Experimental investigation of a tank-in-tank heat storage unit utilizing stable supercooling of sodium acetate trihydrate

A cylindrical heat storage prototype was designed to utilize sodium acetate trihydrate (SAT) composite with 2%wt. extra water and 3%wt. of liquid polymeric solution for combined short and long-term heat storage. It was manufactured with inexpensive standard components of water stores. It contained 150 l of SAT composite in the inner tank and 59 l of water in the mantle surrounding the inner tank and in a spiral heat exchanger going through the inner tank. The concept of stable supercooling of SAT and the heat transfer properties of the store filled with water or the SAT composite were studied. Results showed that 27 kWh of heat was stored between 25 °C and 90 °C, where the energy storage capacity of the composite was determined to be 21.3 kWh. This was 76% higher than for a water heat store of the same volume. After a storage period in supercooled state at ambient temperature, 11.5 kWh (long-term capacity) of heat was discharged when the SAT composite solidified. This value corresponds to a heat of fusion of 207 kJ/kg. During charge and discharge in periods with solidification, the heat exchange capacity rates did not change with increase of flow rates. With discharge flow rates of 2 l/min applied in the mantle surrounding, thermal stratification was utilized. Thus, flow temperatures higher than the average SAT composite temperature resulted in liquid state. By additional use of the spiral, the discharge power reached 15 kW. During solidification the heat transfer was constantly decreasing, which resulted in a rather low discharge power. In building applications, heat transfer limitation could be overcome by discontinuous discharge via the mantle with intervals of 2–24 hours. Thus, thermal power of up to 4 kW was achieved and the outlet temperature was close to the average temperature of the SAT composite.