Saving energy on heating, cooling loads
Part 2 of this 3-part series provides details on how to specify energy-efficient heating and cooling systems.
One of the best ways to save money on an electric bill is to reduce the amount of energy used in a facility, either through the use of more energy-efficient devices or through intelligent building-control systems. While specific energy-efficient devices directly cut costs, intelligent building controls leverage new or existing technology to reduce energy consumption through advanced load-management strategies.
Three main load types dominate electric use in commercial facilities: lighting, HVAC, and power plug loads (from office devices and computers). Combined, these three represent almost 85% of all electric energy consumed by commercial customers. According to the U.S. Energy Information Administration (2008), 39% of commercial-building energy is used for lighting, 28% for HVAC, and 33% is used for computers, office devices, refrigeration, and other electrical equipment.
This second part will explore ways to save money by specifying energy-efficient HVAC options.
Adjustable frequency drives (AFDs)
AFDs are advanced motor-control systems that can save dramatic amounts of energy in variable torque applications. They function by converting the ac line power to dc, storing it in large dc capacitors, and then using an inverter to convert back to ac power at a desired frequency.
When voltage frequency is varied dynamically, load requirements can be better met. For variable torque loads, such as HVAC units or water pumps, AFDs allow the flow rate to be varied without having to physically restrict the air or water with a valve or damper.
Estimated opportunity: In the traditional HVAC system, a mechanical damper is used to control the amount of airflow. Thus, if the building only requires 50% of the full capability of the system, the damper will close the duct halfway. The loading on the blower will decrease as the amount of air being supplied to the intake is reduced. However, the motor still has to provide the full load of the blower, such that for 50% load, the system is still using more than 75% of the rated energy.
In Figure 3, the loading for a damper-controlled HVAC system is represented by the top curve. The bottom curve shows the energy used by the system with an AFD-controlled fan. Rather than physically restricting airflow through the duct, the AFD directly controls the speed of the blower motor. To run the blower at half speed and achieve 50% flow, the AFD regulates the electric frequency of the motor to 30 Hz, or half of the nominal. This drastically reduces energy consumption, through what are known as affinity laws between motor speed (S), flow (F), torque (τ), and power (P), as described in the following equations.
From these relationships it can be seen that at 50% loading, the AFD uses only 12.5% of the full load power, providing 63% total energy reduction when compared to the damper-controlled system. This amount of savings will increase as the speed is reduced, and will decrease as the system is operated closer to full load.
Return on investment: By taking into account the cost per kWh of energy, the motor power, the motor efficiency, the AFD efficiency, the cost of the AFD, and the amount of time spent at various levels of output, annual energy savings and payback time can be calculated. For systems in which the speed is controlled frequently and to lower levels, as might be typical for HVAC applications, the savings are very large. For systems with less speed change, the payback period may be longer; however, there are often incentives available from the electric utility.
When used in the proper applications, AFDs have the potential for tangible and rapid payback. Consider Figure 4 as one example; it shows the energy cost for operating a fan system using an AFD compared to an outlet damper or inlet valve at different speed set points throughout the year. The total energy savings for this AFD system equates to almost 35,000 kWh, or $4,127/yr; therefore, a $5,000 AFD may result in a payback period of between 1 and 2 yrs, depending on use.
Daniel J. Carnovale is a senior member of the IEEE and the manager of Eaton’s Power Systems Experience Center (PSEC), where he develops and teaches technical seminars on power systems and power-system analysis and conducts power-quality site investigations for commercial, industrial, and utility power systems. Ansel Barchowsky is a doctoral student in electrical engineering at the University of Pittsburgh, a student member of the IEEE Power Engineering Society and IEEE Power Electronics Society, and a design consultant for Eaton’s PSEC. Brianna Groden is an engineer at Eaton’s PSEC, responsible for teaching power-quality and power-distribution topics, running demonstrations, and designing and managing the installation of new demonstration projects.