Positing the view that ice thermal storage is an ideal solution to improve the performance and efficiency of a district cooling plant, Georges Hoeterickx supports it with a case study.

District cooling plant designers are continuously challenged to match the plant designs with different conflicting criteria, such as space, occupied area, power demand, plant efficiency, and last but not the least, the client’s budget.

An increasing number of designers of large central and district cooling plants consider the possibility of introducing ice thermal storage in their concepts, and depending on the benefits versus cost, decide to implement ice thermal storage in the cooling plant concept.

This was the case for the designers of the King Abdul Aziz University Central Utility Plant 2 in Jeddah, Kingdom of Saudi Arabia, a few years ago, when leading US and local designers were asked to design and build a new cooling plant to serve the University’s facilities having a total cooling, capacity of 158 MW.

A few of the specific design requirements were:

• Occupy minimum area: Although the campus area is huge, the space allocated for the cooling plant was limited, due to some practical reasons.
• Restrict equipment: Because of the limited plant area, the space for the heat rejection equipment was also limited.
• Minimise required installed power and reduce power demand during day time, whenever possible: This was because power supply was limited and costly. In addition, the client wanted to be prepared for the introduction of electricity rates for different times of the day.
• Cost factor: The initial and running cost of the chilled water distribution system needed to be considered while designing the central plant.
• Offer high reliability: Because of the climatic conditions, providing year-round cooling without interruption was a must. In other words, high reliability under all circumstances was a key factor.

After in-depth investigations and evaluating various solutions, like chilled water storage, internal melt versus external melt, ice thermal storage and the use of eutectic solutions, the engineers and the university finally decided to implement ice thermal storage in the central plant’s concept.

The final solution selected was an ice thermal storage system, being the largest of its kind in the world, having a total ice thermal storage capacity of 445 MWh and providing a cooling capacity of 39 MW (melting ice only, excluding the chillers). In addition, the chillers would provide 158 MW cooling.

This cooling capacity was generated during 10 night hours by using 3,078 l/sec of 25% Ethylene Glycol solution (by weight). The average entering and leaving temperatures were –6.0 °C and –2.3°C, with an average chiller capacity during the night ice building mode of 44.5 MW.

To accumulate the 445 MWh, a total number of 432 ice coils model IPCB 338 was needed. The 432 coils were installed in six individual tanks, each 20 metres in length, 8.7 metres in width and 12.2 metres in depth. The total required space to accumulate the 445 MWh was 12,860 m³ or 35 kWh per m³ tank volume. Illustration 1 shows the coils being installed in the tank and Illustration 2 shows the top of the ice storage tank filled with water ready to build ice.

By adding ice thermal storage in this project, the following cooling system enhancements, compared to a conventional central or district cooling plant system, were achieved:

• Reduction in connected power: With the actual design a total cooling capacity of 197 MW (39 MW of melting ice + 158 MW by the chillers) could be provided by only 53 MW electrical chiller motor load. In total, a reduction of 20 MW connected electrical power, compared to a conventional system, was achieved.
• As a result, significant amounts were saved in electrical connection infrastructure and power supply to the plant.
• When using the 1.1°C water available, instead of 5.6°C water available from conventional chillers, the size of the chilled water distribution system was reduced significantly. This was because the lower water flow resulted in much smaller pipe sizes, smaller pumps and lower pumping costs. Comparisons are shown in the Table 1.
• Operating the chillers in a more efficient way: In the adapted design, the chillers and ice storage operate in series during the day time. In other words, the warm return water from the chilled water distribution system is first cooled down by the chillers until 7°C is reached and the following step in the cooling process, reducing the water temperature to 1.1°C, is done by the melting ice. As such, the chillers are always operating fully loaded at better leaving water temperatures than with any other possible system design. Improvement in efficiency during the day when the ambient temperature is at its peak offsets the need for negative glycol temperatures needed to build the ice.
• Stable leaving water temperature: Because the cooling capacity of the melting ice can easily match variable cooling loads instantly, the leaving chilled water temperatures of the central or district cooling plant is stable at the set point under all circumstances.

With regard to the ice storage coils, the option to use elliptical tube design circuits was preferred, because this arrangement allows more kWh ice storage per m³ tank volume and more circuits (and surface) can be installed in a given tank width.

The ice thickness reached at the end of the ice build determined, in combination with the ice build time, the leaving glycol temperatures needed from the chiller during ice build. Therefore, the ice storage coil surface was adjusted to match the chiller capacities during ice build mode and at different lower glycol temperature conditions. This was even more important, as the chillers were of the centrifugal type and the heat rejection happened by means of radiators. This combination proved to be the most cost-effective and efficient method, considering the fact that due to lack of water, the use of open-type cooling towers for heat rejection was not an option.

The alternative solution with chilled water storage was not applied for the following reasons:

• High cost: While chilled water storage design looked quite simple and easy, in this case, taking into account all civil construction costs to accommodate the very large storage tanks made the concept unfeasible.
• Large volume and space: It was about eight times compared to ice storage.
• Chilled water system: No savings compared to a conventional design were possible.
• High maintenance costs: Keeping the storage tanks would have been a costly challenge.

Conclusion

When considering all design aspects of a large central or district cooling system, very often, the integration of ice thermal storage offers unique opportunities to save initial and operating costs, while often other challenges are being taken care of.

The writer is Director of Business Development, Evapco Europe. He can be contacted at hoeterickx_g@evapco.be