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Electrical energy efficiency
Energy conservation in electrical systems sometime could be very tricky leading to problems and net loss if implementation of conservation measures are not carried out after suitable study of the existing system. This article attempts to highlight some of the pit falls normally encountered while implementing energy conservation projects. Here standard areas of energy conservation in the electrical system namely drives, reactive power management, lighting have been considered.
Motors drives
Three phase, induction motors consume more than 60% of the electrical energy used in Industries. In many cases motor never runs at the rated load. Often the motors are so much over rated that with respect to the actual load that they never utilize the full power capabilities, so the operating efficiencies of such motors is generally inferior.
One of the options of improving the operating efficiencies of such motor drives are replacing these over sized motors with adequately sized motors, which essentially has a higher efficiency than the former. Most of the rewinded motors used in industries generally tend to have lower operating efficiency, substantial energy savings can be obtained by replacing these rewinded motors with new ones. However, one needs to carry out proper efficiency tests before deciding on these issues.
Variable speed drives (VSDs) are potential power savers but mostly for cases where there is substantial variation of flow. The potential area where installation of VSD would lead to energy savings are centrifugal loads namely pumps, fans and air washers. Although pumps and fans provide the best applications for VSD retrofits, speed controls are not necessarily cost-effective for all pumps and fans. The best way to determine the cost-effectiveness of a proposed VSD installation is to look at the power needed at each operating condition with and without a VSD. The energy savings can then be calculated by taking the reduction in power at each condition and estimating the savings based on the actual (or expected) operating time at that condition. In general, good applications for variable-speed flow control and in particular VSD control, are those which:
Are fixed at a flow rate higher than that required by the load.
Are variable-flow, where throttling (by valves or dampers) provides the variation and where the majority of the operation is below the design flow.
Use flow diversion or bypassing (typically via a pressure-reducing valve).
Are greatly oversized for the flow required. This situation can occur where successive safety factors were added to the design, where a process changed so that the equipment now serves a load less than the original design, and where a system was overdesigned for possible future expansion.
Have long distribution networks.
Have flow control by on-off cycling. Such systems are usually less cost-effective retrofit candidates than those that use a throttling control.
Have a single large pump or fan rather than a series of staged pumps or fans that come on sequentially as the process needs increase.
Can reduce the pressure at the outlet of the fan or pump at lower flow. For example, a pump that pumps water into a long pipeline that can move the water at a lower pressure when the flows are low (due to the decreased frictional losses into the pipes) would be a good candidate for a VSD. A pump supplying a fire protection system where the piping is oversized and a constant pressure is needed regardless of flow would probably not save enough for a VSD to be cost-effective.
Soft starters cum energy savers
These are motor controllers applied to motors for a smooth start and save energy for a lightly loaded motors. Studies have shown that the energy savings obtained by softstarters are savings obtained by installing soft starters are dependent on the loading of the motor. Saving are pronounced for motor loading between 10-50%. However if the load is higher then 50% of the rated softstarters tend to have the efficiency reduced in comparison with that of the motor alone.
Saving obtained with softstarters are generally small and simple pay back periods are long, so the decision to use soft starters should not be only based on the anticipated cost savings alone. It should be noted that energy saving properties of softstarters may adversely affect the stability of certain drive system due to reduction of the developed torque. Soft starters also distort current drawn from the utility grid, so the decision for installation of soft starters has to be taken after serious consideration of these issues.
Lighting
Relamping, incandescent or mercury vapor lamps with more energy efficient HID lamps such as metal halide, high pressure sodium, or low pressure sodium lamps can be effective way in reducing electrical energy of lighting system. HID lamps however require proper ballasts to operate effectively. These ballasts function in three ways: they provide starting voltage, limit current, and match line to arc voltage. The use of improper ballasts will increase operation and maintenance costs, lower light output levels, shorten the lamp life, and waste energy.
Lighting Controls
Lighting controls are devices for turning lights on and off or for dimming them. The simplest type is a standard snap switch. Other controls are photocells, timers, occupancy sensors, and dimmers.
Snap switches, located in numerous convenient areas, make it easier for people in large, shared spaces to turn off lights in unused areas. Photocells turn lights on and off in response to natural light levels. Photo-cells switch outdoor lights on at dusk and off at dawn, for example. Advanced designs gradually raise and lower fluorescent light levels with changing daylight levels. Dimmers reduce the wattage and output of incandescent and fluorescent lamps. Dimmers also increase the service life of incandescent lamps significantly. However, dimming incandescent lamps reduces their lumen output more than their wattage. This makes incandescent lamps less efficient as they are dimmed. Dimming fluorescents requires special dimming ballasts and lamp holders, but does not reduce their efficiency. The success of any lighting controls installation depends on careful tuning of the system and special attention to the needs of the space's occupants.
So, it is clearly seen that a detail study is required for the success of any energy conservation initiative.
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