Designing for life-cycle costs

March 2006 » Feature Articles
Selecting the location and type of water storage tank is one of the key decisions in designing a water distribution system.
Thomas M. Walski, Ph.D., P.E.

Pumped storage may seem like a good alternative to elevated water storage tanks, but consider energy costs before choosing a system

Selecting the location and type of water storage tank is one of the key decisions in designing a water distribution system.

Floating and pumped storage are the two primary types of facilities. Floating storage, in which the water level in the tank is the same as the hydraulic grade line (HGL) in the system, usually requires construction of an elevated tank, unless there is a nearby hill where a ground-level tank can be placed. Alternatively, a tall standpipe located at a relative high point may be used. Pumped storage, in which the water level in the tank lies below the HGL, usually comprises a ground-level or underground tank with a pump station to move the water from the tank back into the distribution system.

In each system, water is pumped into the pressure zone from a source pump station. In the case of floating storage, water can freely move back and forth into the tank. In the case of pumped storage, water must enter the tank through a control valve—a pressuresustaining valve or throttling-control valve—and be pumped back into the system, usually with a variable-speed pump.

Intuitively, one would expect floating storage to have a lower energy cost because water is not re-pumped as it is in pumped storage scenarios. Nevertheless, there may be opposition to elevated storage because some perceive them as unsightly, the tank site may be located near an airport where tall structures are prohibited, or decision makers may not realize the life-cycle costs of using pumped storage.

Economic comparison Engineers usually base decisions on an economic analysis. In evaluating construction costs, an elevated tank, standpipe, or ground-level tank on a hill with piping will cost more than a ground-level tank or buried tank. However, the pumped storage system also requires construction of a pump station with variable-speed pump drives and backup generators. As a result, in most cases, the construction costs of both systems are comparable in magnitude.

Therefore, engineers will be tempted to choose a system based on small differences in construction costs; however, a good engineering analysis should be based on overall life-cycle costs. The present worth of energy costs often will be the deciding factor.

Related to this issue is the fact that developers often pay for the capital costs of system extensions while the utility pays for ongoing energy costs to operate the system.

Because energy costs increase proportionately with the flow that must be pumped from storage while construction costs exhibit a significant economy of scale, energy costs are a larger proportion of present worth costs in large systems compared with smaller systems. It is the utility’s responsibility to ensure that they do not become stuck with a system tank that will waste energy.

Obviously, energy costs for pumped storage will be greater because every drop of water that enters a ground-level tank through a control valve wastes energy.

Additionally, that water needs to be repumped back into the system. The farther the water level in the ground tank lies below the system HGL, the more energy is wasted when water enters the tank, and then is pumped back into the system. There is no such waste for an elevated tank that floats on the system.

Energy costs can be difficult to estimate manually. Data on pump head and efficiency are available as a function of flow, but determining the exact operating point (flow rate) of a pump, especially in a pumped storage system with variable-speed pumps, is tricky because flow and pump speed (hence efficiency and head) vary throughout the day. Calculating energy cost manually on an hour-by-hour basis is a tedious process.

Using some gross assumptions, an engineer can get a rough estimate of energy costs. However, accurate energy costing can best be obtained using a computer model that accurately simulates variablespeed pump performance and automatically determines energy costs.

Energy cost modeling

Calculations of energy costs for a typical small system using floating and groundlevel storage demonstrate the importance of energy costing. Energy costs are calculated for the floating storage tank (HGL = 120 feet) and two pumped storage scenarios: ground tank (maximum water level = 40 feet) and an underground tank (maximum water level = ground level = 20 feet).

A hydraulic computer model of a typical small distribution system or pressure zone was created using Bentley Systems’ WaterGEMS software program. Average water use in this system is slightly less than 1 million gallons per day (mgd). The system has one source pump station that serves as the primary feed. It has a single storage tank on the opposite side of the system that can be elevated, placed at ground level, or underground. All of the tanks have the same volume and the same range of water level fluctuations.

Pumpage into the system for the elevated and ground-level tank is shown in Figure 2. The red line represents pumpage into the system with the elevated tank, the blue line represents pumping into the system for the ground-level scenario, and the green line represents pumpage from the ground storage tank. It is clear that more pumping (and hence energy use) is associated with the ground-level tanks.

The pressure at a typical point in the distribution system exhibits similar fluctuations for each of the tank configurations. The goal is for the user not to see significant differences in pressures between configurations. The fluctuations in tank levels are comparable between the floating and ground tank scenarios (Figure 4). The elevated tank behaves similarly to the ground tank, as does the underground tank (not shown).

Energy costs for floating and pumped storage were calculated using the WaterGEMS energy costing feature using a 48-hour extended period simulation run.

There is no re-pumping cost for the floating storage scenario. The costs for the two pumped storage alternatives (ground and underground) depend upon the elevation difference between the water level and the HGL. Results of the energy cost analysis are summarized in Table 1 in terms of annual energy cost at 10 cents per kilowatt hour and the present worth of life-cycle energy costs using a 5-percent interest rate and a 20-year time period (series present worth factor = 12.46). Costs for the source pump station were virtually the same for all scenarios. The costs only differed in the pumping from pumped storage.

Underground storage tanks cost more because the water level is farther below the system HGL than in the ground-level tank.

There are many assumptions with regard to layout, unit prices, and operating rules built into the results that affect the cost; however, the same trends should exist in any station. The costs for this sample system are representative for a 1-mgd system. Energy costs should be roughly proportional such that a 10-mgd system will have a present worth of energy costs from pumped storage on the order of $500,000.

The life-cycle energy costs to operate pumped storage can be significantly greater than that of elevated storage. In addition, given the simplicity of operating elevated storage, compared with pumped storage, elevated tanks also offer savings in pump mechanical and electrical costs. The exact cost differences are highly site specific and are not easy to calculate manually.

Therefore, a hydraulic model such as WaterGEMS that can automatically perform energy cost calculations can be valuable as a decision-support tool.

Thomas M. Walski, Ph.D., P.E., is a senior product manager for the Haestad Solutions Center of Bentley Systems, Inc. He can be reached at tom.walski@bentley.com.


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