Suburban Water Utility Pays Off AC Drives with Savings in Electricity

By Scott Anderson, St. Louis Park Water

St. Louis Park Water is typical of many water utilities across the country trying to provide customers quality water at a reduced cost. As an enterprise fund to the city of St. Louis Park, Minn., revenue is generated by water production. A rise in operating costs means cuts have to be made or rates have to be raised. To avoid raising rates or making cuts, the utility constantly asks the question, “How can we do this more efficiently and at lower costs?”

The city was wasting energy by trying to control flow between booster pumps and the well by restriction valves. By installing variable frequency ac drives to match discharge flows and maintain pump levels, it was able to reduce energy consumption without having to raise water rates.

The pump well motors were working at 100 percent at all times–1,400 gallons a minute–because they weren`t able to reduce water flow fast enough. Since the system was designed to run optimally at 1,000 gallons a minute, operators had to close a gate valve and restrict flow to the optimum level. Although the water flow was constricted, the electric motor still required the same amount of electrical energy used at the 1,400 gallons per minute rate.

If the city only had one motor working this way, it would be no big deal. But it had six motors and about half of its system was working with throttled-down gate valves. If the 25 percent of wasted energy from each motor was added up, it nearly equaled the energy consumption of one-and-a-half motors.

Recapturing Expenditure

St. Louis Park Water knew it should upgrade the system, although it wanted to be sure it would receive a return in energy savings for its investment. When the dollar amount spent on the drives was compared to the amount of time it would take to recapture the expenditure, utility personnel found that payback could be done over a short period of time–making the investment worthwhile. Variable speed drives fit right into that equation because the drives could provide better operational control and economical savings.

An energy rebate program from the local utility was the real clincher. Northern States Power`s energy conservation program provided zero percent financing for purchasing and installing the equipment and incorporating demand management principles. The payback for the $333,000 expenditure was initially estimated to take approximately 4 1/2 years, with projected electrical savings of $73,000 a year. The savings in electricity is essentially paying the loan back so there were no out-of-pocket expenses for the drives upgrade. After being in operation for a year, the utility now anticipates the savings to be 20 to 30 percent greater than the $73,000 per year initially estimated.

Quick Pay-Back

With the installation of adjustable speed drives, the city eliminated the need for valve restriction by using the drives to run the pump motors at the desired level. The drives give the option of pumping water to match both low- and high-demand periods with minimum amount of load on the system.

Previously, pump motors were activated by an electromechanical relay that only turned the pump motors on or off. Control of valves regulated the amount of flow to the elevated tanks to prevent system overflow. The sudden starts, stops and flow restriction can wear on mechanical equipment, such as motors, seals and bearings. It also adds to the electrical demand rate. The existing control system didn`t provide data on electrical load, so operators didn`t have any way to monitor electrical demand and energy usage on an ongoing or real-time basis.

Instead of running the pumps at full speed, the new ac drives vary the pump speed to gradually increase or decrease the flow rate as required to meet the water demand of the system. Traditionally, the pumps would only begin filling a tank once the volume reached a predetermined level. Now the drives enable the system to fill the tanks at the same rate at which they are draining, reducing the number of starts and stops and the associated wear on the water system`s mechanical equipment.

Using drives to vary pump motor speed also reduces the amount of electrical load on the system and energy the pumps consume. Running the motors at full speed demands a great deal of energy–in most cases exceeding the amount required by the application. The drives enable users to save power by matching pump speeds with the required load, reducing utility power consumption surcharges. The city found that variable frequency drives were ideal for the situation, because they can now operate a 100 hp motor at only 75 hp–saving money and energy and providing more control of the system.

Demand Management System

To develop a program to control energy usage, the city enlisted the help of St. Paul, Minn.-based consulting engineering firm Toltz, King, Duvall, Anderson & Associates (TKDA). TKDA went to great lengths to ensure that the city`s operators would be involved in the demand management system design process. The involvement of the operators was a key in the success of this project because it gave them a sense of ownership over the new system.

“Operators typically don`t have access to electrical bills,” said Vern Jacobson, TKDA senior professional engineer. “A total control solution, like that at St. Louis Park, gets the operator interested and knowledgeable about how the billing process works. This gives them a plan that they can follow to meet water needs and save money. The operator now reviews the bills and takes an interest in controlling the demands on a daily basis.”

Operator involvement goes hand-in-hand with the demand management system St. Louis Park implemented which provides operators instantaneous readout of the entire system`s operations. “Our first priority was to implement a demand management system, then install equipment to allow operators more flexibility in controlling the pumping rates,” said Jacobson. “Operators use the plan as a tool for reducing demand and eliminating the need for valve restrictions.”

The city also implemented a power monitoring algorithm to help balance power demands across the system and avoid fees incurred if the city went over its specified energy usage limit. If an increase in water demand requires the activation of another pump, the control system will automatically attempt to balance the electrical load with other pumps in the system. The algorithm will require manual intervention if the water usage demand cannot be met, while staying below the electrical power threshold.

After working in training mode for the first 18 months, an automatic system will be installed where the operator simply plugs in a base operating plan and works within that plan. The training mode allows operators to become comfortable with how the system works–utilizing 603 kW on-peak and 789 kW off-peak. If the energy demand rises above the set level, the system simply alarms the operator. At this time the operator can adjust individual pumps to meet the demand and stay within the system`s kW limits.

The city of St. Louis Park has become more efficient and reduced costs by installing Allen-Bradley variable frequency ac drives and implementing a demand management system. The added benefit to the utility was a quick payback associated with increased energy savings.

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