Solar robotics: Designing autonomous tracking systems

Solar robotics: Designing autonomous tracking systems

The benefit of providing energy has renewed interest in tracking and positioning systems to capture and harness the sun's power

BY JON GABAY

Perhaps one of the oldest technologies humans have mastered is tracking and predicting the locations of celestial objects, particularly the sun and the moon. The benefit of providing energy has renewed interest in tracking and positioning systems to capture and harness the sun's power.

Energy extracted from the sun is directly proportional to the angle of incidence of the suns rays. Following a standard well-known Gaussian curve, the peak extracted energy occurs when the sun's rays are at a 90° angle to the surface. The extractable energy falls off sharply as the angle deviates from 90°. Tracking increases the available energy at all times to maximum (see Figs. 1a and 1b ). This makes tracking an important part of the energy equation, especially if space is constrained or maximum efficiency is required.

Fig. 1. Increased energy from tracking translates to higher power output seasonally.

Tracking technology is not cheap. A typical two-axis tracking system for an array of photovoltaic panels costs around $15,000. This leaves room for designers to use good technology to implement clever designs that reduce power needed and increase efficiencies.

Architectures

When designing a tracking system, several factors need to be addressed. Foremost is the type of energy to be extracted. The vast majority of solar-tracking applications target photovoltaic systems, and that is what we will focus on in this article. Most solar heating systems don't use tracking. Some exceptions are parabolic (footnote 1), heliostat (footnote 2), and trough systems (footnote 3).

Regardless of the type of energy extracted, the weight, size, and tracking mechanics determine how powerful a motor is needed. Gearing ratios, run times, and on-time duty cycles play into motor selection as well. Once torque and rotational rates are determined, the motor type and power characteristics will determine the ideal motor control technology to use.

“We use highly efficient Italian ac motors and all quality metal worm gears,” says Gerald Whipple, president and founder of Solar Array Mounting Systems LLC in Cedar City, UT. “Our trackers handle up to 13.4 kW and are designed to tie to the grid power so ac power is always available, eliminating the need for a battery.” The 120-W motors can drive 415 to 520 sq ft of photovoltaic panels with very short run times. “The longest run time we see is the return to park operation at the end of the day which can take 3 minutes and 30 seconds to return to 55° East,” continues Whipple.

Dc motor are the motors of choice for isolated power systems that do not have grid power available anywhere. Here a battery is required, as is an amount of power from the array to maintain the battery charge.

“Our one-size-fits-all photovoltaic tracking system can handle up to 16 250-W panels. A small 20-W panel can completely maintain our battery for our dc motors,” Says Ken Sexton, operations manager at Patriot Solar Group, Albion, MI. The company uses dc worm drive motor for azimuth and a dc linear actuator for elevation.

Almost exclusively, worm gear types of drives are used for the azimuth angle rotations because the drives have an inherent gear reduction while they provide a self-locking mechanism. It is virtually impossible to manually move a worm gear, making it ideally suited to handle wind.

Speaking of wind, both Patriot and Sun A Ray trackers (as do many other top-of-the-line types of trackers) have wind sensors and retreat features that sense wind power and fold up to minimize the wind profile under heavy windy conditions. This is another design consideration that needs to be taken into account. First, the amount of travel needed to transit to the hide position, and appropriate wind speed sensors must be chosen.

To sense or to position

Another key architectural decision is whether or not to use light sensors or to implement geopositioning. Light sensing is an older technology, but is still effective in many applications. Sensor technology is lower cost than geopositioning, which requires precise motor control, precise position feedback, and a more sophisticated embedded processor. Light-sensor-based systems on the other hand, can use simpler microcontrollers or even be pure analog using timers and op amps instead of microcontrollers.

An example of an open-architectured low-cost sensor-based tracker design comes from a company called Redrock Energy in Minnesota (footnote 4). As with other sensor based systems, a transistor H bridge is used to drive the motor in either direction, based on the balance of signals from the sensors. In this case, LEDs are used as the sensors instead of photocells or photodiodes/transistors. Redrock uses a set of these types of sensors for both azimuth and for elevation tracking.

Geopositioning systems rely on precise setup alignments, precise position feedback, and precise motor control. “We use a GPS-based algorithm to precisely identify a location and to track accurately,” states Sexton. “This is increasingly more important as concentrated solar panel (CSP) technology is used, requiring more precise tracking (to within 1° typically),” he continued.

CSP systems basically use focusing elements to increase the solar flux density incident to a specific cell. These are available from several manufacturers, but basically all use lenses with fixed focal points that increase light intensity typically 500 times (see Fig. 2 ).

Fig, 2. Concentrated Solar Panels provide very high efficiencies, but require more precise tracking.

“Our triple-junction gallium arsenide solar cells can operate up to 1,500 to 1 ratios,” says Christopher Larocca, chief operating officer at Emcore. “Because the gallium arsenide is so much more resilient than silicon, only passive heatsinking is required,” he continues.

Emcore makes its own gallium arsenide cells used for concentrated applications. The other manufacturer is Boeing who makes the Spectrolab cell. These were developed for space and satellite applications and are very resilient and tough.

Emcore uses its proprietary CTJ photovoltaic cells, which boast 39% efficiency under concentrated illumination. The triple-junction high-output cells are built on germanium substrates and incorporate a proprietary anti-reflective coating that provides low reflectance over a wavelength range of 0.3 to 1.8 µm. The high-efficiency solar cells are characterized and optimized for terrestrial applications under concentrated incident illumination (up to 1,500 suns) and high current densities.

CSP technologies need tracking and locations where there is high direct natural illumination (DNI) such as in the Southwest USA, Middle East, or equatorial locations. CSP cells do not work on cloudy days or off angle light incidence as well as silicon cells and, as such, require precise tracking to take advantage of their high efficiencies.

Both Whipple and Sexton agree that, overall, a typical performance increase of around 40% more power is achievable with dual-axis and around 20% to 25% with single-axis tracking. This can seem wrong when you think about the relatively short amount of time a fixed solar panel is at 90° to the solar flux, but looking at Fig. 1 shows how the Gaussian curve is spread, especially seasonally adjusted.

On target

Thanks to low-power microcontrollers, networking, and a variety of energy-efficient motor control chips and systems from the semiconductor industry, the cost of efficient tracking electronics is coming down. While absolute or incremental shaft encoders by themselves are more expensive than light sensors, the more highly functional capabilities of the control electronics has the promise of making solar tracking easy to install, easy to use, and maximizing available power.

It will be the cost of energy that ultimately dictates the cost effectiveness of autonomous tracking systems for wide spread applications. This is a technology to keep a light on. ■

Non-photovoltaic references / examples:

1. http://www.gyroscope.com/d.asp?product=SOLARSTIR

2. http://www.redrok.com/archeliostat.htm

3. http://www.redrok.com/images/smith_ron4.jpg

4. Linear H Bridge Sensor track schematic: http://www.redrok.com/images/led3xc1.gif