As scientists and researchers continue to seek new ways to reduce dependency on fossil fuels and decrease the amount of CO2 put into the air, one area that is sometimes overlooked is how heat is stored. For example, although much research has gone into collecting and using solar and wind energy, little has gone towards answering the question of how to store excess energy for the times when the sun is down or the wind isn’t blowing.
Traditionally, the answer to this storage question has been batteries or, in some cases, pumped hydroelectric storage – two solutions that are far from perfect. In fact, some of their inefficiencies can actually cancel out any environmental benefit created by using solar and wind energy in the first place.
One possible solution to the storage question is to convert the energy into thermal energy and store it in thermal energy storage facilities, which have been shown to be generally more efficient, better at capturing wasted heat and able to provide more low-cost energy. However, before we can make widespread use of thermal energy, research into the development of cost-effective, high-density storage technology is needed.
Thinking outside the box
The project’s objective was to develop new phase change materials (PCM) for seasonal thermal energy storage applications (STES) in the range of medium temperatures. Researchers wanted these materials to be low-cost, environmentally sound, safe and easy to use. Furthermore, the materials would have to be able to serve as a long-term storage solution that provided significantly lower levels of thermal loss than currently available options.
This was an innovative approach to the problem as PCM are not typically viewed as a STES option. This is due to their insufficient energy densities and their high risk of solidification during storage – a phenomena caused by inadequate insulation for maintaining temperatures beyond the melting point.
The magical material
For SAM.SSA, the magic material for overcoming the PCM problem was sugar alcohol, a common and abundantly available waste product of the food industry. Also known as molecular alloys based on sugar alcohols (MASA), this material allows for the adjustment of the melting point and, as a result, significantly increases energy density. According to a recent article on the project published in ‘The Journal of Physical Chemistry’, sugar alcohols, when mixed with carbon nanotubes, create a material capable of storing renewable energy as heat.
The project focused on sugar alcohols as they permit high levels of undercooling, which minimises the risk of spontaneous PCM solidification. They also reduce the requirements for insulation, as well as thermal loss during long-term storage. With the application of a local thermal shock or ultrasound, nucleation and subsequent crystallisation is induced, allowing for an easy and efficient discharge of the energy from the storage system.
Laying the foundation
When carbon nanotubes of varying sizes are mixed with two types of sugar alcohols – erythritol and xylitol – researchers found that, with one exception, heat transfer within a mixture would decrease as the nanotube diameter decreased. They also found that, as a general rule, higher density combinations meant better heat transfer. These findings are significant as they lay the foundation for the future design of sugar alcohol-based energy storage systems.
At the time of the project’s conclusion, researchers had created an early-stage prototype for treating pure sugar alcohol or sugar alcohol blends to achieve crystallisation. With basic concepts for further prototypes already in the works, along with exploitation strategies, SAM.SSA researchers are optimistic about the use of MASA as an answer to the problems of thermal energy storage.