A Study In Chilled Water System Retrofits
With fluctuating energy prices, Loma Linda University searched for a cooling solution that would benefit the sprawling campus. Three different thermal energy storage systems were analyzed to find the best fit.
Founded in 1905, Loma Linda University (LLU) in Loma Linda, CA, is a growing educational health-science institute and medical center. Featuring over 100 academic programs and certificate programs, the university provides medical and dental education to more than 3,000 students from 80 countries and nearly all 50 states. The main campus of the university comprises approximately 50 buildings ranging in size from hundreds of square feet to hundreds of thousands of square feet. These buildings include academic classrooms, laboratories, dormitories, administration centers, and libraries. In addition to the teaching and research functions of the university campus, LLU also maintains a hospital, children’s hospital, emergency room, and medical offices. To view a map of the campus, click here.
The LLU campus has an elevation of 1,300 ft above sea level. Located about 70 miles east of Los Angeles, the University enjoys a Mediterranean/desert climate with sunny days virtually year round. Temperatures on campus vary from below the freezing point of water to more than 100°F. To meet building cooling loads, LLU provides chilled water (CHW) from a central plant. The CHW is pumped from the central plant through piping in underground tunnels to each of the served buildings.
In the early 2000s, California state residents and businesses experienced a period of instability in electricity prices where spot electricity prices escalated from around $30 per megawatt-hour (mWh) to more than $300 per mWh, a tenfold increase. Since then, spot electricity prices have returned to levels comparable to those seen before the electricity price escalation, but the volatility in energy prices still made difficult times for those with significant energy budgets. Even though the university had its own cogeneration plant that generated approximately 80% of annual campus electricity, the capacity of the cogeneration facility could not meet the peak electrical demands of the campus most of the year. As a result, the university had to purchase electricity from the local utility company to meet electrical demands. Additionally, LLU was planning for the addition of up to 6 million sq ft of conditioned building space to the campus. The unstable energy prices and forecasted increase in energy use were important reasons for the university to decide on exploring energy cost-saving options, including the installation of a thermal energy storage (TES) system.
The university commissioned a TES feasibility study to determine the best method of meeting the growing demand for central plant cooling on the university campus. When this study was conducted, the central plant CHW system served 24 buildings around campus. The electric demand resulting from the operation of the existing electric-drive chillers and ancillary equipment within the central plant was estimated to account for about 25% of total campus electric demand.
Since the campus utilized a time-of-use (TOU) electric billing schedule, university staff realized that TES could greatly reduce the amount of power that would need to be purchased by shifting cooling production from the on-peak period to the off- and mid-peak periods, when campus electric demand was significantly lower, loading the cogeneration plant more effectively. The university also realized that an additional benefit of TES would be in providing a means for expanded cooling capacity and wanted to determine the most cost-effective method of providing for the existing and future cooling loads.
The central plant consists of three adjacent areas: the original central heating and cooling plant, an expanded chiller plant, and a cogeneration facility. The central plant contained three old absorption chillers with a total nominal cooling capacity of approximately 3,100 tons and five electric-drive centrifugal chillers with a total cooling capacity of 5,750 tons, as well as cooling towers and CHW and condenser water pumps. Although the total installed cooling capacity was approximately 8,850 tons, the total CHW plant capacity was limited to approximately 7,600 tons due to poor chiller performance.
The study team developed an estimated 24-hr cooling load profile for the existing peak day, with the peak cooling load estimated at approximately 7,200 tons and occurring around 5:00 p.m. The estimated 1,500-ton minimum load occurred 12 hours earlier at 5:00 a.m. The team estimated the total peak day cooling load to be approximately 100,000 ton-hrs.
The team estimated a future CHW cooling load increase of approximately 4,500 tons to be added incrementally, as new buildings were connected to the cooling distribution system, including a new 700,000-sq-ft university hospital. The team incorporated the future cooling loads into the existing cooling load profile and prepared an estimated future 24-hr cooling load profile for the peak day. The estimated future peak cooling load increased to 11,700 tons, and the estimated minimum cooling load on the peak day increased to 2,400 tons. The team estimated the total future peak day cooling load to increase by about 60% to 160,000 ton-hours.
The team developed and evaluated three central chiller plant alternatives to meet the project future cooling load increase and to determine if TES was technically and economically feasible for the campus. The alternatives included a conventional chiller plant with no TES system, a full storage TES plant, and a partial storage TES plant.
The conventional chiller plant alternative considered adding three new 2,500-ton electric centrifugal chillers to meet the future peak load. These new centrifugal chillers would replace the existing absorption chillers. The biggest advantage of this alternative was that no land would be required to build a TES tank. The major disadvantages of the ‘base case’ included that it had the largest chiller capacity of the three alternatives and that the central plant chillers serve the peak cooling load during the more expensive on-peak period of the day.
The full storage TES alternative consisted of installing a 71,000 ton-hour CHW TES tank and replacing the existing absorption chillers with three new 1,200-ton electric centrifugal chillers. The required CHW TES tank would be approximately 9.5 million gal in volume capacity, given a 12°F differential temperature (delta-T) between the CHW supply and CHW return temperatures. The major advantages of the second alternative were that the chillers would run at non-peak hours resulting in high energy savings, and that smaller chillers were required than those required in conventional chiller plants. The disadvantages of this alternative were the higher construction costs associated with constructing a large tank and the concern over tank visibility from nearby streets.
The partial storage TES alternative included the installation of a 40,000-ton-hr CHW TES tank and the consideration of replacing the existing absorption chillers with two new 1,250-ton electric centrifugal chillers. The required CHW TES tank for this alternative would be approximately 5 million gal in volume capacity given a 12°F CHW delta-T. The greatest advantages of this alternative were that the chiller sizes and capacities were smaller than those in conventional chiller plants and less space was required. As with the full storage alternative, the disadvantages included higher construction costs than the ‘base case’ and the concern over tank visibility from nearby streets; however, due to the smaller TES tank size these disadvantages were, relatively, minimized. Another disadvantage of the partial storage TES system is chiller plant operation during the more expensive on-peak period.
Table 1 shows the TES sizing table, illustrating how the peak-day cooling loads would be met in each alternative, as well as how the TES tank sizes were calculated.
For both TES alternatives, the central plant CHW pumping configuration required conversion from primary-only to primary-secondary. At the same time, the pumps would be converted from constant volume to variable volume.
The team prepared energy models of each alternative to simulate chiller operation and estimate energy costs. The team also estimated budget construction costs for all three chiller plant alternatives for use in a lifecycle cost analysis. The estimated budget construction costs of the alternatives included site work and concrete work, mechanical work, electrical work, and instrumentation and controls work. The team further estimated maintenance costs associated with each alternative. Using a 6% discount rate and the estimated construction costs, the team calculated the 20-year net present value (NPV) of each alternative. The results of this effort are provided in Table 2.
It can be readily observed in Table 2 that the alternative with the lowest NPV cost was not the same as the lowest first-cost alterna-tive or even the lowest energy cost alternative. In fact, the only additional category where the lowest NPV cost alternative was also best was in lowest maintenance costs, which is to be expected due to the partial storage TES system allowing for the smallest plant capacity. The team estimated that at the end of the 20-yr period, the partial storage TES alternative would save LLU over $2.5 million NPV when compared to not installing a TES system.
Based on the results of the study, the team recommended that LLU proceed with the partial storage TES alternative. To alleviate con-cerns over visibility, and due to space constraints near the existing central plant, the selected tank location was nearly one-half mile from the central plant. To accommodate this remote location, the team recommended that LLU install 30-in underground pipes from the central plant to the tank in order to fully utilize the tank’s capacity. The team also recommended converting the CHW pumping system from constant volume primary to variable volume primary-secondary and increasing the campus CHW delta-T to increase capacity and save energy.
At the conclusion of the study, LLU elected to move forward with the recommended partial storage TES alternative. One challenge immediately presented itself: the potential of overflowing the TES tank. Typical CHW systems are closed loop, meaning that the vertical return pipe of a tall building is pressurized and water flow through this pipe is primarily driven by pressure. An atmospheric TES tank changes a closed loop system to open loop, leading to the flow through the vertical return pipes of a building taller than the TES tank water level being primarily driven by gravity. To prevent overflowing the TES tank and introducing air into the pipe system, it is important to maintain pressure in any piping and coils above the TES tank water level. This was a concern at LLU since the hospital trauma tower height was significantly taller than the anticipated height of the TES tank, and a loss in cooling capacity in the hospital was considered unacceptable. (While loss of cooling in a university campus classroom building would likely yield complaints from students and faculty, it would be a small inconvenience in comparison with loss of cooling in surgery suites and other sensitive hospital areas.)
Different methods exist to maintain pressure in coils and areas of piping above the TES water surface. Pressure sustaining valves can be used, along with booster pumps, to ensure that the pressure in elevated piping remains at operating levels. Another method is to incorporate pressure-recovery with water turbine driven pumps. Heat exchangers can also be used to create a separate closed loop, isolated from the open loop system.
The design team considered these methods, and ultimately decided that failure of sustaining valves and booster pumps was possible, and said failure could lead to draining of the hospital trauma tower with potentially disastrous consequences. Instead, the design team selected heat exchangers as the solution method. The heat exchangers would be located at the TES tank, isolating the tank from the rest of the campus cooling system. The addition of heat exchangers reduces efficiency due to the approach temperature, reducing delta-T and the capacity of a TES tank with a fixed volume. Once this challenge was resolved, the design team divided the project into three separate phases: TES tank, heat exchanger and pump package, and site work.
The TES tank portion of the work covered the actual tank itself, as well as the diffusers, sensors, and other appurtenances. The soils report indicated that the soil would not support the load of the TES tank, full of water, limiting the maximum tank height. Additionally, to offset the poor soil condition, the team elected to install stone columns underground to support the tank.
The heat exchanger pump package portion of the project involved sizing and selection of heat exchangers, CHW TES pumps, and CHW distribution pumps. The team designed a portable building to enclose all of the heat exchangers and pumps and serve as a satellite plant. The selected equipment to be housed in this package included four 24,000-MBH plate-and-frame heat exchangers, four 125-hp CHW TES pumps, and five 250-hp CHW distribution pumps.
The site work consisted of the underground connection of CHW piping and electrical utilities from the heat exchanger pump package to the existing distribution system. The 30-in underground CHW piping connected to an existing vault in the LLU tunnel system.
Tank construction is now complete and LLU is preparing for the final phase of implementing TES on campus. Design of the 30-in CHW pipe connecting the tank to the central plant is about to begin. Once construction is complete for this underground piping project, LLU will be able to realize the full capacity of the TES tank, as well as some of the other serendipitous benefits TES has to offer, including: reduced chiller energy consumption due to nighttime operation, higher system reliability, and emergency backup potential.