Cool Thermal Energy Storage: Shifting Cooling Load Off
In California, electrical power demand reaches its peak during
the hottest summer days, mostly due to air conditioning loads,
which account for almost 28% of California's peak electrical
demand. A cool storage thermal energy storage system (TES) provides
a means for shifting all or part of a facility's cooling energy use
to off-peak hours, when energy costs are lower and cooling systems
can potentially run more efficiently. A TES system uses cooling
equipment at night to remove heat from a thermal reservoir of
chilled water or ice, which can then be used for space cooling
throughout the day (see Figure 1).
There are thousands of documented TES systems in existence, and
some have been operating for up to 25 years serving schools,
hospitals, universities, airports, churches, government facilities,
and office buildings. While TES strategies may serve either heating
or cooling loads, this e-News focuses on cool storage
In the past, TES systems were large, custom-designed and built
central plant systems, but today packaged TES systems are more
readily available-particularly systems suitable for light
commercial applications or for adding capacity in existing
buildings. One report from Southern California Edison and Pacific
Gas and Electric predicts that in California alone, TES
installations over the next few years could save enough source
energy to supply the annual electricity consumption of
approximately 200,000 homes.
To shift demand from on-peak periods, a cool storage system runs
the refrigerant compressor at night, and stores either chilled
water or ice. During peak periods, the system uses the stored ice
or chilled-water to cool the building and the refrigerant
compressor is turned-off.
Benefits of Cool Storage Systems
Thanks to cooler nighttime temperatures, cooling equipment
operates more efficiently at night than during the daytime. The
benefits of nighttime operation vary depending on the performance
characteristics of the cooling equipment and the specifics of each
particular climate, though this savings will be more significant in
climates with large diurnal temperature swings.
Baseload power plants, which operate throughout the night,
typically operate at higher efficiencies than peaking power plants
that are brought online to meet demand. In addition, transmission
electrical losses tend to be lower during off-peak periods due
primarily to lower electrical demand on the transmission lines and
therefore, lower electrical heating of the wires. A California
Energy Commission 1996 study suggests that replacing peak power
consumption with off-peak consumption using cool storage can reduce
overall power plant energy consumption.
The hourly difference in the cost of electricity is part of
California's Title 24 performance-based compliance methodology. The
efficiency regulations use a concept called Time Dependent
Valuation (TDV) that values energy use differently for each hour of
the year, correlated with the cost of electrical generation and
distribution during that hour. With TDV, a kWh used during a
weekday afternoon in the summer costs more than a kWh used during
an off-peak hour. A properly designed and operated cool storage
system will decrease the facility's electrical demand during
periods of high power costs and high TDV values. Even if the system
uses the same kWh each day, the total TDV usage could drop
The California utilities' Savings By Design New Construction
incentive program uses TDV as the compliance metric, so load
shifting technologies such as cool storage are useful tools for
earning these incentives.
Additional benefits of cool storage systems include:
- By shifting operation to lower cost, off-peak periods and
reducing peak demand charges, a properly design TES system can
reduce building electricity costs.
- Less chiller capacity may be required with a properly sized and
operated cool storage system. The cool storage system may help
ensure the chiller plant operates near its optimal level. In
addition, auxiliaries such as fans and pumps can be smaller if a
cold air distribution design is used.
- Lower peak building demand leads to less strain on the power
grid, reducing the need for additional power plants that only serve
- Chilled water storage tanks may lower fire insurance
Cool Storage System Types
Cool storage systems are classified according to the medium used
for storage. Cool storage media include chilled water and other
liquids (sensible heat storage), or ice and phase-change materials
(latent heat storage). The storage mediums differ in their heat
storage capacities, the temperatures at which energy is stored, and
the physical and chemical properties of the materials. According to
the American Society of Heating, Refrigeration, and Air-Condition
Engineers (ASHRAE), 87% of the cool storage systems in use in the
U.S. are ice-based systems, 10% are water-based, and 3% use
phase-change materials (PCM) as the storage medium.
Figure 2. Ice Harvesting
Chilled water-based systems cool and store water at 39ºF to 42ºF
and then use this cold reservoir to provide on-peak cooling.
Phase-change material (PCM) systems take advantage of the latent
heat capacity of materials such as eutectic salts and ice that
absorb and release energy during their temperature induced phase
changes. Ice storage systems, such as ice-on-coil or ice harvester
systems (see Figure 2), typically require smaller tanks than their
chilled water counterparts. Ice harvester systems have the
additional advantage that the compressor plant can supplement the
daytime cooling load if needed. Another commercially available ice
storage system uses a relatively small tank filled with water and
flexible, plastic coils wound inside the tank. A glycol solution
circulates through the coils, cooled by a centrifugal chiller
plant, to freeze the water at night.
|Case Studies: TES in practice
The William and Flora Hewlett Foundation Office Building
in Menlo Park, California, was the first building ever
awarded LEED® Gold certification in California. Cool storage was
part of the strategy used to earn five points in the LEED Energy
& Atmosphere credit area. "The planning stages for the building
took place at the height of the California energy crisis," noted Jo
Carol Conover, a principal of Benning & Conover and the
building's project manager. "Since the cost of California energy
and its availability were at a premium, the Hewlett Foundation put
great emphasis on both its cost reduction and conservation. The
environmental benefit is realized in that thermal storage reduces
the need for additional generating capacity."
The Los Angeles Community
College - Southwest Campus employs a cool storage system
comprised of 72 storage tanks and two chillers. The system supplies
about 9,900 ton-hours of cooling capacity to the campus over the
course of a typical design day. The chillers each have a capacity
of 910 tons during the "daytime mode" and 640 tons when in
ice-making mode, operating with an average cooling supply
temperature of 23°F and return temperature of 30°F.
Ice is a storage medium capable of storing and releasing 144
BTU/lb of heat when freezing and melting. When ice storage is
combined with a cold air distribution system, less indoor fan
energy is needed to achieve the same amount of space cooling. Cold
air distribution can reduce indoor fan energy by 30 to 40%, duct
size by 20 to 40%, and air handlers by 30-50%, saving up-front
costs, and space required for mechanical rooms.
A principal advantage of ice systems over chilled water systems
is their compact storage size. Ice tanks are only a fraction of the
size of comparable chilled water tanks. On the other hand,
ice-based cool storage systems require lower refrigerant suction
temperatures to make ice and can consume 50% more energy by the
chilled water plant compared to a chilled water-based TES system.
Hence, chilled water TES systems may be better suited for large
central plant applications, or large buildings and campuses, which
have the available space for large water storage tanks.
Building and mechanical designers should carefully consider the
net energy impact of using a cool storage system. The principal
consideration for a cool storage system is the amount of cooling
load shifted to off-peak periods and the proposed mode of
operation. A full storage system is sized and operated to disable
the chiller during peak demand periods. By contrast, a partial
storage system uses both the cool storage system and the chiller to
meet the cooling load during peak periods, resulting in a lower
peak period demand reduction.
All TES systems incorporate a storage vessel, which will be
subject to thermal energy losses of about one to five percent per
day. Therefore, thermal performance of a cool storage system varies
depending on the inventory of ton-hours stored and the rate of
discharge. The total capacity of the storage system depends on the
cooling load profile imposed upon it. ASHRAE Guideline
4 presents thermal performance guidelines for cool storage systems.
For more detailed system design, thermal energy storage systems can
be accurately modeled in DOE-2. Additionally, EnergyPlus
incorporates a module that simulates ice-on-coil based thermal
energy storage (either in series or parallel).
Cool storage systems are often effective in applications with
one or more of the following characteristics:
- A high ratio of peak cooling load to average cooling load
- Either high peak demand charges, or a large difference between
the on-peak and off-peak energy rates.
- A need for additional cooling capacity, in which case adding
cool storage may avoid the addition of another chiller
- Limited total electrical capacity, in which case a cool storage
system could avoid the cost of upgraded transformers and
- Applications requiring back-up cooling systems (for example,
data centers and processes such as electronics manufacturing)
The economic feasibility of a cool storage system depends upon
several factors, including the utility rate structures, the demand
profile of the specific building, and the space required for the
storage system. A cool storage system will increase the initial
building and mechanical design costs. However, the total installed
equipment costs will vary depending on whether the final TES system
design results in any significant cost savings due to a downsized
chiller plant and auxiliaries.
Operations and Maintenance
One of the most important factors for achieving operating costs
savings with a cool storage system is the ability of the operating
plant staff and the control system to operate the system correctly.
When high peak demand charges exist in an electrical rate tariff,
failing to shut off the chiller on time could negate an entire year
of operating cost savings. Hence, it is imperative that the
operating staff thoroughly understand the operation of the cool
storage system, and have available to them a running forecast of
the facility's cooling loads, based on forecast temperatures and
prior cooling demand, preferably a day ahead of time.
Commissioning the cool storage, along with the entire HVAC
plant, is essential to ensure proper operations and realize
operating cost savings. Thereafter, maintenance of cool storage
systems must follow the manufacturer's recommendations, as with all
HVAC equipment. Monitoring ongoing performance against the initial
commissioned parameters can provide a continuing report of the
system's dynamic performance.
California utilities offer outstanding educational opportunities
that focus on the design, construction and operation of energy
efficient buildings. Listed here are a few of the many upcoming
classes and events; for complete schedules, visit each utility's
Title 24 Energy Code, Nonresidential HVAC
This course covers the HVAC provisions of the 2008 California
Building Energy Efficiency Standards. It includes an introduction
to the HVAC standards, as well as 2008 changes to the standards,
including automatic fault detection diagnostic systems, new
requirements for single-zone variable air volume equipment,
refrigerated warehouse requirements, and changes to control system
standards. This course is offered 10/27/09 in Sacramento.
Demand Response - Calculating Load
This class presents how to accurately calculate demand response
load reduction for commercial and industrial businesses. It also
presents how to perform a high-level demand response audit and how
to identify demand response opportunities.
This class covers the processes for calculating potential
electricity load reduction, and includes example calculations that
demonstrate concepts and procedures. This course is offered
11/12/09 in San Diego.
Low Energy HVAC Approaches for Nonresidential Buildings
- Online Course
This course offers a guide for architects on low-energy mechanical
systems, including design implications and architectural
integration. This course is offered 11/18/09 in San
read more >
Chilled Water System Efficiency
This seminar features chilled water system efficiency in large
commercial and industrial facilities. Managers, owners, and
facility engineers will learn how new technologies can reduce
energy costs. Topics include chiller machinery, refrigerant
options, the impact of cooling towers using variable speed
compressors, and variable chilled water flow. This course is
offered 10/20/09 in Irwindale.
read more >
Don't miss future issues - to sign up for a free email subscription, please visit our
newsletter subscription page. Send letters to the editor, suggestions on topics for future
issues, or other comments to the e-News editor via our
Comments & Feedback form.
e-News is published by Energy Design
an online resource center for information on energy efficiency design practices in
Savings By Design (www.savingsbydesign.com)
offers design assistance and incentives to design teams and building owners in California
to encourage high-performance nonresidential building design and construction.
Energy Design Resources and Savings By Design are funded by California utility customers
and administered by Pacific Gas and Electric Company, Sacramento Municipal Utility
District, San Diego Gas and Electric, Southern California Edison and Southern California
Gas Company, under the auspices of the California Public Utilities Commission.