Municipal sludge-based low-carbon phase-change composite towards energy storage: Fabrication and investigation

• Municipal sludge is used to produce phase-change composite. • Optimum mass ratio of skeleton material to KNO 3 is 5 to 5. • Maximal energy storage density of 330.34 J/g is reached for optimum mass ratio. • There is excellent chemical compatibility between components. • Phase-change composites fabricated in this work has low cost and carbon emission. The integration of municipal sludge with phase change materials for composite energy storage material fabrication benefits to sludge significant reduction and recycling, heavy metal fixation, and minimizing environmental pollution. This groundbreaking work proposed municipal sludge-derived phase-change composites utilizing potassium nitrate as phase change material to produce cost-effective, low-carbon thermal energy storage materials. Five samples with varying mass fractions of municipal sludge incineration ash and potassium nitrate were prepared through a cold-compression & hot-sintering process and then thermal properties, microscopic structure, mechanical strength, and chemical compatibility between components was investigated extensively to explore the feasibility of municipal sludge recycling for composite energy storage material fabrication. The results revealed successful encapsulation of heavy metals within the phase change composites, and the composite with a 50:50 ratio of ash to potassium nitrate exhibited superior overall performance, which boasts a thermal energy storage capacity of 330.34 J/g with temperature rising from 100 up to 380 °C, a peak thermal conductivity of 1.04 W/(m·K), remarkable mechanical strength of 153.78 MPa, and excellent thermal stability. In addition, the components of the municipal sludge incineration ash and potassium nitrate displayed exceptional chemical compatibility. This novel phase change composite holds significant potential for engineering application compared to traditional alternatives.