Salt and solar heat can be used to replace fossil fuels for heating houses. A new PhD thesis is testing two prototypes, where solar energy is stored in a salt compound and can be released when needed.
There is a huge potential in using renewable energy sources such as solar energy to heat buildings. Still, the vast majority of buildings are heated by fossil fuels emitting greenhouse gases harmful to the environment and climate.
In order to use renewable energy as an efficient source of energy, it is necessary to store the excess energy produced, so it can be used later on. Exactly storage is a challenge that researchers and companies have ruminated about for many years.
PhD student Mark Dannemand from DTU Civil Engineering has taken up the challenge in his thesis. His proposal for a solution to the heat storage problems is a salt compound that can be heated and act as a passive hot-water bottle and activated when the need to use the stored energy arises.
“Storage of solar heat is a key component in order to get solar heating systems to cover the needs when the sun is not shining. Therefore there is a need to develop an efficient way to store energy, so we can access it when the need arises, “says Mark Dannemand.
Inspiration from hand-warmers
The idea behind Mark Dannemand’s research was inspired by hand-warmers, which have been on the market since the 1970’s.
The hand-warmers contain a salt compound, are boiled for ten minutes and then charged with heat, which can be triggered when the need arises. The heat is released by breaking a metal plate inside the hand-warmers, which initiates a chemical process, and heat is emitted. This gives you a portable heat storage, which can be triggered when the need arises.
“The principle behind the hand-warmers has been known for a long time, but no one has ever tested the principle in a large scale in practice before. Therefore, I and my colleagues developed a prototype of a heat storage device which I am testing in my thesis, “tells Mark Dannemand.
The heat storage in Mark Dannemand’s tests is conducted by melting the salt compound Sodium Acetate Trihydrate at 58 degrees Celsius. By melting the salt compound, a significant amount of heat is absorbed; this is called the heat of fusion. The salt has the property to cool down to below the melting point to the surrounding temperature without solidifying again. This process is known as supercooling. By using this process, the heat is stored without emitting further heat loss to the environment. When a heat demand arises, a solidification of the salt compound is induced, wherein the heat of fusion is released.
Full-scale test shows great potential
For his PhD thesis, Mark Dannemand has tested two different designs of prototype heat storage devices in a laboratory: one design developed at DTU Civil Engineering, and one developed by an industry partner.
“The tests have demonstrated that the storage principle works in full scale. We have achieved a stable supercooling and thus a stable long-term storage of heat for devices with 220 kg of salt for a period of two months. In order to prevent spontaneous cooling, we have designed and controlled stocks so there is no pressure built up during the heating and cooling process. The heat content of the salt has been optimized by using additives to prevent phase separation of the salt. We have also developed salt mixtures with increased thermal conductivity to create an improved performance, “explains Mark Dannemand.
In the longer run, long-term storage of heat from summer to winter could mean that houses can become self-sufficient in solar energy all year round. Although laboratory tests prove that the storage method works at full scale, there is still a way to go in order to convert method to common practice. With the current method, 2 tons of salt hydrate will be needed to keep a smaller house self-sufficient in just one week during winter.
The research is part of the EU project COMTES which focuses on heat storage.