Author: Danny

What Are the Core Chilled Water Pumping Schemes/Strategies for the Modern District Cooling System?

by Danny

Introduction

In every district cooling network, the pumping system serves as the circulatory system, ensuring chilled water—the “life-blood” of the system—reaches every building, even those at the hydraulic periphery. The choice of pumping scheme directly influences energy efficiency, operational reliability, and long-term sustainability. While often overlooked, these systems are pivotal in minimizing energy consumption, reducing carbon footprints, and maintaining cost-effective operations.

This article explores the foundational and advanced pumping strategies that define modern District Cooling Systems (DCS), emphasizing their role in achieving sustainable urban infrastructure. From constant-speed setups to intelligent distributed pumping, we’ll dissect how engineering ingenuity shapes efficient cooling networks.

Why Pumping Matters in District Cooling

Energy Consumption and Cost Impact

Pumping chilled water typically accounts for significant percentage of a DCS’s total energy use. However, inefficiencies such as “low delta T (ΔT) syndrome”—where the temperature difference between supply and return water reduces due to poor coil performance or control valve misoperation, can increase energy demand substantially. For instance, doubling flow rates to compensate for low ΔT can increase pumping power eightfold, owing to the cubic relationship between flow and power in centrifugal pumps. This not only escalates energy costs but also strains the entire cooling infrastructure.

System Efficiency and Heat Gain

Excessive pumping introduces parasitic heat gains into the chilled water loop. This “waste heat” forces chillers to work harder, compounding energy use. In large distribution networks, this parasitic heat gain can be substantial. Optimal pumping minimizes these losses, ensuring that the cooling capacity is used efficiently and that the system operates as close to its design intent as possible.

Reliability and Performance

Consistent pressure and flow are essential for uninterrupted cooling. Poorly designed pumping systems risk cavitation, pressure surges, or inadequate flow to remote buildings, undermining occupant comfort and equipment longevity.

Core Pumping Schemes in District Cooling

The choice of pumping scheme forms the fundamental operational strategy for a DCS, with each approach offering distinct advantages and disadvantages.

Constant Speed (CS) Primary Pumping

In constant speed primary pumping systems, chillers operate in parallel with fixed-speed pumps maintaining a steady flow through evaporators. While simple and low-maintenance, this approach is not energy efficient under part-load conditions due to the following factors:

Over-pumping that occurs when demand drops. Due to the fixed speed pumping, the excess flow is bypassed from the chilled water supply line to the return line. This results in wasted pumping energy. At the same time, the excess flow that is bypassed to the return line reduces the ΔT that is crucial to maintaining chiller efficiency.
Reduced ΔT results in additional energy penalties, as lower temperature lift forces chillers to operate inefficiently.

Constant speed primary pumping is best suited to small, stable-load systems but struggles in dynamic environments.

Variable Speed (VS) Primary Pumping

Variable speed primary pumping is a growing trend due to its modest energy and first-cost savings advantages and smaller footprint. In this scheme, pumps adjust their speed to maintain a minimum differential pressure at hydraulically remote points in the system, adjusting flow based on actual cooling demand, thus resulting in efficient pump operation.

This approach is particularly beneficial in larger DCS plants, as it ensures that chillers receive the highest possible entering return water temperatures, enhancing overall efficiency. By reducing over-pumping, VS primary pumping also helps mitigate the low ΔT syndrome, making it a preferred choice for modern systems. VS systems thrive in medium-to-large DCS but require precise control algorithms to avoid instability.

Primary-Secondary (Decoupled) Pumping

This common configuration hydraulically separates the chiller (primary) loop from the building distribution (secondary) loop using a bypass pipe or “decoupler.” It allows constant flow through the chillers while enabling variable flow to the loads, providing flexibility and stable chiller operation. Although this scheme may incur higher installation costs due to additional pumps and piping, it offers reliability and ease of control, making it a popular choice for many DCS applications.

With this pumping scheme, it is relatively easy to integrate chilled water storage tanks into the system by connecting the tanks to the decoupler line. See schematic for more details.

Primary-Secondary Pumping with Chilled Water Storage Tank

Advanced and Specialized Pumping Strategies

Primary-Distributed Secondary Pumping

In this scheme, chilled water is primarily circulated by central plant pumps, but individual customer buildings house their own secondary pumps that manage flow through their internal HVAC systems. This reduces pressure within the central plant and avoids secondary pump surge, but it introduces installation and energy costs at each Energy Transfer Station (ETS). This strategy is attractive when development loads are well-known and the network length is not extensive. However, it can limit flexibility if future load requirements are uncertain, as the distributed pumps may not easily adapt to changes in demand.

Series Chiller Arrangements

Chillers in Series-Counterflow Connection

Chillers can be arranged in series-counterflow, meaning both evaporators and condensers are in series. This arrangement increases overall chiller-module efficiency because the compressor lift is split between two units, particularly when dealing with large CHW ΔTs (greater than 16°F or 8.9°C). Such designs can greatly reduce pumping costs due to lower required flow rates on both the chilled water and condenser water sides. By minimizing flow, series arrangements also reduce pipe sizes and pumping energy, making them an efficient choice for systems with high cooling demands.

Booster Pumps for System Expansion

For very large distribution systems or interconnections of multiple subsystems, booster pumps can be used at strategic points. These pumps allow chilled water transmission over longer distances as an alternative to increasing distribution pipe size or for enhancing capacity in existing constrained systems. However, booster stations involve significant capital investment and increased pumping energy costs, so a thorough life-cycle cost analysis is essential to justify their use. When properly implemented, booster pumps can extend the reach and capacity of a DCS without the need for extensive infrastructure upgrades.

Optimizing Pumping Performance Through Design and Control

Maximizing Delta T (ΔT)

Achieving a high ΔT—the temperature difference between supply and return water—is critical for reducing flow rates, which in turn allows for smaller pipes and lower pumping energy. Best practices to maximize ΔT include:

Using variable flow on both the district and customer-sides.
Specifying cooling coils with a minimum of six rows to maximize heat transfer.
Eliminating three-way valves in favor of high-quality two-way pressure independent control valves (PICVs) at terminal units. These measures ensure that the system operates efficiently, even under varying load conditions.

Intelligent Control and Automation

Sophisticated District Cooling Instrumentation and Control Systems (DCICS) are essential for optimizing pump dispatch, especially for variable-speed pumps. Control logic should incorporate time delays and hysteresis to minimize pump cycling and pressure fluctuations. Integration with Building Management Systems (BMS) allows for dynamic adjustments, such as linking pump speed to return temperature, ensuring efficient operation across the entire network. Advanced controls not only enhance energy efficiency but also improve system reliability by preventing issues like pump hunting and flow instability.

Overall System Design

Pumping arrangements must be designed with the entire DCS in mind, including the central plant, distribution network, and consumer interconnections. Early planning and detailed hydraulic analyses are crucial for optimal pipe sizing, balancing capital costs with long-term pumping energy and future expansion capabilities. A well-designed system ensures that pumping strategies align with the overall goals of the DCS, providing a scalable and efficient cooling solution for years to come.

Conclusion

The evolution of pumping strategies—from rudimentary constant-speed setups to AI-driven distributed systems—reflects the industry’s push for sustainability and resilience. By embracing variable-speed drives, high ΔT design, and leveraging intelligent controls, engineers can significantly enhance energy efficiency, reduce operational costs, and contribute to a more sustainable urban environment.

References:

Ashrae Owner’s Guide for Buildings Served by District Cooling
Ashrae District Cooling Guide
IDEA District Cooling Best Practice Guide
Trane Applications Engineering Manual – Chiller System Design and Control

What is the recommended design chilled water delta T for a modern district cooling system?

by Danny

Introduction

In modern district cooling systems, optimizing the chilled water temperature differential (ΔT) is a fundamental design consideration for improving energy efficiency, reducing operational costs, and minimizing infrastructure demands. A higher ΔT—the difference between supply and return water temperatures—allows for smaller pipe diameters, lower pumping energy, and increased system capacity. However, achieving an optimal ΔT requires careful planning, from chiller selection and coil design to flow control strategies. This article explores the recommended ΔT ranges for district cooling applications, examines real-world case studies, and outlines key design principles to maximize performance. Whether designing a new system or retrofitting an existing one, understanding these factors ensures a cost-effective and sustainable cooling solution.

Recommended Design ΔT Values and Ranges:

The selection of an appropriate chilled water temperature differential (ΔT) is a critical design decision that directly impacts the efficiency, cost, and performance of district cooling systems. While conventional systems often operate at modest ΔT values, modern designs increasingly target higher differentials to optimize energy use and reduce infrastructure requirements. This section outlines industry-recommended ΔT ranges for various applications, from standard district cooling networks to specialized thermal storage systems. By understanding these benchmarks and the factors that influence them, engineers can make informed decisions that balance operational performance with economic and sustainability objectives.

The following recommendations are based on industry standards, case studies, and proven best practices in system design.

General Range: Typical design ΔT values for district cooling systems generally fall within the range of 12°F to 16°F (6.7°C to 8.9°C) at full load. ¹
Achieving Higher ΔT: With designs that emphasize maximizing ΔT, values as high as 20°F (11°C) or more are possible.² For new buildings specifically designed for district cooling service, a successful design should be able to achieve a ΔT of 16°F to 22°F (8.9°C to 12.2°C).³
Chilled Water Storage Systems: For chilled water storage installations, it is conventionally recommended to select a ΔT range between 18°F to 24°F (10°C to 13.3°C).⁴ Some systems have even installed differentials above 30°F (17°C).⁵ Increasing the ΔT significantly enhances storage capacity.

Factors Influencing and Enabling High ΔT:

Achieving and maintaining an elevated chilled water temperature differential (ΔT) requires careful consideration of multiple interdependent factors across system design and operation. While higher ΔT values offer significant advantages in energy efficiency and infrastructure savings, their successful implementation depends on optimized chiller performance, proper equipment selection, and intelligent system controls. This section examines the key technical and operational elements that contribute to effective high-ΔT operation, from building-side HVAC configurations to plant-side pumping strategies. Understanding these critical factors enables engineers to design systems that consistently deliver superior ΔT performance while maximizing cost savings and sustainability benefits throughout the district cooling network’s lifecycle.

Selecting an optimal chilled water ΔT requires balancing multiple engineering and economic considerations. The following factors critically influence achievable temperature differentials and system performance in district cooling applications:

Cost-Effectiveness: Maintaining a high ΔT is the most cost-effective approach, as it allows for smaller pipes in the distribution system and reduces pumping energy consumption.⁶
Chiller Performance: The pumping arrangement directly affects chiller performance. Designs that allow for the widest temperature differentials in both chilled-water and condenser-water circuits often result in the best overall plant efficiency gains.⁷ Variable flow in both distribution and chiller loops ensures chillers receive the warmest possible entering return water temperatures, optimizing chiller and system performance.⁸
Building Design and Equipment:
- New Construction: Designing in-building HVAC equipment in new buildings to achieve acceptable ΔTs is feasible and less costly than retrofitting existing buildings.
- Cooling Coils: Selecting cooling coils with a minimum of six rows and 12 to 14 fins per inch (5 to 6 fins per cm) can achieve a ΔT of 12°F to 16°F (6.7°C to 8.9°C) or higher at full load.⁹ For optimal performance, coils should maintain high fluid velocity in the turbulent flow range even at reduced loads.
- Control Valves: The use of two-way modulating control valves, especially pressure-independent control valves (PICVs), is crucial for achieving high ΔT. Eliminating three-way valves from terminal units is also essential.
- Variable Flow: Implementing variable chilled water flow on both the central plant and customer sides is necessary to optimize ΔT and save pump energy.
Thermal Storage: Systems utilizing thermal energy storage, particularly ice-based systems, can result in lower chilled water supply temperatures and thus higher ΔT values.¹⁰ Chilled water storage systems operating with higher ΔT also see reduced required storage volume.¹¹

Conclusion

Achieving an optimal chilled water delta T (ΔT) in modern district cooling systems is critical for maximizing energy efficiency, reducing operational costs, and minimizing infrastructure requirements. By targeting a design ΔT within the recommended range or even higher in thermal storage applications, engineers can enhance system performance while lowering pumping energy and pipe sizing demands. Key factors such as variable flow design, high-performance cooling coils, two-way control valves, and proper chiller sequencing play a pivotal role in maintaining high ΔT values. For new constructions, integrating these principles early in the design phase ensures cost-effective implementation, whereas retrofitting existing systems may require strategic upgrades. Prioritizing a high ΔT not only aligns with sustainable cooling practices but also supports long-term economic and operational benefits in district cooling networks.

References:

Ashrae District Cooling Guide ↩︎
Ashrae District Cooling Guide ↩︎
IDEA District Cooling Best Practice Guide ↩︎
EPRI EM-3981 Commercial Cool Storage Design Guide ↩︎
Ashrae – Design Guide for Cool Thermal Storage ↩︎
ASHRAE Owners Guide for Buildings Served by District Cooling ↩︎
Ashrae District Cooling Guide ↩︎
Ashrae District Cooling Guide ↩︎
Ashrae District Cooling Guide ↩︎
Ashrae District Cooling Guide ↩︎
Ashrae – Design Guide for Cool Thermal Storage ↩︎

Understanding the main components and equipment of the district cooling plant

by Danny

Introduction

The district cooling system consists of the district cooling plant, the piping network, and the energy transfer station. The district cooling plant is the main component of the system that generates cooling energy in the form of chilled water for supply to the consumers. This article describes the main components and equipment of the district cooling plant.

Chilled water production system

Image: District cooling plant chilled water production process — District cooling plant chilled water production process

The chilled water production system is the heart of the district cooling plant, responsible for the production and distribution of chilled water to consumers. The district cooling plant comprises several key components and equipment, including the chillers, condenser cooling system, thermal energy storage system, distribution pumps, electrical system, automatic control system, and the balance of plant equipment, all housed in a plant building structure.

Chillers

The chiller generates cooling energy in the form of chilled water by cooling the water that circulates throughout the system. There are two main types of chillers used in district cooling, namely the vapor compression chiller and the absorption chiller.

Vapor compression chillers

The vapor compression chiller uses a refrigerant (for example, HFOs such as R-1234ze) to absorb heat from the returning chilled water. The heat is then subsequently rejected into the environment. In the chiller, the refrigerant goes through the cycle of:

Evaporation: In the evaporator shell, the low-pressure refrigerant evaporates by absorbing heat from the returning chilled water.
Compression: The vapor leaving the evaporator enters the suction of the compressor, where mechanical work is done to compress the gas and raise the fluid pressure to the working pressure of the condenser.
Condensation takes place in the condenser, where the high-pressure refrigerant vapor is condensed by the condenser, transferring the heat to the heat sink or the environment in the process.
Expansion: In this stage, the high-pressure condensate leaving the condenser goes through an expansion device (either an expansion valve or an orifice) to reduce the fluid pressure before it enters the evaporator, where the whole cycle repeats.

For large-capacity chillers, centrifugal compressors are used to perform mechanical work during the compression stage. The driving force for the compressor can come from electric motors or mechanical drives.

Absorption Chillers

On the other hand, the absorption chiller uses pure water as the refrigerant and lithium bromide solution as an absorbent. Heat is used to drive the absorption process and generate cooling energy. Common sources of heat used are steam, hot flue gas, and hot water.

Condenser cooling system

The chiller generates cooling energy by extracting heat from the chilled water using the vapor compression or absorption process. The extracted heat must be rejected to the heat sink. The function of the condenser cooling system is to transfer the heat from the chiller condenser to the environment. The type of condenser cooling system is dependent on the availability of water.

Air-cooled condensers

In arid locations where there is a lack of water resources, air-cooled condensers can be used to cool the chiller condenser.

Water-cooled condensers.

When the availability of water is not a constraint, the condenser cooling system of choice is water cooling. Water-cooled chillers can be cooled by river water, deep lake water, seawater, or more commonly, cooling towers.

Cooling towers

Cooling towers are used to dissipate the heat from the condenser’s cooling water, mainly through the process of evaporation. The warm water from the chiller condenser is transferred to the cooling tower, where the water is cooled before being returned to the chiller. The warm water is cooled through the process of evaporation, which transfers the waste heat to the air being circulated through the cooling tower. The cooling water pumps circulate the water through the cooling tower and chiller condenser.

Thermal energy storage

In addition to the refrigeration equipment, the district cooling plant may also incorporate a thermal energy storage system. Thermal energy storage is the process of generating and storing cooling energy during periods of low demand. The stored cool energy is subsequently discharged to meet cooling requirements during periods of high demand.

Thermal energy storage effectively decouples the production of cooling energy from the cooling demand. With thermal energy storage, chillers can be operated during off-peak periods when there is a surplus of chilled water production capacity, and the excess cooling energy is stored in a thermal energy storage tank or tanks. During peak periods, the thermal energy storage can be discharged to supplement the chiller production capacity to meet the high cooling demand.

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 as sensible heat or latent heat.

Image: District cooling plant - thermal energy storage operation strategy — District cooling plant – thermal energy storage operation strategy

Chilled Water Storage

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

Ice Storage

The latent heat storage technology used in district cooling is ice storage. In the ice storage system, the energy is stored as latent heat as the storage media undergoes a phase change, transitioning from water to ice. The amount of energy stored in the ice storage system is dependent on the latent heat of fusion of water.

Chilled water pumps

The function of the chilled water pumps is to circulate the water through the chillers and pump the cooling energy to the end-users via the chilled water distribution piping network.

The traditional chilled water pumping system is the primary-secondary pumping system designed to segregate the chilled water production circuit (primary circuit) from the chilled water distribution circuit (secondary circuit). Primary chilled water pumps shall serve the primary circuit, and secondary pumps shall serve the distribution circuit.

3D image of pumps in a district cooling plant — 3D image of chilled water pumps in a district cooling plant

Primary chilled water pumps

Typically, dedicated primary chilled water pumps are provided to serve the electrical centrifugal chillers, heat exchangers, and chilled water storage tanks. The pump flow rate shall be based on the chilled water flow rates of the individual equipment.

Secondary chilled water pumps

The function of the secondary chilled water pumps is to supply chilled water to the consumers. The total volume of water supplied from the district cooling plant is dependent on the cooling demand from the consumers. The pump flow rate shall be modulated to match the requirements of the consumer cooling demand.

Condenser cooling water pumps

The function of the condenser cooling water pumps is to circulate the condenser cooling water between the chiller condensers and the cooling towers. The cold condenser cooling water from the cooling towers is supplied to cool the chiller condensers. The warm condenser cooling water from the chiller condensers is then returned to the cooling towers, where the heat is then transferred to the ambient air.

Air Separation System

Air in the chilled water system causes corrosion and noise and can hinder the heat transfer process, which reduces the efficiency of the chilled water production system. Air can exist in the chilled water system in the form of stagnant air bubbles, entrained air, and dissolved gases. The devices commonly used to mitigate the issue of air in the district cooling plant are:

Air vents: properly installed air vents can effectively remove stagnant air bubbles or air pockets.
Air separators are used to remove entrained air in the system.
Vacuum degassers are used to remove dissolved gas from the system.

Chilled Water Expansion Tanks

The chilled water system is a closed-loop recirculating water system. The chilled water in the system is subject to thermal expansion and contraction due to temperature fluctuations. In the absence of a feed and expansion system, constant expansion and contraction will result in pressure fluctuations, which can potentially be damaging to piping and equipment.

The expansion tank serves the following functions:

Accommodating the thermal expansion of the chilled water in the closed recirculating system.
Ensuring positive pressure is maintained at all points in the chilled water system.
Supporting the chilled water pumps by maintaining a net positive suction head.

This is achieved through the use of compressed air, which enables the tank to accept and expel the changing volume of water as it heats and cools. Makeup water pumps for chilled water are provided to replenish the water lost through leakages and drains.

Water treatment system

In a district cooling plant, the water treatment system maintains the quality of the chilled water and cooling water systems within acceptable operating parameters. The system typically consists of water filtration units, a blowdown system, and separate chemical treatment skids for the chilled water and cooling water loops.

Water filtration equipment such as side stream filters serves to remove suspended solids in the open-loop cooling water system.
The cooling water blowdown system serves to remove dissolved solids by occasionally draining a portion of the cooling water. Fresh water with a much lower total dissolved solids concentration is then supplied to make up for the water loss.
The chemical treatment system for chilled water typically includes a corrosion inhibitor and a biocide to prevent microbiological growth.
The chemical treatment system for the cooling water typically includes corrosion and scale inhibitors, biocide, and biodispersant to prevent the growth of bacteria and algae.

The water treatment system is critical to ensuring the long-term reliability and efficiency of the cooling system.

In-plant piping system

The in-plant piping system serves the function of transporting the working fluid in the chilled water and cooling water systems. The components of the piping system include pipes, valves, fittings, strainers, and insulation.

The pipes are made of carbon steel and serve to convey the chilled water and cooling water in the plant.
Valves are used to serve the functions of flow isolation, flow regulation, pressure regulation, and backflow prevention in the piping system.
Fittings are used to enable changes in the flow direction, such as tees and elbows, and to connect piping of different sizes, such as reducers and expanders.
Strainers are used to protect downstream equipment by removing debris from the water.
Insulation is used to prevent condensation on the piping surface and reduce heat gain.

Plant Building

All the main systems and equipment of the district cooling plant are housed in a building structure that provides protection from the weather and a secure environment conducive to plant operation. The district cooling plant can be either a standalone structure or an integrated part of a larger building complex. It can be above-ground or below-ground, depending on the location and circumstances. As with any industrial plant, the building is equipped with all the supporting building services such as HVAC, water supply and drainage, fire protection system, lighting and small power, security system, telecommunications, overhead cranes, maintenance hoists, and water storage tanks.

Plant automatic control system

The plant’s automatic control system functions as the brain of the plant to ensure the smooth operation of each component of the district cooling plant. It consists of the following components:

Local plant controllers: Industrial-grade controllers such as distributed control systems or programmable logic controllers serve the function of controlling and monitoring the overall operation of the district cooling plant.
HMIs, or Operator Interface Terminals, function as human-machine interfaces.
Field Devices: Control valves, flow meters, and temperature sensors are examples of the main field devices used to measure and control process variables such as water flow rate, temperature, and pressure.

Image: District Cooling Plant Automatic Control Architecture Diagram — District cooling plant automatic control architecture diagram

Plant electrical system

The electrical system for a district cooling plant consists of the following components:

The incoming power supply feeder comes from the power utility.
Switchgears are designed to provide electrical protection from electrical faults, electrical isolation, and control of the electrical system.
Transformers perform the function of stepping down the higher voltage of the upstream supply to a lower voltage suitable for downstream usage.
Motor control centers perform the function of controlling electrical motors by providing power and protection to the electrical motors.
Emergency generators provide emergency power supplies to essential services.

Conclusion

In conclusion, the district cooling plant or the central chiller plant in a district cooling system consists of the following main components:

Chillers
Cooling towers
Pumping system
Thermal energy storage
Water treatment system
Chilled water expansion tanks
Air removal system
In-plant piping system
Plant Building
Plant electrical system
Plant automatic control system

Within the district cooling plant, each of these systems plays a crucial role in the production and distribution of cooling energy in the form of chilled water to the consumers. Therefore, it is important for engineers to fully understand the role and function of each of these major components and how they are integrated to form a reliable and efficient district cooling plant.

District cooling system as a utility infrastructure business – Contractual obligations of the district cooling operator

by Danny

Featured Image: District Cooling as a Utility Infrastructure Business - Contractual Obligations

Introduction

The district cooling system, according to IDEA’s “District Cooling Best Practice Guide”, is a long-term utility service business. By their very nature, district cooling systems are often structured as utility infrastructure. In this article, we shall explore the key characteristics of district cooling that distinguish it as a utility infrastructure business, as well as the contractual obligations of the district cooling system operator or owner.

Definition of a utility infrastructure business

A utility infrastructure business invests in physical infrastructure assets that provide utility services to consumers. The infrastructure usually consists of generation assets (such as power plants, which generate electricity) and transmission assets (such as power transmission lines, which transmit and distribute power to the consumers). The utility company commits to heavy upfront investment in these physical infrastructure assets in return for long-term supply contracts with the consumers. Examples of businesses in the infrastructure sector range from water, gas, and electricity to transportation and even heating and cooling energy. Due to the extended life span of the infrastructure, there are significant recurring costs for the repair and maintenance of these assets.

Key features of long-term utility infrastructure businesses include:

High barriers to entry: The requirement for a significant initial capital investment serves as a potent barrier to entry for new competitors. Established utility companies usually enjoy a significant first-mover advantage over newer players seeking to enter the business.
Monopolistic or oligopolistic market structure: Many long-term utility infrastructure businesses operate in markets that are monopolistic or oligopolistic in nature, meaning that there are only a few dominant players in the market.
Long-term contracts: Utility companies enter into long-term contracts, usually lasting decades, with their consumers. The long time horizon provides a stable stream of cash flow, allowing the servicing of debt payments and the distribution of profits to shareholders, representing a return on their investment and the return of investors’ capital.
Regulatory oversight: Utility infrastructures often provide essential services to the community and have significant impacts on the local economy. For these reasons, many utility infrastructure companies are subject to regulatory oversight.

District Cooling Plant vs In-Building Chiller Plant: Understanding the Key Differences

by Danny

Key Differences Between the District Cooling Plant and the In-Building Chiller Plant

Introduction

The United Nations Environmental Program, in its publication titled “District Energy in Cities: Unlocking the Full Potential of Energy Efficiency and Renewable Energy“, stated that:
• Cities account for over 70 percent of global energy use and 40 to 50 percent of greenhouse gas emissions worldwide.
• Half of the cities’ energy consumption is for heating and cooling.
• Building up modern district energy in cities is one of the least expensive and most effective ways to cut down on emissions and primary energy use.

UNEP Report - District Energy in Cities — United Nations Environmental Program (UNEP) Report – District Energy in Cities

Based on the above numbers, there is no doubt that urban comfort cooling in cities has an effect on greenhouse gas emissions around the world. The United Nations Environmental Program (UNEP) is advocating district cooling in cities as a viable solution for decarbonizing urban comfort cooling. The district cooling plant offers many benefits over the conventional in-building chiller plant. In this article, we will explore the key differences between the district cooling plant and the in-building chiller plant.

Definition of a District Cooling Plant

A district cooling plant is a centralized cooling system that provides cooling energy to multiple end-user buildings in a district through a network of chilled water distribution piping. The central chiller plant produces chilled water, which is then circulated through the distribution piping to the buildings that require cooling. The chilled water is then used in the buildings’ HVAC systems for comfort cooling or process cooling.

Definition of an In-Building Chiller Plant

An in-building chiller plant is part of a building’s HVAC system that provides comfort cooling to the building’s occupants. It consists of one or more chillers, pumps, cooling towers, and other components necessary to produce and distribute chilled water throughout the building.

Main Components of a District Cooling System

District Cooling is a Utility Infrastructure

The district cooling plant is part of a utility infrastructure that supplies chilled water to multiple end-user buildings for the purpose of comfort cooling or process cooling. This makes it different from the in-building chiller plant, which is part of the building’s HVAC system. The district cooling plant is operated and maintained by a third-party provider, whereas the in-building chiller plant is typically managed by the building owner or operator.

Profit Center vs Cost Center

To a district cooling operator, the district cooling plant is a profit center that generates revenue and net income through the sale of cooling energy. The operator is responsible for the operation, maintenance, and repair of the district cooling plant. On the other hand, the in-building chiller plant is part of a building’s HVAC system, and the cost of air conditioning is usually built into the gross rental, calculated on a per-square-foot basis. Therefore, to a building owner, the in-building chiller plant is a cost center that incurs operation and maintenance costs in addition to capital depreciation charges.

Industrial Grade vs Commercial Grade Equipment

A district cooling plant is built to the standards of an industrial plant and utilizes high-efficiency industrial-grade equipment for its process. This robust equipment is designed to operate continuously and reliably with high availability. In contrast, an in-building chiller plant is part of a commercial building and is made up of commercial-grade equipment, which may not be as robust as the equipment used in a district cooling plant.

District Cooling operates 24×7

A district cooling plant is required to operate 24×7 without interruption or disruption. This is because the end-user buildings rely on the district cooling plant for their cooling needs. Any downtime at the district cooling plant can have a significant impact on the end-user buildings. Due to stringent availability and reliability requirements, the district cooling plant is equipped with robust, industrial-grade equipment and provided with adequate equipment redundancy to prevent unscheduled outages. In contrast, an in-building chiller plant usually operates according to the normal office hours or the operating hours of commercial retail tenants. It is not uncommon to see the building chillers switched off during the night.

Cooling as a service vs on-premise chilled water generation

The district cooling plant is part of an energy utility business that offers cooling as a service, while the in-building chiller plant is essentially an on-premise chilled water generation asset. From a building owner’s perspective, opting for cooling energy as a service from a district cooling plant is an asset-light strategy. The building owner does not have to commit to a substantial initial capital investment in an in-building chiller plant to get access to cooling energy. Instead, they pay for the cooling energy on a “pay as you go” basis, usually billed on a monthly basis. Furthermore, the operation and maintenance of the cooling energy plant are outsourced to the district cooling operator, allowing the building owner to focus and concentrate on their core business.

Economy of Scale Translates into Cost and Energy Efficiency

The district cooling plant serves a much larger cooling demand compared to the in-building chiller plant. Hence, the economy of scale enables the district cooling plant to utilize technology options such as thermal energy storage and high-efficiency series-counterflow chillers, which lower operation costs and increase energy efficiency. This translates into a lower cost of cooling energy for the end-users and a more sustainable energy solution for urban comfort cooling.

Conclusion

The key differences between the district cooling plant and the in-building chiller plant have been explored. Understanding these key differences is essential for engineers who are involved in the planning, design, development, operation and maintenanace of district cooling systems. The district cooling plant offers many advantages, including being part of a utility infrastructure, utilizing industrial-grade equipment, operating 24×7, and offering cooling as a service. The economy of scale enables the district cooling plant to be a more cost-effective and sustainable energy solution for urban comfort cooling.

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

by Danny

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.

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.

District cooling improves the energy efficiency while reducing the carbon footprint of urban comfort cooling

by Danny

District cooling improves the energy efficiency while lowering the carbon footprint of urban comfort cooling.

District cooling has the potential to improve the energy efficiency while also lowering the carbon footprint of comfort cooling in urban areas. The following discussion demonstrates how district cooling technology can be used to achieve these two important objectives:

How does district cooling improve the energy efficiency of cooling energy production?

Due to economies of scale, the district cooling system can adopt energy-efficient technology such as industrial grade high-efficiency chillers, series-connected chiller modules, thermal energy storage, and cogeneration or combined heat and power.

Thermal energy storage shifts cooling energy production from peak hours to off-peak hours.

Thermal Energy Storage Technologies used in District Cooling

by Danny

Thermal Energy Storage Technologies commonly used in District Cooling

Thermal Energy Storage Classification

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.

What is Thermal Energy Storage in District Cooling?

by Danny

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

What are the different applications of District Cooling?

by Danny

Introduction

District Cooling is a cost-effective and energy-efficient technology for urban comfort cooling. Due to its many technological, environmental, and economic benefits, District Cooling is adopted in a variety of different applications. This article seeks to show selected examples of the different applications of District Cooling around the World.