Energy

Energy processing involves an array of chemical engineering equipment and can be divided into three stages: Generation, storage, and usage. Generation of energy is the process of harvesting energy from a particular source, energy storage involves the system used to contain the harvested energy, and the stored energy is then sent to its final destination to be used and conserved in a multitude of different applications.

Generation

Energy can be generated from a variety of sources, including fossil fuels, wind, solar, hydro, tides, geothermal, and nuclear power. These energies can be categorized into renewable and nonrenewable energy types.

Nonrenewable Energy

This type of energy refers to those from sources formed naturally over time that cannot be regenerated in the short term. Such energy sources come mainly from fossil fuels and are processed to produce coal, petroleum, natural gas, and various plastics.

Coal Powered Energy Generation

Coal Processing

A multitude of processes are involved in coal processing but the bulk of it involves material handling and chemical and mechanical separations. Coal is first extracted from an open-pit mine and transported to a mobile primary sizer to reduce the product down to a manageable size. The coal exits the sizer by flowing on the mine’s conveyor belts to a  secondary sizer, where it is mixed with another coal source from underground mining operations that travels through a mine shaft system. This product is then sent to a drum before it is drained, rinsed, and screened by a screener .

The fine coal settles in a slurry then enters a magnetic separator that further separates magnetic minerals and other unwanted material from the coal. The product from this process then undergoes coal classification by being washed in hydrocyclones , and classifiers. Hydrocyclones wash and clean the coal to allow a density-driven separation to occur, as minerals are denser than the coal. The hydrocyclone is designed to segregate the finer coal particles as they exit the bottom of the conical equipment. These particles are filtered further by screeners before proceeding to a coal dryer . The product is sent to a grinding mill for the final size reduction before being used for coal power generation.

Coal Power Generation

Fine and pulverized coal retrieved from processing plants is burned in the combustion chamber of a boiler , where it is then mixed with hot air. This generates steam through the boiler, and the high pressure steam then passes through a turbine that converts thermal energy into rotational energy. The turbine is connected to a generator that produces electricity as the turbine rotates in a strong magnetic field. After passing the turbine, the steam is condensed back into liquid and is recycled back to the boiler.

Natural Gas Combined Cycle (NGCC) Energy Generation

In the United States, many coal power plants are being converted into natural gas combined cycle power plants to meet the Clean Power Plan and other EPA emissions regulations. The transition gives a greener incentive as natural gas releases much less carbon dioxide into the atmosphere than coal per unit output and requires less back-end emissions treatment.

Nuclear Energy Generation

(Copyright U.S. Nuclear Regulatory Commission)

This type of energy generation involve nuclear power plants that include a nuclear reactor , and various types of heat exchangers such as the cooling towers and condensers , described in their respective modules. Nuclear power plants transform water into steam using the heat released from nuclear fission in the reactor. The steam generated turns the turbine blades where its rotational energy is transformed into electrical energy with the aid of generators. The steam then passes through the cooling tower,is cooled down into a liquid state and is transported back to the power plant.

Fossil Fuels Energy Generation

The process of petroleum generation and corresponding refinery processes is described in the petroleum refining module.

Renewable Energy

Renewable energy is garnered from sources that are naturally replenished. Hydro, solar, wind, and geothermal power are the important renewable energy technologies currently being implemented.

Hydropower

(Bureau of Reclamation, Hoover Dam, NV)

Hydropower is generated by the use of turbines installed in dams that generate electrical power as the water rushes through them.

Solar Power

The basis of solar power is the conversion of sunlight into usable energy. The vast majority of solar power processes create usable energy in the form of electricity using a system of concentrating mirrors and voltaic cells. More primitive and less widely-used solar power generation uses sunlight to directly heat a liquid medium for use in a turbine.

The process occurs in two main steps: collection of sunlight using solar panels, and conversion of the energy present in the rays into electricity. Solar panels focus sunray into a high-energy photon beam using mirrors and angled lenses. This light is then directed at a length of metal or silicon- or selenium-based semiconductor. As the photon collides with the metal or semiconductor, electrons are excited from their ground state orbitals. The electrons either fall back into their ground states, dissipating the energy as heat, which is not preferred, or travel through the material towards an electrode. The motion of negatively-charged electrons generates a current in the semiconductor.

SOLAR CELLS

Solar cells are multiple panel-conductor-electrode systems which, when joined with several others, can generate a tangible and significant amount of electrical current. There are three main types of solar cells, which differ primarily in the material used for the semiconductor and electrode.

Monocrystalline Silicon

Monocrystalline silicon is made from silicon ingots which have been processed to form a single, large crystal. Raw silicon undergoes the Czochralski process, which cultivates a single large crystal from melted silicon through the use of a seed crystal and maneuvering of the growing crystal, often by spinning the crystal as it is drawn through the liquid silicon. This material has the highest solar-to-electricity conversion efficiency and consequently the best space efficiency, but they are the most expensive to manufacture and can decrease in efficiency in colder weather.

Polycrystalline Silicon

Polycrystalline silicon is made from silicon that was not processed to form a single crystal. Rather, it is composed of multiple small coagulated crystals and still functions through the same mechanism as monocrystalline silicon, albeit at a different efficiency. The cost of manufacturing is much lower and they are slightly more weather resistant compared to monocrystalline silicon, but their efficiency is lower.

Amorphous/Thin-Film

Amorphous or thin-film cells involve coating a surface, such as glass, a cheap metal, or other material, with the metal/semiconductor. The conductor, generally silicon or some combination of two or more of silicon, cadmium, telluride, indium, selenide, copper, and gallium, is laid in thin sheets on the surface and functions under the same mechanism described above. These are the cheapest and easiest type of solar cells to mass-produce, but they have an extremely poor space-efficiency compared to the monocrystalline and polycrystalline cells.

Wind Power

(Copyright NASA, Washington, DC)

This type of power generates electrical energy through the implementation of wind turbines, which can be found in the wind section of the turbines module.

Geothermal Power

Geothermal power plants use heat from the reservoirs of hot water found under the Earth’s surface.

(Schematic of an Air Cooled Binary Geothermal Power Plant)

The major components of a geothermal power plant are the cooling towers, electrical substations and chemical engineering equipment such as heat exchangers , condensers , and turbines . A geothermal power plant is analogous to oil drilling rigs in that wells are injected into the earth’s surface to reach a geothermal reservoir with an injection well. This allows hot geothermal fluid to flow up to the power plant so that its heat can be converted to electrical energy. In the power plant, the hot pressurized fluid is first flashed into steam and water. The water is returned to the hot reservoir, and the steam enters a turbine and expands as it generates mechanical energy. This rotational energy then spins through a magnetic field in a generator to create an electrical current, which is then sent to an electrical substation consisting of a step up transformer. The expanded fluid travels through a condenser to be cooled into a liquid before returning to the hot reservoir.

A geothermal power plant also uses a secondary working fluid, such as ammonia, in a closed loop system. This working fluid is heated in the hot reservoir and is further vaporized in a heat exchanger. The working fluid assists in electrical energy generation by driving the turbine. It is then condensed back to liquid form before being sent back to the start of the loop by an injection pump. The hot pressurized fluid is separated from the secondary working fluid to prevent any environmental impact.

Storage

The ability to store energy after it is generated is critical to its successful implementation, so that it’s available when needed. Energy sources that are not stored in mechanical energy storage systems take the form of alternating current (AC) electrical energy, which are later converted into direct current (DC) electrical energy for storage. Each type of storage system is composed  of a storage medium, a power conversion system (PCS), and the balance of the plant (BOP).

The storage medium is an energy reservoir that can take the form of chemical, mechanical, or electrical potential energy, with the type of storage medium chosen depending on the technology’s capacity and its application. The PCS consists of the power electronics that allow the conversion between AC and DC electrical energy and vice versa. It also controls the power during conversion to prevent damage to the storage unit and electrical system. The BOP includes the facility that houses the equipment, the environmental control units, and the electrical units that connect the power grid to the storage medium through the PCS. Examples of storage units include:

Pumped-Hydroelectric Storage (PHS)

(Bureau of Reclamation, Hoover Dam, NV)

This type of storage unit facility is the oldest and the most abundant in the world. Energy is stored as mechanical potential energy, where gravity is the driving force. Its conventional configuration consists of two vast reservoirs, one located at a higher elevation than the other, between 30m to 650m above base level with a 300m difference between the two reservoirs. Water is released from the upper reservoir to the lower reservoir, passing through hydraulic turbines that generate electrical power. These facilities typically run daily with a 60% efficiency that depends primarily on its impellers and variable speed motors, with either freshwater or seawater as their working medium.

Compressed Air Energy Storage (CAES)

(Compressed Air Energy Storage Schematic)

This type of storage unit uses off-peak power supply to pressurize air into an underground reservoir. The air is later released during peak hours to power gas turbines to generate electricity. The components of CAES include a generator, air compressors , a turbine train that functions at varying pressures, controls for combustion and equipment operations, and the balance of plant auxiliary equipment systems. This technology substitutes the expensive natural gas fuel used to power a gas compressor with a lower-cost energy that is available from an off-peak facility such as wind power or nuclear power facilities. Power is generated when the compressed air is exhausted from the underground chamber through an expander. Efficiency of the CAES is increased by installing a recuperator, a heat exchanger that preheats the gas prior to releasing it from the underground reservoir. It allows the air to be the needed temperature and pressure to the combustor unit.

Flow Batteries

Flow batteries store and release energy through a reversible electrochemical reaction between two electrolytes. They are typically made up of four subsystems: Cell stacks, an electrolyte tank system, a control system, and power conversion systems (PCS). Cell stacks are comprised of a cathode, and an anode that may or may not include bipolar plates. As illustrated in the Fuel Cells module, the types of electrolyte tank systems used will depend on the type of flow battery used, such as alkaline, phosphoric acid, or direct methanol, among others. The common types of flow batteries are zinc-bromine, redox, vanadium redox, and chromium ion. Energy storage capacity, measured in megawatt-hours(MWh), is determined by the size of the electrolyte, while the power, measured in megawatts (MW), is dependent on the type of cell stack.

Flywheels

Flywheels store energy by accelerating a rotor to a high speed rate and maintaining it as rotational kinetic energy. Its components consist of a rotor, motor, bearing systems, vacuum housing, and power conversion systems (PCS). When power is required, the flywheel releases its stored energy through usage of the motor as a generator and it eventually slows down from its initial rotating speed. This allows the flywheel to be used for deep discharges without damaging the storage unit because energy is stored mechanically instead of chemically. To maintain the energy in the system, any resistance is minimized by using magnetic bearing systems and by keeping the rotor system inside a vacuum chamber to reduce frictional losses and minimize heat transfer in and out of the unit.

Electrochemical Capacitors

Capacitors are energy storage devices that use electrostatic charges on the opposite surfaces of two plates. This device closely resembles a battery in that it contains two electrodes typically immersed in an electrolyte separated by a porous separator. Capacitors separate the positive and negative charges from the electrolyte ions into the respective plate surface and are able to move electrical charges through solid materials, unlike batteries that require an electrochemical reaction. This allows them to be cycled many times and to not be affected by deep discharges, unlike chemical batteries. The amount of charge stored is referred to as the capacitance.

Superconducting Magnetic Energy Storage

These units use the magnetic field produced from the flow of direct current in a coil of a cryogenically cooled superconducting material to store electrical energy. Key components of this system include a superconducting coil, a power conditioning system, a Cryogenic Refrigerator and a vacuum vessel to maintain the coil in a superconducting state at low temperatures. At low temperatures, superconducting coil that is typically made up of niobium-titanium is cooled to 4.2 K by liquid helium. The low temperature environment enables the superconducting coil to carry large current with little loss of power while avoiding the loss of magnetic energy from resistance in the wire.This storage unit is suitable for short interval discharges, as a high energy output can only be produced in a brief amount of time.

Thermal Energy Storage (TES)

This type of storage unit is mostly used with a building’s existing cooling unit, using either water or ethylene glycol during off-peak hours that will act as a supplementary cooling medium later during peak hours in the day. It serves as an optimizer to the cooling unit of the building. TES systems consist of a heat exchanger unit with helical plastic or metal coils stored inside an insulated chamber, a refrigerant pump, and a condensing unit. At night, the refrigerant is chilled, since cooling is not needed by the building facility. In the daytime, the chilled refrigerant is circulated throughout the building by the chiller system.

Usage

Energy can be categorized  into primary and secondary energy. Primary energy, such as coal or petroleum, is the initial form of energy that has not been transformed into secondary or tertiary forms of energy, such as electricity.. In the United States, the industrial, transportation, residential, commercial, and electric power sectors primarily consume primary energy. In addition, the industrial, transportation, residential, and commercial sectors also consume secondary electrical energy produced by the electric power sectors. In 2016, 81% of energy consumption in the United States came from fossil fuels, petroleum and natural gas, with only 10% coming from renewable energy and the rest from nuclear energy.

Particularly for solar energy, the produced electricity can be used in two ways. First, the electricity can be directed into a wire and used immediately to power electrical devices, machinery, etc. Alternatively, the electricity can be used to charge a fuel cell, as described in the Fuel Cells module. For large-scale solar power generation, the latter option is preferred, as often the energy is not needed immediately and can be transported or stored for long periods of time.

References

Baxter, Richard. Energy Storage: A Nontechnical Guide . PennWell, 2006.

Bouma, John. Shaping A Geothermal Power Plant . 1980.

FLSMIDTH. Equipments & Systems for Coal Preparation . Salt Lake City: FLSMIDTH, 2011. http://www.flsmidth.com/. 20 Apr. 2011. Web.

U.S. Energy Information Administration, Monthly Energy Review. Table 1.3 and 10.1. April 2017.

Acknowledgements

U.S. Department of the Interior, Bureau of Reclamation, Boulder City, Nevada.

Aaron Napier

Andrew Campbell

Developers

Nuramani Saiyidah Binti Ramli

James Rivard