Off peak cooling using an ice storage system

Abstract

The electric utilities in the United States have entered a period of slow growth due to a combination of increased capital costs and a staggering rise in the costs for fuel. In addition to this, the rise in peak power demand continues almost at historical levels resulting in lower plant utilization. Current rate schedules do little to improve the utilities' load factors and,in fact, encourage consumption. Time of day rate structures have been suggested as one load management device. This thesis investigates the impact of commercial cooling systems on the utilities supply picture and describes an off-peak cooling system which would enable a building operator to shift chiller operation to off-peak hours. The chillers draw heat from a water/glycol coolant, cooling it to 20°F. The coolant circulates through a series of coiled pipes inside a water filled storage tank. As heat is drawn from the water, ice forms around the pipe heat exchanger. With a cool ant temperature of 20°F the ice cylinder will form out to a diameter of 3.4" in 10 hours. Optimum pipe spacing is 3.5" on center. Polyethylene pipe is preferred to copper pipe for cost and fabrication reasons. The plastic pipes are grouped in discrete modules which allow flexibility in design. Building cooling loads are managed by circulating the remaining 32°F tank water through a heat exchanger coupled to the air handling units cooling coils. The warm water is returned to the tank where the heat is absorbed by the ice. Economic analysis using the present electric schedules indicate a favorable return on investment Time of day rates would make the system look even more desirable.