Cool storage offers a reliable and cost-effective means of cooling facilities – while at the same time – managing electricity costs. Shown is a 1.0 million gallon chilled water storage tank used in a cool storage system at a medical center. (Image courtesy of DN Tanks Inc.)
One challenge that plagues professionals managing large facilities, from K-12 schools, colleges and offices to medical centers, stores, military bases and data centers, is finding a more cost-effective, environmentally friendly strategy for using and consuming energy. This problem is compounded by the fact that the cost of electricity is at its highest during the day, when demand for power is at its peak. As a result, storing less expensive off-peak electricity has been a viable solution for many firms and institutions.
The U.S. Energy Information Administration noted that nearly 40%(i) of the electricity consumed by commercial facilities in the United States is dedicated toward heating, ventilation and cooling systems. As a result, addressing the costs associated with cooling a facility has huge potential for generating cost-savings and decarbonization. Specifically, a “buildings’ ability to shift energy demand away from peak periods, such as on hot summer afternoons, can greatly reduce both cost and GHG emissions by allowing utilities to reduce the need for their least efficient and most polluting power plants… and mitigating problems associated with the intermittent output of wind and solar energy systems, ” according to the Department of Energy.(iii)
Managing Costs with Cool Storage Technology
A technology called, “cool storage” offers a reliable, cost-effective means of managing electricity costs while ultimately helping to limit greenhouse gas emissions. The technology allows facilities to take advantage of less costly electricity available at night and functionally save that energy for use at a later time. Cool storage achieves this performance by using ice or chilled water as a medium for storing and deploying energy.
A cool thermal energy storage system uses stored ice or chilled water as a medium for deploying energy. (Image courtesy of Trane.)There is hot and cold thermal energy storage. Hot TES would include the water heater in your home. This article focuses on cool thermal energy storage including chilled water storage and ice storage.
The right time to decide on cool storage varies, but may be most attractive when at least one of the following are present:
- It’s time to invest in a chiller plant.
- Back-up cooling is needed.
- Increased cooling loads.
- Reaching electrical service limit.
- Limited space for fans and ducts require colder air temperature applications.
- The facility is seeking to reduce its environmental impact.
- The building has a poor load factor (peak load is higher than the average load).
- The utility offers favorable rates. Many utilities charge peak demand charges that can make up 30 to 70 percent of a building’s electric bill. (iv)
Chilled Water and Ice Storage Each Offer Unique Benefits
Each facility has its own unique energy needs and challenges to contend with. That’s why there are multiple strategies for cool thermal energy storage solutions on the market. Ice storage and chilled water storage make up the two most prominent technologies available - taking a closer look at the advantages of each strategy will reveal which application is the best fit for an organization interested in deploying energy storage.
Ice storage and chilled water have plenty in common. Both are reliable energy storage solutions that have been deployed for decades, and both are capable of making it easier for facilities to efficiently operate their cooling systems. Both have superior benefits over traditional cooling. They offer energy costs savings, back up cooling capacity, may extend an existing system capacity at less cost than conventional non-storage, make renewables more viable, reduce greenhouse gas emissions and lower transmission line losses. On average 5%(v) of the electricity traveling across the grid can be lost by inefficiencies in the system and can go higher during the summer daytime periods. Storing energy at night and then cost-effectively deploying cool thermal energy storage during the day puts less pressure on the grid - the system is most vulnerable during peak demand periods when an entire regions attempt to draw power at the same time.
Evaluating Cool Thermal Storage Technology
Ice storage and chilled water technologies also have a long list of differences with regards to the installation process, design parameters, expected long-term maintenance and operating cost-savings.
There are different types of cool thermal storage technologies, including ice storage and chilled water technologies. Ice storage including the Trane® Thermal Battery™ air-cooled chiller plant is shown here. (Image courtesy of Trane.)
Facility managers should think through a few of the following considerations as they evaluate the advantages each cool thermal storage technology can bring to the table:
Markets - Both chilled water and ice storage work for large facilities such as schools, hospitals and offices. If the building has loads with a very short duration (30 minutes to 2 hours) then chilled water storage may be a better choice due to the quicker discharge rates. Data centers where there is sufficient space for a large tank (3,000 tons and up) would be a great fit for chilled water storage. Whereas, ice storage would be a better applied on large and taller multi-story buildings, chilled water storage tanks operate at atmospheric pressure, complicating the interface with pressurized building piping.
Siting locations - Where can the storage system be located? Ice storage may be buried, put on roofs, placed indoors, in the basements or outdoors. Chilled water storage is mostly sited outdoors, and can be above ground or buried, however may not be applied on roofs due to its mass and largely due to atmospheric pressure limits.
Expansions and permanence of installation – Does building ownership anticipate eventually changing location chosen for storage tank deployment or adding additional cooling capacity? If so, it makes sense to compare the installation permanence of ice storage and chilled water applications. Ice storage tanks like Trane® Ice Bank® units are modular and re-deployable, making it a simple task to change their location with respect to the needs of the business while conveniently staying as a permanent structure for the life of the system if needed. Modular ice storage tanks can be easily added to an existing ice storage facility.
A chilled water tank, on the other hand, is only designed to be a permanent structure and may be considered if storage locations are expected to stay consistent for the foreseeable future.
Retrofits – Most screw and scroll chillers can produce ice making temperatures with very few exceptions. If the facility only has centrifugal compressors that have NOT been designed to produce ice making temperatures and has no plans to add new chillers, the energy storage design would lean towards chilled water storage. Any type chiller can be used with chilled water storage including existing centrifugal chillers.
Tank size - What kind of space is available for installation? Chilled water storage tanks are significantly larger than those used for ice applications like Trane Ice Bank® tanks. Typically, chilled water installations are sized anywhere from 8 to 10 times larger than ice tanks, often approaching 30 feet or taller in height. By comparison, some tanks stand at just 8-and-half feet tall, providing much more flexibility during the installation process. The shorter tank size is preferable for indoor applications as well as outdoors.
Short and long-term value – How does an organization’s ownership experience differ depending on the cool thermal storage solution chosen for deployment? The first costs of ice and water systems should be compared. In smaller (< ~ 10,000 ton-hours), ice storage can be installed for less $/ton-hour than water storage. With larger installations, water storage can be less expensive.
Once at the end of the product life cycle, large water storage tanks can be a stranded asset, i.e., not used at another location. Ice storage may be reused and installed at different facilities.
Performance and reliability - What does the track record of an energy storage tank provider communicate about the performance of their technology? Of importance is a long history of successfully designing and installing cool thermal storage solutions. A diverse portfolio of projects also makes it simple for building ownership to identify a frame of reference and estimate the potential advantages of thermal storage applications.
Construction – How quickly do you need your energy storage system up and running? Do you need to deploy a simple, fast installation or are you prepared for a more complex project? Ice storage tanks can be easily installed and operating in less than one week. Many chilled water storage solutions, on the other hand, are architecturally challenging, making it difficult to site large tanks and may require months of construction. Companies are needed to civil engineer the project, including testing load-bearing capability of the ground. Chiller water storage projects are handled by multiple companies that develop the storage tank, control systems and coordinate installation. Accounting for the reliability of each component is more difficult and time consuming than investing in a single predictable system.
Ease and cost of maintenance - Will the maintenance of a cool thermal storage system impact the ownership experience? Building managers are tasked with keeping HVAC systems performing at optimal levels and will be responsible for keeping an eye on the organization’s energy storage applications as well. The simpler a system is the better, as there are fewer opportunities for components to age or become damaged. Some chilled water tanks (concrete type) are built to require virtually no maintenance. Also, a company with a single-source provider eases not just the purchase but any commissioning and maintenance.
In the case of ice thermal storage, some energy storage tanks are built to minimize the amount of water that needs to be treated and reduce the volume traveling between the tank and the building. Other maintenance considerations may include the number of tanks and size.
System design – Is series architecture important to your chiller plant design? Chilled water storage systems operate the ice and chiller at the same time with flow for charging and discharging traveling back and forth within the same piping. This flow reversal for charge/discharge is awkward and may be complex to design. Whereas, in an ice storage system flow travels in only one direction and series configurations are possible allowing for flexible configurations to take advantage of utility rate structures.
For systems designed for a larger delta T, lower temperature requirements can offer the greater benefit. Low temperature coolant from ice systems can provide wider delta T’s, increasing pumping efficiency. Installed costs for piping and pumps are significantly less due to less flow and extra energy used to make the ice. Plus extra energy needed to make ice may be offset by less pumping energy. In a chilled water storage system, low ice temperatures are not feasible, however wide delta T’s are still possible. For optimal efficiency it is critical to maintain system delta T at all load levels. Any water that returns to tank below design temperatures represent lost storage capacity.
Cutaway of a CALMAC® Ice Bank® energy storage tank. (Image courtesy of Trane.)
Two Sides of the Same Coin
Both ice and chilled water storage have their merits. Chilled water storage can be incorporated into a fire protection system and is more suitable for data centers due to fast discharge rates. Also, chilled water storage plants don’t need to produce ice making temperatures, so existing centrifugal machines that aren’t ice ready can be used. On the other hand, Ice storage arrives factory assembled and installs in far less time than chilled water storage.
In larger installations, chilled water storage can be less expensive, even though it takes longer to install. Ice storage can pay back in as little as three to five years, requires less space, is modular, reusable and simple to maintain and control. You have the flexibility to run the ice and chiller at the same time or run just one or the other and at the end of the system life, it’s a non-stranded asset.
Yet despite all their differences, no matter which solution is chosen, cool storage shares many powerful benefits and fundamentally changes how and when facilities draw electricity from the grid. More efficient lights and equipment, daylighting and fans are all great ways for facilities to improve efficiency, however, reducing peak demand by storing energy for when you need it is just as important. In fact, cool storage is such a novel solution because it allows building managers to fundamentally change how and when the facility draws electricity from the grid, slashing cooling costs and leveling loads for greener, smarter buildings.
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About the Author
Paul Valenta serves as the North America product manager for Trane’s CALMAC® energy storage portfolio. He is responsible for bringing ice thermal energy storage products and systems to market. Valenta graduated from the University of Nebraska with a Bachelor of Science in Electrical Engineering and is also member of ASHRAE® and USGBC®. In 1991, he began working with CALMAC and now has over 30 years of energy storage experience. Valenta grew up in North Dakota, so he knows what cold can do. Email: Paul.Valenta@tranetechnologies.com.
Trane® is a world leader in air conditioning systems, services and solutions. Trane helps customers succeed by providing innovative solutions that optimize indoor environments through a broad portfolio of energy-efficient heating, ventilating and air conditioning systems, building, contracting and energy services, parts support and advanced controls for homes and commercial buildings. For more information, visit www.trane.com.
All photos courtesy of Trane and DN Tanks.
To read similar chiller and cooling system assessments articles, visit https://coolingbestpractices.com/system-assessments/chillers.
(i) EIA. Use of Energy Explained. Sept 28, 2018. https://www.eia.gov/energyexplained/use-of-energy/commercial-buildings-in-depth.php
(iii) DOE. AN ASSESSMENT OF ENERGY TECHNOLOGIES AND RESEARCH OPPORTUNITIES. Sept 2015.
(iv) National Renewable Energy Laboratory. Identifying Potential Markets for Behind-the-Meter Energy Storage: A Survey of U.S. Demand Charges. Aug 2017.
(v) EIA. How much electricity is lost in electricity transmission and distribution in the United States? Dec 31, 2019.