1. EU. Council Directive 2009/119/EC of Sep. 14, 2009 imposing an obligation on member states to maintain minimum stocks of crude oil and/or petroleum products. Brussels, Belgium: European Union; 2009.
2. EU. Regulation (EU) No. 994/2010 of the European Parliament and of the Council of Oct. 20, 2010 concerning measures to safeguard security of gas supply and repealing Council Directive 2004/67/EC. Brussels, Belgium: European Union; 2010.
3. EC. The future role and challenges of energy storage. DG ENER Working Paper. Brussels, Belgium: European Commission; 2013. http://ec.europa.eu/energy/sites/ener/files/energy_storage.pdf
4. Sachverständigenrat für Umweltfragen. 100% erneuerbare Stromversorgung bis 2050: klimaverträglich, sicher, bezahlbar. Berlin: Stellungnahme; 2010, pp. 59.
5. Gay FW. Means for storing fluids for power generation. US Patent 2,433,896; 1948.