Due to continuously growing energy demand and environment protection renewable energy sources would be an alternative to conventional energy sources like fossil fuels (coal, petroleum oil, natural and shale gas). The simplest definition of a renewable energy source (RES) is: a source that delivers energy from  naturally replenished (renewable) energy. There are several main kinds of RES, including: biomass, wind energy, solar energy, hydropower, geothermal energy.

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Biomass

All plants utilize solar radiation, CO2, water and minerals to grow. For this reason, in case of energy, biomass produced by plants is RES. However, biomass is raw material for energy production, there are many different pathways of biomass conversion to energy like: small scale combustion, large industrial combustion units, gasification, pyrolysis, fermentation or estrification. The energy of biomass can be used as heat, electricity and transportation fuel.

Wind energy

Wind energy entirely avoids CO2 emission. It has no resource constraints: the ‘fuel’ is free and endless. The rotors of most wind turbines face into the wind and actively deflect to follow changes in wind direction. Wind farm can be as small as a single turbine or as large as several hundred megawatts. Sited in the area of good wind energy potential, a single 1 MW turbine can power 600 households. Disadvantage of wind turbine is non-continuous work with strong dependency on wind velocity. Main problem with wind farms development in Central European regions is condition of power grids   which can be overloaded and are not suited to great amount of electricity produced during high wind velocity.

“The European Commission is convinced that there is a huge potential for wind energy in Europe", Christopher Jones, European Commission, Introduction to Pure Power, November 2009.

Solar energy

The sun energy can be used in two main ways. First is utilization of solar radiation to heat water for heat and electricity production. Second can be direct conversion of light into electricity at the atomic level (photovoltaic).

Photovoltaic (PV) systems use cells made of semi-conducting materials to convert light into electricity. Some materials exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity. Solar thermal systems (STE) are based on a simple principle known for centuries: the sun heats up water contained in a dark vessel. Solar thermal technologies are efficient and highly reliable, providing solar energy for a wide range of applications like domestic hot water and space heating in residential and commercial buildings, industrial process heat, desalination and swimming pools.

Hydropower

Mechanical power of water flow can be simply transformed into electricity without any air pollution. Small hydropower stations are widely distributed across many countries in the European Union. The principle of hydropower operation is based on a simple process, taking advantage of the kinetic energy and pressure freed by falling water of rivers, canals, streams and water networks. The water drives a turbine, which converts the water’s pressure and motion into mechanical energy. Then the mechanical energy is converted into electricity by a generator. Generated power is proportional to   the difference between up- and downstream of water levels , the quantity of water which goes through the turbines in a unit of time, and finally to the efficiency of the turbine.

Geothermal energy

Earth’s inner energy which is accumulated in rocks and groundwater can be used for heat and electricity production. Resources of geothermal energy range from shallow hot springs, hot rocks occurring a few kilometers beneath the Earth's surface to the molten rocks having extremely high temperatures, located even deeper. Geothermal energy is called a renewable energy source because groundwater is replenished by rainfall and the heat is continuously produced deep within the Earth.

Geologists use different methods to find geothermal reservoirs, but the only way to be sure there is a reservoir is to drill a well and measure the temperature in it.  Deep drillings are still very expensive, so for this reason the most popular application is using hot springs  for heating purposes.

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