Renewable Energy Sources for Electricity Generation

In July 2017, US Energy Information Administration (EIA) announced that renewable power generation passed nuclear power generation for the first time since 1984. The main cause of the growth of renewable electricity generation is attributed to the emerging new wind turbines and solar farms. Also another contribution came from the big increase of hydropower electricity generation because of the high amount of precipitation during the winter of the western states. According to the stats, more than 60% of the total electricity was generated by wind and solar power combined.

On the other side, due to their high cost, building new nuclear reactors has been greatly slowed down in the recent decades. Also the reduction of building new nuclear power plants is due to the louder and louder voice of objection from public. Up to date, almost all the nuclear power plants were built during the period from 1970 to 1990. Some of the old outdated reactors have been abandoned.

Even though, the amount of electricity generate by renewable energy sources exceeds nuclear power now, it is predicted that nuclear power will eventually exceed renewable power by the year total, because water flow passing the dam during the summer will be reduced without much rain fall in the summer. Many nuclear power plants will also pick up speed after the routine maintenance and inspection during the spring and fall when demand for electricity is normally lower that the summer and winter.

It is a great advance in the development of renewable power generation to exceed nuclear power. The reality is that we still have a long road to go towards clean and sustainable energy sources.  In the US, about 2/3 of the electricity comes from fossil fuel power generation, such as natural gas and coal. Nuclear power and renewable sources are basically making the same amount of electricity, which is less than 20% for each in the rest 1/3 of total electricity.

Renewable energy sources are existing everywhere, solar, wind, water, geothermal and bioenergy are among the most explored. Driving by the increasing demand for clean and responsible energy generation, more and more investments have been put in development and education that will lead to the revolution of clean energy.

Electricity generation with non-renewable fossil fuels is the largest industrial pollution. With decades of development of green energy technologies, power generation using clean energy sources is becoming available to more population. The installation of renewable power generation plants has been accelerated and the trend of more electricity generated by renewable energy sources becomes more significant. The result of the transition will be cleaner air and reserved natural resources. The other benefits we will gain by choosing clean energy sources include:

  • Environmental stewardship
  • Public image
  •  Customer loyalty
  •  Employee pride
  • Power portfolio management
  •  Power reliability
Solar Energy

Solar radiation is the easiest to get energy source on the earth. It is also inexhaustible and abundant. About 29% of the sunlight that reaches the top of the atmosphere is reflected back into the space by clouds, atmosphere particles, sea and ice cover. About 71% of the sunlight get through the atmosphere but 23% is absorbed in the atmosphere.  About 48% of the sunlight reaches the surface of the earth and gets absorbed by the surface. Even though the biggest advance of utilizing solar power via photovoltaic effect was made during the recent decades thanks to the development of photovoltaic cells (or called solar cells), solar energy has been long used in human history. For example, cook and heat with sunlight, which is the simplest way of utilization of the so called solar thermal energy.

Photovoltaic cells are made of photo-sensitive semiconductor materials that are doped with specific impurities to get desirable photovoltaic characteristics. Photovoltaic technologies are defined by using different types of materials, either crystalline silicon in the form of wafer or thin films of other materials coated on certain substrates. These semiconductors include CIS (Copper Indium (Di)selenide), CISGS (Copper Indium Gallium (Di)Selenide), CdTe (Cadmium Telluride) and GaAs (Gallium Arsenide).

A photovoltaic cell consists of two parts that are made of P type silicon and N type silicon respectively. The N-type silicon is basically phosphorus-doped and the P-type is Boron-doped. The majority of the charge carries in P-type silicon is called holes which is positive charge, while N-type silicon contains mostly negative electrons. When the P type and N type materials are bring in contact to each other, the recombination process between electrons from the N-type and holes from the P-Type silicon cause a depleted region in the joint boundary and an electric field is created. This physical boundary is so called PN Junction and it has a unique electrical characteristics, unidirectional conduction which allows electrons to flow only from N to P.

A typical photovoltaic cell consists of several layers from the top to the bottom, ARC (Antireflective coating) layer, N-type layer, P-N Junction, P-type layer and the back contact layer.

When the photons with high energy in the sunlight hit the cell surface, the electrons in the N-type silicon get activated to move from N to P and create a voltage difference between the two contacts, which normally measures about 0.5V. In applications, multiple layers are stacked to create practically high voltages. If a load is added to the cell, a current will flow.

The measure for photovoltaic cell performance is based on the efficiency of transferring sunlight into electricity. Today, with current photovoltaic technology, the commercial solar cells typically have efficiency of about 15%, which means only 1/6 of the sunlight received by the cell is converted to electricity. Theoretically, the highest efficiency of a solar cell based on current technology will be in the neighborhood of 30%.

Maximum Power Point Tracking (MPPT) by engineering.com

How the solar panel performs is measured by how efficient it converts the solar energy into electricity. Solar energy depends on many factors, such as the weather, the solar cell temperature, the size of solar panel and the time during the day, etc. The engineers who design the solar power system must ensure the system can convert maximum solar power from the solar panels despite all the factors that affect the efficiency.

From the above diagram, the red curve represents the voltage-power characteristics and the green curve is the current-voltage characteristics. To obtain the maximum power, we need find the largest voltage and current at the same time. The dashed line in the above plot represents MPP (maximum power point) at which the cell current begins to decline. The power is the product of voltage and current.

Wind Energy

Wind energy is actually originated from sunlight. The sunlight reaches the surface of the earth and absorbed. Therefore the surface is heated up. The air near the land becomes warmer and rises, while the heavier and cooler air over the water comes to occupy the vacancy. This displacement creates wind. The same process is reversed during night when the land cools down faster than the water.

Wind energy origination

Wind energy has been long used. Windmills are used to pump water or drive machinery to grind grains. People ride on sail boats to voyage across the oceans. With the development of science of materials and technology of manufacturing, windmills from old ages are reinvented to be bigger and faster. It becomes more efficient in converting the kinetic energy of the spinning windmill into electricity. In order to generate more electricity, the tall wind turbines are generally built in open areas or at high altitudes because of higher speed of the wind. Similar to solar energy, wind energy is completely renewable and clean.

Wind turbine structure (www.eia.gov)

The wind turbine typically has three long blades installed around a rotor. The propeller shaped blades can be easily pushed by wind. To understand how a wind turbine generates electricity, it will be helpful to image how an electrical fan works. The wind turbine works the opposite of the fan. The spinning blades bring the long shaft that is connected to rotor to rotate. The speed of the shaft is then increased by a set of gears to drive the generator to generate electricity.

How do wind turbines work (energy.gov)
Advantages of wind energy
  • Wind energy is renewable and clean; wind is green;
  • Wind turbines can be installed in areas without power grid;
  • In contrast to solar energy, winds are available not only during the day, but also the night;
Disadvantages of wind energy
  • Wind turbines cause noise that impacts the people in the nearby;
  • Wind turbine output can be intermittent because wind availability during the day varies and wind speed also changes;
  • Wind farm occupies large area of lands so it is normally built in remote areas;
  • Wind turbine blades can be dangerous;

 

Geothermal Energy

Geothermal energy is the heat stored deeply in the Earth. It was originated from the formation of the Earth and the decay of radioactive materials. The geothermal gradient reflects the gradual difference of temperature from the center of the Earth to its surface.

Thermal energy inside the Earth (geo-energy.org)

Geothermal energy has been existing from the birth of the planet about 4.5 billion years ago. The heat radiated from the inside of the Earth has been long used for thousands of years. It is clean and renewable. This deeply trapped heat is continuously brought up to the surface by water and air flow through the heat sources.  People from many places of the world have used hot springs for cooking and medical purposes for years. They also pump the hot water from the springs and tapped it for use in houses, such as heating the buildings.

In recent years, as techniques for collecting geothermal becomes more feasible, use of geothermal energy becomes practical and affordable.

Geothermal power generation is widely accepted as a renewable electricity generation technology.  A viable geothermal resource requires water, temperature (300 to 700 F) and permeability to be practical for electricity generation. For practical use of geothermal energy for power generation, the sources of geothermal energy or geothermal reservoir must be tested for temperature and water flow rate. A continuous flow of water into the reservoir is necessary for heater exchange. Permeability of the rock bed close to the geothermal heat is also required for water to contact with the heat. As seen in the nature, steam and hot water are two forms of geothermal resources. To extract the geothermal resources, we need drill either the dry steam wells or hot water wells. A new technology called EGS (Enhanced Geothermal Systems) has been widely used to inject water to the hot subsurface for efficient heat exchange.

Commercially, geothermal power plants exist in three types, flash, dry steam and binary.

Flash geothermal power plant

Currently, there are about 30% of the geothermal power plants are flash geothermal power plants.

Flash Steam Power Plant (energy.gov)

Shown in the figure below, the water that has been heated up to 360 F is pumped by high pressure and release in the flash tank at the surface at much lower pressure than the pumping pressure. Therefore rapid water vaporization occurs due to the pressure reduction. This process is called flash. The steam is then fed to drive the turbine which drives the power generator.  Normally, there is water remains in the flash tank without being used, therefore a second flash tank is used to enable a flash again to extract more energy from the heated water.

Dry Steam Power plant

Unlike the flash power plant, a dry steam power plant directly uses steam from the geothermal reservoir to drive a turbine for generating electricity.  Dry steam power plant does not need the flash tank and steam separator.

Dry Steam Power plant (energy.gov)
Binary cycle power plant

In a binary cycle power plant, the hot water or steam pumped from the production well is used to heat another fluid called working fluid that has much lower boiling point in the heat exchanger. After the heat exchanger, the working fluid is vaporized at a temperature as low as 14 C and drives the turbines to generate electricity. Binary cycle power plant is a closed-loop system. All fluids are enclosed and cycled. No fluid is released to the atmosphere except some leaked water vapor. Working fluid is typically isobutene or pentafluoropropane. The advantage of binary cycle power plant is the relatively lower working temperature than flash and dry steam power plants by using fluids boiling at lower temperature. It’s found that most geothermal resources are below 300 F that is most suitable for binary cycle power generation. Therefore, most newly built geothermal power plants in the US are binary cycle power plants.

Binary Cycle Power Plant (energy.gov)
Hydrogen

Hydrogen fuel cell technology has been shown a viable green solution to our future energy need especially in automobile applications. Hydrogen is abundantly exists in the water molecules. There are many ways have been studied to produce hydrogen with low cost from renewable resources, such as biological water splitting, fermentation, biomass and wastes, photoelectrochemical water splitting and solar thermal water splitting. The cleanest way to produce hydrogen is using sunlight to split water molecules into molecular hydrogen and oxygen. The PEC (photoelectrochemical) technology developed by NREL (National Renewable Energy Laboratory) can reach an efficiency of 12.4% conversion from solar to hydrogen.

Individual Fuel Cell utilizes hydrogen to generate electricity

Hydrogen productionHydrogen production process can be used as storage for the renewable energy from solar, water and wind sources. The electricity generated from renewable sources is used to split water into hydrogen and oxygen. The energy stored in hydrogen can be easily transported and used later when solar or wind energy source is low.

Hydrogen is clean fuel producing only water when consumed. The impact of hydrogen to environment is minimal. When hydrogen is used as fuel by cars, it produces no emissions.

Biomass

People have used biomass energy or bioenergy or biofuel for thousands of years. Biomass include variety of organic materials, such as crops, garbage, wood, alcohol fuels and landfill gas, etc. Wood is burned for cooking food and keeping the house warm. Wood is the largest resources of biomass energy. The plants on the Earth get energy from the Sun to grow. The organic matters in the plants can be used as fuel to burn. The energy released by burning the residue of plant can be used to produce electricity. Burning biomass is just one of the many ways to release the energy of biomass. Many techniques have been used to convert biomass to other forms of energy, such as methane gas, ethanol and biodiesel. Burning biomass is still the easiest and most efficient way to harness this kind of renewable energy. When the biomass is burned, the chemical energy stored in the plant is converted to heat to keep house warm and cook food. Many power plants that use biomass for electricity generation have been built; they are called waste-to-energy power plants. In these power plants, large amount of biomass is burned to heat water to steam that pushes  the turbines to drive an electricity generator.

Type of common biomass (www.eia.gov)
Biomass Power Plant (www.calbiomass.org)

Using biomass for electricity generation can substitute the use of traditional fossil fuels, thus decrease sir pollution and greenhouse gas release. Unlike fossil fuels, biomass is a renewable energy source. Using biomass to produce electricity gives us advantages as:

  • Renewable resource
  • Reduce greenhouse gas and improve global climate
  • Utilizing biomass reduce landfill
  •  Cleaning out biomass from forest helps prevent wild fires
  • Using the by-product of biomass, methane gas to eliminate odor and air pollution

 

 

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