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One needs a tall pole because the low corner of the array tilts and turns far below the gimbal.
The seasonal adjustment angle A and the daily rotation angle B along with the length and width of the array determine how low the corner goes.
The seasonal tilt lowers the centerline ½ the Length times the Sin of angle A below the pole top.Seasonal Tilt = L/2 SIN A The Daily Rotation
The maximum allowable rotation of the Tracker from center is 45º. The daily rotation turns the corner of the array lower yet by ½ the Width times the Cosine of angle A times the Sine of angle B where the angle B is the angle the Tracker has turned about the axle from noon (usually 45º).Daily Rotation = W/2 (COS A) SIN B
A F-Series-168 Track Rack™ with a maximum rack dimension of L= 192” (North, South dimension) and W= 169”
(East, West dimension) is set permanently with a seasonal tilt of 30 degrees.
How tall should the pole be if the corner can reach to within 12” of the ground?
The seasonal tilt of 30º turns the array down:
= (96) x .5
A daily rotation of 45º turns the array corner down:
= 84.5 (.6124)
For the corner to always be held at least 12” above the ground, the pole would be 111-3/4” long.
= 48” + 51-3/4” + 12”
The same Track Rack™ is tilted 45º during the winter. How long a pole would it need then?
(96”) Sin 45º = 67.88”
Daily rotation –
(84.5”) Cos 45º Sin 45º = 42.25
Grand Total = 110.13”
Add 12” for the ground clearance and the pole height would need to be 110.13 + 12” = 122.13” tall.
IMPORTANT NOTE: ZOMEWORKS CORP. ASSUMES NO LIABILITY FOR THE STRUCTURAL INTEGRITY OF THE POLE AND ITS INSTALLATION. SOIL AND WIND CONDITIONS VARY. IF THERE IS ANY DOUBT, CONSULT WITH A LOCAL STRUCTURAL ENGINEER.
For installations in sandy or muddy areas, for tall mounting poles, or for any mounting different from the situations described in these instructions, you will need to consult a local structural engineer. Large TRACK RACKSÔ can receive significant wind loads, so a strong mounting pole and foundation is very important also be aware of turbulence near buildings or sudden changes in terrain. The site should receive the maximum possible sunlight from AM to PM, in the winter and summer. Avoid shade from buildings and trees, including shade that may occur in other seasons. The height of the pole should result in adequate ground clearance for the mounted modules.
CAUTION – BE CAREFUL WORKING AROUND THE RACK AFTER INSTALLATION ON THE POLE. SOME OF THE STRUCTURAL MEMBERS MAY BE AT HEAD LEVEL.
- Customer provided poles are recommended to be a 2-1/2” (nominal) schedule 40 steel pipe with a 2-7/8” OD (UTR-020 only). 3” (nominal) schedule 40 steel pipe with a 3-1/2” OD (UTRK-040 only). 6” (nominal) schedule 40 steel pipe with a 6-5/8” OD (UTRF-64, UTRF-90 & UTRF-120) or a 8” (nominal) schedule 40 steel pipe with a 8-5/8” OD (UTRF-168 only).
- Note: Heavier schedule steel pipe schedule 80, schedule 160 can be used as long as the OD is the same as the recommended schedule 40 pipe”.
- Pole can be black or galvanized steel pipe
- Using the chart below, find the MINIMUM acceptable pole height above your finished grade. It is strongly recommended that the pole height be to our minimum specifications.
- The minimum recommended hole depth is 1/3 the pole length, 1/3 in the ground, 2/3 above the ground. Example: 5’ below grade, 10’ above grade, total of a 15’ pole.
- Center the pipe in the hole, and using a level, make sure pole is vertical.
- Fill the hole with concrete (3000-psi minimum strength), and check level of the pole.
- The pole may be filled with concrete for added strength (only to approximate ground level).
- Allow concrete and pole to set for a MINIMUM of 36 hours BEFORE installing the Track Rack
Mounting Pole Recommendations:
Place a lone tracker where there is no shading. Remember that the sun rises and sets north in summer and south in winter of an East-West line. Installing more than one tracker requires placing each where it least shades the others. Trackers east and west must shade one another some times. Trackers North and South need not. Trackers are best placed, suitably spaced, in North/South lines. The longest shadow cast North is at noon on the winter solstice.
The pole spacing must keep the lower, southern tip of one tracker beyond the northern edge of its neighbor’s shadow. The poles must be farther apart than the sum of the north-south footprint and the shadow length. North-south footprint = (north-south array length) cos A. Shadow length = (tracker rise) (cot B) = (sin A) (north-south array length) (cot B). The solar elevation at noon on the winter solstice is 90° - latitude – 23.5° (solar declination).
Tracker’s 8’ wide in North-South direction set on California/Oregon border 42°
degrees north. Seasonal tilt set @ 40°
Footprint = (8’) cos 40° = 6.13’
Solar elevation is:
90° - 42° - 23.5° = 24.5°
North-South pole spacing must exceed:
6.13’ + [(sin 40°) 8’] cot 24.5 =
6.13’ + [5.142’] cot 24.5° = 17.41’
Trackers arranged in other than north-south lines must be far apart, 4 or 5 times E/W width, or have their angles of rotation restricted to avoid shading. A careful study should be made to optimize such an installation. It is beyond simple analysis unless one constructs a scale model with an included sundial for positioning.
Trackers cast long shadows that may shade others to the east and west at the beginning and end of the day. In the last example, the low corner dipped 110” below the gimbal at the beginning and end of the day, which means the high corner rose the same distance above the gimbal for a total of 220” or 18’4”. This elevation difference casts a long shadow from the high point of one towards the low point of another. The sun 15º above the horizon casts a shadow 68.4’ long when the elevation difference is 18’4”. PV arrays are unduly sensitive to shading. Shading of a few cells may cause the whole array to shut down. For this reason, space Trackers far apart or restrict motion.
The performance of an orchard of trackers that shades itself at the beginning and end of the day can be improved by restricting seasonal tilt or daily rotation or both to reduce shading. Daily rotation can be reduced by expanding the bumper with a radiator hose or by use of chains or ropes from the pole to the canisters.
Figures 1 – 5 show the gain from tracking through greater or smaller angles and illustrate diminishing returns as the angle increases.
Zomeworks can do engineering shading studies by special arrangement. Please call for pricing and details (505) 242-5354.
All racks with solar panels are vulnerable to wind. Passive trackers are especially so because they can flail in the wind if dampening is insufficient.
Wind blows stronger high above the ground. The rougher the terrain, the greater the affect the wind has on the racks.
The graph below shows wind speeds at different elevations. A 90mph wind at 30 feet drops to 50mph at 2.5 feet over rough ground. Smooth ground allows the same wind to maintain a 70mph velocity at 2.5 feet.
A large tracker may have some panels 15 feet and lower panels within a few feet of the ground. The panels higher in the air may be pushed by a 75mph wind at 22.5psf. Meanwhile, below the axle at 5 feet, the wind is 58mph and pushes at 13.5psf. The different forces, surging and slackening with wind gusts cause the tracker’s motion. Even at much slower speeds, it can make it impossible to track. In order to avoid wind problems, keep the trackers mounted as close to the ground as possible. Attaching a tracker pole to the corner of a building invites problems with wind turbulence and should be avoided. A high wind kit is also available to help protect the trackers.