A heliostat is a mirror that automatically tracks the sun across the sky and reflects sunlight onto a fixed target. The word comes from the Greek “helios” (sun) and “stat” (stationary), referring to the fact that the reflected beam stays in one place even as the sun moves. Heliostats are the core technology behind solar power towers, where hundreds or thousands of these mirrors work together to concentrate sunlight intensely enough to generate electricity.
How a Heliostat Works
The sun moves across the sky throughout the day and shifts its arc seasonally. A heliostat compensates for this by rotating on two axes: one horizontal (up and down) and one vertical (side to side). Motors adjust the mirror’s angle continuously so that reflected sunlight always hits the same spot, typically a receiver mounted on top of a central tower.
The tracking system relies on a controller that calculates exactly where the sun is at any given moment based on time, date, and geographic coordinates. Some newer systems use cameras and light sensors to detect the sun’s position directly and fine-tune the mirror’s angle in real time. The industry standard for tracking accuracy is 1 milliradian or better, which translates to the reflected beam drifting less than about 1 meter for every 1,000 meters of distance to the target.
Core Components
Every heliostat has three main parts: a reflective surface, a drive system, and a support structure.
- Mirror facets: The reflective surface is made of one or more flat mirror panels called facets. Most commercial heliostats use thin glass mirrors (around 1 mm thick) bonded to a backing material, achieving over 95% reflectivity. Some large heliostats have a combined mirror area exceeding 178 square meters, roughly the size of a tennis court. Mirror facets and their supports currently cost about $24 per square meter.
- Drive system: Two motorized drives tilt and rotate the mirror along its two axes. This is one of the most expensive components, accounting for up to 30% of a heliostat’s total cost. The azimuth drive (the one that rotates the mirror left and right) is the pricier of the two.
- Pedestal and foundation: A vertical steel tube, either round or rectangular, supports the mirror assembly. It’s anchored into the ground with a concrete foundation large enough to handle wind loads pushing against what is essentially a large sail.
Heliostats in Solar Power Towers
The primary use of heliostats is in concentrated solar power (CSP) plants, specifically the solar power tower design. A field of heliostats surrounds a tall central tower, and all of them aim their reflected sunlight at a receiver near the top. The U.S. Energy Information Administration notes that this arrangement can concentrate sunlight by as much as 1,500 times its normal intensity.
That extreme concentration produces serious heat. Most solar tower receivers operate above 700°C (roughly 1,000 K), and advanced designs using gas turbines or combined-cycle systems push working temperatures into the range of 900°C to 1,300°C. At the high end, these temperatures enable power conversion efficiencies above 50%, which is competitive with fossil fuel plants. The heat typically warms a fluid like molten salt, which can be stored in insulated tanks and used to generate steam for electricity hours after sunset. This built-in energy storage is one of the key advantages solar towers have over photovoltaic panels.
How Heliostats Compare to Parabolic Troughs
Parabolic troughs are the other major CSP technology. Instead of flat mirrors pointing at a tower, troughs are long curved mirrors that focus sunlight onto a pipe running along their center. Troughs track the sun on only one axis, which makes them mechanically simpler, and they tend to be more thermally efficient overall. A 2024 comparative study in the steel industry found parabolic trough systems averaged a total system efficiency of 0.54, while solar towers averaged 0.28.
That efficiency gap is somewhat misleading, though, because heliostats and towers have different strengths. Solar towers reach much higher temperatures, which unlocks more efficient power cycles and better thermal storage. Parabolic troughs max out around 400°C, limiting them to conventional steam turbines. For industrial processes that need extreme heat, such as steelmaking or hydrogen production, solar towers with heliostat fields are the more practical choice.
Cost and the Path Forward
Heliostats are the single largest cost in a solar power tower plant, so bringing their price down is the central challenge for the industry. The current installed cost sits slightly above $100 per square meter. The U.S. Department of Energy has identified $50 per square meter as the target needed to make CSP competitive with other energy sources, and estimates that reaching this number would account for more than 40% of the total cost reductions required.
Getting there means cheaper mirror facets, lighter support structures, and less expensive drive systems. Researchers at the National Renewable Energy Laboratory are developing composite facets that replace heavy glass-and-steel designs with lighter materials while maintaining the high reflectivity needed for efficient operation. Smaller heliostats are also gaining attention: they’re easier to manufacture in volume and can be assembled with lighter, less expensive components, even though more individual units are needed to cover the same area.