Tom Denton discusses radiant heating in Electric Vehicles in his latest blog....
Vehicle manufacturers have been steadily enhancing the electric range of their vehicles, yet these claimed ranges are typically based on ideal thermal conditions. The true challenge lies in delivering the same performance under extreme thermal conditions that can affect the efficiency of lithium-ion battery packs.
During hot summer weather, using air conditioning in electric vehicles results in a modest reduction in range. However, in cold winter weather when the cabin heater is in use, there can be a significant drop of 30 to 40% in range. Until now, most efforts to improve EV heating performance have focused on electric seat heaters, electric steering wheel heaters and heat pumps integrated into the HVAC refrigeration system. However, heat pump performance is limited by the fact that refrigerant flow slows down in low winter temperatures. As a result, heat pumps must switch to less-efficient PTC heaters when ambient temperatures drop below freezing.
Figure 1 Vehicle interior (Source: Toyota Media)
Radiant heating
Toyota recently showcased its testing of an electric radiant heater installed in a Prius Prime. This system offers a more efficient and rapid way of providing supplementary cabin heat compared to other methods. The Prius Prime, which already features seat and steering wheel heaters, was used as a testbed for addressing the challenge of maintaining cabin heat while preserving range.
During testing, the use of the radiant heater led to a 5.3% reduction in fuel consumption while significantly improving driver comfort within two minutes of entering the cabin. The radiant heater operates by emitting invisible infrared waves that quickly warm objects in their path. When directed towards the driver in the cabin, the system warms the front part of the driver’s body, complemented by a seat heater covering the back of the body from shoulders to knees. Additionally, a steering wheel heater warms the hands, while the HVAC’s floor outlet gradually provides warmth to the driver’s knees, feet and the rest of the body. The seat and steering wheel heaters employ conductive heating, where direct contact with a warm surface transfers heat, while the HVAC utilises convective heating through the movement of liquids to a heater core, such as coolant from the engine or the liquid cooling system of an EV battery pack and vehicle drive system.
This heating system focuses on providing warmth primarily to the driver’s feet and lower legs up to the knees. While it still takes some time to reach the desired operating temperature, other electric heaters bridge the initial gaps. During EV-only operation, additional heat needs to be generated at a faster rate. The electric radiant heater, which is mounted below the steering wheel in the dashboard is therefore used.
Tests were carried out on the system first using radiant heat and then not using it. The additional electricity consumed by the radiant heater was determined to be insignificant. Within two minutes of entering the vehicle and activating all systems, the driver in the test vehicle experienced significant comfort, except for the arms, head and feet. In contrast, most of the driver’s body without radiant heat remained cold, except for the back and head. Within five minutes, the test driver with the radiant heater engaged was almost fully comfortable, with only the feet, which are typically harder to warm, slightly below the comfort level. Without the radiant heater, it took 10 minutes for the driver to reach a barely comfortable level for almost the entire body, with the feet slightly below the comfort level. However, when the radiant heater was engaged, all parts of the driver’s body were in the comfort zone after 10 minutes. By the 20-minute mark, both systems had reached fully comfortable levels.
Figure 2 Radiative, conductive and convective heating (Source: Toyota Media)
Summary
Like all new technologies there is a difference between prototypes and development systems. However, radiant heat is not a new concept as it is used by way of infrared lamps to heat large spaces, well, to be more accurate, heat the bodies in those large spaces. Time will tell.
