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Compost Heat Recovery Systems: Global Warming Potential impact estimation and comparison through a Life Cycle Assessment approach

Compost Heat Recovery Systems (CHRS) represent an innovative technology to provide domestic decentralized thermal energy, recovering the heat naturally produced during the aerobic biodegradation of waste biomass, coming from gardening/farming/forestry activities. CHRSs represent an alternative to centralized grid-connected power systems and are usually installed (combined with most traditional systems) to power underfloor heating systems (UHS) or domestic hot water systems (DHWS), lowering impacts and costs of thermal energy production.
In this study, the Global Warming Potential (GWP) of CHRSs (measured as kgCO2-eq/kWh) was investigated using life cycle assessment (LCA) approach, considering the whole life cycle of an average plant. CHRSs showed a negative Net value of GWP impact, equal to -0.268 kgCO2-eq/kWh, as full balance of positive (0.062 kgCO2-eq/kWh) and negative (-0.329 kgCO2-eq/kWh) emissions. Negative emissions are related to avoided primary materials, replacement of natural gas used as traditional thermal energy production and replacement of mineral fertilizers. Considering only the positive emissions (0.062 kgCO2-eq/kWh), CHRSs emerged to be in line with Solar Hot-Water Systems (0.061 kgCO2-eq/kWh mean value) and slightly higher than Geothermal Systems (0.019 kgCO2-eq/kWh mean value). 
Along with GWP impact, other midpoint and endpoint impact indicators were assessed and all showed a negative Net value: Particulate Matter PM (-2.36E-5 kgPM2.5-eq/kWh), Fresh Water eutrophication FWE (-6.78E-06 kgP-eq/kWh), Fresh Water ecotoxicity FWec (-2.10E-01 CTUe/kWh), Human Toxicity cancer effect HTc (-5.68E-09 CTUh/kWh), Human Toxicity non-cancer effect HTnc (-3.51E-09 CTUh/kWh) and Human Health HH (-5.22E-08 DALY/kWh). These results demonstrate that CHRS is extremely convenient considering both environmental and human health consequences.