Future connected vehicles are characterized by clusters of densely packed electronics. Thermoelectric modules can keep the temperature of these hot spots at manageable levels.
Andrew Dereka | Laird Thermal Systems
Advances in automotive electronics tend to come in small packages where dense circuitry squeezed into a pint-sized footprint generates lots of heat and thermal challenges for designers. The usual way of cooling electronics is to use convection heat transfer to a heat sink and then go to more drastic measures if necessary. But a heat sink alone will only cool to just above ambient temperatures. If the heat sink has a high thermal resistance, its hot-side temperature will be several degrees above ambient, a situation that happens frequently.
A greater temperature reduction can often be accomplished via spot cooling — incorporating a Peltier cooler close to the electronics generating heat. The Peltier cooler, also known as a thermoelectric cooling module (TEM), can reduce the temperature of its cold side by as much as ~30°C below that of its hot side.
A Peltier cooler is comprised of p- and n- type semiconductor materials brought in contact to form a junction. Electrons flow when the device connects to a dc power source. At the cold junction, the electrons absorb energy (heat) and move from a low-energy state in the p-type semiconductor element to a higher energy state in the n-type semiconductor element. At the hot junction, energy is expelled to a heat exchanger (usually a heat sink) as electrons move from a high-energy level to a lower energy level. Reversing the direction of current flow reverses the direction of heat pumping. Thus the TEM can provide both cooling and heating with a simple reversal of the current.
A point to note is that heat rejection at the hot side is critical – if the hot side saturates, heat will flow back into the Peltier cooler and heat it up.
This is where the Peltier cooler coefficient of performance (COP) applies. COP is a function of the material dimension, the temperature of the hot side and cold side, and the dc current to the module. A given thermoelectric material has an optimum dc current for maximum COP if the hot-side and cold-side temperatures are fixed.
The design of a thermoelectric cooling system usually starts from a given temperature difference across the hot and cold sides of the module and the amount of cooling capacity necessary (in Watts). Cooling capacity varies with operating voltage and module current. TEMs can be configured to run on a variety of dc voltages through selection of a series or parallel configuration for the TEM’s internal construction. The heat sink’s thermal resistance is also a factor. The module selection process is often iterative and is aided by online calculation wizards.
Peltier coolers, like the HiTemp ET Series from Laird Thermal Systems, maintain a high coefficient of performance (COP) to allow for maximum heat rejection into the air environment even with poor heat sinking. Peltier coolers are available in a wide range of heat pumping capacities, form factors, and input voltages to support a wide range of applications.
An additional point to note is that Peltier coolers don’t outgas – they are basically comprised of ceramics. Designers need to avoid materials that can release any kind of gas that can form a coating over time near laser or imaging sensor optics, so Peltier coolers are an ideal solution for these applications.
However, a thermoelectric module is generally a more expensive option than passive cooling. The temperature sensing involved may necessitate a closed-loop feedback control, and circuits cooled to below the dew point may need considerations for condensation. But TEMs operated with a Proportional-Integral-Derivative (PID) controller typically have operating lifetimes exceeding 70,000 hours. A thermoelectric based controller can drive the temperature of an enclosure to the pre-set temperature. PID control can also adjust the net power to the TEM to a precise degree, often within 0.5°C of a set point, allowing a fast and accurate response to heat load fluctuations.
It can be illustrative to examine applications that are now candidates for thermoelectric cooling. One is laser-based smart headlight technology. Smart headlights automatically adjust the direction of their light output. For example, they may direct the high beam away from oncoming traffic to avoid blinding the other driver or illuminate at an angle when the car turns to enhance the driver’s viewable area.
In a smart headlight, a single laser focuses on a grid of tiny oscillating mirrors that generate the headlight beam. This setup operates efficiently at temperatures up to 70°C, but the performance deteriorates if temperatures rises beyond this limit. The compact form factor of the headlight compartment combined with the heat generated from the engine and the outside environment may boost headlight enclosure temperatures to as high as 115°C. Use of Peltier coolers in the assembly can keep the temperature of the laser within its operational limits.
Another automotive application where spot cooling can be helpful is in high-end sensors. Some sensors now under development can capture high-resolution images using a portion of the spectrum outside that visible to the human eye. Due to thermal noise, the quality of the image resolution deteriorates as the temperature rises above 60°C. In automotive applications, the operating temperature can reach up to 85°C. Moreover, the heat rejection path for passive cooling of these devices tends to be inefficient. Active spot cooling using thermoelectrics can keep the imaging sensors cool in the presence of hot surroundings.
Head-up displays are other automotive application where thermoelectrics can be an ideal cooling solution. Advanced driver-assistance systems (ADAS) now on the drawing board make more extensive use of windshield projection technology than present-day technology, most of which has a projection area of just tens of square inches. Larger head-up displays dissipate more power and generate more heat. The compact nature of the projection electronics makes head-up displays candidates for active spot cooling using thermoelectric cooling.