Navy Medical Center Uses Structured Analytical Approach to Renovate Central Energy Plant
August 1, 2002
At the request of the Department of the Navy Public Works Center, San Diego, California, and FEMP's Technical Assistance Program, DOE's Pacific Northwest National Laboratory evaluated the economic sizing and operation of the central energy plant (CEP) equipment at the Navy Medical Center in San Diego.
The Navy operates the CEP to provide cooling, heating, and electric power to the Medical Center. With aging equipment, uncertain loads, and volatile energy prices, the Navy was facing critical issues regarding replacement equipment sizing and operating strategy for all equipment at its facility.
Major existing equipment at the Navy Medical Center CEP includes the following:
- three 800-kilowatt turbine generators with heat recovery steam generators (HRSGs);
- one 800-ton single-stage absorption chiller;
- two 800-ton electric centrifugal chillers;
- one 1,200-ton electric centrifugal chiller;
- three 25,000-pounds-per-hour boilers; and
- four 1,200-ton cooling towers.
The three turbine generator HRSG sets were to be replaced with similar equipment. The 800-ton absorption and electric centrifugal chillers were to be replaced with two double-effect absorption chillers. The immediate issue for the Navy was to determine the optimum sizes for the new equipment. Of equal importance were the optimum equipment operating strategies for the renovated CEP.
Equipment sizing and operating decisions are often complicated at combined cooling, heating, and power plants, and this was certainly the case at the Navy Medical Center CEP. Electricity can either be self-generated and/or purchased from the grid. Steam can be provided from the boilers and/or the gas turbine HRSGs. Chilled water can be generated from absorption and/or electric chillers. The self-generation decision affects the amount of steam available from the HRSGs. The marginal costs of HRSG and boiler steam are different, causing the marginal cost of operating the absorption chillers to vary. The Medical Center's demand for electricity, steam, and chilled water varies with the season, the day of the week, and the hour of the day. Finally, grid electricity and natural gas prices have been volatile in recent years, especially in California. The equipment sizing and operating decisions for the Navy Medical Center CEP warranted a structured analytical approach.
A spreadsheet model was developed to determine the economically optimal size of new turbine generators and absorption chillers, and the economic operating strategy of the entire CEP. First, the Medical Center's cooling, electric, and steam loads were defined and cost and performance characteristics were then developed for existing and prospective CEP equipment. Alternative electricity and natural gas price scenarios were defined in conjunction with the Navy. Optimal equipment sizing and operating strategies were then determined simultaneously for each of the energy price scenarios.
The optimal gas turbine capacity was found to be the minimum constraint (three units at 1,250 kilowatts each) at the baseline energy price scenario and for other scenarios with a relatively small difference between electricity and natural gas prices (i.e., the "spark spread"). As the spark spread increases, the optimal gas turbine capacity increases above the minimum constraint, but was always less than 2 megawatts per each of three units for the energy price scenarios investigated.
The optimal absorption chiller capacity was also found to be near the minimum constraint (two units at 750 tons each) at the baseline energy price scenario. In contrast to the optimal gas turbine capacity, the optimal absorption chiller capacity was found to correspond more with electricity price than the spark spread. The highest electricity prices investigated pushed the optimal absorption chiller unit size up to about 900 tons. Although higher (grid) electricity prices favor absorption chiller operation, self-generation of electricity keeps electrical centrifugal chillers competitive in these scenarios.
The optimal equipment operating strategy was to run the turbines to the extent possible to meet the Medical Center's and CEP's electrical loads. The absorption chillers should be run preferentially to meet the cooling load, with the electric chillers used when absorption chiller capacity is insufficient or when HRSG steam is insufficient for the absorption chillers to meet the cooling load.
Installation of the new equipment will significantly improve the efficiency of the CEP. Compared to alternatives investigated, implementation of the recommended equipment sizes and operating strategies would save the Navy several million dollars over the life of the equipment. Thanks go to Bill Gage, John Icenhower, Jim Mugg, and John Thomas of the Navy Public Works Center, San Diego, for their help throughout this project.