Solar, for all its promise as a source of abundant and cheap energy, has a problem – there is still no cost-effective and efficient way to store it.

In terms of output and efficiency, solar batteries are still in their infancy – ranging in size from wall-mountable units the size of microwaves (storing 1 kilowatt hour of energy) to floor-standing units as big as small fridges (storing 14kWh).

Who needs another bulky appliance, right? One that produces just enough energy to boil your kettle 120 times? Sure, it’s a lot of hot tea, but to market that as a serious consumer solution just sounds like a lot of hot air.

The missing link

For a long time, this has seemed like an intractable problem, until a group of Swedish scientists came up with an alternative to current battery design – a liquid known as ‘solar thermal fuel’ that they claim can store sun energy for over a decade.

Solar thermal fuel works similarly to a rechargeable battery, but instead of electricity, sunlight enters it and produces heat on demand.

At atomic level, the fluid is made up of molecules comprising carbon, hydrogen and nitrogen atoms. When hit by sunlight, the molecule’s bonds rearrange, producing an energised version of itself called an isomer. Energy is trapped within the isomer’s bonds and stays there, even when the liquid cools down.

When the energy is needed (at night or during cold periods), the fluid is drawn through a catalyst that returns the molecule to its original form, releasing heat energy.


A prototype of this system consists of a concave reflector with a pipe in the centre, which tracks the Sun like a satellite dish.

It works in circular fashion. The fluid is pumped through transparent tubes and heated by the sun, turning system molecules (norbornadiene) into their heat-trapping isomer (quadricyclane).

The fluid is then stored at room temperature with minimal energy loss. When it is needed again, the fluid is filtered through a catalyst that converts the molecules back to their original form, warming the liquid by 63 degrees Celsius.


The researchers have put the fluid through this cycle more than 125 times, picking up heat and dropping it off without significant damage to the molecule.

They claim the fluid can currently hold 250 watt-hours of energy per kilogram – double the energy capacity of Tesla’s PowerWall batteries.

With the right manipulations, heat of at least 110 degrees Celsius can be generated by the system. It is hoped it can be used for domestic heating systems within 10 years.