e-News #67: Cool Thermal Energy Storage
Cool Thermal Energy Storage: Shifting Cooling Load Off Peak
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 systems.
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.
Often cool storage involves large, custom-engineered systems. However, packaged systems are now available that are suitable for small commercial and retrofit options. These systems add cool storage capacity to an existing air conditioning system. Intended for light commercial air conditioning and industrial process cooling applications, the systems can be used with existing air conditioning systems sized between 5 and 25 tons.
In September of 2007, Pacific Gas & Electric announced upfront rebates and annual incentives to customers for such systems, including additions to both existing and new air conditioning equipment.
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 considerably.
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 peak demand.
- Chilled water storage tanks may lower fire insurance premiums.
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).
2008 ASHRAE Handbook - HAVC Systems and Equipment, Chapter 50.
Source Energy and Environmental Impacts of Thermal Energy Storage. California Energy Commission, 1996.
Thermal Energy Storage at a Federal Facility, Federal Energy Management Program, 2000.
Cool Thermal Energy Storage, ASHRAE, 2006.
Thermal Energy Storage, Cogeneration Technologies.
Thermal Energy Storage Strategies for Commercial HVAC Systems, Pacific Gas and Electric, 1997.
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 switchgear
- 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 website.
Title 24 Energy Code, Nonresidential HVAC Overview
Demand Response - Calculating Load Reduction
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
Chilled Water System Efficiency
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 Resources (www.energydesignresources.com), an online resource center for information on energy efficiency design practices in California.
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.