In many respects there is no such thing as a standard air conditioning system design, with different approaches required depending on the type of application.
For instance a leisure centre, retail outlet, cinema and office will have differing needs, which are governed by the particular space application requirements. Consideration must be given to such variables as number of occupants, air distribution, heating and cooling requirements, not forgetting the constraints of the building itself, and the budget available.
When planning a sustainable air conditioning system it is important to fully understand the effect of each of the above – as each can impact on efficiency – as well as the overall life time costs. Firstly, lets consider the plant.
Avoid oversizing
Often, as a comfort factor, there is sometimes a tendency to slightly oversize the plant. This is generally to provide some contingency should there be a possible miscalculation at the design stage, or to provide added flexibility if problems should occur during installation, and pipe or duct layouts should need to change.
For example, if this view is applied to all plant items, provided in a chilled water fan coil system, then the combination of the chiller and the fan coils in practice may mean the air conditioning system is oversized for the building requirements. While this is fully understandable from a design perspective, on the other hand, when considering energy efficiency, plant is most efficient when working at full load conditions.
In simple terms, if plant equipment is oversized it will be more likely to operate under a part load condition, which means that it will not perform to maximum efficiency, which of course leads to increased costs, on two fronts. Taking the previous example, if the plant is oversized, this not only involves higher costs for initial capital investment, which is bad news for the customer, it also leads to increased running costs, which will adversely affect the life time cycle calculation.
Ductwork selection
When an air distribution system is selected as the heat transfer medium, the ductwork selection is also important. Depending on the design criteria applied, a decision must be made with regard to the specification of the ductwork, again depending on application, air volumes and noise criteria.
A fine balance must be struck; if they are oversized there is a higher capital investment, but this can be offset by achieving savings through the lower fan power requirement, leading to a reduction in the energy used and consequently positively effecting life time cycle costs. On the other hand if the ductwork is undersized, leading to a higher static resistance in the system, fan power is increased, and more energy is used.
Consideration must also be given to noise specification, which will also influence the type and size of ductwork selected. A similar picture can be painted of course with pipe work design.
Insulation for protection
If we develop this a stage further, however, the specification and type of insulation applied to both water and air systems brings with it its own issues. Insulation provides both thermal and condensation protection qualities, so the right specification for a particular project is important, with insulation selected to meet the needs of each application.
Insulation with a low thermal conductivity will of course mean that less heat is lost through hot water distribution systems, which will reduce heat lost in the system and can reduce cycling of plant. The inclusion of a buffer or hot water tank too helps offset peak load requirement and reduce cycling of plant. On chilled water systems the dew point of the water condition will dictate the thermal properties required to prevent condensation. Higher chilled water temperature can mean a less expensive insulation can be used and vice versa.
If, however, a lower thermal insulation is used on low dew point chilled water systems, this inevitably will mean condensation will form on the exterior and of course heat will be absorbed from the surrounding area, impacting on the energy used by the plant. So again, careful consideration is required to ensure that insulation is best suited to the application, which can offer cost advantages too if correctly specified.
Finally, as part of the product selection process, it is important to consider the control package. Here it is essential to provide controls that will suit the end user, otherwise there is the danger the air conditioning system will not be operated correctly. This can potentially mean it is not run at an optimum level, reducing efficiency and leading to increased energy bills.
Controls suitable for the user
The package required will very much depend on the end user, the application and the expertise in-house. For instance a hospital may require a quite sophisticated bems to ensure the best use of energy, and that the equipment is operating efficiently. On the other hand the manager of a retail outlet will likely just need a set temperature across one zone, which can be operated simply by any member of staff.
In essence in this type of application the customer interface must be kept as simple as possible. The control package can also save energy by utilising various energy saving features that are currently available, which includes optimisation/compensation as well as control/time zone control.
To ensure long-term sustainability is achieved, maintenance of the air conditioning system is crucial. Just like a regular car, if it is not maintained it will not run as effectively, will not last as long and is liable to break down at any time. A planned preventative maintenance programme of any air conditioning system will ensure it continues to operate efficiently and effectively, improving its life span as well as reducing on-going running costs.
As part of this process it's important that, for example, the filters are changed regularly otherwise they can become clogged meaning the fans have to work harder, increasing the amount of energy used. To create the most efficient system a planned regular programme of maintenance should be implemented, which can help extend the life of equipment.
Free cooling
Of course, where possible, a simple and effective method of reducing energy in cooling systems can be to add free cooling to the system. Free cooling is when air is drawn into the system from outside, when the external air temperature is lower than that of the required supply air temperature, negating the requirement to provide mechanical cooling from the refrigeration system.
This free cooling capability will ensure that the refrigeration system efficiency will be optimised. This cool air is then supplied into the system for distribution throughout the conditioned space. This is more efficient because the air does not need to be cooled mechanically, meaning the plant refrigeration system will operate less, reducing energy consumption.
When planning a new system the choice of refrigerant should also be carefully considered to ensure the design is as environmentally friendly as possible. A great deal of coverage has already been devoted to this subject, so I won't go into detail here, but one such refrigerant to use is the ozone benign hfc, R407C which is one of the preferred options adopted across the industry.
However, it is not just the design of an air conditioning system which is important, the efficiency of plant itself is equally relevant and many manufacturers are now developing equipment with sustainability in mind. For instance at Lennox we have recently launched three new chiller ranges, the EcoLean, EcoLogic and EcoMax (from 7 kW to 1600 kW), which have been developed to provide maximum cooling efficiency with low running costs and power consumption.
This has been achieved through the application of the latest scroll, screw and reciprocating compressor technology and heat pump capability, which helps ensure a minimal impact on the environment. The Climatic control package has also been carefully designed, and is capable of achieving energy savings up to 30%.
The sizing of plant, distribution of air and the control package can all impact on the effectiveness and efficiency of the system. In terms of life cycle costing it is also easy to forget that without regular maintenance efficiency can decrease significantly leading to higher energy usage and costs.
Increasingly manufacturers are also considering green issues when designing air conditioning equipment and are applying the latest technology to ensure maximum sustainability for the future.
For more details contact Julian Brunnock of Lennox at 01604 599400.
Source
Building Sustainable Design
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