Dual-Axis Solar Tracker Torque requirements

[LukeTanner]

TL;DR

I would like a step-by-step formula explanation on how to determine the required torque to move a Solar panel in both the azimuth and elevation directions.

Hello,

I am building a dual-axis solar tracker for educational purposes and I need to determine the required torque to move the solar panel in both the azimuth and elevation directions in order to determine what size motors to use. I have attached a simple sketch illustrating the mechanical structure for this solar tracker.

The solar panel dimensions are (1480x670x40)mm and weights approximately 12.5 kg. I have no mechanical engineering background so a step-by-step formula explanation would be greatly appreciated.

Regards,
Luke

 

[Baluncore]

Welcome to PF.
This is an azimuth-elevation system.
We would need to know the mass and the position of the centre of mass in order to calculate actuator torque.

You should consider the static balance about the elevation axis.
If you use only one panel then consider two balance weights on short arms, attached to and mounted behind the solar panel, either side of the elevation actuator.
You could place one panel on each side of the elevation drive. The two panels would be setback to balance the array.
Both those techniques could achieve a static balance and reduce the motor torque required to maintain position.

 

[LukeTanner]

Welcome to PF.
This is an azimuth-elevation system.
We would need to know the mass and the position of the centre of mass in order to calculate actuator torque.

You should consider the static balance about the elevation axis.
If you use only one panel then consider two balance weights on short arms, attached to and mounted behind the solar panel, either side of the elevation actuator.
You could place one panel on each side of the elevation drive. The two panels would be setback to balance the array.
Both those techniques could achieve a static balance and reduce the motor torque required to maintain position.

Thank you for your response.
I am working on the assumption that the center of mass is in the center of the panel and the elevation point of rotation is in line with the center of mass and 100 mm away for the center of mass. The azimuth point rotation is in line with the center of mass and 150 mm away for the center of mass.

If I were to add weights on the short arms to reduce the elevation torque would that not increase the torque required by the azimuth motor? How would I take the force of wind on the panel face into account for the torque requirements?

Again, thank you for your assistance. It is greatly appreciated.

Regards,
Luke

 

[Baluncore]

One problem with an Az-El tracker is that in the tropics there will be times when the Sun passes close to directly overhead, then the tracker must change Az very rapidly by 180° in order to track the Sun.
Are you in the tropics, at a latitude within 23.5° of the equator?

If I were to add weights on the short arms to reduce the elevation torque would that not increase the torque required by the azimuth motor?

Yes, but only when accelerating. If the weights exactly balance the panel, then the El torque will also be zero.

You show the pole rotating. I would move the Az actuator to the top of the pole. That would reduce the AZ actuator stress in strong winds and allow the pole to be more firmly fixed.

How would I take the force of wind on the panel face into account for the torque requirements?

For winds from any direction, the design should have the same wind drag on either side of the Az axis. The array should offer the same wind drag above, as there is below the elevation axis.

A design like this will need to sense the array orientation and drive to the required position. If a strong gust of wind overcomes the servo, something should slip, (not break), then when the gust ends the panel should recover to the correct orientation and continue to track again.

What will the panel do during hail storms?
What Az-El should the panel face at night, so the rain can wash the panel surface?
How will you stop birds sitting on the top edge of the panel?

 

[LukeTanner]

I am indeed near one of the tropics. Would the momentum of the panel face during this rapid movement near to be taken into account in the determination of the torque? If so, how so?

Yes, but only when accelerating. If the weights exactly balance the panel, then the El torque will also be zero.

You show the pole rotating. I would move the Az actuator to the top of the pole. That would reduce the AZ actuator stress in strong winds and allow the pole to be more firmly fixed.

I see your point here, thank you for this suggestion.

For winds from any direction, the design should have the same wind drag on either side of the Az axis. The array should offer the same wind drag above, as there is below the elevation axis.

A design like this will need to sense the array orientation and drive to the required position. If a strong gust of wind overcomes the servo, something should slip, (not break), then when the gust ends the panel should recover to the correct orientation and continue to track again.

This makes sense to me. So as opposed to resisting the force of the wind, the system can be displaced by it then re-adjust after the gust is gone. Thank you for this suggestion.

What will the panel do during hail storms?
What Az-El should the panel face at night, so the rain can wash the panel surface?
How will you stop birds sitting on the top edge of the panel?

In the case of hailstorms and rainfall I intend to research the topic of linking the system to a weather app and adjust the systems orientation accordingly. In the case of birds I intend to mount a simple reflective bird deterrent alongside the panel face.

 

[Baluncore]

I am indeed near one of the tropics. Would the momentum of the panel face during this rapid movement near to be taken into account in the determination of the torque? If so, how so?

You would need to define the maximum slew rate of the tracking system, and avoid the controller locking up at the zenith.
You could consider an equatorial mount where the principle axis is not vertical, but parallel to Earth's axis. That would be driven by a gear motor like a clock, with declination adjusted daily for the season. The maximum tracking rate is then one revolution per day = 0.0007 RPM, and there is no zenith trap to avoid.
https://en.wikipedia.org/wiki/Equatorial_mount

 

[anorlunda]

I'm probably overthinking this, because I'm visualizing my friend's 144m² 2-axis installation. But it is a fun exercise in engineering design at any scale.

This makes sense to me. So as opposed to resisting the force of the wind, the system can be displaced by it then re-adjust after the gust is gone. Thank you for this suggestion.

That is indeed a clever suggestion. But severe thunderstorms frequently rip the roofs off buildings and could destroy your solar installation. In many countries, local building codes give the required strength for roofs. I think you need to design your panel for the same parameters as an equivalent area roof in your location. Since your area of 1 m² is small, the numbers should be reasonable.

My friend has a 2-axis solar system. It senses wind speed, and when speed goes above maximum, the system automatically moves to horizontal. That is called the "feather" position. Horizontal is the least resistance to wind. Then it needs to lock in that position so that the servo can not be overcome by wind forces blowing it back toward the vertical.

Just a guess, but I would expect wind forces to be an order of magnitude more important than mass and friction in your system. Instead of calculating the minimum strength required, I would calculate the worst case and then apply a safety factor of 4x or more in sizing the motion system.

But better than any calculation is the local building code. Here is a wind load calculator used for roof design. You could maybe try it for several angles.
https://www.buildingsguide.com/calculators/structural/ASCE705W/

Since 1m² is small, you might consider an alternative to strong+smart. You could attache the panel to the mount with bolts designed to shear if the wind load is too high. Then tether the panel to the ground with a steel cable. If the wind gets too strong, the panel breaks away, and falls to the ground, but it does not blow far away because of the tether. With that, you could return to where you started considering only mass and friction. But don't skip the tether because a rigid panel blowing in the wind becomes a lethal missile with sharp corners.