A concentrator must follow the sun because the sun crosses the sky at 15° an hour, or 0.25° a minute, and with a 0.6 m focal length every degree of pointing error moves the focus about 1 cm. A 5 cm spot is lost after roughly 10 minutes of drift, so a dish needs tracking by hand every few minutes, by a clock drive, or a target large enough to forgive the drift.
Part of Concentrated Solar Power
Tracking is the turning of a concentrator to keep it aimed at the sun as the sun moves, so that its focus stays on the target.
The Sun Moves 15° An Hour
The Earth turns 360° in 24 hours, so the sun appears to move 360 ÷ 24 = 15° per hour, which is 0.25° per minute. The parent page’s example: a dish aimed at the sun at noon is 30° off by 2 PM (2 hours × 15°), which for a high-concentration system means zero useful output.
How fast the focus runs away depends on the focal length. For a pointing error ε, the focus shifts by about f × tan ε. Inputs: f = 0.6 m (the 1.5 m dish of the dish pages).
| Pointing error | Focus shift | Time at 0.25°/min |
|---|---|---|
| 1° | 0.6 × tan 1° = 1.05 cm | 4 min |
| 2.4° | 0.6 × tan 2.4° = 2.5 cm | 9.6 min |
| 5° | 0.6 × tan 5° = 5.2 cm | 20 min |
A 5 cm spot has a radius of 2.5 cm, so the focus leaves it after about 2.4° of drift, about 10 minutes. The parent page’s manual interval of 15-30 minutes is enough for a large target and not for a small one.
Three Ways To Track
- By hand: adjust every few minutes (Manual Sun Tracking).
- By clockwork: a weight-driven drive turning one axis at 15° an hour (Gravity-Driven Clock Tracker).
- By season: the second axis needs adjusting only slowly, as the sun’s height changes through the year (Seasonal Angle Adjustment).
A trough tracks in one axis (parent page); a dish in two.
What You Need
- A mount that turns smoothly on two axes (dish) or one (trough)
- Locks that hold the aim against wind
- A sighting aid — a shadow pin or a pair of pegs
- A cover and a stow position
Common Mistakes
- Choosing a mount that turns easily but will not hold. Wind rotates it.
- No alignment mark. Without a quick sighting aid, each adjustment is guesswork.
- Forgetting the focus moves when the dish is parked. A parked dish points at where the sun will pass, not away from it.
Safety
A drifting focus sweeps across everything near it
Between adjustments the focus moves; at 15° an hour a 0.6 m focus moves at about 16 cm an hour for each axis, and it will pass over whatever is beside the target, igniting it. Concentrated sunlight also blinds permanently and painlessly. A tracking concentrator left unattended is a hazard, and so is an un-tracked one in the sun. Cover the reflector when you step away.
What’s Next
- Learn next: Manual Sun Tracking — the simplest tracker is you
- Combine with: Parabolic Dish — the shape that most needs it