Chiller system motors and fans are often too big for the job. Larger motors are typically more efficient, but they may waste a significant amount of energy if they frequently operate at less than their rated capacity. Variable speed drives (VSDs) save energy by reducing fan and motor speed to match the lower or varying demand.
VSDs and chiller systems
Motor power consumption is proportional to the cube of motor speed; a decrease in speed leads to a larger decrease in energy use. For example, a 20% reduction in speed will yield roughly 45% in energy savings.
VSDs can help reduce the energy use of four major chiller components:
- Compressor motors (refrigerant loops)
- Pumps (water loops)
- Cooling tower fans
- Air handling unit fan motors
The application of VSDs typically reduces a chiller’s annual energy consumption by up to 30%, according to Johnson Controls. The savings can be much higher, depending on the type of facility and equipment.
By reducing the rotational speed of a compressor motor in response to off-peak, low-load conditions, VSDs can reduce the energy use of chillers. For water-cooled chillers, the use of VSDs can also reduce pump energy use. Especially at low loads, both types of VSD chillers can operate using significantly less energy.
Many manufacturers offer VSD-driven chillers as an option. Most existing chillers can also be modified unless the manufacturer doesn’t offer a VSD retrofit kit.
The many benefits of VSDs
The advantages of installing VSDs go beyond energy savings. Additional benefits include:
- Less stress and wear on fans and motors, improving their service life
- Reduction in the number of chillers and related equipment
- Decreased chilled water pumping costs
- Minimize damage to sensitive electrical equipment (less electrical current on startup)
- Reduction in size and cost of emergency generators (soft start capabilities)
As changes in the weather affect the temperature of the condenser water supply, VSDs will adjust chiller performance to compensate for variations in the compressor load.
VSDs can provide a good return on investment, especially in areas with high electricity rates. If your facility is charged for low power factor, VSDs can save you money by improving your power factor rating. Applying VSDs to induction motors can also help increase their power factor.
VSDs take a “byte” out of energy costs at data center
At a 100,000 square-foot data center, VSDs were installed on all mechanical cooling pumps, chilled water pumps and data floor air-handling units. As a result, fan speeds decreased by an average of 42%. At a total installation cost of about $45,000, the VSDs resulted in an annual savings of 385,000 kWh or more than $42,000 for a payback slightly over one year.