01 thermal energy storage using ice slurry

Wahid H. Mohamed BSc, LEED AP BD+C
,

Mechanical Project Engineer

TECHNICAL SUPPORT SERVICE DIVISION
MECHANICAL SECTION

Thermal Energy Storage
Ice Slurry

Thermal Energy Storage Using Ice Slurry

CONTENTS:
 Factors Favoring Thermal Energy Storage
 Attractive Prospects
 Thermal Energy Storage Introduction
o Sensible Energy Storage
o Latent Cool Storage Technology and it’s Devices
o Ice Slurry Definition
o Ice Slurry System Schematics
o Ice Slurry Heat Transfer Benefits
o Ways To Produce Ice Slurry
o Advantages of Ice Slurry compared to Solid or Flake Ice
 Typical Applications
 District Cooling Systems
 Ice Storage Strategies
o Partial Storage System
o Full Storage System
 Case Study
 Manufacturers
 References

Factors Favoring Thermal Energy Storage
Thermal storage systems offer building owners the potential for substantial operating cost savings by
using off-peak electricity to produce chilled water or ice for use in cooling during peak-hours. The
storage systems are most likely and particularly attractive to be cost-effective in situations where one
or more of the following conditions are present:
•

New Investment in Chiller Plant Required
Thermal storage systems generally require smaller refrigerating equipment than non-storage
systems. Air and water distribution equipment may also be smaller, and electrical primary
power distribution requirements may be significantly smaller.

•

Peak Load Higher Than Average Load
Thermal storage can provide significant equipment size reductions and cost savings for
applications with high peak load.

•

Favorable Utility Rates
On-peak demand and energy charges are higher than off-peak charges, reflecting the utility
company’s increased cost of meeting high daytime power demands High demand charges and
ratcheted rates, as well as large differences between on-peak and off-peak energy charges,
favor the use of cool storage to reduce operating costs.

•

Back-Up or Redundant Cooling Desirable
Thermal storage can provide cooling if the emergency back-up power supply is sized to run
just the pumps. Thermal storage can also reduce the size of a full back-up power supply,
because of the reduced power of the thermal storage system compared to a traditional full
chiller system.
Thermal storage in combination with conventional chiller(s) can provide a larger portion of the
daytime cooling required if one chiller goes down.

•

Cold-Air Distribution Desirable
Allows smaller fans and ducts to cool the spaces, and the system can maintain a lower space
humidity.

•

Environmental Benefits Sought
Thermal storage provides the opportunity to run a heating or cooling plant at its peak efficiency
during 100% of its operating period, for a much higher dynamic operating efficiency, because
equipment in a non-storage system has to follow the building load profile, the majority of its
operation is at part-load conditions, which, for most systems, is much less efficient.
Most thermal energy storage systems reduce the size of refrigerating equipment needed.
In addition to the potential reduction in energy use at the building site, thermal storage
systems can reduce source energy use.
Thermal storage also has beneficial effects on combined heat and power systems by matching
the thermal and electric load profiles.
The system awards points for various sustainable design features, including reductions in
annual energy cost compared to a standard reference building. Thermal storage is a good way
for a building to reduce energy cost and comply with the LEED rating system.

PREPARED BY: WAHID H.MOHAMED

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−

Real reasons Off-Peak Cooling is Green:

1. It is much more Energy Efficient to create and deliver a kWh of Electricity at night than
during the hot of the day.
• Research from the California Energy Commission on 2 Cal. Utilities reports 8 to 34%
savings in raw fuel when comparing On and Off Peak Operation.
• Heat Rates for Base Load Plants ~7,800 Btu/kWh vs.
Peaking Plants ~9,400 to 14,000 Btu/kWh
2. The last power plants to come on during peak hours are normally the dirtiest per kW
• Ashok Gupta (Director of Energy, NRDC) in NY Times article “Peak Shifting results in
lower emissions because some of the plants used to meet demand peaks are among
the dirtiest in the city”
• New CA Report by Greg Kats The Costs and Financial Benefits of Green Buildings
states Peak power in CA is twice as dirty as Off Peak Power.
Attractive Prospects
•

Electricity energy charges vary significantly during the course of a day.

•

Electricity demand charges are high or ratcheted.

•

The average cooling load is significantly less than the peak cooling load.

•

The electric utility offers other incentives (besides the rate structure) for installing cool storage.

•

An existing cooling system is expanded.

•

There is new construction.

•

Older cooling equipment needs replacing.

Thermal Energy Storage Introduction
Sensible Energy Storage
Water is well suited for both hot and cold sensible energy storage applications and is the most
common sensible storage medium, in part because it has the highest specific heat [4.18
kJ/(kg·K)] of all common materials.
Latent Cool Storage Technology and it’s Devices
Latent cool storage systems achieve most of their capacity from the latent heat of fusion of a
phase-change material, although sensible heat contributes significantly to many designs. The
high energy density of latent storage systems allows compact installations and makes factorymanufactured components and systems practical.
Latent storage devices are available in a wide variety of distinct technologies:
Internal-melt ice-on-coil,

External-melt ice-on-coil,

Encapsulated,

Ice harvester, and

Ice slurry.


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Ice Slurry Definition:
Ice slurry is a suspension of ice crystals in liquid. An ice slurry system has the advantage of
separating production of ice from its storage without the control complexity and efficiency
losses.
Slurries also offer the possibility of increased energy transport density by circulating the slurry
itself, rather than just the circulating secondary liquid. However, a heat exchanger is usually
used to separate the storage tank flow from the cooling load distribution loop.
Slurry systems have very high discharge rates and provide coolant consistently close to the
phase-change temperature. In general, the working fluid’s liquid state consists of a solvent
(water) and a solute such as glycol, depresses the freezing point of the water and buffers the
production of ice crystals.
The result is that smaller refrigeration systems (chillers) can be used, usually with a capacity
of 20–50% of the peak cooling load. Additionally, operational savings can be made where offpeak electricity tariffs are available.
The orbital rod evaporators are typically arranged in vertical banks above the storage tank,
and the slurry can also be pumped into an adjoining tank.
An ice slurry-based air conditioning system usually employs three independent circuits:
−

A refrigerant circuit,


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−

An ice slurry circuit between the ice slurry generator, the storage tank & a heat
exchanger and

−

A chilled water circuit between the ice slurry system and the load.

Figure below illustrates the components of an Ice Slurry Generator System consisting of:


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Slurry System Schematics:
The ice slurry generator (ISG) has been installed in several different configurations, some of
those illustrated below.

The supply water from the base chiller is diverted around the plate heat exchanger (PHE) at night and the ice slurry
generator is building and storing ice. The following day, the supply water from the base chiller is provided to the PHE at
a temperature above the desired supply temperature, and the ice solution provides the additional drop in temperature
required by the load. Note that the efficiency of the base chiller improves as the supply temperature increases.

The base chiller can meet night loads while ice is being made and stored. The following day the stored ice can be used to
provide low temperature supply water to the load with only the base chiller running or both could run under high load
conditions. The base chiller would operate at high supply temperatures and, therefore, at high efficiency.


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This system provides efficient operation of both the base chiller and the welded plate chiller. The ice storage tank is used
to drop the supply water temperature of the chillers to the desired value; therefore, the chillers operate more efficiently
and at greater capacity. The PHE could be at the central plant or at each building in a district cooling system. This
configuration results in very high efficiency of operation for the chillers and may show both a first cost savings, and
operating cost savings, when compared to a conventional chiller system.
Ice Slurry Heat Transfer Benefits:
The ice slurry plant was found to have higher energy consumption than the conventional plant,
but hydronic and air distribution costs were found to be lower for the ice slurry plant.
Simple rules of thumb were used to establish the size of the cool storage system required to
reduce peak electricity demand by either 5% or 10% as savings in the overall energy
consumption of the building.
The important benefit of ice slurry is the increased “cooling” capacity compared with that with
conventionally chilled water system at 6 deg. C.
In conventional chilled water systems, the enthalpy difference of water between 6 and 12 deg.
C results in a cooling capacity of about 30 kJ/kg, so large volumes of water need to be
pumped for a given load.
The use of ice slurry significantly decreases the volumetric flow requirements.
The latent heat carried by ice particles in the water adds great cooling capacity to the flow.
The additional cooling capacity of ice slurry relative to conventional chilled water system at 6
deg C and water at freezing point is shown in Figure A. For example, compared with chilled
water with supply/return temperatures of 6/12 deg C, the cooling capacity of ice slurry
operation at 0/13 deg C with ice fraction of 20% equals to 144kj/kg (4.8 x 30 = 144). By the
way, at 20-25% ice concentration, ice slurry flows like conventional chilled water while
providing 5 times the cooling capacity.


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Fig. B shows the relationship between the heat transfer coefficient and the ice fraction with
various ice slurry mass flux at the same conditions. At low mass flux range, it can be seen
from the figure that the heat transfer coefficient did not change with ice fraction until it reached
to certain ice fraction value (about10%in this condition). However, a sharp increase in the heat
transfer coefficient occurred at the ice fraction above this point. Instead, the heat transfer
coefficient increases gradually with the ice fraction at the high mass flux range.
Ways to Produce Ice Slurry
1. Crushing or grinding solid ice, and mixing the created sludge with salted or sweetened water.
2. Using a tube-in-tube evaporator or crystallizer.
3. Bubble Slurry Ice, which contains ice crystals of 5 µ (0.005 mm, or 0.0002 in) in size and are
formed at a moderate refrigerant evaporating temperature of -12°C to -17°C (+10°F to +1°F),
is produced by second generation tube-in-tube evaporator, in which ice crystals are formed
inside the entire space of the inner tube.

Advantages of Ice Slurry compared to Solid or Flake Ice
•

Slurry Ice can be pumped through regular hoses or pipes

•

Slurry Ice surrounds the to-be-cooled products completely, leaving no air pockets, and
therefore capable to extract heat better, faster and more efficiently.

•

Slurry Ice is soft and has no jagged or sharp edges, thus preventing injuries or damages to the
product-to-be-cooled.

•

Slurry Ice requires less energy, space, and money to produce.

•

Because of its optimal cooling efficiency as a result of 100% use of its cooling surface area, a
Slurry Ice mix requires less (usually 30-50% less) pure ice crystals than pure Flake Ice.


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Typical Applications
Particularly well suited to thermal storage are sports and entertainment facilities, convention centers,
churches, airports, schools and universities, military bases, and office buildings, because they have
widely varying loads. The average cooling load for these buildings is much lower than the peak load. In
addition, using high temperature differentials for chilled-water and air distribution can significantly
reduce construction costs, and the lower electrical demand can also reduce the size of emergency
back-up generators.
Health care facilities, data centers, and hotels having loads with only a slight variation between day
and night.
District Cooling Systems
District cooling systems benefit from thermal storage in several ways. The cost of the distribution
system in district cooling applications is a large percentage of the total system cost. Life-cycle costs
can be lower by using smaller pipes, valves, pumps, heat exchangers, motors, starters, etc. Energy
costs of chilled-water distribution are thereby reduced. The lower electrical demand may reduce the
size of emergency back-up generators. During off-peak cooling load conditions, pumping energy costs
can be greatly reduced, because the flow rate can be reduced proportionate to the reduced load.
The ice slurry generator requires a freeze depressant for ice production; therefore, the ice is made at
solution temperatures of about 27°F, down to about 24°F. The result is that 30°F temperature is
possible from the ice slurry storage tank. Reference Two makes the point that 34°F system supply
temperature provides several operating and cost savings advantages. An ice slurry system can
provide 30°F system supply temperature, thus it offers additional savings in the distribution system and
equipment costs.
The ice slurry generator system provides flexibility in design. As a system grows from Phase I to Final
Phase, the combination of centrifugal chillers and ice machines can be optimized for the given load.
The Final Phase cooling loads of a project may be significantly different than originally planned. If so,
the flexibility of an Ice Slurry System could prove to be very cost effective.


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System load profile analysis; is a basis to ensure that cool storage suppliers provide equipment with
the appropriate performance; the load profile must be specified. The shape of the load profile also
affects the performance characteristics of storage systems.
Ice Storage Strategies
Partial Storage System; only portion of the daily load is generated during the previous off-peak period
and put into storage. During the peak-load period, the overall daily load duty is satisfied by balancing
the operation machine and the stored energy (Fig. Ca,b). The chiller will always be smaller than for a nonstorage system.

(Fig. Ca) Partial Storage System

(Fig. Cb) Partial Storage System


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Full Storage System; all the daily load is generated during the previous off-peak period and put into
storage. During the peak-load period, the overall daily load duty is satisfied by the stored energy (Fig.
D). The chiller may be smaller or larger than for a non-storage system, depending on the design load
profile and the length of the on-peak period.

(Fig. D) Full Storage System


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Case Study:


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The peak cooling demand of 2,087 tons would create a peak cooling system electric demand of 1,835kW for a
water chilling system [including the electrical demand of the chiller compressor, cooling water pumps and
cooling tower fan (if water-cooled) or condenser fan (if air-cooled)] with a COP of 4.0.
The cool storage system chillers need only provide 1,252 tons of cooling during the on-peak period, so the
electrical demand of its water chilling system components would be 1,100 kW at a COP of 4.0 or a reduction of
1186 kW.
Note: The peak electrical demand for the chilled water pumps will be approximately the same for the two

systems and any difference can be ignored for the screening study.
Differences in supply and return water temperatures, flow rates, and pumping energy should be
considered when refining the analysis for alternatives passing the initial screening.
Clearly, these rules-of-thumb for estimating the annual demand charge savings should only be applied for
initial screening purposes and not for the more refined analyses that follow.
Even for the initial screening, additional analysis of cooling loads is required where the differential between onpeak and off-peak energy charges is thought to be significant.
At a minimum, the annual cooling load must be segregated into that occurring during peak and off-peak
periods for each alternative cooling system. System COPs should be estimated for both periods to determine
kWh consumption and the applicable energy charge applied to estimate energy costs and the savings relative
to the reference non-storage system. An evaluation based on hourly cooling loads for the full length of the
cooling season is a requisite.


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Manufacturers:
• Paul Mueller Co. manufactures both ice harvesting and ice slurry systems.
• Chicago Bridge & Iron Co. supplies large-scale chilled water thermal storage systems and has worked
on the development of ice slurry systems.
• Baltimore Aircoil Co. is a leading supplier of external melt ice-on-coil thermal storage systems.
• Calmac Manufacturing Corp. supplies internal melt ice-on-coil systems.
• Dunham-Bush offers internal melt ice-on-coil thermal storage systems.
• Turbo Refrigeration makes ice harvesting equipment for a wide range of applications including cool
thermal storage.

References:
• ASHRAE Handbook 2008, HVAC Systems and Equipment, CHAPTER 50.
• Slurry-Ice Based Cooling Systems Application Guide.
• Heat Transfer Characteristics of The Ice Slurry At Melting Process In A Tube Flow.
• ARI GUIDELINE T – 2002, Guideline For Specifying The Thermal Performance of Cool Storage
Equipment.
• Federal Energy Management Program.
• 1992 Ice Slurry Cooling Development and Field Testing, Report DE92015226.


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