Solar power is potentially the greatest single energy source outside of controlled nuclear fusion, but the Sun is literally a fair weather source that relies on daytime and clear skies. To make solar energy a reliable, 24-hour source of energy, a team of scientists at Sweden's Chalmers University of Technology in Gothenburg is developing a liquid energy storage medium that can not only release energy from the Sun on demand, but is also transportable.

The Chalmers team has been working on variants of its system, called a MOlecular Solar Thermal (MOST), for over six years, with a conceptual demonstration in 2013. It differs from other attempts to store solar energy in things like heated salts and reversing exothermic reactions in that the MOST system stores the energy directly in the bonds of an organic chemical.

In this case, the scientists exposed a hydrocarbon called norbornadiene to light. This alters the chemical bonds, turning it into quadricyclane. Altering the temperature of the quadricyclane or exposing it to a catalyst reverses the effect and energy in the form of heat is released and carried off by a water jacket.

According to the team, the present system converts 1.1 percent of sunlight directly into chemical bonds, which is 100 times more efficient than the 2013 version that could only manage 0.01 percent. In addition, the new liquid storage system replaces ruthenium, a rare metal, with carbon-based elements that are much cheaper. Additionally, it can go through over 140 store and release energy cycles without noticeable degradation.

"The technique means that we can store the solar energy in chemical bonds and release the energy as heat whenever we need it." says team leader Kasper Moth-Poulsen. "Combining the chemical energy storage with water heating solar panels enables a conversion of more than 80 percent of the incoming sunlight."

The teams says the system allows solar energy to be stored and transported before being released as heat when and where it's needed.

The research was published in Energy & Environmental Science.