MEBS6008 Environmental Services II
http://www.hku.hk/mech/msc-courses/MEBS6008/index.html
Thermal Storage Systems - One
Ir. Kelvin Tam
Department of Mechanical Engineering
The University of Hong Kong
1
The Course
MEBS6008 Environmental services II
Thermal Storage Systems - One
Thermal Storage Systems - Two
Sea Water Heat Rejection System & Heat Recovery Systems
Heat Pump Systems
Acoustic Treatment and Vibration Control - One
 Acoustic Treatment and Vibration Control - Two
Assessment: 100% by examination
2
Review to Environmental Services I
 Your Constructive Feedback (after
lecture) is preferred to any comments
made an the evaluation form.
 Examination of Environmental Services I
(your feedback)
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3
Content
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Why Bother Thermal Storage
Definition of Thermal Storage
Types of Thermal Storage Systems
Situations favour the use of Thermal Storage Systems
Unit of Cold Thermal Storage
Cold Storage Media
Typical Applications of Thermal Storage
Benefits of Cold Thermal Storage
Disadvantages of Cold Thermal Storage System
Operating Strategies
Examples of reduction in equipment size by storage
Chiller priority, storage priority & constant proportion
Systems Schematics
Church Example
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Why Bother Thermal Storage ?

Primary energy source -Hydro,
Gas, Coal and Nuclear fuels
transformed directly into
Electricity as a power source
for industrial and household
appliances.

In principle, electricity
generation has to be balanced
with the exact time of the
consumption to satisfy the
fluctuating demand at the
lowest possible cost.
Thermal Storage - 1
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Why Bother Thermal Storage ?

Fluctuating seasonal and
specific time demands
outside their control and

The essential specific
running time requirement of
electricity generation plants
which do not necessarily
match the demand.

Utility companies generate
electricity using different
types of primary energy
sources to offset peak
demands and a typical UK
electricity generation
pattern.
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Why Bother Thermal Storage ?

Almost every modern society has a
mid-day or late evening peak
electricity demand.

This essential demand force utility
companies to build new additional
peak demand power stations ->
considerable investment that operate
only during peak demand periods and
shut down the rest of the time.

They use expensive primary energy
sources and are subject to the
standard cost of maintenance,
consequently production cost per
kWh is 3-4 times higher than the
standard base load electricity
production cost.
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Definition of Thermal Storage
Thermal storage for HVAC applications
Storage at various temperatures associated
with heating or cooling.
Energy may be charged, stored, and discharged
daily, weekly, annually, or in seasonal or rapid
batch process cycles.
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Types of Thermal Storage Systems
1.
Cold storage
2.
Fabric and slab energy storage
3.
Solar storage
4.
ground source
5.
Packed Rock Beds
6.
Low Temperature CO2 Storage System
7.
Thermochemical Energy Storage
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Types of Thermal Storage Systems
Cool storage
Storage receiving and accumulating cooling capacity output from the
refrigeration plant.
Release cooling capacity to the load at some different time and rate.
Fabric & Slab energy storage
Building materials absorbed heat/ cooling during a particular period
and release it at another period.
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Types of Thermal Storage Systems
Solar Storage
Solar collector along with its associated pump to convert solar radiation
into heat.
The store which receives the heated water from the collector delivers
heated water to the space heating heat exchanger.
May contribute to the building's hot water requirements of between 6%
and 12%.
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Types of Thermal Storage Systems
Ground source
Systems may be closed loop or open loop, and both types typically
take water from a borehole, river or well.
It is required to assess the characteristics of ground sources as this
can vary widely.
Heat pump selection needs to match these characteristics as well the
energy requirements of the building.
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Types of Thermal Storage Systems
Packed Rock Beds
A packed rock bed utilises the available thermal energy by means of
circulating through a packed rock bed to add heat to or remove heat from
the system for charging and discharging respectively.
The energy can be transferred from a fluid but the most common systems
utilise air due to the high heat transfer coefficient between air and rock.
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Types of Thermal Storage Systems
Low Temperature CO2 Storage System
Carbon Dioxide offers the most compact latent heat storage system
due to the commercially obtainable triple point which allows the
utilisation of a single substance as static latent heat of fusion
storage.
Carbon Dioxide can be stored at it‘s triple point of -57 Deg C and 518
kPa with solid fraction of 70-80 % by mass and the system can
provide 140 kJ/kg thermal storage capacity within the required
volume of 166.6 MJ/m3.
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Types of Thermal Storage Systems
Thermochemical Energy Storage
Recent research shows that various alcohols and ketones are
potential thermochemical storage media but due to the relative cost
and complexity, no commercially viable systems have yet emerged.
Typical examples are the mixture of Sulphuric Acid and water, and
alternatively Sodium Hydroxide and water.
Systems in which the water is separated by the heat input to the
mixture and as soon as the two substance are mixed, the chemical
reaction of the substances liberates heat.
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Situations favour the use of Thermal Storage Systems
The storage systems are most likely to be cost-effective in situations : A facility's maximum cooling load is much greater than the
average load; An existing tank is available;
 Limited electric power is available at the site;
 Backup cooling capacity is desirable;
 Loads are of short duration, infrequently, cyclical in nature
 Loads are not well matched to the availability of the energy
source
 Energy costs are time-dependent (e.g., time-of-use energy rates
or demand charges for peak energy consumption)
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Situations favour the use of Thermal Storage Systems
Large daily temperature swing
Variations between day and night
time ambient temperatures reaches
10-15oC
Heat rejection equipment i.e. Air
Cooled Chiller and Air Cooled
Condenser operate more efficiently.
The head pressure (Condensing
Pressure) changes proportionally with
the ambient temperature.
The lower the ambient temperature,
the lower the condensing pressure
Thermal Storage - 1
A typical example of a cooling system operating data
Vs condensing temperature
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Situations favour the use of Thermal Storage Systems
Cold air distribution would be advantageous
It may be adopted in case of ice storage system.
Low temperature distribution systems supply air to the occupied zone at 4oC -
10oC.
Low temperature supply air systems are often used with an ice storage
system to take advantage of the low chilled water temperature.
The supply air temperature achieved depends on the chilled water
temperature and the characteristics of the cooling coil, the supply fan heat
gain, air leakage paths, insulation condition, ductwork length, etc.
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Situations favour the use of Thermal Storage Systems
Cold air distribution would be advantageous (Cont’d)
ASHRAE suggests a differential of 3 to 6K between chilled water supply
temperature to the coil and air temperature leaving the coil.
Leakage from cold air ducts must be considered as this can cause condensation
problems. Air handling units must be insulated from the mixed air section to
the supply air outlet.
As the temperatures involved are lower than the conventional applications, the
performance of the diffusers prevent cold air dumping.
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Situations favour the use of Thermal Storage Systems
Cold air distribution would be advantageous (Cont’d)
Air and water distribution costs can be reduced by 14-19% when the supply air
temperature is reduced from 13oC to 7oC
Decreased floor to floor height requirements due to smaller ducts
Improved comfort due to lower RH in the occupied zone.
Reduced fan energy consumption - reduced air flow rate requires smaller fans. AHU
energy consumption reduced by 20-30%.
Increased cooling capacity for existing distribution systems - an ideal solution where
internal heat gains have increased.
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Situations favour the use of Thermal Storage Systems
An existing cooling system is being expanded;
Any future or additional cooling/heating demand can be easily satisfied by
means of changing the thermal storage strategy for the system.
The additional capacity can be provided by shifting from a full storage to a
partial storage or even weekly storage system depending on the required
additional capacity over the existing capacity limits.
Utility rebates, tax credits, or other economic incentives
are provided for the use of load-shifting equipment
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Situations favour the use of Thermal Storage Systems
 The utility rate structure has high demand charges or a
high differential between on-and off-peak energy rates;
Some electric utilities of foreign countries charge less during the
night or weekend off-peak hours than during the time of highest
electrical demand
Electric rates are normally divided into a demand charge and a
consumption charge.
The monthly demand charge is based on the building’s highest
recorded demand for electricity during the month.
The consumption charge is based on the total measured use of
electricity in kilowatt-hours (kWh) over a longer period and are
generally representative of the utility’s cost of fuel to operate its
generation facilities.
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Situations favour the use of Thermal Storage Systems
In England, the off-peak period is between 12.00 pm and 7.00 am at an
average rate of average 2.68 p/kWh against the standard charge of 7.35
p/kWh.
In the USA, due to the large air conditioning load this structure has been
generally divided into Winter and Summer charges but still offers similar
incentives : -
Winter
Summer
Lower demand
charges
2.75 cents per
kWh
3.40 cents per kWh
Standard charges
5.45 cents per
kWh
6.75 cents per kWh
Off-peak cooling running costs are almost half of those of a conventional system
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Situations favour the use of Thermal Storage Systems
Is there any special Rate offered by
Power Companies on using ice-storage
system in their premises in HONG KONG ?
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Situations favour the use of Thermal Storage Systems
Hong Kong Electric – Maximum Demand Tariff – Demand Charge
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Situations favour the use of Thermal Storage Systems
Hong Kong Electric – Maximum Demand Tariff - Energy Charge
"FCA" means Fuel Clause Adjustment.
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Situations favour the use of Thermal Storage Systems
China Light & Power – Ice Storage Tariff
– Demand Charge
Thermal Storage - 1
China Light & Power – Bulk Tariff
– Demand Charge
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Situations favour the use of Thermal Storage Systems
China Light & Power – Ice Storage Tariff
– Energy Charge
Thermal Storage - 1
China Light & Power – Bulk Tariff
– Energy Charge
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Unit of Cold Thermal Storage
The ton-hour, or ton-h (kWh), is the unit of stored
refrigeration.
One ton-hour is the refrigeration or heat absorption of
12,000 Btu (3.516 kWh) performed by a refrigeration system
during a 1-h period.
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Cold Storage Medium - Chilled water
Chilled-water storage systems
They use the sensible heat capacity of water to store cooling capacity.
They operate at temperature ranges compatible with standard chiller
systems and are most economical for systems greater than 2,000 ton-hours
in capacity.
The capacity of a chilled-water thermal energy storage system is
increased by storing the coldest water possible and by extracting as much
heat from the chilled water as practical (thus raising the temperature of
the return water).
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Cold Storage Medium - Ice
Ice thermal storage systems
They use the latent heat of fusion of water to store cooling capacity.
Storing energy at the temperature of ice requires refrigeration equipment that
can cool the charging fluid (typically, a water/glycol mixture) to temperatures
below the normal operating range of conventional air-conditioning equipment.
Special ice-making equipment or standard chillers modified for low temperature
service are used.
The low temperatures of the chilled-water supply allow the use of lowtemperature air distribution, meaning smaller fans and ducts are needed.
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Cold Storage Medium-Eutectic salts.
Eutectic salts
They are also known as phase-change materials.
They use a combination of inorganic salts, water, and other elements to
create a mixture that freezes at a desired temperature.
The material is encapsulated in plastic containers that are stacked in a
storage tank through which water is circulated.
The most commonly used mixture for thermal storage freezes at 8.3°C,
which allows the use of standard chilling equipment to charge storage.
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Cold Storage Medium – a Comparison
Chilled water systems
They require the largest tanks, but they can easily interface
with existing chiller systems.
Ice systems
They use smaller tanks and offer the potential for the use of
low- temperature air systems, but they require more complex
chiller systems.
Eutectic salts
They can use existing chillers but usually operate at the warmest
temperatures.
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Cold Storage Medium – a Comparison
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Cold Storage Medium – a Comparison
Chilled water
Ice
Temperature
difference of 10°C
2.2 kg of chilled
water can store 19
kJ of thermal energy
2.2 kg of ice can
store 178 kJ
Density
997 kg/m3
920 kg/m3
Storage volume
1
0.12
Chilled water supply
temperature
1.1 to 1.7°C
4 to 7°C
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Typical Applications of Thermal Storage
Churches, Sports Facilities, Horse racing, Coliseum, theatres
1.
The load is short in duration and there is a long time between
load occurrences,
2.
They have a relatively large space-conditioning load for fewer
than 6 hour per day and only a few days per week.
3.
The relatively small refrigeration plant for these applications
would operate continuously for up to 100 h or more to
recharge the thermal storage.
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Typical Applications of Thermal Storage
Industrial Process - food processing, dairy, brewery, processing
and gas turbine air inlet gas cooling for industrial applications.
Bakeries, 10 to 15 minutes of cooling every 2.5 hours to
stop yeast fermentation
Tire manufacture 2 minutes of cooling every 15 minutes to
stop a vulcanizing process
Dairies, 6 hours of cooling every 24 hours to cool milk
after pasteurization.
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Benefits of Cold Thermal Storage
Reduced Equipment Size
Equipment can be downsized to meet an average load rather than the peak
load.
Chillers for thermal storage applications are generally 30-60% smaller than
the conventional system chillers (longer running periods and large latent
heat storage capacity).
Chiller(s) run most of their expected life span running at full load during
the charging mode and supplement the operation for partial storage
strategy.
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Benefits of Cold Thermal Storage
Capital Cost Savings
Due to equipment downsizing and utility cash incentive programs.
Downsizing cooling equipment offset the cost of the storage.
Cool storage integrated with low-temperature air and water distribution
systems provide an initial cost savings (smaller chillers, pumps, piping, ducts, and
fans.)
For systems having heating or cooling peak loads of extremely short duration.
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Benefits of Cold Thermal Storage
Energy Cost Savings
The significant reduction of time-dependent energy costs such as electric demand
charges and on-peak time-of-use energy charges
Energy Savings
Chillers operate more at night with lower condensing temperatures-> improve
efficiency
Operation of equipment at full-load, avoiding inefficient part-load performance (may
reduce annual energy consumption by up to 12%)
Improved HVAC Operation
Decoupling of the thermal load profile from the operation of the equipment.
=>increased flexibility, reliability, or backup capacity for the control and operation.
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Benefits of Cold Thermal Storage
Full Stand-by Capacity
The stored thermal energy can provide reasonable safety periods for any
regular and/or emergency repair works without disturbing the system.
Full Stand-by capacity becomes quite essential for industrial and continuous
space conditioning applications.
Allow Free Cooling
In a climate where the night ambient temperature drops below the thermal
storage temperature, the storage system can be charged by means of free
cooling.
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Disadvantage of Cold Thermal Storage System
Distribution and storage vessel thermal losses that would not occur with a
conventional system - pumping to both charge and discharge the store.
Operation of chiller plant to produce ice requires a chiller capable of
depressing its evaporating temperature to say, -6oC as opposed to the
+6oC with conventional chiller plant. This reduces the chiller coefficient of
performance (COP).
Ice storage systems use 15% more energy than conventional plant due to
the lower operating COP and additional pumping energy requirements.
CIBSE Technical Memorandum states that the efficiency of ice storage
relative to producing chilled water at 5oC is around 85% to 90%.
Inevitable heat loss in pipework and storage tank.
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Full-storage operating strategy
A full-storage, or load-shifting, strategy shifts the entire on-peak cooling load to
off-peak hours.
This strategy is most attractive where on-peak demand charges are high or the
on-peak period is short.
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Partial storage load: Load-leveling operating strategy
Chiller runs at its full capacity for 24 hours on the design day.
Load < chiller output, surplus cooling is stored.
Load > chiller output, additional requirement is discharged from storage.
Most effective : Peak cooling load >> the average load.
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Partial-storage load: demand-limiting operating strategy
The chiller runs at reduced capacity during on-peak hours
Controlled to limit the facility's peak demand charge.
Demand savings and equipment costs :
Demand-limiting system > load-leveling system
Demand-limiting system < full-storage system.
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Example of reduction in equipment size –
Non-storage System
kWH from 6am to 6pm = 6120 kWh
Storage :
Chiller Capacity: 660 kW (peak
demand).
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Example of reduction in equipment size –
load-leveling partial storage system
The design-day cooling load >
255kW (that is, 3060 kWh)
from storage.
Chiller :255 kW
The cost of storage < saving
by downsizing equipment (i.e.
initial cost).
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Example of reduction in equipment size –
full storage system
The entire peak load from storage.
Chiller: 360 kW
Initial cost of full storage> loadleveling system, full storage offers
large reduction on operating costs
(demand shifted to off-peak
period).
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Chiller priority, storage priority & constant proportion
Chiller priority control strategy
Cooling Load < Chiller capacity : operates the chiller, up to its available capacity.
Cooling Load < Chiller capacity : Cooling capacity from storage.
Demand limit : Available capacity of chiller < maximum capacity.
Load > the chiller capacity => supply temperature > setpoint => some flow is diverted
through storage to provide required additional cooling.
Storage priority control strategy
Load < discharge rate: from storage up to its available discharge rate.
Load > discharge rate: Chiller operates to meet the remaining load.
Storage discharge rate limit : available discharge rate < maximum discharge rate.
Ensure storage not depleted too early in the discharge cycle. Else loss of control
of the building or excessive demand charges or both. Need correct load
forecasting.
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Chiller priority, storage priority & constant proportion
A constant proportion control strategy
It divides the load between chiller and storage.
Load : divided equally or in some other proportion.
The proportion may change with time in response to changing conditions.
A limit on chiller demand or storage discharge may be applied.
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Chiller priority, storage priority & constant proportion
On-peak energy cost >> off-peak energy cost, the use of
stored energy should be maximized => a storage priority
strategy.
On-peak energy NOT >> off-peak energy, a chiller priority
strategy
If demand charges are high, some type of demand-limiting
control should be implemented.
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Refrigeration Design and Thermal Storage
Chiller operates at a greater percentage of the operating hours at lower
ambient temperatures: Need a condensing temperature that maintains
compressor differential.
The lower suction temperature necessary for making ice imposes a higher
compression ratio on the refrigeration equipment.
Positive displacement compressors (e.g., reciprocating, screw, and scroll
compressors) are usually better suited to these higher compression ratios than
centrifugal compressors.
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Cooling Load and Cold Storage
2.5% design temperatures would be used for a non-storage design, the 1% values
are recommended for a cool storage design => full storage system can fall back
to partial chiller operation if design loads exceeded.
Load profiles must be calculated for the entire design charge-discharge cycle.
The most common cycle is 24 h long (Weekly cycles in some situations)
Calculation of the design load profile requires accurate estimation of schedules
of occupancy, lighting, and equipment use.
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Cooling Load and Cold Storage
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Systems Schematics
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Church Example
Load Profile
Church operates for 3 hours on Sunday morning. Load steady for each hour.
Instantaneous peak hour load of 40 ton. Chiller capacity at 40 ton if no storage.
The integrated cycle of cooling load is 40 Tons x 3 = 120 ton-hours.
Day Cycle with Partial Storage
Plant operates 24 hours => Chiller Capacity is ? tons
Storage capacity is ? ton-hours
If plant cost is $4,800/ton => the saving is $?.
If storage cost is $560/ton hour=> storage is $?
Cost saving = $?.
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Church Example
Day Cycle with Full Storage - 3 hour load period was the on-peak period
Plant operates 21 hours => Chiller Capacity is ? tons
Storage capacity is ? ton-hours
Storage requirement increases by ? tons
As plant cost is $4,800/ton and storage cost is $560/ton-hour
Increase in storage capacity comparing with partial storage is ? tons
The increase in plant capacity comparing with partial storage is ? ton
Total increase in cost in comparison with partial storage = $?.
Church Example - Weekly Cycle
Church Example -Weekly cycle- Partial storage plant
Operates for 168 hours at ? ton
Storage capacity = ? ton-hours
Church Example -Weekly cycle- Full storage plant
The plant Capacity = ? ton
Storage = ? ton-hours
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Church Example - Conclusion
Advantages learnt from this church example

This plant can have reserve to meet any expansion of the load

There is reserve to handle an error in the original load
calculation.

By operating on weekly cycle, there is no need for owners and
operators to meet longer hours of operation by operating
cooling equipment for a longer time. The day cycle can at least
meet a load of 120 ton-hours in each day.
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Next Lecture
Ice storage and chilled water storage systems
Typical ice storage and chilled water storage systems are as follows:Ice storage
Static Ice Production Systems
Ice-on-coil, internal-melt ice storage system
Ice-on-coil, external-melt ice storage system
Encapsulated ice storage system
Dynamic Ice Production Systems
Ice-harvesting ice storage system
Ice slurry system
Chilled water storage
Stratified chilled water storage system
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Question and Answer
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