星空传媒

Designing hvac systems today is all about compromise: balancing the clients' needs, the cost of the hvac equipment, installation costs, running costs and environmental impact. The emergence of completely integrated systems, where all components are supplied by one manufacturer in a pre-optimised, fully controllable package, has extended the choice of systems. However, market trends, legislative influence and new research suggests that understanding what such systems offer simplifies the options. Until recently the market has had to rely upon vrf systems for totally integrated systems but with the introduction of water based technology, which allows direct communication from the chiller/heat pump directly to fan coil units, it is possible to create systems that work with all existing building services systems.

It is gradually becoming clear to everyone in the industry that with the advent of Part L, and authoritative recommendations on good and best practice in terms of energy consumed, that considerably more design time is going to be spent on calculating load profiles and producing a wide variety of different system options than ever before for any given situation. However, it seems likely that more systems offering higher energy savings will eventually be chosen by end-users in the coming months and years.

The latest research indicates that this will support the growing enthusiasm of the market place for integrated air conditioning systems. That is systems where the chiller, the hydronic unit and the air delivery systems communicate with each other, are optimised to work together and in which some of the components are even installed in the same unit. Some manufacturers have created chillers that incorporate the hydronic unit in the same cabinet as the compressors and pumps.

Predicting the actual annual energy consumption of any air conditioning system is a complex and difficult undertaking that needs to consider a number of related factors. The building type, the building load profile, the system performance at part-load and 'off-design' conditions, and the local weather pattern, all have a significant influence on the client's annual energy bills.

Despite these difficulties, system designers are now obliged to make estimates of the energy consumption and to ensure that this meets certain targets. Unsurprisingly, the methodologies available for making these estimates are somewhat simplistic and tend to concentrate on the performance of the plant at the design condition. In practice it has been shown that equipment operates for over 95% of its time at 'off-design' conditions, and this can be at substantially improved efficiencies, as figure 1 illustrates.

Early results from a recent independent long-term study of the energy consumption of a wide variety of types of system in situ and under installed conditions are going to offer designers and owners some useful, practical guidance on the performance of systems, when fully published1. They also indicate that well designed integrated systems can offer energy efficiencies considerably better than for stand-alone systems and can meet or exceed current recommendations of best practice (figure 2).

The report also highlights the importance of operational factors in determining the energy performance of a system. Once again emphasising the value, in energy efficiency terms of fully integrated systems. With their built-in comprehensive time clock, scheduling options and set back facilities, energy saving regimes reflecting the conditions actually pertaining day-to-day in the building environment can easily be applied to an integrated system.

HVAC systems that are totally integrated provide direct communication between all the components of the system. This also allows the controls to react instantaneously to changing demands. However, this is not the whole story as some critical and frequently overlooked factors affecting the comfort of the occupants and the power consumed by the system include building orientation, internal zoning and, of course, the eventual use of the building.

Orientation and zoning
Accurate hvac system control is reliant on appropriate and well thought through zoning. Zoning considerations should reflect a building's orientation as well as taking into account occupancy use and time schedules.

Terminal fan coil units that serve zones with similar thermal characteristics should communicate with the same chiller and will then provide the best performance and comfort conditions. Mixing types of zone demand will increase inefficient system performance. Now that small integrated chiller systems are so efficient this will often mean that it is more energy efficient to use more systems to cover a building that might previously have been covered by a single large chiller with a supposed capital plant cost saving. The same applies to units grouped on the same or similar time schedules. Examples of this practice would be to group areas such as photocopier facilities, open plan areas or offices taking into account the exposure to solar effect. Such complex building usage patterns can only be coped with by using a number of integrated hvac systems and when refurbishment opportunities arise it will become good practice to substitute a single large chiller by several smaller systems.

Very high load areas such as computer rooms and restaurant areas should be reviewed to see if they can be fitted in the system design or whether they should be a separate system, which can take in to consideration the most cost-effective and energy efficient solutions for these areas.

Set point control
Temperature set points on air conditioning systems are an area where considerable energy savings can be obtained if there is a facility to adjust them to reflect everyday conditions rather than the design extreme.

Integrated systems offer this facility, and what is more enable a very accurate reflection of demand as the in-the-space fan coils communicate user needs. Making adjustments relative to the return water temperature gives the most energy efficient results.

Figure 3 demonstrate that a 6掳C temperature adjustment on the chiller (about 2掳C in the occupied space) can produce a 22% improvement in the coefficient of performance.

Research into installed system efficiency and ways to improve it suggests that there are benefits to be gained not just from installing fully integrated heat pump based systems but also creating regimes which integrate all types of hvac system whatever their component base. Integration can be applied to previously non-integrated systems using external control or by substituting one part of the system by an integrated ac unit with wider control capabilities. This can be a cost-effective retrofit.

Two-pipe cooling only systems
This type of system is predominantly used where high internal gains are expected within the building or where there is some form of heating system already in use within the structure. The main challenge that designers face is the integration of the physically separate cooling and heating systems to prevent each competing with the other and performing inefficiently in terms of space conditions and energy consumed during operating periods.

The system must be carefully zoned to prevent over-heating or over-cooling of the building. This type of direct integration has previously only been achievable using very complex and expensive bems controls and was therefore only found on larger systems and buildings. Now it should be considered as an option for medium and smaller buildings too.

The correct design and installation of two-pipe cooling and heating systems with electric heaters can provide simultaneous heating and cooling in individual areas within a building. It lends itself ideally for offices, hotels, and restaurants up to 2500 m². As has been indicated, to ensure optimum operation the system should be zoned with alike areas or needs grouped together.

The system should be set for cooling priority operation. This will ensure that the system satisfies those areas requiring cooling while also allowing low-power electric heaters (0路75 W) to provide heat to those areas demanding heating. If the demand for heating increases above the total input power of the heat pump, the system switches the unit to heat mode until the number of zones demanding heating reaches zero. In very cold temperatures (below 2-3掳C) the system can use both hot water and electric heaters, the priority being given to water over electric heating.

Temperature control for this type of system is paramount to occupant comfort. Carrier Aquasmart installations have been designed to control space conditions to 1 掳C and to do so with lower energy consumption than systems using the latest direct expansion technology.

This traditional system is generally used in larger building applications where the cost of installing separate boiler and chiller systems becomes more affordable within the overall building services cost. It has long been accepted practice that it is difficult to provide an integrated design solution for this type of installation. The more so if no bems system is present. The main challenge facing designers is with the control of this installation type. Integrating the cooling and heating plant to prevent systems competing against each other in the intermediate seasons is the difficulty.

But there are now hydronic systems on the market, exemplified by the Carrier Aquasmart that have been designed with integration in mind from the start. An inbuilt chiller-based electronic system manager system can manage either two-pipe and four-pipe systems and incorporates the fresh air handling unit and the boiler time clock control to ensure comfort within the building environment.

Each fan coil in such a system controls its local space temperature through modulation of control valves and fan speed to achieve ideal conditions within the space. This flexibility and quantity of management means that Aquasmart can replace the more expensive bems in many applications, reducing the total system cost considerably. In addition, the communication and fault monitoring capabilities available argue in favour of integrating a system in this way.

Integrated system popularity
Many thousands of integrated ac systems, both hydronic and dx, have been installed across Europe in the last three years. The popularity of the hydronic systems is higher among consulting engineers in Europe than in the UK. Perhaps there is a greater commitment to environmentally sensitive solutions on the continent or perhaps the UK has not caught on to how much work, as well as energy, can be saved by specifying an integrated system where fan coil performance is optimised to match the chillers with which they can so easily communicate. This is surely set to change.