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Mon 1 Jan 2007 08:06 PM

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Ice cold conditioning

The need to meet cooling demands in an energy efficient way is yet more vital with the increasing scale of projects in the Middle East. Baltimore Aircoil International marketing manager Georges Hoeterickx explains to MEP Middle East an icy method that is being installed at the Burj Dubai Developmnent.

Finding efficient ways to provide the cooling capacity required in the Middle East is increasingly important as the vast scale of construction work further raises demands. The electricity used for cooling purposes currently accounts for over 40% of the UAE’s demand for the utility.

The majority of this power is consumed during a few daytime hours, however to ensure that the peak demand can be met the electrical generating and distribution systems must be designed and built to meet this capacity. Add to this the fact that the efficiency of the gas turbines in power generation plants is lowest during the warmer periods of the day and this leads to a non-economical way to provide power.

Two decades ago the USA and many European countries introduced financial incentives in an attempt to motivate users to shift the demand for electrical power to non-peak hours. Methods included using differential tariffs according to the time of day or season. Putting such measures in place provides power cost savings for both the utilities firms and the users. However as some applications by their nature must operate at specific times of the day, alternative ways to meet the cooling demand at lower cost or by efficiently ‘shifting’ the peak demand for the capacity must be found.

One technology that is already being successfully used to serve the region’s air conditioning systems is district cooling. But how long can we continue to build conventional district cooling plants and expect to receive unlimited power supplies?

One of the most cost-effective and technically acceptable ways of peak demand shifting for cooling applications is the use of ice thermal storage systems. This involves the use of a store of ice slurry to reduce the temperature of the cooling water. The ice is created during periods of reduced cooling load (night-time) and stored within a thermal storage unit for use when needed.

The two main methods of extracting the cooling energy are external and internal melt. With external melt there is direct contact between the ice and the water that circulates to the cooling load. Coils are immersed in a tank filled with water and by feeding a refrigerant or cold glycol to the coil, cylinders of ice are formed around the tubes. During melt-out, warm return water from the cooling load is pumped through the ice tank, melting the ice and cooling the water.

With internal melt systems the medium for ice build and melt-off is a glycol solution. The ice coils are again immersed in a tank filled with water and during the ice build mode cold glycol is circulated through the coils to freeze the water. During the melt cycle warm water from the cooling load is fed through the ice coils and cooled indirectly by the ice through the coil tube walls as the ice closest to the coil is melted first. The cooled glycol is recirculated to the cooling load or an intermediate heat exchanger until the ice in the tank is fully melted.

Ice storage has been used for decades in industrial applications and since the early 1980s it has been commonly used in Europe and the USA for commercial and district cooling applications. Countries in the Far East such as China have also begun adopted the technology to solve the steep increase in power demand for cooling purposes.

Diagrams one and two, above, illustrate the principles of an ice storage system in relation to a typical cooling demand pattern. In a conventional system, all of the equipment installed in the system including chillers, cooling towers, transformers and switchgear must be sized to meet the peak demand. However the full capacity of this equipment may only be used for a few hours of the day.

In an ice storage system the total installed chiller capacity can be reduced by around 35%. The latent energy stored in ice is eight times greater than that possible with a similar volume of chilled water, which makes ice storage more favourable for large district cooling plants.

Ice thermal storage plants can deliver chilled water at lower temperatures than are normally available without any penalty. This lower water temperature allows the consultant to size the system for a lower leaving water temperature and as such a larger temperature change in the chilled water loop. This enables all auxiliary equipment to be reduced in size.

A further difference from a conventional system is that the chillers will not be switched off during the night in an ice storage system. By changing the setpoint they will start to deliver a glycol/water mixture at -4°C instead of chilled water of 4-6°C in daytime mode. This is then circulated through the coils in the ice storage tanks, freezing the surrounding water to form ice; ice enables storage of about 16 tonne hours per m³ tank volume.

During daytime, cooling will be provided by the chillers and supported by the melting ice as water coming from the cooling load is circulated through the ice storage tanks. The water leaving the ice storage tanks will be close to 0°C during the complete melt cycle.

Often the factor that limits the maximum capacity of a district cooling plant is the size of the heat rejection equipment on the roof. If this can be reduced by a factor of 35% by using an ice thermal storage system, the plant output can be increased by the same degree.

So what are the risks involved in selecting an ice thermal storage system for use in the Middle East? Firstly, the use of the systems in Europe and the USA over the past two decades has meant that the systems have now been well developed and any early issues are resolved.

Designers and manufacturers have learned to better understand ice thermal storage techniques and have improved the solutions, so that the systems now offered can be applied without risks when dealing with people who have a track record of successful designs and installations. This is of importance when a new concept is introduced in a market like the UAE.

On a recent project in the Kingdom of Saudi Arabia, Baltimore Aircoil undertook full design responsibility under its scope of works in addition to acting as the supplier of the ice thermal storage systems. This assured that proper designs and a single source of responsibility were applied in an initial phase of introducing the concept into the market.

Specialist equipment and operators are not needed to install the equipment. The systems use standard equipment such as chillers, heat exchanger and pumps, but operators need to have a solid understanding of the principles of ice storage systems prior to operating the plants as required.

Combining creative engineering practices with a lot of good common sense will provide a new generation of district cooling plants with integrated ice storage systems, which will provide major benefits for owners and also allow the power generation and distribution authorities to better match demand and production capacities and as such save money for everyone in the country.

The first application of a large ice thermal storage system in the UAE is currently being completed at the Burj Dubai Development. The Dubai Mall, Burj Dubai Square and Burj Tower all currently under construction will receive chilled water from a district cooling plant that includes a combination of conventional cooling techniques and ice storage.

The system involves over 50,000 tonne hours of ice storage that can be used to supply chilled water in case of a catastrophic plant failure for several hours, depending on the load requirement. The system was designed by Cairo-based firm Allied Consultants, which has extensive knowledge and experience in the design of all types of district cooling plants, including ice storage systems.

In this case one of the driving forces to use ice was to limit the power demand of the cooling load, which has been reduced by 25%. Other benefits being sought included the desire for lower noise during daytime as less mechanical equipment operates, a smaller plant footprint, low temperature water available from the ice storage which resulted in pumping cost savings, better daytime efficiency and the provision of a back-up system for cooling and a reduction of the plume effect. Despite higher capital costs, the combination of these benefits made the concept feasible for the client.

The construction of this installation is almost complete and in around two months time it will start generating chilled water and will be the largest ice storage system in the region.

Ice thermal storage – making the choice

There are several factors that should be considered before opting for the use of a thermal ice storage system in the Middle East. The factors affecting the decision and reasons for using such systems differ from those in more temperate climates such as are experienced in Europe.

Advantages of integrating ice storage in a district cooling plant:

• the size of the installed chiller capacity can be reduced by 30-40 %;

• the size and cost of all the auxiliary equipment can be reduced by 30-40%;

• it is possible to obtain more cooling capacity per m2 of plant area;

• it is possible to deliver chilled water at lower temperatures than would normally be available from chillers, enabling the use of smaller equipment such as pipes and pumps, hence lower capital costs;

• standby cooling capacity will remain available if chiller operations are interrupted.

Disadvantages of applying ice thermal storage:

• the space required for the ice storage – a 1m
3

tank is needed per 16 tonne hours of refrigeration. As the chillers are operating during the night to produce glycol/water at -4°C, they are operating at a lower efficiency than during the day when they temperature of chilled water needed may be +5°C, for example.

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