Lately, renewable energy systems are increasingly being used for electricity generation, either at small-scale decentralized systems with capacity in the kW scale or even medium-scale systems (often called utility-scale) with capacity of a few MW.
However, the large-scale systems with capacity of some hundreds of MW are still using conventional technologies based on fossil fuels (natural gas, coal, oil, lignite, etc.).
These very large plants operate at high loads (operation at a range of 50 % to 100 % of their capacity), having high capacity factors (operation of many hours annually), and covering the base load needs of the electricity grid.
One of the most important disadvantages of conventional technologies is the environmental impact.
The combustion of fossil fuels leads to the inevitable production of carbon dioxide (CO2), while most of the times harmful emissions are produced, such as carbon monoxide (CO), nitrogen oxides (NOX), sulfur oxides (SOX), unburned hydrocarbons (HC), and solid particles.
Another critical disadvantage of conventional technologies is that they need continuous fuel supply to operate, which contributes to the operating costs. This cost depends on various local and global parameters, such as fuel availability and type, fuel purity, world economic conditions, local prices, etc.
On the other hand, renewable energy technologies do not require any fossil fuel during normal operation. Their operation is based on the exploitation of natural resources, such as the sun and wind, having relatively lower operating costs, although they still require some maintenance.
The most important disadvantage of renewable energy technologies is the fluctuation of their power output, which depends on the intermittency of the particular natural resource, which can have a significant variation, even on an hour-to-hour basis.
This aspect has brought many second thoughts and skepticism on their wider implementation when energy supply must be secured, irrespective of any possible fluctuations in renewable energy resources availability.
Additionally, the installation cost (in terms of cost per installed kW) of renewable technologies is usually much higher than that of fossil fuel fired plants, bringing some restrictions on their scale and their total capacities for electricity generation.
Although this cost is rapidly decreasing during the recent years, securing a lower specific energy cost (cost per kWh), and slowly approaching grid parity (specific cost equal to the electricity price for an end-user), there are still many issues to be resolved, such as excess energy storage and their safe and efficient integration to the electricity grids.
