ENERGY SAVINGS WITH VARIABLE SPEED DRIVES ABSTRACT K M Pauwels Energy auditor, Laborelec, Industrial Applications, Belgium This paper focuses on the economic benefits that can be obtained by replacing mechanical flow control of pumps and fans with variable speed drives. TECHNOLOGY First, different topologies of variable speed drives are discussed: Rectifier topologies (diode, thyristor, IGBT) Control of invertors by Pulse Width Modulation (PWM). A short overview of the advantages for the different topologies is given. Some restrictions on Variable Speed Drives are briefly mentioned. CASE STUDIES The flow regulation methods are: 1) load/unload regulation, 2) valve regulation, 3) frequency regulation. The 3 regulation methods are compared from an energy efficiency point of view. Depending on the load profile, energy savings up to 41 % can be obtained by applying frequency regulation instead of load/unload regulation. CASE STUDY 3 Case study 3 shows the case of a convertor in a stainless steel factory. Two 8 kw fans blow air into the convertor. The air flow is controlled by a valve. Replacing the valve with a VSD implies a yearly saving of 4.188.277 kwh. The core of the paper consists of three case studies. The case studies are taken from our energy consulting experience in Belgian industry. CASE STUDY 1 Case study 1 discusses VSD installation on the circulation pumps of the electrocoat bath in a car assembly factory. Before, flow was controlled by valve regulation. Installing a variable speed drive generated a yearly saving of 176.4 kwh for one pump. The payback time amounts to approximately 14 months. CASE STUDY 2 Case study 2 explains the economic benefits of VSD on lubricated screw compressors. Measurements on three screw compressors with different flow regulation were effectuated.
ECONOMIES D ENERGIE GRACE A LA VARIATION DE VITESSE - RESUME K M Pauwels Energy auditor, Laborelec, Industrial Applications, Belgium, kenneth.pauwels@laborelec.be Ce résumé est axé sur les avantages économiques qui peuvent être obtenus en remplaçant le contrôle mécanique de débit des pompes ou des ventilateurs par une régulation de vitesse (VSD). TECHNOLOGIE Tout d abord, différentes topologies des régulateurs de vitesse sont examinées: Topologie des redresseurs (diode, thyristor, IGBT) Contrôle des circuits par modulation de largeur ( Pulse Width Modulation, PWM). Un bref aperçu des avantages des différents techniques est donné. De même, certaines limitations des VSD sont mentionnées. CAS 2 Il explique l intérêt économique du VSD sur les compresseurs à vis lubrifiés à l huile. Des mesures sur 3 compresseurs équipés de systèmes de régulation différents ont été réalisées: 1) régulation charge/décharge 2) régulation par vanne 3) régulation de vitesse Les 3 méthodes sont comparées du point de vue rendement énergétique. Suivant le profil de charge, des gains énergétiques allant jusqu à 41 % ont été obtenu avec le VSD par rapport à la régulation charge/décharge. CAS 3 ETUDES DE CAS La base de ce document est constituée de 3 études de cas extraites de notre expérience en consultance énergétique en Belgique. Il expose le cas d un convertisseur dans une usine de production d acier inoxydable. Deux ventilateurs de 8 kw soufflent l air dans le convertisseur. Le remplacement de la vanne par un VSD a entraîné une économie annuelle de 4.188.277 kwh. CAS 1 Il traite de l installation d un VSD sur la pompe de circulation d un bain d electro-coating dans une usine d assemblage de voitures. Avant, le débit était contrôlé par une vanne. Le VSD a rapporté un gain annuel de 176.4 kwh pour une seule pompe. Le retour d investissement est d environ 14 mois.
ENERGY SAVINGS WITH VARIABLE SPEED DRIVES K M Pauwels Energy auditor, Laborelec, Industrial Applications, Belgium SUMMARY TECHNOLOGY This paper focuses on the economic benefits that can be obtained by replacing mechanical flow control of pumps and fans with variable speed drives. Firstly, different topologies of variable speed drives are discussed: rectifier topologies (thyristor, IGBT) as well as the control of invertors with Pulse Width Modulation. A short overview of the advantages for the different topologies is given. The most important restrictions are also mentioned. The core of the paper consists of three case studies, which are taken from our energy consulting experience in Belgian industry. Case study 1 discusses VSD installation on the circulation pumps of the electrocoat bath in a car assembly factory. Before, flow was controlled by valve regulation. Installing a variable speed drive instead generated a yearly saving of 176.4 kwh for one pump. The payback time amounts to approximately 14 months. Case study 2 explains the economic benefits of VSD on lubricated screw compressors. Depending on the load profile, energy savings up to 41 % can be obtained by applying frequency regulation instead of load/unload regulation. Case study 3 shows the case of a convertor in a stainless steel factory. Two 8 kw fans blow air into the convertor. The air flow is controlled by a valve. Replacing the valve with a VSD implies a yearly saving of 4.188.277 kwh. A variable speed drive (VSD) essentially consists of two main parts: rectifier and invertor (fig. 1). Fig. 1 concept of a VSD Rectifier Nowadays, two main topologies are used for medium power rectifier units: diode rectifiers M The rectifying unit consists of a diode bridge. This means that the DC link voltage is fully depending on the AC supply voltage. Some typical characteristics of diode bridges are: - Good power factor: phase shift between voltage and current is very low. - No control needed. - Introduction of line-side harmonics: a diode bridge is a non-linear load. This means that a non-sinusoidal current is taken from the feeding line. Current harmonics cause voltage harmonics. These voltage harmonics can disturb nearby charges. IGBT rectifiers To eliminate the disadvantages of diode (and thyristor) rectifiers, new solutions are developed.
These solutions are all based on the principle that the diode bridge is substituted by an inverter consisting of IGBT components (Fig. 2). IGBT stands for Insulated Gate Bipolar Transistor. Invertors with IGBT rectifiers are called Active Front End (AFE) invertors. Some features of Active Front Ends are: - good power factor, even better then diode rectifier bridge - rectifier is controlled by means of Pulse Width Modulation (PWM) - PWM technology reduces harmonic distortion to a very low level Fig. 3 Pulse Width Modulation RESTRICTIONS In spite of all the advantages, some problems can occur with Variable Speed Drives. The problems are all related to a deteriorated power quality. Production of harmonics Fig. 2 active front end Invertor The invertor uses the DC voltage from the DC bus to create an AC voltage with variable frequency (and amplitude). Nowadays, all invertors are equiped with IGBT (or similar) components. PWM control (Pulse Width Modulation) is widely used for control of the IGBT switches. PWM control consists in rapidly switching on and off the IGBT switches in such a way that pulses with variable width constitute a sinusoidal waveform (Fig. 3). Power electronics always take a distorted current from the feeding line. This causes line-side current harmonics. Some problems that may occur with lineside harmonics are: Harmonics cause supplementary heating in transformers, cables, This fastens the aging process. Nearby capacitor banks can be damaged severely by harmonics. Older types of thyristor control (based on switching at zero voltage) can be disturbed. Next to line-side harmonics, motor-side harmonics are also generated. One of the big problems caused by motor-side harmonics is overvoltage: overvoltage appears due to reflections of the voltage waveform at both ends of the transmission cable. Transmission line theory tells us that matching impedances for all components are required to reduce reflection as much as possible. Sensitivity to voltage dips and short interruptions An invertor basically consists of a rectifying and an inverting unit, linked by a DC-circuit (Fig. 1). In the DC-circuit, the capacitor is the only energy storing element. Even with a big capacity, the stored energy is low.
Example: the stored energy in a 1 mf capacitor (6 V DC) is,1 6 2 2 = 18J =, 5kWh Furthermore, the capacitor in the DC-link is limited to avoid too big inrush currents at switching on. Most important consequence of the low capacity to store energy is the fact that invertors are very sensitive to short interruptions and voltage dips. In the worst case, the control of the VSD simply blocks. Nowadays, a lot of constructors provide different kinds of techniques that improve this weak point. However, none of this solutions guarantees complete protection to short interruptions and voltage dips. CASE STUDY 1: CIRCULATION PUMP Short description (of the original set-up) A car assembly factory paints the body works by means of an electrocoat bath. The paint is attached to the body work by applying a DC voltage to the body work. To maintain temperature and concentration of the paint in the bath, the paint is continuously circulated by pumps, even when there s no production. Flow regulation is effectuated by means of valves.. Measurement results An ultrasonic flow rate measurement (case of valve regulation) revealed a flow rate of 194 m 3 /h. Taking into account the nominal flow rate of 28 m 3 /h, the frequency of the drive needed to be set to 34.8 Hz (nominal frequency = 5 Hz) in order to obtain the required flow rate. In the case of valve regulation, the absorbed power amounts to 36 kw. In the case of frequency regulation, the absorbed power amounts to 15 kw. Gain payback time The pump functions continuously during 5 weeks a year. The use of VSD thus implies a yearly saving of: (36 15)kW x 24 h/d x 7 d/w x 5 w/y = 176.4 kwh/y. For this case, the payback time (investment + installation) amounts to approximately 14 months. Measurements To be able to know the energy saving with Variable Speed Drive, the following parameters have to be measured: 1) flow rate: In order to have a reliable comparison between the two regulation methods, the flow has to be the same in the two cases. 2) Power: for the two regulation methods, the power has to be measured. The measurements were limited to one circulation pump. Characteristics of the pump Nominal power: 45 kw Nominal flow rate: 28 m 3 /h CASE STUDY 2: LUBRICATED SCREW COMPRESSORS Introduction In industry, lubricated screw compressors are widely used. For these compressors, three regulation methods exist: 1) Load/unload regulation Air flow is regulated by opening and closing the inlet vane. During the time that the vane is closed, the motor is running unloaded. 2) Valve regulation Air flow is regulated by a valve that can be put in all positions. 3) Frequency regulation Air flow is regulated by controlling the speed of the electric motor. To compare the energy consumption for the three methods, measurements were carried out.
To do so, three compressors of the same installed power were taken from one constructor, each with a different flow regulation. The power of the compressors is 45 kw. Load profiles Each of the compressors has been tested for three different load profiles: 1) profile A: compressed air consumption all day, even during weekends (fig. 4) 2) profile B: compressed air consumption all day, not during weekends (fig. 5) 3) profile C: compressed air consumption during working hours (5 am to 2 pm), not during weekends (fig. 6) compressed air flow rate (l/s) 12 11 1 9 8 7 6 5 4 3 2 1 Fig. 6 profile C Weekly profile (Sunday to Saturday) These profiles have been simulated by using control valves of different flow rate connected to a compressed air vessel (2 l). The control was made by PLC (fig. 7). compressed air flow rate (l/s) 12 11 1 9 8 7 6 5 4 3 2 1 weekly profile (Sunday to Saturday) Fig. 4 profile A 12 Compressed air flow (l/s) 11 1 9 8 7 6 5 4 3 2 1 Fig. 5 profile B Weekly profile (Sunday to Saturday) Fig.7 picture of the vessel + valves Measurements For each of the three compressors, the three load profiles were simulated during one week. This makes a total measuring time of 9 weeks (3 compressors 3 profiles). The power (1/4 h average) was measured and registered for each of the 9 situations.
Results Measurements Figure 8 gives an overview of the obtained results: Depending on the load profile, a valve regulation gives a saving from 3,9 % up to 8,8 % in comparison with load/unload regulation. The average saving amounts to 8,1 %. Depending on the load profile, a variable speed drive gives a saving from 15,7 % up to 41,1 % in comparison with load/unload regulation. The average saving amounts to 37,2 %. During one week, the position of the valve of 1 fan was registered. The opening of the valve appeared to be 6 % on average. Software simulation Simulation on software (fig. 9) gives a yearly saving of 4.188.277 kwh (for one fan). saving compared to load/unload (%) 45 4 35 3 25 2 15 1 5 41.1 37.2 31.3 15.7 8.8 8.1 7.1 3.9 profile A profile B profile C average valve regulation VSD Taking into account an investment of 15. euro, the payback time amounts to 8 months. Including the estimated installation cost, the payback time comes to 14 months. hours/year 9 8 7 6 5 4 3 2 1 9 8 7 6 5 4 3 2 kw Fig. 8 overview savings 1 1 CASE STUDY 3 Flow (%) uren/jaar Ventilator smoorklep P = f(debiet)^3 Introduction Fig 9 software simulation 8 kw fan A large stainless steel manufacturer uses an AC arc furnace to melt the steel. The molten steel goes from the arc furnace in a convertor to reduce the percentage of carbon. Essentially, air is blown into the convertor to oxidise the carbon. Two 8 kw fans blow the air into the convertor. The air flow is regulated by valves. CONCLUSIONS Invertor technology is improving continuously: more performant components in combination with more performant control increase accuracy and speed of control, and reduces current harmonics. The three case studies taken from industry show clearly that installing variable speed drives generates important energy savings. Screw compressors, centrifugal pumps and fans with mechanical flow regulation are especially suited for variable speed regulation. However, one has to be aware of the fact that electronic speed regulation also implies some restrictions, especially in relation with deteriorated power quality. Line-side as well as motor-side harmonics are created, invertors are in the mean time very sensitive to short interruptions or voltage dips.