Active cooling solutions keep medical equipment below their maximum operating temperature to ensure proper performance.
Andrew Dereka, Laird Thermal Systems
Thermoelectric coolers are active solid-state heat pumps that use the Peltier effect to move heat away from sensitive electronics. Also referred to as Peltier coolers, these devices are a more efficient alternative to compressor-based cooling systems for a wide range of medical applications.
Designed with size, efficiency, a low power requirement and continuous reliable operation in mind, thermoelectric coolers enable medical OEMs to meet thermal design challenges, including thermal stability and precise temperature control. With the ability to cool well below ambient temperatures, thermoelectric coolers protect medical electronics from heat generated by medical products including lasers, imaging equipment, sample storage chambers and thermal cycling equipment for DNA amplification.
How they work
During operation, DC current flows through the thermoelectric cooler to create heat transfer and a temperature differential across the module. One side of the thermoelectric cooler is cold, while the other side is hot. Heat absorption and heat dissipation mechanisms are connected to the thermoelectric cooler, which are usually fans or heat sinks. The cold side heat sink absorbs heat from inside the cabinet, while the hot side heat sink rejects heat to the ambient environment.
A standard single-stage thermoelectric cooler can achieve temperature differentials of up to 70°C and transfer heat at a rate of up to 350 watts. Colder temperatures can be achieved, down to -100°C, by using a multistage thermoelectric cooler in a vacuum environment. Thermoelectric coolers are also environmentally friendly, as they do not require refrigerants to dissipate heat.
When the polarity on the thermoelectric cooler is reversed, the device can also provide heat with temperature control, which is a requirement in a number of medical applications such as reagent storage. This also eliminates the need for a resistive heater that would be used in traditional compressor-based systems, resulting in lower costs.
Thermoelectric cooler assemblies
At their core, thermoelectric cooler assemblies consist of an array of thermoelectric coolers, a heat exchanger and a fan. Thermoelectric cooler assemblies offer a cooling capacity spectrum from approximately 10 watts to 500 watts and use multiple heat-transfer mechanisms, including air-to-air (convection), liquid-to-air (re-circulating liquid) or direct-to-air (conduction) methods. These assemblies remove the passive heat load generated by the ambient environment in order to stabilize the temperature of sensitive components used in medical equipment.
The most commonly used type of thermoelectric cooler assembly is the air-to-air configuration, which uses fans to increase the air exchange between the hot and cold sides to maximize heat transfer.
Precise temperature control
When combined with an advanced bi-directional temperature controller, thermoelectric cooler assemblies can deliver temperature control to within ±0.5°C. Temperature controllers can turn on the thermoelectric cooler assembly to cool once the upper-temperature limit has been reached and turn off a few degrees below the temperature set point. Similarly, the controller can turn on and provide heat once the low-temperature limit has been reached and turn off a few degrees above this limit. A hysteresis setting is used in conjunction with the temperature limit set points to set the desired degree range for the application.
Temperature controllers may also provide monitoring and alarm functions, including identification of a problematic fan, over-temperature thermostat and temperature sensor failure, all of which are critical to maximizing the amount of time medical equipment can operate. These controllers require minimal programming and can be easily integrated with a thermoelectric cooler assembly. They may also lower operational noise, as fan speeds can be reduced once the specified temperature has been reached.
Thermoelectric coolers and assemblies offer several advantages when compared to alternative cooling technologies, including:
- Compact size.
- Solid-state construction providing robust reliability with few moving parts.
- Ability to heat and cool for precise temperature control.
- Use no hazardous CFC refrigerants.
- Can be mounted in most any orientation.
- Reduce development time.
- Reduce operating cost.
- Offer options for customization.
Thermoelectric coolers and assemblies in combination with temperature controllers are ideal for medical thermal management applications that require active cooling to below ambient temperatures and have cooling capacity requirements of up to 500 watts. Peltier devices provide precise temperature control in an efficient and compact form factor, all at a lower total cost-of-ownership.
Andrew Dereka is product director at Laird Thermal Systems. He has 18 years of experience in engineering, sales and product management.
The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design and Outsourcing or its employees.