Thermal energy storage is a principle where cooling energy is produced and stored in the low cooling demand period of the day (typically at right) and utilized during the high demand period of the day (typically in the afternoon time).

The two basic strategies of thermal energy storage are load leveling and load shifting.

• Load leveling, or peak shaving, is when the chillers’ capacity is sized to meet the average daily load.
• Load shifting is when the chillers are shut off during peak hours and only thermal storage is utilized. Load shifting would require a large thermal energy storage system and large chillers capacity.

The first strategy is the one typically utilized in UAE due to a flat structure electric rate. In the Kingdom of Saudi Arabia, though load shifting was utilized as a requirement of the electric utility to shut down the air conditioning system during peak hours or for inlet turbine cooling.

The different storage technologies comprise the following, ranked from the most popular to the least utilized.

• Stratified chilled water thermal energy storage system
• External ice melt
• Encapsulated ice
• Ice harvesting
• Internal melt ice on coil
• Phase change materials

The most challenging criterion facing the designer is to be able to determine the typical day cooling load profile, which is highly dependent on the weather conditions and on the internal use of the buildings or development.

A lot of software are available to predict load profile; however, very few are reliable. This is due to non availability of full weather data and due to the fact that actual peak day, or days, will have different patterns each year.

However, the best data source is the actual daily load profile, which the operating district cooling companies start to have in the Middle East. A typical field data load profile for a 10,000 TR district cooling plant room is in the chart, below.

Some benefits of load – leveling thermal energy storage are listed here below.

• Lower chilled water plant capacity by around 20% is typical, which when considering the thermal energy storage system, the district cooling plant capital cost saving of around seven percent of the total plant cost may be achieved.
• Reduced electric demand by around 20% due to peak shaving.
• Reduced Electric Utility Infrastructure cost (power generation plant and electric distribution) by around 20%.

The main drawbacks of thermal energy storage are..

• More land or plant space is required to accommodate the storage system.
• Heat exchangers or pressure-sustaining valves are often required to transfer the cooling load when glycols are used when the highest point of the building connected load is statically higher than the highest point of the storage system.

The economics of a thermal energy storage system and the technology utilized can be quite complex, which is why many designers or district cooling utility providers shy away from implementing thermal storage. However thermal energy storage is picking up in the GCC. and more thermal energy storage capacity is being designed or installed.

The writer is CEO of DC Pro Technologies, UAE.