Payback times and multiple midpoint/endpoint impact categories about Building-Integrated Solar Thermal (BIST) collectors.

The purpose of the present article is the evaluation, by means of life cycle assessment, of a system which consists of vacuum-tube solar thermal collectors. The system is appropriate for building integration and it has been developed in France. The methods ReCiPe and USEtox have been adopted. Regarding life-cycle results, according to the scenario "without recycling" and for 30-year system lifespan, ReCiPe payback time was calculated to be 18.14 years based on France's electricity mix whereas by using Spain's electricity mix (hypothetical scenario) it was found to be 4.03 years. Recycling offers a ReCiPe-payback time reduction of 2.66 years based on France's electricity mix and 0.59 years based on Spain's electricity mix. All the studied cases show ReCiPe payback times much lower than an assumed system-lifespan of 30 years. On the basis of ReCiPe midpoint and by considering material manufacturing of the 16 collectors and the additional elements of the system (scenario "without recycling"), among glass-, aluminium-, copper- and steel-based components, the copper-based ones present the highest impact in 15 of the 18 impact categories. For instance, for Freshwater eutrophication, the copper-based elements have a score that is around 30 times higher comparing to that of the aluminium-based ones. The USEtox findings, for the material manufacturing of the 16 collectors and the supplementary elements of the system and for the scenario "without recycling", reveal that the material with the highest total score in terms of: i) human toxicity/cancer is copper (6.7E-09 CTU), ii) human toxicity non-cancer is propylene glycol (4.0E-08 CTU), iii) ecotoxicity is copper (2.06 CTU). Recycling of the metals, according to USEtox, offers an impact reduction of 20-95%. A discussion about factors that influence the environmental profile of building-integrated solar systems is also provided.