4. Conclusion
According to the ReCiPe Endpoint evaluation method, the solar-only operation of the Concentrate Solar Power plant produced the best environmental performance from a life cycle point of view, even considering that the power generation is lower than in the hybrid mode. Hybridisation significantly affected the environmental performance of the plant, while renewable fuels were the best environmental option to hybridise, having less than half the impact than that of fossil fuels. The coal scenario, whose main impacts derived from the coal mining spoil, was the worst case.
Single endpoint score impact presented similar results for the three renewable fuel case scenarios; however, their performance in characterised environmental categories presented significant differences in the categories of human toxicity, climate change, marine ecotoxicity and natural land transformation. Terrestrial acidification in the biomethane scenario was similar to the other biomass derived fuels, yet a high sensitivity has been observed in this category as well as in that of climate change, according to the allocation method for manure digestate. The main contributor for environmental impact in the wheat straw scenario was the cultivation of wheat, representing 25% of single score impact. The main environmental impacts in the wood pellets scenario were derived from the pellets manufacturing process and emissions during combustion. However, natural land transformation was the category most affected due to the impact associated with wood acquisition from forests.
Energy results indicated the lowest Cumulative Energy Demand (1158 MJ/MW h) in the solar-only mode, and hence, the lowest Energy Payback Time (1.44 yr). Cumulative Energy Demand results for the six hybridisation scenarios more than doubled the results for the solar-only mode, increasing its value from 2.4 to 2.7 times. The fuels with more embodied energy were biomethane and wood pellets. The straw scenario had the lowest impacts both in the environmental single point indicator and the energy indicators.