The communications system -- which may include SCADA equipment, transceivers, and backup batteries - affiliated with a utility's Transmission and Distribution lines will require protection from the elements, thermal management to maintain the proper operating temperature ranges, and security to prevent malicious access and tampering. In many cases, the installation of a thermally managed outdoor enclosure will be more cost effective and practical than a new building or shelter.
Thermally-managed outdoor enclosures now have micro-processors controlling their interior environment, which covers a wide range of thermal management systems, power efficiency, acoustic management, and security and intrusion prevention. These features and capabilities have a major impact on the total cost of ownership over the life of the deployment -- typically many years -- therefore understanding and leveraging them can result in significant operational expense reductions, as well as increased reliability and longevity versus other enclosures that do not support these capabilities.
Most commercial electronic equipment is specified to operate in a - 40 °C to + 65°C range. For electronic components, for every 10 °C rise in temperature, the average reliability is decreased by 50 percent. In terms of Mean Time Between Failure (MTBF), the MTBF will double if the operating temperature is lowered 10 °C. Thermal management systems are available in a number of technologies and performance ranges to accommodate the heat load from contained equipment and solar radiation.
The design of the thermal management system must take into account the operating temperature range of the enclosed electronic equipment, the range of seasonal temperatures and humidity, and address the heat dissipation from that equipment plus the total heat load contribution of solar radiation from exposure to direct sunlight. More than any other feature, the thermal management system could have the largest impact on the operational expenses.
Enclosure manufacturers offer a wide range of thermal system technologies and capacities to provide an optimal combination of technology, performance, reliability and cost. The four most common thermal management technologies Purcell Systems uses for outdoor enclosures are -- Direct Air Cooling (DAC), Air-to-Air Heat Exchanger (HEX), Air Conditioning (A/C), and Thermoelectric Cooling (TEC).
Direct Air Cooling (DAC) -- DAC systems provide excellent above ambient thermal management performance. DAC systems are open-loop systems that bring outdoor ambient air into the interior of the enclosure for cooling purposes. DAC systems typically employ inexpensive mesh filters to prevent particulate contamination from entering the enclosure, or alternatively can also be equipped with high-performance hydrophobic filters that prevent moisture entry into the enclosure. DAC systems are highly reliable, consume very little energy, and have long life expectancy as the only moving parts are the fans.
Heat Exchanger (HEX) -- HEX's provide superior above ambient temperature thermal management. HEX units are closed loop systems meaning that they isolate the outdoor ambient air from the air inside the enclosure. Closed loop systems prevent intrusion of any particulate contaminates, such as dust, moisture and humidity from entering the enclosure. HEX's are highly reliable and have long life expectancy because the only moving parts are the fans to circulate air. HEX's require no maintenance and consume very little energy.
Air Conditioner (A/C) -- A/C offers the highest performance thermal management technology, able to support very high heat loads and cool the interior of the enclosure far below ambient air temperatures. Most A/C units deployed in support of outdoor enclosures are closed-loop systems based on vapor compression cycle where a refrigerant undergoes a change of state (from a liquid to a gas) that absorbs thermal energy from within the enclosure and transfers it to the outdoor air. This process also removes humidity from within the enclosure.
While A/C offers the highest performance cooling, it also comes at a significant price in terms of the relative cost of the technology, high energy consumption, required regular preventative maintenance, and poor reliability and life expectancy, due mainly to many complex components and moving parts operating at high temperatures and pressure.
Due to the high-performance capabilities of A/C systems, many outdoor enclosure vendors tend to apply it for all cooling requirements, which typically results in significantly more cooling capacity than what is actually required. When deployed in this manner, A/C systems will have higher than normal failure rates due to short-cycling and higher energy consumption.
Thermoelectric Cooler (TEC) -- TEC's use the Peltier-Effect, by which current applied across two dissimilar materials causes a temperature differential. TEC's offer variable and scalable incremental cooling or heating in a compact form factor. TEC's have very high reliability and long life expectancy because the only moving parts are fans to circulate air. TEC's require no maintenance and consume far less energy to operate than A/C.
Equipment Segregation into Cooling Zones
The interior layout of the enclosure, and all of its affiliated accessories, should accommodate multiple temperature zones. This design method enables the enclosure to keep one zone at a different temperature than another. By understanding the operating ranges of the groups of equipment in the enclosure, the manufacturer can design the thermal management system to keep each group of equipment within its specific operating temperature range.
This capability is especially useful for enclosures that house both batteries and electronics. Batteries should be stored as close to room temperature as possible, as any elevated temperatures substantially shorten cycle life. Electronics are more tolerant of higher temperatures, and can operate comfortably between -40 and + 65°C. Typical cooling configurations would be a TEC for the battery zone and a HEX for the electronics zone. Without the application of zone cooling, a common chamber containing batteries would probably need A/C to maintain the required temperature range. Note that zone cooling applies to any configuration that includes equipment with different thermal characteristics, not just configurations with batteries. The enclosure must support the inclusion of a horizontal insulated partition, as well as the mounting of up to three different cooling systems on the door.
A critical consideration when selecting a thermal management system is an intelligent controller. An intelligent controller, in combination with multiple temperature sensors, should control the operating speed of the air-movers. These variable-speed controllers regulate the air flow, maintain a more even temperature within the enclosure, reduce energy consumption, and reduce acoustic emissions outside the enclosure. An intelligent controller should also provide alarm reporting and communication capabilities for the thermal system.
Thermally-managed enclosures maximize equipment up-time, as well as extend service life by providing an optimal operating environment for communications equipment embedded within a utility's infrastructure. Additionally, ongoing heating and cooling costs can be minimized by selecting the optimal mix of thermal management technologies.
The features and capabilities reviewed in this article have a major impact on the total cost of ownership over the life of the deployment -- typically many years -- therefore understanding and leveraging them can result in significant operational expense reductions, as well as increased reliability and longevity versus other enclosures that do not support these capabilities.