MEBS 6008
Thermal Storage Two
Thermal Storage Two
1
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
Thermal Storage Two
2
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
Thermal Storage Two
3
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
Thermal Storage Two
4
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Thermal Storage Two
5
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, aircooled chillers may sometimes be more efficient than water-cooled chillers.
Thermal Storage Two
6
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
System - 2
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.
Thermal Storage Two
7
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.
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Brine and Glycol Solution – 2
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.
Thermal Storage Two
9
ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Ice Storage Tank – 1
In an ice-on-coil, internal-melt ice
storage system, ice is produced, or
charged, in multiple storage tanks.
There are closely spaced multicircuited polyethylene or plastic tubes
surrounded by water in these storage
tanks.
Thermal Storage Two
10
ICE-ON-COIL, INTERNAL-MELT ICE
STORAGE SYSTEM
Ice Storage Tank – 2
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.
Thermal Storage Two
11
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.
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Example on Control Strategy - 1
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.
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT
ICE STORAGE SYSTEM
Example on Control Strategy - 2
Off-Peak.
Ice storage system in the
following operating sequence:
1.
Open control valves and
close control valves as
shown on the right diagram
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Example on Control Strategy - 3
Off-Peak
2.
Reset the temperature of the glycol solution leaving the chiller to about
-6°C
3.
Reset the load limit of both chillers to 100 percent.
4.
Start the chiller and condenser pumps.
5.
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.
6.
Chiller pumps operate at low speeds in direct cooling mode.
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT
ICE STORAGE SYSTEM
Example on Control Strategy - 4
Off-peak
7.
Start chillers 1 and 2 following
the lead / lag sequence. After
chillers are started, open control
valves shown with ARROWS on
the diagram.
8.
Start the building chilled water
circulating pumps in sequence.
Thermal Storage Two
16
ICE-ON-COIL, INTERNAL-MELT ICE
STORAGE SYSTEM
Example on Control Strategy - 5
Off-peak
9.
Modulate control valves as
shown on the right diagram, and
maintain a 1°C glycol solution
supply temperature to the airhandling units.
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Example on Control Strategy - 6
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.
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT ICE STORAGE SYSTEM
Example on Control Strategy - 7
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:
Thermal Storage Two
19
ICE-ON-COIL, INTERNALMELT ICE STORAGE SYSTEM
Example on Control Strategy - 8
On-Peak
1.
Open control valves and close
control valves as shown on the
right diagram.
2.
Open control valve serving
chiller 1 and close control
valve serving chiller 2 if chiller
1 is required to operate.
3.
Vice versa for 2 above.
Thermal Storage Two
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ICE-ON-COIL, INTERNALMELT ICE STORAGE SYSTEM
Example on Control Strategy - 9
On-Peak
4. Modulate control valves shown on
left from normal open positions.
5. Reset chilled water temperature
leaving the chiller to 0°C.
6. Set the load limit of the
operating chiller.
7. Start one condenser pump.
8. Start chiller pumps 1 and 2 at
low speed. Both pumps will
operate during ice burning.
9. Start one chiller according to
the lead/lag sequence.
Thermal Storage Two
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ICE-ON-COIL, INTERNAL-MELT ICE
STORAGE SYSTEM
Example on Control Strategy - 10
On-Peak
10. Modulate control valves marked
with M to maintain a 1.1°C chilled
water supply temperature to the
air-handling units.
11.
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.
Thermal Storage Two
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ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Thermal Storage Two
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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.
Thermal Storage Two
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ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
System Description - 2
Schematic diagram of ice-on-coil, external melt ice storage system
Thermal Storage Two
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ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
System Description - 3
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.
Thermal Storage Two
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ICE-ON-COIL, EXTERNAL-MELT ICE
STORAGE SYSTEMS
System Description - 4
Thermal Storage Two
27
ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Ice Builders -1
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.
Thermal Storage Two
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ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Ice Builders -2
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.
Thermal Storage Two
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ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Refrigerant Feed - 1
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.
Thermal Storage Two
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ICE-ON-COIL, EXTERNAL-MELT ICE STORAGE SYSTEMS
Ice-Charging Control - 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 iceburning 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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
Thermal Storage Two
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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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
System description - 2
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 shortcircuit 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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
Chiller at upstream or downstream of Storage Tank - 1
Chiller upstream
Thermal Storage Two
Chiller downstream
35
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.
Thermal Storage Two
36
ENCAPSULATED ICE STORAGE SYSTEMS
Chiller at upstream or downstream of Storage Tank - 3
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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
Controls - 1
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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
Controls - 2
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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
Controls - 3
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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
Charging and Discharging - 1
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.
Thermal Storage Two
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ENCAPSULATED ICE STORAGE SYSTEMS
Charging and Discharging - 2
For an entering chilled water temperature at the beginning of
charging of 0oC and a chilled water temperature at the end of
charging of -7 to -3oC, a typical range of charging
temperatures is between 2 and 7oC 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 0oC and ends at a discharge temperature of 3 to 7oC.
Thermal Storage Two
42
ICE-HARVESTING ICE STORAGE SYSTEMS
Thermal Storage Two
43
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.
Thermal Storage Two
44
ICE-HARVESTING ICE STORAGE SYSTEMS
System - 2
Schematic diagram of a typical ice-harvesting ice storage system.
Thermal Storage Two
45
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 0oC.
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.
Thermal Storage Two
46
ICE-HARVESTING ICE STORAGE SYSTEMS
Ice Making or Charging - 2
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.
Thermal Storage Two
47
ICE-HARVESTING ICE STORAGE SYSTEMS
Ice Making or Charging - 3
The time required to melt the ice in the storage tank is less than onetenth 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, iceharvesting systems need more headroom than other ice storage systems.
Thermal Storage Two
48
ICE-HARVESTING ICE STORAGE SYSTEMS
Chiller Operation - 1
Brine at a temperature of 1oC is supplied to the air-handling units to cool
the air to a supply temperature of 6 to 7oC during direct cooling.
It is then returned to the ice harvester at 10 to 16oC and distributed over
the evaporator plates directly.
After falling from the evaporator plates, brine is again cooled to a
temperature of 1oC 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.
Thermal Storage Two
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ICE-HARVESTING ICE STORAGE SYSTEMS
Chiller Operation - 2
The temperature of brine from the storage tank of the ice harvester
can be lowered to 1oC, which is 1oC 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.
Thermal Storage Two
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SLURRY ICE SYSTEM
Thermal Storage Two
51
SLURRY ICE SYSTEMS
System - 1
Ice slurry systems are also known as binary ice.
The ice slurry system is a 'dynamic ice storage system' as
ice is transported around the system in direct contact
with the working fluid, as opposed to the static ice
storage systems previously described.
One method of producing ice slurry is a direct contact
heat exchange method, which is exploited to produce an
ice slurry.
The refrigeration plant is used to cool a heat transfer
fluid down to a temperature below 0oC, and the heat
transfer fluid and the water are brought in direct contact
with each other.
Thermal Storage Two
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SLURRY ICE SYSTEMS
System - 2
This results in the water freezing to form ice slurry,
which floats to the top of the storage tank.
The ice store is discharged by circulating the return
system water from the air conditioning or process plants
through it.
The ice machine consists of a shell and tube heat
exchanger in a vertical orientation with the refrigerant
contained in the outside shell where it evaporates.
Fluid falling through the tubes freezes to form ice
crystals on the tube surface.
Thermal Storage Two
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SLURRY ICE SYSTEMS
System - 3
The ice crystals, continuously removed by an orbital rod, flow
from the bottom of the evaporator into an accumulator vessel
from where the ice slurry is fed into the storage tank.
As there is no heat exchanger surface, and the ice crystals
change phase instantaneously when heated, the cooling rate is
only limited by the temperature rise and flow rate of the chilled
water.
The outlet temperature from the store is claimed to remain
virtually constant until the store is fully depleted.
Thermal Storage Two
54
SLURRY ICE SYSTEMS
System - 4
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.
Thermal Storage Two
55
SLURRY ICE SYSTEMS
System - 5
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.
Thermal Storage Two
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SLURRY ICE SYSTEMS
System - 6
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:
Thermal Storage Two
57
SLURRY ICE SYSTEMS
System - 7
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).
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58
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Thermal Storage Two
59
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 7oC.
The stored chilled water offsets the building refrigeration
load during on-peak hours to shift the load to the offpeak 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.
Thermal Storage Two
60
STRATIFIED CHILLED WATER STORAGE SYSTEMS
System - 2
Chilled Water Storage System
Thermal Storage Two
61
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
m3/kWh compared to about 0.027 m3/kWh for ice storage systems.
Thermal Storage Two
62
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Charging and Discharging - 1
Charging is the process of filling the storage tank with chilled water
from the chiller, usually at a temperature between 4 and 7oC.
Meanwhile, the warmer return chilled water from the air-handling
units or terminals, usually at a temperature between 11 and 16oC, 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 7oC, 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.
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63
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Loss of Cooling Capacity during Storage - 1
During the storage of chilled water, the following processes result
in losses in cooling capacity:
1.
Stored chilled water is warmed by direct mixing of warmer
return chilled water and stored colder chilled water.
2.
Heat from previously stored warmer return chilled water is
transferred from the warmer tank wall to the stored chilled
water.
3.
Heat is transferred through the tank wall from the warmer
ambient air.
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64
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Figure of Merit - 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
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65
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Figure of Merit - 2
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.
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66
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tanks - 1
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.
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67
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tanks - 2
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.
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68
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tanks - 3
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.
Thermal Storage Two
69
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Temperature Gradient and Thermocline - 1
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-andheavier stored chilled water, thermocline, and top warmer-and lighter return
chilled water.
Thermal Storage Two
70
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Temperature Gradient and Thermocline - 2
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.
Thermal Storage Two
71
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Temperature Gradient and Thermocline - 3
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.
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72
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Temperature Gradient and Thermocline - 4
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.
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73
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Diffusers - 1
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.
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74
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Diffusers - 2
Mixing on the inlet side of thermocline depends on the inlet
Reynolds number Rei and Froude number Fri.
The inlet Reynolds number is closely related to the inlet velocity and is
defined as
Re i 
q
vw
Where
q
= Volume flow rate per unit diffuser length (m3/s.m)
vw
= Kinematic viscosity of water (m2/s)
When Rei < 850, loss due to mixing and loss of cooling capacity during
discharge can be significantly reduced.
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75
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Diffusers - 3
The inlet Froude number Fri is defined as
Fri 
q
 3 (i   a ) 
 ghi

i


0.5
Where
q
= Volume flow rate per unit diffuser length (m3/s.m)
g
= Acceleration of gravity (m/s2)
hi
= Inlet opening height (m)
ρ
i
= Density of inlet water (kg/m3)
ρ
a
= Density of ambient water (kg/m3)
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76
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Diffusers - 4
hi 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 hi indicates the vertical
distance between the tank floor and the top of the opening of the diffuser.
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.
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77
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Diffusers - 5
Self-balancing should be achieved on evenly distribution of the flow by:
The piping design should be symmetric.
1.
Branch pipes should be equal in length.
2.
Flow splitters should be added at the appropriate points.
3.
Pipe diameter reduction should be combined with the flow splitter.
4.
Long-radius elbows should be used.
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78
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Storage Tank Insulation - 1
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.
Thermal Storage Two
79
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Charging and Discharging Temperature versus Tank Volume - 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.
Thermal Storage Two
80
STRATIFIED CHILLED WATER STORAGE SYSTEMS
Charging and Discharging Temperature versus Tank Volume - 2
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.
Thermal Storage Two
81