Energy saving opportunities-
| In industrial applications, motors account for 60% of the energy consumed |
A number of different measures can be adopted to save energy (see Fig. K7).
- Reduce energy use
These measures try to achieve the same results by consuming less (e.g. installing highly energy-efficient lights which provide the same quality of light but consume less energy) or reduce energy consumption by taking care to use no more energy than is strictly necessary (e.g. another method would be to have fewer lights in a room which is too brightly lit).
- Save energy
These measures reduce costs per unit rather than reducing the total amount of energy used. For example, day-time activities could be performed at night to in order to take advantage of cheaper rates. Similarly, work could be scheduled to avoid peak hours and demand response programmes.
- Energy reliability
As well as contributing to operational efficiency by avoiding lost production, these measures avoid the energy losses associated with frequent restarts and the extra work generated when batches of products go to waste.
Fig. K7: An overall strategy for energy management
Everyone immediately thinks of equipment for transforming energy (motors, lighting/heating devices) when considering areas where savings can be made. Less obvious, perhaps, are the potential savings offered by the various control devices and programmes associated with this type of equipment.
Motors
Motorised systems are one of the potential areas where energy savings can be made.
Those wishing to improve passive energy efficiency often consider replacing motors as a starting point. There are two reasons for this:
- To benefit from the advantages offered by new high-performance motors (see. Fig. K8)
Fig. K8: Definition of energy efficiency classes for LV motors established by the European Commission and the European Committee of Manufacturers of Electrical Machines and Power Electronics (CEMEP)
- To rectify oversizing
Motors operating for long periods are obvious candidates for replacement by high-performance motors, particularly if these existing motors are old and require rewinding.
Depending on the power they generate, high-performance motors can improve operational efficiency by up to 10% compared to standard motors. Where motors have undergone rewinding, efficiency is reduced by 3% to 4% compared to the original motor.
By contrast, replacement with high-performance motors will not prove to be cost effective if the existing standard-efficiency motor – particularly if it has not undergone rewinding – experiences low or moderate levels of use (e.g. less than 30,000 hours per year). It is also important to ensure that the new motor’s critical performance characteristics (such as speed) are equivalent to those of the existing motor.
- As well as being inefficient, oversized motors are more expensive to buy than correctly sized motors. Motors are at their most effective when operating at between 60% and 100% of their nominal load. Efficiency reduces rapidly at loads below 50%. In the past, designers tended to develop oversized motors in order to provide an adequate safety margin and eliminate the risk of failure, even in conditions which were highly unlikely to occur. Studies show that at least a third of motors are clearly oversized and operate at below 50% of their nominal load. The average load for a motor is around 60%.
Larger motors also tend to have lower power factors, which can lead to charges being levied for reactive power. When deciding whether to replace a motor, it is essential to take these factors, as well as the motor’s remaining life cycle, into consideration. It is also important to remember that the expense of replacing an admittedly oversized motor may not be justified if its load is very small or it is only used infrequently.
All things considered, every parameter needs to be taken into account before making a decision on replacing a motor.
Other approaches are also possible, as far as motors are concerned:
- Improving active energy efficiency by simply stopping motors when they no longer need to be running. This method may require improvements to be made in terms of automation, training or monitoring, and operator incentives may have to be offered. If an operator is not accountable for energy consumption, he/she may well forget to stop a motor at times when it is not required.
- Monitoring and correcting all the components within the drive chains, starting with those on the larger motors capable of affecting overall efficiency. This may involve, for example, aligning shafts or couplings as required. An angular offset of 0.6 mm in a coupling can result in a power loss of as much as 8%.
- Paying special attention to pumps and fans, because:
- 63% of the energy used by motors is for fluid propulsion in components such as pumps and fans.
- Flow control often uses valves, dampers and throttles, all of which cause energy to be lost by blocking ducts whilst motors are
operating at full speed.
- Effective project planning can often recoup investments in less than ten months.
Speed variation
| 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.
