How advanced heliostat systems are enhancing photovoltaic plant efficiency and creating new possibilities for solar energy
Since ancient times, humans have been fascinated with capturing and directing sunlight. Historical accounts suggest that as early as the 3rd century BC, the Greek scientist Archimedes proposed using shields to reflect sunlight and set fire to invading Roman ships. While this story may be more legend than fact, it demonstrates our long-standing fascination with harnessing the power of the sun. Today, that ancient concept has evolved into sophisticated technology called heliostats—mirrors that track the sun and reflect its light with precision 1 .
The concept of directing sunlight dates back to Archimedes in the 3rd century BC, showing humanity's long-standing interest in solar energy manipulation.
Today's heliostats are computer-controlled mirrors that precisely track the sun's movement, representing a technological evolution of ancient concepts.
In modern solar energy, heliostats have primarily been used in concentrated solar power (CSP) plants, where thousands of mirrors focus sunlight onto a single receiver tower to generate heat, which is then converted into electricity. Now, a new application is emerging: using heliostats to enhance large-scale photovoltaic (PV) plants. This innovative approach, which we might call "Prothelios" (from "pro" meaning forward and "helios" meaning sun), represents an exciting fusion of technologies that could significantly boost the efficiency and output of solar farms. By directing additional sunlight onto PV panels during early morning and late afternoon hours, heliostats can extend daily energy production and maximize the utility of existing solar infrastructure 1 3 .
A heliostat is essentially a computer-controlled mirror that follows the sun's apparent movement across the sky throughout the day. The name comes from the Greek words "hēlios" (sun) and "statos" (standing) 1 . Unlike regular mirrors, heliostats continuously adjust their position to maintain reflection of sunlight toward a fixed target—in this case, PV panels rather than a central tower.
The fundamental principle governing heliostats is simple but precise: the mirror surface must remain perpendicular to the bisector of the angle between the directions of the sun and the target as seen from the mirror. This ensures that incoming sunlight is reflected precisely toward the PV panels regardless of the sun's position 1 .
Sunlight → Reflection by Heliostat → Enhanced PV Panel Output
Most modern heliostats use a two-axis tracking system (azimuth and elevation) controlled by computers. The computer is programmed with the heliostat's geographical coordinates (latitude and longitude) and the current date and time. Using astronomical algorithms, it calculates the sun's exact position—both its compass bearing and elevation angle—and then determines the proper mirror orientation to reflect sunlight toward the target 1 .
This sequence of calculations and adjustments is repeated frequently throughout the day to maintain proper alignment. In large installations, a single computer typically controls an entire field of heliostats, coordinating them to work in unison 1 .
| Design Type | Key Features | Advantages | Applications |
|---|---|---|---|
| Conventional Glass/Metal | Steel support, adhesive, copper layer, silver reflection, glass protection | Durability, high reflectivity | Large-scale CSP plants |
| Stellio Pentagonal | Radial steel structure, 10 cantilever arms, central hub | Reduced wind oscillation, better optical efficiency | CSP towers, potential for PV enhancement |
| Lightweight Composite | Thin glass mirrors embedded in composite materials | Lighter weight, lower structural cost | New installations where weight matters |
| Polymer Reflectors | Silvered polymer surfaces | Reduced weight, lower manufacturing cost | Experimental, cost-sensitive applications |
Table 1: Comparison of Heliostat Design Approaches
In 2021, a collaborative effort called the Heliostat Consortium (HelioCon) was formed to advance heliostat technologies in the United States. Led by the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories, with funding from the U.S. Department of Energy, this initiative aims to lower costs and improve performance of heliostat technologies 7 8 .
Heliostats represent 30-50% of the initial capital investment for solar power tower plants 1 . Similarly, for proposed heliostat-PV hybrid plants, reducing heliostat costs is essential for economic viability.
An adaptable quality-control tool that measures slope and canting in heliostats using deflectometry with a commercial camera 7 .
An open-source platform serving as a collaborative environment where the CSP community can share code, data, and computer-aided design models 7 .
These advancements are crucial for making heliostat technology more accessible and affordable for integration with large PV plants.
While heliostats traditionally work only during daylight hours, Dr. John Sandusky of Sandia National Laboratories has pioneered research into giving them a "night job"—finding asteroids 2 5 . This innovative application demonstrates the versatility of heliostat technology and represents the kind of creative thinking that could lead to further dual-use applications for heliostat-PV hybrid facilities.
The concept is both simple and revolutionary. At night, heliostats can be repurposed to reflect starlight rather than sunlight. When a heliostat sweeps across the night sky, stars appear stationary in the reflected image, but asteroids—being much closer and moving relative to Earth—create distinctive streaks or patterns that can be detected with sensitive instruments 5 .
"The heliostat fields don't have a night job. They just sit there unused. The nation has an opportunity to give them a night job at a relatively low cost for finding near-Earth objects."
Enhance PV output by reflecting additional sunlight
Detect asteroids by reflecting starlight
Dual-purpose operation maximizes infrastructure utilization
In his groundbreaking experiment conducted at the National Solar Thermal Test Facility, Dr. Sandusky used just one of the facility's 212 heliostats to test his theory. The experimental procedure involved:
Using existing control software to oscillate the heliostat's direction relative to the stars
Using standard optical instruments to detect faint starlight reflected by the heliostat
Gathering data points throughout the night with regular intervals
Applying frequency analysis to detect moving objects against stationary stars
"Solar towers collect a million watts of sunlight. At night, we want to collect a femtowatt, which is a millionth of a billionth of a watt of power of sunlight that's scattered off of asteroids" 5 .
This experiment represents a classic example of repurposing existing infrastructure for new scientific applications—a concept highly relevant to integrating heliostats with existing PV plants.
Dr. Sandusky's initial experiments, conducted during summer nights at the National Solar Thermal Test Facility, successfully demonstrated that heliostats can indeed reflect starlight and be precisely controlled for astronomical observations. While the initial test didn't discover new asteroids (which wasn't its primary goal), it validated the core concept and methodology 5 .
The research showed that the relative motion of asteroids compared to stars could be detected through frequency shifts in the reflected light, similar to techniques used in radio communication. "If I can map all of the stars to one frequency, anything moving relative to the stars will appear at a neighboring frequency but still be separable," Dr. Sandusky explained 2 .
This successful proof-of-concept opens the door to potentially using heliostat fields at solar farms for multiple purposes—enhancing PV production by day and monitoring near-Earth objects by night, creating a more economically viable model for both applications.
| Error Type | Cause | Impact | Correction |
|---|---|---|---|
| Azimuth Rotational Axis Tilt | Improper installation | Systematic tracking inaccuracy | Drift prediction models |
| Mirror Alignment/Canting | Manufacturing imperfections | Reduced concentration efficiency | ReTNA measurement system |
| Encoder Reference Position | Calibration errors | Consistent angular deviation | Regular calibration protocols |
| Seasonal Drift Variations | Changing solar declination | Variable performance across seasons | Predictive adjustment algorithms |
Table 2: Heliostat Drift Error Analysis and Correction
The journey from experimental concept to commercial implementation requires rigorous validation. Recent heliostat designs like the Stellio pentagonal heliostat have undergone extensive testing at facilities like CIEMAT's outdoor testing lab in Spain and DLR's Institute for Solar Research in Germany 3 .
"When you look at the deformations from the wind of a classic rectangular heliostat, you can see that the corners are weaker and you have a strong deformation there, which is not optimum. The ideal would be a circular one and for us this was something in between" 3 .
These structural advantages translate directly to improved performance and lower operating costs—critical factors for the economic viability of heliostat-PV hybrid plants.
The advancement of heliostat technology depends on specialized tools and materials that enable precise measurement, control, and optimization.
| Component | Function | Current Developments |
|---|---|---|
| Mirror Facets | Reflect sunlight to target | Transition from thick glass to thin glass composites and polymers |
| Support Structure | Provides stability and positioning | Lightweight designs to reduce material costs |
| Drive Mechanisms | Enable precise two-axis movement | Transition from slew drives to linear actuators in some designs |
| Control Systems | Calculate sun position and adjust mirrors | Advanced algorithms for predictive tracking |
| Sensors | Provide feedback on position and alignment | Integration of GPS, encoders, and optical reference systems |
| Communication Systems | Coordinate field-wide operations | Wireless systems to reduce installation costs |
Table 4: Essential Materials and Components for Heliostat Systems
The potential applications for heliostat-PV hybrid plants extend beyond simply enhancing electricity production. Researchers are exploring how this technology could contribute to:
Using concentrated solar energy to split water molecules 1 .
Providing high-temperature heat for manufacturing processes 8 .
Creating sustainable alternatives to fossil fuels 3 .
Tracking objects in cislunar space between Earth and Moon 5 .
"When the 2030 performance goal for the heliostat-based CSP systems is met, it would have great potential in non-electricity sectors as well" 8 .
Current focus on extending PV production hours with heliostat reflection
Nighttime use of heliostats for near-Earth object monitoring
Integration with solar water splitting for green hydrogen
Full integration of multiple applications in hybrid solar facilities
The journey from specialized solar component to versatile energy tool represents the next chapter in heliostat technology. As research continues and costs decline, we may see increasing integration of heliostat technology with conventional PV plants, creating hybrid systems that offer greater efficiency, extended operation, and multiple revenue streams.
This vision of multi-purpose solar infrastructure—where heliostats enhance PV production by day and contribute to scientific research or other applications by night—exemplifies the innovative thinking needed to accelerate our transition to renewable energy and maximize the utility of our solar resources.
As these technologies continue to evolve, the ancient dream of precisely controlling sunlight for practical purposes is becoming a sophisticated tool for addressing both our energy needs and scientific challenges. The future of heliostat technology shines brightly, limited only by our imagination and commitment to innovation.