Solar Panel Mirrors: How Do Heliostats Work?

Legend has it that Greek inventor Archimedes used mirrors to concentrate the sun’s rays on an approaching Roman army. As the beam of sunlight focused on the invaders’ wooden ships, the fleet caught fire, turning the Roman soldiers to ash before they could set foot on land. Although this story is likely nothing more than a legend, it perfectly illustrates how mirror solar panels work. 

What Is Concentrated Solar Power?

Ordinary photovoltaic panels absorb sunlight and convert it into electricity. Like leaves, they’re designed to maximize solar absorption rather than reflect it. 

In contrast, heliostats — which get their name from Helios, the Greek god of the sun — look like traditional solar panels but are actually giant mirrors. Engineers group them together at concentrated solar power (CSP) plants. Flying over the Crescent Dunes Solar Energy Project outside Tonopah, Nevada, you’d be forgiven for thinking this perfectly round sea of glass was actually a lake, so clearly does it reflect the sky. 

These solar mirrors reflect beams of sunlight onto a single, concentrated point on a receiver to generate enormous amounts of heat, much like using a magnifying glass to burn paper. The receiver sits at the top of a tower to increase optical efficiency and reduce shadowing. 

The tower contains fluids — which could be sodium, helium, air, water or the molten salt of nitrates — that reach temperatures ranging from 1,472° Fahrenheit to 3,632° Fahrenheit as they absorb the sunlight. 

CSP systems usually use this scorching heat not to burn enemy ships, but rather to drive a steam turbine connected to an electric generator. The spinning generator creates electricity just like in a traditional coal or oil power plant. The difference is that it’s a renewable power source. 

History of Concentrated Solar Power

Giovanni Francia designed and built the world’s first CSP plant in 1968. Situated near Genoa, Italy, the system featured a solar receiver in the middle of a field of mirror solar panels. Then, in 1981, engineers developed the Solar One power plant in Southern California, which ran until 1999. 

Today, the largest CSP station in the world is the Ouarzazate Solar Power Station, also called Noor Power Station. It is located near Ouarzazate, Morocco and generates over 500 megawatts of electricity at any given moment.

Pros and Cons of Mirror Solar Panel Arrays

Every hour and a half, enough sunlight strikes the Earth’s surface to power humanity’s energy needs for a year. CSP stations harness this abundant energy supply slightly differently than traditional solar panels but with the same overall goal.

If solar mirror arrays generate so much power, why do we hear so little about them? What advantages and disadvantages do they have when compared to traditional solar panels? 

Downsides

Like all technologies, concentrated solar panel systems have a few flaws. The first is that they take up a lot of space. 

Requiring a large, relatively level area that receives abundant sunlight, solar mirrors can’t be installed on rooftops or in residential spaces. This quality can present problems when it comes to preserving the environment. Desert ecosystems are fragile, and development often involves scraping and grading large desert sites to install the structures that support the solar mirrors. The heat coming off the solar mirrors can also kill passing birds and bats. 

Because of their size, CSP arrays have higher upfront costs than rooftop solar panels and even solar farms. Maintenance costs are higher, too — running hot salts or other corrosive fluids through metal pipes can wear down the system over time. There’s also the cost associated with the AI software that keeps the heliostats turning to face the sun. 

Additionally, most CSP plants use a lot of water to cool their steam engines. Wet cooling is expensive and can evaporate more water per unit of electricity than nuclear or coal power plants. Because most heliostat arrays are located in the desert, this high water consumption raises questions about responsible resource usage. It is also costly to transport water to the CSP plants. 

One potential solution is to implement air cooling systems. A 2022 paper in the journal Energy Storage and Saving concluded that daytime-only radiative cooling could reduce water consumption by 40% to 60% in CSP plants located in the southwestern U.S. Round-the-clock radiative cooling could reduce water usage by 65% to 85% overall. 

Benefits

One advantage of CSP plants is that they can store massive amounts of energy without a battery. Some systems use underground rocks or oil as a heat sink, storing energy day and night to power civilization around the clock. 

Like all solar power systems, CSP plants don’t create carbon dioxide or other greenhouse gas emissions, nor do they generate arsenic, lead, sulfur dioxide, mercury, nitrogen oxides or nuclear waste as byproducts. They carry none of the environmental risks associated with extracting natural gas, like fracking and water pollution. 

Heliostat arrays work well in regions with ample sunlight and low population densities. It’s theoretically possible to build them on disturbed land in remote desert areas to minimize ecological and human impacts. 

CSP plants could use the massive amounts of heat they generate to power high-energy industrial processes currently relying on fossil fuels. For example, they could power electrolysis for green hydrogen production. Or, a solar thermochemical hydrogen production system could use heat-based chemical processes to generate hydrogen. 

Currently, green hydrogen — created with no fossil fuel inputs — is expensive to produce. Most hydrogen production requires the use of methane gas and generates carbon dioxide and carbon monoxide as air-polluting byproducts. Concentrated solar power arrays offer a potential way to lower the cost of green hydrogen synthesis.

The Rise of Solar Energy

As a source of universal renewable energy, solar power produces no emissions, reduces dependence on foreign oil and lowers energy expenditures. Concentrated solar plants generate energy by focusing the sun’s energy on a single point. Whether or not these mirror solar panel arrays become common, solar power is still on track to overtake fossil fuels in the near future. It’s only a matter of time.