system. But it needs to generate more heat, at a faster rate, particularly during EV-only operation. To fill this time gap, Toyota proved the value of the electric radiant heater, which is mounted in the dashboard below the steering wheel. EVs would, at this point, operate a conventional electric heater, typically a PTC or resistance type, but Toyota’s radiant heater provides instant warmth and is more efficient because it covers only a small zone. It will be used on Toyota’s first BEV, the 2023 bZ4X, and might also be installed under the glovebox to assist passenger-side heating, according to engineering sources.
Feet still need heat
Research proved the benefits of the radiant heat design on the Prime, where a Denso-developed thermostatically controlled 150-watt heater was installed on the driver’s side. A cold-weather test then was performed by a test driver trained in comfort-level sensing, in a test chamber per the EPA Cold FTP driving schedule. The HVAC was set in Auto/ 22°C (72°F) and the chamber temperature was maintained at 6.7°C (19.9°F).
This was compared with HVAC set to the same temperature with reduced HVAC airflow volume and radiant heater activation to heat just the anterior thighs and shins to the ankles instead of waiting for convective heat from HVAC. The strategy was for HVAC to heat just the feet and legs well below the knees for fast engine warm-up and use radiant heat primarily above that. The additional electricity consumed was deemed insignificant.
Within two minutes of vehicle entry and activation of all systems, the driver in the test vehicle with the radiant heater engaged was largely comfortable except for arms, head and feet, whereas most of the driver’s body without radiant heat was still cold, except for back and head. Within five minutes, the test driver with the radiant heater engaged was almost fully comfortable, with only the hard-to-warm feet close to but below the comfort level.
With the radiant heater disengaged, it took 10 minutes for the driver to reach a barely comfortable level for almost the entire body (unsurprisingly, with the feet slightly below comfort level), whereas all parts of the driver’s body with the radiant heater engaged were in the comfort zone after 10 minutes. At 20 minutes into the test, both systems reached fully comfortable levels.