Why is there an energy cost associated with a tank?

Product(s): WaterGEMS, WaterCAD
Version(s): 08.11.XX.XX, 10.XX.XX.XX
Area: Modeling

Problem

In the "Storage" section of the Scenario Energy Cost tool, what does the "Cost" represent for a tank and how is it calculated?

Solution

This cost analysis not only calculates the cost of the energy being used by the pump, it also adjusts the reported daily cost based on the effects of storage within the network. To illustrate this, let's say you have a tank in the network that has an initial level of 10 ft, and during the course of the extended period simulation this level falls to 5 ft. Realistically, this translates to an energy loss because at some point the pump will have to expend energy to fill the tank back up to its original level. Conversely, if the water level in the tank at the end of the simulation is greater than the initial level, the cost associated with the additional energy expenditure will be subtracted from the final daily cost.

The effect of this additional consideration in the cost analysis is that the estimate provided by WaterCAD will be much more realistic than an estimate based solely on the cost of running the pump. For instance: If you ran an extended period simulation in which the tank was able to meet the demands of the network for a 24 hour period without requiring additional water, and ran a cost analysis without accounting for storage gains/losses for this 24 hour period, the program would calculate that the daily energy cost for this network is zero - the pumps did not run, so no energy was consumed. This is obviously incorrect, as energy will be required to fill the tank again to recoup the losses of the previous day.

The tank Cost ($) is figured by taking the sum of the Energy Use Cost (Total) and dividing the result by the sum of the Volume Pumped (Total).  This gives you the Tank Cost per Unit Volume.

Multiply the Cost per Unit Volume times the Volume Change to get the Cost ($).  If the Hydraulic Grade (End) is greater than the Hydraulic Grade (Beginning), the Cost ($) will be a negative number.  If the Hydraulic Grade (End) is less than the Hydraulic Grade (Beginning), the Cost ($) will be a positive number.

See Also

Carbon Emission Analysis and Energy Cost Analysis

Energy Cost Analysis in WaterGEMS V8i SELECTseries 4

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