Pumping systems comprise almost 25 percent of all motor energy usage in building and HVAC, water and wastewater, oil and gas extraction and transportation, and irrigation applications. Efficient pumping systems can save users more than 50 percent in potential energy costs.
In addition, electrical energy costs represent 40 percent of the total cost of ownership (TCO) of pumping systems (see Figure 1). Yet organizations struggle to leverage cost reduction through efficiency improvements. The following major barriers need to be recognized and addressed:
- Energy efficiency has traditionally not been a primary focus for assessing system performance. In most organizations, energy utilization has only just been introduced as a corporate metric.
- A lack of awareness in overall pump system energy efficiency opportunities is common. As a result, potential savings and other benefits have been missed.
- Lack of funding has resulted in operations personnel struggling to present attractive investments to their senior management.
An energy management plan reduces pumping systems’ TCO while maintaining sustainability objectives.
Figure 1. Typical pump life cycle cost profile (Courtesy of Hydraulic Institute and Pump Systems Matter)
Energy Efficiency Management
The Department of Energy is currently developing minimum efficiency standards for pumping systems. In addition, industry trade and government organizations such as the Air Conditioning, Heating, and Refrigeration Institute (AHRI) and the American Society of Heating, Refrigeration, Air Conditioning Engineers (ASHRAE) have joined the DOE in introducing standards to mandate minimum efficiency requirements for motors and variable speed drives (VSDs). The DOE Energy Policy Act and ASHRAE 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings) are some of these standards.
In pumping systems, most inefficiency comes from:
- A mismatch between the pump deployed and the actual system requirement (i.e., undersized or oversized)
- The improper use of throttling valves and flow-restriction technologies to control the volume and pressure of liquids
Figure 2. Energy saved with variable versus fixed speed drives at 100 percent and 60 percent flow, according to the static head and pump sizing. The operating point is represented as the intersection of the pump curve with the system curve.
The example in Figure 2 compares two installations (one with a variable speed drive and one with a fixed drive throttled system) in which static heads (height difference between the source and the end use) are different.
- At fixed speed (in the throttled system example), a throttle valve must be added to the hydraulic circuit. This adjusts the flow by increasing or decreasing the flow resistance. This will modify the system curve. However, the speed remains the same, so the pump curve does not change. The flow rate is matched, but the head is much higher than required, resulting in poor energy savings.
- If a variable speed drive is deployed, the system curve does not change. The pump curve is modified according to flow speed and affinity laws (rules of hydraulics that express the relationship between variables involved in pump performance such as head, volumetric flow rate, shaft speed and power). Adjusting the speed matches the process requirement and results in significant energy savings.
Energy savings depend on the static head: the lower the static head, the bigger the energy savings (and speed variation range). Pumping action requires generating enough power to overcome the static head. The friction head is the amount of head required to push the liquid through the pipe and fittings. It depends on flow rate, pipe size, pipe length and viscosity.
- Scenario 1 (see Figure 2, left): The static head represents 50 percent of the system head, and the pump is rated for the head and flow of the system. At 100 percent flow, the power consumed by the pump is the same at both fixed speed and with a variable speed drive. At 60 percent flow, the energy savings resulting in the variable speed drive use is 46 percent.
- Scenario 2 (see Figure 2, right): The static head represents 85 percent of the system head, and the pump is oversized by 20 percent. Therefore, a variable speed drive saves 20 percent of energy at 100 percent flow and saves 36 percent energy at 60 percent flow.
Changing the operating point on the pump curve also changes the efficiency of the pump. The pump performs at maximum efficiency at its full capacity, which corresponds to its best efficiency point (BEP). The objective is to work as closely as possible to the BEP. By varying the speed, the pump efficiency remains roughly the same but is applied to a new flow rate. At fixed speed, reducing the flow rate quickly deteriorates the pump efficiency (because it works far from the BEP) while adjusting the speed keeps the efficiency close to the BEP (see Figure 3).
Figure 3. Comparison of two efficiency scenarios at different flow rates: 8 to 9 percent more efficient with variable speed drives at 60 percent flow
Physical assets, such as pumps, need to be maintained on a regular basis. Maintenance costs represent 25 percent of TCO (see Figure 1), so examining maintenance practices can make a positive contribution to energy-influenced savings.
The challenge behind reducing maintenance costs is ensuring system integrity remains stable, but with the right steps, operators and maintenance specialists can strike an affordable balance.