Optimal design and operation of a hybrid gas/electric chilled water plant

Large commercial buildings and many industrial facilities use central chilled water plants to provide chilled water for air conditioning and process cooling requirements. Equipment for producing chilled water can be driven by electric motors, natural gas fueled engines, or by thermal energy through absorption cooling cycles. The chilled water plant equipment is expensive and pricing structures for electricity, natural gas, and thermal energy are often complex. In order to minimize the cost associated with owning and operating a central chilled water facility, the design approach and operating strategy must consider equipment cost, cooling requirements for the facility, utility cost structures, and equipment performance characteristics. A mathematical model of the chilled water plant design and operation problem is developed. The model minimizes the life cycle cost of a chilled water plant through selection of chilled water plant equipment and specification of an optimal operating schedule for the equipment. The problem is formulated as a mixed integer linear programming problem where non-linear chiller performance curves are transformed into linear constraints through the use of integer variables. The model is decomposed into design and operation problems. The design problem is solved heuristically with the underlying operational problem solved using branch and bound techniques. The solution approach is demonstrated through application to a case study. Results indicate that the approach can be used to establish a plant configuration and an operating strategy for all-electric, all-gas, and hybrid chiller plant configurations with the optimal design dependent upon the utility cost structure.