The BAC team can provide in-depth knowledge on the design and operation of your air-conditioning system.
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.
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.
Lower water temperatures available from ice thermal storage allows supply air to be generated at 45°F (7°C), compared to the traditional 55°F (13°C) supply air. With colder supply air, air volume can be reduced, and fans and ducts can be downsized, resulting in substantial reductions in air distribution system costs. As shown in the following equation, the percentage decrease in the volume of air from the 45°F (7°C) system compared to the 55°F (l3°C) system is proportional to the ratio of the temperature difference between the supply air and the room air for the two systems. The equation shows that the use of 45°F (7°C) supply air can reduce the system air volume by 33%. Consequently, the number and/or size of air handling units can be reduced significantly.
% Reduction of Air = 33%
Since the relationship of air volume to fan power is the power of 3, fan energy consumption can be reduced by as much as 70%.
For a typical cold air system, 38°F (3°C) chilled water is circulated to air handling units in the building. Air handling units supply 45°F (7°C) cold air to VAV terminal control boxes with direct supply of primary air to zones.
Ceiling mounted cold air diffusers are typically used to supply 45°F (7°C) air into room space. The cold air diffusers supply air through small openings to produce horizontal air jets that significantly increase induction and mixing compared to typical ceiling diffusers. To prevent condensation from forming on the diffusers under all possible operating conditions, all parts of the diffuser are constructed of a self-insulating material.
To prevent condensation on glycol and chilled water piping, closed cell insulation with water stops should be used, and the insulation thickness should be increased by 0.5 in (12.7 mm). To keep the heat loss the same in a cold air design as in a conventional design, the ductwork insulation thickness should be increased 50% to offset the 50% increase in the air-to-room-temperature difference that is normally used in cold air systems. The added insulation will keep the ductwork surface temperature the same, and actually reduce the potential for condensation on the duct insulation surface because of the lower space RH and lower corresponding dew point.
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 below:
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.
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.
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.
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.
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.
Below are examples of piping schematics tor ice thermal storage systems.
The control of the ice thermal storage system should include automatic control of chillers, cooling towers, heat exchanger, thermal storage units, and system pumps to provide building chilled water at the required temperature at the most economical operating cost. This control system directs operation of the ten (10) modes of operation as follows:
The HVAC control is programmed to energize the applicable mode of operation based on a time schedule matching electrical use of facility and utility rate schedule.
The HVAC control communicates with the chiller’s on-board micropanel, which provides information on the operation of the chillers and with the building automation system (BAS) to obtain a cooling required signal.
Ice Build – Whenever the HVAC control schedule indicates the ice build mode, and the operating control on the BAC ICE CHILLER® Thermal Storage Unit indicates there is a need for additional ice, the HVAC control will operate the chiller at a reduced set point to make ice. the chiller’s load limit set point will be set at 100% (maximum output).
The ice build mode continues until the operating control on the BAC ICE CHILLER® Thermal Storage Unit indicates a full charge of ice has been built or the HVAC control time schedule indicates a change in mode of operation.
Ice Build and Cool with Base Water Chiller – Whenever the BAS selects cooling required and the predetermined time schedule indicates ice build and cool with base water chiller, the glycol chiller will operate the same as in ice build mode. The base water chiller micropanel will regulate the chilled water temperature leaving the chiller at the discharge temperature required. Chiller will be operated at capacity that will not exceed predetermined peak electrical demand.
Cooling – Glycol Chiller Only – Whenever the BAS selects cooling required and the predetermined time schedule indicates cooling – Glycol Chiller Only Mode, the chiller micropanel will regulate chilled glycol temperature leaving the chiller at the discharge temperature required (usually around 46°F. 7.8°C). Chiller will be operated at capacity that will not exceed predetermined peak electrical demand.
Cooling – Base Chiller Only – Whenever the BAS selects cooling required and the predetermined time schedule indicates Cooling – Base Chiller Only Mode, the chiller micropanel will regulate chilled water temperature leaving the chiller at the discharge temperature required. Chiller will be operated at capacity that will not exceed predetermined peak electrical demand.
Cooling – Ice Only – Whenever the BAS selects cooling required and time schedule indicates Cooling – Ice Only Mode, the HVAC control processor will operate to de-energize the glycol chiller and allow ice to provide full cooling.
Cooling – Glycol Chiller and Base Chiller Only – Whenever the BAS selects cooling required and the predetermined time schedule indicates Cooling – Glycol Chiller and Base Chiller Only Mode, the chillers’ micropanels will regulate chilled glycol and water temperatures leaving the chillers at the discharge temperatures required. Chillers will be operated at capacity that will not exceed predetermined peak electrical demand.
Cooling- Glycol Chiller and Ice Only – Whenever BAS selects cooling required and time schedule indicates Glycol Chiller and Ice Only Mode, the chiller micropanel will regulate chilled glycol temperature leaving the chiller at the discharge temperature required. Chiller will be operated at capacity that will not exceed predetermined peak electrical demand.
At peak load, the total cooling load is met by the combined operation of the glycol chiller and the depletion of the ice storage. At partial load conditions, the chiller operation will he automatically reduced allowing ice to satisfy demand.
Cooling – Base Chiller and Ice Only – Whenever the BAC selects the cooling required and the predetermined time schedule indicates Cooling – Base Chiller and Ice Only Mode, the chiller micropanel will regulate chilled water temperature leaving the chiller at the discharge temperature required. The base chiller will be operated at capacity that will not exceed predetermined peak electrical demand. The HVAC control processor will operate to de-energize the glycol chiller.
At peak load, the total cooling load is met by the combined operation of the base chiller and the depletion of the ice storage. At partial load conditions, the chiller operation will be automatically reduced allowing ice to satisfy demand.
Cooling – Ice, Glycol Chiller and Base Chiller – Whenever the BAS selects cooling required and the predetermined time schedule indicates Cooling — Ice, Glycol Chiller and Base Chiller Mode, the chillers’ micropanels will regulate chilled glycol and water temperatures leaving the chillers at the discharge temperatures required. Chillers will be operated at capacity that will not exceed predetermined peak electrical demand.
At peak load, the total cooling load is met by the combined operation of the chillers and the depletion of the ice storage. At partial load conditions, the chillers operation will be automatically reduced allowing ice to satisfy demand.
Standby –Whenever the Cooling Is Not Required Signal is received from the BAS System and time schedule is not in Ice Build Mode, the control system will be in the Standby Mode. In this mode, chiller and chiller support equipment will be OFF.
The rectangular design of the Modular BAC ICE CHILLER® Thermal Storage Unit maximizes the ton-hours per ft2 of available plan area. The product is designed specifically for tight installations where access is limited. The 7’-10” (2.4 m) wide units are designed so that they can be installed through interior double-door openings. Units are designed to be installed either indoors or outdoors.
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.
Evaporative condensers offer energy savings by providing lower system horsepower than conventional air-cooled and water-cooled condensing 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.