**1. Introduction**

Energy has been a fundamental need of a human society. On the other hand, energy consumption has increased exponentially due to rapid growth in population and modernization [1]. The population of world has grown after Second World War, from two billion to seven billion in the 21st century [2, 3]. Currently fossil fuels are the major source for the primary energy of the world (**Figure 1**) [4, 5].

According to the International Energy Outlook 2013 set by the U.S Energy Information Administration [6, 7], the total energy consumed in 2010 was 5.5282 × 1020 J, which is predicted to rise further to 8.6510 × 1020 J by 2040. Accordingly, the total world energy consumption will grow by 56% between 2010 and 2040; as given in **Figure 2**. The mismatch between the energy supply and energy demand has increased dramatically all over the world.

The limited fossil fuelsand the associated problems such as energy security environmental issueshave emphasized the need for sustainable, reliable renewable energy sources.

**Figure 1.** *Global energy consumption in 2013 [3].*

**Figure 2.** *Total world energy consumption, history and projection [7].*

In the view of the current energy scenario, renewable energy sources could be fantastic choice for the world to meet the increasing energy demand and socio economic development. Renewable energy sources are gaining much attention due to their non-toxicity, biodegradability and low emissions profile as compared to petro diesel [8, 9]. According to US energy information administration, there are seven countries Paraguay (100), Iceland (100%), Costa Rica (99%) Norway (98.5%) Austria (80%), Brazil (75%) and Denmark (69.4%) in the World to have or very near to 100 percent renewable energy sources. Resources of renewable energy are available on large scale such as hydropower, solar, biomass, wind and geothermal energy **Figure 1**. The fossil fuels substitution with renewable energy sources will have very positive effect on greenhouse gases emissions. It has been reported that 2% replacement of fossil fuels with renewable energy sources will result in 1.8% reduction of emissions of CO2 while replacement of 100% will lead to 90% reduction [10]. In the
