OR-20-010 -- Effect of Water Consumption on Optimal Control of Chiller Plants with Variable-Speed Cooling Tower Fan PDF

A generic component-based model is developed for largeelectrically-driven chillers. Models of components of thechiller plant are developed on first principles and acceptedempirical relations formulated to be valid over a wide rangeof operating conditions. For the illustrated case, componentsinclude a single-stage centrifugal compressor with capacitycontrolled by inlet guide vanes, shell and tube evaporator andcondenser, and counterflow cooling tower cells with variablespeed fan. Each component model is calibrated using one ormore physically meaningful parameters to achieve good matchbetween measured and modeled performance. The chillermodel is able to predict input power at specified capacity andoperating conditions. A tricubic is fit to the solutions obtainedover a wide range of cooling water temperature and capacity,with RMSE of about 1% to permit fast performance evaluationsof subsequent optimization. A plant model that simultaneouslysolves the chiller tricubic model and the cooling towereffectiveness-NTU model is then used to determine the optimalcontrol settings for any given load and wet bulb temperature.

To illustrate the impact that water use can have on plantoperating cost, an year’s operational data was obtained for oneof four 2500 ton chillers of a district cooling plant situated in AbuDhabi (UAE). Model-based control is developed for this typicaldistrict cooling plant and its operation is assessed with and withoutwater cost included in the objective function. Combinedannual electricity and water costs are determined for

1. the system as it was operated

2. optimal control considering only electricity cost

3. optimal control considering both electricity and watercost