Simultaneous effect of biochar-additive and lightweight heat exchanger on phase change material for low-grade thermal energy storage

In this paper, we report the experimental performance of a low to medium temperature (20–50 °C) latent heat heat storage (LHS) system. The power and temperature ranges of the LHS have relevance to various applications such as heat sink and storage for high-power electronics devices and low-temperature district heating (LTDH). This new LHS includes a lightweight heat exchanger (LHE) with a complex heat transfer area for organic phase change materials (PCMs). The model PCMs provide a medium-low phase transition temperature range, namely decanoic acid (DA) and a commercial PCM. To facilitate the storage performance of the system, biochar (BC) additive is further investigated to simultaneously enhance the thermal properties of the PCM within the LHE. The PCMs are characterized using common material characterization methods such differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy. The system investigation focuses on the effect of the heat exchanger, the PCM type and the additive concentration on the duration and power of charging-discharging cycle as well as the storage capacity. Two weight percentages (1 % and 2 %) of BC were studied, while the addition of 1 % BC to DA resulted in the most effective storage performance in both the system (444 kJ storage capacity) and characterization (170.5 kJ/kg melting enthalpy) experiments. The lightweight structure of the grid enabled loading of a large PCM amount (1.84 kg) as well as swift charging (430 W during 25 min). The system indicated a cooperative enhancement with BC additive as the charging power increased by 32 % (570 W) and charging time decreased by 33 % (17 min). A conceptual modular design of the investigated system is proposed to heat up the floor or the bench of a smart city bus stop. The modular unit of ten integrated LHEs filled with BC enhanced DA is estimated to provide 1.2 kWh storage capacity sufficient for heating the bus floor for 6 h or the bus bench for 15 h. This is a potential solution for reclaiming the excess heat from LTDH substations, while buffering their undesired temperature variations.