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6.2 RENEWABLE ENERGY RESOURCES
Today, utility providers currently rely heavily on coal, natural gas, and oil for their energy. Fossil fuels are nonrenewable and finite resources that will eventually dwindle, becoming increasingly expensive and environmentally damaging to retrieve. In contrast, renewable energy resources are sources of energy that are constantly being replenished. These energy resources include: biomass, geothermal, hydrogen, hydroelectric, solar, and wind.
Most renewable energy resources come from the sun either directly or indirectly. For example, solar energy is either used directly with the use of solar panels or solar heat exchangers. However, solar energy drives winds, and plants convert solar energy into carbohydrates which later are harvested for food or fermented to create transportation fuels like hydrogen and ethanol.
Today, an emphasis is being placed on creating clean and renewable energy resources as a means of reducing the carbon footprint and its effects on climate change. In 2015, the United States generated about seven million Gigawatt (GW) hours of electricity. About two thirds of the electricity generated was from fossil fuels.
Major energy sources and percent share of total U.S. electricity generation in 2015.
- Coal = 33%
- Natural gas = 33%
- Nuclear = 20%
- Other renewables = 7%
- Biomass = 1.6%
- Geothermal = 0.4%
- Solar = 0.6%
- Wind = 4.7%
- Hydropower = 6%
- Petroleum = 1%
- Other gases = <1%
When people think of renewable energy, they generally think of wind and solar. However, biomass, derived from plant material and animal waste, is one of the oldest sources of renewable energy. This energy source absorbs energy from the sun and regrows over a short period of time.
Compared to fossil fuels, which take millions of years to form, biomass has a clear advantage in the time it takes to be processed and used. Similar to fossil fuels, biomass is mainly used as a means to create heat through the process of burning material or creating transportation fuels for use in generators and engines.
In the United States, biomass fuels provide about 1.6% of the energy used for electricity generation and about 2.5% of the energy used in transportation fuels. Of this, 46% of the energy from biomass was from wood and wood products, 43% from biofuels, and 11% from municipal waste.
Examples of biomass energy include:
- Wood and Plant Wastes—burned to heat buildings, produce heat, and generate electricity
- Agricultural Crops —burned or converted to liquid biofuels
- Biodegradable Garbage—burned to generate electricity in power plants
- Animal Manure and Human Sewage—converted to biogas and burned as a fuel to generate electricity
184.108.40.206 BIOMASS POWER GENERATION
In biomass power plants, municipal waste, wood and wood waste, and biogas (predominantly methane) are burned to heat water and produce steam that runs a turbine and generates electricity. Additionally, biomass is burned to provide heat to industries and homes; burning wood in a fireplace is a great example.
Burning biomass isn’t the only way to use its energy. Biomass can be converted to other forms of energy. Transportation fuels such as ethanol and biodiesel are used to power automobiles, trains, and even ships. In the United States, corn and sugar cane are used as a sources of ethanol. To create ethanol, carbohydrate-rich crops are grown and fermented. The ethanol is then combined with gasoline. Today, most cars can run on 10% ethanol with some flex fuel cars capable of running on E85 (85% ethanol).
Geothermal energy is heat that is generated from below the Earth’s crust. Molten rock, called magma, contains 50,000 times more energy than all of the natural gas and oil resources in the world. Since access to geothermal energy is ubiquitous, significant advancements in technology have been made to tap into this renewable resource. These technologies range from complex power stations to small and simple pumping systems, each providing significant advantages over traditional energy resources. Methods to utilize these resources include: geothermal powerplants and heat pumps/heat exchangers.
220.127.116.11 GEOTHERMAL POWER PLANTS
As of 2014, the United States has more than 3,300 Megawatts (MW) of installed generation capacity and is a global leader in this energy category. Eighty percent of this capacity is located in California, where more than 40 geothermal plants provide nearly 7% of the state’s electricity.
Geothermal power plants follow three basic designs. The first and simplest design, known as dry steam, uses steam directly from the geothermal source and transfers it through a turbine to generate electricity.
The second and most uncommon approach depressurizes hot water, which is then flashed into steam. The steam is then used to drive a turbine and generate electricity. Due to the limitations of deep drilling, this approach is both expensive and difficult to use.
The third approach, called a binary cycle system, passes hot water through a heat exchanger. The heat exchanger passes the heat where a second liquid (isobutene) is converted to steam and used to drive a turbine. This approach, commonly referred to as a closed loop system, prolongs the life of the geothermal source by retaining the super-heated water and reducing waste. Below is an illustration of all three approaches.
18.104.22.168 GEOTHERMAL HEAT PUMPS
Almost everywhere, the upper 10 feet of the Earth’s surface maintains a nearly constant temperature between 50° and 60°F. Geothermal heat pumps tap into this resource to heat and cool buildings. These systems consist of a heat pump, an air delivery system or ductwork, and a heat exchanger. In the below figure, the heat pump pulls ground temperature water through a compressor, which, depending on the season, either condenses or evaporates the water to heat or cool a working fluid. In the winter, a heat pump moves heat by pumping water from the ground through a condenser and into the building to heat the air. In the summer, this process is reversed and the hot water from the building is moved through an evaporator to the ground loop and back to the building to cool the air.
In regions with temperature extremes, ground-source heat pumps are the most environmentally clean and energy-efficient heating and cooling systems available. Far more efficient than electric heating and cooling, these systems can circulate as much as five times the energy they consume in the process.
The U.S. Department of Energy conducted a study and found that heat pumps can save average households hundreds of dollars in energy costs each year. The system typically pays for itself in eight to twelve years. Tax credits and other incentives can reduce the payback period to five years or less.
Today, more than 600,000 ground-source heat pumps supply climate control in U.S. homes and in other buildings. Although this is a significant, it is still only a small fraction of the U.S. heating and cooling market with several barriers the market must overcome. For example, despite their long-term savings, geothermal heat pumps have higher upfront costs. In addition, installing them in existing homes and businesses can be difficult, since it involves digging up areas around a building’s structure. Finally, many heating and cooling installers are simply not familiar with the technology.
Hydrogen is the simplest of all the elements and consists of only one proton and one electron. Despite its simplicity, it’s the most plentiful element in the universe; however, hydrogen doesn’t occur naturally as a gas on Earth. It’s always combined with other elements in the form of molecules. Water, for example, is a molecule made up of hydrogen and oxygen to form (H2O).
Hydrogen is also found in many organic compounds, notably the hydrocarbons that make up many fuels, such as gasoline, natural gas, methanol, and propane. Most of the hydrogen produced today is separated from natural gas using a process called gas reforming. Additionally, an electrical current can also be used to separate water into its components of oxygen and hydrogen. This process is known as electrolysis.
Hydrogen in pure form is high in energy and can be burned to release that energy. The key advantage of using pure hydrogen as a fuel is when hydrogen is burned it produces almost no pollution, making it a clean and renewable source of energy. Currently the energy from hydrogen is produced using two main methods: hydrogen fuel and hydrogen fuel cells.
22.214.171.124 PURE HYDROGEN
In the 1970’s, NASA was looking for a fuel source high in energy and clean burning. When combined with oxygen, hydrogen was used to power rockets and space shuttles. Today, some cars are using liquid hydrogen as a fuel with promising results.
126.96.36.199 FUEL CELLS
A fuel cell combines hydrogen and oxygen to produce electricity, heat, and water. Similar to batteries, fuel cells convert the energy produced by a chemical reaction into usable electricity. Unlike batteries, as long as hydrogen is supplied, the fuel cell never loses 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 can even be fueled directly with methanol, without using a reformer.
188.8.131.52 THE FUTURE OF HYDROGEN
In the future, hydrogen will 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 could also be used as a fuel for zero-emissions vehicles, to heat homes and offices, and to fuel aircraft. However, before hydrogen can play a bigger role in energy production and become a widely used alternative to gasoline, many new facilities and systems must be built.
On Earth, water is constantly moving around in various forms. As an example, water evaporated from oceans combines to form clouds, which eventually condense and precipitate in the form of rain and snow. All this movement provides an enormous opportunity to harness useful energy. Hydroelectric generators take advantage of this movement to create electricity.
184.108.40.206 CONVERTING MOVING WATER TO ELECTRICITY
In the United States, hydropower has grown steadily, from 56 Gigawatts (GW) of installed capacity in 1970 to more than 78GW in 2011. In order to generate electricity from the kinetic energy in moving water, the water has to move with sufficient speed and volume to spin a turbine. Roughly speaking, one gallon of water per second falling one hundred feet can generate one kilowatt of electricity. To increase the volume of moving water, impoundments or dams are used to collect the water. An opening in the dam uses gravity to drop water down a pipe called a penstock. The moving water causes a turbine to spin, which causes magnets inside a generator to rotate and create electricity.
Since hydropower depends on rivers and streams for generation, the potential to use hydropower as a source of electricity varies across the country. For example, the Pacific Northwest (Oregon and Washington) generates more than two-thirds of its electricity from hydroelectric dams.
In addition to very large plants in western states, the United States has many smaller hydropower plants. In 1940 there were 3,100 hydropower plants across the country, though by 1980 that number had fallen to 1,425. Since then, a number of these small plants have been restored. As of 2013, 1,672 hydro plants (not including pumped storage) were in operation.
Hydropower can also be generated without a dam, through a process known as run-of-the-river. In this case, the volume and speed of water is not augmented by a dam. Instead, a run-of-river project spins the turbine blades by capturing the kinetic energy of the moving water in the river. Hydropower projects that have dams can control when electricity is generated because the dams can control the timing and flow of the water reaching the turbines. Therefore, these projects can choose to generate power when it is most needed and most valuable to the grid. Because run-of-river projects do not store water behind dams, they have much less ability to control the amount and timing of when electricity is generated.
220.127.116.11 PUMPED STORAGE
Another type of hydropower technology is called pumped storage. In a pumped storage plant, water is pumped from a lower reservoir to a higher reservoir during off-peak times when electricity is relatively cheap, using electricity generated from other types of energy sources. Pumping the water uphill creates the potential to generate hydropower later on. When the hydropower is needed, it is released back into the lower reservoir through turbines. Inevitably, some power is lost, but pumped storage systems can be up to 80% efficient. Currently more than 90GW of pumped storage capacity is available worldwide, with about 20% of that in the United States. The need to create storage resources to capture and store for later use the generation from high penetrations of variable renewable energy (e.g. wind and solar) could increase interest in building new pumped storage projects.
18.104.22.168 THE FUTURE OF HYDROPOWER
Advances in ‘fish-friendly’ turbines and improved data collection techniques to increase the effectiveness of fish passage technologies create exciting new opportunities for the hydropower industry. If constructed and operated in a manner that minimizes environmental and cultural impacts, hydropower projects can provide low-cost, clean sources of electricity to urban and rural areas throughout the world. Harvesting the power from our rivers can be part of a smart and diverse set of solutions for reducing our dependence on fossil fuels, and the impact they have on our climate and public health. The ability to ramp up and down hydropower generation is a valuable source of flexible generation on the electricity grid, which can directly displace coal and natural gas, and help integrate larger amounts of variable renewable energy resources, like wind and solar power.
Solar energy is energy from the sun that is converted into thermal or electrical energy. This form of energy is the cleanest and most abundant renewable energy source available. A variety of technologies convert sunlight into usable energy. The most commonly used solar technologies for homes and businesses are solar photovoltaics, concentrated solar heating, and passive solar heating.
22.214.171.124 SOLAR PHOTOVOLTAICS
Traditional solar cells are made from silicon and tend to be the most efficient. Second-generation solar cells are made from amorphous silicon or non-silicon materials such as cadmium telluride, and are called thin film solar cells. Thin film solar cells use layers of semiconductor materials that are only a few micrometers thick. Due to their flexibility, thin film solar cells can double as rooftop shingles and tiles, building facades, or the glazing for skylights. Solar cells, also called photovoltaic (PV), convert sunlight directly into electricity. PV gets its name from the process of converting light (photons) to electricity (voltage), which is called the PV effect. The PV effect was discovered in 1954, when scientists at Bell Telephone discovered that silicon created an electric charge when exposed to sunlight. Soon, solar cells were being used to power space satellites and smaller items like calculators and watches.
126.96.36.199 CONCENTRATED SOLAR HEATING
Fossil fuels are used by many of today’s power plants as a heat source to generate electricity. However, a new generation of power plants with concentrated solar power systems uses the sun as a heat source. The three main types of concentrated solar power systems are: linear concentrator, dish/engine, and power tower systems.
- Linear concentrator systems collect the sun’s energy using long rectangular, curved mirrors. The mirrors are tilted toward the sun, focusing sunlight on pipes (receivers) that run the length of the mirrors. The reflected sunlight heats a fluid flowing through the pipes and is used to boil water in a conventional steam generator.
- A dish/engine system uses a large mirrored dish similar to a satellite dish. The dish-shaped surface directs and concentrates sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to the steam generator.
- A power tower system uses a large field of flat, sun-tracking mirrors known as heliostats to focus and concentrate sunlight onto a receiver on the top of a tower. A heat-transfer fluid heated in the receiver is used to generate steam and used in a steam generator.
188.8.131.52 PASSIVE SOLAR HEATING
Commercial and industrial buildings can use the same solar technologies used for residential buildings: photovoltaics, passive heating, daylighting, and water heating. Nonresidential buildings can also use solar energy technologies that would be impractical for a home. These technologies include ventilation air preheating, solar process heating, and solar cooling.
Solar water-heating systems are designed to provide large quantities of hot water for nonresidential buildings. A typical system includes solar collectors that work along with a pump, heat exchanger, and/or one or more large storage tanks. The two main types of solar collectors used for nonresidential buildings—an evacuated-tube collector and a linear concentrator—can operate at high temperatures with high efficiency. An evacuated-tube collector is a set of many double-walled, glass tubes, and reflectors to heat the fluid inside the tubes. A vacuum between the two walls insulates the inner tube, retaining the heat. Linear concentrators use long, rectangular, curved (U-shaped) mirrors tilted to focus sunlight on tubes that run along the length of the mirrors. The concentrated sunlight heats the fluid within the tubes. Many large buildings need ventilated air to maintain indoor air quality. In cold climates, heating this air can use large amounts of energy. However, a solar ventilation system can preheat the air, saving both energy and money. This type of system typically uses a transpired collector, which consists of a thin, black metal panel mounted on a south-facing wall to absorb the sun’s heat. Air passes through the many small holes in the panel. A space behind the perforated wall allows the air streams from the holes to mix together. The heated air is then sucked out from the top of the space into the ventilation system.
Humans have been harnessing the wind’s energy for hundreds of years, from old windmills used for pumping water or grinding grain, to ships using sails to move. Today, wind energy is captured by the natural wind in our atmosphere and converted into mechanical energy used to drive a generator that creates electricity.
Wind is the movement of air from an area of high pressure to an area of low pressure. This is caused by the uneven heating of the atmosphere by the sun, irregularities of the earth’s surface, and rotation of the earth. In an effort to capitalize on wind power, wind turbines are installed in areas where wind gusts are consistent all year round.
184.108.40.206 WIND TURBINES
Each turbine is equipped with wind assessment equipment and will automatically rotate into the face of the wind, and angle or “pitch” its blades to optimize energy capture.Wind turbines, like windmills, are mounted atop a steel tubular tower up to 325 feet, which supports both a “hub” securing wind turbine blades and the “nacelle” which houses the turbine’s shaft, gearbox, generator, and controls. Usually, the hub will have two or three propeller-like blades which are mounted on a shaft to form a rotor. Each blade acts like an airplane wing. When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade which causes the rotor to turn and triggers an internal gearbox to spin. The gearbox then steps up the rotation speed of the rotor and spins an internal shaft which is connected to a generator that produces electricity.
Wind is a clean source of renewable energy that produces no air or water pollution. And since the wind is free, operational costs are nearly zero once a turbine is erected. 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 system. Mass production and technology advances are making turbines cheaper, and many governments offer tax incentives to spur wind-energy development.
In the United States, Texas has the most wind Farms (42) with a combined wind generated capacity of 17,713 MW. Nevertheless, the wind energy industry is booming. Globally, generation more than quadrupled between 2000 and 2006. At the end of 2015, global capacity reached more than 432,419 MW.
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