Abstract:
A pyranometer for measuring solar irradiance at a selected site comprises a light detector adapted to produce an output signal in response to incident solar radiation, a reversible motor having a curved shadowband attached to its output shaft, means for mounting the light detector and motor so that the shadowband is at a fixed distance from the light detector; and a motor controller that is adapted to periodically cause the motor to rotate the shadowband from one to the other of two limit positions, with the shadowband acting to momentarily shadow the light detector during its movement between its two limit positions. A datalogger stores and processes the light detector&#39;s signal output to provide a measure of total horizontal, direct normal and horizontal diffuse solar irradiance.

Description:
This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/392,389, filed Jun. 29, 2002 for “Rotating Shadowband Pyranometer”. 

   FIELD OF THE INVENTION 
   This invention relates to apparatus for measuring solar radiation and more particularly to a pyranometer adapted to measure direct normal and horizontal diffuse solar irradiance. 
   BACKGROUND OF THE INVENTION 
   Photovoltaic (PV) power is a desirable complement to conventional electric power generation delivery systems. In assessing the value of photovoltaic power generation, electric power planners require solar energy resource data for specific areas where photovoltaic modules are to be installed. Solar energy reaches the earth surface along two paths-irradiance directly from the sun and diffuse irradiance from the sky. Both of those components are required in order to estimate the energy produced by PV systems in a given location. Direct normal and horizontal diffuse solar irradiance data and temperature data are used to estimate the energy produced by fixed and tracking PV systems. Those estimates support utility planning and PV system design studies by comparing the performances of various fixed and tracking array concepts. The data acquired can be used to compute the DC and AC power output for any fixed or tracking PV system at a given location. 
   Various forms of apparatus have been conceived for measuring solar radiation. Circa the year 1995 Ascension Technology Inc. (ATI), located in Waltham, Mass., introduced to the market a rotating shadowband pyranometer that was adapted to measure direct normal and horizontal diffuse irradiance. The ATI pyranometer is illustrated in  FIGS. 1 and 2 . It utilized a single light sensor  2  to determine total horizontal, direct normal and horizontal diffuse irradiance. The light sensor  2  was mounted on a bracket  4  attached to the end of a stationary tube  6  that was secured to a hollow stationary shaft  8  that in turn was affixed to one wall of a housing  10 . The signal output of the light sensor was coupled to an electronic data logging system (not shown) by a cable (also not shown) that passed through tube  6  and shaft  8 . 
   A shadowband in the form of a curved opaque strip  12  had one end attached to a member  14  that was secured to one face of a gear  16  that was mounted for rotation on shaft  8 . Gear  16  meshed with a second gear  20  affixed to the output shaft  22  of an electric motor  24 . The latter was mounted to a bracket  26  that had a pair of perforated ears  28  (only one of which is visible in  FIG. 1 ) located along its opposite sides. Ears  28  that were used to pivotally mount the bracket to a yoke  30  mounted on a vertical post  32  that was fixed to the ground or to a building. Yoke  30  was rotatable on post  32  to permit it to be positioned to a selected azimuth position for optimum monitoring of sunlight. 
   Rotation of the shadowband was achieved by energizing motor  24 , with operation of the motor being initiated and stopped by an electronic controller (not shown). The controller periodically caused the motor to rotate the shadowband unidirectionally through 360°, with rotation of the shadowband taking approximately one second. During that one-second interval the data logger sampled the pyranometer signal approximately 700 times. The sampling irradiance data was then analyzed to provide an estimate of the electrical power that could or should be produced by an existing or planned PV system. Typically the data logger and controller were mounted in a common box-like enclosure  36  that also contained a battery power supply (not shown) for the pyranometer, and a PV module  38  was attached to yoke  30  and connected to the battery so as to keep the latter charged by sunlight-derived electric power. 
   The ATI rotating shadow band pyranometer functioned well but suffered from the limitation that the mechanical structure for supporting and driving the shadow band was complicated and hence expensive. Friction in the gear system would cause it to wear out and bind over time. Also water penetration from rain affected the reliability and life of the drive system for the shadow band and/or associated electrical and electronic components, and repair or replacement of one or more components typically involved removal of substantially the entire drive system. Removal of the drive system included removal of the irradiance sensor which was assigned a unique calibration number. As a consequence of removing the drive system, the calibration number factor in the datalogger calculations needed to be adjusted. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   The object of the invention is to provide a rotating shadow band pyranometer that overcomes the limitations of the ATI device. A more specific object is to provide a rotating shadow band pyranometer that employs a direct drive relationship of the electric motor and the shadow band with the shadow band and the light sensor being separable from the electric motor, thereby facilitating replacement of a disabled motor at lesser cost than previously. 
   Another object of the invention is to provide a rotating shadow band pyranometer wherein the shadow band rotates periodically from one stowed position to another stowed position, with the angle of rotation measured from one stowed position to the other being approximately 300°. 
   Another object of the invention is to provide a rotating shadow band pyranometer that permits three degrees of adjustment, i.e., adjustment on three axes, of the position of the light sensor. 
   A more specific object is to provide a rotating band pyranometer that is simpler, less expensive and more reliable than the Ascension Technologies pyranometer. 
   A further specific object is to provide a pyranometer for the purpose described that comprises a motor housing that permits ready access to the motor that rotates the shadow band and is characterized by weep holes for draining any moisture that may tend to accumulate in the motor housing. 
   The foregoing objects are achieved by providing a rotating shadow band pyranometer that comprises a motor enclosure containing a motor, a disk with a magnet mounted for rotation with the output shaft of the motor, a curved shadow band, means connecting one end of that band to the motor&#39;s output shaft so that the band will rotate with the output shaft, and two Hall-effect sensors that interact with the magnet to sense the rotational location of the band and determine first and second limit positions for the band that are spaced apart by an angle of approximately 300 degrees. The motor enclosure is affixed to a bracket that is pivotally attached to a yoke that is pivotally mounted to a support structure to permit angular orientation about a first axis, with the pivot connection between the bracket and the yoke providing a second pivot axis. The bracket includes an extended portion to which is attached a tongue that carries a light sensor. The end of the tongue is pivotally mounted to the bracket so as to permit the light sensor to undergo pivotal adjustment on a third axis that extends at an angle to the other two axes. 
   The motor and the light sensor are connected to a controller that periodically causes the motor to rotate the band in a first or second direction between the first and second limit positions. The controller causes the motor to rotate the shadow band until one of the Hall-effect sensors detects the near presence of the magnet embedded in the disk, whereupon the controller halts the rotation with the shadow band stowed in one of the two limit positions. The shadow band then remains stationary until the controller causes the motor to rotate in the opposite direction to where the magnet is sensed by the other Hall-effect sensor, whereupon the controller halts rotation of the shadow band in the other limit position. The band remains stowed in the second limit position until the controller again initiates operation of the motor in a direction opposite to its previous operation. The controller is preferably situated in a separate enclosure with a data logger that stores and processes the signal output of the light sensor. Preferably the data logger controls operation of the motor controller, periodically initiating a start up signal to the motor. 
   Other features and advantages of the invention are described or rendered obvious by the following specific description that is to be considered together with the accompanying drawings. 

   
     THE DRAWINGS 
       FIG. 1  is a side view in elevation of the Ascension Technology Inc. pyranometer discussed above; 
       FIG. 2  is an enlargement of a part of  FIG. 1 , with portions broke away to illustrate the mechanical drive for the shadow band; 
       FIG. 3  is a side view in elevation of a pyranometer apparatus embodying the present invention; 
       FIG. 4  is an enlarged side view in elevation of a portion of the apparatus shown in  FIG. 3 , with a portion of the motor housing omitted; 
       FIG. 5  is an enlarged sectional view illustrating the shadow band motor and associated components; 
       FIG. 6  illustrates the pyranometer mounting bracket; 
       FIG. 7  illustrates one of the two parts of the motor housing; 
       FIG. 8  illustrates the second part of the motor housing; 
       FIG. 9  illustrates the motor mount bracket; 
       FIG. 10  is a view of the magnet-carrying disk looking from right to left in  FIG. 5 ; 
       FIG. 11  illustrates the L-shaped bracket that supports the light sensor; 
       FIG. 12  is a schematic illustration of the electrical system for the apparatus of  FIGS. 3-11 ; 
       FIG. 13  is a diagrammatic representation of the operation of the motor controller; and 
       FIG. 14  is a graph illustrating the measurements during a single band rotation on a clear day. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 3 , a vertical post  40  supports an enclosure  42  that houses a data logger apparatus, a PV controller, and a battery that serves as a power supply for the data logger and the PV controller. The data logger, controller and battery are not shown. The same post supports a U-shaped yoke  44  that in turn supports a PV module  46  and a pyranometer support bracket  48 . The crossbar section of the yoke is attached to the post by a connector  50  that permits the yoke to be rotated relative to the post to a selected azimuth position. The PV module is connected to the battery (by wire cable not shown) and serves to generate d. c. power that keeps the battery charged. Bracket  48  serves as a support for a motor housing  54  and a sun light sensor  56 . 
   Referring to  FIGS. 4 ,  6  and  11 , bracket  48  is essentially cross-shaped, comprising a pair of laterally-extending arms  58  with upturned ears  60  at their opposite ends. Each ear  60  has a threaded hole  62  for receiving screws  64  that serve to pivotally connect bracket  48  to the two vertically extending arms of yoke  44 . The screws can be tightened to lock bracket  48  in a selected angular position relative to the horizon. Bracket  48  also comprises a narrow extension  66  with a bent end portion  68 . The latter is provided with a circular hole  70  and an arcuate hole  72  that is essentially concentric with hole  70 . The opposite ends of hole  72  are equidistant from the end edge of bent end portion  68  and the center portion of hole  72  is furthest from that end edge. Mounted to the bent portion  68  is an L-shaped bracket  74  and attached to one end of bracket  74  is light sensor  56 . The other end of the bracket  74  is provided with a first threaded hole that receives a screw  76  that passes through hole  70 , and a second threaded hole that receives a screw  78  that passes through arcuate hole  72 . The first screw  76  serves as a pivot shaft for bracket  74  to rotate relative to bracket  48 . The second screw  78  serves to secure the bracket  74  in a selected angular position relative to bracket  48 , with the elongated arcuate hole  72  coacting with screw  78  to determine the magnitude of the angle through which the bracket can rotate about its pivot screw  76 . The sunlight sensor  56  is supported so that it is located in line with the axis of rotation of the shadow band  136  described hereinafter. 
   Referring to  FIGS. 4-9 , motor housing  54  comprises two housing parts  80  and  82 . The  80  is generally of U-shaped cross-section, comprising a base panel  84 , opposite side panes  86 , inturned flanges  88  at the edges of the side panels, and end flanges  90  and  92  at the opposite ends of base panel  84  and side panels  86  respectively. The other housing part  82  also is of U-shaped configuration, comprising a longitudinally extending base panel  94  and opposite end panels  96  having aligned holes  98  and  99  respectively. Housing part  80  is attached to bracket  48 . Housing part  82  fits over housing part  80  so that base panel  94  engages flanges  88 . In this connection it is to be noted that housing parts  80  and  82  are provided with multiple threaded holes  100  for receiving fasteners (not shown) for securing them together and/or to bracket  48  or motor mount bracket  104  described hereinafter. Also, one end of housing part  80  is provided with two weep holes  106  located at opposite corners (only one weep hole  106  is visible in  FIG. 7 ) for allowing escape of any moisture that may accumulate in the housing through leakage or condensation. With reference to the orientation of the housing as shown in  FIG. 3 , the weep holes are located at the lower end of the motor housing, thereby assuring escape of any moisture. 
   Referring again to  FIGS. 4 ,  5 , and  9 - 11 , an L-shaped motor mount bracket  104  is attached to base panel  94  of housing part  82  by screws (not shown. Attached to the inwardly projecting portion  108  of bracket  104  is a reversible d. c. motor  120 . The inwardly projecting portion of bracket  104  has three holes  110 ,  112  and  114 . Hole  110  is sized to rotatably accommodate a forward portion of a d. c. motor  116  having an output shaft  118 . Mounted in the two other holes  112  and  114  are two Hall-effect sensors  120  and  122 . Mounted on motor shaft  118  is a circular disk  126  ( FIG. 10 ) having embedded therein a circular magnet  128 . The Hall-effect sensors are spaced from one another so that the angle between them measured with reference to the axis of motor shaft  118  is approximately 300° measured clockwise in  FIG. 10  from sensor  120  to sensor  122 , and the disk is sized and the magnet positioned on the disk so that the magnet will pass in front of each of the sensors as the disk is rotated back and forth by motor  116 . The motor and the two Hall-effect sensors are connected to the controller in enclosure  42  by electrical cables (not shown) that pass through hole  99  in housing part  82 . As described hereinafter, the motor is stopped when the magnet is sensed by either of the two Hall-effect sensors, and remains in that stopped position (the “stowed” position) until the motor is again energized by operation of the controller. 
   Referring to  FIGS. 5 and 10 , disk  126  has a hub  130  that extends through a hole  98  in housing part  82 . Hub  130  has several tapped holes  132  for receiving screws  134  that serve to releasably secure one end of a circularly curved shadow band  136  to the hub. The band extends lengthwise through an angle of approximately 133° and has a 5 inch radius of curvature, with its forward or free end extending far enough so that when the motor is operated, some portion of the band can pass through a line extending directly from the optical axis of the light detector  56  to the sun. The distance between the light-receiving surface of light detector  56  and the band is constant throughout the entire length of the band. The width of the shadow band is such that it can cast a shadow wide enough to fully obscure the light-receiving surface of the sensor  60  at some position of the shadow band relative to the sun. 
   In the foregoing construction the pivot axis provided by screw  76  extends normal to the longitudinal center axis of light detector  56 , with the distance between that detector and the shadow band remaining constant regardless of the position of screw  78  along the arcuate hole  72 . The pyranometer construction also provides two degrees of tilt leveling. One degree is afforded by screws  76  and  78  which allow pyranometer bracket  74  with detector  56  to pivot a short distance around pivot screw  76 . A second degree of tilt leveling is afforded by the pivot connection between yoke  44  and bracket  48 . Additionally, the connector  50  allows for adjustment of azimuth pointing. 
     FIG. 12  illustrates the electrical system of the apparatus shown in  FIGS. 1-11 . The system includes an electronic datalogger represented schematically at  140  and a motor controller  142  that includes motor  116 . Details of the datalogger are not provided since such apparatus is well known and may take various forms, e.g., a digital computer. Although not shown, it is to be appreciated that the d. c. power supply for light sensor  56 , datalogger  140 , motor controller  142 , motor  116  and Hall Effect sensors  120  and  122  may be a battery (not shown), e.g., one that is kept charged by d. c. current derived from a PV module as shown at  38 , or from some other power source. 
   The controller supplies the logic for applying power to rotate the motor in either direction. Essentially the controller comprises a pair of and gates  144  and  146  each having one input terminal connected to a common output port of the datalogger, with the datalogger output signal C 1  being applied to and gate  144  via an inverter  148 . The other input terminals of gates  144  and  146  are tied to Hall-effect sensors  120  and  122  respectively. Each Hall-effect sensor produces a high (positive) output signal when the magnet is not aligned with it and a low (negative) output signal when the magnet is aligned with it. The output signals of sensors  120  and  122  are identified hereinafter as S 1  and S 2  respectively. The output terminals of gates  120  and  122  are connected to semiconductor logic circuits that are represented graphically as latching relays  150  and  152 . The latter have a positive d. c. voltage terminal contact, a grounded terminal contact and an output terminal contact, with the latter contact of relays  150  and  152  being connected to opposite sides of motor  116 . 
   Operation of the motor controller will now be described with reference to  FIGS. 12 and 13 . The datalogger output is a pulse train with a selected repetition rate. In the preferred embodiment of the invention the repetition rate is set at one minute, and the pulse width is approximately one second, which is the transit time desired for the motor to rotate the shadow band  136  from one limit position characterized by the magnet being aligned with Hall-effect sensor  120  and a second limit position characterized by the magnet being aligned with the other Hall-effect sensor  122 . Controller  142  has four states as follows: 
   First state: C 1  is low; magnet aligned with sensor  120  so S 1  is low. 
   Second state: C 1  is low; magnet not aligned with sensor  120  so S 1  is high. 
   Third state: C 1  is high; magnet aligned with sensor  122  so S 2  is low. 
   Fourth state: C 1  is low; magnet not aligned with sensor  122  so S 2  is high. 
   Accordingly the operating action of the motor as determined by controller  142  is as follows: (1) in the first state, the input to and gates  144  and  146  from the datalogger are high and low respectively and the inputs to the same gates from sensors  120  and  122  are low and high respectively, with the result that the relays are in the states shown in  FIG. 12  with the motor stopped since both of its terminals are connected to ground; (2) in the second state, the output from sensor  120  goes high and the output from sensor  122  remains high, with the result that the output from and gate  144  causes relay  150  to change states and thereby initiate operation of the motor; (3) in the third state, the input signals to and gates  144  and  146  from the datalogger are low and high respectively, the sensor signal S 1  is high and the sensor output S 2  is low, resulting in the motor being stopped due to the relays again being in the state shown in  FIG. 13 ; and (4) in the fourth state, the inputs signals to and gates  144  and  146  from the datalogger are low and high respectively and the sensor signals S 1  and S 2  are high and low respectively, resulting in relay  150  switching to connect its high voltage terminal to motor  116 , whereupon the motor is energized to run in reverse to its direction of operation during the second state. 
   To summarize, assuming that the motor is stopped with magnet  128  aligned with sensor  120  and that the datalogger is operating to generate a pulse train as described above, when the output from the datalogger goes low, the motor controller will keep the motor de-energized if the magnet  128  is aligned with sensor  120  and will energize the motor to run in a first (e.g., clockwise) direction if the magnet is not aligned with sensor  120 , and when the datalogger output goes high, the motor controller will keep the motor de-energized if the magnet it aligned with sensor  122  and will energize the motor to run in a second opposite direction if the magnet is not aligned with sensor  122 . With the datalogger producing a continuous pulse train as described, the system will sequentially rotate the shadowband from a first limit position determined by alignment of magnet  128  with sensor  120  to a second limit position determined by alignment of magnet  128  with sensor  122 , hold the shadowband in that position for a limited time, then rotate the shadow band back to the first limit position, hold the shadowband in that position for a limited time, and then repeat the foregoing cycle of start and stop movement. 
   The datalogger of the above-described rotating shadowband apparatus uses the single light sensor (pyranometer) to measure three components of sunlight, namely, total horizontal, direct normal and horizontal diffuse irradiance. These are related by the equation:
 
 T   h   =Dif   h   +Dir   n  cos( Z ),
 
which expresses T h , the total irradiance measured on a horizontal surface; Dif h , the diffuse irradiance (skylight) on a horizontal surface; Dir n , the direct normal irradiance (sunlight directly incident on a surface facing the sun); and Z, the sun&#39;s zenith angle, the angle measured from straight overhead down to a sight-line to the center of the sun.
 
   In the preferred mode of practicing the invention, the datalogger is programmed so that once per minute the shadowband (band) passes over the light sensor, traveling approximately 300 degrees from a first stowed (limit) position wherein the magnet  128  is in front of Hall-effect sensor  120  around to the other side of the bracket  48  to a second stowed (limit) position wherein the magnet is in front of Hall-effect sensor  122 . On the next rotation, the band rotates in the reverse direction. One pass takes approximately one second. During this one-second period the pyranometer signal is sampled by light detector  56  about 700 times. The minimum pyranometer reading occurs when the sun is completely occluded by the band. The stream of high-sample-rate irradiance data is processed to measure the horizontal diffuse irradiance. With T h , Dif h  and Z known, Dir n  is calculated. 
   Referring now to  FIG. 14  which is a graph illustrating measurements during a single band rotation on a clear day, the pyranometer views the full sky while the band travels from its stowed position below and to one side of the sensor, up to the horizon (A). As the band traverses its path above the horizon, it blocks a small strip of the sky, reducing the diffuse irradiance falling on the sensor (B). The irradiance measurement drops dramatically when the band shades the sensor from direct sunlight (C). A symmetrical pattern occurs as the band completes its revolution, ending in its stowed position below and to the other side of the pyranometer (D). 
   An advantage of the invention herein described is that it is susceptible of modifications. Thus, for example, the motor housing may be constructed otherwise than as herein illustrated and described without affecting the mode of operation. Similarly the two Hall-effect sensors may be mounted in a different manner. Another advantage is that different forms of light sensors may be utilized to measure radiation, although a silicon photovoltaic solar cell is preferred. It is to be noted also that the motor controller represented in  FIGS. 12 and 13  may be a component part of the datalogger or a separate device as shown. The motor controller may be in the form of hard-wired logic, but preferably the result represented in  FIGS. 12 and 13  is achieved by appropriately programming a digital processor. 
   Other advantages of the invention herein described is that it overcomes the limitation of the ATI pyranometer described above and has a modular construction. The disk  126  helps protect against water penetration, since the disk hub is sized to pass through hole  98  with only enough gap to avoid binding. Inside the motor enclosure the enlarged diameter of disk  126  helps provide a barrier between the motor and any water that may penetrate the housing hole  98 . The motor, sensors  120  and  122  and the assorted wiring are all located behind disk  126  and are further protected by the barrier presented by bracket  98 . The disclosed motor housing also accommodates the electrical connections for the motor and the sensors  120  and  122  so that they also are protected from the weather. An important aspect is the provision of weep holes  106 , whereby when the motor housing is oriented as shown in  FIG. 3 , any water that may enter the housing around the hub of disk  126  will flow down to the weep holes and exit the housing via those weep holes. Two other advantages are that the motor enclosure is mounted on the bracket  48  so that the geometry of the band with respect to the pyranometer  56  is fixed, yet the motor can be removed for viewing without disturbing the pyranometer  56  which can continue to function to measure solar radiation. Other advantages and modifications will be obvious to persons skilled in the art.