Pumps & Systems, October 2008
Water and wastewater systems in the United States use a tremendous amount of power. The EPA estimates that these systems use 50 trillion watt-hours annually at a cost of $4 billion. Combined with electric rate increases upward of 20 percent in a single year, water and wastewater system operators are left with an enormous strain on their budget.
Industrial Power Bills
Although the cost of power continues to increase, there are ways to reduce monthly energy costs. Begin by examining the components of your typical industrial power bill. Power bills usually have four components; as an example, we will use numbers from the Connecticut Light and Power Company (CLPC) Rate Schedule 57. The main components are:
- 1. Customer Charge is rated loosely on the connection size. The CLPC rate is $950/month for small industrial, $2,000/month for mid-range and $4,000/month for larger industrial users.
- 2. Actual Power Used, in kilowatt-hours (kWh), is currently $0.17/kWh from 12 PM to 8 PM and $0.13/kWh for off-peak times (a 24 percent savings from peak rates).
- 3. Demand Charge of $10.04 is added for the maximum usage period of kilovolt-amps (kVA) over a 30-minute window in a revolving calendar year. Note: kVA is commonly called apparent power, and includes both real power used and reactive power on the system.
- 4. Power Factor (PF) is a penalty for variations in apparent power or the power used. The CLPC sets 0.90 as a minimum PF requirement for users, meaning real power used must be 90 percent or higher when compared to the apparent power. When a user is below 90 percent, CLPC will correct the PF and charge the consumer for the improvements; often utilities will impose a monthly penalty rather than make physical changes.
To reduce a system's power consumption, it is critical to locate places where power is wasted in the treatment process, including:
• Lightly loaded motors
• Flow control with valve or vanes
• Motors are constantly turning on and off
• Motors working against each other
This list frames the types of problems, but it does not include all possible waste areas. Because systems are designed for peak flows along with a growth factor, motors are regularly oversized for their actual use, resulting in short run cycles or valves to control speed. The use of multiple smaller motors in place of a single large motor allows users to better match the motor load to the required flow, which reduces the need for flow control valves and their potential inefficiencies.
Let's look at specific power bill components and the savings potential.
By changing the timing of pumps, a significant savings can be realized on both demand charges and changing from peak to off-peak rates (actual power used costs). Demand charges focus on the 15 or 30 minute window of maximum power used in the month; this normally occurs on the hottest day of the year for large air conditioning loads, the wettest day for storm water collection or while restarting the systems after a shut down in an industrial facility. Even if this peak is abnormally high or only occurs a single time during the month, the utility will still charge based on this peak usage.
By locating buffers in a system where there are reserves or holding capacity and then staging the motors, many processes can maintain output while reducing peak power used. Motor staging refers to the delay in starting a second or third motor until the first motor has completed its cycle; rather than filling several tanks at one time, a staged system would fill them in sequence. Remember, this does not actually reduce the power used, and the motor staging requires 15 to 30 minutes of separation to reduce the measured demand. Based on the CLPC rate schedule, this would mean a 100-hp motor shifted out of the peak demand window would save $750/month during the following 12 months (100-hp ≈ 746 kVA x $ 10.04/kVA ≈ $750). Because the CLPC used the highest month period in a rolling 12 months, that could add up to $9,000 in savings during a calendar year.
Actual Power Used
Moving loads also requires additional buffering capacity, as the load needs to move outside the peak rate window from the utilities. While the CLPC rate schedule only has two levels, some rate schedules provide a third tier. With CLPC rates, running the 100-hp motor off-peak instead of on-peak means a $30 per hour savings. Many cities already take advantage of these savings by filling water tanks at night and allowing the tanks to run down during the peak rates. While certain high-demand days may not allow shifting the load, these savings are available whenever the motor load can be shifted to off-peak rates and is not directly tied to the demand charge. However, moving a load from the peak demand period and peak rates could save on both line items on the utility bill.