BEIJING — Highly efficient power supply has kept the combination of the Tianzhou-2 cargo craft and China's space station core module Tianhe operating stably.

China sent into space the Tianhe module on April 29, kicking off a series of key launch missions that aim to complete the construction of the space station by the end of 2022.

The country launched Tianzhou-2 on May 29, which successfully docked with Tianhe in about eight hours to deliver supplies, equipment and propellant.

Both the core module and cargo craft are equipped with independent power supply systems. After docking, the two systems can form a connected grid and achieve mutual transmission of electric power.

"The two independent systems work like two smartphones," said Yu Lei, an engineer in charge of power supply to the Tianzhou-2 project. "They can undergo both separate and mutual charging."

When docking with the space station, Tianzhou-2 is in sleep mode and has relatively lower power demand. Its solar panels can provide abundant power most of the time, and the surplus is stored to be used for astronaut activities and scientific research that requires higher energy consumption.

"Its solar panels not only fulfill the power demand of Tianzhou-2 directly, but also charge its batteries," said Wang Zhenxu, a designer of the power supply system of the Tianzhou-2 project.

Researchers estimate that it takes 91 minutes for the space station to go around Earth in the low-Earth orbit, during which it can receive sunlight for about 54 minutes. The solar panels thus experience the light-shade cycle about 16 times every day.

The three sets of lithium ion batteries power the entire cargo craft when the combination enters the shadow areas. The solar panels work again after it moves into the sunlight, and continue supplying power and charging.

The core module Tianhe is powered by third-generation flexible GaAs (gallium arsenide) solar panels, which provide high power output, are light, small when folded, have long in-orbit life, and can be repeatedly folded and unfolded.

The thickness of a solar panel is less than one millimeter, only half of the weight per unit area of the traditional solar arrays.

The power area of Tianhe's solar panels covers 80 square meters, which can provide 20 KW power to the core module, said Yu Hui, designer of the solar panels.

The researchers carried out a large number of ground simulation tests to verify the environmental protection technology used in the solar panels to help them endure almost 90,000 high and low temperature alternating cycles ranging from 100 to minus 100 degrees Celsius in orbit as well as the complex space environment.

The design life of the solar panels is up to 15 years, and they can work effectively for the space station project, Yu Hui said.