Andrew Dereka, Product Director at Laird Thermal Systems
Temperature control of sensitive telecom electronics in unattended mobile base stations and cell towers is vital for the operation of primary and back-up systems. Heat can significantly degrade the performance and operating life of telecom cabinets, energy storage systems and back-up battery systems. Mobile base station and cell tower equipment operate 24/7 with a continuous load that generates heat. Operating outdoors, mobile base stations and cell towers are also exposed to daily temperature and humidity fluctuations. Thermoelectric coolers offer temperature stabilization that protects critical telecommunication equipment to ensure consistent operation and reduce maintenance cost.
Bulky compressor-based air conditioners have traditionally been used for cooling communications equipment installed in base station and cell tower enclosures. However, these air conditioners consume large amounts of energy, when constantly operating throughout the year. Electronic cabinets found in base stations and cell towers are often cooled needlessly with these expensive compressor-based air conditioners. Because standard air-to-air temperature control systems with vertical mounts are often too large to fit inside an enclosure, they are mounted on an exterior wall to provide temperature control to the entire space inside. Thermoelectric coolers, also referred to as Peltier coolers, offer a smaller, more efficient option to precisely cool or heat vital electronics in telecom enclosures, energy storage and battery backup cabinets.
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Figure 1a and 1b. Due to the limited access for repair and maintenance of base station and cell towers, long life operation is required.
Requirements & Challenges
Remote monitoring and control of the cooling system is vital to ensure the working condition of the machines distributed in different base stations. When the power to a cellular antenna tower goes out, emergency batteries provide power for a limited time. Battery back-up systems are susceptible to degradation when exposed to elevated temperatures or when exposed to very cold temperatures. Cooling below ambient is necessary to extend the life of back-up batteries, and temperature stabilization is required to maintain peak performance. Many base stations and cell phone towers are found in isolated locations with limited access for repair and maintenance. Long life operation is required in wireless base station and cell tower applications to maximize uptime and maintain low cost of ownership. Another thermal challenge that needs to be taken in consideration is humidity control. Since dry air will make static to burn the communication equipment in base station and cell towers, humidity control is as important as temperature control.
Thermoelectric coolers are solid-state heat pumps that operate using the Peltier effect. When an electric current is driven through a circuit containing two dissimilar materials, heat is absorbed at one junction (the cold side) and released at the other junction (the hot side). The design of most thermoelectric coolers requires the use of both a n-type and p-type semiconductor. Since heat naturally flows down a temperature gradient from hot to cold, a Thermoelectric cooler’s ability to move heat from cold to hot in a solid-state fashion is unique. By reversing the polarity of the applied DC current, heating is also possible. This feature is especially useful for applications requiring both cooling and heating for precise temperature control.
Thermoelectric Cooler Assemblies
Thermoelectric cooler assemblies are compact, efficient units that can control the temperature in mobile base stations and cell towers. Consisting of a thermoelectric cooler, heat exchanger and fan combination, Thermoelectric cooler assemblies serve a cooling capacity spectrum from approximately 10 to 400 Watts and can cool by removing heat from control sources through convection, conduction, or liquid means. This thermal solution offers several advantages over other cooling technologies. For example, conventional fan trays do not cool to below ambient and require an air exchange with the outside environment. Thermoelectric cooler assemblies can cool to well below ambient and protect electronics inside enclosures from outside contaminants, and also limit moisture exposure from the outside environment. Thermoelectric cooler assemblies provide precise temperature control and accuracies to within 0.01˚C.
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Figure 2. AA-230 Outdoor Cooler thermoelectric cooler assembly with SR-54 bi-directional thermostatic controller
Thermoelectric cooler assemblies not only offer a more compact size and lower weight compared to conventional compressor-based systems, but they are also mountable in any orientation. This simplifies integration and lowers installation costs. In addition, fans, fan trays and compressor-based systems generate noise and vibration. Thermoelectric cooler assemblies feature a solid-state construction that does not create noise or vibration, offering a longer life cycle compared to competing technologies. This results in a significantly lower maintenance requirement, which reduce the total cost of ownership. Thermoelectric cooler assemblies are also more environmentally friendly as they do not use CFCs and are RoHS compliant.
Thermoelectric cooler assemblies designed for harsh and remote environment applications, including electronic cabinets and battery cabinets in mobile base stations and cell towers, combine superior heat pumping capability with minimal power consumption. Air-to-air Thermoelectric cooler assemblies, like the Outdoor Cooler Series from Laird Thermal Systems, absorb and dissipate heat through custom designed heat exchangers with high aspect ratio, air ducted shrouds and high-performance fans. With heat pumping capacity of up to 480W at dT = 0°C, Tambient = 35°C, custom designed Thermoelectric coolers achieve a high coefficient of performance (COP) of 1 to minimize power consumption. This means the thermoelectric cooler can run longer on backup battery power during power outages compared to other cooling units.
In addition, the thermoelectric cooler assemblies have been designed to pass rigorous Telcordia test requirements, such as earthquake resistance, salt fog, wind-driven rain, high temperature exposure and dust contaminants. This is due to the selection of world-class components such as brand fans with the highest degree of environmental protection and lifetime guaranteed waterproof connectors, heavy duty anodization on the high-density heat sinks, overheat protection, and double environmental seals for the Thermoelectric coolers.
The temperature control specification for a battery back-up application is normally ± 2°C or greater. This allows hysteresis to be designed in, reducing cycling between cooling and heating or on/off when the enclosure is at its set point temperature. This range is suitable for thermostatic control, but a tighter tolerance requires a proportional type of control.
A thermoelectric-based controller can drive the temperature of an enclosure to within 0.5°C of the set point temperature. This is accomplished with the integrated bi-directional proportional (PID) control, adjusting the net power to the Thermoelectric cooler allowing fine tuning and rapid response to component or environmentally-induced heat load fluctuations. A bi-directional thermostatic controller can operate in both heating and cooling modes to achieve precise temperature control. Controllers with a bi-directional output are used with Thermoelectric cooler assemblies for maintaining a constant temperature within a system surrounded by an ambient environment with large temperature fluctuations, i.e., back-up battery storage or climate control in an outdoor environment.
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Figure 3. AA-480 Outdoor Cooler thermoelectric cooler assembly
The operating range for a typically Thermoelectric cooler is -40°C to +65°C for most systems, while compressor-based systems are usually designed for operation between 20°C and 55°C. This range is useful for most enclosure applications and operating environments. If heating is required, a separate heater and switching circuit must be used or if higher or lower temperatures are desired, a special compressor for that range (refrigerant, hardware) must be designed.
Engineers must consider thermal management early in the product design process. Thermoelectric assemblies provide different cooling capacity and deliver temperature stabilization that ensures sensitive battery backup systems operate at maximum efficiency. Offering precise temperature control and accuracy to within 0.01˚C, Thermoelectric cooler assemblies offer bi-directional control in one unit, making it ideal for sensitive telecom electronics including telecom cabinets, energy storage systems and back-up battery systems.