1. Introduction
The current financial and economic crisis as well as the wider environmental pressures, including climate change and security of energy supply put energy back at the ‘‘heart’’ of the European Union’s (EU) action. In this context, ambitious targets have been set for 2020 (‘‘20–20–20’’ energy policy package) aiming to foster European economy to more sustainable energy paths [1]. This policy is a first resolute step towards achievement of the low-carbon economy ultimate goal, whilst making at the same time the consumed energy more secure, competitive and sustainable.