Thermal Storage and Perpetual Savings
TES can shift a significant portion of a hospital's energy demand away from the peak-cost period to a lower-cost period. Utilities often support this technology, because it relieves pressure to construct new and costly power plants. Shifting a large energy demand away from the peak period enables the utility to maximize its generating plants, so it saves money for both the customer and the utility.
The Dallas, Texas Veterans Administration (VA) Medical Center is one of those hospitals intrigued with the idea of TES. It now relies on a stratified water thermal storage system, with a 3.3-million-gal tank, to balance energy demands during peak needs.
Another hospital that installed a TES system is the West Haven (Connecticut) Campus of the Veterans Affairs Connecticut Health Care System (VACHS). As part of its extensive energy conservation renovation, a hybrid central cooling plant was installed, which combines a 1,000-ton steam absorber, an 800-ton electrical centrifugal chiller, primary/secondary pumping, and 6,000 tons of thermal storage.
Both hospitals have seen significant energy savings since installing their TES systems.
Deep in the Heart of TexasThe Dallas, TX, VA Medical Center has the distinction of being the first VA medical facility in the nation to use TES technology to reduce operating costs. A partnership with Texas Utilities Electric Company (TU Electric) made implementing this technology possible.
But the thermal storage is just one component of an ongoing renovation that is still taking place at the medical center. "In 1996, we built a 1 million-sq-ft clinical addition to the hospital, effectively doubling our size. At that point, we built a new energy center, which is comprised of four 32,000-lb-mass boilers. We also have two Trane steam absorbers at 750 tons and four York electrical centrifugals running at 1,000 tons apiece," said John Ashby, facility engineer, Dallas VA Medical Center.
The new energy plant services both the addition and the older buildings, some of which still have equipment that was installed in the 1960s. "What we're in the process of doing now is basically upgrading those 1960s systems as we get the money. Those systems were constant-volume systems, with induction units and fancoil units," said Ashby. Of the 85 existing air handlers, approximately 15 still need to be replaced.
The idea of installing a TES system didn't come about until later in the renovation project, when the facility engineers were looking at ways to decrease their electrical loads. TU Electric subsequently said it was willing to look at energy projects that would reduce its peak demand loads during the summer when it was having the hardest time meeting the loads of the area with its generation capacity.
In early 1996, the VA Medical Center contracted with TU Electric for financial support of the project. It was agreed that TU Electric would build the tank, finance the cost, and let the medical center pay for the thermal storage on its electric bill. The utility provided $500,000 of the total cost of $2.2 million required for design and installation. Savings resulting from the installation will allow the VA to recoup its investment within seven years.
Ashby notes that the VA looked at a number of different thermal storage options, focusing on estimated construction costs, maintenance costs, utility savings, and life-cycle cost analysis. The final system chosen was a stratified water thermal storage system with a 3.3-million-gal tank at its core, which has 3,000 tons of chiller capacity for 8 hours. (The tank is considerably larger than an Olympic-size swimming pool.) The system became operational in the fall of 1996.
The storage tank is filled with chilled water every night. Then every day, from noon until 8 p.m., the hospital draws its chilled water from the tank to meet its cooling load, instead of requiring continuous electric service to operate on-site chillers. As an added feature, the thermal storage system doubles the capacity of the hospital's central plant. If the hospital expands, the VA could use the thermal storage system and its existing cooling system at the same time, avoiding additional capital investment.
Deciding to install a TES system didn't make everyone happy at first. "When you do this, you're moving from a closed type of system to an open type of system, which created some challenges with the designers, who designed the whole building to be a closed-loop system. It was a new thing at the time, and people were concerned about what was going to happen with this drastic change," said Ashby.
The modifications weren't so bad after all. Pressure-sustaining valves, which keep the water from running out of the system all at once, were installed at the top of the loop. Balancing the system was also a little different than what people were used to, but it turned out better than anyone expected, noted Ashby.
The final results can be seen in the amount of money saved by the TES system. In the summer of 1997 thermal storage reduced peak demand by 2,934 kW, cutting annual electricity costs by $223,650. "We've seen a lot of energy savings," noted Ashby. "And because we have this type of thermal storage system, we have a lot of opportunities to do things that most people aren't able to do."
Onto the East CoastThe VACHS is a complex of 28 buildings spread out over 24 acres. Its space includes 1,125,000 sq ft, 1,200 people, 350 beds, plus specialty care and blind rehabilitation centers. The complex dates back to the 1920s, and a central chilled water system was installed in 1964.
The age of the systems, as well as a federal mandate to reduce energy consumption in federal buildings by 30% by the year 2005, prompted VACHS to look at replacing many of its mechanical systems. After reviewing the requirements of the complex, it was decided in the early 1990s to institute two phases of energy conservation measures.
The first phase involved replacing approximately 8,000 incandescent and fluorescent fixtures with modern energy-efficient fixtures. Two inefficient 885-ton centrifugal chillers were replaced with a 1,000-ton two-stage steam absorption chiller and an 800-ton R-123 electric centrifugal chiller. The aging cooling tower and chilled water and condenser water pumps were all replaced with new equipment and energy-efficient motors. A new ddc system was also installed to control the new chiller plant.
Between the lighting retrofit and the chiller plant replacement, VACHS is now seeing an annual savings of $600,000. The total capital cost of Phase One was $2.5 million, which is being paid for from the energy cost savings and amortized over a 15-year period.
The second phase of the project expanded the plant capacity with a 6,600-ton/hr ice thermal storage system. During peak demand times for cooling, ice is utilized to lower the discharge chilled water temperature from 44 degrees F to 40 degrees. With the lower chilled water temperature, the need for increasing the chilled water distribution system was not required, providing a first-cost savings of $280,000.
Initially, the TES system was not considered. "When a major addition was under design in 1995, our counterparts in Washington were thinking about building a separate cooling plant just for that addition. We thought we could do it a little bit differently, save some money, and actually simplify the chiller plant by enhancing what we already had," said Robert Palazzi, chief design and development and energy manager for VACHS.
A study was performed with the assistance of the local utility, United Illuminating, which demonstrated that a TES system could provide the necessary cooling for the additional square footage for about the same price as building a separate standalone plant.
"This way we could use existing equipment so far as chilled water piping and some of the mechanical components that we already had in place. We would have to add some equipment anyway, regardless of the system chosen, so instead of the traditional chillers, we put in an ice system," noted Palazzi.
The ice thermal storage system saved $650,000 in first cost and is saving approximately $100,000 annually in operating costs. There are other intangibles that factor into the savings as well. As Palazzi points out, the VA is saving a lot of money in maintenance costs by utilizing the existing chiller plant and its primary pumping system and auxiliary equipment, eliminating the need to have additional equipment installed.
Palazzi said a TES system is a natural choice for health care facilities and should be considered more often. "It really makes good economic sense when you can create and store energy at an off-peak time. Just in our case, the use of electricity during the normal work hours, which is the peak time, costs about $0.11 to $0.12/kWh. But on the off-peak, the rate goes down to $0.04/kWh. There's a substantial savings dollar-wise if electricity is used at night."
In addition, health care facilities usually have load profiles that are rather steady. Unlike other buildings that shut down at night, thus varying the loads, health care facilities have a continuous baseline, because they're running 24 hours a day, 7 days a week, 365 days a year.
"We have no down time. We have load profiles to show that our peak demand is at 12 or 1 p.m. By shifting the ice thermal storage to off-peak hours and using it during on-peak hours, we cut a good chunk off of our peak demand," he said.
Palazzi also noted that taking advantage of a performance contracting situation can also help with capital costs, as well as other potential problems. "The burden is predominantly on the design-build contractor. They have to produce a product that will perform at a level that you indicate in your contract. It's not like a traditional job where they'll install it to your plans and specs and make it run for a year and then after that you're on your own. With a performance contract, they have to make it work and prove it."
In both cases outlined here, TES technology has helped hospitals to save a significant amount of money in their utility costs. Palazzi added a few more words of wisdom for engineers who might be considering similar projects: "Do the cost analysis. Take the time and run the numbers prior to making that decision. If it makes economic sense, then your administration as well as your financial people will back it up and say it makes economic sense to do this." ES