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Design/Selection
District Cooling Bookmark and Share

Why Ice Thermal Storage for District Cooling Projects?

BAC has been involved in the design and manufacture of ice thermal storage for over 25 years.

The BAC team can provide in-depth knowledge on the design and operation of your district cooling system.

Basic Information

Getting Started

A daily load profile is the hour-by-hour representation of cooling loads for a 24-hour period. Most District Cooling applications use a daily load profile to determine the amount of storage required. For conventional systems, chillers are selected based on the peak-cooling load. For ice storage systems, the chillers are selected based on the ton-hours of cooling required and a defined operating strategy.

The Air-Conditioning & Refrigeration Institute (ARI) has published Guideline T, “Specifying the Thermal Performance of Cool Storage Equipment.” The purpose of Guideline T is to establish the minimum user-specified data and supplier-specified performance data. Design data provided by the customer includes: System Loads, Flow Rates and Temperatures. Table 1 details the user-specified data.

The Ice Advantage

It is important to take full advantage of the low temperatures available from the ice to reduce first cost and maximize energy savings. The savings available are shown in the Benefits Section of this web site under Lowest First Cost Design.

The use of ice storage makes it possible to design the system with 36°F (2°C) supply glycol or 38°F (3°C) supply chilled water to the building. A system temperature difference of 20°F (11°C) or more is now possible. System first cost can be reduced by using smaller chilled water distribution piping and pumps, smaller chillers and heat rejection equipment, and smaller electrical distribution equipment.

The buildings on the district cooling loop can take advantage oft he colder chilled water available and design their air distribution systems with a higher temperature differential and smaller ductwork. Existing buildings can take advantage of the lower supply chilled water temperatures to increase the cooling capacity of their air -conditioning systems without installing new piping or air distribution systems.

The use of less pump horsepower leads to energy savings. In addition, the chillers now operate at full load instead of part load which increases their energy efficiency and counteracts any extra use of energy to build the ice.

Selection Optimization

BAC has done extensive research and testing on the building and melting characteristics of ice storage. This research and testing has resulted in selection capabilities unmatched by any other company in the industry.

BAC can predict the temperatures required on an hour-by-hour basis for building ice on standard and custom products, over a variety of conditions and build times. BAC can optimize the design of an ice storage system to meet load profile, discharge temperatures, chiller capacity, operating sequences and layout restrictions.

For internal melt systems, BAC can provide and hour-by-hour analysis of the ice system build and melt temperatures as shown in the printout below.

Operating Strategies

The next step in selecting thermal storage equipment is to define an operating strategy. Choices include either full or partial storage. Partial storage operating strategies can be categorized as either demand limiting or load leveling. The operating strategy used is dependent upon the load profile, utility rate structure, energy cost and equipment first cost.

Full storage systems eliminate the need to operate the chiller during the utility on-peak period by storing the required cooling capacity during off-peak periods. This strategy shifts the largest amount of electrical demand and results in the lowest operating costs. However, the equipment first cost is considerably higher than partial storage systems due to larger chiller and storage requirements.

Unlike full storage systems, the chiller must operate during the on-peak period when a partial storage operating strategy is used. There are two types of partial storage operating strategies. The first strategy is demand limiting. With the demand limiting operating strategy, the non-storage system loads establish the peak demand for the facility. Items that contribute to the non-storage system loads include lights, equipment, appliances, fans, motors etc. The thermal storage equipment is selected so the chiller operation does not increase the facility’s non-storage demand. This operating strategy provides the lowest operating costs for partial storage systems. This strategy also requires less storage capacity and smaller chillers than a full storage design. The disadvantages of the demand limiting operating strategy are that the storage requirement and chiller capacity is larger than required for a load leveling operating strategy. This results in a longer payback period.

The second partial storage operating strategy is load leveling. By distributing the cooling load equally over a 24-hour period, this operating strategy reduces the size of the thermal storage equipment and chiller when compared to either full storage or demand limiting strategies. This results in the lowest possible first cost and shortest payback period. Since the chiller operates fully loaded during the on-peak period, operating costs are higher than either demand limiting or full storage.

Melt Discharge Strategies

Internal Melt

When ice is built on a coil using cold glycol on the inside of the coil, and melted without direct contact with the ice, using warm glycol on the inside of the coil. The warm return glycol is circulated through the ice storage coils and is cooled indirectly by the ice through the coil as the ice closest to the coil is melted first. As the melt out continues, the cylinders of ice crack and the 32°F (0°C) tank water comes in contact with the ice coil. The internal melt design is shown below.

TES Design Internal Melt

Typical melt temperatures for a BAC internal melt system are graphed below. It is important when designing an ice storage system using internal melt that the leaving glycol temperatures from the ice storage are known on an hour-by-hour basis as specified in ARI’s Guideline T, “Specifying the Thermal Performance of Cool Storage Equipment”. This is because glycol melting through a barrier such as a steel or plastic tube can result in a rapidly rising temperature of the chilled glycol as the melt cycle proceeds.

Internal Melt.JPG

External Melt

When ice is built on a coil using glycol or refrigerant inside the coil, and melted by water on the outside of the coil with direct ice contact. The warm return water is in direct contact with the ice and a constant low leaving water temperature is provided during the entire melt cycle. The external melt design is shown below.

TES Design External Melt.JPG

Typical melt temperatures for a BAC external melt system are graphed below. System designs which require 34°F (1°C) to 36°F (2°C) supply water or rapid melting of the ice are best suited for external melt.

External Melt.JPG

Schematics

Modes of Operation

Internal Melt

The modular BAC ICE CHILLER® Thermal Storage Unit can operate in any of five distinct operating modes. These modes of operation provide the flexibility required by building operators to meet their daily HVAC cooling requirements.

ICE BUILD –

– In this operating mode, ice is built by circulating a 25% solution (by weight) of inhibited ethylene/propylene glycol through the coils contained in the BAC ICE CHILLER® Thermal Storage Unit. The graph below illustrates typical chiller supply temperatures for 8-, 10- and 12-hour build cycles. For a typical 10-hour build time, the supply glycol temperature is never lower than 22°F(-5.5°C). As the graph illustrates, for build times exceeding 10 hours, the minimum glycol temperature is greater than 22°F (-5.5°C). For build times less than 10 hours, the minimum glycol temperature will be lower than 22°F (-5.5°C) at the end of the build cycle. This performance is based on a chiller flow rate associated with a 5°F (2.8°C) range. When a larger temperature range is the basis of the chiller selection, the chiller supply temperatures will be lower.

Different Build Times.JPG

ICE BUILD WITH COOLING –

When cooling loads exist during the ice build period, some of the cold glycol used to build ice is diverted to the cooling load to provide the required cooling. The amount of glycol diverted is determined by the building loop set point temperature. BAC recommends that this mode of operation be applied on systems using primary/secondary pumping. This reduces the possibility of damaging the cooling coil or heat exchanger by pumping cold glycol, lower than 32°F (1°C), to this equipment.

COOLING – ICE ONLY –

In this operating mode, the chiller is off. The warm return glycol solution is cooled to the desired set point temperature by melting ice stored in the modular BAC ICE CHILLER® Thermal Storage Unit.

COOLING – CHILLER ONLY –

In this operating mode, the chiller supplies all the building cooling requirements. Glycol flow is diverted around the thermal storage equipment to allow the cold supply glycol to flow directly to the cooling load. Temperature set points are maintained by the chiller.

COOLING — ICE WITH CHILLER –

In this operating mode, cooling is provided by the combined operation of the chiller and thermal storage equipment. The glycol chiller pre-cools the warm return glycol. The partially cooled glycol solution than passes through the BAC ICE CHILLER® Thermal Storage Unit where it is cooled by the ice to the design temperature.

External Melt

The BAC ICE CHILLER® Thermal Storage Unit for external melt can operate in five distinct operating modes. These modes of operation provide the flexibility required by operators to meat their cooling loads.

ICE BUILD –

In this operating mode, ice is built by circulating glycol or refrigerant through the ice storage coils. The glycol or refrigerant temperature required to build the ice is dependent on the length of the ice build and the thickness of the ice build. The “Typical Ice Build Performance for Different Build Times” graph above illustrates the typical chiller supply temperatures for 8-, 10- and 12- hour build cycles. The graph below, “Ice Thickness vs. Chiller Supply Temperature” shows typical chiller supply temperatures for 1.5, 2 and 2.5 in (38, 51 and 64 mm) ice builds with a 12-hour build cycle.

Ice Thickness.JPG

ICE BUILD WITH COOLING –

When cooling loads exist during the ice build period, it is best to handle these loads with a base water chiller.

COOLING – ICE ONLY –

In this operating mode, the glycol chiller (or refrigeration machine) is off. The warm return water is cooled to the desired set point temperature by direct contact with the ice.

COOLING – CHILLER ONLY –

In this operating mode, the glycol chiller (or refrigeration machine) supplies all the building cooling requirements. Either a base water chiller or the glycol chiller through a heat exchanger cools the warm return water. Temperature set points are maintained by the chiller.

COOLING – ICE WITH CHILLER

In this operating mode, cooling is provided by the combined operation of the base water chiller, glycol chiller (or refrigeration machine) and ice thermal storage equipment. The base water chiller and glycol chiller (through a heat exchanger) pre-cools the warm return water. The partially cooled warm return water then passes through the BAC ICE CHIILER® Thermal Storage unit where it is cooled by direct contact with the ice.

Below are examples of piping schematics tor ice thermal storage systems.

Internal Melt

  • IM-1A Single Loop, Glycol Chiller Upstream
  • IM-1D Single Loop with Heat Exchanger, Glycol Chiller Upstream, Optional Base Water Chiller in Parallel
  • IM-3A Primary/Secondary Loop, Glycol Chiller Upstream
  • IM-3B Primary/Secondary Loop, Glycol Chiller Upstream, Optional Base Glycol Chiller in Parallel
  • IM-3D Primary/Secondary Loop with Heat Exchanger, Glycol Chiller Upstream, Optional Base Water Chiller in Parallel
  • IM-2A Single Loop, Glycol Chiller Downstream

External Melt

  • EM-1A Single Loop, Open System
  • EM-2A Closed System, Optional Base Water Chiller
  • EM-3A Closed System, Chiller Upstream, Optional Base Water Chiller in Parallel
  • EM-4A Closed System, Dual Evaporator Chiller Upstream, Optional Base Water Chiller in Parallel

BAC will custom manufacture ICE CHILLER® Thermal Storage Coils to meet a projects specific requirements. BAC has done extensive research and testing on the building and melting characteristics of ice storage. This research and testing has resulted in selection capabilities unmatched by any other company in the industry.

BAC can predict the temperatures required on an hour-by-hour basis for building ice on custom coils, over a variety of conditions and build times. The physical space available, load profile, discharge temperatures, chiller capacity and operating sequences can be evaluated to find the design that best meets the application.

The ICE CHILLER® Thermal Storage Coils are constructed of continuous 1.05 in (26.7 mm) O.D. all prime surface serpentine steel tubing, with no intermediate butt welds. The coils are assembled in a structural steel frame designed to support the weight of the coil stack with a full ice build. After fabrication, the coils are tested for leaks using 375 psig (2,600 kPa) air pressure under water for a minimum of fifteen (15) minutes, then hot-dip galvanized for corrosion protection. This coil construction is the same as BAC uses in their evaporative cooling equipment. An ice storage tank, however is a very low corrosion environment because corrosion decreases as temperature decreases.

For glycol applications, the coils are configured to provide countercurrent glycol flow in adjacent circuits for maximum storage capacity.

Individual coils are factory-assembled into modules of two (2) coils. Glycol manifolds are coated with zinc-rich, cold galvanizing finish at the factory. Necessary support steel and lifting lugs are provided on the modules to allow for lifting into and final positioning within the storage tank.

BAC ICE CHILLER® Thermal Storage Coils can go in a variety of customer-supplied tanks. Concrete, steel, rectangular, round, triangular, above ground, or below ground, we have put coils into it.

Orioles Coil.JPG

Heat Rejection Equipment

Cooling Towers

Cooling towers use an environmentally compatible evaporative cooling process to produce significantly lower water temperatures than achievable with air-cooled equipment for maximum energy savings and operating efficiency in recycled cooling water systems.

S3000_D8

Evaporative Condensers

Evaporative condensers offer energy savings by providing lower system horsepower than conventional air-cooled and water-cooled condensing systems.

Condenser_G2b.JPG

Closed Circuit Cooling Systems

Closed circuit cooling towers use a clean closed loop in a highly efficient evaporative heat transfer process to cool water and other process fluids while isolating the cooling fluid from the outside airstream.

FXV_E10.jpg

Field Erected Tanks

Tank Design

BAC can provide custom sized BAC ICE CHILLER® Thermal Storage Coils to fit in a tank constructed at the project location. When laying out a tank design, you should consider the total volume required, the space available, the most economical shape and height, and ice water and glycol (or refrigerant) piping layout.

An ice storage tank generally requires 3 ft3 (0.08 m3) per ton-hour. The exact size depends on:

  • The actual ice coil configuration
  • Tank access requirements for fluid piping and maintenance
  • Water rise during build
  • Pulldown volume requirements for system piping

In addition, insulation, waterproofing and tank layout should be considered.

Insulation is generally used on the top of concrete underground tanks only and on the sides and tops of above ground tanks. Armaflex, rigid board or spray foam are the most common types of insulation used.

Concrete tanks must be waterproofed, either through additives to the concrete or some type of liner. Commercial Industrial Membrane (CIM 1000) is a popular sprayed on asphaltic liner. EPDM liners are often used on smaller tanks.

Waterproofing

For external melt applications there are several ways to channel the ice water flow through the tank. Often, the water inlet and outlet are on opposite ends of the tank. In large tanks, circuiting the water through the tank helps provide the lowest possible leaving water temperatures. This circuiting can be achieved by using multiple tanks or coil battles. Examples of tank layouts are below.

The Comfort Link® Plant II had limited plan area tor the thermal storage tank. They opted to use tall round tanks to maximize the space available and to minimize the first cost. Below is a picture during construction.

ComfortLink_2.JPG

West Quest Technology Park is located near the Baltimore Washington International Thurgood Marshall Airport. The ice storage tanks are underground. The tanks are buried 18 in. (457 mm) under a fire lane of gravel and grass. Each tank was built with a removable concrete cover to allow installation of the coils after construction was completed. Below is a picture of the coils in the tank.

WestQuest

The Union Station District Energy Center provides 17,000 tons (5,984 MW) of cooling to downtown Houston. The ice storage tank was built as part of the chiller plant. It occupies the west side of the building. The tank walls were insulated with 3 in. (76 mm) of spray-on closed cell foam insulation and waterproofed with a spray-on membrane. The tank was constructed with walkways around the perimeter to allow access to the BAC ICE CHILLER® Thermal Storage Coils and piping. Below is a picture of the tank during construction.

UnionStation

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