In the past, many mechanical cooling systems designed to reduce energy costs and demand were innovative but cost-prohibitive for most applications. Now, however, alternative energy solutions with relatively quick paybacks have been developed. One such solution is thermal energy storage, which is growing in use and reducing electrical costs around the country.

A successful stratified water TES system at Dallas' Veterans Administration Medical Center. Annual estimated demand savings are expected to average $230,000. Payback on the investment in this technology is expected to be seven years.

Limiting and Load Shifting

These thermal energy storage systems utilize ice or chilled water, which is produced and stored during off-peak hours and then is used to cool buildings during the day, when electric utility rates are highest. There are currently several thousand thermal energy storage (TES) systems in operation in the United States. The basic system includes chillers to generate either chilled water or ice, a control system (generally an automated system that provides information to an operator), a storage medium, and a tank.

TES systems work well for facilities ranging from as few as 100,000 sq ft to those with up to several million sq ft of space. They work best in facilities that are not fully operational at night, such as office buildings, schools, airport terminals, convention centers, and sports complexes.

The two primary configurations of TES systems are partial or full storage. In a partial storage system, which is used to limit demand, the chillers operate continuously, charging the storage system at night and directly cooling the facility in the daytime. When a predetermined level of demand is reached, the system draws chilled water from the TES system. On average, a partial storage system reduces electrical demand charges by 25% to 50%.

Full storage systems shift the entire cooling load from peak electrical rates to off-peak rates. At night, when rates are lowest, the chillers charge the TES system. In the daytime, when the highest electrical rates kick in, the chillers are shut down, and chilled water is drawn from the storage system. Once the peak rate has passed, the chillers are restarted and the storage system is recharged. Under many current electricity rate structures, off-peak rates may be less than 25% of on-peak rates. This is increasingly true with the electric industry's continuing deregulation. Such dramatically lower costs make for rapid paybacks for full storage systems. However, full storage systems do have at least one drawback: its greater storage requirements mean the initial cost is higher than that of partial storage systems.



Figure 1. An example, based on actual projects, of how thermal storage reduces on-peak electrical demand by reshaping the load schedule.

Ice or Chilled Water?

The storage medium for TES systems can be either ice or chilled water. The advantage of a chilled-water system is that it uses less energy overall and is more efficient. It is also easier to install than an ice storage system, especially for a retrofit application. Because ice storage requires more controls and more valves - in other words, more things that can go wrong - chilled water systems are more simplistic.

The most significant design problem associated with chilled water storage systems is maintaining separation between stored cold water and warm return water. A number of alternatives have been used, including multiple tank systems, vertical baffled systems, and single tank systems. Modern diffuser designs make the single tank system very reliable and the most economical.

In the last 20 years, ice has become a more common alternative, partly because it requires only 20% to 30% of the space required for water-based storage systems of identical capacities. Ice storage systems work best in facilities that are site-constrained, such as hospitals and large office buildings.

Ice storage systems are also attractive alternatives because they can generate chilled water at much cooler temperatures (e.g., 32°F to 33°F), requiring lower volumes of liquid and less energy to transport. They may also be more economical in the long run in both new construction and renovation, if ductwork and piping can be reduced and smaller, more efficient fans, cooling towers, and pumps can be installed.



Figure 2. The layout of the central utility plant for the Dallas/ Fort Worth International Airport. The plant sits between the northbound and southbound lanes of the airport's main traffic artery, and one ramp will wrap around the storage tank.

Lower Demand and Energy Charges

Whether chilled water or ice is chosen as the storage medium, TES systems offer operators of both individual buildings and district cooling systems two chief advantages: lower demand charges and/or lower energy charges. The driving factors in the increased popularity of thermal storage have been the opportunity for facility users to cut costs and shape their load to reduce on-peak demand in preparation for deregulation.

Ice storage systems do not generally save a significant amount of energy (if any) at the facility level because they require a media temperature about 16 degrees lower than that needed in conventional systems. Thus the savings are derived from the lower cost of electricity rather than from a reduction in energy use.

Some energy savings can be realized in chilled-water or ice systems because chillers run at night, when wetbulb (wb) temperatures and resulting condenser water temperatures are cooler. With colder condenser water, the chillers will operate more efficiently. Also, facilities managers can better pinpoint when to satisfy the load and thus can control the load at which the chillers operate. Some energy is saved by running equipment at full loads rather than at inefficient partial loads. However, these consumption savings are minimal compared to demand savings.

Figure 3. A virtual cross-section of the top of the DFW plant.

The Impact Of Deregulation

In a deregulated energy market, electricity rates will be a commodity, real time price-based rather than set by a rate schedule. This structure and supply/demand guidelines dictate that prices will be highest during the day. Thus, facility managers know that being in firm control of their facilities' energy consumption and electric load profiles will be advantageous if not essential.

When utility deregulation hits, large users operating TES systems with relatively flat load profiles and no major demand spikes will be well-positioned to get the lowest cost, because they can buy less expensive power off-peak. Thermal storage's flexibility also allows facilities managers to manage loads more aggressively. In a deregulated environment, facilities managers can move the chiller load around as needed to fit new tariffs and/or real-time pricing. Moreover, facilities with thermal storage systems can handle major load downturns by using the system's tank, not the chillers, for low loads.



TES And Cold Air Distribution

The growing use of cold air distribution, which typically uses 30% less volume of air than conventional systems of similar load, represents another opportunity for cost savings. TES paired with cold air distribution offers an efficient and often more cost-effective cooling option.

The reduced airflow means that smaller fans, ducts, and risers are required, thus reducing energy consumption and first costs. The savings realized often offset the first-cost premium of a TES system. Another benefit of this combination is the positive impact on a building's energy efficiency and comfort level.

An added advantage of this combination in new construction has been demonstrated in a new high-rise office building in Bellevue, WA. There, a combination of TES and cold air distribution saved so much space during design that building owners were able to add another floor and save $4 per sq ft of rentable space.

The system required smaller ducts, which reduced floor-to-floor height requirements by 4 in. As each floor contributed its savings to the building, the inches added up to what became the 21st floor.

Reducing the size of the air-handling units (ahu) and installing them in the ceiling rather than in separate mechanical rooms allowed a 400-sq-ft increase in net rentable space on each floor. Annual revenue gains for the additional rental income were conservatively estimated at $250,000, based on 1992 rental rates. Other advantages of chilled water TES systems include:

  • Fewer chillers, pumps, cooling towers, and switchgear are needed, reducing central plant capital costs and maintenance costs.
  • If additional capacity is needed due to facility expansions, chillers that are normally off can be put into service during on-peak hours. No physical changes are required to meet the new loads.
  • Maintenance is easier, because repairs to the chillers can be made during the day, when the chillers are scheduled off.



When TES Makes Sense (And When it Might Not)

Thermal energy storage is an excellent choice when a facility can determine that a large percentage of its electrical demand occurs during on-peak hours, and when the occupied cooling load is 12 hrs or less. It also makes sense when the facility's utility billing structure penalizes on-peak kW use through a 12-month ratchet clause or other means.

Other factors that favor a thermal storage solution include new plant development, major expansion, limited availability of electric power at a site, and the possibility of utility rebates.

A thermal storage system may not be advantageous when a facility is in a rate structure that does not have a kW penalty clause. The TES system can still work, but it doesn't provide as good a payback or as clear-cut a way to save money. Why? If there is no on-peak window, the whole day is subject to on-peak rates.

Sometimes the cost of installing the infrastructure required to support a TES system is prohibitive. In other situations, it may be impossible to install the support infrastructure due to site constraints. And sometimes the load profile of individual buildings may make it difficult to design a central plant that would be both economical and efficient.

The Dallas VA Medical Center

The Dallas Veterans Administration (VA) Medical Center was the first VA medical facility in the United States to use thermal energy storage technology to reduce operating costs. In an unprecedented partnership with Texas Utilities Electric Company (now TXU Electric [TXU]) and Carter & Burgess (Fort Worth, TX), the Dallas VA Medical Center built a thermal energy storage system that lowers energy demand during peak demand times.

In early 1996, the VA Medical Center authorized TXU to build the tank and finance the cost. TXU agreed to let the hospital pay for the thermal storage on its electric bill. TXU also provided a Demand Side Management (DSM) incentive of $500,000, and the total project cost was $2.2 million for design and installation.

One of six alternatives developed by Carter & Burgess, the system provides 24,628 ton-hours of thermal storage. The system's 3.3 million-gal tank, which has a peak cooling capacity of 4,500 tons, became operational in the fall of 1996.

Carter & Burgess also provided civil, landscaping, structural, mechanical, and electrical design services required for installing the 98-ft-dia, 58-ft-high precast concrete tank. The tank is partially buried because the soil on the property was not sufficiently stable, so it was necessary to dig down about 26 ft to rock.

This type of open, nonpressurized system requires that special care be taken when connecting it to a high-rise building. In this condition, building mechanical systems are higher than the water level in the open TES tank. The solution is to use pressure-sustaining valves that hold pressure on the chilled water return riser in the building so that the top of the building does not go into a vacuum. Without pressure sustaining valves, the building return riser will have negative pressure, which can cause air ingestion and mechanical pump seal failures.

Related infrastructure requirements for the Dallas VA project included software and hardware upgrades. In addition, the tank and the piping had to be sized to keep pressure drops low.

The system operates on the principle of thermal stratification, using the difference in the density of water at different temperatures to separate the chilled water from the warmer water in the storage tank. The storage tank is charged with 40° chilled water every night. Then from noon to 8 p.m., the hospital draws its chilled water from the tank to meet its cooling load rather than having electric chillers run continuously to meet on-site cooling needs.

Benefits Of The TES System

As a large commercial utility customer, the VA Medical Center incurred annual electrical demand costs based on a rate that was 80% of the highest demand charged during the year (ratchet clause). The obvious major goal of the new system was to lower energy costs. By implementing TES technology, the hospital has been able to shift a significant portion of its energy demand away from the peak cost period to a lower cost period.

The new system has paid off as expected in cost savings. In the summer of 1997, for example, thermal storage reduced peak demand by 2,934 kW, saving the Medical Center $223,650. Annual estimated demand savings are expected to average $230,000. Payback on the investment in this technology is expected to be seven years.

The VA Medical Center's TES system also doubles the capacity of the hospital's central plant. If the hospital expands, the VA can use the TES system and its existing cooling systems concurrently, avoiding additional capital investment.

A Service To The Military

Carter & Burgess also provided engineering analysis and design services as a subcontractor to TXU for a feasibility study related to two energy conservation measures at the Army & Air Force Exchange Service (AAFES) headquarters facility in Dallas.

The study focused on determining and predicting the operating savings and implementation costs of the addition of a TES system to AAFES' already-planned central plant, and replacement of three-way chilled water control valves with two-way valves in the air-handling systems.

The Carter & Burgess team recommended and then provided design and construction management services for the installation of a stratified water TES system and three new chillers (one less than the original design) in the central plant. Simulation concluded that implementation of both energy conservation measures would reduce the energy costs for the central plant by 24 percent, or $67,930 per year. The total construction cost was $2.3 million, of which $540,000 was spent to provide a TES system and the two-way valve installation, consistent with the firm's recommendations.

In comparing the incremental investment of $540,000 to the total energy savings of $67,930 plus $16,875 of maintenance savings (due to elimination of one chiller and all related accessories, pumps, towers, etc.), a simple payback of 6.4 years was achieved.



Tyler Junior College

As part of a performance contract to provide comprehensive campus-wide improvements, Johnson Controls (Milwaukee) recommended the implementation of a thermal storage system to Tyler Junior College, in Tyler, TX. Carter & Burgess provided engineering analysis and design services while Brandt Engineering (Dallas) constructed the chilled-water partial storage system; both were subcontractors to Johnson Controls.

The project, which went online in June 2000, presented no special technical challenges. A routine geotechnical study revealed nothing unusual, with only a little fill required to prepare the site. The system is connected to the college's central plant by a chilled water supply and a return pipe. The system is also self-balancing, meaning that there are no control valves that open or close to direct water in and out of the tank Ð and thus fewer possibilities of mechanical problems.

The 780,000-gal steel-construction thermal storage tank system is 58 ft in diameter and 40 ft in height. The TES system is estimated to pay back in 15 years, with projected yearly savings of $41,000.

"The tank is being utilized, but the overall system is not yet fully automated," said Jeff Schild, project manager for Johnson Controls. "However, the tank supplies the campus with constant 42° water, during the on-peak hours, and meets the college's cooling demands."

"From our perspective, we are looking ahead to deregulation," noted Schild. "The tank will provide the college with a better solution and the opportunity to negotiate with utility companies for the most economical rate. The tank allows the college to shift energy usage to any time they want [in order] to take advantage of the best rates."

In a deregulated energy market, "having a thermal energy storage system gives the college greater flexibility to negotiate and choose a utility company to receive the best rate," he added. "It's a very good negotiating tool."

Johnson Controls is currently negotiating with TXU to reset the ratchet from last year. The college is on the time of day rate where they are on peak from noon to 8 p.m. during the on-peak months (June through September). In order to maintain their relationship with Tyler Junior College, TXU is considering releasing the ratchet penalty from 1999.

The TES system was part of a much larger project at the college. Carter & Burgess also engineered the installation of two new chillers, the expansion of the chilled water and hot water loops to include six existing buildings, and the conversion of the distribution system to variable-volume pumping. Other facets of the project included the implementation of several control strategies, a campus-wide plumbing retrofit, a campus-wide lighting retrofit, and window tinting of several buildings.



Preparing For Liftoff At DFW International

As a subconsultant to HKS Inc. (Dallas) on the Dallas/Fort Worth International Airport (DFW) Central Utilities Plant Upgrades project, Carter & Burgess is providing a number of services, including the installation of a thermal storage system.

One of the primary goals of the system, which is scheduled to be online by March 2002, is to level out the demand for electricity to optimize the size of the airport's on-site generation. Flattening the peak electrical demand over a 24-hr period means that the size of the generators for the central utilities plant can be reduced and the base loading of that equipment can be increased.

"This will enable us to stabilize our operating load for the chilled water system while simultaneously increasing our capacity and reducing our energy costs," said Bob Barker, airport maintenance assistant director for energy and utilities services.



Conclusion

Thermal energy storage is being adopted at DFW International Airport and at a variety of other sites across the nation due to the benefits that it can provide. Energy costs can be reduced by shifting energy loads to off-peak hours when rates are lower. In a deregulated market, the system can allow the flexibility to buy less expensive power off-peak. In addition, if a facility is expanded, the system can meet the new load demand without major physical changes. Maintenance costs can be reduced due to having fewer chillers, pumps, cooling towers, and switchgear associated with other systems. ES