cnrs I international w 24 | Focus magazine Lime, which releases heat when in contact with moisture, is one of the reagents currently under investigation. The idea is to dry lime during the summer and re- humidify it in the winter, to heat buildings, for example. Between the two seasons, lime can be stored with no heat loss, as long as it is shielded against humidity. Thermochemical storage, although a rather complex process whose reactions need to be finely controlled, has many advantages. Compared to other methods, it stores more heat and releases energy at higher temperatures than those needed to stabilize the chemical reagent. Moreover, the reaction can be used to produce both heat and cold. The PROMES researchers have perfected a device that can generate temperatures ranging from –30°C to 300°C using liquid ammonia and a reactive salt. A small quantity of ammonia spontaneously evaporates and reacts with the salt, leading to heat production. This heat induces an increase in ammonia evaporation, which produces cold. “With this process, in contrast to many other solutions, the temperature can be finely controlled by adjusting the pressure level and using specific types of reactive salts,” Py emphasizes. The PROMES device is now being used to cool blood transport boxes, aircondition buildings, and keep food hot or cold on a meal tray. These are only but a few of the many applications that thermal storage will find in our daily lives. J. B. 01. Procédés, matériaux et énergie solaire (CNRS / Université de Perpignan V ia Domitia). 02. A ccording to the International Energy A gency, concentrated solar power should account for 10% of the world’s electrical production by 2050. These solar reactors (13) are used to vitrify the ashes of hazardous industrial waste, resulting in ceramics (14) capable of storing very hightemperature heat energy. 15 Heat storage units made from asbestos-containing wastes. ous industrial waste like asbestos, ashes from incineration plants, or metallurgical wastes. Melted at 1400°C, which renders them wholly inert, these substances are thus recycled for a useful purpose. “We have calculated that the energy costs of manufacturing these materials would be recovered in less than a year of use in a solar power plant,” Py reports. “Furthermore, our ceramics are able to absorb heat up to 1000°C, whereas nitrate salts start to decompose at 600°C.” This is a considerable advantage given that future solar power plants are expected to generate higher temperatures than today, on the order of 900°C. SELF-HEAT ING WA LLS Ceramics could also find other functions. In our homes, integrated into the walls, they could be used to collect ambient heat during the day and release it at night. Indeed, as part of the general effort to save energy, effective heat storage for housing has become a priority. So far, the preferred method is to use phase-change materials (PCMs), which transform from a solid to a liquid state when exposed to heat. Paraffin, for example, the main ingredient in candles, melts at about 70°C. 1 cm © p hotos 13-14-15 : CNRS-PROM ES 13 14 15 Contact i nformation: Xavier Py > py@univ-perp.fr © c old way 16 Coldway, a company launched by CNRS researchers, has developed a thermochemical cooling system that can be used to store heat-sensitive products. 16 The objective is to efficiently recover the heat released as the substance returns to its solid state. Some buildings are already fitted with PCM technology: microcapsules of paraffin embedded in the walls absorb the room’s heat in the daytime and release it at night. The primary advantage of PCMs is their high storage capacity and their stability. They absorb more heat per volume than other materials, and they release it at a constant level corresponding to their phase-change temperature. MORE CHALLENGES AHEAD Unfortunately, PCMs are not always compatible with the temperatures required for a given application. Furthermore, being poor conductors, they cannot store or release heat rapidly. “To broaden the range of accessible temperatures, research is focusing on the development of new materials,” Py explains. “Some polymers or polyols are now available that can store heat between 20°C and 200°C. At the same time, researchers are trying to improve the materials’ thermal conductivity, for example by adding graphite, which is an extremely effective heat transport medium.” Storing heat during the day to release it at night is already a significant step forward, but it is not enough. In the future, the main challenge will be storing surplus thermal energy produced in summer for winter use. To achieve this, researchers are developing heat storage systems based on chemical reactions.
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