Energy saving opportunities - Speed variation
From Electrical Installation Guide
|Savings can be made by sizing motors correctly and using speed control and/or a variable speed drive|
A number of technologies can be used to vary flow or pressure within a system (see Fig. K9). The technology chosen will depend on how the pump and fan have been designed. For example, the pump used may be a displacement or centrifugal pump, and the fan used may be a centrifugal or axial-flow fan.
Fig. K9: Theoretical energy savings based on reducing fan speed by half
Every time a fan or a pump is installed with a view to achieving specific flow or pressure levels, sizing is based on maximum demand. As a result, oversizing is the norm, and the device concerned will not operate efficiently at other speeds. In general, systematic oversizing, combined with the ineffective control methods described above, allows scope for significant energy savings to be made by using control methods aimed at reducing the pump or fan’s supply current during periods of reduced demand.
Systems with fans and pumps are governed by certain correlations:
- Flow is proportional to shaft speed, e.g. reducing speed by half reduces flow by the same amount (see Fig. K10).
Fig. K10: Relationship between energy and flow for different methods of fan control (damper, inlet vanes and variable speed)
- Pressure or head is proportional to the square of the shaft speed; halving the shaft speed reduces pressure by a quarter.
- Energy is proportional to the cube of the shaft speed.
Halving the shaft speed reduces energy consumption by an eighth and, by implication, halving the flow reduces energy consumption by an eighth.
In light of this, energy consumption can be reduced in cases where the fan or the pump does not have to generate 100% of the flow or pressure. The savings involved are significant, even where the flow is only reduced by a small amount
(see Fig. K11). Unfortunately, the efficiency losses incurred by the various components mean that these theoretical values cannot be achieved in practice.
|Control of stopping and starting||This method is only effective when intermittent flow is acceptable.|
|Control valve: a valve is used to control flow by increasing frictional resistance at the pump’s outlet.||Energy is wasted, as the flow produced by the pump is subsequently reduced by the action of the valve. In addition, pumps have an optimal operating level and increasing resistance by this method may force the pump to operate at a less efficient level (with additional energy loss) where it may be less reliable.|
|Bypass device: with this method, the pump turns continuously at full speed and excess fluid at the pump’s outlet is channelled upstream, causing flow to be reduced without the risk of outlet pressure increasing.||The system is very inefficient, as the energy used to pump excess fluid is completely wasted.|
|Multiple pumps or fans: these configurations support ad hoc increases by activating extra pumps or fans, making control difficult.||There is usually a loss in efficiency, as the actual need is often somewhere between the different speeds available.|
|Damper: a similar technology to the control valve in systems with a pump, this reduces flow by partly obstructing the fan’s outlet.||Energy is wasted, as the flow generated by the fan is subsequently reduced by the action of the damper.|
| Overflow valve: a similar technology to the bypass valve in systems with a pump. The fan rotates at full speed continuously and the excess gas flow is evacuated.||The system is very inefficient, as the energy used to propel the air or gas is completely wasted.|
|Fan with adjustable blades: the flow can be changed by adjusting the blades.||Energy is wasted, as the flow generated by the fan is subsequently reduced by the action of the blades.|
|Inlet guide blades: fins are used to obstruct or facilitate gas flow inside a fan, thereby determining its efficiency.||The fan does not generate excess flow, but does not operate at maximum efficiency either.|
Fig. K11: Examples of technologies which may benefit from using a variable speed drive
Using a variable speed drive (see Fig. K12), as opposed to the technologies discussed earlier, constitutes an active energy efficiency method and provides the type of variable efficiency required for optimal pump or fan operation.
Altivar 12 (< 4 kW ) Altivar 21 (< 75 kW) Altivar 71 (< 630 kW)
Fig. K12: Altivar drives with different power ratings
Certain scenarios favour simple solutions:
- When changing the dimensions of the pulleys enables fans or pumps to turn at their optimal speed. This solution does not afford the flexibility associated with variable speed drives, but it involves little work and could well be covered by the maintenance budget without the need for any additional investment.
- When the fan or pump can operate at full speed continuously without the control features referred to above being installed, or with these control features installed but unused (e.g. with dampers and valves fully opened). Under this arrangement, the device will operate at or near optimum efficiency.
In reality, the potential savings will depend on the model of the fan or pump used, its intrinsic efficiency, the size of the motor, annual operating hours and the cost of electricity locally. These savings can be calculated using special software or can be estimated with some accuracy by installing temporary meters and analysing the data obtained.