Towards decarbonization of the agro-food sector: application of a solar-assisted drying process in a juice factory
The use of heat pumps in industrial processes is gaining wide attention in the last years, since it allows to strongly diversify the resources needed by the industrial sector, representing up to 24% of overall worldwide CO2 emissions and 37% of primary energy consumption. Aim of the present work is to assess, through a technical and environmental analysis, the replacement of a drying process powered by gas boilers with a solarassisted heat pump solution. The industrial process targeted is the drying of a wet biomass, which represents the side product from the juice production. As for the case of citrus peel residues, the dried biomass can be used for animal feeding, pectin or other molecules extraction, and also as fuel for bioenergy systems. Currently, drying is only achieved through gas boilers. The alternative solutions proposed to reduce biomass humidity down to less than 15% is a solar-assisted heat pump process: a high-temperature heat pump, using a low-GWP refrigerant, uses solar heat as the low-temperature source and provides hot air for biomass drying. Electricity for heat pump operation is also supplied by solar energy through PV panels. Starting from real data of the juice factory and experimental data from heat pump testing available in the literature, a TRNSYS model was realised. A sensitivity analysis was carried out, assessing the optimal ratio of solar/PV panels for operation of the heat pump, as well as the optimal sizing of the heat pump to reduce the cost of energy to run the entire drying process. The results indicated that the ratios of 1:1 of solar to heat pump powers is the most suitable one. The average heat pump COP under the examined conditions is 2.9 to 3.2, which allows greater exploitation of the primary energy if compared to the efficiency of gas boilers.