Lighting the Olympic flame, generating electricity in space...how many surprises are there in solar energy that you don't know about?

Review expert: Qian Hang, aerospace science expert of the First Academy of China Aerospace Science and Technology Corporation

On July 27th Beijing time (July 26th local time), the 33rd Olympic Games will open in Paris, France. After a century, the Olympic flame will once again ignite the romantic city.

Looking back at the history of the Olympic flame relay, on April 17, the flame of the Paris Olympic Games was successfully collected at the birthplace of the Olympic movement, the site of Ancient Olympia on the Peloponnese Peninsula in Greece. On July 14, the Olympic flame was relayed to Paris.

Olympic flame collection source: Xinhua News Agency

The Sun and the Olympic Flame

The flame-lighting ceremony was held in Ancient Olympia, Greece, the site of the first ancient Olympic Games. The Olympic flame links the ancient and modern Olympic Games together. The Olympic flame has been a symbol of peace and friendship since ancient times, and also represents the positive values ​​of human beings being closely united under the symbol of fire.

On April 16, 2024, at the Paris Olympic flame collection ceremony, an actor who played a priestess held a fire can. Source: Xinhua News Agency

Unlike manual ignition, the flame is collected from sunlight to show its purity. The collection process is very simple. A concave mirror is used to focus the sunlight to ignite the film on the collection torch. A rehearsal will be held before the flame collection ceremony begins. The flame collected during the rehearsal will be saved in case of bad weather conditions on the day of the collection ceremony. The Paris Olympic flame collection encountered cloudy conditions.

On April 15, 2024, Mary Mina (front left), who plays the role of the high priestess, took the fire during the rehearsal of the Paris Olympic flame-collecting ceremony. Source: Xinhua News Agency

At the Paris Olympic flame lighting ceremony, actress Mary Mina, who plays the role of the high priestess, lights the torch from the burning fire can. Source: Xinhua News Agency

The sun and the concave mirror

The key to the Olympic flame is fire. There is no fire in the concave mirror, so how can we get fire to light the torch? And why do we need to consider weather factors and prepare fire in rehearsals even though there is a concave mirror?

This requires talking about the subtle connection between sunlight and concave mirrors.

Concave mirrors, that is, concave parabolic mirrors, play a role in focusing light. A parabola has an important property: straight lines parallel to the axis of symmetry are reflected by the parabola and converge at the focus of the parabola.

The parabola for collecting the holy flame is coated with a reflective layer. When it faces the sun, the sunlight is parallel to the symmetry axis of the parabola. After being reflected by the parabola, the sunlight is concentrated at the focus of the parabola, making the temperature at the focus very high, which can reach the ignition point of the torch fuel. The priestess puts the torch burner at the focus of the concave mirror, so that the reflected sunlight ignites the torch.

A parabola can reflect parallel light rays to a focal point. Conversely, light rays emitted by a light source at a focal point will also become parallel light rays after being reflected by a parabola . For example, the flashlights, car headlights, and searchlights we commonly use all have parabolic reflective surfaces.

What else can the sun do?

The first thing that comes to everyone’s mind is probably solar power generation.

Solar power generation is a power generation method that converts solar radiation energy into electrical energy through water or other media and devices. There are currently two basic ways: solar thermal power generation and solar photovoltaic power generation.

Solar thermal power generation

Solar thermal power generation is the process of first converting solar radiation into thermal energy, and then converting the thermal energy into electrical energy according to a certain power generation method.

This is actually the concave mirror parabola focusing principle . The incident light parallel to the axis will be focused to the focal point after being reflected by the parabola. If a device that receives energy is placed at the focal point, high temperature can be generated. If there is a high-temperature heat source, it is possible to generate electricity.

At present, according to the different ways in which the concentrating system collects solar radiation, solar thermal power generation technology is mainly divided into four types : tower type, trough type, linear Fresnel type and dish type .

China's first solar thermal power generation experimental power station, the Beijing Yanqing Badaling Solar Thermal Power Generation Experimental Power Station , was successfully completed in 2012. This marks that China is the fourth country in the world to achieve large-scale solar thermal power generation after the United States, Germany and Spain!

Solar power generation

Solar power generation is the direct conversion of sunlight into electrical energy through photoelectric devices. The more common solar power generation method is photovoltaic power generation , which can be seen everywhere, such as street lights, calculators, and water heaters.

Photovoltaic power generation technology continued to develop and grow in the first half of the 20th century, but was initially generally used only in the military field. It was not until the first oil crisis in 1973 that people began to transfer the application of photovoltaic power generation to the civilian field .

For example, in the field of aerospace, it can be used to power satellites and space stations, or solar-powered aircraft equipped with photovoltaic cells can fly continuously without interruption. In the future, it may even be possible to build solar power stations in space to wirelessly transmit electricity to ground receiving stations or to provide endurance for aircraft in the air.

The sun, the source of light and heat on Earth, creates wind and rain and nurtures all things. I believe that with the progress of human society, the development of the sun will surely reach a new level in the future!