MEBS 6008
Thermal Storage - I
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Definition of Thermal Storage
Thermal storage for HVAC applications can involve 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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Cool storage
is associated storage receiving and accumulating cooling capacity
output from the refrigeration plant, and the release cooling capacity
to the load at some different time and rate.
High temperature storage
is associated with solar energy or high-temperature heating
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Thermal storage may be an economically attractive approach to meeting
heating or cooling loads if one or more of the following conditions apply:
1.
Loads are of short duration
2.
Loads occur infrequently
3.
Loads are cyclical in nature
4.
Loads are not well matched to the availability of the energy source
5.
Energy costs are time-dependent (e.g., time-of-use energy rates or
demand charges for peak energy consumption)
6.
Utility rebates, tax credits, or other economic incentives are
provided for the use of load-shifting equipment
7.
Energy supply from the utility is limited, thus preventing the use of
full-size nonstorage systems
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Types of Some Thermal Storage Systems
1.
2.
3.
4.
5.
6.
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Ice storage
Warm/cool water store
Fabric energy storage
Embedded pipework slab heating and
cooling
Solar storage
ground source.
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Fabric energy storage
Fabric storage technologies, such as slab cooling passing
air over the slab surface and via hollow slabs for limiting
internal temperatures.
Peak temperatures can be reduced and, in combination
with other passive measures such as solar shading to
provide comfortable conditions.
Slab heating and cooling using embedded pipework
Under floor heating and cooling by embedded pipework
system can provide comfortable conditions in both heating
and cooling modes.
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Active 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%.
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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Situations favorable to the adoption of cold thermal storage system
Large daily temperature swing
This is difference between daily max. DB temp. to that of daily min.
(Compare Hong Kong and USA )
Large difference between off-peak energy charge and on-peak
energy charge
Compare HK - CLP & HKE and USA
Large amount in demand charge
What is demand charge?
Small in ratio of Day-time cooling-demanding hours against nighttime ice-making hours
That is, ice is made during night at low energy charge
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Typical Applications of Thermal Storage
Churches, Sports Facilities, Horse racing, Coliseum
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
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 Thermal Storage
Reduced Equipment Size
If thermal storage is used to meet all or a portion of peak heating or
cooling loads, equipment can be downsized to meet an average load rather
than the peak load.
Capital Cost Savings
Capital savings can result both from equipment downsizing and from certain
utility cash incentive programs.
Even in the absence of utility cash incentives, the savings from downsizing
cooling equipment can offset the cost of the storage.
Cool storage integrated with low-temperature air and water distribution
systems can also provide an initial cost savings due to the use of smaller
chillers, pumps, piping, ducts, and fans.
Storage has the potential to provide capital savings for systems having
heating or cooling peak loads of extremely short duration.
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Benefits of Thermal Storage (Cont’d)
Energy Cost Savings
The significant reduction of time-dependent energy costs such as electric
demand charges and on-peak time-of-use energy charges is a major economic
incentive for the use of thermal storage.
Energy Savings
Cool storage systems permit chillers to operate more at night when lower
condensing temperatures improve equipment efficiency
Storage permits the operation of equipment at full-load, avoiding inefficient
part-load performance. Chilled water storage installations may reduce annual
energy consumption for air conditioning by up to 12%
Improved HVAC Operation
Storage adds an element of thermal capacitance to a heating or cooling
system, allowing the decoupling of the thermal load profile from the
operation of the equipment. This decoupling can be used to provide increased
flexibility, reliability, or backup capacity for the control and operation of the
system.
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Disadvantage of Cold Thermal Storage System
1.
There will be distribution and storage vessel thermal
losses that would not occur with a conventional system pumping to both charge and discharge the store.
2.
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).
3.
Ice storage systems use 15% more energy than
conventional plant due to the lower operating COP and
additional pumping energy requirements.
4.
CIBSE Technical Memorandum states that the efficiency
of ice storage relative to producing chilled water at 5oC is
around 85% to 90%.
5.
There would be inevitable heat loss in pipework and
storage tank.
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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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Church Example
Load Profile
The church operates for 3 hours on Sunday morning with a calculated
instantaneous peak hour load of 40 ton. The load is steady for each of the 3
hours.
The integrated cycle of cooling load is 120 ton-hours.
In a conventional non-storage system, the plant would need to have a
capacity to meet the instantaneous design load of 40 ton.
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Church Example – A summary of plant capacity
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Church Example -Day Cycle
Using a day or 24 hour cycle and a partial storage mode
The plant operates for the full cycle time of 24 hours, and the plant capacity
reduces from the instantaneous load of 40 ton to 5 ton.
The storage capacity in the partial storage mode is the integrated cycle capacity
of 120 ton hours less the plant contribution of 15 ton-hours during the load
period, or a net of 105 ton-hours.
Assuming that the 3 hour load period was the on-peak period, shifting to a full
storage mode reduces the plant output time by three hours.
In full storage the plant operates for 21 hours, requiring a capacity of 5.71 ton
to meet the integrated cycle load of 120 ton hours.
The storage increases to the full capacity of 120 ton-hours since the plant would
no longer contribute during the on-peak period.
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Church Example -Day Cycle
As a first alternative, compare the day-cycle partial storage to the non-storage.
Day cycle partial storage affords a plant reduction of 35 ton or 87.5%.
If plant cost is US$600 per ton, the saving is US$21,000.
This saving is partially offset by the cost of providing the storage. If the cost
of the storage is US$70 per ton hour, the storage cost is US$8400 yielding a
net first cost saving of US$12,600
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Church Example -Day Cycle
For the second alternative, consider changing from day-cycle partial storage to
day cycle full storage.
In full storage, the load is not shared, storage requirement increases by 15 tons
and the plant increases by 0.71 ton. Using the same per-unit costs as the first
alternate, there is a first cost increase of US$1476.
This additional capital expense would have to be supported entirely by the demand
cost savings that could be realized by avoiding the electrical demand equivalent to
the operation of the 5 ton plant in the "on-peak" period.
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Church Example - Weekly Cycle
Extending the church example to weekly cycle, the partial storage plant then
operates for 168 hours at 0.71 ton to produce the integrated cycle load of 120
ton-hours.
The smaller plant contributes less during the load period.
Weekly cycle, therefore, increases the partial storage capacity to 117.8 tonhours.
For weekly cycle full storage, there is little change in the plant and the storage,
with the plant increasing to 0.73 ton and the storage going back to the full 120
ton-hours.
The weekly cycle plant, with a capacity of 0.71 ton capacity is only capable of
producing the integrated cycle load, or weekly load of 120 ton-hours.
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Church Example - Conclusion
This plant has no reserve to meet any expansion of the load, or as is often
the case, no reserve to handle an error in the original load calculation.
Owners and operators are accustomed to meeting longer hours of operation
simply by operating cooling equipment for a longer time.
In the church example, using the minimum chiller on weekly cycle completely
eliminates this reserve.
The day cycle alternative, on the other hand, can at least meet a load of
120 ton-hours in each day.
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Ice Storage and Chilled Water Storage
Two thermal storage media are widely used for air conditioning
systems: ice storage and chilled water storage.
At a temperature difference of 10°C, 2.2 kg of chilled water can
store 19 kJ of thermal energy, whereas 2.2 kg of ice can store 178
kJ.
For the same stored cooling capacity, the storage volume for ice is
only about 0.12 that of the chilled water.
Note the density of water is 997 kg/m3 and the density of ice is
920 kg/m3.
In addition, ice storage systems generally provide chilled water at
a temperature of 1.1 to 1.7°C to produce cold supply air between
5.6 and 9.4°C.
For chilled water storage system, the chilled water is typically
supplied at discharge temperature between 4 to 7°C
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Electricity Tariff
Some electric utilities of foreign countries charge less during the night or
weekend off-peak hours than during the time of highest electrical demand,
which often occur on hot summer afternoons due to air-conditioning use.
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.
In some cases, the consumption charge is lower during off-peak hours because a
higher proportion of the electricity is generated by baseload plants that are
less expensive to operate. Rates that reflect this difference are known as timeof-use billing structures.
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Electricity Tariff
The annual operating cost of each system being considered must
be estimated, including both electrical demand and consumption
costs to compare the costs of different systems,.
To determine demand cost, the monthly peak demand for each
system is multiplied by the demand charge and totaled for the
year.
The electrical consumption cost is determined by totaling the
annual energy use for each system in kilowatt-hours and
multiplying it by the cost per kilowatt-hour.
For time-of-use billing, energy use must be classified by time-ofuse period and multiplied by the corresponding rate.
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Electricity Tariff
In foreign countries (e.g. USA), there are certainly
incentive in respect of different time charges that makes
cold thermal storage favorable for consideration.
Is there any special Rate offered by Power Companies on
using ice-storage system in their premises ?
The answer is currently yes for the Hongkong Electric but
not for China Light and Power.
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Hongkong Electric Co. – Maximum Demand Tariff
Monthly Tariff Charges for the Supply
Demand Charge (a minimum of 100 kVA )
For each of the first 400 kVA of
in the month maximum demand in the month
$42.1/kVA
For each of the next additional kVA
in the month of maximum demand in the month
$41.1/kVA
Energy Charge (Monthly consumption)
For each of the first 200 kWh supplied per month per kVA of maximum
demand (subject to a minimum of 100 kVA) in the month is charged at
$1.023/kWh
For each additional kWh supplied in the month is charged at $0.962/ kWh
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Hongkong Electric Co. – Commercial, Industrial and Miscell.Tariff
Block Rate Tariff
For each of the first 1500kWh supplied per month, the Net Rate
is $1.093/ kWh
From 1,501kWh and above, the Net Rate is $1.179/kWh
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China Light and Power
Tariff for Customer who have installed ice-storage air conditioning systems in
their premises
Monthly max. demand in kVA
On-peak first 650kVA at $66.5 per kVA
On-peak kVA over 650 at $63.5 per kVA
Off-peak kVA up to the on-peak demand is $0.0 per kVA
Off-peak kVA in excess of the on-peak demand is $26.0 per kVA
Off-Peak period (9:00pm to 9:00am)
Energy charge
On peak - $0.689 per kWh for the first 200,000 kWh and $0.674
afterwards
Off peak - $0.614 per kWh
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Full Storage System
All refrigeration compressors cease to operate during on-peak hours,
and the building refrigeration load during that period is entirely
offset by the chilled water supplied from the thermal storage tank
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Partial Storage System – 100% Chillers in Operation
Partial storage, or load leveling (in load-leveling mode) in which refrigeration
compressors are operated at full capacity during on-peak hours
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Partial Storage System – 50% Chillers in Operation
Partial storage, or load leveling (in demand-limited mode) in which building electric
demand limits only part of the refrigeration compressors operated
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Low temperature air distribution and ice storage - part 1
Low temperature distribution systems supply air to the occupied
zone at between 4oC and 10oC, in contrast to most conventional air
distribution systems, which supply at between approximately 14oC
and 18oC.
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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Low temperature air distribution and ice storage - part 2
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 in conventional
applications, the performance of the diffusers must be assessed
accordingly to prevent cold air dumping.
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Low temperature air distribution and ice storage - part 3
The use of cold air distribution technology has a number of benefits including:
Reduced mechanical system costs - smaller air handling units, ducts, pumps,
and coils can be used to achieve the same cooling to the space.
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 relative humidity in the occupied zone.
The lower supply air temperature reduces supply air moisture content which
reduces relative humidity in the occupied area
Reduced fan energy consumption - reduced air flow rate requires smaller fans.
AHU energy consumption can be reduced by 20-30%.
Increased cooling capacity for existing distribution systems - this is an ideal
solution where internal heat gains have increased.
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Example of reduction in equipment size – Non-storage System
If the 6120 kWh load is met by a non-storage air-conditioning system,
a 660 kW chiller is required to meet the peak cooling demand.
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Example of reduction in equipment size – load-leveling partial storage system
An 255 kW chiller meets the demand. The design-day cooling load in excess of
the chiller output (3060 kWh) is supplied by the storage.
The cost of storage approximates the amount saved by downsizing the chiller,
cooling tower, electrical service, etc.
Load-leveling partial storage is often competitive with non-storage systems on
an initial cost basis.
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Example of reduction in equipment size – full storage system
The entire peak load is shifted to the storage. A 360 kW chiller is required.
The size of the chiller equipment may be reduced, but the total equipment
cost including the storage is usually higher for the full storage system than
for non-storage systems.
Although the initial cost is higher than for the load-leveling system, full
storage offers large reduction on operating costs because the entire chiller
demand is shifted to the off-peak period.
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Control Strategies
A thermal storage control strategy defines how the system is controlled in
a specific operating mode. The control strategy defines what equipment is
running and the actions of individual control loops, including the values of
their setpoints, in response to changes in load or other variables.
Operating Mode
Charging Storage
For Cool Storage
Operating cooling equipment to remove
heat from storage
Charging Storage while
Operating cooling equipment to remove
meeting loads
heat from storage and meet loads
Meeting loads from discharging Discharge (adding heat to) storage to
storage only
meet loads without operating cooling
equipment
Meeting loads from discharging Discharging (adding heat to ) storage
storage and direct equipment and operating cooling equipment to meet
operation
loads
Meeting loads from direct
Operating cooling equipment to meet
equipment operation only
loads (no fluid flow to or from storage)
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Control Strategies One
Charging the Storage
Control strategies for charging are generally easily defined.
Typically the generation equipment operates at full capacity with a constant
supply temperature setpoint and a constant flow through the storage.
This operation continues until the storage is fully charged or the period
available for charging has ended.
Under this basic charging control strategy, the entire capacity of the
equipment is applied to charging storage.
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Control Strategies Two
Charging Storage while Meeting Load
A control strategy for charging storage while meeting load may
also operate the generation equipment at its maximum capacity.
The capacity that is not needed to meet the load is applied to
charging storage.
Depending on the system design, the load may be piped either in
series or parallel with storage under this operating mode.
Some systems may have specific requirements for the operating
strategy in this mode.
For example, in an ice storage system with a heat exchanger
between glycol and water loops, the control strategy may have to
address freeze protection for the heat exchanger.
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Control Strategies Three
Meeting Load from Discharging Only.
A control strategy by discharging only (full storage or load shifting
operation) is also straightforward.
The generating equipment does not operate and the entire load is met from
storage.
Control Strategies Four
Meeting Load from Discharging & Direct Equipment Operation
These strategies must regulate what portion of the load at any time will be
met from storage and what proportion will be met from direct generation.
These partial storage strategies have been mostly developed for and applied
to cool storage. Three common control strategies are chiller priority, storage
priority, and constant proportion or proportional.
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A chiller priority control strategy operates the chiller, up to its available
capacity, to meet loads.
Cooling loads in excess of the chiller capacity are met from storage. If a
chiller demand limit is in place, the available capacity of the chiller is less
than the maximum capacity.
Chiller priority control can be implemented with any storage configuration.
However, it is most commonly applied with the chiller in series upstream of
storage.
A simple method of implementing chiller priority control is to set the chiller
setpoint and the temperature downstream of storage to the desired chilled
water supply temperature.
When the load exceeds the chiller capacity, the supply temperature exceeds
the setpoint, and some flow is diverted through storage to provide the
required additional cooling.
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A storage priority control strategy meets the load from storage up to its
available discharge rate.
If the load exceeds this discharge rate, the chiller operates to meet the
remaining load.
If a storage discharge rate limit is in place, the available discharge rate is less
than the maximum discharge rate.
A storage priority strategy must ensure that storage is not depleted too early
in the discharge cycle.
Failure to properly limit the discharge rate could cause loss of control of the
building or excessive demand charges or both. Load forecasting is required to
maximize the benefits of storage-priority control.
Simpler storage-priority strategies using constant discharge rates, or
predetermined discharge rate schedules have been used.
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A constant proportion or proportional control strategy divides the load
between chiller and storage.
The load may be 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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Demand-limiting control
It may be applied to any of the above control strategies (chiller priority, storage
priority, and constant proportion or proportional).
This type of control attempts to limit the facility demand either by setting a
maximum capacity above which the chiller is not allowed to operate or by
modulating the chiller set-point.
The demand limit may be a constant value or it may change with time in response
to changing conditions.
Demand limiting is most effective when the chiller capacity is controlled in
response to the facility demand at the billing meter. In such cases, the chiller
capacity is controlled to keep the total demand from exceeding a predetermined
facility demand limit.
A simpler approach, which generally achieves lower demand savings, is simply to
limit chiller capacity or the chiller’s electric demand without considering the total
facility demand.
Operating strategies that seek to optimize system operation often recalculate
the demand limit and discharge limit on a regular basis during the discharge
period.
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Electricity Tariff
Applicable utility rates and system efficiency in various operating modes
determine the selection of a control strategy.
If on-peak energy cost is significantly higher than off-peak energy cost,
the use of stored energy should be maximized and a storage priority
strategy is appropriate.
If on-peak energy is not significantly more expensive than off-peak
energy, a chiller priority strategy is more appropriate.
If demand charges are high, some type of demand-limiting control should
be implemented.
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Refrigeration Design
For the most part, chillers in chilled water storage systems operate at
conditions similar to those for non-storage applications.
However, a greater percentage of the operating hours occur at lower ambient
temperatures; special consideration should thus be given to providing 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 - 1
Cool storage systems generally have less capacity to recover than non-storage
systems if design loads are exceeded.
For example, in an application for which the 2.5% design temperatures would
be used for a non-storage design, the 1% values are recommended for a cool
storage design.
Designers may elect to use less extreme design weather conditions for full
storage systems, since a full storage system can fall back to partial chiller
operation if design loads are exceeded.
Load profiles must be calculated for the entire design charge-discharge cycle
of the cool storage system.
The most common cycle is 24 h long, but weekly cycles are also applied when
appropriate. Longer or shorter cycles are also possible for certain
applications.
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Cooling Load and Cold Storage - 2
Calculation of the design load profile requires accurate estimation of schedules
of occupancy, lighting, and equipment use schedules.
It is important not to neglect relatively small heat gains that are present for
the entire occupied period or even the entire day. Such loads may be a small
part of the peak hourly load, but a significant portion of the integrated daily
load.
Personal computer equipment represents a particularly important heat gain. It
was found that that even though actual equipment heat gains were generally
less than 50% of nameplate power ratings, a gain of 19 to 27 W/m2 could be
expected in a computerized office.
If computers are left on during unoccupied periods, their heat gain must be
included in the load profile. Turning computers off or using automatic power
reduction is recommended for successful cool storage operations.
If supply air temperatures are to be reduced, latent heat gains due to
infiltration should be calculated based on the expected space relative humidity.
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Ice storage and chilled water storage systems
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
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To be continued in the next lecture………………………..
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