Renewable Energy Systems 5 Buchla, Kissell, Floyd
Chapter Outline Solar Tracking 5 Buchla, Kissell, Floyd 5-1 MOVEMENT OF THE SUN 5-2 COSTS AND BENEFITS OF TRACKING 5-3 SINGLE-AXIS AND DUAL-AXIS SOLAR TRACKERS 5-4 DC MOTORS 5-5 STEPPER MOTORS
5-1 Movement of the Sun The earth is divided by reference lines called latitude and longitude. Latitude lines are imaginary lines that are parallel to the equator. Longitude lines are imaginary lines that form great circles that converge at the poles. The equator is at 0 o latitude and the North Pole is at 90 o N latitude. The prime meridian is the longitude that runs through Greenwich, England and defines 0 o.
5-1 Movement of the Sun Someone at the North Pole has the earth s axis going through their body and pointing to an imaginary pole in space (at their zenith). Likewise, someone on the equator has the projection of the earth s equator (called the celestial equator) going directly overhead and the pole on the horizon. (a) First day of summer at the North Pole (b) At the equator on the day of the equinox
5-1 Movement of the Sun The sun appears to move in the sky along a path known as the ecliptic, which is an imaginary circle inclined to the equator by 23.4 o. The sun appears to travel around the ecliptic over the course of a year. On the day of an equinox, the ecliptic crosses the celestial equator and there are approximately 12 hours from sunrise to sunset.
5-1 Movement of the Sun In the northern hemisphere the sun is north of the ecliptic in summer; in the southern hemisphere the sun is south of the ecliptic during summer.
David Buchla 5-1 Movement of the Sun The analemma is the figure 8 pattern the sun makes in its annual motion if the position is observed regularly at the same location and time of day.
Source:NREL Source:NREL 5-2 Costs and Benefits of Tracking Tracking devices are used with solar concentration systems because they work best with direct radiation. Tracking systems can use one or two-axis tracking. Single-axis tracking is useful for trough collectors Dual-axis tracking is useful for higher concentrations 2015 by Pearson Higher Education, Inc.
Source:NREL 5-2 Costs and Benefits of Tracking For utility scale operations, a type of concentrating collector is the tower that is surrounded by many heliostats. A Heliostat is a device that uses a movable mirror mounted in a manner so as to reflect the sunlight to a fixed target (such as a tower collector). In a large tower array, each mirror needs to move independently to aim the reflection to the top of the tower.
5-2 Costs and Benefits of Tracking Trackers require more space to allow for motion and avoid shading problems; this is a consideration when space is at a premium.
5-2 Costs and Benefits of Tracking The decision to track or not track a flat panel depends on the result of cost/benefit analysis. Tracking with a flat panel will generally produce 20 to 40% more power than not tracking.
Source: NREL 5-3 Single-axis and Dual-axis Solar Trackers Solar trackers can be categorized as either active or passive. An active tracker usually moves by a motor. During the day, the sun moves 15 o per hour from east to west. At sunset, the motors can turn the collectors back to the east.
5-3 Single-axis and Dual-axis Solar Trackers Passive trackers do not need external power to move. A passive tracker can move by shifting weight of a volatile liquid between two canisters. The liquid vaporizes easily and will shift to the other canister if one canister receives more sun. The shadow plates shade the canisters evenly when collector is perpendicular to the sun.
Source: NREL 5-3 Single-axis and Dual-axis Solar Trackers Equatorial trackers align the main driving axis along the north-south line of the earth s axis. Panels need to be mounted in a manner to prevent interference with the daily motion. In the northern hemisphere, the rotating axis points to the north star, which is in line with the earth s pole. This arrangement is good for single axis trackers because the north-south motion of the sun is small compared to the east-west motion.
(Source: NREL) 5-3 Single-axis and Dual-axis Solar Trackers Dual-axis trackers can follow the sun precisely. Most are designed as alt-azimuth mountings, whereby the axes move vertically (the altitude) and horizontally (azimuth). The system shown is a two-axis concentrating mount that was built for the Arizona Public service and uses concentrating lenses.
Source: Tom Kissell 5-4 DC Motors DC motors are well suited for tracking motors, as power from the array can be tapped for power and because they develop high torque at low speeds and can start heavy loads. All motors have two basic parts: the rotating part that is called the rotor, and the stationary part called the stator. The stator of a dc motor on the bench for repair is shown in the photo showing the stationary field coils.
Source: Tom Kissell 5-4 DC Motors The rotating magnet (the rotor) on a dc motor is the armature because power is taken from the rotor. Current is supplied to the rotor coils through the brushes and commutator. The rotor is removed from the dc motor shown in the last slide. A bearing and the commutator is in the foreground. This motor shows obvious signs of wear on the commutator from the brushes making contact.
5-4 DC Motors A simplified view of a dc motor is shown (showing only one rotor winding). Motors typically have a number of low resistance windings, which allow high current (and hence high torque) at startup. Current in the rotor (armature current) creates a magnetic field which interacts with the field from the permanent magnets (in this case).
5-4 DC Motors A general curve of torque and speed is shown. In general, for a Permanent Magnet DC (PMDC) motor, the torque and speed are related with a negative linear correlation as illustrated. The power delivered by the motor depends on the speed and the torque. There is no power developed if the motor is not turning (stall torque) or if there is no torque (noload speed). Between these extremes, the motor delivers power to the load.
Courtesy of Bodine Electric Company 5-4 DC Motors Small geared motors can be useful in tracking drives because the output turns slowly. Gearing reduces the shaft speed and increases the torque, which is important for tracking motors. The motor shown is a PMDC that is smaller because of the strong ceramic permanent magnets. The relationship between torque and power (in SI units) is: T q P T q =torque, in newton-meters (n-m) P=power, in watts (W) =rotational speed, radians/second (rad/s)
Courtesy of Bodine Electric Company 5-4 DC Motors Another type of PMDC motor uses a permanent magnet for the rotor and an electromagnet for the coils. Because the rotor is a permanent magnet, there is no need for brushes or a commutator; hence it is called a brushless PMDC motor.
5-4 DC Motors Three basic types of wound stator motors are the serieswound motor, the shunt-wound motor, and the compound motor. These designations describe the type of connection(s) between the field winding and the rotor winding. In a series dc motor, the field current and the armature current are identical. The starting torque is high, so the motor is useful for starting heavy loads.
5-4 DC Motors In a shunt wound motor, the field winding is in parallel with the armature. In the shunt motor, the torque is proportional to the armature current. Because the voltage is directly across the field, the field current is constant and independent of the load. A dc shunt motor has increased current when the motor is loaded. The torque is low at startup and increases with increased loading. It tends to have fairly constant speed throughout its load range.
5-4 DC Motors In a compound dc motor, there are two field windings: a series winding and a shunt winding. The combination of series and shunt windings allows the motor to have the torque characteristics of the series motor and the regulated speed characteristics of the shunt motor. There are different ways to wire a compound motor; a cumulative compound short-shunt motor is illustrated. Here the magnetic fields of the field coils aid each other.
5-5 Stepper Motors Stepper motors rotate in small steps in response to a pulse signal. They are widely used as drive motors in solar trackers because of their ability to move precisely. The diagrams show a simplified permanent magnet stepper motor, in which the rotor magnets line up with an active pair of electromagnets as they are pulsed.
Courtesy of Kollmorgen Corporation 5-5 Stepper Motors Another type of stepper motor is formed if the magnets are replaced with a soft iron core. The motor moves to minimize the air gap when selected electromagnets are active. This action minimizes the reluctance of the path, so the stepper is known as a variable reluctance stepper motor. A hybrid stepper motor combines the permanent magnet motor and variable reluctance motor. Hybrid stepper motors have good torque and can be run at higher step than other types, so they are popular for various applications.
Selected Key Terms Brushes Celestial equator Commutator Ecliptic In motors or generators, the electrical conductors that provide a path for current from a stationary part to a moving part. The projection of the earth s equator in the sky. The celestial equator traces a line that is at an angle above the horizon that is 90 o L, where L is the latitude. A ring structure that has independent conductors that is designed to switch the current in a rotor coil as the structure rotates. The sun s path in the sky with reference to an observer on earth.
Selected Key Terms Heliostat Holding torque Latitude Longitude A device that uses a movable mirror mounted in a manner so as to reflect the sunlight to a fixed target (such as a tower collector). With reference to a stepper motor, the maximum torque that can be externally applied to a stopped motor without causing it to rotate to the next step position. The angle in degrees formed by a line that extends from the center of the earth to the equator and from the center of the earth to a given point on the globe. Reference circles that are perpendicular to the equator and converge at the poles. Longitude increases in the positive direction in the eastwardly direction.
Selected Key Terms Meridian Prime meridian Rotor Stator Synodic day Zenith A line of longitude that passes through both poles and a point straight overhead. A great circle of longitude that passes through both poles and Greenwich, England. The rotating part of a motor or generator. The stationary part of a motor or generator. The time for the earth to make one revolution on its axis. It is almost exactly 24 hours. A point directly overhead of an observer.
true/false quiz 1. At the North Pole, the latitude is 0 o.
true/false quiz 2. On the equinox, the ecliptic crosses the celestial equator.
true/false quiz 3. In a large tower array, the heliostats all point in the same direction.
true/false quiz 4. To follow the sun, all trackers require electrical power.
true/false quiz 5. Concentrating collectors will have the greatest benefit from tracking.
true/false quiz 6. A commutator uses coils to generate a magnetic field.
true/false quiz 7. Some dc motors do not use brushes.
true/false quiz 8. The current in the rotor and stator is the same in a series wound motor.
true/false quiz 9. A series wound motor has high starting torque.
true/false quiz 10. A hybrid stepper motor combines the permanent magnet motor and variable reluctance motor.
true/false quiz Answers: 1.F 2.T 3.F 4.F 5.T 6.F 7.T 8.T 9.T 10. T