Applying DDC and VFD to Central Chilled Water Plants for Profits

Abstract

The Central Utility Plant (CUP) at D/FW Airport is a facility that was first placed in operation during the Fall of 1973 and has continuously operated since that time. The original plant produced steam for heating and for chilled water generation via condensing turbine type centrifugal chiller drives. Its original service area was 1.4 million square feet. The original utilities costs were 27 cents per MCF for natural gas and 0.85 cents per KWH for electricity. The 1974 energy budget was slightly less than $500,000.00. Currently this same plant, after a continuous series of improvements, serves over 3 million square feet utilizing less chiller capacity and less boiler capacity than was used in 1974. The energy usage per square foot of facility served has been reduced by at least 65% and the value of the accumulated annual energy cost avoidances from January 1, 1975 to present exceeds $17,000,000.00 The present value of those cost avoidances would be $21,718,000.00 if treasury bill interest rates were applied to these cost avoidances as they were being accumulated. The total cost of capital improvements to achieve these savings was less than 2 million dollars of which 1.5 million has been spent during the last two years. In 1984 the total utility cost for operating this plant was less than 1.5 million dollars. The bottom line is that the service area has increased by a factor of 2.15, the cost of natural gas has increased by a factor of 15, the electric service cost increased by a factor of 6 and the utility service cost to the tenant has increased less than 25% on a per square foot basis. The cost avoidances were generated primarily by the elimination of reheat for space conditioning and modifications and improvements to the central heating and cooling equipment. The remainder of this paper will deal with the latter and more specifically with the implementation and utilization of variable frequency drives (VFD) and direct digital controls (DDC) to improve the efficiency of chilled water generation and distribution systems.