This is second part of a two-part blog post. Read part I here.
A couple of weeks ago, an operations manager for a water authority in Central Texas told me that his energy provider called him last summer during the hottest part of the summer to see if he could shut his pumps down to prevent overloading their power generation plant. The operations manager had to tell his power provider that he could not shut down his pumps because his pump station was feeding the power plant with the water it needed to generate power! This is a good example of how water and energy are intrinsically connected and why implementing strategies to take advantage of time-of-use based billing structures can be a challenge.
Issues with Time-of-Use Based Pumping
The first priority of any pump station at a water utility is to deliver water to customers and to keep the system's tanks full. Not every pump station has the operational flexibility to take advantage of time-of-use billing structures. For example, some pump stations maintain a constant pressure in their system's distribution lines. In this situation, these pump stations are actually forced to increase their energy usage during on-peak time periods as water usage tends to increase as energy usage increases.
I mentioned in my previous post that water use and energy use are strongly correlated. Their relationship is commonly called the "water-energy nexus". The "water-energy nexus" represents the fact that a lot of water is evaporated to generate electricity and a lot of electricity is consumed to collect, pump, and treat water. The water-energy nexus means that simply not pumping during peak energy time periods is not always an option.
Taking Advantage of System Storage
Utilities can use system storage (tanks, clearwells, etc.) in their system to help shift their heavy pumping from on-peak periods to mid-peak and off-peak time periods. Ideally, a pump station with time-of-use energy rates would pump as much water as possible during off-peak time periods to completely fill the tank, periodically run pumps during mid-peak time periods to maintain the tank level, and let the tank level drop to an acceptable level during on-peak time periods to minimize pumping. Unfortunately, very specific pumping schedules are difficult to program into a Programmable Logic Controller (a pump station's traditional control device) and can be difficult to implement with station operators.
Figure 1: Specific Energy's Optimization Suite settings page. The bar at the top shows the current billing time period and time until the next billing time period.
Specific Energy's Optimization Suite enables station managers to enforce constraint-based settings for each time-of-use period. Managers can configure their pumps to fill tanks during off-peak time periods, maintain levels during mid-peak periods, and pump only when necessary during on-peak periods.
As shown in Figure 1, utilities can enter their billing schedules and set operational constraints for each billing time period on the settings page of the Optimization Suite. Once the time-of-use settings are set, the controller will automatically adjust the pump station's operation to honor the configured settings during each time period of the day. With the Optimization Suite, a single engineer or operator could manage the time-of-use schedules across a utility's entire system, rather than expending the effort and expense of training operators to implement time-of-use pumping schedules at each pump station. On the settings page, engineers and operators also have the ability to periodically adjust settings during each time period to find and achieve the best operational constraints.
The Optimization Suite allows water utilities to reliably and consistently implement pumping schedules that maximize energy cost savings at pump stations with time-of-use energy billing.
You can read my previous post to learn how the Optimization Suite not only reduces the cost of energy but also the amount of energy used by a pump station. How are you taking advantage of Time of use rates?