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? • Energy Conservation and its Management has become a prime factor for the nation, society and individual due to high cost and non availability of energy. • Non awareness of latest technologies and methods, non-conventional energy sources and renewable energy sources and how to run the plant and equipment in an energy efficient manner.
Our Concern • Though, energy losses in power distribution equipment including switch gear, transformer, transmission and distribution lines etc are also of our concern, but they are smaller amount as compared to losses in electrical motors used for any drive. • Electrical motors when not selected properly can cause energy losses as high as 25% to 30%. • 3.The motors designed in lower frame might not be properly selected and have lower margins when operated at abnormal operating conditions run with very low power factor and efficiency and resultin to high losses and consumption
Electrical Drives 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.
Type of Drives 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.
Individual 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.
Group Drive 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.
Multi-motor Drive 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.
Various Types of Driven Equipments • Pump; • Fan; • Compressors; • Conveyers; • Mill; • Crusher; • Crane; • Hoist; • Traction;
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.
Energy Management 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 consumption. 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.
Energy Conservation Plan 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.
Implementation of energy conservation measures • The energy conservation measures include: • 1. Method of installation i.e. recycling (i.e. using scrap), retrofitting and changing process ( from existing to more efficient one); • 2. Method of heat use (i.e. installation of equipment for waste heat recovery and utilization, waste material utilization and process efficiency improvement); • 3. Changing the equipments with energy efficient i.e. energy efficient motors and drives; • 4. Improving the power factor of the system; • 5. Utilizing the energy during off peak period;
Energy conservation in Mechanical systems • Various Mechanical drives are pumps, fans, compressors, mill, crushers, hoists, tractions system etc. In all these drives, there are mechanical losses like friction and windage losses. • In the bearings, friction loss occurs. In order to reduce the frictional loss the surface are made smooth and bearings are lubricated. The size and grade of bearing is decided based on the torque on shaft. Small and medium range drives are provided with ball and roller bearings while Higher capacity and higher speed drive are always provided with sleeve bearing with pedestals. These bearings are forced oil lubrication. The oil is cooled by water separately. The quantity of oil will depend on the heat developed in the bearing. • The pump and fan has the blades in rotation. While rotating, there is friction windage loss in fan or blower, which is minimized by proper angle of blade design. Similarly, the blade angle of pumps is designed to minimize the friction loss of water in the pump. Similarly, for compressor, the friction occurs because of movement of piston in the cylinder, the friction loss is minimized to have proper lubrication and gap between piston and cylinder. There may be other losses in pulley drive; belt drive and gear drive mechanism which is designed properly to have minimum mechanical losses.
Energy conservation during design 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. • The important design features worth mentioning are: • More copper section in stator and rotor thus reducing I2R loss; • 2. Better quality of stampings and lamination to reduce the iron loss; • 3. Enhanced cooling by adopting improved design of fans and ventilation circuit; • 4. Designing for lower flux density for improving power factor;
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
Observations • From the above table, it can be seen that the efficiency and power factor of Frame 1LA7 710-6 is more than the frame 1LA7 636-6. • The saving in power loss by using 1LA7 710-6 motor is ( 53.6 – 31.66) kW = 21.94 kW. If the motors are running continuously, then the number of unit saved is 24x 21.94 = 526.56 kWh per day per motor or 192194 kWh per year per motor. • Taking the unit charge as Rs 4 per unit; the total saving is Rs 192194 x 4 = Rs 768776. • 4. Let us take 15% interest and depreciation. The extra expenditure per year on the cost is 2x0.15 Lakh = Rs 30000. The pay back period is 30000 x12/768776 months = 0.468. It is less than 15 days only. It may be noted that the working hours were taken 24 hours which is not realistic. Let us take 8 hrs per day. The amount saved per year is Rs 768776x8/24 = 256259/-. Therefore payback period is 0.468x3 = 1.4 months. • 5. If, there are 1x105 ( 1 Lakhs) motors operating every day, we can save power 21.94 x 1x105 kW = 2194 MW.
Variable Speed Drives 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 Let 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.
Energy conservation by Adjustable speed control 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.
3. Synchronous 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 Ic V 2 1 I2 I1
The important disadvantages of low power factor are: • Higher currents require larger size cable, switchgears, transformer and alternators etc. Thus the capital cost of the equipment is increased. This is, uneconomical from the supplier's point of view. • Higher currents give rise to higher copper losses in the system and therefore, the efficiency of the system is reduced. Also, the cost of energy loss (that is running cost) in the system increased. • 3. Higher currents produce larger voltage drop in cables and other apparatus. This results in poor voltage regulation.
Optimization of Power factor correction when Power is same 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
Optimization of Power factor correction when KVA demand is same 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
Power Factor Improvement Using Synchronous Condensers 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. Advantages 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.