1. Introduction In the last 20 years considerable changes have been occurring on the energy markets. The traditional public management has been gradually replaced by private management and the traditional utility monopolies have given place to free and competitive markets. In the European Union (EU), in particular, important steps have been taken to meet liberalisation objectives in the energy market (Ferreira et al., 2005). The idea is to improve economic efficiency and service quality, to guarantee provision and energy availability in better economic conditions, to provide consumers free access to alternative supply sources and to provide a better environmental protection. All these objectives were set on the 96/92/EC and 2003/54/EC Directives and latter on the III Energy Package approved in 2009. Simultaneously, the evolution of the EU-27 energy system has been characterised by a decoupling between energy demand and GHG emissions growth. Between 1997 and 2007 total GHG emissions decreased by 1.2% while final energy consumption grew by 5%. In the same period, final electricity consumption per capita also rose by 17%, continuing the trend observed in preceding years (Eurostat, 2009). This electricity demand is expected to grow about twice faster than the average energy demand (Perez-Arriaga et al., 2005). Thus, further investments in new generation capacity will be needed during the next years. Indeed, between 1997 and 2007 both total installed capacity of electricity generation and power generation increased by 18%. In 2007, thermal power plants provided the majority of capacity--with a share of 58% -, followed by hydro (18%) and nuclear plants (17%). Despite the strongly increase, the renewable energy sources (RES) still contribute to only 7% of total installed capacity of electricity generation. The most important change was registered by wind capacity, although hydro power still remains the largest contributor, with a 62% share of RES, in 2007, followed by wind energy (25%). The power generation showed a similar trend, with thermal power plants contributing for the majority of generation (56%) and RES for only 6%, in 2007. Although electricity generated from RES had verified a 41% increase, between 1997 and 2007, its contribution to total electricity consumption has only increased by 19% over this period, reaching 15.6% in 2007. Furthermore, total energy dependency rate has increased by 8.1 percentage points. The EU-27 is highly dependent on oil (82.6%), natural gas (60.3%) and hard coal and derivatives (58.6%) and the forecasts show the same trend for the next decades (Eurostat, 2009). Additionally, there is an accelerating decline of fossil fuel domestic resources. Consequently, most of the energy consumption growth will need to be met by increasing imports from outside the EU. Conversely, most environmental pressures show an improving trend. From 1990 to 2007 GHG emissions decreased by 9.3% slowing down (0.2%) between 2000 and 2007 (Eurostat, 2009). The changes operated in the fuel mix during that period, together with the restructuring of the Eastern European economies, were the key drivers for this improvement. This environmental enhancement has though an important exception. Total carbon emissions are expected to rise at a short-term rate of 0.3% per year, accelerating from 2015 onwards to a long-term rate of 0.5% per year (Perez-Arriaga et al., 2005). Because the anthropogenic sources of carbon dioxide to the atmosphere are dominated by fossil fuel use, energy and electricity are the main components for the mitigation of those emissions (Edmonds et al., 2006). In Portugal, energy-related activities are the major sources of Greenhouse Gases (GHG) emissions, accounting in 2008, for 70.8% of total emissions, presenting an increase of 37.4% over the 1990-2008 period. By far the most important gas emitted by this sector is C[O.sub.2], with 97.2% of sector emissions (APA, 2010). …