Solution to Example of previous lecture Church Example page 1 Partial Storage 40 ton x 3 hrs = 120 ton hr If such 120 ton hr is to be produced in 24 hrs, we get 120 ton hr/ 24 hr = 5 ton The plant contributes 5 ton x 3 hr = 150 ton hr The partial storage is 120 ton hr – 15 ton hr = 105 ton hr Full Storage The cooling load only served from storage = 120 ton hr / 21 hr = 5.71 tons Reduction in plant capacity = 40 – 5 = 35 tons 35 tons/40 tons = 87.5% Plant cost saving = $600/ton x 35 tons = $21,000 $70 ton hr x 120 hr = $8,400. $21,000 - $8,400 = $12,600
Solution to Example of previous lecture Church Example page 2 Increase in storage capacity comparing with partial storage is 5 ton x 3hr = 15 tons 15 tons x $70/ton = $1050 The increase in plant capacity comparing with partial storage is 5.71 – 5 ton = 0.71ton 0.71 ton x $600/ton = $426. $426 + $1050 = $1,476. Weekly Cycle Partial storage : 120 ton/ (24 hr x 7 days) = 0.71 ton Full storage : 120 ton/ (24hr x 7 days – 3 hrs) = 0.73 tons
Typical ice storage and chilled water storage systems are as follows:- Ice storage Ice-on-coil, internal-melt ice storage system Ice-on-coil, external-melt ice storage system Encapsulated ice storage system Ice-harvesting ice storage system Ice slurry system Chilled water storage Stratified chilled water storage system
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM System - 1 An ice-on-coil, internal-melt ice storage system uses brine flowing inside coils to make ice and to melt ice in the water that surrounds the coil. The central plant (cooling) and a chilled water or brine-incorporated ice storage system consists of chillers, ice storage tanks, chiller pumps, building pumps, controls, piping, and fittings as well as AHUs, terminals, return air system. Centrifugal, screw, and reciprocating chillers are usually used in ice-on-coil internal-melt ice storage systems depending on the size of the plant and types of condenser (water-cooled, air cooled, or evaporatively cooled) used. In locations where the outdoor air temperature during nighttime off-peak hours drops 11.3°C lower than the daytime maximum temperature, air-cooled chillers may sometimes be more efficient than water-cooled chillers .
An ice-on-coil, internal-melt ice storage system is a modular system consisting of many closely packed storage tanks connected in parallel. Such an ice storage system is more flexible during the installation of storage tanks, especially for retrofit projects . Off-peak cooling of the building can be provided by direct cooling from the chillers. During ice burning , melted water separates the tube and ice. Water has a much lower thermal conductivity 0.61 W/m °C than that of ice 2.25 W/m °C, so the capacity of the ice-on-coil, internal-melt ice storage system is dominated by the rate of ice burning or melting. ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM System - 2
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Brine and Glycol Solution – 1 Brine is a salt solution or an aqueous glycol solution used as a heat-transfer medium. Its freezing point is lower than that of water, and depends on the concentration of salt or glycol in solution. Brine is also used as a liquid coolant to absorb or to store heat energy in refrigeration and thermal storage systems. Ethylene glycol and propylene glycol are brines that are colorless, nearly odorless liquids. They are often mixed with water at various concentrations and used as freezing point depressants to lower the freezing point of water. Inhibitors must be added to ethylene and propylene glycols to prevent metal corrosion .
The freezing point of an aqueous ethylene glycol solution with a concentration of 25 percent by mass drops to -12.2°C , and its rate of heat transfer is about 5 percent less than that of water. The freezing point of a propylene glycol solution with a concentration of 25 percent by mass drops to -9.4°C . The physical properties of aqueous ethylene glycol solution are more appropriate for thermal storage systems than those of aqueous propylene glycol solution . ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Brine and Glycol Solution – 2
In an ice-on-coil, internal-melt ice storage system, ice is produced, or charged, in multiple storage tanks . There are closely spaced multi-circuited polyethylene or plastic tubes surrounded by water in these storage tanks. ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Ice Storage Tank – 1
Brine , an aqueous ethylene glycol solution with 25 to 30 percent ethylene glycol and 70 to 75 percent water , circulates inside the tubes at about - 4.4°C . The water surrounding the tubes freezes into ice up to a thickness of about 12.7 mm . Tubes containing glycol solution entering and leaving the tank are arranged side by side alternately to provide more uniform heat transfer. ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Ice Storage Tank – 2
Brine typically leaves the storage tank at -1.1°C. Plastic tubes occupy about one-tenth of the tank volume . Another one-tenth is left empty to accommodate the expansion of ice during ice making. Multiple ice storage tanks are always connected in parallel. During ice burning or ice melting , brine returns from the cooling coils in the air-handling units at a temperature of 7.8°C or higher. This brine melts the ice on the outer surface of the tubes and is thus cooled to 1.1 to 2.2°C. The brine is then pumped to the air-handling units to cool the air again. In the storage tank, the high-pressure brine inside the tubes is separated from the water, usually at atmospheric pressure, surrounding the tubes in the storage tank. ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Ice Storage Tank – 2
A demand-limited partial-storage strategy is used; i.e., one chiller is operated during on-peak hours Ice is burned during on-peak hours to reduce the demand charge. Ice is charged during off-peak hours to reduce energy costs. For summer cooling, the daily 24-h operating cycle can be divided into three periods: off-peak, direct cooling, and on-peak Off-Peak. This is the period from 9 p.m. until the air-handling units start the next morning. During this period, the primary operating mode is ice making and the chillers also provide direct cooling at a small capacity for refrigeration loads that operate 24 h . In this operating mode, the ice-on-coil, internal-melt storage tanks are charged . At the same time, ethylene glycol solution 1.1°C is supplied to the air-handling units for nighttime cooling . ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 1
<ul><li>Off-Peak. </li></ul><ul><li>Ice storage system in the following operating sequence: </li></ul><ul><li>Open control valves and </li></ul><ul><li>close control valves as shown on the right diagram </li></ul>ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 2
<ul><li>Off-Peak </li></ul><ul><li>Reset the temperature of the glycol solution leaving the chiller to about -6°C </li></ul><ul><li>Reset the load limit of both chillers to 100 percent. </li></ul><ul><li>Start the chiller and condenser pumps. </li></ul><ul><li>Chiller pumps operate at high speed during ice making to provide a higher flow rate as well as a greater head to overcome the pressure drop for both the evaporator and the coils in the ice storage tanks. </li></ul><ul><li>Chiller pumps operate at low speeds in direct cooling mode . </li></ul>ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 3
<ul><li>Start chillers 1 and 2 following the lead / lag sequence . After chillers are started, open control valves shown with ARROWS on the diagram. </li></ul><ul><li>Start the building chilled water circulating pumps in sequence. </li></ul>ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 4 Off-peak
<ul><li>Modulate control valves as shown on the right diagram, and maintain a 1°C glycol solution supply temperature to the air-handling units. </li></ul>ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 5 Off-peak
When the sensors detect that the ice storage tanks are 100 percent charged , the ice-making mode is terminated. If nighttime after-hours cooling is not needed, the ice storage system shuts down. If the ice inventory (the amount of stored ice in the tanks) falls below 90 percent , ice making starts again. There are two additional operating modes during this period: 1) Ice making without direct cooling for after-hours use 2) Ice burning for after-hours use with all chillers shut off. ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 6
Direct Cooling Direct-cooling operation lasts from the start of the air-handling units until noon on weekdays . This period has two operating modes: Direct cooling mode : Chillers are operating and are reset to 1.1°C. Direct cooling with ice-burning mode : Both chillers are turned on. When the required refrigeration load exceeds both chillers’ capacity, some ice storage will be discharged to supplement the chillers. On-Peak On-peak hours are from noon until 9 p.m . weekdays. Ice-burning mode, with or without chiller operation, is used in this period. In ice-burning mode, one chiller is operated at the demand limit . The operating sequence is shown on the next page: ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 7
<ul><li>On-Peak </li></ul><ul><li>Open control valves and close control valves as shown on the right diagram. </li></ul><ul><li>Open control valve serving chiller 1 and close control valve serving chiller 2 if chiller 1 is required to operate. </li></ul><ul><li>Vice versa for 2 above. </li></ul>ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 8
<ul><li>On-Peak </li></ul><ul><li>Modulate control valves shown on left from normal open positions. </li></ul><ul><li>Reset chilled water temperature leaving the chiller to 0°C. </li></ul><ul><li>Set the load limit of the operating chiller. </li></ul><ul><li>Start one condenser pump . </li></ul><ul><li>Start chiller pumps 1 and 2 at low speed . Both pumps will operate during ice burning. </li></ul><ul><li>Start one chiller according to the lead/lag sequence . </li></ul>ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 9
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM Example on Control Strategy - 10 <ul><li>On-Peak </li></ul><ul><li>Modulate control valves marked with M to maintain a 1.1°C chilled water supply temperature to the air-handling units. </li></ul><ul><li>Start the brine circulating pumps (building pumps ) in sequence. During on-peak hours, the brine circulating pump needs a greater head to overcome the pressure drop of the coil in the AHU and that of the coil in the ice storage tanks. </li></ul>
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS System Description - 1 The ice-on-coil, external-melt ice storage system is the oldest type of ice storage system. It may be costly and complex . In an ice-on-coil, external-melt ice storage system, ice builds up on the outer surface of coils or tube banks, which are submerged in water in a storage tank. The refrigerant flows and evaporates inside the tubes. When the ice melts , it cools the water at a temperature between 1.1 and 3.3°C for cooling in the AHUs.
Schematic diagram of ice-on-coil, external melt ice storage system ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS System Description - 2
An ice-on-coil, external-melt system consists of chillers, evaporating coils, storage tanks, condenser, heat exchanger, refrigerant pumps, chilled water pumps, air system controls, piping, and fitting. Screw compressors are often used because of their higher efficiency. For an ice-on-coil, external-melt ice storage system with a capacity less than 2400 ton h, a reciprocating compressor may also be used. Evaporatively cooled condensers have a higher system energy efficiency ratio and are often used in many new projects. ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS System Description - 3
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS System Description - 4
Ice builders are large, well-insulated steel tanks containing many coils, usually made of steel pipes of 25 to 30mm diameter. HCFC-22 is currently used as the refrigerant. (HFC for refrigerant?? ) The refrigerant-filled coils are submerged in water in the ice builder and function as evaporators. The ice build up on the coil is between 25 and 64 mm thick . When ice builds up on the coil, the suction temperature of the compressor falls to -5.6 to - 4.5°C. Ice is melted by the water circulating over it. The steel tubes of the coil should be spaced so that the built-up ice cylinders do not bridge each other . If the cylinders are bridged , the paths of water circulation are blocked . Baffle plates are sometimes added to guide the water flow and provide a secondary heat-transfer surface between the refrigerant and water. ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS Ice Builders -1
The storage tanks containing refrigerant coils are usually located at a lower level or on a grade because of their weight. Because the chilled water system in a multistory building is always under a static head at lower levels, a heat exchanger is used to isolate the storage tank brine system from the chilled water system connected to the AHUs. An alternative is to supply chilled water directly to the storage tanks and pressurize the tanks . This arrangement obviates the use of a heat exchanger and a corresponding increase in brine temperature of about 1.7°C. In the storage tank, stored ice occupies only about one-half the volume of the tank, so the ice builder must be larger and heavier. ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS Ice Builders -2
Two kinds of refrigerant feed are widely used in ice-on-coil, external-melt ice storage systems: direct expansion and liquid overfeed. Direct expansion (DX) uses the pressure difference between the receiver at the high-pressure side and the suction pressure to force the refrigerant to flow through the ice builder. Direct expansion is simple, and no refrigeration pump is required. Its main drawback is that 15 to 20 percent of the coil surface is used for superheat and is not available for ice buildup. Liquid overfeed uses a refrigerant pump to feed ice-builder coils about 3 times the evaporation rate they need. Because the liquid refrigerant wets the inner surface of the ice-builder coils, liquid overfeed has a higher heat-transfer coefficient than direct expansion. ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS Refrigerant Feed - 1
The thicker the ice built up on the coils, the greater the amount of ice stored in the tank. The thickness of ice on the coil should be measured to meet ice-burning requirements during on-peak operating hours or in the direct cooling period. Because ice has a higher volume than water , as the ice builder is charged (i.e., as ice builds up on the coil), the water level rises. An electric probe can sense the water level in the tank and thereby determine the amount of ice stored in the tank. ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS Ice-Charging Control - 1
ENCAPSULATED ICE STORAGE SYSTEMS System description - 1 Plastic containers , filled with de-ionized water and ice-nucleating agent, are immersed in a secondary coolant ethylene glycol solution in a steel or concrete tank . Ice is charged and stored when the secondary coolant is at a temperature between -6 and - 3°C circulated through the tank. Ice is melted when the warm coolant returned from the AHUs is circulated through the tank. Chillers can also provide direct cooling at a coolant temperature from 2 to 6°C. An encapsulated ice storage system consists of the following components: chillers, steel tank, encapsulated containers, pumps, air system controls, piping, and accessories.
Two types of encapsulated ice containers are currently available : dimpled spheres of 100mm diameter and rectangular containers approximately35 by 300 by 750 mm. The containers are made of high-density polyethylene and are designed to withstand the pressure due to the expansion during freezing. When containers are put or stacked inside the storage tank, they allow free circulation of fluid and do not provide unwanted short-circuit fluid flow which causes degradation of performance. The storage tank can be an open, non-pressurized type or pressurized type. An open storage tank needs a barrier to keep the frozen containers submerged into the coolant. ENCAPSULATED ICE STORAGE SYSTEMS System description - 2
ENCAPSULATED ICE STORAGE SYSTEMS Chiller at upstream or downstream of Storage Tank - 1 Chiller upstream Chiller downstream
ENCAPSULATED ICE STORAGE SYSTEMS Chiller at upstream or downstream of Storage Tank - 2 The chillers and storage tanks are usually connected in series . When partial storage is used, their relative location can be either in chiller upstream or chiller downstream arrangements. In a chiller upstream arrangement , the chilled water returned from AHUs at 8°C is often first cooled in the chiller to 4°C, and then it enters the storage tank and is cooled down to 1°C. In the chiller upstream arrangement, since the chilled water cooled at the chiller is at a higher temperature, this results in a higher COP at the chiller. However, the usable portion of the total storage capacity will be reduced because of the lower storage tank discharge temperature.
In a chiller downstream arrangement, the chilled water returned from the AHU at 8°C is often first cooled in the storage tank to 4°C, and then it enters the chiller and is cooled down to 1°C. In a chiller downstream arrangement, the COP of the chiller is lower , and the usable portion of the total storage capacity of the ice storage tanks is increased . Usually, during partial storage , the chiller upstream arrangement is often used for higher efficiency in the chiller. ENCAPSULATED ICE STORAGE SYSTEMS Chiller at upstream or downstream of Storage Tank - 3
Ice-charging inventory in the storage tank is measured based on the displacement of water in the tank when the ice is formed inside the encapsulated containers. For open tanks , a static pressure transducer is often used to detect the water level in the storage tank. In pressurized tanks , the expansion of the frozen containers forces the secondary coolant overflowing into a separate inventory tank, and its water level is measured. ENCAPSULATED ICE STORAGE SYSTEMS Controls - 1
Encapsulated ice storage systems with a chiller upstream arrangement are well suited to chiller priority control . When the system refrigeration load is less than the chiller capacity , the chilled water bypasses the storage tanks completely. When the system refrigeration load exceeds the chiller capacity and the chiller leaving temperature increases above the leaving set point, the control system diverts part of the chilled water flow through the storage tanks to maintain the required supply temperature to the AHU. Storage priority is more complicated to achieve. A required refrigeration load prediction algorithm to forecast the chiller cooling is needed each day. Chiller capacity is then limited by increasing the chilled water leaving setpoint, and most of or all the refrigeration load is then met by the ice storage. ENCAPSULATED ICE STORAGE SYSTEMS Controls - 2
A pump is used to pump the overflowing fluid in the inventory tank back into the storage tank after discharging. With non-dimpled spherical containers that expand very little as the encapsulated ice freezes, storage inventory can be monitored based on the integrated flow and temperature measurements . Encapsulated ice storage systems use a storage tank bypass three-way modulating valve to control the chilled water leaving temperature. Chillers should be controlled at full load during charging to prevent the reduction of system efficiency and incomplete charging of ice storage . The chiller leaving temperature setpoint should be set at or below the minimum required charging temperature so that the chiller is fully loaded throughout the charging cycle . ENCAPSULATED ICE STORAGE SYSTEMS Controls - 3
For encapsulated ice storage systems, the charging temperature decreases during the charge cycle as the thickness of ice through which heat is transferred increases. Encapsulated containers are subject to super-cooling , i.e., cooling of the liquid water inside the container below its freezing point prior to the ice formation . Super-cooling occurs only in fully discharged containers and results in a reduced rate of heat transfer at the beginning of the charging process. Super-cooling can be significantly reduced by the addition of nucleating agents. ENCAPSULATED ICE STORAGE SYSTEMS Charging and Discharging - 1
For an entering chilled water temperature at the beginning of charging of 0 o C and a chilled water temperature at the end of charging of - 7 to - 3 o C, a typical range of charging temperatures is between 2 and 7 o C corresponding to a charging period of 8 to 16 h. Encapsulated ice storage systems have a steadily falling discharge rate when the discharge temperature is kept constant, or a steadily rising discharge temperature when the discharge rate is constant. This is due to the decreasing area of ice in contact with the container as the ice melts. The encapsulated ice storage discharge temperature typically begins at 0 o C and ends at a discharge temperature of 3 to 7 o C. ENCAPSULATED ICE STORAGE SYSTEMS Charging and Discharging - 2
ICE-HARVESTING ICE STORAGE SYSTEMS System - 1 Ice is produced in a harvester , which is separate from the storage tank where ice is stored. The evaporator of the chiller is a vertical plate heat exchanger mounted above a water / ice storage tank. Low-pressure liquid refrigerant is forced through the inner part of the plate heat exchanger, in which liquid refrigerant is vaporized , and produces a refrigeration effect . The brine incorporated ice storage system in an ice-harvesting ice storage system consists of the following equipment and main components: chillers, an ice harvester, storage tank, air system controls, piping, and accessories.
Schematic diagram of a typical ice-harvesting ice storage system. ICE-HARVESTING ICE STORAGE SYSTEMS System - 2
ICE-HARVESTING ICE STORAGE SYSTEMS Ice Making or Charging - 1 During ice-making or -charging mode, a chilled aqueous ethylene glycol solution with a concentration of 25 to 30 percent is pumped from the storage tank. This solution distributed over the outer surface of the evaporator plates at a temperature equal to or slightly above 0 o C. It then flows downward along the outer surface of the plate in a thin film. Water is cooled and then frozen into ice sheets approximately 5 to 7.5 mm thick. Periodically, hot gas is introduced into one-fourth of the evaporator plates by reversing the refrigerant flow.
Ice is harvested, or released from the outer surface of the plates, in the form of flakes or chunks and falls into the storage tank below. Ice is formed in 20 to 30 min and is harvested within 20 to 40 s . During harvesting, this section of plate evaporator acts as a condenser. Ice accumulates in the storage tank to occupy slightly less than 60 percent of the volume of the tank. Because the ice flakes are usually smaller than 1500 mm by 1500 mm by 63 mm, there is a large contact area between the return brine from the cooling coils and the ice. ICE-HARVESTING ICE STORAGE SYSTEMS Ice Making or Charging - 2
The time required to melt the ice in the storage tank is less than one-tenth of the time needed in ice making or charging. For a reciprocating compressor using an evaporative condenser, the COP 3.2 to 3.7 is typical for chiller during ice making. Because the evaporator plates must be located above the storage tank, ice-harvesting systems need more headroom than other ice storage systems. ICE-HARVESTING ICE STORAGE SYSTEMS Ice Making or Charging - 3
Brine at a temperature of 1 o C is supplied to the air-handling units to cool the air to a supply temperature of 6 to 7 o C during direct cooling. It is then returned to the ice harvester at 10 to 16 o C and distributed over the evaporator plates directly. After falling from the evaporator plates , brine is again cooled to a temperature of 1 o C before it leaves the storage tank. Because of the higher temperature of return brine distributed over the evaporator plates , the capacity of the ice harvester increases, and its power consumption decreases during chiller operation. During chiller operation, power consumption usually varies between 0.75 and 0.85 kW / ton. ICE-HARVESTING ICE STORAGE SYSTEMS Chiller Operation - 1
The temperature of brine from the storage tank of the ice harvester can be lowered to 1 o C, which is 1 o C lower than in the ice-on-coil, internal-melt ice storage system . Ice-harvesting systems have been successfully used in load shifting and load leveling to reduce electric demand and energy cost. But melting of the ice during the harvesting process decreases the amount of ice harvested and adds an incremental refrigeration load to the system. An ice-harvesting ice storage system with brine system is an open system . More water treatment is required than in an ice-on-coil, internal-melt ice storage system whose brine system is a closed system. ICE-HARVESTING ICE STORAGE SYSTEMS Chiller Operation - 2
Slurry ice is a suspension of very small ice crystals in a liquid. The Binary Ice fluid contains latent energy in the form of ice minute crystals (sizes various from 1/10 – 1/100 mm) More intensive than the single phase fluids (e.g. cold water or brine) in respect of heat transfer and reaction kinetics It changes from the frozen state to the liquid state when heat is absorbed. This phase change is instantaneous (much faster than normal ice melting). It sends warm return fluid at the max. acceptable flow rate through the ice maker which acts as a water chiller during operation of the second loop. The slurry liquid is pumpable . SLURRY ICE SYSTEMS System - 4
Simply defined, an ice slurry is a suspension of ice crystals in liquid. In general, the working fluid’s liquid state consists of a solvent (water) and a solute such as glycol, ethanol, or calcium carbonate. Depending on the specific slurry technology, the initial solute concentration varies from 2% to over 10% by mass. The solute depresses the freezing point of the solvent and buffers the production of ice crystals. The freezing point of an aqueous solutions calcium magnesium acetate decrease with increase in concentrations. Slurry generation begins by lowering the working fluid to its initial freezing point . Extracting additional energy from the working fluid initiates the process of solidification. SLURRY ICE SYSTEMS System - 5
As solidification proceeds , solute is rejected to the solid-liquid interface (only the water is frozen). The solute-rich interface is eventually incorporated into the bulk fluid by convection and diffusion . As freezing progresses, the solute in the bulk field increases and the freezing point needed to sustain ice crystal production decreases. Assuming that none of the solute is incorporated into the solid, a relationship between the solute concentration and the ice fraction can be established by the following equation: SLURRY ICE SYSTEMS System - 6
Thus the ice fraction can be estimated by knowing the initial solute concentration and the solute concentration at any time during slurry production or consumption . The equilibrium solute concentration can be related back to temperature through freezing point curves. By monitoring the liquid temperature during slurry production and slurry consumption, the fraction of ice in a given system can be estimated by knowing the initial solute concentration and the freezing point characteristic of the working fluid. The success of this technique relies on an assumption that all of the solute is being rejected to the liquid phase upon solidification and minimal dilution of the working fluid over time (e.g., due to condensation of moisture from the air in the storage vessel). SLURRY ICE SYSTEMS System - 7
STRATIFIED CHILLED WATER STORAGE SYSTEMS System - 1 A stratified chilled water storage system often uses a large storage tank to store chilled water at a temperature between 4 and 7 o C. The stored chilled water offsets the building refrigeration load during on-peak hours to shift the load to the off-peak hours and reduces the energy cost. Chilled water in the storage tank is stratified into three regions because of its gravity: top warmer return water from the AHUs, middle region of steep temperature gradient, and bottom colder chilled water from the chillers. The chilled water incorporated storage system consist of chillers, a cylindrical storage tank, pumps, piping, air system controls, and accessories.
Chilled Water Storage System STRATIFIED CHILLED WATER STORAGE SYSTEMS System - 2
STRATIFIED CHILLED WATER STORAGE SYSTEMS Basic Considerations for Chilled Water Storage - 1 The stored cooling capacity of a chilled water storage system depends on the temperature difference between the warm water return from the AHUs and the chilled water stored in the tank, and the amount of water stored. The larger the storage tank, the lower the capital cost per unit stored volume. It was found that a chilled water storage system is economical for large capacity (storage capacity exceeds 7000 kWh). Currently, chilled water systems achieve thermal separation between cold charged water and warm return water by stratification, multiple tanks, membrane, diaphragm, and baffles . The stratified tank is the simplest and most efficient method. Chilled water storage systems need a storage tank volume of about 0.169 m 3 /kWh compared to about 0.027 m 3 /kWh for ice storage systems.
Charging is the process of filling the storage tank with chilled water from the chiller, usually at a temperature between 4 and 7 o C. Meanwhile, the warmer return chilled water from the air-handling units or terminals, usually at a temperature between 11 and 16 o C, is extracted from the storage tank and pumped to the chiller to be cooled. In the process of discharging, the chilled water, at a temperature between 5 and 7 o C, from the storage tank is supplied to the terminal units such as air handling units. At the same time, the warmer return chilled water from the coils fills the tank with an aid of storage water pumps. STRATIFIED CHILLED WATER STORAGE SYSTEMS Charging and Discharging - 1
<ul><li>Stored chilled water is warmed by direct mixing of warmer return chilled water and stored colder chilled water. </li></ul><ul><li>Heat from previously stored warmer return chilled water is transferred from the warmer tank wall to the stored chilled water. </li></ul><ul><li>Heat is transferred through the tank wall from the warmer ambient air. </li></ul>During the storage of chilled water, the following processes result in losses in cooling capacity : STRATIFIED CHILLED WATER STORAGE SYSTEMS Loss of Cooling Capacity during Storage - 1
A more easily measured, enthalpy-based figure of merit (FOM) is often used to indicate the loss of cooling capacity of the stored chilled water during the charging and discharging processes in a complete storage cycle. The FOM is defined as STRATIFIED CHILLED WATER STORAGE SYSTEMS Figure of Merit - 1
The smaller the losses of cooling capacity during chilled water storage, the greater the value of FOM . Well-designed storage tanks have figures of merit of 90% or higher for daily complete charge/discharge cycles and between 80 and 90% for partial charge/discharge cycles. STRATIFIED CHILLED WATER STORAGE SYSTEMS Figure of Merit - 2
Chilled water storage tanks are usually flat-bottomed vertical cylinders. A cylindrical tank has a lower surface-to-volume ratio than a rectangular tank. Large cylindrical tanks typically have a height-to-diameter ratio of 0.25 to 0.35. Steel is the commonly used material for above-grade tanks, and concrete is widely used for underground tanks. In certain projects, pre-cast, pre-stressed, cylindrical concrete tanks with watertight steel diaphragms are used for very large chilled water storage facilities. All outdoor above-grade structures should have a minimum of 50mm thick external insulation layer spray-on polyurethane foam, a vapor barrier, and a highly reflective top coating. STRATIFIED CHILLED WATER STORAGE SYSTEMS Storage Tanks - 1
There are many types of storage tanks: stratified tank, membrane tank, empty tank, etc. Stratified tanks rely on the buoyancy of warmer return chilled water, which is lighter than colder chilled water, to separate these two chilled waters during charging and discharging . Diffusers are used to lower entering and leaving water velocity to prevent mixing. In a stratified tank, colder stored chilled water is always charged from the bottom diffusers arranged concentrically. It is also discharged from the same bottom diffusers. The warmer return chilled water is introduced to and withdrawn from the tank through the top lateral diffusers. Field measurements shows thatstratified tanks have a figure of merit between 0.85 and 0.92. STRATIFIED CHILLED WATER STORAGE SYSTEMS Storage Tanks - 2
It was found that there is no significant difference in FOM between stratified tanks and membrane tanks or empty tanks. A membrane tank is a storage tank in which a membrane separates the colder stored chilled water and warmer return water. An empty tank is a storage tank in which walls are used to separate the colder and warmer chilled water. Compared with membrane tanks and empty tanks, stratified tanks have the advantages of simpler construction and control, greater storage capacity, and lower cost . Stratified tanks are widely used in chilled water storage installations. STRATIFIED CHILLED WATER STORAGE SYSTEMS Storage Tanks - 3
Vertical temperature profiles are formed during charging or discharging in stratified tanks at various time intervals. Temperature profiles may be illustrated on a height-temperature diagram at the beginning, the middle, and near the end of the charging process. In the middle of the charging process along the vertical height of the storage tank, chilled water is divided into three regions: bottom colder-and-heavier stored chilled water, thermocline , and top warmer-and lighter return chilled water. STRATIFIED CHILLED WATER STORAGE SYSTEMS Temperature Gradient and Thermocline - 1
A thermocline is a stratified region in which there is a steep temperature gradient. The water temperature often varies from 6 to 16°C. The thermocline separates the colder stored chilled water from the warmer return chilled water. The thinner the thermocline, the smaller the mixing loss. The layout and configuration of diffusers in a stratified tank have a significant effect upon the mixing of the colder and warmer chilled water, as well as on the formation of the thermocline. STRATIFIED CHILLED WATER STORAGE SYSTEMS Temperature Gradient and Thermocline - 2
The purpose of the diffusers and their connecting piping is to distribute the incoming chilled water evenly , so that it flows through the inlet openings with sufficiently low velocity (usually lower than 0.3 m/s) to minimize mixing of colder and warmer chilled water. Warmer return chilled water should be introduced as closely as possible to the top water level in the stratified tank. Colder stored chilled water should be introduced just above the bottom floor of the tank. The inlet temperature of chilled water should be controlled within a narrow band (say about plus or minus 1°C) during charging to avoid additional mixing. STRATIFIED CHILLED WATER STORAGE SYSTEMS Temperature Gradient and Thermocline - 3
Obstructions in the flow crossing the tank, other than diffusers and the connecting piping, should be minimized. The function of diffusers is to reduce mixing. Mixing can occur at at the start of the charging and discharging processes . Mixing also occur at the reformation of the thermocline , and at the inlet side of the thermocline after the thermocline has been formed. Mixing near the inlet diffuser can be minimized if the incoming chilled water initially forms a thin layer of gravity current that travels across the tank. Gravity current slowly pushes the chilled water originally in the tank out of the way so that mixing only occurs at the front of the gravity current when it first crosses the tank. STRATIFIED CHILLED WATER STORAGE SYSTEMS Temperature Gradient and Thermocline - 4
Large stratified tanks usually incorporate linear diffusers. Inlet flow from the top diffusers should be upward or horizontal. Bottom diffusers should flow downward and have slots spreading at 120°. The cross-sectional inlet area of the branch pipe leading to the diffuser should be at least equal the total area of the diffuser openings in that branch. Inlet and outlet streams must be kept at sufficiently low velocities , so that buoyancy forces predominate over inertia forces in order to produce a gravity current (density current) across the bottom or top of the tank. STRATIFIED CHILLED WATER STORAGE SYSTEMS Diffusers - 1
The inlet Reynolds number is closely related to the inlet velocity and is defined as When Re i < 850, loss due to mixing and loss of cooling capacity during discharge can be significantly reduced. STRATIFIED CHILLED WATER STORAGE SYSTEMS Diffusers - 2 Mixing on the inlet side of thermocline depends on the inlet Reynolds number Re i and Froude number Fr i .
The inlet Froude number Fr i is defined as STRATIFIED CHILLED WATER STORAGE SYSTEMS Diffusers - 3
Stratification diffusers must be designed and constructed to produce and maintain stratification at the maximum flow through storage. Designers typically select a diffuser dimension to create an inlet Froude number of 1.0 or less. h i indicates the inlet opening height m. It is the vertical distance occupied by the incoming flow when it leaves the diffuser and forms the gravity current. For the bottom diffusers, inlet opening height h i indicates the vertical distance between the tank floor and the top of the opening of the diffuser. STRATIFIED CHILLED WATER STORAGE SYSTEMS Diffusers - 4
<ul><li>The piping design should be symmetric. </li></ul><ul><li>Branch pipes should be equal in length. </li></ul><ul><li>Flow splitters should be added at the appropriate points. </li></ul><ul><li>Pipe diameter reduction should be combined with the flow splitter. </li></ul><ul><li>Long-radius elbows should be used. </li></ul>Self-balancing should be achieved on evenly distribution of the flow by: STRATIFIED CHILLED WATER STORAGE SYSTEMS Diffusers - 5
Exposed tank surfaces should be insulated to help maintain the temperature differential in the tank. Insulation is especially important for smaller storage tanks because the ratio of surface area to stored volume is relatively high. Heat transfer between the stored water and the tank contact surfaces (including divider walls) is a primary source of capacity loss. Not only does the stored fluid lose heat to (or gain heat from) the ambient by conduction through the floor and wall, but heat flows vertically along the tank walls from the warmer to the cooler region. STRATIFIED CHILLED WATER STORAGE SYSTEMS Storage Tank Insulation - 1
The FOM of a stratified tank is closely related to the temperature difference of the outlet temperature of stored chilled water during discharging and the inlet temperature of stored chilled water during charging . For a complete charging and discharging cycle, the average during discharging is always greater than the average because of the mixing loss and heat gains, provided that the water flow rate is constant. The smaller the difference between outlet temperature of stored chilled water during discharging and the inlet temperature of stored chilled water during charging, the higher the FOM . STRATIFIED CHILLED WATER STORAGE SYSTEMS Charging and Discharging Temperature versus Tank Volume - 1
During the charging process , return chilled water is extracted from the top diffusers of the stratified tank , cooled in the chiller, and charged into the stratified tank again through the bottom diffusers . During the discharging process , stored chilled water is extracted from the stratified tank and supplied to the cooling coils in the air-handling units and terminals. The return chilled water is introduced to the stratified tank through the top diffusers . Both the outlet temperature of return chilled water during charging and inlet temperature of return chilled water during discharging should be controlled between 13 and 16°C so that stratification can be maintained in the storage tank. STRATIFIED CHILLED WATER STORAGE SYSTEMS Charging and Discharging Temperature versus Tank Volume - 2
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