An overview of Renewable Energy sources

Introduction

Renewable energy sources are those sources of energy which are replenished back to the nature such as bio mass, solar, wind etc. They sources of energy does not produce harmful effects in the environment.

The demand of energy is increasing day by day however the reserves of fossil fuels are depleting rapidly. According to a study carried out by FORBES, an oil extracting company, the world is left with 54 years of oil. Not only this, the world is facing danger of severe global warming threatening life on this planet in the future. The main reason of the global warming is due to the burning of fossil fuel. According to an Estimate by UK agency REN21, 86.5% of energy consumed on earth is derived from fossil fuels.

Due to the foresaid problems it is the need of time to shift our power needs towards renewable energy sources. In 2013 and 2014 about 9% of the total energy was produced from renewable energy sources. It was divided as 4.2% as heat energy (biomass), 3.8% hydroelectricity and 2% is electricity from wind, solar, geothermal. The world is focusing on to reduce the dependence on fossil fuel and increase the power production from renewable energy sources.

Types of Renewable Energy Sources

There are several types of renewable energy sources which are being researched and investigated for potential power sources in the future. Most promising and common types are discussed as follows

Solar or Photovoltaics

According to an estimate, earth receives about 174,000 (Tera Watt) of power/energy in the form of solar radiation in the upper portion of atmosphere. According to the scientists, about 35% of energy is reflected back to the space while the rest strikes the earth surface. So there is a great potential which needs further research.

The device which converts solar energy into useful electrical energy is called solar cells or photovoltaics. Solar cells in series and parallel combination constitutes solar panels.  Semiconductor materials like silicon or germanium crystals are used to make Solar cells. When sunlight strikes solar cells, portion of light is absorbed by semiconductor material. Due to the absorption of energy, electrons are detached from their nucleus and start flowing freely. Electrons having sufficient energy are able to cross diode junction and passes through the external circuit. Current obtained from these semiconductor devices is very small. To extract sufficient amount of electric power from solar cells, they are connected in series-parallel arrays.

Detail description of solar cells and thermal collector can be found in my solar thermal collector 

Solar or photovoltaic system is composed of three main components. PV array, inverter and battery bank. Output of solar panel is direct current (DC) however electric power used in home and industry is Alternating (AC). So Inverter is connected which converts this DC into AC.  Battery banks are used to store energy in day time for it to be utilized at night time.

Details relating to storage system for photovoltaics are given in my article basics of storage type for solar panels.

Maximum PV output can be obtained when solar panel is placed perpendicular to sunlight. Tracking technology can be added with a PV array to get maximum electrical output from solar energy. PV technologies are divided into three categories depends upon semiconductor material, crystalline silicon, thin film and multi junction. Efficiency of solar panels lies in the range of 0 to 25 percent.

Some advanced countries are already taking advantage of solar energy. They have employed solar panels to generate green energy from sun. The top 10 countries generating power from solar radiations are as under

Total Solar power Production
	Country	MW
1.	Germany	38,200
2.	China	28,199
3.	Japan	23,300
4.	Italy	18,460
5.	United States	18,280
6.	France	5,660
7.	Spain	5,358
8.	UK	5,104
9.	Australia	4,136
10.	Belgium	3,07

Wind Turbines

Wind energy refers to the process of extracting energy from wind’s kinetic energy.  It is a renewable source. Neither it consumes nor its quantity increases or decreases with its use. It is among the clean and non-pollutant renewable energy sources.

Wind turbine is a machine which converts kinetic energy of breeze (wind) into mechanical energy. This mechanical energy is further converted into electrical energy by induction generator. Wind turbine can be categorized into two types based on the turbine rotating axis. Turbine that rotates in a horizontal axis and in a vertical axis.

Turbine can also be classified on the basis of its rating. Small wind turbines are rated up to 10kW. They are used in homes, farms and remote applications. Intermediate turbines ranges from 10 to 250kW similarly large wind turbines have power capacities from 600kW to 1000kW. Large scale wind turbines are very attractive substitute to conventional grid.

Wind turbines are one of the promising renewable sources of energy. They are non-polluting and do not require fuel provision and transport. On small scale cost per kilowatt is also very economical. Apart from many advantages there are few disadvantages like fluctuating wind speed and storage requirements.

Many countries round the world utilize wind mills to produce green energy. Top 10 among them are as follows

#	Nation	2014	2015
–	European Union	128,752	141,579
1	China	114,763	145,104
2	United States	65,879	74,472
3	Germany	39,165	44,947
4	India	22,465	25,088
5	Spain	22,987	23,025
6	United Kingdom	12,440	13,603
7	Canada	9,694	11,205
8	France	9,285	10,358
9	Italy	8,663	8,958
10	Brazil	5,939	8,715

According to the industry experts, wind energy will contribute around one third of total energy consumption of the world in 2050.

Bio-Mass Energy

Biomass is biological material extracted from recently dead or living organisms.  It is a misconception that it only includes plants. It equally applies to animals and material originated from vegetable.

Bio-mass is combustible material. They can be burnt to produce considerable amount of heat.  There are several methods which can be used to generate electricity from bio-mass. Most common method is to produce heat by burning bio-mass. This heat is utilized to rotate turbine of steam generator to produce electric power.

Other options include gasification, pyrolysis, and anaerobic digestion.

• Bio-mass can be converted into gaseous form in the presence of limited oxygen supply needed for combustion. This synthetic gas can be used like natural gas as a source of power.
• Synthetic bio-oil can also be produced by burning bio-mass in the absence of oxygen.
• Bacteria action on the bio-mass can decompose it and convert into renewable natural gas.

Unlike other renewable energy sources, Bio-mass does not produce pure green energy. Due to combustion, some gases are produced whose quantity must be kept with in allowable limits. Research in combustion engineering has already advanced such that emission from such process is always less than allowable limits.

Other renewable sources like solar and wind have the disadvantage that their available solely depends upon nature. However bio-mass has dispatch-ability that is its availability and controlability is similar to fossil fuels.

Working of bio-mass generating unit is explained in bio mass basics and its energy 

Micro turbines

Micro turbines are small turbine engine integrated with generators and power electronics devices. They generate AC power that is rectified by power electronics device. Micro turbines consume fuel both liquid and gases with very low emissions of nitrogen oxides. The operation of micro turbine is very much similar to steam power plant except that air is used instead of water. Combustion takes place by spraying fuel into the air and igniting it. High temperature gas enters a turbine which rotates the shaft. Turbine shaft is used to drive electric generator Efficiency of Micro turbines are just 25-30 percent range which makes it highly uneconomical. However with the advancement in new technologies in combined cycle gas turbine efficiency up to 60 percent can be achieved.

Fuel Cells

Fuel cell is a device which converts chemical energy from fuel into electrical energy through a chemical reaction. The reaction produces electricity, heat and water as a by product. Fuel cells are effective in a way that, they produce almost no toxic waste and have no moving parts. Operating principle of fuel cell is almost like a battery. It will continuously produce electricity as long as fuel is supplied.

Fuel cells are of different types like Proton exchange membrane fuel cells (PEMFCs), Phosphoric acid fuel cells (PAFC) and Solid Oxide fuel cells (SOFC). Primarily fuel cells are used as power sources in remote locations where conventional power source is extravagant. Fuel cells electric vehicles (FCEVs) have been introduced on commercial basis. According to US department of energy fuel cells are 40-60 percent energy efficient.