The basic raw material of this new material is a xylitol-like sugar alcohol whose thermophysical properties can be altered with the help of polymers.
would be possible to provide the entire heat supply required for a passive house using solar power. Heat storage could also be used in district heating areas. In such cases, the heat stores could be placed in the buildings connected to a district heating network. The stores would be loaded using heat generated at combined heat and power plants in summer, and unloaded in wintertime. The material also has potential in the storage of waste heat generated by industry. Fully-charged heat modules could then be transported to different locations according to need.
An exceptional material
The team’s invention takes advantage of the latent heat involved in phase changes that is characteristic of all materials. Latent heat is the release or absorption of thermal energy. Seppälä illustrates the phenomenon using a familiar example. When ice melts into water, its state changes from solid to liquid. Thermal energy is bound into the water in this conjunction. When the ice once again freezes, the phase transition releases heat. The material now invented behaves exceptionally. When the liquid material cools down, it does not crystallise once it goes below the point of freezing, nor does it release the heat stored during thawing. Instead, the heat is stored in a glass-like, amorphous substance that forms during the process. When you want to convert the material back from the amorphous into the crystalline form, it needs to, somewhat surprisingly, be heated a little bit. Once sufficient “starter heat” has been added, the material begins to crystallize by itself, and thus to release the thermal energy stored in it. In other words, launching the crystallising process requires some
heat, but even this can be taken advantage of. “A material like this has not been developed before, and we don’t know much about its behaviour. Some pure polymers behave similarly, but they lack a correspondingly large energy content. Some studies have observed water bound in polymers also behaving in a similar manner. These cases have, however, only involved smallish amounts of water, and the researchers didn’t manage to get the phenomenon to repeat itself in consecutive crystallisation-thawing cycles, unlike our material,” Seppälä says. The principal components of the triumphant material are erythritol, a sweetener favoured by dieters, whose thermophysical behaviour is altered with a cross-bridged polymer. The material is, in light of current knowledge, safe: erythritol is edible, and the polymer in question is used in diapers. The material is not corrosive. “It’s inexpensive, the present price is around a euro per kilo, but larger applications would require substantial amounts of it. Growing demand could lower prices, however. It should be possible to manufacture erythritol more cheaply in the future from, for example, the sidestreams of biochemical processes.”
Chemistry and physics
Development of the material commenced about a decade ago, when Ari Seppälä’s interest got piqued by the mitt en heaters that had appeared on the market. These heaters contain salt hydrates, whose crystallisation and associated heat-formation is started by clicking a thin metal plate contained in the product. Closer research demonstrated that the salt hydrates lost their operating power in recurring use and would not function at all on a larger scale. The temperature they release is likewise too little to be of use in many applications. The research got a shot in the arm about four years ago, when chemical engineering postgraduate student Salla Puupponen joined the team. She started a doctoral research project with the aim of developing new materials to store thermal energy. The breakthrough was achieved about a year ago. “Salla Puupponen brought her excellent chemistry expertise to the team. However, she transferred to work as a researcher in the business sector this Konsta Turunen (left) and Salla Puupponen observe how the material behaves at different temperatures.
spring, which was a major loss for us. We need another talented chemist who is passionate about research to replace her,” Seppälä says. Follow-up research aims to scale the size of the heat store from the present droplet size to something bigger. Another aim is to increase the material’s energy storage density even further. After this, it will be time to run some pilot trials. Furthermore, the behaviour of the material is associated with some poorly known physical and chemical phenomena, whose study should be inspiring for scientists who focus on fundamental research. Seppälä believes the research will also yield practical applications in the end. These may well be of a shape that differs from what is being imagined now. The phenomena associated with this material are still so new that it is impossible to envision the final results with any accuracy. •
— Helsinki Challenge 2017 The idea competition was realised as a cooperation between ten Finnish universities. Its goal is to discover new, sciencebased solutions to some of the world’s most difficult problems. The competition’s themes – people in change, a sustainable planet and urban future – are based on the UN’s sustainable development objectives. Some 110 proposals were submitted to the competition towards the end of 2016. Following an initial elimination round, the jury selected 20 semifinalists for an accelerator programme. Six teams were named for the final stage of the competition, in addition to one finalist who was voted in by the public. The HeatStock and iCombine teams shared the first prize, meaning each team will receive €187 500 towards the realisation of their idea. The Aalto-led HeatStock team has developed a new kind of material for the longterm storage of thermal energy. The team members are Ari Seppälä, Salla Puupponen, Konsta Turunen, Olli Vartia and Kari Saari from Aalto University, Leena Hupa and Daniel Lindberg from Åbo Akademi University, Kirsi Jouppila from the University of Helsinki as well as Ilkka Hippinen and Kati Laakso of Motiva. The University of Helsinki-led iCombine team is developing a mathematical model, which will help doctors identify optimal medical therapies for cancer patients. AALTO UNIVERSITY MAGAZINE 20 \ 35