◊ This is part of the ‘Electricity Generation’ series of articles ◊
Generating facilities convert energy from one or more primary sources to electricity.
Primary sources:
- Biomass
- Fossil fuel
- Geo-thermal
- Hydro (water)
- Nuclear
- Solar
- Wind
The sources may be classified as renewable or non-renewable.
Renewable
A renewable generator uses a primary source of energy which renews itself through a natural process. Examples of renewable generation include wind, solar, water (hydro), biomass and geo-thermal.
Non-renewable
Non-renewable generators use primary fuels that have finite availability and do not regenerate. Fossil fuel and nuclear are non-renewable generation.
Biomass (biofuel)
The term biomass refers to ‘renewable organic material that comes from plants and animals’, U.S. Energy Administration (EIA).
Biomass may be solid, liquid (biofuel) or gas (biogas). Utilization of biomass for generating electricity involves some form of combustion that produces carbon, however, the source of the biomass absorbs as much carbon during its life as it releases in combustion. The balance of carbon capture and release makes biomass a carbon neutral source of energy.
In solid form, biomass includes wood products such as wood pellets, fire wood and waste products from pulp & paper manufacturing. Electricity production comes from biomass combustion using a Rankine cycle to produce steam and drive a turbine. The steam turbine drives a generator which produces electricity. The biomass electricity production process is similar to coal-fired generation with coal being replaced by some type of biomass. A coal-fired generating facility near Atikokan, Ontario was converted from coal-fired to biomass in 2014 with a rating of 205 MW. It is the largest biomass generating station in North America.
Steven’s Croft power station near Lockerbie, Scotland produces 44MW burning timber waste, coppiced wood and recycled fibre.
In liquid form, biomass is referred to as biofuel. Biofuels are similar to fossil fuels except they come from agricultural sources which are renewable based on the growth cycle of the plants used. Corn, and sugarcane are used to produce ethanol. Soy beans and oil palm trees are used to produce diesel fuel.
Biomass can be used in the form of gas. The biogas can be harvested from various sources including animal manure, sewage and landfill sites. The gas is mostly methane produced from the anaerobic decomposition of a biomass. Electricity generation can be from a fuel cell, internal combustion engine, a gas or steam turbine.
Energy storage
Energy storage is a special case of generation that is noteworthy as it is becoming increasingly available on the modern grid. Stored energy originates from primary sources which may be dedicated or derived from the mix of generation on the grid. Technically, the fuel source for stored energy traces back to a primary source. Energy may be stored in the form of water reservoirs, gravity, batteries, thermodynamic cycles, compressed air or chemical processes (other than battery).
The grid also has stored energy in the form of inductors and capacitors (reactive compensation), however, that category of energy storage is not part of this discussion. See the article – Some physics.
Pumped water storage can provide performance similar to existing hydroelectric generating facilities. Battery storage utilizes inverter-based technology that has the capability of sub-second response times through semiconductor switching (i.e. Uninterruptable Power Supplies). Each energy storage method has its own performance characteristics including efficiency, response capabilities, economics and environmental impact.
The preferred implementation of energy storage will vary according to the need, however, a broad range of solutions are available to accommodate everything from peak shaving to improving grid transient response.
Regardless of the storage method, the primary energy source will fall into the categories described in this article. The technology used to convert the stored energy back into electricity are covered in The technology of electricity generation article in this series.
Fossil fuel
Fossil fuel is a non-renewable energy source which may be coal, natural gas or petroleum products. The fossil fuel may be used to produce steam in a boiler and drive a turbine, or for an internal combustion engine (including gas turbine). Fossil fuel generation can use a Rankine cycle, simple cycle (Brayton cycle), combined cycle or combined heat and power.
Simple cycle uses a single stage combustion turbine to convert energy to electricity. Waste heat from simple cycle generators is dissipated into the environment. These generators operate at around 30% to 45% efficiency.
Combined-cycle generators use an additional stage to recover energy from the waste heat of the simple cycle. Combined cycle increase efficiencies to over 50%.
A variation of the combined cycle generator is the combined heat and power process (CHP). The CHP process directs the heat from the combustion turbine to secondary processes which utilize the steam or hot water produced directly. The process is also called cogeneration.
Coal
Coal-fired generating stations ignite pulverized coal to create steam in a Rankine cycle. The same process is utilized for various primary energy sources.
The boiler in a generating station is optimized for efficiency using its specific form of fuel.
According to the U.S. Energy Information Administration, the average coal-fired generator efficiency in the U.S. is 32%.
Natural Gas Combustion turbine
Simple cycle
Simple cycle, or Brayton cycle is a term used to describe the process of a combustion turbine generator. Electricity is produced by using a combustion turbine to drive an electrical generator. Without any heat recovery, it is considered a simple cycle energy conversion. The generation has a low efficiency but is low cost and provides fast response when generation is required.
Oil may also be used as a fuel for combustion turbines. According to the U.S. Energy Information Administration, the average natural gas generator efficiency in the U.S. was 44%. Oil achieves approximately 30% efficiency.
Combined cycle
Generating facilities that produce electricity through multiple conversion methods or stages are called combined-cycle generators.
The most common example of a combined cycle facility uses a Brayton cycle for primary generation and a Rankine cycle to recover waste heat from the exhaust of the combustion turbine.
Efficiencies of 50 to 60% are typical for combined cycle generators.
Nuclear
Nuclear generators make use of nuclear fission to generate heat that is used to create steam. The steam is then used to drive a steam turbine coupled to an electric generator. There are many types of nuclear generators using variations of the fission mechanism with various materials. Nuclear generators come in a wide range of capacities from the legacy installations of more than 500 MW (Candu) to the newer Small Modular Reactors (SMRs) with capacities ranging from 4 to 470 MW.
Nuclear generators like the Candu are best suited to supply base load as the reactors are designed to operate with a constant output. Reactor startup and shutdown processes are complex and take days. Inadvertently tripping a nuclear generator offline is highly undesirable as it disrupts the steady state nuclear reaction and steam cycle. The nuclear reaction can become unbalanced and poison-out due to the buildup of radioactive isotopes. The reactor’s continuous steam production must be diverted from the turbines while the nuclear reaction is shut down. Once a reactor poisons out, the isotopes produced by the fission process must decay before the reactor can restart. One of the isotopes created by the process is Xenon-135 which has a half-life of about 9.2 hours. The entire process from inadvertent tripping of the generator to regaining full output capability takes several days.
Nuclear generators are the workhorse of the supply mix as they are capable of producing rated output with an extremely high capacity factor. The design capacity factor of the CANDU 6 is 89%
Just don’t inadvertently interrupt the process.
Geothermal
Geothermal electricity generation uses a hot geofluid (or hydrothermal fluid) from within the earth as the primary energy source.
A geofluid may be ground water or brine
The geothermal heat required for steam production is only found deep within the earth and generally isn’t practical or safe to access. In some areas there are anomalies in the earth’s crust where suitable conditions exist closer to the surface to produce steam.
The geothermal energy may be harnessed using a binary cycle, flash steam or dry steam to produce electricity.
The binary cycle transfers heat from a geofluid to an organic working fluid through a heat exchanger. The heat converts the working fluid to a gas that drives a turbine. The turbine is coupled to a generator to produce electricity. The working fluid is then condensed and reheated in a closed loop, sealed system to form a continuous cycle.
The binary cycle is a variation of a Rankine cycle called an Organic Rankine cycle (ORC). The working fluids have a lower boiling point than water to make the process efficient at lower temperatures than a steam cycle. The ORC is effective where the temperature of the geofluid ranges from 120°C to 180°C (approximately).
The Mak–Ban Geothermal Power Plant uses a binary cycle, located in the Philippines. It is part of a 458MW power station complex.
The 100MW Ngatamariki geothermal power plant in New Zealand is another example of a binary cycle plant. The binary cycle draws heat from wells as deep as 3,000 meters at temperatures of 290°C. The working fluid is pentane which has a boiling point of 36°C.
Flash steam geothermal plants utilize underground geofluid pumped at high pressure into a low pressure vessel where it changes to steam (flashes). The steam drives a turbine which is coupled to a generator to produce electricity. The steam turbine and generator are similar to those used in other Rankine-cycle facilities.
The 65MW Ohaaki Power Station in New Zealand is an example of a power plant using flash steam.
Dry steam power plants utilize steam produced within the earth directly. An example of a dry steam facility is the Larderello geothermal power site in Tuscany, Italy which has 34 plants with a total capacity of 800MW.
Additional information on geothermal power generation is available through the U.S. Department of Energy and the Energy Information Administration.
Hydro (water)
Hydro is the grandfather of electricity generation with the first large scale facilities being built in the late 19th century. Hydroelectric generation uses water pressure to drive a turbine coupled to a synchronous generator.
The water pressure may be produced by the elevation difference (head) between a water reservoir and a turbine, or by the velocity of flow in a river. Turbines may be oriented vertically or horizontally depending on the implementation of the hydraulics. Most installations use vertical axis turbine/generator units.
Pumped water storage
Hydroelectric systems may also be used to store energy. Energy storage may be the result of managing water levels in a forebay, or by pumping it into a storage reservoir. In the case of a storage reservoir, water is pumped from a lower water level into an elevated reservoir when electricity demand is low and drained through a turbine to produce electricity when the demand is high.
Examples of pumped storage generating facilities are on the Niagara River downstream of Niagara Falls. Both Canadian and U.S. utilities use pumped storage reservoirs to maximize the use of water on the Niagara River.
Solar
Solar electricity generation can be from photovoltaic cells or a thermal process that concentrates sunlight. Solar generation that combines energy conversion cycles are hybrid thermal solar generators. Energy conversion cycles are combined to increase overall process efficiency.
Solar Photovoltaic (PV) power plants
The most common solar power plants uses photovoltaic (PV) cells which convert sunlight to electricity through their semiconductor material. The PV cell is the building block used in solar generating facilities which are grid connected. An individual cell produces between 0.5 and 0.6 volts dc open circuit and one or two watts of energy. The PV cells produce a dc output and require inverters to produce ac voltage which can couple to the grid connection point.
Cells are connected in series and parallel to produce panels with much higher voltage and energy ratings. Panels used for rooftop solar like the Longi LR4-60HBD typically have 40 volt dc outputs and are capable of producing 375 watts under Standard Test Conditions (STC) with solar irradiance of 1,000 W/M2. Panels are arranged in arrays to increase outputs which are suitable for grid connection.
Solar PV can be scaled from miniature electronics up to massive grid connected arrays.
More information can be found in my solar generation series of articles, or from the U.S. Energy Information Administration (EIA).
Solar thermal power plants (Rankine cycle)
Thermal power plants use solar collectors (mirrors or Fresnel reflectors) to concentrate sunlight and produce heat. Collectors are equipped with tracking mechanisms to maximize the radiant energy from the sun.
The three main types of thermal solar power plants are referred to as:
- Linear concentrating systems – parabolic mirrors and Fresnel reflectors
The heat generated is transferred to a fluid which is used to produce steam through a heat exchanger in a Rankine cycle.
2. Solar power towers
Solar power tower facilities use arrays of mirrors that track the sun to focus the light on a collector tower. The towers contain the liquid media used to absorb the suns energy for steam production in a Rankine cycle. The media used in the towers may be liquid sodium or molten salts.
3. Solar dish-engines
Solar dish engine systems look like a satellite dish made up of many mirrors that focus the sun’s rays on a thermal receiver at the focal point. The thermal receiver transfers the heat to an engine/generator to produce electricity. The most common type of engine for the dish system is a Stirling engine.
Solar hybrid systems
Steam production may be supplemented by using fossil fuel to improve the capacity factor of a thermal solar generating facility.
Additional information is available from my series on solar power, the U.S. Energy Information Administration (EIA), the National Renewable Energy Laboratory (NREL) and Wikipedia.
Wind
Wind generation utilizes a wind-optimized turbine to drive a generator. There are many different types of turbines which can be used to convert wind to useful mechanical energy. The generator also has several variants, each of which has different electrical characteristics.
The most common type of wind generator is the horizontal axis propeller turbine. The propeller-type spins at a relatively low speed that must be multiplied to operate the generator. A mechanical transmission couples the propeller turbine to the generator to produce either a fixed or variable speed. Speed and torque may also be controlled by varying the pitch of the propeller blades. Typically the wind generator operates through a range of rotational speeds which prevents it from directly coupling to a fixed frequency ac grid.
Wind generating system types
Wind generators can be categorized by their physical orientation, turbine blade aerodynamics or generator characteristics.
Physical orientation
The two main classifications for wind generator orientation are horizontal and vertical axis turbines.
Vertical axis wind generators are omnidirectional, meaning the wind may originate from any direction for them to operate.
Horizontal axis generators must face the wind in order to operate efficiently. The horizontal axis turbine must incorporate a yaw-control mechanism to turn the axis into the wind. Turbine speed control is also managed by blade aerodynamics or pitch control. Wind generators operate at variable speeds and most are asynchronous machines with one exception. The type 5 wind turbine employs a synchronous-type generator and couples directly to the grid, however, types 1 through 4 are asynchronous.
Turbine blade aerodynamics
The aerodynamics of the turbine determine its speed and torque output. The two main classifications for turbine blade seed/torque control are stall regulation and pitch regulation. More information is available in my article “The technology of electricity generation”.
Wind generator type by electrical characteristics
The IEC standards organization defines wind generator types for the purpose of modelling their electrical behavior on the power system (IEC 61400-27-1). There are 5 types of wind generators (only 4 modeled in the IEC standard) based on their electrical characteristics.
Type 1 – direct-connected induction machine
A type 1 wind generator is a fixed speed machine which utilizes an induction generator. The induction generator requires separate capacitance for power factor control. The speed of the turbine is limited by the turbine blade aerodynamics (stall regulation)
A type 1 wind generator is a fixed speed machine which utilizes an induction generator. The speed of the generator must be slightly higher than synchronous speed to produce power because of the principles of ‘slip’ in an induction machine. The induction generator requires separate capacitance for power factor control.
Type 2 – variable rotor resistance induction machine
A type 2 wind generator is a variable speed machine that relies on ‘slip’ to generate power. Slip is the difference between the generator and the grid frequency. As with the type 1 generator, the type 2 requires separate capacitance for power factor control. The speed and torque of the turbine is controlled by changing the pitch of the turbine blade (pitch regulation).
Type 3 – doubly fed induction machine or doubly fed asynchronous machine
A type 3 wind generator uses variable frequency rotor excitation to maintain a fixed frequency output connection with the grid. Both the generator and the power converter are grid connected with the converter contributing about 30% to the total power output. The type 3 wind generators are called Doubly Fed Induction Generators (DFIG) or Doubly Fed Asynchronous Generators (DFAG). They are also capable of voltage and VAR control.
The speed and torque of the turbine are controlled by changing the pitch of the turbine blade (pitch regulation).
Type 4 – fully rated frequency converter
The type-4 wind turbine may use synchronous generators, permanent magnet synchronous generators (PMSG) or induction generators. It uses a fully rated converter to couple to the grid which allows a wide range of turbine speeds for power production.
Type 5 – synchronous wind turbine
A type 5 wind generator utilizes a variable speed turbine connected to a torque/speed converter coupled to a synchronous generator. In this configuration, the torque/speed converter manages the variable speed turbine, allowing a synchronous generator to operate at a fixed speed directly connected to the grid. As far as the grid is concerned, the type 5 wind generator looks like a variable output synchronous generator. The mechanical portion of the wind turbine manages speed, enabling the use of a synchronous generator with automatic voltage regulator (AVR) controls.
Wind generator grid connection requirements
The early generations of wind generators were not particularly grid-friendly in that they did not provide VAR support (see Some Physics). As the wind generation share of grid energy increased, standards and reliability organizations became concerned about the impact it would have on grid operation. In 2015 the Federal Energy Regulatory Commission (FERC) in the United States began discussions to eliminate the exemption of wind generation for providing reactive power to the grid. FERC order 827 in June 2016 removed the exemption for wind generators reactive power support. Since that time reliability organizations (i.e. NERC) have revised requirements around renewable resources including wind generators. New wind generator installations must meet more stringent requirements for power factor and voltage control as specified in applicable standards.
The IEEE has developed standard requirements for wind generators that connect to the grid (IEEE 1547-2018 or latest), however, local jurisdictions may have their own requirements.
See also the series on Wind Generation and the US Department of Energy.
previous article > The technology of electricity generation
next article > Connecting Generation to the grid
Derek
