Energy Management and Conservation. By Prof. K.Prasad LNCT, Bhopal. Management is required when there is a Crisis Regulation Competition Waste. Implementation of energy demand side management is to Eliminate the waste; Minimize the losses. . Why Important?.
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The general meaning of a Drive is the system, which is driven by some energy. The source of energy may be any thing like wind, water, oil, steam, solar or electricity etc.
When the source of energy is electricity, the drive is called Electric Drive. In any drive system, we take some output in terms of energy or work done.
Utilization of electricity for driving the mechanical system employs the use of Electric Motors, which gives an output in terms of Mechanical Energy. These electric motors are DC Motors, Synchronous motors or Induction Motors.
Many industrial applications requiring rotating electric drives are normally capable of speed control and often require an equipment to attain a versatile and smooth speed control and make the motor to operate at a desired specific speed torque characteristic. These drives are characterized by the nature of speed torque characteristic such as constant torque drives and constant power drives. These are sometimes characterized by the type of motor used in the drive i.e. dc and ac drives making use of dc and ac motors respectively.
The various types of electric drives used in industries may be divided into three types:
1. Individual motor drive;
2. Group drive;
3. Multi-motor drive.
In individual drive, a single electric motor is used to drive one individual machine.
The machines can be placed in any desired position and can be moved very easily. The machines can be built as an integral part of the complete system, which results in a good appearance, cleanliness and safety. For the purposes where constancy of speed and flexibility of control is required, such as in paper mills and textile industry, individual drive is essential.
By group drive is meant a drive in which a single electric motor drives a line shaft by means of which an entire group of working machines may be operated.
It is also sometimes called the line shaft drive. The line shaft is fitted with multi-stepped pulleys and belts that connect these pulleys and the shafts of the driven machines serve to vary their speed.
This drive is economical in consideration of the first cost of the motors and control gear. A single motor of large capacity costs less than the total cost of a number of small motors of the same total capacity
The efficiency and power factor of a large group drive motor will be higher, provided it is operated fairly 10% overload when being driven by group drive.
This form of drive has become obsolete now-a-days because of its following draw-backs and objectionable features, and the modern trend is to employ individual and multi-motor drives:
1.In group drive, speed control of individual machine is very cumbersome using stepped pulleys, belts etc.
2.Owing to use of line shafting pulleys and belts group drive does not give good appearance and is also less safe to operate.
3.In group drive since machines have to be installed to suit the layout of the line shafting, as such flexibility of layout of the various machines is lost. Also it is not possible to install any machine at a desired place.
4.The possibility of installation of additional machines in an existing industry is limited.
5.If, at any time, all operations are not required, the main motor will work at low capacity and, therefore, operation efficiency will be low.
6. The breakdown of large single motor causes all the operations to be stopped.
It consists of several individual drives each of which serves to operate one of many working members or mechanisms in some production unit.
Such drive is essential in complicated metal-cutting machine tools, paper making machines, rolling mills, and similar types of machinery. The use of multi-motor drive is continuously expanding in modern industry as their advantages outweigh the increase in capital cost as compared to the group drives.
The use of individual and multi-motor drives has enable the introduction of automation in production processes, which in turn has considerably increased the productivity of various industrial organizations. Complete or partial automation helps to operate various mechanisms at optimum conditions and to increase reliability and safety of operation.
Amongst motor used in any modern plant and industry, the maximum number of motors used is 3-phase Induction motor because of their cheapness, robust construction and satisfactory performance. Their maintenance in service is also easier as compared to other types of electrical motors.
The Torque – speed characteristic, inertia and duty cycle of load will mainly determine the electrical characteristic and rating requirements of the driving motor.
The proper selection of motor rating and design will result in a minimum motor cost for a specified motor life expectancy, torque-speed characteristic, inertia and duty cycle of load.
The selection of under size motor for low motor cost however may result in overloads and a consequent reduction in motor life.
Additionally, in actual practice, motors are subjected to abnormal operating conditions because of system inability to maintain the normal operating conditions when some accidents or faults take place.
Any form of abnormal operating conditions may affect the performance, high losses and life expectancy of the motor. The degree of deterioration depends upon the magnitude and nature of abnormal conditions. Therefore, the abnormal operating condition may also be taken in to account in design stage so that its withstand capability is increased. Proper protection shall also be employed to save motor against severe damage.
The energy management is the work for energy manager assigned by the management who will exclusively look about energy matter such as
1. A detailed energy monitoring system.
2. Comparison of specific energy consumption values on a
monthly and yearly basis.
3. Exploring possibility of improvement in energy
The Energy Audit is carried out to critically examine each of the major energy consuming units to determine whether there exists any unwanted use of energy, losses, idle/redundant running etc. All efforts should be made to run the machine at full/optimum capacity.
Specific energy consumption value is the index to determine how effectively the plant and machinery are utilized in any industrial process. The KWH/ton or KWH/unit of production is calculated in each month and energy consumption indices are worked out separately for major equipment and process. These are then compared on a monthly and yearly basis regularly to detect any deviation from the norm (targeted value) and to take necessary correcting steps.
On identification of areas where electrical energy is not efficiently utilized, remedial measures are to be taken to either replace the old equipment with energy efficient or to implement with energy efficient equipment or to implement modifications to make them more energy – efficient.
Induction Motor represents the majority of all electric motors used in industries world- wide. Therefore, there is tremendous amount of energy consumed in operating these motors.
A case study for 750KW, 6.6KV, 6 Pole Squirrel cage Induction motor which is optimally design for the specification as given below:
Starting torque – 90%
Starting current - 600%
Maximum torque - 230%
Temperature rise - 70 0 C
Out of various design, two close designs were selected with the following parameters as shown in Table
In pumps and fans using constant speed motor with conventional control, variation in flow is achieved by means of throttling valve or damper as shown in Fig.
The Power consumed
P Q H/ (m xp)
Where Q = Delivery of pump in m3/sec
H = pressure head (m)
m = motor efficiency
p = pump efficiency
s = m p
Q1 = 4; H1 = 200
Q2 = 3.6; H2 = 240
Let us assume that there is no change in system efficiency. Then
P1 Q1 H1 4 x 200
= = = 0.9259
P2 Q2 H2 3.6 x 240
Therefore P2 = 1.08 P1
Fig.: Pump curve at different speed with constant resistive curve for lower quantity
Q1 = 4; H1 = 200
Q2 = 3.6; H2 = 160
P1 4 x 200
= = 1.39
P2 3.6 x 160
Therefore, P2 = 0.72 P1
It means, the power required by variable speed motor is only 72%. It is also possible to keep the system efficiency same as original one. Even if the system efficiency is down by 1%, the power required by pump system will be 72x85/84 = 72.85%. It can be seen that there is an advantage of having the variable speed motor. We can also see that in case of still lower delivery, the power reduction in percentage will be more because of lower head.
1. DC Motor
DC motor with variable speed by rheostatic control in field and armature would give speed control. Some losses also occur in the resistance. DC motor with Thyristor converter is widely used for efficient and precise speed control in steel mills, Cement Plant, paper and textile mills. With fully controlled bridge, regenerative braking can be achieved.
2. Induction Motor
In case of squirrel cage motor multispeed winding may help the operation at different speed while speed change can be achieved with the help of addition of rotor resistance. Energy conservation in Slip ring Induction Motor with Slip recovering is one of best method.In conventional SRIM, introducing resistance in the rotor circuit varies motors speed. Since power is absorbed by rotor resistance, efficiency of the motor drops as the speed decreases. The power otherwise wasted in rotor resistance can be fed back to the system by using a static converter, which converts the slip frequency power to DC and then converting in to three phase which would be fed to main supply for controlling the speed. This is fed back to the line with a line-commutated inverter. The variable voltage and variable frequency is also adopted for variable speed in Induction motor.
The speed change is achieved by application of Variable frequency variable voltage.
Fans – a case study
To have similarity of measurements, two 210 MW units at Vijaywada Thermal power station of Andhra Pradesh State Electricity Board were selected for carrying out site measurements and subsequent energy consumption comparisons. Unit 3 & 4 are 210 units and having tower type boilers supplied by the same source.
Fixed speed induction motors drive unit 3 ID fans. The fan is coupled to induction motor through a hydraulic coupling so that the fan speed could be varied by scoop tube control.
Unit 4 ID fans are driven by Variable Frequency Drive. The fan is coupled to the motor through a flexible coupling. A synchronous motor fed from Load Commutated Inverter (LCI) was used as a variable Frequency drive.
The study conclusively proves that introduction of Variable Frequency Drive system for flow control application has a definite advantage in terms of substantial energy savings as shown in Table.
Variable Frequency Drive has got the following advantages in addition to power savings:
(1) Increase in life of equipment due to soft start(2)Unlimited number of starts (3)
Assimilation of plant automation system for higher productivity.
Taking the cost of Control Rs 3 to 4 Crores, Payback period can be of the order of 4-5 years only.
Energy conservation by Improving the power factor in case of Induction motor
For improving the power factor, there should be reduction in reactive power. For, inductive load, leading reactive power is required and for capacitive load, lagging reactive power is required.
I1 = current before pf improvement;
I2 = current after pf improvement;
1 = pf angle before pf improment;
2= pf angle after pf improvement.
P = Power consumed = V I1Cos 1= V I2Cos 2
If x is the annual cost per KVA of maximum demand then annual saving in the KVA demand charges
= x ( S1 – S2) = x P(Sec 1 – Sec 2)
If y is the annual cost per KVAr of the power factor correction equipment then annual cost of the power factor correction equipment.
CPF = y Qc = y P ( Tan1 – Tan 2)
The total annual saving ,
Cs = CD - CPF = x P(Sec 1 – Sec 2) - y P ( Tan1 – Tan 2)
Condition for optimization is
Sin 2 = y/x
For the same kVA, the power can be conserved or productivity can be increased.
Let us assumed that z is the annual cost per KW of the installation, then the annual saving due to increased power out would be
= z S ( Cos 2 – Cos 1)
Let y is the annual cost per KVAr of the pf correction equipment then the annual cost of the power factor correction equipment is given by
CPF = y Qc = y S (Sin 1 – Sin 2)
The net saving would be
Cs = z S ( Cos 2 – Cos 1) – y S ( Sin 1 – Sin 2)
For maximum annual saving
Tan 2 = y/Z
When the KVAR requirement is small, it can be met through static capacitors. However when requirements exceed 10,000 KVAR, it is generally more economical to use the synchronous condensers.
A synchronous condenser is essentially an over excited synchronous motor. Generally, it does not supply any active mechanical power. The excitation of the machine is varied to provide the necessary amount of the leading KVAR.
1. By the use of synchronous condenser a finer control is possible than by the use of static capacitors.
2. A synchronous condenser can be overloaded for short periods but a static capacitor cannot be overloaded.
3. A momentary drop in voltage causes the synchronous condenser to supply greater KVAR to the system whereas in the case of static capacitor, the KVAR supplied is reduced.
4. The inertia of the synchronous condenser improves the system stability and reduces the effect of sudden changes in load.