The potential of renewable or green energy sources is enormous that can meet the increasing demand of energy worldwide. Brief introduction to different green energy resources is as fallow.
Biomass is defined as organic material from plants, algae, trees and crops which is generated as a result of photosynthetic activity by using sun’s energy. Biomass energy is utilization of biomass into useful forms of energy such as heat by direct or indirect burning, electricity generation and conversion of biomass into liquid fuels.
Biomass for bioenergy is obtained directly form land as well as from energy crops or from leftover residual waste material generated during processing of food or other material such as pulp and paper from wood industry. Other sources for biomass are demolition wood from buildings, pallets and Municipal Solid Waste. The conversion of biomass to bioenergy system is the flow of solar generated materials in a proper managed path that include the synthesis of intermediate energy carriers called biofuels such as charcoal, ethanol, or biogas .
Biomass has low energy density as compared to coal which is 28 GJ/ton, mineral oil has 42 GJ/ton, and liquid natural gas has 52 GJ/ton whereas biomass has only 8 GJ/ton of energy density. The solar energy conversion efficiency of plant is low which is 1 percent in real practice.
Efficient and reliable systems are required for storage of energy. One such system is solar hotwater where solar energy is extensively used in water heating. Solar water heaters are getting popularity as they are inexpensive and simple to fabricate and maintain. Solar water heater can avoid 50 tons of carbon dioxide emissions in 20 years period.
Solar water heaters can be classified into three types on the basis of energy collectors that are flat plate, evacuated tube, and concentrating collectors. Solar water heaters can be active and passive that is based on operating conditions. Active system has electric pump to circulate fluid however passive system does not have that pump.
The efficiency of solar water heater is directly related to efficiency of collector which is defined as the ratio of the amount of energy transferred from collector to the heat transfer medium to the incident radiant energy on the collector.
Electrical energy can be produced from various sources such as nuclear, coal, diesel etc and renewable sources like wind, solar and geothermal energy as well. Many power plants use steam to generate electricity, Most power plants use fossil fuel, and coal to boil water and make steam. For geothermal power plants steam is generated by utilizing reservoirs of hot water found few miles below Earth’s surface.
Steam temperature for fossil fuel driven electrical plant is 550 0C whereas the steam temperature of geothermal plants ranges from 80-250 0C. This temperature variation is the possible reason for low efficiency of geothermal electricity generation which is 10% to 17% and 3 times lower then fossil fuel driven steam electric plants.
Hydropower is word that is derived from Hydrogen oxide (water) and power (electricity generation). Electricity generated by the utilization of conversion of potential energy of water stored to kinetic energy which helps to run turbines and operate huge generators. Hydropower is the largest known renewable energy resources used for electricity. About 700GW of hdro capacity is in operation worldwide. There are two types of hydropower plants, large scale hydro plants and small scale hydro plants.
Hydropower is the most efficient way to generate electricity as modern hydro turbines has ability to convert 90% of energy into electricity whereas in comparison to that only fossil fuel plants are 50% efficient.
Ethanol is clean burning high efficiency an alcohol which is synthesized from biomass using fermentation process and can replace conventional liquid petroleum fuels such as gasoline or kerosene. Many countries are producing ethanol from food crops like corn as well as sugarcane. Ethanol is mostly used as an additive to boost octane level or reduce the use of fossil fuel. The production of ethanol from lignocellulosic biomass such as wood, straw and grasses is under consideration and is being given serious attention.
An alternative of diesel fuel is Biodiesel which is synthesized from vegetable oil or animal fats. Biodiesel is biodegradable and nontoxic as it does not release sulphur on combustion that is why it is recognized as beneficial for environment. Methods that are used to synthesize biodiesel include Direct use and Blending, Microemulsion Thermal cracking also called as pyrolysis and Tansesterification or Alcoholysis. The cost of biodiesel is the main factor that limit its commercialization
Wind is a clean energy source widely available throughout the world and is producing no pollution during electricity or power generation. Wind energy is the most cost competitive renewable green energy technologies. Cost of electricity generation has through wind has fallen by more than 80%. Powerful wind is required to generate electricity from wind miles. Some problems that are associated wind is siting wind turbines, acoustic noise emission, visual impact on the landscape, impact on bird’s life, shadow caused by rotor and electromagnetic interference for radio, TV and radar signals.
Solar energy can be used by two different methods which are first solar photovoltaic (PV) modules that covert sunlight into electricity directly and solar thermal power systems that is used to produce steam from water which is then used to run turbine for electricity generation. Solar PV are solid state semiconductors also called as silicon cells. They are widely used for satellites. Globally PV module production is 150 MW. Recent research has decreased the cost for PV modules production. Solar thermal power system needs intermediate fluid to transfer heat and boil water to generate steam and run conventional turbines 
Energy from oceans can be obtained in the form of heat, water currents, waves and tides. Different technologies have been proposed for the conversions of thermal, wave, tidal and offshore wind energy of oceans. Renewable energy sources from oceans can be exploited without harming the marine environment if projects are sited appropriately .