Understanding the thermal issues involved in cooling outdoor enclosures can help designers and engineers select suitable cooling techniques and equipment.



Internet, cable, satellite, cellular, wireless, PC, electrical/utility, control and surveillance systems increasingly have a substantial portion of equipment that is housed outdoors. Because most systems do not have environmentally hardened designs, the enclosure must provide an environment in which they can survive.

To protect all types of temperature- and moisture-sensitive equipment outdoors, designers have been pushing for sealed enclosures. These designs allow for little or no exchange of the enclosure air with the outside air by using nearly airtight construction, high-pressure door-closure designs and closed-loop thermal systems. Thermal management thus becomes paramount. Not only must the internal heat generated be removed from the equipment, but also the effects of solar loading, which can be substantial depending on the size of the enclosure and its orientation toward the sun.

Figure 1. Outdoor enclosures are designed to house a variety of equipment configurations with dissipating heat rates raging from 500 to 10,000 W, depending on the size and type of the equipment.

Thermal Management System Design

Outdoor enclosures are designed to house equipment configurations with dissipating heat rates ranging from 500 to 10,000 W, depending on the size and type of the equipment (figure 1). These enclosures are installed in various environmental conditions and are typically fitted with either air-conditioning units or heat exchangers as needed.

The main goal of the enclosure is to maintain the peak temperatures below the level prescribed by the equipment manufacturer. Humidity levels also are considered, but because most enclosures are either sealed or have temperatures much higher than the air's dewpoints, humidity is generally not a problem after the transient effect of opening and closing the enclosure has been eliminated.

Outdoor temperatures in most environmental conditions range from -40 to 131 oF (-40 to 55oC). The air temperatures within the enclosure are a function of:1

  • The amount of heat generated by all of the equipment in the enclosure.
  • The amount of heat generated by auxiliary and cooling equipment (fans, etc.).
  • The ambient conditions (outdoor air), particularly temperature, solar radiation and wind speeds.
  • The objects surrounding the enclosure (shading, ground reflections, buildings, trees, etc.).
  • The enclosure design (surface area, shape, paint's radiation characteristics, etc.).
  • The air exchange with the outside air, either passive by infiltration, or active by fans or blowers.


Cooling/Heating Load Calculations. The design temperature of an outdoor enclosure is the temperature that the enclosure air will attain when a heat balance is achieved among the internal heat dissipation, the solar heat load and the amount of heat removed by the cooling system. The solar load is a complicated term because it includes contributions from all modes of heat transfer -- radiation, convection and conduction. Normally, the radiated heat transfer will always be positive (toward the enclosure), but the convected and conducted heat transfer can either be positive or negative, depending on the enclosure's temperature. If the balance is not zero, the temperature is either higher or lower than the set temperature, and the enclosure is either losing or gaining heat by convection or conduction.

Click for larger image
The solar load can be calculated using the Sol-air method recommended by ASHRAE,2which takes into account some convection and reradiation effects: (Please see image)

Because incident solar radiation varies during the daylight hours, the designer must decide whether to conduct a steady-state or transient analysis. Additionally, because radiation is a complex term that includes solar declination; latitude; time of year; solar azimuth; atmospheric absorption; atmospheric clearness; reradiation from other walls, buildings and the ground; and incident wall surface properties, some simplifying measures must be taken into account.3The result is that a designer can effectively double or triple the amount of heat flux being added to the enclosure, depending on the calculation method.

When calculating cooling loads, designers typically include the space heat gain, space cooling load and space heat extraction rate.2The space heat gain is the rate at which heat enters or is generated within the space at any given instant. This figure includes heat transferred into the conditioned space from the external walls and roof due to solar radiation, convection and temperature differential.

Designers also typically include instantaneous solar radiation effects and, for thick-walled enclosures, the delayed effects, which include the slow buildup of energy that the external walls accumulate as they absorb solar radiation. The delayed effects are not included for thin-walled enclosures. Another component of heat gain is latent heat due to moisture infiltration. For most sealed outdoor enclosures, the equipment is kept in an airtight enclosure with negligible contribution.

The Sol-air method involves calculating heat loads using an external temperature that lumps radiation effects and sensible air temperature. It does not take into account solar inclination and radiation intensity variations during a day-cycle, and the enclosure's solar load is calculated for the worst condition. Hour-by-hour calculations can be carried out using BIN data, but these calculations are not necessary in most cases because outdoor enclosures are not large and therefore have a low thermal mass.

To calculate heat transfer into the conditioned space, use

Qsolar-load= U A (Te- Tin)

where Qsolar-loadis the solar heat load, U is the overall heat transfer coefficient for the wall and A is the surface area for the wall. The term U includes convective and radiation effects by the internal and external airflow (see ASHRAE's Fenestration Chapter for more details2) and the wind outside, in addition to conduction through the walls.

Figure 2. These PCM-filled pouches are placed in cooling devices to absorb heat.

Typical Cooling Systems for Enclosures

Once the heat rate to be removed has been calculated, then a cooling system must be matched to the outdoor enclosure. Cooling systems for outdoor enclosures are typically fully active, assisted (semi-active) or passive.

Fully Active: Air-Conditioning/Refrigeration Units. If the enclosure air temperatures must be kept below the maximum ambient (outside) conditions, the preferred method is to install air-conditioning units.

For example, consider an enclosure that measures 7' (2 m) high and has a footprint of 10' x 3' (3 m x 0.75 m). The installed equipment dissipates 1,500 W and must be kept below ambient conditions. All walls are insulated by a 1" layer of polyurethane with a thermal conductivity of 0.026 W/m-K. Thus, this value will be U ~ 1 W/m2-K. The total solar load is around 500 W, and the total load that the cooling unit must dissipate is 2,000 W (Qsolar-load+ Qequipment). The area here refers to the three surfaces that can be illuminated simultaneously, with the roof always included.

In this case, an air-conditioning unit rated at 2,000 W should be installed. These calculations do not include capacity for cooldown because the calculations were carried out for steady-state operation; that is, the system does not include cooling capacity to bring the system to the designed inside temperature from starting conditions that are 131oF or above. Transient effects also are not included. This is a realistic assumption because the highest conditions are likely to occur for a very short time, and the system is unlikely to be started during the hottest period. Air conditioners usually have an outlet air temperature of around 59oF (15oC) or below to achieve the cooling required.

Assisted (Semi-Active): Air-to-Air Heat Exchangers, Water-to-Air Heat Exchangers. If the enclosure air temperatures do not have to be kept below the maximum ambient (outside) conditions and the load is not too high, an air-to-air or water-to-air heat exchanger is the preferred system. Heat exchangers still allow for sealed compartments but have much lower operating and maintenance costs compared to air-conditioned enclosures. For the enclosure described previously, the heat exchanger would need to transfer around 1,500 W with a maximum outside (cooling air side) temperature of 68oF (20oC) and a maximum enclosure air temperature of 86oF (30oC).

Unfortunately, due to the reduced space availability in most enclosure chambers, it is difficult to design a heat exchanger that meets all of the specifications. Unlike air conditioners, a heat exchanger's heat removal capabilities change as a function of the cooling air and enclosure air values. If off-design temperatures are encountered, the enclosure either overheats or overcools. A typical variation might be 9oF (5oC) above or below maximum conditions. However, thermal inertia effects due to variations in ambient conditions might compensate for the shortfall. To determine the feasibility of using heat exchangers for an outdoor enclosure, a 24-hour simulation can be carried out using a computational fluid dynamics (CFD) software package or actual prototype testing.4

A CFD study divides a typical day into 4-hour intervals. During these intervals, the outside temperature and flux are varied. This simulation allows thermal inertia effects to be taken into account because the two key parameters -- solar radiation and the ambient temperature -- fluctuate during a 24-hour cycle. Of these two, the ambient temperature is of paramount importance as it controls, in conjunction with the enclosure temperature, the heat removal rate of the heat exchanger and, thus, its cooling effectiveness.

Passive: Natural Convection. Smaller outside enclosures (wall-mounted control boxes, for example) in which relatively high temperatures can be tolerated can be cooled by passive means. Passive methods include primarily natural (free) convection and phase-changing materials (PCMs). Natural convection is the transport of heat by buoyancy-induced fluid flows. Hotter fluid -- heated, for example, by a wall exposed to the sun -- rises and displaces colder fluid. In an enclosure, the hotter fluid moves up along the heated wall, then travels to the colder wall and descends as it loses heat to the wall. The fluid will then make a recirculating closed loop that effectively transports heat from the hot wall to the cold wall. The fluid in the middle remains relatively undisturbed.

The situation becomes more complex as power electronics are added, but there are ways to let the heat generated by the equipment be carried away by natural convection. The body of scientific literature on natural convection within enclosures is vast. However, the designer must always keep in mind that the overall goal is to transfer to the outside as much heat as possible by natural convection to keep internal temperatures low.

Another passive method that has become popular is the use of PCMs, which change phase, most often from solid to liquid, as they absorb heat. Typical PCMs for high-temperature applications are waxes, salts and paraffins, and ice (water) for low-temperature applications. A variation is to use a substance with a high heat capacity, such as water, that absorbs large amounts of heat without changing phase. Figures 2 and 3 show examples of PCM packaging and devices.

These materials are kept inside the enclosures in appropriately sealed devices and take advantage of thermal inertia and phase change effects. For example, in an enclosure with PCMs, solar heat will be absorbed by the PCM device during the daylight hours and will not be allowed to heat up the enclosure air. At night, the enclosure will release the stored energy to the cooler environment. While these processes take place, heat will continue to be transferred in and out through the enclosure walls.

Some systems incorporate a thermosiphon (or natural convection) loop to enhance cooling. A closed-loop conduit is connected to a large reservoir inside or outside the enclosure, and water heated by solar radiation flows up naturally by buoyancy forces. The hotter fluid moves up, then travels to the colder wall, and descends as it loses heat to the wall. The fluid effectively transports heat from the hot wall to the cold wall, in addition to absorbing heat as thermal inertial storage. In some cases, the reservoir might contain a PCM that melts and begins to circulate in thermosiphon fashion until it returns to the reservoir.

Figure 3. This PCM cooling device for small compartments is filled with PCM capsules. Courtesy of PCM Thermal Solutions

Thermoelectric Coolers

In 1822, Seebeck conducted experiments in which he determined that if a closed circuit were made of two dissimilar metals, an electric current flowed in the circuit if the two metals were kept at different temperatures. Later, in 1834, Peltier witnessed the reverse effects. He observed that if a closed circuit were made of two dissimilar materials and a current was made to flow through the circuit, heat was either absorbed or given off. The former is called the Seebeck effect, and the latter the Peltier effect.

A complete circuit, called a thermoelectric couple, consists of two dissimilar thermoelectric materials. Today's thermoelectric materials are made of semiconductor materials and can be considered heat pumps as heat is transported from the cold junction to the hot junction as a function of the input DC current. The use of thermoelectric coolers is growing at a good rate due to the low cost of these devices.

Similar to a mechanical air conditioner, thermoelectric coolers require energy input to remove heat from the conditioned space. (Without this energy, the heat would naturally flow from the higher- to lower-temperature regions.) For refrigeration systems, the coefficient of performance (COP) is used to define a system's efficiency.

The main advantage of thermoelectric coolers is that they contain no moving parts for producing cold, therefore making them very reliable. Furthermore, they are inexpensive, lightweight, intrinsically safe and provide accurate temperature control. Unfortunately, these systems have a low COP. Additionally, because these units only transport energy from the cold to hot side, heat sinks or fans must be used to move the heat away from the unit, thereby adding cost and complexity to the cooling assembly. Small enclosures might be good candidates for thermoelectric coolers if their internal loads are small (less that 10 to 15 W) and solar loads are completely eliminated. In fact, if internal temperatures are much lower than the hottest day average temperature (during which even PCMs are not suitable), then thermoelectric coolers are the only choice.

In conclusion, managing the thermal performance of outdoor enclosures will become increasingly important as components and systems become more complex. By understanding the various requirements and alternatives, system designers and engineers can be sure to select the right methods and equipment for their applications.

References

1. McKay, J.R., “The Effect of Solar Radiation and Wind Speed on Air Temperature Rise in Outdoor Enclosures Containing Telephone Equipment,”10th International Telecommunication Energy Conference (INTELEC), San Diego, Calif., Nov. 1998.

2.ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigeration and Air-Conditioning Engineers, Atlanta, Ga., 1981, 1986.

3. Kreider, J.F., and Kreith, F.,Solar Energy Handbook, McGraw-Hill Book Co., New York, N.Y., 1981.

4. Marongiu, M.J., “Some Issues in Experimental Testing and Methodologies in the Thermal Management of Telecommunication Components, Systems and Enclosures,” presented at the 17th International Telecommunication Energy Conference (INTELEC), The Hague, Holland, Oct. 29-Nov. 1, 1995.

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