Finned coil-type energy storage unit using composite inorganic hydrated salt for efficient air source heat pumps

The imbalance between the variable power load during day and night and the energy supply of office building heating can supplement a staggering electricity economy. In this study, a novel composite inorganic hydrated salt phase change material (PCM) was fabricated with a melting temperature of 50.3 °C using CH₃COONa·3H₂O (the primary material), KNO₃ (the eutectic material), Na₂HPO₄·12H₂O (the nucleating agent), and carboxymethyl cellulose (CMC, the thickener) in a mass ratio of 89 %:8 %:1.5 %:1.5 %. Moreover, we developed a modular finned coil-type energy storage unit (ESU) with a PCM charging capacity of 1200 kg and a theoretical heat storage capacity of 315 MJ. Subsequently, we created an ESU test system for an air source heat pump (ASHP) operated at the valley electricity period from 23:00 to 7:00. The heat storage, heat release, and defrosting functions were tested, and the operational cost of heating for the office building was evaluated. The results reveal that the composite PCM exhibited stable thermal properties, but more accurate working medium flow matching is required to improve the heat transfer tail phenomenon. Furthermore, the ASHP-ESU reduces the defrosting time by 63 % and improves the efficiency by 6–9 % compared to conventional ASHP. The operating costs are the lowest at 4.66 CNY/month/m² with valley electricity operation compared with the other three heating modes. Therefore, the ASHP-ESU system for office building heating in off-peak operation is an economical and sustainable approach for the power system.