How do variable speed drives reduce energy costs?
Many plant owners are confused how variable speed drives (inverters) save them money: after all if they were that good everyone would use them, wouldn’t they? A frequently asked question is “What’s the difference between using inverter drives and my ventilation fans being thermostatically controlled?”
According to estimates, motors consume 70% of the energy used by European industry. Therefore, using variable speed drives within HVAC systems is a major component of the energy-efficiency and cost-savings game. The UK Governemnt recognises this and offers tax incentives for installing inverter drives
Centrifugal fans and pumps
Most HVAC systems are designed to keep the building cool on the hottest days and warm on the coldest days. Therefore, the system needs to work at full capacity only about 10% of the time. For the rest of the time it can operate at a reduced capacity. Standard electric motors run at a fixed speed and airflow is controlled by dampers. The analogy is to drive a car with the engine running constantly at 5000rpm and controlling the speed using the brakes-noisy expensive and wearing.
The inverter reduces the speed of the motor controlling the fan or pump when full flow is not required, thereby reducing the power and the electrical energy used. What is not understood by many facilities managers is why reducing the speed by say 10% reduces the energy consumption by 27% and not 10%. Enter the cube law
Cube Law
It’s obvious that reducing the speed of a motor reduces the power consumed by the motor. As motor speed falls, the reduction in energy required to drive the motor is determined by the cube law. Simply put, reducing the motor speed by 10% reduces the power required by the motor to 90% x 90% x 90% =72.9% of full speed: a saving of 27.1%. By the same law, if you reduce the motor speed to 60% then 60% x 60% x 60% = 21.6% or a reduction of 78.4%.
Payback time
Now consider a modern 22kW IE2 (91.6% efficient) motor running at full load for 10 hours a day 250 days a year. This will use 22 x 10 x 250/91.6% = 60,000kW of electricity in a year. Assume £0.10 per kW that’s a running cost of about £6,000. From the previous paragraph, by running it at 90% of full speed you save 27.1% of your energy consumption or £1626 a year for as long as the motor runs. By coincidence, £1626 is about the cost of a 22kW inverter from 999 Inverters, so you know the payback time too.
Other benefits
Variable speed drives also contribute to the building’s overall comfort level by optimising and regulating air flow and temperature into the building’s occupant space. Unlike a fan that runs either full speed or off, a variable speed drive can run at all speeds in between, allowing facilities managers and building occupants more control to adjust the temperature.
In addition, VSDs reduce the long-term wear and tear on equipment. The drives provide a soft start instead of slamming motors on at full speed—so motors are quieter, filters and drives belts last longer, and maintenance and unscheduled downtime are also reduced.
HVAC equipment offering opportunities for energy savings including centrifugal air handler fans and exhaust fans; centrifugal pumps for hot and chilled water; cooling tower pumps and fans.
Go here for our ranges of HVAC drives
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