Scientists from Finland’s Aalto University have created a prototype of an unusual energy storage device that uses ordinary quartz sand instead of lithium-ion batteries. This device is a so-called Carnot battery, in which electricity is converted into heat, after which the stored heat is reused to generate energy. The new battery uses sand as a heat accumulator. After sand gets heated by electric heating elements inside an insulated tank, the accumulated heat activates a Stirling engine, which converts the temperature difference into electricity.
For the purposes of their experiment, the researchers assembled a unit with a power output of about 1 kW. It consisted of a 0.2-cubic-meter tank containing quartz sand with grain sizes ranging from 0.6 mm to 2 mm. A commercial Stirling engine made by Microgen served as an energy converter, with standard sauna heating elements used for heating.
During tests, the sand was heated to 300–350°C, after which it gradually transferred its heat to the engine. At 300°C, the unit could generate electricity for about 9 hours, and at 350°C, it produced electricity for up to 14 hours. Its peak output reached 690 W.
So far, the system’s overall efficiency has been relatively low. During experiments, the scientists only managed to convert 4.4–8.3% of the energy expended into electricity. As a point of comparison, modern lithium-ion batteries typically return over 90% of their stored energy. The disparity turned out to be caused not by the sand itself, but by heat losses. A significant portion of the energy was lost through the unit’s walls and the engine cooling system. For instance, after receiving 53 kWh of electricity in one cycle, the system returned only 2.33 kWh. Another 16.5 kWh was used to cool the engine, and over 34 kWh was simply dissipated into the environment.
Another problem is the low thermal conductivity of sand. Heat within it spread slowly, as a result of which the engine periodically experienced a lack of heat flow and would shut down temporarily. To solve this problem, the scientists are proposing the addition of metal elements or special heat-conducting structures to the sand in order to accelerate heat transfer to the engine.
Nevertheless, the Carnot sand battery concept has demonstrated its viability and could, according to the researchers, become significantly more efficient with further design refinement. Overall, sand has proven to be a good material for heat storage: it is inexpensive, widely available and non-toxic, and it can withstand temperatures exceeding 1,700°C without degradation. Furthermore, the service life of these systems could potentially be much longer than that of chemical batteries.
