Clean Energy Options for Power Generation

'Clean Energy' focuses on major types of Renewable energy available to us, taking into account a future involving a reduction in fossil fuel consumption and the rapidly increasing carbon dioxide in our atmosphere. Efforts to harness the renewable energy catching up with especially developed countries, the genesis of the problem should not be neglected at this juncture. Stringent measures have to be adopted to monitor and control the population explosion on the planet earth.

Geothermal Energy

Geothermal energy is one of the cleaner forms of energy now available in limited quantity. Use of geothermal energy greatly reduces greenhouse gas emissions and other forms of air pollution. Most power plants use steam to generate electricity and this steam rotates a turbine that activates a generator, resulting in the production of electricity. Many power plants still use fossil fuels to generate the required quantity of steam. Geothermal power plants, however, use steam produced from reservoirs of hot water found 10 to 15 kilometers beneath the earth surface. There are three types of geothermal power plants: Dry steam, Flash steam, and Binary cycle.

Dry steam power plants draw steam from underground resources directly. The steam so drawn is directed into a turbine, generator assembly to produce electricity. Flash steam power plants use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine, generator assembly. Binary cycle power plants operate on water at lower temperatures of about 225°-360°F (107°-182°C). These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine.

Geothermal power plants use steam produced from reservoirs of hot water found 10 to 15 kilometers beneath the earth surface. There are three types of geothermal power plants: Dry steam, Flash steam, and Binary cycle.

Temperature increases below the earth surface at the rate of about 300C per kilometer in the first ten kilometers. This internal heat of the earth is the infinite store of energy that can be tapped. It is not oil, coal or natural gas but the energy from the rocky, hot interior of 4200C or more. This intense heat, known as the 'Geothermal Energy' turns underground water into the steam that erupts from the geysers such as those found in Philippines. It also powers steam turbines built over these geysers. Holes or bores drilled 15 kilometer deep into the ground can reach Geothermal energy and create steam anywhere a power plant is needed.

Tapping of Geothermal energy can done using single bore of about 1 meter diameter or multiple bores of smaller diameter through which cold water is made to go down through these holes and returns to the surface as high pressure steam. This steam is used to generate the electricity subsequently. Existing technology does not yet allow recovery of heat directly from magma, the very deep and most powerful resource of geothermal energy.

Solar Energy

Solar energy technologies use the sun's energy and light to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and
industry. There are a variety of technologies that have been developed to take advantage of solar energy."

The world's solar resource is huge, with the total radiation intercepted being approximately 8000 times the total primary energy needs of humanity. As a diffuse resource, with around 1000 watts falling on a square meter of earth surface at any given day time, the real challenge is to capture this abundant energy and convert it cost effectively. Solar energy technologies use the sun's energy and light to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. There are a variety of technologies that have been developed to take advantage of solar energy. These include:

The Photovoltaic process is the direct conversion of solar radiation into electricity by the Photovoltaic Cells. Considerable research and development is being undertaken to improve the technology and reduce the manufacturing costs. They are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic (PV) effect.

Solar cells are typically combined into modules that hold about 40 cells; a number of these modules are mounted in PV arrays that can measure up to several meters on a side. These flat-plate PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. Several connected PV arrays can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system.

Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Thin film technology has made it possible for solar cells to now double as rooftop shingles, roof tiles, building facades, or the glazing for skylights or atria. The solar cell version of items such as shingles offer the same protection and durability as ordinary asphalt shingles.

Some solar cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. This approach has both advantages and disadvantages compared with flat-plate PV arrays.

The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. But because the lenses must be pointed at the sun, the use of concentrating collectors is limited to the sunniest parts of the country.

Some concentrating collectors are designed to be mounted on simple tracking devices, but most require sophisticated tracking devices, which further limit their use to electric utilities, industries, and large buildings.

The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain energies will work efficiently to create electricity, and much of it is reflected or absorbed by the material that makes up the cell. Because of this, a typical commercial solar cell has an efficiency of 15%-about one-sixth of the sunlight striking the cell generates electricity. Low efficiencies mean that larger arrays are needed, and that means higher cost. Improving solar cell efficiencies while holding down the cost per cell is an important goal of the PV industry, NREL researchers, and other U.S. Department of Energy (DOE) laboratories, and they have made significant progress. The first solar cells, built in the 1950s, had efficiencies of less than 4%.

Photovoltic Solar Panels (1.5mm thick)-from SOPREMA, CANADA.

Mr. JK Chauhan, MD, of MJ Techno consultants, representing SOPREMA, Canada, says; there are two primary PV panel technologies being heavily deployed right now. There is thin-film (or amorphous silicon) and there is hard-panel (or crystalline silicon).

Crystalline panels are either mono or poly crystalline. They vary in size and wattage, ranging from 100 to 300 watts per panel. The average sized panel used is 230 watts. They do have more watts per square foot, but they are also heavier and require a racking system be mounted on the roof surface. The racking is either attached to structural supports through the roof or by using ballasts.

Thin-film panels perform extremely well in high wind conditions. They are also much more hail resistant versus than the glass of the hard panels while maintaining their durable and flexible characteristics. Thin-film is also extremely lightweight at only 0.7 pounds per square foot, versus 5-6 pounds per square foot for hard panels and 12-14 pounds per square foot for hard panel ballasted solutions.

Two of the most significant differences between thin-film and hard panel is this film's greater output at lower light levels and in high heat plus better performance in shaded or soiled conditions. It has been scientifically proven that amorphous silicon outperforms crystalline panels in high heat conditions. The power output of hard panels decreases as the temperature rises, while the power output of thin-film remains constant. Thin-Film also produces more power in low light conditions. The reason thin film is less affected by shading and soiling is due to the bi-pass diodes design. Each thin-film panel has 22 cells and there is a bypass diode between every cell. Therefore, if there is some shading on one of the cells, only the power output of that single cell is affected. Hard panels typically have 72 smaller cells and have only 2 bi-pass diodes between the three sets of 24 cells. If there is some shading, the bi-pass diode will trip and reduce the power output of the entire hard panel by one-third.

Below is a plot graph showing the impact on power output from increases in temperature, for two different types of hard panels and the thin-film. The two black lines represent the two hard panels and it shows that the power output decreases as the temperature increases. However, the power output of the thin-film remains constant as the temperature increases. On roofs where temperatures can become quite hot, this can be significant.

Hydrogen Energy

Hydrogen is the simplest element. An atom of hydrogen consists of only one proton and one electron. It's also the most plentiful element in the universe. Despite its simplicity and abundance, hydrogen doesn't occur naturally as a gas on the Earth - it's always combined with other elements. Water, for example, is a combination of hydrogen and oxygen (H2O).

Hydrogen is also found in many organic compounds, notably the hydrocarbons that make up many of our fuels, such as gasoline, natural gas, methanol, and propane. Hydrogen can be separated from hydrocarbons through the application of heat - a process known as reforming. Currently, most hydrogen is made this way from natural gas. An electrical current can also be used to separate water into its components of oxygen and hydrogen. This process is known as electrolysis. Some algae and bacteria, using sunlight as their energy source, even give off hydrogen under certain conditions.

Hydrogen is high in energy, yet an engine that burns pure hydrogen produces almost no pollution. NASA has used liquid hydrogen since the 1970s to propel the space shuttle and other rockets into orbit. Hydrogen fuel cells power the shuttle's electrical systems, producing a clean byproduct - pure water, which the crew drinks.

A fuel cell combines hydrogen and oxygen to produce electricity, heat, and water. Fuel cells are often compared to batteries. Both convert the energy produced by a chemical reaction into usable electric power. However, the fuel cell will produce electricity as long as fuel (hydrogen) is supplied, never losing its charge.

Fuel cells are a promising technology for use as a source of heat and electricity for buildings, and as an electrical power source for electric motors propelling vehicles. Fuel cells operate best on pure hydrogen. But fuels like natural gas, methanol, or even gasoline can be reformed to produce the hydrogen required for fuel cells. Some fuel cells even can be fueled directly with methanol, without using a reformer.

In the future, hydrogen could also join electricity as an important energy carrier. An energy carrier moves and delivers energy in a usable form to consumers. Renewable energy sources, like the sun and wind, can't produce energy all the time. But they could, for example, produce electric energy and hydrogen, which can be stored until it's needed. Hydrogen can also be transported (like electricity) to locations where it is needed.

Wind Energy

We have been harnessing the wind's energy for hundreds of years. From old Holland to farms in the United States, windmills have been used for pumping water or grinding grain. Today, the windmill's modern equivalent - a wind turbine - can use the wind's energy to generate electricity.

Wind turbines, like windmills, are mounted on a tower to capture the most energy. At 100 feet (30 meters) or more aboveground, they can take advantage of the faster and less turbulent wind. Turbines catch the wind's energy with their propeller-like blades. Usually, two or three blades are mounted on a shaft to form a rotor.

A blade acts much like an airplane wing. When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade. The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift. The force of the lift is actually much stronger than the wind's force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller, and the turning shaft spins a generator to make electricity.

Wind turbines can be used as stand-alone applications, or they can be connected to a utility power grid or even combined with a photovoltaic (solar cell) system. For utility-scale sources of wind energy, a large number of wind turbines are usually built close together to form a wind plant. Several electricity providers today use wind plants to supply power to their customers.

Stand-alone wind turbines are typically used for water pumping or communications. However, homeowners, farmers, and ranchers in windy areas can also use wind turbines as a way to cut their electric bills.

Small wind systems also have potential as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.


Biopower, or biomass power, is the use of biomass to generate electricity. There are six major types of biopower systems: direct-fired, cofiring, gasification, anaerobic digestion, pyrolysis, and small, modular.

Most of the biopower plants in the world use direct-fired systems. They burn bioenergy feedstocks directly to produce steam. This steam is usually captured by a turbine, and a generator then converts it into electricity. In some industries, the steam from the power plant is also used for manufacturing processes or to heat buildings. These are known as combined heat and power facilities. For instance, wood waste is often used to produce both electricity and steam at paper mills.

Wind turbines, like windmills, are mounted on a tower to capture the most energy. At 100 feet (30 meters) or more aboveground, they can take advantage of the faster and less turbulent wind.

Many coal-fired power plants can use cofiring systems to significantly reduce emissions, especially sulfur dioxide emissions. Cofiring involves using bio-energy feed stocks as a supplementary energy source in high efficiency boilers.

Gasification systems use high temperatures and an oxygen-starved environment to convert biomass into a gas (a mixture of hydrogen, carbon monoxide, and methane). The gas fuels what's called a gas turbine, which is very much like a jet engine, only it turns an electric generator instead of propelling a jet.

The decay of biomass produces a gas - methane - that can be used as an energy source. In landfills, wells can be drilled to release the methane from the decaying organic matter. Then pipes from each well carry the gas to a central point where it is filtered and cleaned before burning. Methane also can be produced from biomass through a process called anaerobic digestion. Anaerobic digestion involves using bacteria to decompose organic matter in the absence of oxygen.

Methane can be used as an energy source in many ways. Most facilities burn it in a boiler to produce steam for electricity generation or for industrial processes. Two new ways include the use of micro-turbines and fuel cells. Micro-turbines have outputs of 25 to 500 kilowatts. About the size of a refrigerator, they can be used where there are space limitations for power production. Methane can also be used as the "fuel" in a fuel cell. Fuel cells work much like batteries but never need recharging, producing electricity as long as there's fuel.

In addition to gas, liquid fuels can be produced from biomass through a process called pyrolysis. Pyrolysis occurs when biomass is heated in the absence of oxygen. The biomass then turns into a liquid called pyrolysis oil, which can be burned like petroleum to generate electricity. A biopower system that uses pyrolysis oil is being commercialized.

Several bio-power technologies can be used in small, modular systems. A small, modular system generates electricity at a capacity of 5 megawatts or less. This system is designed for use at the small town level or even at the consumer level. For example, some farmers use the waste from their livestock to provide their farms with electricity. Not only do these systems provide renewable energy, they also help farmers and ranchers meet environmental regulations.

Small, modular systems also have potential as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.


Flowing water creates energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower.

The most common type of hydroelectric power plant uses a dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. But hydroelectric power doesn't necessarily require a large dam. Some hydroelectric power plants just use a small canal to channel the river water through a turbine.

Another type of hydroelectric power plant - called a pumped storage plant - can even store power. The power is sent from a power grid into the electric generators. The generators then spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the power is stored. To use the power, the water is released from the upper reservoir back down into the river or lower reservoir. This spins the turbines forward, activating the generators to produce electricity.

A small or micro-hydroelectric power system can produce enough electricity for a home, farm, or ranch.

Ocean Energy

The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.

Oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world.

Methane can be used as an energy source in many ways. Most facilities burn it in a boiler to produce steam for electricity generation or for industrial processes.

Ocean thermal energy is used for many applications, including electricity generation. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems.

Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. As a result, tides and waves are intermittent sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.

A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.

This article is compiled from various sources to give an overview of clean energy options for readers' interest.


1. website of National Renewable Energy Laboratory and the Department of Energy USA,

2. Collected articles and papers from Future Energy; edited by Trevor M Letcher.

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