Optimum control strategy for all-variable speed chiller plant

Abstract

The optimum control strategy and the saving potential of all variable chiller plant under the conditions of changing building cooling load and cooling water supply temperature were investigated. Based on a simulation model of water source chiller plant established in dynamic transient simulation program (TRNSYS), the four-variable quadratic orthogonal regression experiments were carried out by taking cooling load, cooling water supply temperature, cooling water flow rate and chilled water flow rate as variables, and the fitting formulas expressing the relationships between the total energy consumption of chiller plant with the four selected parameters was obtained. With the SAS statistical software and MATHEMATICA mathematical software, the optimal chilled water flow rate and cooling water flow rate which result in the minimum total energy consumption were determined under continuously varying cooling load and cooling water supply temperature. With regard to a chiller plant serving an office building in Shanghai, the total energy consumptions under different control strategies were computed in terms of the forecasting function of cooling load and water source temperature. The results show that applying the optimal control strategy to the chiller plant can bring a saving of 23.27% in power compared with the corresponding conventional variable speed plant, indicating that the optimal control strategy can improve the energy efficiency of chiller plant.

Keywords

chiller plant / control strategy / variable speed / cooling water flow rate / chilled water flow rate

References

[1] HartmanT. B.. All-variable speed centrifugal chiller plants [J]. ASHRAE J, 2001, 43(9): 43-52

[2] HartmanT. B.. Designing efficient systems with the equal marginal performance principle [J]. ASHRAE J, 2005, 47(7): 64-70

[3] YuF. W., ChanK. T.. Environmental performance and economic analysis of all-variable speed chiller systems with load-based speed control [J]. Applied Thermal Engineering, 2009, 29(9): 1721-1729

[4] TaylorS. T.. Degrading chilled water plant delta-T: Causes and mitigation [J]. ASHRAE Trans, 2002, 108(1): 641-653

[5] GordonJ. M., NgK. C., ChuaH. T.. How varying condenser coolant flow rate affects chiller performance: Thermodynamic modeling and experimental confirmation [J]. Applied Thermal Engineering, 2000, 20(13): 1149-1159

[6] ZhengJ., WangS.-wei.. Energy efficient control of variable speed pumps in complex building central air-conditioning systems [J]. Energy and Buildings, 2009, 41(2): 197-205

[7] BahnflethW. P., PeyerE.. Comparative analysis of variable and constant primary flow chilled-water-plant performance [J]. Heating, Piping, Air Conditioning Engineering, 2001, 73(4): 41-50

[8] BahnflethW. P., PeyerE.. Energy use and economic comparison of chilled-water pumping systems alternatives [J]. ASHRAE Transaction, 2006, 112(2): 198-208

[9] QureshiT. Q., TassouS. A.. Variable-speed control in refrigeration systems [J]. Appl Therm Eng, 1996, 16(2): 103-113

[10] HydemanM., ZhouG.. Optimizing chilled water plant control [J]. ASHRAE J, 2007, 49(6): 44-54

[11] YuF. W., ChanK. T.. Optimization of water-cooled chiller system with load based speed control [J]. Appl Energy, 2008, 85(10): 931-950

[12] BraunJ. E.. A generic control algorithm for cooling towers in cooling plants with electric and/or gas-driven chillers [J]. Int J HVAC&R Res, 2007, 13(4): 581-598

[13] LongF., DingG.-l., SuZ.-jian.. Steady-state simulation of screw liquid chillers [J]. Applied Thermal Engineering, 2002, 22(15): 1731-1748

[14] ZhaoL., ZaheeruddinM.. Dynamic simulation and analysis of a water chiller refrigeration system [J]. Applied Thermal Engineering, 2005, 25(14): 2258-2271

[15] KouryR. N. N., MachadI. K. A. R.. Numerical simulation of a variable speed refrigeration system [J]. Int J Refrig, 2001, 24(2): 192-200

[16] ZhangW.-y., LongW.-ding.. Application of surface water source heat pump systems in Shanghai Expo and the environment effect analysis [J]. Heating Ventilating & Air Conditioning, 2007, 32(2): 38-41