Thermal Storage Systems - One

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MEBS6008 Environmental Services
II http//www.hku.hk/mech/msc-courses/MEBS6008/ind
ex.html
Thermal Storage Systems - One
Ir. Kelvin Tam Department of Mechanical
Engineering The University of Hong Kong
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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

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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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Content

• 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.

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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
  -gt 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 its
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

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
  (Contd)
• 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
  (Contd)

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 buildings
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 utilitys 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 Bulk Tariff Demand Charge
China Light Power Ice Storage Tariff
Demand Charge
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Situations favour the use of Thermal Storage
Systems
China Light Power Ice Storage Tariff
Energy Charge
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 low-temperature 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 8.3C, 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 10C 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 1.7C 4 to 7C
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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-gt
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. gtincreased 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 lt chiller output, surplus
cooling is stored. Load gt chiller output,
additional requirement is discharged from
storage. Most effective Peak cooling load gtgt
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 gt
load-leveling system Demand-limiting system lt
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 gt 255kW (that is,
3060 kWh) from storage. Chiller 255 kW   The
cost of storage lt 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 storagegt load-leveling
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
lt Chiller capacity operates the chiller, up to
its available capacity. Cooling Load lt Chiller
capacity Cooling capacity from storage.
Demand limit Available capacity of chiller lt
maximum capacity.   Load gt the chiller capacity
gt supply temperature gt setpoint gt some flow is
diverted through storage to provide required
additional cooling.
Storage priority control strategy Load lt
discharge rate from storage up to its available
discharge rate. Load gt discharge rate Chiller
operates to meet the remaining load. Storage
discharge rate limit available discharge rate lt
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 gtgt off-peak energy cost, the
use of stored energy should be maximized gt a
storage priority strategy. On-peak energy NOT gtgt
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 gt 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 gt Chiller Capacity is ? tons
Storage capacity is ? ton-hours If plant cost is
4,800/ton gt the saving is ?. If storage cost
is 560/ton hourgt 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 gt
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 ?.
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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-harvest
ing ice storage system Ice slurry system Chilled
water storage Stratified chilled water storage
system
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Question and Answer