Patent Publication Number: US-2019199276-A1

Title: Systems and methods for rotating photovoltaic modules

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation under 35 U.S.C. § 120 of U.S. patent application Ser. No. 15/154,763, filed May 13, 2016 and entitled “Systems and Methods for Rotating Photovoltaic Modules,” the entire contents of which are incorporated by reference herein, which claims the benefit of the following applications, the entire contents of each of which are incorporated by reference herein: 
     U.S. Provisional Patent Application No. 62/163,258, filed May 18, 2015 and entitled “Systems and Methods for Rotating Photovoltaic Modules;” 
     U.S. Provisional Patent Application No. 62/191,176, filed Jul. 10, 2015 and entitled “Systems and Methods for Rotating Photovoltaic Modules;” 
     U.S. Provisional Patent Application No. 62/191,980, filed Jul. 13, 2015 and entitled “Systems and Methods for Rotating Photovoltaic Modules;” and 
     U.S. Provisional Patent Application No. 62/307,125, filed Mar. 11, 2016 and entitled “Single-axis tracker.” 
    
    
     FIELD 
     This application relates to rotating photovoltaic modules. 
     BACKGROUND 
     It can be useful to rotate arrays of photovoltaic (PV) modules, e.g., as the sun moves relative to the array over the course of a day. However, rotating multiple photovoltaic modules of a given array can be challenging. For example, individually rotating the modules can require providing each module with its own actuator, and appropriately controlling such actuators. 
     Hence, it is desirable to improve techniques for rotating PV modules. 
     SUMMARY 
     Embodiments of the present invention provide systems and methods for rotating photovoltaic modules. 
     Under one aspect, a system is provided for rotating photovoltaic modules arranged in a row. The system can include an elongated structural member extending along and parallel to the row; protrusions coupled to the elongated structural member; an actuator; and drive mechanisms coupled to the photovoltaic modules. Actuation of the actuator can move the elongated structural member, the movement of the elongated structural member can move the protrusions, the movement of the protrusions can move the drive mechanisms, and the movement of the drive mechanisms can rotate the photovoltaic modules. 
     Optionally, the elongated structural member can include a cable or belt. Additionally, or alternatively, the protrusions optionally can include balls, knots, or barrels that traverse the row responsive to actuation of the actuator. Additionally, or alternatively, the drive mechanisms optionally can include gearboxes, worm drives, belt drives, ratchets, or geneva mechanisms. Additionally, or alternatively, the system optionally further can include a common structural element attaching together the photovoltaic modules. 
     Additionally, or alternatively, the protrusions optionally can convert motion of the elongated structural member into rotary motion of the photovoltaic modules. Additionally, or alternatively, the protrusions optionally engage spaces within the drive mechanisms. Additionally, or alternatively, the drive mechanisms optionally can include curved racks each arranged perpendicularly to the photovoltaic modules. Optionally, the curved racks can include gear teeth. Additionally, or alternatively, the photovoltaic modules optionally rotate about a pivot point at a radial center of the curved racks. Additionally, or alternatively, the drive mechanisms optionally can include distributed gearboxes. 
     Additionally, or alternatively, the system optionally further can include an elongated concrete ballast extending along and parallel to the row and upon which the photovoltaic modules are disposed. Optionally, the elongated concrete ballast is split into discrete tracks each parallel to the row. Additionally, or alternatively, the elongated concrete ballast optionally is formed by slip forming, precasting, or is cast-in-place. Additionally, or alternatively, the elongated concrete ballast optionally further can include one or more control joints. 
     Additionally, or alternatively, the photovoltaic modules optionally are arranged in a plurality of independent tables. Each table optionally can include one or more of the drive mechanisms and can extend parallel to the row. Additionally, or alternatively, the system optionally further can include a purlin extending parallel to the row and joining together the photovoltaic modules of a corresponding one of the tables. Optionally, the system further can include a purlin arm coupled to the purlin and to one of the drive mechanisms corresponding to that table. Additionally, or alternatively, each table optionally is disposed on a discrete portion of an elongated concrete ballast, the discrete portions being separated from one another by control joints. 
     Additionally, or alternatively, the elongated structural member optionally can include a drive tube. Optionally, the drive tube includes flexible couplings that allow articulation of the drive tube. 
     Additionally, or alternatively, the system optionally can include equal numbers of protrusions and drive mechanisms. Additionally, or alternatively, the protrusions optionally can include spur gears, and the drive mechanisms can include curved rack gear mechanisms that engage with the spur gears. Additionally, or alternatively, the actuator optionally can include a slew drive actuator or a gearbox in a ganged configuration. 
     Additionally, or alternatively, the system optionally can be configured to rotate photovoltaic modules arranged in a second row, the second row being parallel to the row and laterally offset from the row in a direction orthogonal to the row. For example, the system optionally further can include a second elongated structural member extending along and parallel to the second row; protrusions coupled to the second elongated structural member; and drive mechanisms coupled to the photovoltaic modules arranged in the second row. Optionally, actuation of the actuator can move the second elongated structural member, the movement of the second elongated structural member can move the protrusions coupled to the second elongated structural member, the movement of the protrusions coupled to the second elongated structural member can move the drive mechanisms coupled to the photovoltaic modules arranged in the second row, and the movement of the drive mechanisms coupled to the photovoltaic modules arranged in the second row can rotate the photovoltaic modules of the second row. Optionally, the first and second elongated structural members are discrete from one another. Additionally, or alternatively, the system optionally further can include a torque transmission mechanism configured to transmit torque from the actuator to the second elongated structural member. Optionally, the torque transmission mechanism can include a rotating driveshaft. 
     Additionally, or alternatively, the system optionally further can include A-shaped uprights supporting the elongated structural member, the protrusions, and the drive mechanisms. 
     Additionally, or alternatively, the system optionally further can include a bridge and a post, the bridge extending between first and second support surfaces, the post extending vertically from the bridge and supporting the elongated structural member, the protrusions, and the drive mechanisms. 
     Additionally, or alternatively, the system optionally can be configured to rotate photovoltaic modules arranged in a second row, the second row being parallel to the row and laterally offset from the row in a direction orthogonal to the row. For example, the system further can include a first elongated concrete ballast extending along and parallel to the row and upon which the photovoltaic modules of that row are disposed; a second elongated concrete ballast extending along and parallel to the second row and upon which the photovoltaic modules of that row are disposed; and a linking member extending perpendicular to and connecting together the first concrete ballast and the second concrete ballast. 
     Additionally, or alternatively, the system optionally further can include an elongated concrete ballast extending along and parallel to the row and upon which the photovoltaic modules are disposed, the elongated concrete ballast can include first and second vehicle support surfaces; and a maintenance robot that can include first and second wheels respectively contacting the first and second vehicle support surfaces and configured to maintain the system. Optionally, the elongated concrete ballast is split into first and second discrete tracks each parallel to the row, the first track can include the first vehicle support surface, and the second track can include the second vehicle support surface. Optionally, the maintenance robot can include a body coupled to the first and second wheels and disposed between the first and second discrete tracks. 
     Additionally, or alternatively, the system optionally further can include stop members configured to inhibit rotation of the photovoltaic modules beyond a preselected angle. Optionally, the stop members include flexible members that are pulled taut when the photovoltaic modules reach the preselected angle, or include fixed members that the photovoltaic modules contact when reaching the preselected angle. 
     Under another aspect, a system is provided for rotating photovoltaic modules arranged in a row. The system can include a drive tube extending along and parallel to the row. The drive tube can include a plurality of discrete sections coupled together with flexible couplings. The system also can include an actuator; and drive mechanisms coupled to the photovoltaic modules. Actuation of the actuator can rotate the discrete sections of the drive tube and the flexible couplings, the rotation of the discrete sections of the drive tube and the flexible couplings can rotate the drive mechanisms, and the rotation of the drive mechanisms can rotate the photovoltaic modules. 
     Optionally, the photovoltaic modules are arranged in a plurality of independent tables. Each table can include one or more of the drive mechanisms and extending parallel to the row. Optionally, at least one of the flexible couplings is disposed between each of the tables. Optionally, at least two of the flexible couplings are disposed between each of the tables. Additionally, or alternatively, the flexible couplings optionally allow articulation of the discrete sections of the drive tube between the tables. Optionally, the system further can include an elongated concrete ballast extending along and parallel to the row and upon which the photovoltaic modules are disposed. Optionally, the elongated concrete ballast can follow an irregular geological topology, and the drive tube can follow the irregular geological topology via the articulation of the discrete sections of the drive tube. Additionally, or alternatively, the flexible couplings optionally allow articulation of the discrete sections of the drive tube. Additionally, or alternatively, the flexible couplings transmit torque from the actuator to the drive mechanisms. Additionally, or alternatively, the flexible couplings optionally transmit longitudinal forces to compensate for thermal expansion or contraction or seismic effects. Additionally, or alternatively, each flexible coupling optionally can include a first flange coupled to a first discrete section of the drive tube; a second flange coupled to a second discrete section of the drive tube; and one or more fasteners coupling the first flange to the second flange. Additionally, or alternatively, each flexible coupling optionally can include a sleeve can include a first end, a second end, and a lumen connecting the first and second ends, the lumen at the first end receiving a portion of a first discrete section of the drive tube, the lumen at the second end receiving a portion of a second discrete section of the drive tube. Additionally, or alternatively, each flexible coupling can include a fastener comprising a pin slidably disposed through an aperture of a first discrete section of the drive tube and through a slotted aperture of a second discrete section of the drive tube. Additionally, or alternatively, each flexible coupling can include a fastener comprising a pin, a bearing, and a collar. The bearing can be disposed within a first aperture of a first discrete section of the drive tube, the pin can extend through the bearing, through a second aperture of a second discrete section of the drive tube, and through the collar, and the collar can be slidably disposed within the bearing. 
     Under yet another aspect, a system is provided for rotating photovoltaic modules arranged in a row. The system can include a torque tube extending along and parallel to the row. The torque tube can include a plurality of discrete sections coupled together with flexible couplings, the plurality of discrete sections being coupled to the photovoltaic modules. The system also can include an actuator. Actuation of the actuator can rotate the discrete sections of the torque tube and the flexible couplings, and the rotation of the discrete sections of the torque tube and the flexible couplings can rotate the photovoltaic modules. 
     Optionally, the photovoltaic modules are arranged in a plurality of independent tables, each table being coupled to a discrete section of the torque tube and extending parallel to the row. Optionally, at least one of the flexible couplings is disposed between each of the tables. Optionally, at least two of the flexible couplings are disposed between each of the tables. Optionally, the flexible couplings allow articulation of the discrete sections of the torque tube between the tables. Additionally, or alternatively, the system optionally further can include an elongated concrete ballast extending along and parallel to the row and upon which the photovoltaic modules are disposed, wherein the elongated concrete ballast follows an irregular geological topology, and wherein the torque tube follows the irregular geological topology via the articulation of the discrete sections of the torque tube. Optionally, the flexible couplings allow articulation of the discrete sections of the torque tube. Additionally, or alternatively, the flexible couplings optionally transmit longitudinal forces to compensate for thermal expansion or contraction or seismic effects. Additionally, or alternatively, each flexible coupling optionally can include a first flange coupled to a first discrete section of the torque tube; a second flange coupled to a second discrete section of the torque tube; and one or more fasteners coupling the first flange to the second flange. Additionally, or alternatively, each flexible coupling optionally can include a sleeve can include a first end, a second end, and a lumen connecting the first and second ends, the lumen at the first end receiving a portion of a first discrete section of the torque tube, the lumen at the second end receiving a portion of a second discrete section of the torque tube. Additionally, or alternatively, each flexible coupling can include a fastener comprising a pin slidably disposed through an aperture of a first discrete section of the torque tube and through a slotted aperture of a second discrete section of the torque tube. Additionally, or alternatively, each flexible coupling can include a fastener comprising a pin, a bearing, and a collar. The bearing can be disposed within a first aperture of a first discrete section of the torque tube, the pin can extend through the bearing, through a second aperture of a second discrete section of the torque tube, and through the collar, and the collar can be slidably disposed within the bearing. 
     Under yet another aspect, a system is provided for rotating photovoltaic modules arranged in a plurality of rows. The system can include a plurality of drive tubes extending along and parallel to the rows; drive mechanisms coupled to the photovoltaic modules; an actuator configured to rotate the photovoltaic modules via the drive tubes and drive mechanisms; and a wind fence disposed parallel to and adjacent to at least one of the rows. 
     Optionally, the wind fence includes a first portion, a second portion, and a joint disposed between the first and second portions. The first portion can be substantially vertical, and the second portion can be articulable via rotation of the joint between a vertical position and a folded position. Optionally, articulation of the second portion to the folded position reduces shading of at least one of the rows. Additionally, or alternatively, the wind fence optionally can include panels can include mesh, fabric, or solid material. 
     Under still another aspect, a method for mounting photovoltaic modules is provided. The method can include casting or slip-forming an elongated concrete ballast; wet-setting uprights into the elongated concrete ballast; curing the elongated concrete ballast with the uprights therein; and supporting, with the uprights, a drive tube extending along and parallel to the elongated concrete ballast, and drive mechanisms coupled to the photovoltaic modules. The photovoltaic modules can be rotatable via the drive tubes and drive mechanisms. 
     Optionally, the uprights are A-shaped. Additionally, or alternatively, the uprights optionally each can include a bridge and a post, the bridge contacting first and second surfaces of the elongated concrete ballast, the post extending vertically from the bridge and supporting the drive tubes and drive mechanisms. Additionally, or alternatively, the uprights optionally each can include first and second feet that each are wet-set into the elongated concrete ballast. Additionally, or alternatively, wet-setting the uprights optionally can include vibrating the uprights. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A-1I  schematically illustrate components of exemplary systems for rotating photovoltaic modules arranged in a row, according to some embodiments. 
         FIGS. 2A-2B, 3A-3B, 4, 5, 6A-6C, 7A-7B, 8A-8J, 9A-9J, 30A-30B, 31, 32, 35, 36A-36B ,  37 , and  38 A- 38 C schematically illustrate components of exemplary mechanisms for rotating photovoltaic modules, according to some embodiments.  FIGS. 8I-1 and 8I-2  are collectively referred to herein as  FIG. 8I . 
         FIGS. 10A-10I  schematically illustrate exemplary mechanisms that can be used to inhibit rotation of a photovoltaic module, according to some embodiments. 
         FIGS. 11A and 11B  schematically illustrate exemplary options for following irregular terrain in a system for rotating photovoltaic modules arranged in a row, according to some embodiments. 
         FIGS. 12A-12E, 13A-13C, 14A-14X, 33, and 34A-34B  schematically illustrate exemplary flexible couplings that can be used in a system for rotating photovoltaic modules arranged in a row, according to some embodiments, and exemplary components of one non-limiting example of a drive tube and coupling, e.g., for use in an arc-drive configuration, are illustrated in  FIGS. 12B-12F . 
         FIGS. 15A-15G  schematically illustrate exemplary wind fences that can be used in a system for rotating photovoltaic modules, according to some embodiments. 
         FIGS. 16A-16E  schematically illustrate exemplary vehicles that can be used with a system for rotating photovoltaic modules, according to some embodiments. 
         FIGS. 17, 18A-18F, 19A-19E, 20A-20C, 21A-21C, 22A-22J, 23, 24A-24E, 25, 26A-26R, 28A-28C, and 29A-29B  schematically illustrate optional arrangements of components in a system for rotating photovoltaic modules in a row, according to some embodiments, and  FIGS. 26Q-26S  illustrate still further exemplary embodiments of exemplary flags that can be used with the arrangement of  FIG. 25 . 
         FIGS. 27A-27E  schematically illustrate views of exemplary structures formed during steps of a method for wet-setting uprights in a system for rotating photovoltaic modules, in some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention provide systems and methods for rotating photovoltaic modules. Illustratively, some aspects of the systems and methods provided herein relate to certain arrangements of components for transmitting rotational forces from an actuator to a row of photovoltaic modules so as to rotate the modules of the row, e.g., at different times than one another, or concurrently with one another. Still other aspects of the systems and methods provided herein relate to elongated drive tubes that couple an actuator to a row of photovoltaic modules, and that include flexible couplings that allow for angular misalignment of different modules in that row relative to one another. Still other aspects of the systems and methods provided herein relate to wind fences that can reduce wind loads on certain arrangements of photovoltaic modules. Yet other aspects of the systems and methods provided herein relate to methods of mounting photovoltaic modules. It should be appreciated that any suitable combination of one or more aspects provided herein optionally can be used with one another, but need not necessarily be used with one another. For example, the presently provided arrangements of components for transmitting rotational forces from an actuator to a row of photovoltaic modules so as to rotate the modules of the row optionally can be, but need not necessarily be, used in combination with one or more of the presently flexible couplings, wind fences, and/or methods of mounting photovoltaic modules. As another example, the presently provided flexible couplings optionally can be, but need not necessarily be, used in combination with one or more of the present components for transmitting rotational forces, wind fences, and/or methods of mounting photovoltaic modules. As still another example, the presently provided wind fences optionally can be, but need not necessarily be, used in combination with one or more of the present components for transmitting rotational forces, flexible couplings, and/or methods of mounting photovoltaic modules. As yet another example, the presently provided methods of mounting photovoltaic modules optionally can be, but need not necessarily be, used in combination with one or more of the present components for transmitting rotational forces, flexible couplings, and/or wind fences. 
       FIGS. 1A-1I  schematically illustrate components of exemplary systems for rotating photovoltaic modules arranged in a row, according to some embodiments. For example,  FIG. 1A  schematically illustrates a perspective view of exemplary system  100  including a plurality of rows  110  of photovoltaic modules  111  and row rotation mechanism  120  which can be coupled to any suitable number of rows  110 , e.g., to each of rows  110 , to only one of rows  110 , or to more than one of rows  110 , so as to rotate photovoltaic modules  111  of that row. Optionally, system  100  can further include a common structural element attaching together the photovoltaic modules (not specifically illustrated in  FIG. 1A , but can be configured similarly as a purlin such as described with reference to  FIG. 1B ) Exemplary features of system  110  include one or more of the following: ballasted (non-penetrating); central actuation and control (low cost); low force transmission (low cost, low loads, simple components); distributed gearbox (non-backdrivable); easy to install (manually installed or automated install); ship as a unit, or assembly of pieces on-site (manual/automated); and/or robotically cleanable. 
       FIG. 1B  schematically illustrates a plan view of certain components of a non-limiting embodiment of row rotation mechanism  120 . Row rotation mechanism  120  includes elongated structural member  121  extending along and parallel to at least one row  110 ; one or more protrusions  122  coupled to elongated structural member  121 ; actuator  123 ; and drive mechanisms coupled to the photovoltaic modules of the at least one row (drive mechanisms not specifically illustrated in  FIG. 1B , but optionally configured such as described with reference to  FIG. 2A-2B, 3A-3B, 4, 5, 6A-6C, 7A-7B, 8A-8E , or  9 A- 9 H). Optionally, row rotation mechanism  120  includes rotatable posts or pulleys  124  configured so as to guide elongated structural member in a continuous loop along and between multiple rows  110 . In some embodiments, actuation of actuator  123  moves elongated structural member  121 , the movement of elongated structural member  121  moves protrusion(s)  122 , the movement of protrusion(s)  122  moves the drive mechanisms, and the movement of the drive mechanisms rotates the photovoltaic modules  111  of the one or more rows  110 . For example, protrusion(s)  122  can convert motion of elongated structural member  121  into rotary motion of the photovoltaic modules  111  of row  100 . Elongated structural member  121  optionally can include a cable or belt. Additionally, or alternatively, protrusion(s)  122  optionally can include balls, knots, or barrels that traverse the row responsive to actuation of the actuator. Additionally, or alternatively, the drive mechanisms (not specifically illustrated in  FIG. 1B ) optionally can include gearboxes, worm drives, belt drives, ratchets, or geneva mechanisms. Additionally, or alternatively, protrusion(s)  122  can engage spaces within the drive mechanisms in a manner such as described with reference to  FIG. 2A-2B, 3A-3B, 4, 5, 6A-6C, 7A-7B, 8A-8E , or  9 A- 9 H. 
     Illustratively, in the non-limiting example shown in  FIG. 1B , actuator  123  turns and moves cable or belt  121  (elongated structural member) that traverses multiple rows  110  or a single row  110  of panels  111 . The belt or cable  121  has a single or multiple balls or knots or barrels  122  (protrusions) attached to it that traverses the row(s)  110 . As the ball or knot or barrel  122  passes by each panel  111 , the angle of the panel is changed by a mechanism (drive mechanism). This mechanism can be a gearbox, worm drive, belt drive, ratchet, or geneva mechanism. The mechanism is not backdrivable (cannot be reversed by loads applied to the panel; this non-backdrivable nature can be due to the mechanism itself (such as a worm drive) or due to a mechanism that keeps the mechanism from being backdrivable (such as a ratchet or catch). The belt or cable  121  can be driven in one way to tilt in one direction, and in the other way to tilt in the other direction. Alternatively, the cable  121  may only travel in one direction and the panel will reverse tilt automatically at the end of travel. 
       FIG. 1C  schematically illustrates a perspective view of exemplary system  100 ′ including a plurality of rows  110 ′ of photovoltaic modules  111 ′ and one or more row rotation mechanisms  120 ′ which can be coupled to any suitable number of rows  110 ′, e.g., to each of rows  110 ′, to only one of rows  110 ′, or to more than one of rows  110 ′, so as to rotate photovoltaic modules  111 ′ of that row. Optionally, system  100 ′ can further include a common structural element  125  attaching together at least some of the photovoltaic modules  111 ′ of each row  110 ′, such as a purlin. In an exemplary, non-limiting embodiment, row rotation mechanism  120 ′ includes elongated structural member  121 ′ extending along and parallel to at least one row  110 ′; one or more protrusions  122 ′ coupled to elongated structural member  121 ′; actuator  123 ′; and drive mechanisms  126  coupled to the photovoltaic modules of the at least one row. In some embodiments, actuation of actuator  123 ′ moves elongated structural member  121 ′, the movement of elongated structural member  121 ′ moves protrusion(s)  122 ′, the movement of protrusion(s)  122 ′ moves drive mechanisms  126 , and the movement of drive mechanisms  126  rotates the photovoltaic modules  111 ′ of the one or more rows  110 ′. For example, protrusion(s)  122 ′ can convert motion of elongated structural member  121  into rotary motion of the photovoltaic modules  111  of row  100 . Additionally, or alternatively, protrusion(s)  122 ′ can engage spaces within drive mechanisms  126  in a manner such as described with reference to  FIG. 2A-2B, 3A-3B, 4, 5, 6A-6C, 7A-7B, 8A-8E , or  9 A- 9 H. 
     In one optional configuration of modular installation including tables, such as illustrated in  FIG. 1C , the overall installation optionally can include multiple independent groups of PV modules, or “tables”  112 . Such tables  112  optionally can be arranged in multiple rows  110 ′ that can be parallel to one another in a manner such as illustrated in  FIG. 1C . In one example, the modules  111 ′ of a given table  112  and/or the tables  112  of a given row  110 ′ share a common foundation (e.g., elongated concrete ballast  127 ), a common drive rail (e.g., elongated structural member  121 ′) (which can include multiple drive rail portions coupled together by couplings), and a common rotation mechanism (e.g., drive mechanism  126 ). In one example, one motor (e.g., actuator  123 ′) at the center of each row  110 ′ or other suitable location is responsible for rotating the modules  111 ′ (or tables  112  of such modules) in that row  110 ′. Exemplary components of one non-limiting example of a single-axis tracker are illustrated in  FIG. 1C . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. 
     Power, actuation, and control system (PAC). In some embodiments, actuator  123 ′ optionally is configured as part of a PAC. In some embodiments, each PAC unit includes a gearbox and motor (e.g., actuator) assembly that turns the drive tubes (or other elongated structural members  121 ′) that can be connected from either side. In some embodiments, each PAC unit optionally can be mounted on a specialized or standard section of the track foundation  127 . In some embodiments, between tables  112  the drive tubes (or other elongated structural members  121 ′) optionally are connected with flexible couplings, e.g., such as described with reference to  FIGS. 11A-11B . In some embodiments, each table  112  can be fully or partially isolated from adjoining (neighboring) tables, e.g., using the flexible couplings and/or control joints (breaks)  128  in the track (foundation)  127 . These optional control joints  128  can help to protect the single-axis tracker (system) from thermal, settlement, and/or seismic effects, for example, by providing controlled, acceptable places for the concrete foundation to crack. In some embodiments, the optional flexible coupling also or alternatively can accommodate such movement and/or can continue to transmit torque and/or motion to table  112 . In some embodiments, the drive tube can be configured to slide within bearings on the uprights so as to provide still greater accommodation, or can be fixed (non-sliding), optionally with compensation for such movement in other components (such as the drive tube couplings). 
     It should be understood that such flexible couplings and control joints readily may be used with any other embodiments or configurations provided herein. 
       FIG. 1D  schematically illustrates a plan view (non-limiting example of site layout) of exemplary system  100 ″ including a plurality of rows  110 ″ of photovoltaic modules  111 ″ and a plurality of row rotation mechanisms (not specifically labeled) that each is coupled to only one of rows  110 ″. In an exemplary, non-limiting embodiment, the row rotation mechanism includes an elongated structural member extending along and parallel to at least one row in a manner such as described with reference to  FIG. 1A-1C, 1F-1H, 6A-6C, 8A-8J , or  11 A- 11 B, and an actuator  123 ″. In some embodiments, actuation of actuator  123 ″ moves the elongated structural member, which rotates the photovoltaic modules  111 ″ of the corresponding row  110 ″. For example, the elongated structural member can include a belt or cable that moves the photovoltaic modules of row  110 ″ via a ball, knot, or barrel, or a drive tube that moves the photovoltaic modules of row  110 ″ via a drive mechanism, or a torque tube that is directly coupled to the row of photovoltaic modules. Optionally, system  100 ″ includes one or more wind fences  131  (e.g., an optional west windfence),  132  (e.g., an optional east windfence) disposed parallel to and adjacent to at least one of rows  110 ″ in a manner such as described with reference to  FIGS. 15A-15G . Additionally, or alternatively, system  100 ″ includes one or more of the following features: optional water tank  133 ; optional autonomous cleaning vehicle home base  134 ; power, actuation, and control system  135 ; and optional row-to-row track or foundation  136  for optional autonomous cleaning vehicle (SPOT).  FIG. 1D  shows optional configurations including one or both of a windfence ( 131  and/or  132 ) and SPOT integrated O&amp;M (operation and maintenance) vehicle, e.g., configured for cleaning and/or vegetation management, and/or remote inspection and/or life/performance enhancing material application. 
       FIG. 1E  schematically illustrates a plan view of exemplary system  100 ′″ including a plurality of rows  110 ′″ of photovoltaic modules  111 ′″ and a plurality of row rotation mechanisms (not specifically labeled) that each is coupled to actuator  123 ′″. In an exemplary, non-limiting embodiment, the row rotation mechanism includes an elongated structural member extending along and parallel a plurality of rows in a manner such as described with reference to  FIG. 1A-1C, 1F-1H, 6A-6C, 8A-8J , or  11 A- 11 B, which member is coupled to a shared or ganged actuator  123 ″. In some embodiments, actuation of actuator  123 ′″ moves each of the elongated structural members, which rotates the photovoltaic modules  111 ′″ of the corresponding rows  110 ″. For example, the elongated structural member can include a belt or cable that moves the photovoltaic modules of each row  110 ′″ via a ball, knot, or barrel, or a drive tube that moves the photovoltaic modules of row  110 ′″ via a drive mechanism, or a torque tube that is directly coupled to the row of photovoltaic modules. Optionally, system  100 ′″ includes one or more wind fences  131 ′,  132 ′ disposed parallel to and adjacent to at least one of rows  110 ′″ in a manner such as described with reference to  FIGS. 15A-15G . 
     Ganging options. In some embodiments, rather than having a PAC unit  135  (of which actuator  123 ′″ can be a part) on each row  110 ′″ such as illustrated in  FIG. 1D , the present tracker (system) optionally can be configured so as to include a central actuator unit  123 ′″ that moves groups of rows  110 ′″. For example, a torque transmission mechanism can transmit the torque from the actuator  123 ′″ to the rows  110 ′″ of modules  111 ′″. For example, in some embodiments, the tracker (system) can include a rotating driveshaft that traverses multiple rows to provide motion and torque to slew drives for each individual row. Such a driveshaft optionally can be powered by a single motor (actuator)  123 ′″ and controller, and provide torque and motion to many rows  110 ′″ (for example, 2 or more rows, 5 or more rows, 10 or more rows, 20 or more rows, of 60 or more rows). Note that such ganging options are compatible with use of any suitable elongated structural member(s) for use in rotating the photovoltaic modules of the ganged rows, such as a cable or belt, drive tube, or torque tube. 
     Additionally, or alternatively, In some embodiments, the present single-axis tracker (system) optionally can include one or more structural members that span between rows such as shown in  FIG. 1F . Such supports (structural members) can help to provide a suitable foundation for PAC unit(s).  FIG. 1F  schematically illustrates a perspective view of a non-limiting embodiment in which the system is configured to rotate photovoltaic modules  151  arranged in multiple rows  150  (e.g., in first and second rows) that are parallel from one another and laterally offset from the row in a direction orthogonal to the row. The system can include, for each row  150 , an elongated concrete ballast  177  extending along and parallel to the row and upon which the photovoltaic modules  151  of that row are disposed; and a linking member  144  extending perpendicular to and connecting together the concrete ballasts. For example, in some embodiments, the torque(s) applied to the panels  151  can be transferred from actuator  143  to a single torque tube  161  (such as in the non-limiting example shown in  FIG. 1F ) or to a drive tube via an arc drive (such as described with reference to  FIGS. 8A-8F ). The torque tube or drive tube then can transfer this torque to a slew drive or gearbox actuator, which can be connected foundation (e.g., elongated concrete ballast  177 ). The foundation has the option of handling this torque, for example, by including the foundation already present, by including an additional mount of ballasted foundation, by including a post-driven foundation, or by including an element (such as linking member  144 ) that connects the foundation of one row to the foundation of adjacent rows (thus spreading the torque over a wider base and making it easier to resist). Optionally, actuator  143  can be disposed on linking member  144 , optionally in a region where linking member  144  optionally is disposed on elongated concrete ballast  177 . Additionally, or alternatively, elongated concrete ballast  177  optionally can include a plurality of control joints  178 , e.g., optional foundation control joints, e.g., concrete ballast control joints. 
     In some embodiments, optional flexible couplings and/or optional track (concrete ballast) control joints can facilitate the present single-axis tracker to tolerate thermal and seismic effects. In some embodiments, the optional flexible couplings can act as universal joints. For example,  FIG. 1G  schematically illustrates a side view of another exemplary configuration of a row  150 ′ of photovoltaic modules  151 ′ for use in the present systems and methods. Row  150 ′ includes elongated concrete ballast  177 ′ optionally including control joints  178 ′, torque tube  161 ′ coupled to photovoltaic modules  151 ′ and optionally including flexible couplings  179 ′, and actuator  143 ′. Optionally, unsupported sections  180  of torque tube  161 ′ can include suitable coupling(s) to accommodate larger misalignments. Optionally, each section of track (foundation or elongated concrete ballast)  177 ′ can be bonded to any suitable number of uprights  190 , e.g., to one upright, or to more than one upright, wherein the uprights support at least photovoltaic modules  151 ′ and torque tube  161 ′. 
       FIG. 1H  schematically illustrates a perspective view of yet another exemplary configuration of a row  150 ″ of photovoltaic modules (solar panels)  151 ′ for use in the present systems and methods. Row  150 ″ includes elongated concrete ballast  177 ″ optionally including control joints (not specifically illustrated), torque tube (torque transmitting tube)  161 ″ coupled to photovoltaic modules  151 ″ and optionally including flexible couplings (not specifically illustrated), and an actuator (not specifically illustrated). Optionally, elongated concrete ballast  177 ″ can be bonded to any suitable number of uprights (tube supports)  190 ″, e.g., to one upright, or to more than one upright, wherein the uprights support at least photovoltaic modules  151 ″ and torque tube  161 ″. Optionally, fold-out panel supports  162  support and couple photovoltaic modules  151 ″ to torque tube  161 ″. Optionally, row  150 ″ includes one or more of the following features: all plastic, frameless modules  151 ″ (no grounding necessary, except metal torque transmitting tube  161 ″); drive tube (torque transmitting tube  161 ″) is grounded (metal); works with SPOT cleaning robot (tracker rotates to a specific tilt). 
     It should be appreciated that the actuators for rotating one or more rows of photovoltaic modules suitably can be powered and controlled using a suitable power, actuation and control system (PAC). In some embodiments, the PAC systems can be powered by the grid and/or by battery storage. In embodiments in which battery storage is used, a PAC unit optionally can be fitted with a solar panel, for example, to charge the battery or to use inductive/parasitic power to charge the batter.  FIG. 1I  schematically illustrates an exemplary block layout for PAC  190  configured to rotate one or more rows  191  of panels. PAC  190  includes actuator  192  that includes a gearbox, motor, and connections for coupling to an elongated structural member for rotating row  191 , such as torque tube connections. PAC  190  also includes controls  193  including a feedback monitor for sensing the tilt of the panels of row  191 , motor controller, an inverter pad including master/monitoring controller configured to control the rotation of multiple rows and a supervisory control and data acquisition system (SCADA) that collects data on site, and a weather data acquisition subsystem (MET). PAC  190  also includes power source  194 , such as a solar charged battery. The components of PAC  190  suitably can be coupled together with wires and/or wirelessly, e.g., such as shown in  FIG. 1I . 
     In systems such as described herein with reference to  FIGS. 1A-1I , any suitable drive mechanism can be used so as to couple an elongated structural member to photovoltaic modules of a row, so as to rotate those modules responsive to actuation of an actuator. For example, in some embodiments, the elongated structural member can include a cable or belt. Additionally, or alternatively, the drive mechanism can include protrusions that include balls, knots, or barrels that traverse the row responsive to actuation of the actuator. Additionally, or alternatively, the drive mechanism can include gearboxes, worm drives, belt drives, ratchets, or geneva mechanisms. Additionally, or alternatively, the protrusions can convert motion of the elongated structural member into rotary motion of the photovoltaic modules. For example,  FIGS. 2A-2B  schematically illustrate side views of a non-limiting embodiment of a mechanism  200  for converting cable  230  motion into rotary motion. Mechanism  200  can act directly on the solar module (not specifically illustrated) to tilt it, through a supporting device (e.g., panel supporting structure  250 ) or through a separate mechanism to tilt the solar panel. In mechanism  200  illustrated in  FIGS. 2A-2B , ball  210  passes through cable guide or constrained space  260  and engages space  221  within wheel  220  that turns with ball passage or other device that causes it (the wheel) to turn a specified amount. In some embodiments, mechanism  200  can be driven in reverse by the cable  230 , but wheel  220  cannot move backwards on its own. Mechanism  200  can be attached to concrete  240  or ballast or other mounting surface or to the solar panel or to the supporting structure  250 . There can be one mechanism  200  per solar panel to provide tilting, or one mechanism for multiple solar panels, or multiple mechanisms per solar panel. In embodiments that include a single ball, mechanism  200  can be considered to include equal numbers of protrusions and drive mechanisms. 
       FIGS. 3A-3B  schematically illustrate side views of a non-limiting embodiment of a mechanism  300  for converting cable  330  motion into rotational motion. In mechanism  300  illustrated in  FIGS. 3A-3B , ball  310  passes through constrained/constraining space  360  and engages space  351  within panel supporting structure  350  to tilt the panel (not specifically illustrated) a specified amount. In some embodiments, mechanism  300  can be driven in reverse by the cable  330 , but wheel (panel supporting structure  350 ) cannot move backwards on its own. Mechanism  300  can be attached to the concrete or ballast (not specifically illustrated) or to the solar panel or to the supporting structure  350 . There can be one mechanism  300  per solar panel to provide tilting, or one mechanism for multiple solar panels, or multiple mechanisms per solar panel. In embodiments that include a single ball, mechanism  300  can be considered to include equal numbers of protrusions and drive mechanisms. 
       FIG. 4  schematically illustrates a perspective view of a non-limiting embodiment of a mechanism  400  for rotating at least one photovoltaic panel (module)  411 . Mechanism  400  can include panel supporting structure  450 , worm gear interface  460  to panel supporting structure  450 , cable  430 , ball(s)  410 , space constrainer or cable guide  460 , and wheel  420 . Panel supporting structure  450  may include, or consist of, multiple parts that pivot relative to one another. Cable  430  with one or more balls  410  may drive wheel  420  or other device that turns a worm gear  460  or other device that causes the panel to tilt by a specified amount. The motion of ball(s)  410  through mechanism  400  may be constrained in place, e.g., by space constrainer or cable guide  460 , as it (the ball) passes through mechanism  400 . The worm drive  460  or other mechanism is not backdrivable, in certain embodiments. 
       FIG. 4  schematically illustrates a perspective view of a non-limiting embodiment of a mechanism  400  for rotating at least one photovoltaic panel (module)  411 . Mechanism  400  can include panel supporting structure  450 , worm gear interface  460  to panel supporting structure  450 , cable  430 , ball(s)  410 , space constrainer or cable guide  460 , and wheel  420 . Panel supporting structure  450  may include, or consist of, multiple parts that pivot relative to one another. Cable  430  with one or more balls  410  may drive wheel  420  or other device that turns a worm gear  460  or other device that causes the panel to tilt by a specified amount. The motion of ball(s)  410  through mechanism  400  may be constrained in place, e.g., by space constrainer or cable guide  460 , as it (the ball) passes through mechanism  400 . The worm drive  460  or other mechanism is not backdrivable, in certain embodiments. 
       FIG. 5  schematically illustrates a perspective view of a non-limiting embodiment of a mechanism  500  for rotating at least one photovoltaic panel (module)  511 . Mechanism  500  can include curved rack portion of solar panel supporting structure  550 , vertically oriented wheel  520 , cable  530 , and ball  510 . Panel supporting structure  550  may include, or consist of, multiple parts that pivot relative to one another. Cable  530  with one or more balls  510  may drive vertically oriented wheel  520  or other device that directly or indirectly engages the pivoting part of solar panel support structure  550 . This engagement may be a curved rack or other mechanism. The motion of ball  530  through mechanism  500  may be constrained in place as it (the ball) passes through mechanism  500 . Mechanism  500  may be inherently nonbackdrivable or may incorporate features such as a ratchet to make it non-backdrivable. Mechanism  500  may be operating in the forward or reverse direction by changing the direction of travel of cable  530 . 
       FIG. 6  schematically illustrates a perspective view of a non-limiting embodiment of a mechanism  600  for rotating a plurality of rows  610  of photovoltaic modules  611  that can be mounted on mounting surfaces  627  that can include concrete, asphalt, or other mounting surface and that may or may not be ballasted. Mechanism  600  includes cable(s)  630 , pulleys  660 , and panel rotational mechanisms  620 . Panel rotational mechanisms  620  each are coupled to one or more photovoltaic modules  611  and can include vertically oriented wheel  621  that is moved by cable  630  or a ball on cable  630  that causes panel  611  to tilt; worm drive or other mechanism  622 ; and secondary belt system  623  that tilts panel (module)  611  indirectly, e.g., responsive to rotation of wheel  621  by motion of cable  630  or ball attached thereto, which causes rotation of worm drive or other mechanism  622 , which causes motion of secondary belt system  623  so as to move photovoltaic modules  611  through multiple tilts such as shown in  FIG. 6C . Cable(s)  630  move from one row of solar panels  611  to another using pulleys  660 . 
       FIGS. 7A-7B  schematically illustrate perspective views of a non-limiting embodiment of a mechanism  700  for rotating at least one photovoltaic module  711  that can be disposed on structure mounting surface  727 , such as an elongated concrete ballast. Mechanism  700  can include solar panel support structure  750 , e.g., a curved rack that can be coupled to module  711  via a folding hinge  751  into the back of the panel, cable  730 , ball  710 , and mechanism  720  that moves as ball  710  passes through. Panel supporting structure  750  may include, or consist of, multiple parts that pivot relative to one another. The tracking mechanism  700  and support structure  750  may fold relative to the back of panel  711  by hinged mechanism  751 . The mechanism then may occupy the space created by the mounting structure or by additional components that create a space for folded support structure  751 . 
     In still other embodiments, the drive mechanisms of a system such as illustrated in  FIGS. 1A-1I  can include curved racks each arranged perpendicularly to the photovoltaic modules. Optionally, the curved racks can include gear teeth and/or the photovoltaic modules rotate about a pivot point at a radial center of the curved racks. Additionally, or alternatively, the drive mechanisms can include distributed gearboxes. Illustratively, in some embodiments, the present single-axis tracker (system) utilizes a pinion (also known as a spur gear) and a curved rack gear mechanism (which together can be called the arc drive, or radial arc drive (RAD)) so as to rotate rows and/or tables of solar panels (e.g., groups of adjacent solar panels) so as to track the sun as it moves from east to west. These types of systems can be referred to as “trackers.” 
     In some embodiments, a solar block can include a collection of tracker rows. In some embodiments, each tracker row can include multiple tables. In some embodiments, these tables can include independent structures that can articulate to accommodate terrain variation at the installation site, such as slope and/or offset and/or angular variation that can be either in place during installation or that can occur due to phenomena such as settlement and/or erosion at the installation site over the lift of the product. 
     In some embodiments, the present single-axis tracker can be configured so as to turn such rows and/or tables to track the sun. In some embodiments, a drive tube, which in some embodiments can be controlled by a central slew actuator (or, as another example, a gearbox in a ganged configuration), optionally can be the only structural component that connects the tables, other than for a rail upon which the table(s) may sit. The drive tube optionally can include flexible couplings further enable the tables to articulate such as described herein. In some embodiments, the drive tube transmits torque and motion to a pinion gear, which in turn engages a curved rack gear to turn the table. In some embodiments, there may be one or many of these arc drives per each table. 
     Exemplary foundations for the present single-axis tracker can include ballasted, and traditional post-driven systems. The ballasted foundation (e.g., elongated concrete ballasts) can be monolithic or split into two parts, and can be referred to as tracks, rails, or pontoons. In some embodiments, the ballasted foundation can be formed by slipforming or precasting. Additionally, or alternatively, the ballasted foundation can be cast-in-place, e.g., using re-usable forms. 
     For example,  FIGS. 8A-8D  schematically illustrate perspective views of exemplary mechanisms for rotating photovoltaic modules using curved racks. Exemplary components of one non-limiting example of a table  812 , including a foundation and a tracker including an arc-drive configuration, are illustrated in  FIG. 8A . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added.  FIG. 8A  illustrates an exemplary embodiment of a single table such as can be used in systems such as illustrated in  FIGS. 1A-1I . A plurality of such tables can be provided in a given row and such tables can be independent of one another, e.g., can be joined together only by an elongated structural member (such as a drive tube) for rotating the tables and/or by an elongated concrete ballast upon which the table can be disposed. Illustratively, each table can be disposed on a discrete portion of an elongated concrete ballast, the discrete portions optionally being separated from one another by control joints. Optionally, the drive tube can include flexible couplings that allow articulation of the drive tube, such as described elsewhere herein. Each table can include one or more drive mechanisms coupled to such an elongated structural member. 
     Exemplary table  812  illustrated in  FIG. 8A  includes a plurality of photovoltaic modules  811  (e.g., two or more modules, three or more modules, four or more modules, five or more modules, six or more modules, seven or more modules, eight or more modules, nine or more modules, or ten or more modules); first and second optional common structural elements  813 ,  814  attaching together the photovoltaic modules, e.g., purlins; and uprights  815  coupled to elongated concrete ballast  818  extending along and parallel to the row and upon which the photovoltaic modules are disposed. Optionally, each photovoltaic module  811  can include multiple photovoltaic panels, e.g., two or more, three or more, or four or more photovoltaic panels that are joined together using stiffeners. Optionally, the modules  811  are frameless, and clips can be used so as to clamp the glass frames together. Alternatively, the modules  811  can be framed, and the frames optionally can be attached to purlins  813 ,  814 . The modules  811  can have a portrait orientation such as illustrated in  FIG. 8A , or can have a landscape orientation such as illustrated in  FIGS. 1G and 1H . 
     In the non-limiting embodiment illustrated in  FIG. 8A , elongated concrete ballast  818  is split into discrete tracks each parallel to table  812  and/or to the row of which table  812  can be a member. Alternatively, elongated concrete ballast  818  can be monolithic. Elongated concrete ballast  818  optionally can be formed by slip forming, precasting, or cast-in-place. Optionally, elongated concrete ballast  818  further includes one or more control joints such as described elsewhere herein. In the non-limiting embodiment illustrated in  FIG. 8A , uprights  815  include bridge  816  and post  817 . The bridge  816  can extend between first and second support surfaces such as provided by elongated concrete ballast  818 , and the post  817  can extend vertically from bridge  816  and can support rotational mechanism  820 . In the exemplary embodiment illustrated in  FIG. 8A , rotational mechanism  820  includes elongated structural member  860 , protrusions that are coupled to elongated structural members  860  and that engage with drive mechanisms  862  (e.g., teeth of spur gear  861 ), and drive mechanisms  862  (e.g., curved rack gear) coupled to photovoltaic modules  811 . However, it should be understood that uprights  815  can have any suitable configuration. For example, in other embodiments such as described elsewhere herein, A-shaped uprights can be used so as to support elongated structural member  860 , protrusions (e.g., teeth of spur gear  861 ), and drive mechanisms  862 . 
     In some embodiments, actuation of an actuator (not specifically illustrated in  FIG. 8A ) moves elongated structural member  860 , e.g., drive tube; the movement of elongated structural member  860  moves protrusions that engage with drive mechanism  862  (e.g., rotates spur gear  861  which causes the teeth of spur gear  861  to rotate); the movement of the protrusions moves drive mechanisms  862  (e.g., rotates a curved rack gear such as illustrated in  FIG. 8A ); and the movement of drive mechanisms  862  rotates photovoltaic modules  811  (e.g., rotates purlin arms  863  which are coupled to drive mechanisms  862  and to purlins  813 ,  814  which are coupled to photovoltaic modules  811 ). 
     Referring still to  FIG. 8A , the actuator to which elongated structural member, e.g., drive tube,  860  is coupled optionally can include a slew drive actuator or a gearbox in a ganged configuration such as described elsewhere herein. Illustratively, the system further can be configured to rotate photovoltaic modules arranged in a second table or row, the second table or row being parallel to the row of which table  812  is a member and laterally offset from the row in a direction orthogonal to the row. The system can include a second elongated structural member extending along and parallel to the second row (e.g., a second drive tube configured similarly as drive tube  860  illustrated in  FIG. 8A ); protrusions coupled to the second elongated structural member (e.g., second spur gears configured similarly as spur gears  861  illustrated in  FIG. 8A ); and drive mechanisms coupled to the photovoltaic modules arranged in the second row (e.g., second drive mechanisms configured similarly as drive mechanisms  862  illustrated in  FIG. 8A ). Actuation of the actuator moves the second elongated structural member, the movement of the second elongated structural member moves the protrusions coupled to the second elongated structural member, the movement of the protrusions coupled to the second elongated structural member moves the drive mechanisms coupled to the photovoltaic modules arranged in the second row, and the movement of the drive mechanisms coupled to the photovoltaic modules arranged in the second row rotates the photovoltaic modules of the second row. The first and second elongated structural members optionally can be discrete from one another. The system optionally further can include a torque transmission mechanism, such as a rotating driveshaft, configured to transmit torque from the actuator to the second elongated structural member. 
     It should be understood that the present mechanisms can include any suitable number of drive mechanisms per table or row. For example,  FIG. 8B  schematically illustrates a non-limiting embodiment of a table  812 ′ that can be configured similarly as table  812  and that includes a single rotational mechanism  820 ′, e.g., a single drive mechanism  862 ′. 
     In some embodiments, embodiments of the present tracker, arc drive configuration (rotational mechanism), and/or table can provide one or more of the following features, e.g., any suitable combination of the following features: 
     High stiffness—for example, a table including any suitable number of photovoltaic (PV) modules, e.g.,  6  PV modules, can be supported on any suitable number of purlins that are attached to any suitable number of purlin arms, e.g., can be supported on two purlins that are attached to two purlin arms; and/or 
     No damper required—high natural frequency, does not depend on row length; and/or 
     Reduced number of parts—for example, a table can be supported using any suitable number of purlins, such as two purlins configured or optimized for load bearing and torsion. The modules can be mounted directly to the purlins (e.g., with clamps). In some embodiments, no other support necessarily is required or included for the PV modules; and/or 
     More modules per motor/actuator can be provided than a design utilizing a torque tube. In one nonlimiting example, the drive shaft (drive tube) can have a smaller diameter than an equivalent stiffness torque tube, e.g., about 1/10 diameter of equivalent stiffness torque tube; and/or 
     Lower cost components—in some embodiments, drive shaft (drive tube) connections (couplings) can operate using lower load capacity and size than a torque tube design, e.g., in one example can require 1/10 load capacity and size than in torque tube design. Torque tube and couplings can be major cost components; and/or 
     Installed system can follow the terrain—for example, in some embodiments, modular tables can accommodate uneven ground at the installation site; and/or 
     The present tracker, arc drive configuration, and/or table can be compatible with an autonomous cleaning/operation and maintenance (O&amp;M) vehicle (SPOT) that travels along the track foundation(s). Exemplary embodiments of SPOT are described in U.S. Patent Publication No. 2015/0144156, the entire contents of which are incorporated by reference herein. Additional exemplary embodiments of SPOT are described further herein with reference to  FIGS. 16A-16E . 
     Local and distributed options. In some embodiments, such as illustrated in  FIGS. 8A and 8B , the present single-axis tracker includes a distributed foundation (e.g., one or more concrete tracks). In some embodiments, the mechanisms that support the table, rotate the table (e.g., arc drive), and support the modules can be distributed along the foundation. Additionally, or alternatively, each table is provided as an independent unit in which torque and motion are delivered by the drive tube. Additionally, or alternatively, in some embodiments, the strength of individual supports can be reduced as there can be any suitable number of supports to handle the load (the ability for structures to handle loads can change with their spatial distribution). 
     In some embodiments, the present single-axis tracker can include one or more tracks, e.g., concrete tracks, that can act as ballast. Such ballasting optionally can eliminate the need for a ground-penetrating foundation or can be used with a post-driven system or with a hybrid of the two approaches. 
     Split ballast/track options. In some embodiments, a table of modules optionally can be supported by a track, e.g., a split track that includes two sections such as in the non-limiting example shown in  FIGS. 8A-8B , or by a track that includes one monolithic section. Such optional configurations can accommodate adhesive connection between the uprights and the concrete, or wet-set (e.g., embedment of a component or structural element into the body of the concrete before curing that is securely attached upon or after curing of the concrete. Exemplary methods for wet-setting uprights into an elongated concrete ballast are described elsewhere herein. 
     Number of arc drives per table options. The non-limiting exemplary table shown in  FIG. 8A  can include an arc drive (drive mechanism  862 ) at each upright. Alternative configurations optionally can include an arc drive at only some of the uprights, e.g., at only one of two uprights, or at another location of the table, e.g., at the center of the table, rather than at the uprights (one non-limiting example of such an embodiment being shown in  FIG. 8B ). In one example, the curved rack and pinion/spur gears at one of the uprights shown in  FIG. 8A  can be removed. 
     Bumper under pivot options. In some embodiments, padding optionally can be placed between the module and pivot point so as to protect the module in case of deflection. For example,  FIG. 8C  illustrates a non-limiting embodiment in which optional padding 819 is placed between module  811 ′ and pivot point  864  so as to protect the module in case of deflection. For example, in some embodiments, padding 819 can include rubber or other relatively soft material configured so as to prevent or inhibit the module from directly contacting the support or drive mechanism, e.g., purlin arm or arc-drive. Alternatively, or additionally, a support with adhesive can be used to limit panel deflection and constrain movement. Alternatively, or additionally, the pivot  864  can be located between panels, or far enough below panels that the deflection of panel  811 ′ does not come into contact with the support or drive mechanism, e.g., pivot components. 
     Square drive tube geometry options. It should be appreciated in that any suitable embodiment provided herein, the elongated structural member, e.g., drive tube, can have any suitable geometry, e.g., can have any suitable cross-sectional geometry. For example, the non-limiting configuration illustrated in  FIGS. 8D-8E  includes a tube with a square cross-section.  FIGS. 8D-8E  schematically illustrate perspective views of components of another exemplary table  812 ″ including exemplary rotational mechanism  820 ′. Mechanism  820 ′ can include bearing  863 ″, e.g., split plastic bearing, which is coupled to square drive tube  860 ″, spur gear  861 ″, and curved rack gear  862 ″. In the non-limiting configuration shown in  FIGS. 8D-8E , the square drive tube  860 ″, bearing (e.g., split plastic bearing)  863 ″, and gear  861 ″ can be mechanically interlocked without the need for additional fasteners. Exemplary components of one non-limiting example of a drive tube, bearing, and gear, e.g., for use in an arc drive configuration, are illustrated in  FIGS. 8D-8E . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. Another exemplary tube  3500  that can be used as a drive tube or a torque tube in a system or method provided herein is illustrated in  FIG. 35 . 
     Folding and/or A-shaped uprights options. Optionally, in some embodiments, one or more of the uprights that support the table can be A-shaped and/or can be hinged in a manner such as illustrated in  FIGS. 8D-8E . For example, table  812 ″ illustrated in  FIGS. 8D-8E  can include uprights  815 ″ that are A-shaped and/or hinged. Such hinging can facilitate the uprights to fold during shipment and/or to accommodate different track (foundation/elongated concrete ballast) widths. Alternatively, or additionally, in some embodiments, the uprights can be shipped independent of one another and/or can be nested together, and then assembled (with or without an optional arm that pivots and attaches to the purlins), e.g., by inserting a pin. Alternatively, or additionally, the uprights can be A-shaped and can support the elongated structural member (e.g., drive tube), the protrusions (e.g., teeth of a spur gear), and drive mechanisms (e.g., curved racks).  FIG. 31  schematically illustrates alternative table  3100  including alternative A-shaped uprights  3110 . 
       FIG. 8F  schematically illustrates a side view of another exemplary embodiment of a mechanism for arc drive integration with a track foundation. Mechanism  830  illustrated in  FIG. 8F  includes photovoltaic module  831 , optionally which can be in portrait orientation; curb track  838 ; module-driven DC motor  841  every third or fourth module (having, for example, an estimated cost of $100 each); arc mechanism  842  that is movable responsive to actuation of motor  841  so as to tilt module  831 ; and spacer strut  844 . Exemplary components of one non-limiting example of an arc-drive configuration are illustrated in  FIG. 8F . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. 
       FIGS. 8G-8H  schematically illustrate perspective views of a non-limiting embodiment of torque tube mechanism  845 . In this example, mechanism  845  includes row  850  of PV modules  851  coupled to concrete track foundation  858  via uprights  852  coupled to panel supports  853 ; power, actuation, and control system (PAC)  871 ; torque tube  870 ; and optional flexible couplings  872  disposed along torque tube  870 . Exemplary components of one non-limiting example of a single-axis tracker are illustrated in  FIGS. 8G-8H . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. Optionally, foundation  858  includes one or more control joints  859  such as described elsewhere herein. 
     Illustratively, mechanism  845  can be included in a system for rotating photovoltaic modules  851  arranged in row  850 . The system (e.g., mechanism  845 ) can include torque tube  870  extending along and parallel to row. Torque tube  870  can include a plurality of discrete sections coupled together with flexible couplings  872 , the plurality of discrete sections being coupled to photovoltaic modules  851 . The system or mechanism also can include an actuator, e.g., provided as part of PAC  871 . Actuation of the actuator can rotate the discrete sections of torque tube  870  and flexible couplings  872 . The rotation of the discrete sections of torque tube  870  and flexible couplings  872  can rotate photovoltaic modules  851 . In a manner similar to that described elsewhere herein, photovoltaic modules  851  optionally can be arranged in a plurality of independent tables, each table being coupled to a discrete section of torque tube  870  and extending parallel to row  850 . At least one of the flexible couplings  872  optionally can be disposed between each of the tables, e.g., at least at least two of the flexible couplings can be disposed between each of the tables. The flexible couplings  872  can allow articulation of the discrete sections of the torque tube between the tables. Additionally, or alternatively, he system/mechanism further can include elongated concrete ballast  858  extending along and parallel to row  850  and upon which the photovoltaic modules  851  are disposed. Optionally, elongated concrete ballast  858  follows an irregular geological topology, and torque tube  870  can follow the irregular geological topology via the articulation of the discrete sections of the torque tube. Illustratively, flexible couplings  872  can allow articulation of the discrete sections of torque tube  870  and/or can transmit longitudinal forces to compensate for thermal expansion or contraction or seismic effects. 
     Exemplary embodiments of flexible couplings suitable for use in mechanism  845  are described herein, such as with reference to  FIG. 12A-12F, 13A-13C, 14A-14Y , or  32 . For example, each flexible coupling  872  optionally can include a first flange coupled to a first discrete section of torque tube  870 ; a second flange coupled to a second discrete section of torque tube  870 ; and one or more fasteners coupling the first flange to the second flange. As another example, each flexible coupling  872  can include a sleeve that can include a first end, a second end, and a lumen connecting the first and second ends, the lumen at the first end receiving a portion of a first discrete section of the drive tube, the lumen at the second end receiving a portion of a second discrete section of the drive tube. 
       FIG. 8I  illustrates another exemplary embodiment of a single-axis tracker. More specifically,  FIGS. 8I-1 and 8I-2  (collectively referred to herein as  FIG. 8I ) illustrate a plurality of rows  850 ′, each of which can include any suitable number of panels  851 ′ (PV modules) per row, e.g., 20, or 40, or 60 or more panels per row (1, 2, 3 strings of panels per row, or more), and any suitable number of rows  850 ′. Torque-transmitting tube  870 ′ (torque tube) transmits torque between actuator(s)  871 ′ and panels  851 ′. Concrete ballast  858 ′, which optionally includes one or more concrete control joints  859 ′, supports tube supports (uprights)  852 ′ and optionally also supports actuators  871 ′. Tube supports  852 ′ can connect torque tube  870 ′ to concrete ballast  858 ′, can allow rotation for pivoting to track the sun, and/or can allow sliding (e.g., of torque tube  870 ′) for thermal expansion/contraction. Panel supports  853 ′ can connect panel(s)  851 ′ to tube  870 ′ (e.g., rigidly attached). Torque tube  870 ′ optionally can include universal joints  872 ′ or other torque-transmitting, flexible joint(s) such as described elsewhere herein. Such joints  872 ′ can allow for differential concrete settlement and/or other ground support disruption. In one non-limiting example, tube supports  852 ′ and/or panel supports  853 ′ (which collectively can be referred to as support parts) can be made of, or include, plastic, and can have high lubricity and do not need to be grounded. Additionally, or alternatively, in one non-limiting example, torque tube  870 ′ can include metal tube parts, which can have a single grounding path and/or a high load carrying capacity (torque and shear). 
     Optionally, actuators  871 ′ can be connected across rows, e.g., using linking member  872 ′ which can be configured similarly as other linking members provided herein, so as to provide torsional resistance to loads. Optionally, the connection provided by linking member  872 ′ can happen on or be provided on the ground (like a speed bump) so that it can be driven over by typical construction equipment. Optionally, actuator  871 ′ control wiring and power plant power wiring can also be housed in this speed bump (linking member  872 ′). Optionally, actuator  871 ′ can include a worm drive or slew actuator, and optionally is non-backdrivable. Torque tube  870 ′ can come out of both sides of actuator  871 ′. 
     Options for torque tube configurations such as illustrated in  FIGS. 8G-8I  include one or more of the following: 
     The non-limiting, exemplary torque tube configuration such as shown in  FIGS. 8G-8I  optionally can include a single row of modules in landscape orientation. In some embodiments, the tracker can be configured to include two modules in landscape or a single row of modules in portrait; and/or 
     In some embodiments, an optional windfence can be installed around the perimeter of PV arrays. This eliminates the need to upsize components on array edges to resist higher wind loads; and/or 
     In some embodiments, the tracker optionally can include one or more masses that can act as a counterweight to the modules, for example, so as to reduce the torque demands on the motor and torque tube; and/or 
     The tracker optionally can be configured so as to include a restraint mechanism to inhibit contact between the modules and the uprights; and/or 
     This tracker optionally can include one or more flexible couplings, e.g., so as to accommodate undulations in terrain, as described elsewhere herein. 
     Exemplary configurations for counterweights, restraint mechanisms, and windfences are described elsewhere herein. 
     Gear ratio stiffness options. In various embodiments provided herein, e.g., arc-drive based embodiments, there is a gear ratio between the spur gear and the arc drive (for example, in one configuration the ratio is 1:10). In some embodiments, any torque applied to panels of a table can be resisted by the arc drive. In some embodiments, the arc drive connects the panels on the table to the drive tube using a gear reduction. Thus, in some embodiments, applying torque to a panel on the table can transmit an amount of torque reduced by the gear ratio to the drive tube. In some embodiments, the actuator holds the drive tube stationary in one location. Thus, in some embodiments, the drive tube can experience some amount of rotation along the length of the drive tube, also called a torsional deflection. In some embodiments, this torsional deflection can cause a movement of the table, and thus the panels, with this movement being the same as the drive tube rotation but reduced by the gear ratio. As a result, in some embodiments, the torsional stiffness of the panels on the table (which can be defined as the movement of the panels divided by the torque applied to the panels) can be the torsional stiffness of the drive tube multiplied by the gear ratio squared. (In the case of a 1:10 gearbox, the stiffness increases by 100×). Exemplary relationships between such components are schematically illustrated in  FIG. 8J . 
     For example, a local gear ratio optionally can provide more stiffness with less structure, and/or optionally can provide by a distributed foundation via rotational stiffness, which optionally can be a constraint on tracker design and/or optionally can increase with the square of the gear ratio. In one non-limiting example of the present single-axis tracker, a gear ratio can be 1:10, the stiffness multiplier can be 100×, a tube diameter can be 2 inches, and the number of panels per row can be 120. Exemplary components and configurations are described. It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. 
       FIGS. 30A-30B  respectively schematically illustrate exemplary actuators  3010 ,  3011  that can be used in the present systems and methods, e.g., so as to rotate a drive tube or a torque tube, optionally in a ganged configuration. 
       FIGS. 9A-9J  schematically illustrate views of various exemplary components of an arc drive mechanism such as suitable for use in certain systems and methods provided herein.  FIGS. 9A-9B  schematically illustrate perspective views of one exemplary embodiment of rotational mechanism  900  including arc drive  901  and spur gear  902 . Protrusions of spur gear  902  (e.g., teeth of spur gear) can engage with spaces within arc drive  901 . For example, spur gear  902  and arc drive  901  can mesh together in such a manner that rotation of spur gear  902  caused by rotation of drive tube  903  coupled to drive tube  902  causes arc drive  901  to rotate so as to rotate PV module  904  coupled thereto. In some embodiments, arc drive  901  can include, or can be made of, a single piece or multiple pieces assembled together. Optionally, one or more components of arc drive  901  can include stainless steel, e.g., so as to reduce corrosion at locations where arc drive  901  and spur gear  902  meet. One or more other components of arc drive  901  optionally can include galvanized steel. Optionally, spur gear  902  can include stainless steel, aluminum, or a polymer. In some embodiments, spur gear  902  can be coupled to drive tube  903  via slide interface  905  which is configured so as to permit spur gear  902  to shift laterally along drive tube  903 , e.g., responsive to thermal expansion or contraction of drive tube  903  over the course of the day, while maintaining rotational engagement of spur gear  902  with drive tube  903 . Alternatively, spur gear  902  can be laterally fixed relative to drive tube  903 . For example, drive tube  903  could include a hole, and spur gear  902  could be locked into place using a pin in a manner such as described herein with reference to  FIGS. 36A-36B . Exemplary embodiments of one non-limiting example of an arc-drive configuration are illustrated in  FIGS. 9A-9B . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. Illustratively, although drive tube  903  is illustrated in  FIGS. 9A-9B  as having a circular cross-section, the drive tube instead could have a square cross-section such as described elsewhere herein. 
       FIGS. 36A-36B  schematically illustrate side and cross-sectional views of another exemplary embodiment of rotational mechanism  3600  including arc drive  3601  and spur gear  3602 . Protrusions of spur gear  3602  (e.g., teeth of spur gear) can engage with spaces within arc drive  3601 . For example, spur gear  3602  and arc drive  3601  can mesh together in such a manner that rotation of spur gear  3602  caused by rotation of drive tube  3603  coupled to drive tube  3602  causes arc drive  3601  to rotate so as to rotate a PV module coupled thereto (not specifically illustrated). In some embodiments, arc drive  3601  can include, or can be made of, a single piece or multiple pieces assembled together. Optionally, one or more components of arc drive  3601  can include stainless steel, e.g., so as to reduce corrosion at locations where arc drive  3601  and spur gear  3602  meet. One or more other components of arc drive  3601  optionally can include galvanized steel. Optionally, spur gear  3602  can include stainless steel, aluminum, or a polymer. In some embodiments, spur gear  3602  can be coupled to drive tube  3603  via an interlocking mechanism  3605 , such as a pin which is configured so as to lock spur gear  3602  to drive tube  3603  in such a manner as to inhibit rotation of spur gear relative to drive tube  3603 . Optionally, spur gear  3602  can be laterally fixed relative to drive tube  3603 . Bushing  3606  can support drive tube  3603  relative to upright  3607 . Exemplary embodiments of one non-limiting example of an arc-drive configuration are illustrated in  FIGS. 36A-9B . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. Illustratively, although drive tube  3603  is illustrated in  FIGS. 36A-9B  as having a circular cross-section, the drive tube instead could have a square cross-section such as described elsewhere herein. 
     Arc drive options. The arc drive can have many other configurations, some exemplary options of which are schematically illustrated in  FIGS. 9C-9F . In some embodiments, the gear profiles (and thus the optional spur/pinion gear) can be on the inside of the curved rack (e.g., such as shown in  FIGS. 9C and 9F ) or on the outside of the curved rack (e.g., such as shown in  FIGS. 9A-9B, 9D, and 9E ). Additionally, or alternatively, in some embodiments, the optionally curved rack gear teeth profiles optionally can be made by bending pieces of metal or other material in the shape of a gear (e.g., such as shown in  FIG. 9E ), and/or by cutting holes in the sheetmetal itself (e.g., such as shown in  FIG. 9D ) to allow for the spur gear teeth to engage, and/or by cutting gear teeth shapes into the sidewalls of the curved rack side flanges (e.g., C- or V-shaped cross section) or central flanges (e.g., T-shaped cross section). Additionally, or alternatively, the gear shaped teeth can be cut into flat pieces of steel or other material and used as a single layer of material or combined with other layers and materials into a multi-layered or composite assembly. 
     Gear options. The gears and arc drive mechanisms can have many different configurations, some options of which are schematically illustrated in  FIGS. 9G-9J . For example,  FIGS. 9G and 9H  schematically illustrate exemplary gear tooth profiles suitable for use in a spur gear in one of the present systems and methods. As another example,  FIGS. 91 and 9J  schematically illustrate alternative embodiments of arc drive mechanisms that respectively include a friction wheel and a wire and capstan. Exemplary variations include different types of gear shapes, including roller gears, involute gears, friction drives, and/or wire and capstan drives (tensioned or untensioned). 
     Another exemplary arc drive mechanism  3200  is illustrated in  FIGS. 32 and 38A-38C . Mechanism  3200  includes pinion gear  3201  that engages with arc drive  3207  such that rotation of drive tube  3206  responsive to actuation of an actuator causes rotation of the arc drive. Pinion gear  3201  can be coupled to drive tube  3206 , which can have a generally square cross-section, via clamps  3202 ,  3203 , round/square adapter  3205 , and bushing  3204 . Optionally, the assembly can include a spacer to keep bushing  3204  from backing out. As shown in  FIGS. 38A and 38C , exemplary clamp  3202  can be coupled to drive tube  3206  (clamp  3203  optionally can be configured similarly). In one example, clamp  3202  can include a structurally stiff material, such as sheet metal, that is shaped so as to receive drive tube  3206  and that can be securably engaged to drive tube  3206  via suitable fastener  3208 , such as a bolt/bobtail. Optionally, pinion gear  3201  can include a square aperture that is sufficiently large as to permit lateral/longitudinal movement of pinion gear  3201  along drive tube  3206 , and that is sufficiently small as to inhibit significant rotational movement of pinion gear  3201  relative to drive tube  3206 . As shown in  FIG. 38B , clamps  3202 ,  3203  respectively can inhibit lateral movement of pinion gear  3201  beyond selected lateral points of drive tube  3206 . Bushing  3204  can support drive tube  3206  on a corresponding aperture (not specifically illustrated) through upright  3209  while permitting rotational movement of drive tube  3206  responsive to actuation of an actuator (not specifically illustrated). As such, arc drive mechanism  3200  can decouple lateral/longitudinal constraints of pinion gear  3201  from rotational constraints. 
     Yet another exemplary arc drive  3707  is illustrated in  FIG. 37 . Arc drive  3707  engages with a corresponding pinion gear (not specifically illustrated) such that rotation of a drive tube (not specifically illustrated) responsive to actuation of an actuator causes rotation of the pinion gear, which causes rotation of the arc drive. In the non-limiting example illustrated in  FIG. 37 , arc drive  3707  includes first and second trusses  3708 ,  3709 , each of which can include one or more structural members providing a V-shaped support that contacts the curved portion  3707 ′ of arc drive  3707  at one or more points, e.g., at one or both of points  3710 ,  3712  for first truss  3708 , and at one or both of points  3711 ,  3713  for second truss  3709 . Optionally, the photovoltaic module to which arc drive  3707  is coupled can be stowed by rotating the drive tube so as to rotate arc drive  3707  (via the pinion gear) to a position at which the pinion gear is disposed adjacent to one of points  3710 ,  3711 ,  3712 , or  3713  (e.g., adjacent to point  3710  or point  3711 ), at which point the corresponding one of trusses  3708 ,  3709  can provide additional support to arc drive  3707  and to the pinion gear engaged therewith. 
     Optionally, any of the embodiments provided herein can include one or more stop members configured to inhibit rotation of the photovoltaic modules beyond a preselected angle. As one example, the mechanism illustrated in  FIG. 8C  optionally can include pin  865  or other component interference to limit rotation and/or transmit wind loads (including but not limited to stow wind loads) directly into the structure instead of through the torque/motion transmitting elements (e.g., arc drive and drive tube). 
     Pivot stops and/or restraints options. In some embodiments, the table of modules reaches its limit of travel when the purlin contacts the uprights. In some embodiments, this provides a secure position for stowing the tables during high wind events. In some embodiments, the contact between the purlin and uprights sends the stow loads through the structure. Additionally, or alternatively, some embodiments include a pin that attaches to each arc drive (e.g., pin  865 ). In some such embodiments, the limit of travel is reached when the pin contacts the upright. 
     Alternatively, the stop members can include flexible members that are pulled taut when the photovoltaic modules reach the preselected angle, or include fixed members that the photovoltaic modules contact when reaching the preselected angle. For example, in the non-limiting, exemplary configuration shown in  FIGS. 10A-10B , the limit of travel of table  1001  optionally can be controlled by an optional limit travel arm  1002  (e.g., counterweight and limit travel arm) and/or an optional restraint wire, cable, or chain  1003 . In some embodiments, wind loads trying to rotate the module  1001  and structure can be predominantly in the clockwise rotation shown in  FIG. 10A  (wind load  1004 , in which constraint of wire, cable, or chain  1003  limits rotation) and less in the counterclockwise direction shown in  FIG. 10B  (wind load  1005 , in which constraint of wire, cable, or chain  1003  does not limit rotation). Note that embodiments such as illustrated in  FIGS. 10A-10B  can be used with drive tube based embodiments and/or with torque tube based embodiments. 
     Counterweight options. In some embodiments, such as illustrated in  FIGS. 10A-10B , the present single-axis tracker optionally can include counterweights (e.g., included in optional limit travel arm  1002 ) so as to balance the mass of the tables around the pivot points. In some embodiments, such counterweights can reduce torque on system components. In some embodiments, optional counterweights (e.g., included in optional limit travel arm  1002 ) can include pipes filled with a suitable material, e.g., can include steel pipes filled with concrete. 
       FIGS. 10C-10H  illustrate another embodiment of an exemplary rotation mechanism including a stop member (locking device). The rotation mechanism can be for converting cable  1010  motion into indexing motion (linear or rotational; direct or indirect action on panel). In one example, the passage of cable  1010  is allowed when no ball  1011  is present. The indexed device  1012  is not allowed to move since locking device  1013  does not allow that movement. When ball  1011  enters the mechanism, the locking device  1013  is pushed out of the locking position, and unlocks the indexed device  1012 . Ball  1011  then moves the indexed device  1012  a specified amount, and is constrained by the locking device  1013 . Ball  1011  then exits the mechanism and allows the locking device  1013  to re-engage, which stops further motion of the indexed device  1012 . The operation of the locking device  1013  may include multiple levels, which may include a locking level (e.g., locking teeth  1014 ) and a wire groove level (e.g., wire groove  1015 ). In the embodiment illustrated in  FIG. 10I , multiple levels may exist, including locking and moving levels, or bands. In one example, locking band  1020  and moving band  1021  can be the same part as one another.  FIG. 10I  illustrates transitions from a locked position to a moving position, and again to a locked position. The indentations in the locking band/moving band  1020 ,  1021  can be long enough for full entry and unlocking, and can include locking key slop. 
     Additionally, or alternatively, any suitable embodiments provided herein can be adapted so as to follow irregular terrain, e.g., uneven ground, at an installation site. The ground can be uneven to begin with, or can become uneven due to settling, erosion, or seismic activity, for example. In one example, a system for rotating photovoltaic modules arranged in a row can include a drive tube extending along and parallel to the row. The system can include the drive tube including a plurality of discrete sections coupled together with flexible couplings; an actuator; and drive mechanisms coupled to the photovoltaic modules. Actuation of the actuator can rotate the discrete sections of the drive tube and the flexible couplings, the rotation of the discrete sections of the drive tube and the flexible couplings can rotate the drive mechanisms, and the rotation of the drive mechanisms rotates the photovoltaic modules. Exemplary drive tubes, actuators, drive mechanisms, and flexible couplings are described elsewhere herein. In some embodiments, the photovoltaic modules are arranged in a plurality of independent tables, each table including one or more of the drive mechanisms and extending parallel to the row. In some embodiments, at least one of the flexible couplings is disposed between each of the tables. For example, at least two of the flexible couplings can be disposed between each of the tables. The flexible couplings can allow articulation of the discrete sections of the drive tube between the tables, can transmit torque from the actuator to the drive mechanisms, and/or can transmit longitudinal forces to compensate for thermal expansion or contraction or seismic effects. The system further can include an elongated concrete ballast extending along and parallel to the row and upon which the photovoltaic modules are disposed, wherein the elongated concrete ballast follows an irregular geological topology, and wherein the drive tube follows the irregular geological topology via the articulation of the discrete sections of the drive tube. 
     For example,  FIGS. 11A and 11B  schematically illustrate exemplary options for following irregular terrain in a system for rotating photovoltaic modules arranged in a row, according to some embodiments. Any suitable combination of options, such as described below or elsewhere herein, can be used. 
     Independent tables options. In some embodiments, the present single-axis tracker includes any suitable number of independent groups of PV modules, or “tables.” In the non-limiting example illustrated in  FIG. 11A , each table  1110  includes 6 modules  1111 , but the tables can have more or fewer modules per table (e.g., one, two, three, four, five, six, seven, eight, nine, ten, or more than ten modules  1111  per table  1110 ). 
     Control joints and/or ballast sections options. In some embodiments, optional control joints  1121  or cuts on the foundation tracks  1120  (elongated concrete ballast) can isolate each table  1110  from neighboring tables, or optionally can isolate uprights (not specifically illustrated) from each other. In one nonlimiting example, there can be two uprights per section of ballast (foundation)  1120  between control joints  1121 , or one upright for every section of ballast  1120  between control joints  1121 . Optionally, the section of track (foundation  1120 ) under a given table  1110  can contain a control joint  1121 , e.g., if the concrete is not reinforced; in some embodiments of such an option, each section of track (foundation)  1120  supports one upright. Optionally, the section of track (foundation)  1120  under a given table may not necessarily need to include a control joint, e.g., if the concrete is reinforced such as with rebar, fibers, or another reinforcing material. Alternatively, or additionally, the foundation  1120  can be reinforced or otherwise configured such that the foundation can span multiple tables or entire rows without control joints. 
     Terrain following options. In some embodiments, optionally independent tables combined with optional flexible couplings in the drive tube (such as described elsewhere herein) can facilitate the single-axis tracker to follow terrain such as schematically shown in  FIG. 11A  Such a configuration also can make the tracker resistant to thermal and seismic effects. 
     In some embodiments, the present single-axis tracker can accommodate uneven terrain such as schematically shown in  FIG. 11B . For example, in one non-limiting embodiment, each table  1110 ′ is supported by any suitable number of uprights  1112  (e.g., two uprights) such that drive tube  1113  for that table can remain substantially straight between the uprights  1112  without bending. In some embodiments, sections of drive tube  1113  optionally can be connected with flexible couplings  1114  that can act to transmit torque (and potentially longitudinal forces to compensate for thermal expansion/contraction or seismic effects) but also can allow for angular misalignment, e.g., such as can arise from irregular terrain at the installation site. Illustratively, placing one or more flexible couplings, e.g., two flexible couplings, between uprights  1112  can allow for one or both of angular misalignment and lateral and/or vertical offsets. In some embodiments, each flexible coupling includes a first flange coupled to a first discrete section of the drive tube; a second flange coupled to a second discrete section of the drive tube; and one or more fasteners coupling the first flange to the second flange, e.g., such as described herein with reference to  FIGS. 12A-12F . In other embodiments, each flexible coupling can include a sleeve that includes a first end, a second end, and a lumen connecting the first and second ends, the lumen at the first end receiving a portion of a first discrete section of the drive tube, the lumen at the second end receiving a portion of a second discrete section of the drive tube, e.g., such as described herein with reference to  FIGS. 13A-13B . 
     Continuous drive tube options. In some embodiments, the present drive tubes can transmit torque over a relatively long distance (the drive tube optionally can be the only component of the system that transmits torque over such a distance). In some embodiments, the torque on the drive tube can be reduced or minimized based on the gear ratio between the drive tube and the arc drives on the tables in a manner such as described elsewhere herein. Additionally, or alternatively, the effect of deflections can be reduced by the gear ratio. 
       FIGS. 12A-12E, 13A-13C, and 14A-14Y  schematically illustrate exemplary flexible couplings that can be used in a system for rotating photovoltaic modules arranged in a row, according to some embodiments. Any suitable number of such flexible couplings can be included along one of the present elongated structural members, e.g., along a drive tube or torque tube. For example,  FIG. 12A  illustrates detail of one exemplary embodiment of components of a drive tube, e.g., in an embodiment of an arc drive-based rotational mechanism that includes drive tube  1201  including one or more flexible couplings  1202  coupling discrete segments  1203 ,  1204  of the drive tube to one another. Exemplary components of one non-limiting example of a drive tube and flexible coupling, e.g., for use in an arc-drive configuration, are illustrated in  FIG. 12A . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. 
       FIGS. 12B-12F  illustrate an optional configuration of flexible coupling  1202  illustrated in  FIG. 12A . Flexible coupling  1202  can include optional flanges optionally welded to drive shaft (drive tube)  1201 , e.g., flange  1205  coupled (for example, welded) to segment  1203  of drive shaft  1201 , and flange  1206  coupled (for example, welded) to segment  1204  of drive shaft (drive tube)  1201 . Flanges  1205 ,  1206  can be coupled to one another, for example, using 6× (or other suitable number) of bobtail fasteners or other suitable fasteners.  FIG. 12D  illustrates an optional stainless steel (SS) disc pack  1207 , e.g., that can be disposed between flanges  1205 ,  1206 . Drive shaft segments  1203 ,  1204  each optionally have “half” of the coupling assembled to each end, e.g., in the factory or field, such as flange  1205  or  1206 . The optional flange  1205 ,  1206  can be attached to optional disc pack  1207  with any suitable number of suitable fasteners, e.g.,  3 X bobtail fasteners. The couplings optionally are attached in the field or factory with any suitable number of fasteners, e.g.,  3 X bobtails or other fasteners. Exemplary components of one non-limiting example of a drive tube and coupling, e.g., for use in an arc-drive configuration, are illustrated in  FIGS. 12B-12F . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. 
     Flex coupling configuration options. The optional flexible couplings optionally can be welded onto sections of drive tube. The couplings also, or alternatively, can be configured so that they attach with suitable fasteners, e.g., bolts or set screws, or adhesive, e.g., in a direct attach method (e.g., bolted/glued together) or by clamping (e.g., the bolt clamps the two pieces together, either directly or indirectly by themselves or by the introduction of a third component). Additionally, or alternatively, the pieces can be fit together while at elevated or depressed temperature, and then allowed to return to normal temperature to clamp them together. Additionally, or alternatively, the optional flexible couplings can be mounted on any suitable shape of drive tube, e.g., round, square, pentagon-shaped, hexagonal, octagonal, or other shape drive tube, using any suitable attachment, such as but not limited to bolting, clamping, adhesive, or thermal methods of attachment. Alternatively, or additionally, these couplings can be made rigid (e.g., do not allow for angular misalignment). Alternatively, or additionally, the drive tube itself can be flexible enough to account for angular misalignment. 
     Additional exemplary flexible couplings are illustrated in  FIGS. 13A-13C . For example,  FIG. 13A  schematically illustrates a perspective view of flexible coupling  1301  including sleeve  1310  that can include first end  1311 , second end  1312 , and lumen  1313  connecting the first and second ends. Lumen  1313  at first end  1311  receives a portion of a first discrete section (segment)  1315  of the drive tube, and lumen  1313  at second end  1312  receives a portion of a second discrete section (segment)  1314  of the drive tube. In one non-limiting example, a coupling between sections of a square drive tube can have an appearance such as shown in  FIG. 13A . In one non-limiting example, the coupling is secured with bolts or any other suitable fastener. Exemplary components of one non-limiting example of a drive tube and coupling, e.g., for use in an arc-drive configuration, are illustrated in  FIG. 13A . It should be understood that any suitable combination of such components can be included. For example, one or more components can be modified. In another example, one or more components can be removed. In yet another example, one or more components can be added. 
       FIGS. 13B and 13C  schematically illustrate a perspective view of flexible coupling  1301 ′ including sleeve  1310 ′ that can include first end  1311 ′, second end  1312 ′, and lumen  1313 ′ connecting the first and second ends. Lumen  1313 ′ at first end  1311 ′ receives a portion of a first discrete section (segment)  1315 ′ of the drive tube, and lumen  1313 ′ at second end  1312 ′ receives a portion of a second discrete section (segment)  1314 ′ of the drive tube. Flexible coupling  1301 ′ optionally can include first and second flanges  1316 ,  1317  that suitably can be secured to one another, e.g., using fasteners such as described herein with reference to  FIGS. 12A-12F . 
       FIGS. 14A-14X  schematically illustrate additional options for flexible couplings. For example,  FIG. 14A  schematically illustrates an embodiment with an expanding bushing  1401 , in which tightening bolt  1402  draws conical shapes  1403  together, filling in and aligning holes in tubes (bushing  1401 ). One or both of conical shapes  1403  can be coupled to a spring.  FIG. 14B  schematically illustrates an embodiment in which segments  1404  of torque tube or drive tube generally follow segments  1405  of elongated concrete ballast, addressing the challenge of torque tube alignment.  FIG. 14C  schematically illustrates an exemplary universal (flexible coupling) between torque tube or drive tube segments  1406 ,  1407  using pin  1408 .  FIG. 14D  schematically illustrates angular and linear offset that can occur between segments of a torque tube or drive tube or between segments of an elongated concrete ballast.  FIG. 14E  schematically illustrates an exemplary bridge type embodiment in which certain segments  1404  of torque tube or drive tube are used to bridge misalignments between other segments  1404  and/or between segments of concrete  1505 .  FIG. 14F  schematically illustrates an exemplary embodiment of a flexible coupling in which a smaller section  1410  of a torque tube or drive tube is fitted into a larger section  1411  of a torque tube or drive tube; optionally, such an embodiment can include interlocking features  1412  that engage with one another so as to inhibit relative rotation of sections  1410 ,  1411 .  FIG. 14G  schematically illustrates an exemplary bellows type embodiment in which segments  1413 ,  1414  of a torque tube or drive tube are flexibly coupled to one another using expandable and flexible (bellows-like) segment  1415 .  FIG. 14H  schematically illustrates an exemplary interlocking embodiment in which the end of a first segment  1416  of a torque tube or drive tube is shaped so as to engage with and interlock with the end of a second segment  1417  of a torque tube or drive tube. 
       FIG. 14I  schematically illustrates another exemplary flexible coupling (universal) that includes tube grounding through the universal, e.g., a sheetmetal piece to link tube-coupler-tube together. The coupling illustrated in  FIG. 14I  includes torque transmitting tubes  1422 ,  1423 ; sheetmetal  1420 , which can be welded into place (e.g., spot welded) for easy assembly and connection to coupler  1424 ; bolts  1421 , which respectively can ground sheetmetal  1420  to tubes  1422 ,  1423 , e.g., since the tube(s) have welded threads; and coupler  1424 . 
       FIG. 14J  schematically illustrates another exemplary flexible coupling (universal) that can take up rotational tolerance, e.g., using an elastomer. The flexible coupling illustrated in  FIG. 14J  can include elastomer  1418  with steel center  1419 . The steel center  1419  can provide strength, and elastomer  1418  can fill in the hole completely, with the elastic (elastomeric) component compensating for hole size variation. 
       FIGS. 14K-14N  illustrate an exemplary embodiment of a flexible coupling (universal) with expanding or crush features, such as including thin walled nut tube crush features. The universal can include an expanding bushing to take up rotational tolerance and a welded in threaded part for reduced tolerance and easier assembly. For example, the coupling illustrated in  FIGS. 14K-14N and 14S  can include expanding sleeve  1429  that takes up rotational tolerances and expands or crushes to fill in excess space from tolerances; thick-walled coupler that can handle pin stresses, can be short, so as to save on material cost, and that can receive the ends of thin walled long tubes  1425 ; thin-walled long tubes  1425  that can save on material costs and can be good at transmitting torque; internally threaded welded shaft  1428  that can be welded in place to 1) reduce tolerance stack and 2) transmit torque to thin walled tubes  1425 ; and crush cone  1427 , optionally which can include plastic. Alternatively, normal bolt  1430  illustrated in  FIG. 14N  can be used in place of crush cone  1427 . 
       FIGS. 14O-14R and 14T-14X  illustrate still further exemplary embodiments of flexible couplings or components thereof. In the example shown in  FIG. 14O , segments of a torque tube or drive tube are articulably coupled together so as to follow angular variations in an elongated concrete ballast that includes control joints. In the example shown in  FIG. 14P , segments of a torque tube or drive tube are articulably coupled together with a larger sleeve, e.g., using fasteners. In the example shown in  FIG. 14Q , segments of a torque tube or drive tube are articulably coupled together with sleeves, e.g., using fasteners.  FIG. 14R  illustrates an exemplary coupling that can include cut slots to allow for a little flex.  FIG. 14T  illustrates segments of a torque tube or drive tube articulably coupled together with an exemplary sleeve that is stronger and without cuts.  FIG. 14U  schematically illustrates an additional structural member that can be added inside of a torque tube or drive tube so as to facilitate fastening a flexible coupling thereto.  FIG. 14V  schematically illustrates an example of a flexible coupling in which components of the coupling are disposed inside of the segments of the torque tube or drive tube that are being coupled together.  FIG. 14W  illustrates another coupling using a shaped thin pipe overlay that can be used to couple segments of a torque tube or drive tube to other another in a manner such as illustrated in  FIG. 14X . 
     As another option, each flexible coupling comprises a fastener comprising a pin slidably disposed through an aperture of a first discrete section of the drive tube or torque tube and through a slotted aperture of a second discrete section of the drive tube or torque tube. For example,  FIG. 33  schematically illustrates a perspective view of another exemplary flexible coupling  3300  that includes drive tube or torque tube  3310 , coupling segment  3320  having reduced cross-sectional area relative to tube  3310  (e.g., that slidably fits within tube  3310 ), and fastener  3330 , such as a shaft and cotter pin. As another example,  FIGS. 34A-34B  schematically illustrate assembled and exploded perspective views of another exemplary flexible coupling  3400  that is similar to coupling  3300  and that includes fastener  3430  coupling drive tube or torque tube segment  3410  to drive tube or torque tube segment  3420 . Non-limiting embodiments such as illustrated in  FIGS. 33 and 34A-34B  provide for universal joints in which sections of the drive tube or torque tube need not necessarily be angularly aligned relative to one another, and that can compensate for thermal expansion/contraction. For example, as shown in  FIG. 34B , fastener  3430  includes large diameter pin  3431  that can fit relatively snugly within aperture(s) of drive tube or torque tube segment  3420  and relatively loosely within slotted aperture(s)  3435  of drive tube or torque tube segment  3410 ; one or more (e.g., two) securement pins  3432 ; one or more (e.g., two) collar(s)  3433 , optionally which can have a generally diamond shape; one or more (e.g., two) bearing insert(s)  3434  that can fit relatively snugly within apertures  3435  of drive tube or torque tube segment  3410 ; and optional spacer(s)  3436 . In the example shown in  FIG. 34B , pin  3431  fits relatively snugly within aperture(s) of collar(s)  3433 , optional spacer(s)  3436 , and drive tube or torque tube segment  3420 . Collar(s)  3433  slidably fit within bearing insert(s)  3434  in such a manner that when fastener  3430  is assembled and secured using pins  3432 , collar(s)  3433  move laterally and/or angularly in conjunction with lateral and/or angular movement of drive tube or torque tube segment  3420 , and slidably move within bearing(s)  3434  so as to permit such lateral and/or angular movement of drive tube or torque tube segment  3420  relative to drive tube or torque tube segment  3410 , e.g., responsive to thermal expansion/contraction, seismic activity, settling, or other causes of lateral and/or angular variations along the drive tube (such as irregular geological topology). Optionally, pin  3431  is hollow and/or is of such a diameter as to sufficiently distribute stresses of fastener  3430  among other components of the fastener. Collar(s)  3433  can include any suitable material(s), such as bronze. Bearings(s)  3434  can include any suitable material(s) that optionally do not corrode responsive to contact with collar(s)  3433 , such as stainless steel or bronze. Optional spacers can include a material selected to inhibit corrosion that otherwise may arise responsive to contact between collar(s)  3433  with drive tube segment  3420 , such as stainless steel. The optional generally diamond shape of collar(s)  3433  can increase contact area with bearing(s)  343  and thus reduce or distribute stress within fastener  3430 . 
     Wind fence options. As noted further above, such as with reference to  FIGS. 1D-1E , a wind fence optionally can be used, e.g., so as to reduce wind loads on photovoltaic modules. For example, in some embodiments, a wind fence at one or more respectively suitable location(s), e.g., at one or more of the edges of the present single-axis tracker (system), can reduce wind loads on the tracker. Such wind fence(s) can have any suitable configuration, e.g., can be ballasted or post-driven, and/or can be articulated or fixed, and/or can be solid or perforated. The optional wind fence(s) can be approximately the height of the pivot (or slightly lower), the height of a fully tilted panel, or some height between, or any other suitable height. This wind fence(s) can be on all sides of the tracker, only on the east-west sides, or some hybrid, including being on the east-west sides, and part of the north-south sides. 
     For example, a system for rotating photovoltaic modules arranged in a plurality of rows can include a plurality of drive tubes extending along and parallel to the rows; drive mechanisms coupled to the photovoltaic modules; an actuator configured to rotate the photovoltaic modules via the drive tubes and drive mechanisms; and a wind fence disposed parallel to and adjacent to at least one of the rows. Optionally, the wind fence includes a first portion, a second portion, and a joint disposed between the first and second portions, the first portion being substantially vertical, the second portion being articulable via rotation of the joint between a vertical position and a folded position. Optionally, articulation of the second portion to the folded position reduces shading of at least one of the rows. Additionally, or alternatively, the wind fence can include panels that include mesh, fabric, or solid material. 
     For example,  FIGS. 15A-15G  schematically illustrate exemplary wind fences that can be used in a system for rotating photovoltaic modules, according to some embodiments.  FIG. 15A  schematically illustrates a perspective view of exemplary wind fence  1500  including panels  1501  supported by vertical support members  1502 , e.g., posts. Panels  1501  can include a solid material, or can be perforated, e.g., can include a mesh or fabric.  FIG. 15B  schematically illustrates a perspective view of exemplary wind fence  1510  that can be configured similarly as wind fence  1500 , and in which vertical support members  1512  supporting panels  1511  can be coupled to ballast  1513 , e.g., an elongated concrete ballast.  FIG. 15C  schematically illustrates a perspective view of exemplary wind fence  1520  that can be configured similarly as wind fence  1510 , and that is approximately half the height of a photovoltaic module.  FIGS. 15D-15F  schematically illustrate an exemplary wind fence  1530  that includes substantially vertical first portion  1532 , second portion  1531 , and joint  1533  disposed therebetween. Second portion  1531  is articulable via rotation of joint  1533  between a vertical position (e.g., such as illustrated in  FIG. 15D ) and a folded position (e.g., such as illustrated in  FIG. 15F ).  FIG. 15G  schematically illustrates a side view of a non-limiting example of a wind fence configuration including one wind fence  1550  on the east side of a present system  1560 , and one wind fence  1540  on the west side of system  1560 . Fences  1540 ,  1550  independently can have any suitable configuration, e.g., each can be configured similarly as any of fences  1500 ,  1510 ,  1520 , or  1530  provided herein. 
     Additionally, or alternatively, embodiments of the present single-axis trackers (systems) are compatible with the SPOT autonomous cleaning vehicle. For example, a system such as provided elsewhere herein further can include an elongated concrete ballast extending along and parallel to the row and upon which the photovoltaic modules are disposed, the elongated concrete ballast comprising first and second vehicle support surfaces; and a maintenance robot that can include first and second wheels respectively contacting the first and second vehicle support surfaces and configured to maintain the system. Illustratively, the elongated concrete ballast optionally can be split into first and second discrete tracks each parallel to the row, the first track including the first vehicle support surface, the second track including the second vehicle support surface. The maintenance robot optionally can include a body coupled to the first and second wheels and disposed between the first and second discrete tracks. 
     For example,  FIGS. 16A-16E  schematically illustrate exemplary vehicles that can be used with a system for rotating photovoltaic modules, according to some embodiments. For example,  FIG. 16A  schematically illustrates an embodiment of an exemplary SPOT vehicle  1600  for tracker and fixed tilt systems, that optionally includes water filtering. In some embodiments, the optional SPOT vehicle  1600  can include multiple tubes for liquids such as cleaning fluids (such as water) or other fluids (such as material application), including one or more tubes  1601  for cleaning fluid storage and one or tubes  1602  for treatment of such fluids (such as filtering, e.g., water filtration, or chemical additives). As shown in  FIG. 16A , SPOT vehicle  1600  can include a plurality of wheels, each configured to engage a vehicle support surface, such as can be provided by an elongated concrete ballast upon which the photovoltaic modules of a row or table can be disposed. For example,  FIGS. 16B-16D  schematically illustrates SPOT vehicle being used together with different types of systems such as provided elsewhere herein. 
     SPOT between ballasts options. In some embodiments, an optional vehicle, e.g., an autonomous maintenance vehicle such as SPOT, can be configured to travel between the uprights of the present single-axis tracker. The maintenance vehicle can, for example, trim vegetation and/or perform other operation and maintenance (O&amp;M) tasks. For example,  FIG. 16E  schematically illustrates an arrangement including elongated concrete ballast  1620  extending along and parallel to the row or table  1630  and upon which photovoltaic modules are disposed, such as using A-shaped upright  1640 . Elongated concrete ballast  1620  can include first and second vehicle support surfaces  1621 ,  1622 . Maintenance robot (e.g., SPOT)  1610  includes at least first and second wheels  1611 ,  1612  respectively contacting first and second vehicle support surfaces  1621 ,  1622  and configured to maintain the system. In the non-limiting embodiment illustrated in  FIG. 16E , elongated concrete ballast  1620  optionally can be split into first and second discrete tracks each parallel to the row or table  1630 , the first track including first vehicle support surface  1621 , the second track including second vehicle support surface  1622 . Maintenance robot  1610  optionally can include body  1613  coupled to first and second wheels  1611 ,  1612  and disposed between the first and second discrete tracks. 
       FIGS. 17, 18A-18F, 19A-19E, 20A-20C, 21A-21C, 22A-22J, 23, and 24A-24E  schematically illustrate optional arrangements of components in a system for rotating photovoltaic modules in a row, according to some embodiments. 
     Sheet of material between uprights options. In some embodiments, material optionally can be included between the uprights, such as shown in the non-limiting example in  FIG. 17 , so as to inhibit or prevent vegetation growth. The material can, in some embodiments, include thin sheets of concrete or thin sheets of metal or other suitable material. For example,  FIG. 17  schematically illustrates an arrangement including elongated concrete ballast  1720  extending along and parallel to the row or table  1730  and upon which photovoltaic modules are disposed, such as using A-shaped upright  1740 . Elongated concrete ballast  1720  optionally can be split into first and second discrete tracks  1721 ,  1722  each parallel to the row or table  1730 . Sheet of material  1750  can be disposed between, and optionally coupled to, tracks  1721 ,  1722  in such a manner as to inhibit growth of vegetation between tracks  1721 ,  1722 . 
       FIGS. 18A-18C  schematically illustrate an optional arrangement of components in a system  1800  for rotating photovoltaic modules  1810  in a row, according to one non-limiting embodiment. System  1800  includes elongated concrete ballast  1820 , e.g., a tall concrete curb with a generally triangular profile; photovoltaic modules  1810  (solar panels), which can be in landscape or portrait orientation; control joints  1830  including a pivot hinge provided by arcuate members  1831  to which photovoltaic modules  1810  suitably are coupled, elongated structural member  1832  coupled to an actuator (not specifically illustrated) and to arcuate members, and base  1833 . The actuator can rotate member  1832 , which causes arcuate members to rotate, which causes photovoltaic modules  1810  to rotate. Optionally, the elongated concrete ballast  1820  (e.g., tall concrete curb) can act as a wind dam to lower the wind forces on panel/module  1810 . The panel pivot point (provided by elongated structural member  1832 ) can integrate a hinge for the pivoting, including the pivot being a part of the concrete or the pivot being attached to the concrete. 
       FIGS. 18D-18F  schematically illustrate additional optional arrangements of components in a system for rotating photovoltaic modules in a row, according to some non-limiting embodiments. In the optional arrangement illustrated in  FIG. 18D , system  1801  includes concrete curb (rail or ballast)  1821  to which photovoltaic modules  1811  can be coupled via pivoting gearbox mechanism  1851  or other suitable mechanism such as provided elsewhere herein. Elongated structural member  1841 , e.g., a cable, drive tube, or torque tube, is coupled to an actuator (not specifically illustrated) and rotates photovoltaic modules  1811  via pivoting gearbox mechanism  1851  or other suitable mechanism responsive to actuation of the actuator. In the optional arrangement illustrated in  FIG. 18E , system  1802  includes concrete curb (rail or ballast)  1822  to which photovoltaic modules  1812  can be coupled via a pivoting gearbox mechanism or other suitable mechanism (not specifically illustrated). Elongated structural member  1842 , e.g., a cable, drive tube, or torque tube, is coupled to an actuator (not specifically illustrated) and rotates photovoltaic modules  1812  via the mechanism responsive to actuation of the actuator. In the optional arrangement illustrated in  FIG. 18F , system  1803  includes concrete curb (rail or ballast)  1823  to which photovoltaic modules  1813  can be coupled via pivoting gearbox mechanism  1853  or other suitable mechanism such as provided elsewhere herein. Elongated structural member  1843 , e.g., a cable, drive tube, or torque tube, is coupled to an actuator (not specifically illustrated) and rotates photovoltaic modules  1813  via pivoting gearbox mechanism  1853  or other suitable mechanism responsive to actuation of the actuator. In embodiments such as illustrated in  FIGS. 18D-18F , as well as other embodiments provided herein, the concrete curb, rail, or ballast can extend in the same direction of pivoting or in the orthogonal direction. In addition, the elongated structural member, e.g., cabling, drive tube, or torque tube, can run along the concrete rail or orthogonal to it. The solar panels (modules) can be attached together using a common structural element, such as joining structural element  1860  illustrated in  FIG. 18E . 
     Other concrete shapes may exist, including one with driving surfaces for vehicles (such as an installation, cleaning, or maintenance vehicle, such as SPOT; such vehicle may be automated). Additionally, or alternatively, the cable or other elongated structural member may include, or be constructed of, two lines, and may have a link that connects the two lines and acts in the same way the ball acts. This link may operate the mechanism that tilts the solar panel. 
     For example,  FIGS. 19A-19E  schematically illustrate additional optional arrangements of components in a system for rotating photovoltaic modules in a row, according to various non-limiting embodiments. For example,  FIG. 19A  schematically illustrates two lines  1900  that make up an elongated structural member, e.g., cable, optionally including a link or actuating link  1901  coupling the two lines together.  FIG. 19B  schematically illustrates wheels  1902  of a vehicle, such as SPOT, disposed on vehicle support surfaces  1903  of an elongated concrete ballast  1904 .  FIG. 19C  schematically illustrates smaller wheels  1905  of a vehicle, such as SPOT, disposed on vehicle support surfaces  1906  of an elongated concrete ballast  1907 .  FIGS. 19D-19E  schematically illustrate use of elongated concrete ballast  1908  as a wind dam for photovoltaic module  1909  coupled thereto via hinge  1910  providing a line between the height of ballast  1908  and tilt. Optionally, the embodiment illustrated in  FIGS. 19D-19E  can be manually installed, e.g., without the use of a robot. One or more vehicle support surfaces can be included, e.g., for SPOT. Grass can be cut around ballast  1908 . 
       FIGS. 20A-20C  schematically illustrate additional optional arrangements of components in a system for rotating photovoltaic modules in a row, according to some non-limiting embodiments. In the example illustrated in  FIG. 20A , a damper can be added to rows  2000  of modules  2001  to deal with dynamics, e.g., wind dynamics. Additionally, or alternatively, wiring and/or structural connection  2002  can be provided between and connecting rows  2000 . In the example illustrated in  FIG. 20B , elongated concrete ballast  2003  can include a space between vehicle support surfaces  2008  for receiving wiring  2004 , e.g., house power wiring, and/or elongated steel member  2005 . Torque tube(s)  2006  can be supported by and rotated by actuator(s)  2007 , e.g., worm drive(s). In the example illustrated in  FIG. 20C , more than one actuator  2009  can be provided per row  2010  of photovoltaic modules. 
       FIGS. 21A-21C  schematically illustrate additional optional arrangements of components in a system for rotating photovoltaic modules in a row, according to some non-limiting embodiments. The exemplary arrangement in  FIG. 21A  includes groups of panels that are attached at two points, e.g., at two different panels of the group, to a pipe (e.g., torque tube or drive tube). Such an arrangement optionally provides straight pipes for panels and readily can be taken apart (e.g., two panels can be detached to disassemble one table or panel from the pipe). The exemplary arrangement in  FIG. 21B  includes groups of panels that are attached at two points, e.g., at two different points on a single panel of the group, to a pipe (e.g., torque tube or drive tube). The exemplary arrangement in  FIG. 21C  includes panels attached to a support in such a manner that the panels can be rotated and the arrangement can handle changes in temperature, e.g., can deal with thermal expansion/contraction. 
       FIGS. 22A-22J  schematically illustrate additional optional arrangements of components in a system for rotating photovoltaic modules in a row, according to some non-limiting embodiments. For example, the embodiment illustrated in  FIG. 22C  accommodates for torsion twist and dead weight. 
     In still other embodiments, the panel may tilt about a pivot, which may be offset. The tilting mechanism may include, or be made of, a ratcheting mechanism or other mechanism that pushes the panel or supporting structure upwards or downwards. Gravity may help push the panel up or pull the panel down. Some or all of the tilting mechanism and/or panel support structure may fold underneath the panel. When folded, some or all of the tilting and/or supporting mechanism may fit within the frame of a solar panel or within the space provided by packaging for the solar panel or a device attachment to the underside of the solar panel. For example,  FIG. 23  schematically illustrates another optional arrangement of components in a system for rotating photovoltaic modules in a row, according to one non-limiting embodiment. In  FIG. 23 , device  2320  can be attached to the underside of solar panel (photovoltaic module)  2310 , e.g., attached to or adjacent to hat channel  2360  and/or hinge  2361 . Supporting structure  2330  may be fixed or moving, and optionally can include ratcheting mechanism  2340 . 
     Exemplary embodiments of stiffener attachments (panel supports) are schematically illustrated in  FIGS. 24A-24E . In these examples, a stiffener attachment (panel support) ships with panel  2410 , folds out, and/or is all plastic. The stiffener attachment can include stiffener  2420  that attaches to panel  2410  and fold-out support  2430  that attaches to a tube via aperture  2470 , such as a drive tube or torque tube such as described elsewhere herein, as well as to stiffener  2420 . Optionally, stiffener  2420  is symmetric. In some embodiments, fold-out support  2430  is rotatably attached to stiffener  2420  via a hinge, and optionally could lock/snap at location  2460  (or is the hinge itself). In the example shown in  FIG. 24E , extender  2440  can act as a stop, stack, and backsheet protection, and can be provided as a portion of fold-out support  2430 . One or more bumper(s) or felt  2450  can be coupled to a distal end of fold-out support  2430 . 
       FIG. 25  schematically illustrates a side view of another exemplary arrangement that includes a stiffener attachment (panel support). In this example, the stiffener attachment attaches to a tube, such as a drive tube or torque tube such as described elsewhere herein, via a flag, and/or is all plastic. Illustratively, torque transmitting tube (torque tube)  2520 , optionally which can be coupled to one or more additional segments of torque transmitting tube via couplers  2570  (such as flexible couplers described elsewhere herein) can be coupled to panel  2510  via flag  2530  (e.g., part of the torque transmitting tube) and via panel support  2540  (which attaches to the flag by a rivet, bolt, or other suitable fastener). Tube supports  2550  can be coupled to concrete  2560 , e.g., to an elongated concrete ballast such as provided elsewhere herein. 
       FIGS. 26A-26S  schematically illustrate exemplary components and arrangements that optionally can be included in the arrangement of  FIG. 25 . For example,  FIG. 26A  schematically illustrates a perspective view of an exemplary flag that can be used in the arrangement of  FIG. 25  and includes aperture  2580  for receiving a tube and flange  2583 ′ for securably engaging the tube.  FIG. 26B  illustrates front, side, and perspective views of an exemplary tube support that can be used in the arrangement of  FIG. 25 .  FIG. 26C  schematically illustrates a side view of an alternative arrangement of features that can be used instead of, or in combination with, the arrangement of  FIG. 25 .  FIG. 26D  schematically illustrates a side view of an exemplary coupling between a flag  2530 ′ and panel support  2540 ′ using rivet  2541  or other suitable fastener.  FIG. 26E  schematically illustrates a perspective view of an exemplary coupling between a flag  2530 ″ and tube  2520 ′ via aperture  2580 ′.  FIG. 26F  schematically illustrates a front view of an exemplary flag  2531  including key  2501  which is movable downwards aperture  2581 , e.g., so as to securably engage a tube disposed therethrough.  FIG. 26G  schematically illustrates a front view of an exemplary flag  2531 ′ including spines  2501 ′ which are arranged about aperture  2581 ′, e.g., so as to securably engage a tube disposed therethrough.  FIG. 26H  schematically illustrates a side view another exemplary arrangement of features that can be used instead of, or in combination with, the arrangement of  FIG. 25 .  FIG. 26I  schematically illustrates a perspective view of an exemplary flag  2531 ″ that includes rivet  2581 ′ and optionally includes two opposite flags through which tube  2521  passes.  FIG. 26J  schematically illustrates a perspective view of an exemplary flag  2532  that includes flange (pinned sleeve)  2503  and first and second fasteners  2502  (such as self-driving screws) for securably engaging tube  2522  passing through flag  2532 .  FIG. 26K  schematically illustrates a perspective view of an alternative flange  2503 ′ that can be coupled to a flag for securably engaging a tube passing therethrough.  FIGS. 26L-26M  schematically illustrate perspective and side views of an alternative flag  2532 ″ including alternative flange  2503 ″ for securably engaging tube  2522 ″ passing through the flag.  FIG. 26N  schematically illustrates a side view of another exemplary flag  2533  having tube  2523  passing therethrough.  FIG. 26O  schematically illustrates a side view of another alternative arrangement of features that can be used instead of, or in combination with, the arrangement of  FIG. 25 .  FIG. 26P  schematically illustrates a perspective view of use of a Band-It sleeve  2504  for securably engaging pipe  2523 ′ through flag  2533 .  FIGS. 26Q-26S  illustrate still further exemplary embodiments of exemplary flags that can be used with the arrangement of  FIG. 25 . 
     Under yet another aspect, the systems and methods provided herein optionally can include wet-setting one or more components, such as one or more uprights to which the present rotation systems can be coupled, into concrete, such as into an elongated concrete ballast described elsewhere herein. For example, a method for mounting photovoltaic modules can include casting or slip-forming an elongated concrete ballast; wet-setting uprights into the elongated concrete ballast; curing the elongated concrete ballast with the uprights therein; and supporting, with the uprights, a drive tube extending along and parallel to the elongated concrete ballast, and drive mechanisms coupled to the photovoltaic modules, the photovoltaic modules being rotatable via the drive tubes and drive mechanisms. Optionally, the uprights can be A-shaped in a manner such as described elsewhere herein, or can include a bridge and a post, the bridge contacting first and second surfaces of the elongated concrete ballast, the post extending vertically from the bridge and supporting the drive tubes and drive mechanisms, in a manner such as described elsewhere herein. Optionally, the uprights each can include first and second feet that each are wet-set into the elongated concrete ballast. Wet-setting the uprights can include vibrating the uprights. Systems produced by such a method are provided. 
     For example, a wet set ballast can include a support that into concrete and/or can be all plastic. Optionally, a plastic tube support has a feature that engages with concrete (such as/perhaps by aggregate features, or chemistry that promotes adhesion with concrete, and/or is placed in uncured concrete (such as/perhaps by the use of a vibrating tool/wand); when the concrete cures, the tube support is structurally locked/mated to the concrete ballast. 
     Further wet-setting uprights options. In one non-limiting embodiment, the bases of the uprights or other system components (such as bolts or other components) optionally can be placed into the wet concrete of the foundation at the time of installation. For example, a bridge-based upright such as illustrated in  FIGS. 8A-8C  can be wet-set into an elongated concrete ballast. In another example, an A-frame shaped upright such as illustrated in  FIGS. 8D-8E  can be wet-set into an elongated concrete ballast. 
     Illustratively,  FIGS. 27A-27E  schematically illustrate views of exemplary structures formed during steps of a method for wet-setting uprights in a system for rotating photovoltaic modules, in some embodiments.  FIG. 27A  schematically illustrates a front cross-sectional view of an exemplary upright  2701  that includes tube aperture  2721  (e.g., an aperture for receiving a torque tube or drive tube such as described elsewhere herein) and that is wet-set into concrete  2711  (e.g., an elongated concrete ballast such as described elsewhere herein). Upright  2701  can include first and second feet  2701 ′,  2701 ″. Optionally an aggregate can be simulated so as to strengthen attachment between one or both of feet  2701 ,  2701 ″, e.g., by a chemical bond (such as promotes adhesion between foot  2701 ″ and concrete  2711 ) or mechanical interlock.  FIGS. 27B-27D  schematically illustrate front cross-sectional views of steps in an exemplary method for wet-setting non-limiting upright  2702  into concrete (e.g., elongated concrete ballast)  2712 . Concrete  2712  could also be asphalt. In the illustrated example, upright  2702  includes first and second features to engage with concrete, such as feet  2702 ′ that optionally include one or more apertures  2702 ″ therethrough; first and second features to define an amount of penetration, such as flats  2703 ; and tube aperture  2722 . Concrete  2712  can be wet, and can be formed using slip-forming, cast-in-place, or pre-forming. The first and second features to engage with the concrete, such as feet  2702 ′, can be positioned over the concrete  2712  such as illustrated in  FIG. 27B . A vibrating tool/wand  2730  can be brought into contact with one or both of features, e.g., feet  2702 , such as illustrated in  FIG. 27C . Pressure can be applied vertically to upright  2702  while actuating tool/wand  2730 , such as by bringing a vise down, so as to set the features, e.g., feet  2702 , into the concrete  2712  such as illustrated in  FIG. 27D . The first and second features to define an amount of penetration, such as flats  2703  can define the amount of penetration of feet  2702  into the concrete and can assist in holding upright  2702  vertically while the concrete cures.  FIG. 27E  illustrates a perspective view of one exemplary structure resulting from such a method. 
     An exemplary assembly process for an assembly of components such as illustrated in  FIG. 25  can include one or more of the following steps, and optionally can include each of the following steps, in any suitable sequence (e.g., in the numerically listed sequence): (1) drop onto spreader jig, e.g., temp spreader jig and support  2580 ; (2) thread tube  2520  and coupler (e.g., flag  2530 ) through tube supports  2540  and panel supports  2550 ; (3) attach tube  2550  and coupler (e.g., flag  2530 ) to previous coupler, e.g., one or both of flexible couplings  2570 ; (4) attach panel  2510  to tube  2520 , e.g., via rivet, bolt, other suitable fastener; and/or (5) remove jig. Such a process optionally can be automated, can accommodate for thermal expansion/contraction, and/or can provide suitable torque characteristics, such as for use in a photovoltaic module rotation system or method provided herein. 
     Other attachment methods options. In some embodiments, the uprights optionally can be attached to the foundation (e.g., concrete tracks, or elongated concrete ballast) by one or more of the following: adhesive/epoxy, powder-actuated fasteners, concrete anchors, or bolts. 
     Corrosion protection options. In some embodiments, the surfaces of the upright that are in contact with the foundation (e.g., concrete tracks, or elongated concrete ballast) can experience corrosion depending on the chemistry of the concrete or component material or other environmental considerations. So as to prevent or inhibit such corrosion, the bases of the uprights can include a different material that is less susceptible to corrosion and/or can be covered with a protective coating. 
       FIGS. 28A-28C  illustrate still further optional arrangements of photovoltaic modules that can be used with the present systems and methods for rotating photovoltaic modules in a row, according to some embodiments. For example,  FIG. 28A  schematically illustrates an upside-down view of a 2-up configuration 2-wide configuration  2888  that includes assembly  2800  including first and second solar panels (photovoltaic modules)  2810 ,  2811  arranged in the same plane as one another on opposing sides of tube  2830 , e.g., drive tube or torque tube such as described elsewhere herein; and stiffener  2820  coupled to each of panels  2810 ,  2811  and to tube  2830 . Optionally, stiffener  2820  includes a fold-out support that bridges across two panels, e.g., each of panels  2810 ,  2811 , and optionally uses adhesive or tape to attach thereto. Tube  2830  can include a drive tube or a torque-transmitting tube of sufficient length to be coupled to, and to rotate, any suitable number of assemblies  2800 , such as to be coupled to assembly  2800  and assembly  2800 ′ which is configured similarly as assembly  2800 , e.g., 1 tube  2830  for four solar panels.  FIG. 28B  illustrates right-side up view of the configuration of  FIG. 28A  on a concrete (or asphalt) ballast  2840 , in which tube  2830  is coupled to the ballast using any suitable number of tube supports (uprights), e.g., first and second tube supports  2840 ,  2840 ′, such as two tube supports for every 4 panels. Optionally, ballast  2840  can include one or more control joints such as described elsewhere herein and/or can have the same width as the collective width of the panels coupled thereto, e.g., can be two panels wide in the example illustrated in  FIG. 28B .  FIG. 28C  illustrates an embodiment in which first and second assemblies  2860 ,  2860 ′ each configured similarly as assembly  2888  (e.g., include solar panels  2810 ′, stiffeners  2820 ′, and one torque transmitting tube  2830 ′ for four panels) are coupled to one another via tube connection  2850 , which optionally can include a flexible coupling such as described elsewhere herein. 
     An exemplary way to manage the need for torque transmitting tube to both (e.g., to multiple sections of a torque tube or drive tube such as described elsewhere herein, e.g., such as illustrated in  FIGS. 28A-28C ) can include one or more of the following features:
         transmit torque;   flex along length to handle uneven terrain or differential settlement in terrain;   the tubes that the panels are attached to are constrained, since two supports attach to the ballast; settlement or uneven terrain can shift adjacent panel supporting tubes so that their axes are no longer aligned; to keep from overconstraining, two “universals” (components that transmit torque, but allow rotation in two directions) and a connecting tube can be used, and optionally, are required, between panel supporting tubes (similar to the drive shaft in a car/truck); and/or   does not have rotational slop, since the bolts clamp the tabs together.       

       FIGS. 29A-29B  schematically illustrate side and perspective views of an exemplary arrangement of elements that can be used in a torque tube or drive tube, according to some embodiments.  FIG. 29A  illustrates exemplary assembly  2900  including segments of tube (e.g., segments of torque tube or drive tube)  2910 , universals  2920 , and short tube  2930 . Tube segments  2910  can correspond to the tube that panels attach to, and can be constrained. Short tube  2930  can correspond to a connecting tube. Universals  2920 , e.g., first and second “universal” tubes, respectively can couple tube segments  2910  to short tube  2930  in such a manner as to transmit torque and/or to provide sufficient flex along the length of assembly  2900  as to accommodate uneven terrain and/or thermal expansion/contraction.  FIG. 29B  illustrates further detail of exemplary features of assembly  2900 . In the illustrated example, tubes  2910 , universals  2920 , and short tubes  2930  each include bolt tabs  2940  via which the tubes  2910  or  2930  respectively may be coupled to universals  2920  using suitable fasteners  2950 , such as nuts and bolts. Bolt tabs  2940  can be or include a flange that sticks off the end of the respective tube or universal. A bolt can go through to connect that is tightly clamped and does not allow rotational slop. The relatively thin thickness of the bolt tab allows twisting and/or bending in the other one/two directions, allowing it to act like one part of a “universal” joint, or the entire “universal” joint. Illustratively, bolt tabs  2940  can flex just enough to pivot slightly and to transmit torque and also has no slop. The bolt tabs  2940  at one end of a tube  2910  or universal  2930  optionally can be arranged in an opposite direction (e.g., orthogonally) to the bolt tabs at the other end of the tube or universal. As an optional alternative to the universal tube  2920 , the bolt tab  2940  connection may flex in two directions, and the bolt tab tube connection can create a universal by itself (e.g., instead of two bolt tab connections and a universal tube). 
     Accordingly, in some embodiments provided herein, exemplary components of the present tracker (photovoltaic module rotation system) include one or more of the following features: ballasted: non-penetrating; tube-based transmission; non-backdrivable worm drive gearbox; easy to install: manually and/or automated install; ship as a unit, or assembly pieces on-site (manual/automated); and/or robotically cleanable. Exemplary embodiments include any suitable combination of the following features: 
     High level (general arrangement);
         any suitable number of rows, any suitable number of panels per row, e.g., 20/40/60 panels per row, or more, e.g., 80 or more, 100 or more, or 120 or more;   connect actuators together across rows using speed bump; and/or   speed bump has wiring       

     Actuator;
         worm drive; and/or   connects to tube drive that goes in both directions       

     Tube drive;
         slides to accommodate thermal expansion/contraction;   universals to accommodate ballast settling;   grounding happens through the tube (only metal part); and/or   rotates relative to plastic ground support       

     Universal;
         expanding bushing to take up rotational tolerance; and/or   welded in threaded part for reduced tolerance and easier assembly       

     Fold-out stiffener attachment;
         ships with panel;   attaches to tube via flag; and/or   all plastic       

     Wet set ballast; and/or
         vibrate support into concrete; and/or   all plastic       

     Other
         all plastic, frameless modules (no grounding);   drive tube is grounded (metal); and/or   works with SPOT (goes to a specific tilt) (exemplary embodiments of SPOT cleaning robot described in U.S. Patent Publication No. 2015/0144156 to French, the entire contents of which are incorporated by reference herein). Other exemplary embodiments of SPOT are described elsewhere herein.       

     In one example, a system for rotating photovoltaic modules arranged in a row can include an elongated structural member extending along and parallel to the row; protrusions coupled to the elongated structural member; an actuator; and drive mechanisms coupled to the photovoltaic modules. Actuation of the actuator can move the elongated structural member, the movement of the elongated structural member can move the protrusions, the movement of the protrusions can move the drive mechanisms, and the movement of the drive mechanisms can rotate the photovoltaic modules. Exemplary embodiments of such a system are described herein, for example, with reference to  FIGS. 1A-1I, 2A-2B, 3A-3B, 4, 5, 6A-6C, 7A-7B, 8A-8J, 9A-9J, 10A-10I, 11A-11B, 12A - 12 F,  13 A- 13 C,  14 A- 14 X,  16 A- 16 E,  17 ,  18 A- 18 F,  19 A- 19 E,  20 A- 20 C,  21 A- 21 C,  22 A- 22 J,  23 ,  24 A- 24 E,  25 ,  26 A- 26 S,  27 A- 27 E,  28 A- 28 C,  29 A- 29 B,  30 A- 30 B,  31 ,  32 ,  33 ,  34 A- 34 B,  35 ,  36 A- 36 B,  37 , and  38 A- 38 C. 
     In another example, a system is provided for rotating photovoltaic modules arranged in a row. The system can include a drive tube extending along and parallel to the row. The drive tube can include a plurality of discrete sections coupled together with flexible couplings. The system also can include an actuator; and drive mechanisms coupled to the photovoltaic modules. Actuation of the actuator can rotate the discrete sections of the drive tube and the flexible couplings, the rotation of the discrete sections of the drive tube and the flexible couplings can rotate the drive mechanisms, and the rotation of the drive mechanisms can rotate the photovoltaic modules. Exemplary embodiments of such a system are described herein, for example, with reference to  FIGS. 11A-11B, 12A-12F, 13A-13C, 14A-14X, 29A-29B, 33, 34A-34B, and 35 . 
     In another example, a system for rotating photovoltaic modules arranged in a row can include a torque tube extending along and parallel to the row. The torque tube can include a plurality of discrete sections coupled together with flexible couplings, the plurality of discrete sections being coupled to the photovoltaic modules. The system also can include an actuator. Actuation of the actuator can rotate the discrete sections of the torque tube and the flexible couplings, and the rotation of the discrete sections of the torque tube and the flexible couplings can rotate the photovoltaic modules. Exemplary embodiments of such a system are described herein, for example, with reference to  FIGS. 11A-11B, 12A-12F, 13A-13C, 14A-14X, 29A-29B, 33, 34A-34B, and 35 . 
     Under yet another aspect, a system is provided for rotating photovoltaic modules arranged in a plurality of rows. The system can include a plurality of drive tubes extending along and parallel to the rows; drive mechanisms coupled to the photovoltaic modules; an actuator configured to rotate the photovoltaic modules via the drive tubes and drive mechanisms; and a wind fence disposed parallel to and adjacent to at least one of the rows. Exemplary embodiments of such a system are described herein, for example, with reference to  FIGS. 1D, 1E, and 15A-15G . 
     Under still another aspect, a method for mounting photovoltaic modules is provided. The method can include casting or slip-forming an elongated concrete ballast; wet-setting uprights into the elongated concrete ballast; curing the elongated concrete ballast with the uprights therein; and supporting, with the uprights, a drive tube extending along and parallel to the elongated concrete ballast, and drive mechanisms coupled to the photovoltaic modules. The photovoltaic modules can be rotatable via the drive tubes and drive mechanisms. Exemplary embodiments of such a system are described herein, for example, with reference to  FIGS. 25 and 27A-27E . 
     While various illustrative embodiments of the invention are described herein, it will be apparent to one skilled in the art that various changes and modifications may be made therein without departing from the invention. For example, the present systems and methods are not limited to use with photovoltaic modules, and instead can be applied to rotating any type of solar module (e.g., a module such as used with a concentrated solar power system, such as a parabolic trough or heliostat), or to rotating any other type of surface. The appended claims are intended to cover all such changes and modifications that fall within the true spirit and scope of the invention.