Improving energy efficiency — efficiently
How can you reduce the cost of heating large spaces by up to 40%. And how do you achieve that feat using the minimal amount of extra energy to comply with ever-more demanding EU legislation? Over to Alan Macklin of Elta Group.
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| Especially with a mezzanine floor in this Toyota showroom, high temperatures are to be avoided at high level. Note the two destratification fans. |
For anybody involved in the HVAC industry, thermal stratification is certainly not a new concept. The fact that warm air rises means that the lower areas of an interior space, particularly in facilities characterised by large open expanses, have been cooler than those higher up.
The most common response by building owners and operators thus far has been to increase the heat to compensate for this, over-heating a facility to achieve a comfortable temperature in the area where people are located, i.e. at floor level. However, with the focus on energy efficiency getting ever stronger and the costs of heating a space (or cooling it) ever greater, there is an increasing need to identify ways in which the effects of stratification can be minimised.
Destratification, the mixing of internal layers of air to equalise temperatures by eliminating stratified layers, is seeing something of a renaissance. With organisations such as the Carbon Trust identifying destratification as one of the top three methods of reducing carbon-dioxide emissions from buildings, many heating and ventilation engineers are looking to destratification fans as an important contributor in achieving energy reductions.
The legislation
The global commitment to reducing energy consumption is being implemented throughout Europe through the Ecodesign Directive 2009/125/EC, which lays out the framework defining the rules for setting product-specific requirements and legislation on energy efficiency. The US Energy Information Administration reports that across all commercial applications, HVAC systems are estimated to account for 51% of a buildings’ fuel consumption, with space heating alone accounting for 36%. The biggest consumer of electricity in the UK is motor-driven systems, with about 22% of industrial motor energy usage being attributable to fans. It is therefore perhaps not surprising that the new breed of destratification fans is not only addressing its role in reducing the energy required to heat or cool a building, but also the energy consumed in doing so.
The zone of occupancy (ZOO), is the focus for destratification, recognising that it is the areas where people are going about their daily business where heating or cooling needs to be directed. The higher the ceiling, the greater the temperature differential between floor and ceiling is likely to be — and the greater the potential savings in energy. Computational Fluid Dynamics (CFD) has been used to demonstrate the ‘columns’ of air that are generated by destratification fans and their effects on an environment, with tests demonstrating energy savings for space heating ranging from 20 to 40%. Likewise, savings for air conditioning of 10 to 20% can be achieved by reducing the energy required to condition the air in an occupied space.
Destratification fans can also be used as spot coolers to provide a cooling effect in a given area by increasing the air speed. The sensation of increased air flow allows thermostats to be set higher in air-conditioned spaces. For example, airflow of 1 m/s can equate to a cooling sensation of 3 to 4 K, depending on factors such as humidity, without sacrificing occupant comfort. A destratified space also has lower demands for heating and air-conditioning capacity design.
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| The benefits of thermal destratification as demonstrated by a CFD analysis of a 12 m square space with a 7.6 m flat ceiling deck and a ceiling mounted heat source with a thermostat 1.5 m above the floor set at 21°C. The middle picture shows what happens when a ZOO fan (white silhouette) is turned on to achieve (third picture) an even temperature throughout the zone of occupancy to improve comfort noticeably and reduce energy consumption — often significantly. |
Fan efficiency
But that is not the whole story. There is also new legislation regarding the efficiency of the fans themselves. The ErP Directive, the first phase of which becomes mandatory in January 2013 with the second phase coming into effect two years later, has also had a direct impact on the fans being used in destratification applications.
Fan manufacturers such as ourselves are now required to review their product ranges in order to attain new efficiency levels. Inefficient fans are being removed from production by legislation, in the same way that inefficient, old-style light bulbs are being phased out for energy-efficient ‘compact fluorescent lamps’ (CFLs) as part of the EU’s eco-design for energy-saving products regulations.
The new legislation presents a number of issues to UK fan users who are used to working in static pressure, fan shaft power and static efficiency, which ignores the efficiency of the motor. That is no longer the case.
Speed control is a further factor in destratification fans. The process of destratifying an air mass consumes more energy than simply maintaining equilibrium once it is achieved. Once the air is destratified, the fan speed can be reduced, typically by 30 to 60%. This provides major energy savings as the fan power is proportional to the cube of the fan speed, for example, a 20% reduction in fan speed results in a 50% reduction in fan absorbed power.
Ongoing improvements
Advances in fan manufacture will need to continue — in destratification as well as all other applications. More efficient impellers and more technologically advanced fans will be necessary to meet the ever-increasing environmental and sustainability challenges posed by modern construction. The benefit to end users of such advances will be reduced running costs as well as conserving the finite amount of available energy, which is an issue for us all.
Alan Macklin is group technical manager with Elta Group
