Many energy sources are not only renewable, but are more efficient alternatives to a conventional approach. There are many forms of renewable energy. Most of these energies depend in one way or another on sunlight. Wind and hydroelectric power are the direct result of differential heating of the Earth's surface which leads to air moving (wind) and precipitation forming as the air is lifted. Solar energy is the direct conversion of sunlight using panels or collectors. Biomass energy is stored sunlight contained in plants. Other renewable energies that do not depend on sunlight are geothermal energy, which is a result of radioactive decay in the crust combined with the original heat of accreting the Earth, and tidal energy, which is a conversion of gravitational energy.



This form of energy relies on the nuclear fusion power from the core of the Sun. This energy can be collected and converted in a few different ways. The range is from simply heating water for domestic use, to the complex technologies of direct conversion of sunlight to electrical energy using mirrors and boilers or photovoltaic cells. Solar energy is one the most resourceful sources of energy for the future. One of the reasons for this is that the total energy we receive each year from the sun is around 35,000 times the total energy used by man. However, about 1/3 of this energy is either absorbed by the outer atmosphere or reflected back into space.Solar energy is presently being used on a smaller scale in furnaces for homes and to heat up swimming pools. On a larger scale use, solar energy could be used to run cars, or power plants. 



Hydroelectric Energy
This form uses the gravitational potential of elevated water that was lifted from the oceans by sunlight. It is not strictly speaking renewable since all reservoirs eventually fill up and require very expensive excavation to become useful again. At this time, most of the available locations for hydroelectric dams are already used in the developed world.


This is the term for energy from plants. Energy in this form is very commonly used throughout the world. Unfortunately the most popular is the burning of trees for cooking and warmth. This process releases copious amounts of carbon dioxide gases into the atmosphere and is a major contributor to unhealthy air in many areas. Some of the more modern forms of biomass energy are methane generation and production of alcohol for automobile fuel and fueling electric power plants. 


Hydrogen and Fuel Cells
These are also not strictly renewable energy resources but are very abundant in availability and are very low in pollution when utilized. Hydrogen can be burned as a fuel, typically in a vehicle, with only water as the combustion product. This clean burning fuel can mean a significant reduction of pollution in cities. Or the hydrogen can be used in fuel cells, which are similar to batteries, to power an electric motor. In either case significant production of hydrogen requires abundant power. Due to the need for energy to produce the initial hydrogen gas, the result is the relocation of pollution from the cities to the power plants. There are several promising methods to produce hydrogen, such as solar power, that may alter this picture drastically. 


Wind Power


Wind power is another alternative energy source that could be used without producing by-products that are harmful to nature. Like solar power, harnessing the wind is highly dependent upon weather and location. The average wind velocity of Earth is around 9 m/sec. And the power that could be produced when a windmill is facing the wind of 10 mi/hr. is around 50 watts.

The fins of a windmill rotate in a vertical plane, which is kept perpendicular to the wind by means of a tail fin. As wind flow crosses the blades of the windmill, it is forced to rotate and can be used to generate electricity. Another type of wind power generator is the two hollow half-drum-type wind collectors. This wind collector rotates in a single vertical axis, making this device independent of the wind direction, which may generate more electricity. 

The movement of the atmosphere is driven by differences of temperature at the Earth's surface due to varying temperatures of the Earth's surface when lit by sunlight. Wind energy can be used to pump water or generate electricity, but requires extensive areal coverage to produce significant amounts of energy.


Geothermal Power

 Geothermal power is energy left over from the original accretion of the planet and augmented by heat from radioactive decay that seeps out slowly everywhere, everyday. In certain areas the geothermal gradient (increase in temperature with depth) is high enough to exploit to generate electricity. This possibility is limited to a few locations on Earth and many technical problems exist that limit its utility. Another form of geothermal energy is Earth energy, a result of the heat storage in the Earth's surface. Soil everywhere tends to stay at a relatively constant temperature, the yearly average, and can be used with heat pumps to heat a building in winter and cool a building in summer. This form of energy can lessen the need for other power to maintain comfortable temperatures in buildings, but cannot be used to produce electricity.  Geothermal energy is obtained from the internal heat of the planet and can be used to generate steam to run a steam turbine. This in turn generates electricity, which is a very useful form of energy. The radius of the Earth is about 4000 miles, with an internal core temperature of about 4000 degrees celsius at the center. The mantle surrounds the outer core and is only about 45 miles below the surface, depending on location. The temperature at the mantle-surface crust boundary is about 375 degrees celsius. 

So, how does this help us? It turns out that if we drill down only three miles we can reach temperatures of 100 degrees, celsius, which is enough to boil water to run a steam-powered electric power plant. Drilling three miles through the earth is possible, but not easy, so luckily there are easier routes to access this power source, known as geothermal hotspots.


Geothermal hotspots are volcanic features that are found all around the world. Basically a hotspot is an area of reduced thickness in the mantle that transmits excess internal heat from the interior of the earth to the outer crust. These hotspots are well known for their unique effects on the surface, such as the volcanic islands of Hawaii, the mineral deposits and geysers in Yellowstone National Park, or the hot springs in Iceland. These geothermal hotspots can easily be used to generate electricity! 

Some systems pump hot water into permeable sedimentary hotspots found underground and then use the steam to generate electricity. Then the used steam is condensed and sent back down to the permeable sedimentary stream. Another system utilizes volcanic magma, which is still partly molten at around 650 degrees celsius, to boil water which would generate electricity. Also there is a system that uses hot dry rock, which is just hardened magma, but still is extremely hot. To recover this heat from these rocks, a system is used which circulates water through the rock and transfers the heat up to a steam generator. The first system listed here is not as useful as other methods because of the acidic nature of the fluids found under the ground. These acidities require a lot of maintenance and upkeep on the equipment, and this cost reduces the economic effectiveness of the system. Therefore, geothermal energy systems are more inefficient than other alternative energy sources because of the costs required in upkeep and the shortage of potential sites.