Lab membrane with food-waste biochar: 96.1% higher heat-storage rate

A paper membrane now carries an unlikely load: carbon made from food waste, graphene and a heat-storing material. In tests, the optimized sheet showed a 96.1% higher heat-storage rate than a pristine paper membrane, while still allowing water vapor to pass through.

The researchers first carbonized mixed food waste at 752°F, then activated the resulting biochar with potassium hydroxide at temperatures between 1,112°F and 1,472°F. The version activated at 1,292°F developed a surface area of 323.1 square meters per gram, with mesopores accounting for 82.8% of its pore structure. Those pores provide room for docosane, a phase-change material that stores heat as it melts and releases it when it solidifies, while helping contain the liquid phase.

Graphene added another layer to the design. The graphene-engineered composites increased latent heat storage by as much as 72% compared with composites made with pristine engineered biochar. The optimized FK7G/C22 composite reached a phase-change enthalpy of 93.1 J/g at 116.1°F and retained up to 90.2% of that enthalpy after 1,000 heating and cooling cycles.

So what changes in practice? A membrane with several jobs could help energy-recovery ventilation systems exchange heat and moisture between incoming and outgoing air, instead of relying only on conventional heat transfer. The paper system retained 80.2% of the bulk composite’s latent heat, and its equivalent air-layer thickness was 0.71, below the reported critical threshold of 1.0 for water-vapor permeability. The researchers also said it met relevant ISO 12572 and ASTM performance criteria.

That is a promising building block, not a product in service. The team says the material could eventually serve in energy-recovery ventilation, building facades and thermal-comfort systems, but larger-scale deployment still requires work on production energy requirements, manufacturing costs and techno-economic performance. The findings were published in Biochar.