Silviya Boycheva

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1. Introduction The growing demands of modern society for an increasingly high standard of living, together with intensive industrialization, have led to a high consumption of energy and natural resources.

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited. For more information, please refer to https://www.mdpi.com/openaccess.