1. German Biogas Association (FVB). Biogas market data in Germany 2016/2017, 〈https://www.biogas.org/edcom/webfvb.nsf/id/DE_Branchenzahlen/$file/17-10-13_Biogasindustryfigures-2016-2017.pdf〉, viewed in June; 2018.
2. European Biogas Association (EBA). EBA Statistical report 2017; 2018. 〈http://european-biogas.eu/2017/12/14/eba-statistical-report-2017-published-soon/〉, viewed in June.
3. German Federal Government. Energy concept for an environmental friendly, reliable and affordable energy supply, Deliberation of the Federal Ministry, September 28th.
4. Tietjen O, Arikas D, Austrup T, Bals C. Warum sich die Energiewende rechnet. Eine Analyse von Kosten und Nutzen der erneuerbaren Energien in Deutschland, Germanwatch e.V., Bonn, p.29, May 2011; 2011.
5. Renewable Energy Sources Act- EEG 2014. Part 4, §44, 21st July 2014, 2014, p. 23.