The technique is ideal for climates where there is a large temperature swing over the daily cycle, which is exploited by using water as a medium to store the coolness of the night and use it to moderate temperatures throughout the day.
In recent years however, hybridised forms of passive cooling have been developed which significantly extend the location and application possibilities of this technology. These can extend the capability of the cooling technology to applications in Europe, in equatorial regions and offshore.
INTERTEC employs two main techniques to extend the scope of passive cooling. One is the use of micro pumps – which can be powered by small solar panels – to improve the circulation of the cooling media. The second is the augmentation of the performance of the cooling media – which is typically water – by using cooler or chilled water. At the heart of these solutions is the use of highly insulated enclosures, which utilise composite constructions of GRP inner and outer walls – plus a thick core of embedded insulation.
Small 12VDC-powered pumps are available to improve the circulation of the cooling media. As a rule of thumb, this can double the performance of a passive cooler. The electrical energy consumption of these pumps is typically only around 10W. The required electrical energy might be available on site, but it can also be provided by a small solar panel if required.
Using forced circulation extends the location potential of passive cooling. This is because passive cooling relies on a reasonable temperature differential to power the natural convection cycle that moves the water around the system. Micro pumps reduce the dependence on a wide day/night temperature difference – opening up applications in areas beyond traditional arid climates.
Such systems are used often on SCADA shelters along pipelines. Figure 1 shows an example on a drinking water pipeline in the UAE. Control and instrumentation at remote wellheads is another major application.
A cooler solution
A second technique to improve the performance of water-based passive cooling is to exploit the availability of any nearby cooler water – which might come from a river, underground source, or the sea. Such water only needs to be cooler than the required interior temperature by 2-3°K to provide a good solution. This type of design can significantly reduce the size of the cabinet or shelter – or even eliminate the need for a water storage tank – and extend the geographical areas in which this type of cooling is applicable.
A high profile example of this technology is on the Prelude floating liquefied natural gas vessel, which will be sited in an equatorial region near Timor. INTERTEC has supplied some 120 instrumentation and analyzer cabinets for the vessel, which will be cooled using ‘cold’ water from a 150m long pipe into the ocean. The cabinets’ internal rear walls are fitted with a high efficiency heat exchanger (Figure 3), comprising one or more aluminium cooling plates and stainless steel coolant pipes. Heat dissipated by the equipment in the cabinets is absorbed by the water and transferred to the vessel’s main water cooling system, from where it is dissipated to the environment. In this application, the power dissipation of the cabinets range from 140 to 900 W.
INTERTEC has also used the same technique but with underground water – to cool electronics systems sited in mainland Europe. One further extension of this concept is the use of a water chiller mounted externally to the cabinet or shelter to augment performance: this is explained later.
Advantages of hybridisation
There can be a number of advantages to system building with semi-passive cooling. The size of semi-passive shelters – compared to pure passive systems which require a large water tank – can be reduced significantly. The use of liquid cooling means that conductive heat transfer can be employed – which is much more efficient than convection cooling.
Unlike conventional HVAC, no air ducts are required, providing protection against dust and sand ingress – which can be a major issue in some environments (HVAC systems will often also need to be explosion proof – raising costs substantially).
The need for electrical power can be greatly reduced. The semi-passive or passive cooled cabinet or shelter requires little or no electrical power of its own and is virtually maintenance-free, making it ideal for long life-cycle applications.
Semi passive cooling also opens up the intriguing possibility of providing ultra-reliable cooling strategies for critical equipment. A current example that INTERTEC is involved with is the cooling of cabinets that house critical PLC-based control functions at a refinery. In this application, there is usually plenty of electrical power available, but the client needs more than one layer of protection redundancy against power loss – as a system failure could shut down the entire process. So, three separate strands of cooling are provided. In normal operation, when power is available, an electrically powered chiller feeds cool water into the cabinet’s internal heat exchanger-based passive cooler (the PLCs are cooled by an efficient conductive plate, which is connected directly to the aluminium PLC enclosure). If the chiller should fail for any reason, there is also an electrically powered air conditioning unit that will switch on. If both should fail, the passive cooling system retains enough capacity to keep the cabinets cool for several days, allowing plenty of time for repairs to take place.
The trend towards RIEs
The need for such high reliability is increasing substantially. More and more control and instrumentation systems are being sited deep inside processing plants, much closer to the process – eliminating a lot of the cabling and marshalling style cabinets previously used. Such remote instrument enclosure (RIE) installations are often then connected using fibre optics to the control system, and they need to be ultra-reliable. The protection challenges here for the enclosure provider can include dealing with corrosive atmospheres and media, blast and fire protection, and cooling in hazardous areas, among others.
Composite GRP-based cabinets and shelters are ideal for this purpose. GRP offers exceptional resistance to corrosion – as it does not rust or degrade in any meaningful way. INTERTEC has developed many additional processes to extend GRP’s natural protective advantages, by combining special grades of high-quality GRP with composite layers to achieve extra degrees of protection. The most commonly specified forms provide embedded insulation to optimize energy consumption and the efficiency of heating or cooling, anti-static properties, and protection against ultraviolet exposure and abrasion. But this technique can also be used to provide fire safety. The intrinsic flexibility of GRP also makes it simple to design cabinets and shelters with the appropriate resistance to blasts, which can handle forces up to 1200 mbar or more.
An example of this is RIE shelters produced to house the automatic emergency shutdown and fire suppression systems on offshore gas production platforms. The shelters’ basic GRP construction protects against corrosion. The key protection capability however is an extremely high degree of fire resistance that will maintain internal temperatures below 60 degrees C for up to two hours – even in the presence of a high temperature hydrocarbon fire. This protection is provided by novel composite layered construction combining GRP sheets and mineral wool insulation. The lightweight shelters are also blast resistant.