Passively-cooled shelters for remote equipment in hot and arid climates

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1 Passively-cooled shelters for remote equipment in hot and arid climates The reliability of electronics equipment is dramatically shortened if it operates in very hot conditions, which is why much electronic equipment incorporates fan cooling. Equipment builders designing for arid climates have always faced considerable environmental protection problems. Traditional cooling techniques require a power supply, which often poses severe difficulties. The problem is growing bigger as the topology of SCADA evolves towards more distributed and unmanned systems, and new solutions are being sought. Although solar cells and DC powered air conditioning techniques are making progress, their relative lack of efficiency, together with the problems of battery lifecycles in hot climates, continues to make environmental protection complicated and expensive. One of the most powerful weapons in the battle to protect remote instrumentation and electronics against high temperatures is passive cooling. The swing between day and night temperatures in arid and desert climates can be exploited by using the energy storage capacity of a medium such as water to reduce temperature fluctuations. Water-based passive cooling pioneered more than 30 years ago and used extensively in the last decade has met with considerable success, with demand continuing to grow today. However, it is often seen as only being suitable for larger requirements such as shelters, and for the arid and desert climates such as are prevalent in the Middle East region. Today, the field instrumentation protection company Intertec is challenging these perceptions with developments that make passive cooling technologies applicable to a much wider range of climatic conditions and problems. Cooling without power Water-based passive-cooling systems powered by natural convection is a commonly-used cooling solution for equipment in the Middle East. The technique has the huge benefit of not requiring an electricity supply - making it ideal for cooling remote equipment such as instrumentation and computer/communications electronics of SCADA systems on pipelines. Pure passive cooling systems (PCSs) work by exploiting the difference between day and night temperatures in regions with high swings - such as arid deserts - using the energy storage capability of a coolant fluid. Water is most commonly used as the coolant because of its high thermal capacity, and circulates by means of natural convection. The process is inherently reliable because there are no moving parts. Passively-cooled shelters typically comprise an insulated housing with two closed-loop internal and external heat exchanger circuits connected to a single thermally stratified heat storage tank. The major components of a passive cooling system for shelters.

2 During the day no thermo-siphon circulation occurs in the external circuit because the external heat exchanger is higher than the storage tank and contains water which is hotter and therefore less dense. However, thermo-siphon circulation causes cold water to be displaced from the bottom of the tank and drawn up through the internal circuit/heat exchanger, cooling the shelter's interior by air convection. At night, water in the external heat exchanger becomes colder than that in the tank, causing circulation. The heat energy stored in the tank is dissipated to the environment by the external heat exchanger circuit, while the internal circuit continues operating normally. This process consumes no energy, and smooths temperature peaks and troughs over 24 hours. The external heat exchanger is roof-mounted and incorporates a sunshade for the shelter, to improve the overall efficiency of the cooling process. The nearby figure shows the temperature profiles of a test shelter built by Intertec to compare different cooling technologies. With a footprint of 2.4 x 5.4 m, the 3 m high shelter is constructed from panels of Intertec s low thermal conductivity GRP material - with twin 4 mm walls of 'sandwiching' a 76 mm PU foam core (see 'GRP out-performs other materials for environmental protection' for more information on composite GRP). The heat dissipation of equipment inside the shelter is 1 kw, and the ambient 29 C night and 58 C day temperatures represent worst-case summer values in the UAE. Good insulation ensures that internal temperature is barely affected by the high daytime ambient, and passive cooling keeps its maximum to within 7 C of the ambient temperature of the preceding night. Passive cooling smooths out temperature peaks and troughs over 24 hours. Key parameters to consider when configuring a PCS are the heat dissipation of the equipment, maximum permissible temperature inside the shelter, the outside air temperature of the night preceding the hottest day, and the difference between night and day temperatures. System performance is limited by the sum of the temperature differences, or delta-t, which must be at least 6 K for natural convection. However, if the delta-t is too small, 'hybrid' variations of the passive cooling system - with forced circulation of the media for instance - can offer a cost-effective solution. Hybridization boosts performance Forced circulation of cooling media effectively doubles PCS performance, allowing operation with a delta- T of just 3 K. One way this can be realised is by using small low-power pumps driven by a rechargeable battery and solar panel. Another major benefit of forced circulation is that water can be routed via heat conduction manifolds instead of an internal water/air heat exchanger. As thermal conduction is about five times more effective at heat transfer than air convection, direct cooling of manifold-mounted analyzer electronics is considerably more efficient than using air conditioning (AC) to circulate cooled air, reducing cooling requirements by as much as 80 %. This lowers capital and operating costs dramatically, and decreases shelter sizes. Another 'hybrid' method of cooling shelters that is gaining ground combines a passive cooler with a small external water chiller that only operates on demand, to further reduce the water temperature in the heat storage tank. The chiller forms part of a closed-loop system, feeding a heat exchanger coil in the bottom of the tank. It s also possible to build hybrid systems to exploit cool water from any available nearby source, provided its temperature is at least 3 K lower than the shelter s maximum allowable internal temperature.

3 A hybrid cooling solution with passive cooling aided by an external water chiller. Designers can easily scale cooling capacity to the application. The passive element can be sized such that in winter its cooling power is sufficient to maintain internal shelter temperatures. However, in summer months when the ambient night time temperature is too high, the chiller can be switched on by a thermostat until the tank water is cool enough (a timer ensures it only operates at night). Intertec s test results show that hybrid cooling with a 1 kw chiller ensures that the test shelter s internal temperature never exceeds 27 C, even after the hottest night. The chiller s capital costs are much less than an AC system, and its energy consumption is about 30 % lower because it does not have to run when ambient temperature is very high. Hybrid cooling systems also offer increased reliability and reduced maintenance costs, through built-in redundancy. In the event of chiller failure or power loss, even at the hottest time of year a tank s cold water can keep a shelter below the night time temperature for two successive days; provided the analyzer electronics are backed by an uninterruptible power supply they can operate normally throughout this period. On the other hand, equipment cooled by conventional AC invariably has to be shut down or revert to a low power mode with limited functionality immediately cooling ceases, to prevent overheating. The fault then needs to be rectified very quickly, which carries high cost overheads. Traditionally, passive cooling technology has been restricted to arid regions. However, these hybrid or 'semi passive' cooling systems from Intertec can extend the range of this technology to provide solutions for equipment in equatorial regions. Intertec's design service has a large library of proven design templates for passive/hybrid cooled shelters at its disposal to speed and simplify projects - allowing example proposals to be provided easily and quickly to EPCs and project engineers. Economics Shelters with passive or hybrid cooling systems incur slightly higher capital costs than those with standard air conditioning (AC). As an example, the total cost of a passively-cooled version of Intertec s test shelter is approximately 20 % higher than a similar shelter with AC, while a test shelter with hybrid cooling costs about 28 % more. However, by far the most important parameter for SCADA installations in remote locations is long term cost of ownership. While a PCS has very low maintenance requirements and requires no power, in extreme environments such as the deserts of South Australia (where daytime temperatures might reach the high 40s Centigrade), the shelter s room temperature could reach 39 C permissible for electronics but uncomfortable for personnel. A hybrid cooling system capable of maintaining the shelter at 25 C under the same conditions would have a power consumption of about 5,800 kwh/year, whereas an AC system of comparable performance would consume over 13,000 kwh/year, require regular maintenance and be susceptible to breakdown at peak temperatures. System integrators seeking a reliable, scalable and cost-effective long term solution for protecting analyzer equipment against high temperatures should take a look at passive cooling.

4 Table. Cooling options for remote equipment in arid environments Advantages Disadvantages Heat Peltier cooler No moving parts Requires power Low efficiency Small rating (suitable for cooling problem spots) Produces heat that needs to be transported away (fan required) Explosion proof versions very difficult to produce Passive cooling (using water) Passive cooling (using phase change materials) No moving parts No power required Intrinsically explosion proof No moving parts No maintenance No power required Intrinsically explosion proof Requires large space Efficiency depends on wide day-night temperature swings Fan cooling Low costs Dust ingress Requires power supply Fans/cool air purging Low costs dissipation to ~100 W ~ W High Limited to smaller-scale electronics Up to 10 W Low Temperature remains above ambient Requires compressed air supply (sometimes available in larger plants) Systems can be very large Systems can be very large Relative cost Low Low Low- Medium Air conditioning High cooling capacity (can achieve very low temperatures, even subzero) Moving parts Maintenance required Requires power supply. AC systems are the norm, but DC systems (powered via solar cells) starting to become available) Vortex cooling No moving parts Requires compressed air (power supply) Limited in size Low efficiency (a lot of compressed air is required for large systems) ~500 W to almost any size ~50 W (maybe higher if cool air is available) Medium High Water-cooled heat exchange Can cool to low temperatures Requires cool water source such as a well or a chiller Usually makes sense for larger systems Up to many kw High

5 Project-enabling shelter solutions for harsh desert environments Projects in locations with extreme climates such as deserts face severe environmental challenges for instrumentation, which can increase project costs dramatically. The most significant of these is usually cooling. This problem is often exacerbated by remote locations, and the lack of a reliable electricity supply. Intertec has developed specialized techniques that can be applied singly or in combination to meet such challenges. In addition to offering cooling via conventional air conditioning technology, Intertec has pioneered a range of innovative passive cooling technologies for field mounted enclosures and shelters. These cooling techniques can operate either without power, or with the assistance of tiny amounts of electricity from small-scale solar power installations. This avoids the need to use fan cooling which can be problematic in regions prone to dust and sand storms. These technologies are helping to make many of today's most innovative projects become possible, from remote desert locations in arid climates such as the Middle East, to many other world regions such as offshore applications in equatorial regions. Of course, desert environments also pose other general protection problems, such as the very high levels of UV, and abrasion from dust and sand. The composite-grp construction possibilities offered by Intertec allow these challenges to be met with ease, using techniques such as thick gel-coat surface protection layers. Key threats for shelters in desert environments Threat Intertec protection possibilities GRP sandwich structure Extreme heat Abrasion dust/sand Ultraviolet radiation Unreliable power supply Extra thick embedded insulation No thermal short cuts Sunshades Gel-coat surface layer IP65 Cooling without the use of a fan Gel-coat surface layer Passive or 'hybrid' semi-passive cooling

6 GRP out-performs other materials for environmental protection A field shelter protects valuable and sensitive electrical and/or electronic equipment from harsh external environmental conditions - so that it can work efficiently in a controlled environment. In the Middle East, there are many environmental hazards that can damage the shelter and its cooling system - which in turn reduces the life of whole system. Threats include high humidity and temperatures, salt-laden atmospheres, aggressive and corrosive chemicals and gases that can be found in the atmosphere of chemical/petrochemical plants, etc. So, it is very important to choose the right construction material. Over 50 years, GRP (glass reinforced polyester) has proven to be a highly superior material for building enclosures to protect field-based equipment. The main competitive material is sheet metal. Intertec's GRP is produced using long-fibre glass strands which makes it almost as strong as stainless steel, yet around 75% lighter. In addition, GRP offers exceptional resistance to corrosion - it does not rust or degrade in any meaningful way. GRP is also an excellent insulator, and is intrinsically flexible. In its basic form, these properties make GRP a superb material for manufacturing robust outdoor shelters and enclosures - allowing maintenance-free lifecycles of 25 years or more. Intertec has developed many processes to extend GRP's natural protective advantages, by combining special grades of GRP with composite layers to achieve extra degrees of protection. The most commonly specified forms provide embedded polyurethane (PUR) foam insulation to optimize energy consumption and the efficiency of heating or cooling, anti-static properties, and protection against UV exposure and abrasion. Further composite techniques are employed to meet other specialised application demands, including fire safety and EMC shielding. Different grades of GRP may also be used at different layers of a composite, to optimize the protective properties of interior and exterior faces. Good insulation is critical to almost all outdoor applications. The basic GRP sheet material used in Intertec shelters has a very high thermal resistance compared to metal, with an efficiency that is over 1,000 times better. GRP sheets are fabricated easily into composite 'sandwich' forms, enclosing high performance insulation. As Intertec enclosures do not utilize any material with poor thermal conductivity, they are almost perfect insulators - with no 'thermal short cuts' between interior and exterior. Intertec also provides special accessories to ensure process connections do not degrade overall performance. This holistic approach has particular benefits in avoiding 'cold-spots' (or 'hot-spots') that lead to problems. It delivers extremely stable operating environments for instrumentation and electronics which can be crucial in many process control applications. Thanks to these properties, passive cooling systems work extremely efficiently with GRP shelters. Thanks to the insulation efficiency, the number of hours required by the system for cooling water at night is reduced substantially compared with a metal shelter. A field enclosure transfers heat to/from the atmosphere in two main way: by gradual leakage through the structural fabric; and by conduction through any fixings or structural components that hold the enclosure together or the access ways for tubes, cables etc. These conductive points create 'thermal short-cuts' which usually account for a majority of losses. Intertec GRP construction techniques have been refined over decades to create enclosures with almost perfect thermal performance. The structural materials are all very good insulators. Insulation is bonded into place eliminating the need for interior-exterior fixings.

7 Phase-change materials extend passive cooling to small applications Novel materials with higher melting/solidifying points are capable of storing and releasing large amounts of energy during phase change, which can be exploited to provide passive cooling for smaller-scale field equipment enclosures. Phase change materials (PCMs) that operate in the 28 to 34+ degrees C range are particularly attractive for remote equipment such as pipeline instrumentation, remote terminal units, etc. To date, PCMs tend to have been employed as an element of the environmental protection system for remote shelters to help minimize the size of back-up diesel-powered generators for example but advances in packaging techniques means that we now seeing these materials being applied to cool small-scale electronics systems. PCMs provide a heat storage mechanism when they change from solid to liquid and back again. Initially, a PCM absorbs heat as temperature rises. However, when the material reaches melting point (or phase change temperature) it absorbs large amounts of heat at an almost constant temperature. The PCM continues to absorb heat without a significant rise in temperature until all the material is transformed into liquid. When the ambient temperature then falls, the PCM resolidifies, releasing its stored latent heat. The effect of this can be seen in the test results shown: an enclosure fitted with a tank containing a PCM is subjected to a temperature swing over 24 hours. The PCM - with a phase change temperature of around 34 degrees C - 'smooths' out the internal enclosure temperature. This has the beneficial effect of capping peak daytime temperatures by some 10 degrees C. Similarly, the mechanism releases heat through the night time - protecting against extreme temperature drops. A PCM-based passive cooler system inside a small enclosure. Careful design of the PCM storage tank, which controls the efficiency and duration of the phase change process, allows small enclosures suitable for many typical remote instrumentation nodes to be cooled entirely passively. In the example shown, a 100 liter enclosure provides a cooling ability that is able to dissipate 10 W of heat and reduce the swing of internal temperatures by a total of around 20 degrees C (10 degrees lower than maximum, and 10 degrees higher than the minimum). This heat dissipation rating and the space available are more than adequate for many typical remote nodes. For instance, the average process transmitter dissipates around 2 W, and an average communication module around 5 W. A 'sunshade' is a critical additional passive element of such enclosure designs, to minimize the heat absorbed from the sun, and the environment (e.g. nearby heat sources such as rocks). In this enclosure, the PCM is located at the back, behind a mounting panel. The electronics device or equipment to be protected is attached to this panel with as large a contact area as possible to aid heat conduction. A range of different phase change materials are available to handle different climates. Passively cooled enclosures greatly simplify the design, and reduce the costs, of environmental protection. Although a small degree of power will still be required to operate the electronics equipment used in these small enclosures, this can usually be achieved with a relatively small-scale solar cell array and battery storage system. A further important advantage of passive cooling such as this is the elimination of fans or vents, so an enclosure's environmental ingress protection (IP rating) against dust and water remains unchanged, avoiding maintenance problems that might arise as a result of weather events such as sandstorms. If an alternative cooling strategy such as an HVAC system is employed, the size and cost of the design can increase considerably. The nearby table provides an overview of the key techniques available today (with judgements of capability based on Intertec's experience). If explosion protection is also required this can escalate costs dramatically: a multiplier of five is a reasonable rule-of-thumb. Some of the cooling techniques are only viable in arid climates in particularly favorable situations such as at an existing plant but are included for completeness.

8 Walk-in shelters for larger-scale protection applications Intertec offers a design and construction service that is unparalleled in its flexibility. Shelters of almost unlimited size and shape can be produced using high-integrity single-piece composite-grp walls. This flexibility is made possible by very large investments in specialized GRP sheet processing machinery at Intertec's two shelter manufacturing plants in Europe and North America. This investment includes large-area multi-function CNC machinery that can cut, rout and drill GRP sheets in quantities as small as one, and in single-piece shapes of lengths to >12 meters. The manufacturing processes allow the grade and make-up of the composite GRP materials to be adapted and varied to optimize protective qualities, and give immense flexibility for for larger enclosures such as shelters. The number and thickness of insulation layers can be varied to suit the application, including a choice of PUR foam for thermal insulation, and/or mineral wool for fire resistance. Large-bed presses and CNC shaping/fabricating machines are used to produce custom compositelayered panel shapes. Application-specific size, shape and protection characteristics can greatly reduce process design and lifecycle costs. Among many challenges that can be met are shelters for remote locations without gridsupplied power, shelters for extremely cold or extremely hot climates, lightweight and corrosion-resistant shelters for offshore applications, and custom size/shape shelters for skid-mounted packages. Intertec shelters are produced using sandwich-construction panels can incorporate strong pultruded- GRP posts. This construction principle greatly increases load-bearing capability by transferring the weight of the interior equipment to support posts - via infinitely adjustable equipment mounting rails. Pultruded-GRP posts can be used to provide very high load-bearing capability.