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Sustainability: Electric efficiency

Sustainability: Electric efficiency

23/07/21

Electricity is what drives a HVAC system. Compressors, fans, circulation pumps… Everything that moves is electrically driven. It’s not hard to imagine how much energy consumption can be reduced if those electromotors are replaced by more efficient ones. This blog has the potential to save you a great deal in costs!

A HVAC system is nothing more than a number of physical processes being driven by various main components. The use of these drive components is responsible for 75% to 95% of the HVAC’s total electricity consumption. Considerable savings can be enjoyed by making use of efficient solutions such as:

  • Brushless DC motors;
  • Magnetic bearing compressors.

Brushless DC motor

Brushless DC electro motors – also known as electronically commutated motors (ECMs) – have many advantages. They require no maintenance, have instantaneous speed control and are low on noise levels. Unlike their brushed counterpart, ECMs do not require electrical conducts (brushes) that slide against the shaft as it rotates. This brushing normally results in friction and sparks, which is why ECMs are ideal for use in potentially explosive environments.

Like conventional electric motors, brushless motors use magnets and a magnetic field. A current that runs through one of the coils activates a magnetic field and attracts the (permanent) magnets – which are fixed on the rotating part – to start moving. Like a carrot on a stick, the current jumps over to the next coil so the magnets keep going after it.

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The position of the rotor and the speed at which the coils are activated is determined by a controller, which is often equipped with a microprocessor. This allows the motor to rotate at variable speeds/flows and replaces the use of a frequency inverter.

Unfortunately, the maximum power provided by a brushless DC motor is limited and the threshold for ECMs lies at around 4 kW. When converting electricity into mechanical power in this range, the brushless motor is more efficient than a conventional electro motor with electric efficiencies of around the 92% mark compared to the 80%-85% of conventional three-phase motors.

Magnetic bearing compressors

One of the systems that weighs heavily on the e-load are the chillers and provision cooling plants. The main electric energy consumer is the compressor that basically sets the whole cooling cycle in motion. There are many different types of compressors, ranging from screw to piston and scroll. The problem with conventional compressors is that a lot of energy is lost due to friction. Mechanical parts involved in the process rub and twist against each other and oil is used as a lubricant in order to keep those losses to a minimum.

Choosing a magnetic compressor eliminates the friction problem. This type of compressor has its shaft hanging in a magnetic field. The magnetic bearings allow the compressor to operate without the use of oil for lubrication, reducing any energy losses due to friction. The absence of oil has another advantage which is that the heat transfer efficiency is much higher without a film layer of oil in the evaporator and condenser.

Conventional compressor electromotors of the same size have efficiencies of around 92%. Magnetic compressors reach electric efficiencies up to 97%. The advantage of a magnetic compressor is that it delivers the same cooling capacity with a lower electrical power as it does not suffer from frictional resistance.

Final thoughts

Drive components – the electromotor which converts electricity into mechanical energy and powers all physical processes in the HVAC installation – consume the most energy. Electricity is not evenly transferred into mechanical energy due to heat and friction. Efficient solutions like the brushless motor and magnetic compressor have a higher transfer rate because they mainly reduce the friction.

Electro motor efficiency improvements from 85% to 92% (fans/pumps) or 92% to 97% (compressors) may look like marginal gains, but on top of other sustainable measures, the total savings will be significant. Moreover, these solutions offer other improvements in addition to e-motor efficiency such as an absence of oil, low maintenance costs and reduced noise levels.


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