What are the benefits of chilled water storage in district cooling?

What are the benefits of chilled water storage in district cooling?

Introduction

Chilled Water Storage, being a form of sensible energy storage, utilizes a large insulated tank as a storage vessel for chilled water.  In District Cooling Plants, Chilled Water Storage is used to store the excess chilled water generated by the chillers during periods of low cooling demand. During peak periods, when the cooling demand exceeds the chiller operating capacity, the chilled water tank is discharged, releasing the stored cooling energy to meet the shortfall in the chiller operating capacity.

Schematic – chilled water storage tank in a district cooling plant

The chilled water storage system has the following characteristics:

  • Chilled Water Storage is the process of generating and storing cooling capacity, in the form of chilled water, during off-peak hours to meet parts of the peak cooling demand.
  • Decouples chiller operation from cooling demand.
  • Reduces peak chiller capacity.
  • Increases chiller load factor.
  • Transfers energy consumption during peak hours to off-peak hours.

Chilled water storage in a district cooling plant reduces the installed chiller capacity and enables capital cost savings

A chilled water storage system supplements the cooling capacity of the chillers during peak hours, thereby allowing a substantial reduction in the required operating capacity of the chillers. The smaller chillers will also be able to operate at higher capacity loads for longer hours, resulting in optimum asset utilization.

Figure 1: Chilled water storage decouples cooling energy production from cooling demand

Chilled water storage effectively decouples cooling energy production from cooling demand. As shown in the graph above, chillers are operated continuously during off-peak hours, from 2200 to 0800 hours, even when cooling demand is less than the chiller production capacity. The excess cooling energy from the chillers is stored in the chilled water storage tank. During peak hours, from 0800 to 2200 hours, cooling demand exceeds chiller operating capacity, and the stored energy is discharged to supplement chiller production capacity in meeting the higher cooling demand.

Chiller Sizing in a Traditional Chiller Plant (Non Thermal Energy Storage)
Figure 2: Chiller sizing in a traditional chiller plant without chilled water storage

In a traditional central chiller plant system, without chilled water storage, the chiller operating capacity must be chosen to match the maximum cooling load on the design day. Using the cooling load profile shown in the chart above as an example, the chiller operating capacity must match the peak cooling load of 16,975RT, which occurs only once per 24-hour cycle. Every other hour, the chiller plant will run at a lower part load. This is not conducive to the efficient use of production assets.

Chiller Sizing with Thermal Energy Storage
Figure 3: Chiller sizing with chilled water storage

With the inclusion of a chilled water storage system, the total chiller operating capacity does not have to match the maximum design day cooling demand. Furthermore, the chiller operation can be decoupled from the end-user cooling demand, allowing the total chiller operating capacity to be sized considerably smaller than the maximum cooling load. As shown in the chart above, proper sizing of the chiller operating capacity allows the chillers to run continuously throughout the 24-hour cycle. The chilled water storage system is charged during off-peak hours and then discharged during peak hours to supplement the chillers’ chilled water production.

The incorporation of chilled water storage allows the district cooling plant to have a smaller installed chiller capacity, resulting in substantial capital cost savings in terms of chillers, cooling towers, the balance of plant equipment, and electrical and control systems.

The potential capital cost savings from chilled water storage include the following:

  • Smaller chiller capacity and ancillary equipment
  • Smaller cooling towers and ancillary equipment
  • Smaller electrical system
  • Reduced plant size
  • Reduced piping costs

Chilled water storage in a district cooling plant reduces operation and maintenance costs

In addition to the capital cost savings, the District Cooling plant will also be able to reduce the operation and maintenance costs of the plant.

There will be fewer chillers, cooling towers, pumps, and other ancillary equipment to operate and maintain with a chilled water storage system, lowering the overall operation and maintenance cost of the district cooling plant significantly.

Many electricity utility companies use chilled water storage as a demand management strategy to shift demand for power generation from peak to off-peak hours.

Power generation during off-peak hours is advantageous to the electric utility for the following reasons:

  • Base load power generation is more efficient than peaking power generation plants.
  • Demand shifting increases the load factor of base load generating plants while decreasing demand for expensive and inefficient peaking plants.
  • Transmission and distribution losses are lower during off-peak hours.

Many electrical utilities offer incentives to encourage the adoption of thermal energy storage technology such as chilled water storage due to its obvious benefits in demand management. Differential electricity energy charges (higher peak hour energy charge and lower off-peak hour energy charge) and longer off-peak hours for charging the storage system are among the incentives offered.

Electricity tariff incentives for chilled water storage in Malaysia
Figure 4: Electricity tariff incentives for chilled water storage in Malaysia

These utility incentives have the potential to significantly reduce the electrical utility bill for a district cooling plant that uses chilled water storage, making chilled water storage a viable option for Malaysian district cooling systems.

Chilled water storage in a district cooling plant increases energy efficiency and reduces carbon dioxide emissions

In a District Cooling Plant, chilled water storage also enables the chillers to operate at a higher and more constant load continuously throughout the day. This leads to improved asset utilization efficiency and higher average chiller COP.

Thermal energy storage enables more chillers to operate at night when the ambient wet-bulb temperature is lower which allows for lower cooling water temperature to be supplied to the chiller condenser. The lower compressor lift will increase chiller COP and improve overall chiller plant efficiency.

  • Increased on-site energy efficiency
    • The load leveling or peak shaving operation mode shifts a significant portion of chiller operation from peak hours to off-peak hours. Because of the lower ambient temperature, chillers operate more efficiently during off-peak hours than during peak hours. Chilled water storage allows chillers to operate continuously at close to full capacity and optimum efficiency, improving chiller energy utilization even further.
    • The low flow, high chilled water delta T design also helps to improve chilled water pumping efficiency.
  • Increased source energy efficiency
    • From the standpoint of the power grid, shifting power demand from peak hours to off-peak hours is beneficial for a number of reasons. Lowering peak power demand reduces power generation from the inefficient peaking power generators. With the shift in demand to off-peak hours, there is greater demand for power generation from the base load power generators, which are significantly more energy efficient than peak generators.
    • Furthermore, transmission and distribution losses are lower during off-peak hours, contributing to the energy efficiency improvement of the power grid.
  • Lowering CO2 emissions
    • Increased energy efficiency, both on-site and at the source, will result in lower CO2 emissions.

Chilled water storage in a district cooling plant reduces carbon footprint throughout the life cycle of the system

By lowering the installed chiller capacity in a district cooling plant, chilled water storage helps to lower resource utilization throughout the district cooling system’s life cycle, including construction, operation and maintenance, repair, and disposal. Consequently, a district cooling system will have a lower life-cycle carbon footprint than individual in-building chiller plants serving the same end-user space.

Additional benefits of incorporating chilled water storage in a district cooling system

Additional advantages of chilled water storage in district cooling include the following:

  • System redundancy – A chilled water storage system can provide critical backup cooling for mission-critical applications.
  • Operational and maintenance flexibility – By decoupling chilled water production from cooling demand, a chilled water storage system adds operational and maintenance flexibility to a district cooling plant.
  • Increase district cooling system capacity without adding more chillers – In a brownfield district cooling system, a chilled water storage system can be installed as a satellite plant to supplement cooling demand during peak hours, thereby alleviating peak load bottlenecks.

Thermal Energy Storage Technologies used in District Cooling

Thermal Energy Storage Technologies commonly used in District Cooling

Thermal Energy Storage Classification

Thermal energy storage technologies commonly used in the district cooling industry can be classified according to the form of energy stored in the system. Cool energy can be stored either in the form of sensible heat or latent heat.

Sensible Heat Storage

In a sensible heat storage system, the energy is stored as sensible heat associated with the change in temperature of the storage media. The storage media does not undergo a phase change. The amount of energy stored in a sensible heat storage system is dependent on the sensible heat capacity of the media and the degree of temperature change during the charging process. In district cooling systems, the most popular form of sensible heat storage is the chilled water storage system.

Latent Heat Storage

In a latent heat storage system, the energy is stored as latent heat as the storage media undergoes a phase change, transitioning from liquid to solid form. The amount of energy stored in a latent heat storage system is dependent on the latent heat of fusion of the media.
In district cooling systems, the most popular form of latent heat storage is the ice storage system.

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What is Thermal Energy Storage in District Cooling?

What is Thermal Energy Storage

What is thermal energy storage in a district cooling system?

Thermal energy storage, in the context of district cooling, is the process of producing and storing cooling energy during periods of low demand. The stored cool energy is then discharged to meet cooling requirements during periods of high demand. Depending on the type of thermal energy storage technology, the cool storage medium can be in the form of chilled water, ice, or some other form of phase change media.

Thermal energy storage system in a district cooling plant
Thermal energy storage system in a district cooling plant

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