Optimum trade - off between chiller power and pump power for a chiller plant serving a commercial building

This paper explains how to enhance the energy performance of a chiller plant designed with chillers of equal size or unequal size and having superior coefficient of performance (COP) at part load operation. A simulation study was carried out on an air-cooled centrifugal chiller plant serving an office building. Four design options with respect to the number and nominal capacity of chillers were investigated (OP1: 6 x 1124kW; OP2: 8 x 810kW; OP3: 4 x 1213kW + 2 x 810kW; OP4: 2 x 1350kW + 2 x 1107kW + 2 x 801 kW). For each design option, the annual electricity consumption of chillers and pumps was assessed using a chiller model capable of determining how the chiller COP changes at various operating conditions. It is found that prolonging the full load operation of chillers helps reduce the pumping energy, but fails to minimize the energy use of chillers having maximum COP at a part load ratio of around 0.8. An annual electricity saving of 5.2% can be achieved by using the design option OP2 with which each of the chillers operate frequently with maximum COP at their optimum loading points. This highlights the need to ascertain at what loading point the trade-off between the chiller energy and pumping energy can be optimized to minimize the overall energy consumption of a chiller plant. The results of this paper will give engineers and researchers a better idea about how to select the right size and combination of chillers and how chiller part load performance curves can be used to evaluate improvements in the performance of a chiller plant with alternative designs.