Nontechnical guide to Power Station Characteristics

KEY LEARNING POINTS:

Electricity can be generated as either alternating current (AC) or in direct current (DC)

As explained in previous Module , alternating current fluctuates between positive and negative voltage, with electrons moving backwards and forwards within a circuit. Direct current is driven in a single direction by a steady voltage level. As explained in further detail below, many electricity generation technologies use rotating turbines to generate electricity, which result in an alternating current. Some technologies, such as wind and solar PV, however, generate a direct current. This can be converted into an alternating current using a device called an inverter.

Different electricity generation technologies have different operating characteristics

Key characteristics of plants include their emissions intensity, capital and operating costs, availability to generate electricity in a given time period, the share of their total potential power output that can be supplied over a given time period, and their flexibility to increase and decrease power output over different timescales (varying from sub-seconds to hours). The following specific metrics are important when considering power plant characteristics:

?   Emissions intensity. A measure of the CO2 (and other pollutant emissions) released to the atmosphere for each unit of electricity generated, commonly measured in gCO2/kWh when referring to CO2 emissions.

?   Capital cost. The cost of constructing and preparing the plant for operation.

?   Operating cost. The cost of generating a unit of electricity, which takes into account any fuel costs, as well as costs of labour, water and other utilities, as well as other resources required to support the plant’s operation.

?   Levelised cost of electricity (LCOE). A measure of the average cost of electricity produced by a plant over its lifetime, taking into account both capital and operating costs. Further information on how the LCOE is calculated, as well as its use, is given here.

?   Availability factor. The portion of time that a plant is available to generate electricity over a given time period, considering factors such as reliability, as well as downtime for maintenance. Most plants have availability factors above 70% and many low-maintenance renewables like wind and solar PV have factors close to 100%.

?   Capacity factor. The ratio of the power generated by a plant to the plant’s theoretical maximum power output, averaged over a given time period. IN spite of having high availability factors, weather-dependent renewables have relatively low capacity factors owing to the variance in wind and sunshine.

?   Ramp rate. The speed at which the output can be changed. It can vary from sub-seconds to hours, depending on the plant technology.

The capacity factor of a plant can be a useful metric to compare different generators. To calculate the capacity factor, you divide the actual power that is produced by the plant over a given time period, by the maximum potential output. For example, a wind turbine of 1MW capacity would have a maximum output of 8760 MWh/year, as there are 8760 hours in a year. In reality, the wind varies, and the turbine would produce less: in this scenario 3240 MWh. To calculate the capacity factor, we divide the electricity produced (3240) by the maximum theoretical output (8760) to give 0.37, meaning our capacity factor is 37%. Conventional hydropower and fossil fuel power plants may be able to produce much nearer their maximum output. Output may be lower however, as the capacity factor over a given time period will also depend on the proportion of time certain power plants are needed on the grid. A peaking plant will therefore have a much lower capacity factor than one used for baseload generation.

Thermal Power Stations

In coal and oil plants, chemical energy stored in the coal and oil is released as heat when these fuels are burned.

This heat is used to convert water into extremely hot (superheated) steam in specially designed boilers. The superheated steam is piped to a turbine, making it rotate. The steam is then condensed back into water for re-use. The process of generation of steam from water, and its subsequent condensation back to water, is known as the steam cycle. As the turbine rotates, magnets attached to its axel rotate, inducing an alternating current in coils of wire that surround the magnets. This current is fed into the electricity grid at a given frequency and voltage (Figure 1 below). Thermal power plants using fossil fuels can in general be ramped up and down depending on demand. The speed at which this can happen will depend on the type of fuel and plant. For example, traditionally coal plants have been slow to ramp up and down, and it can take many hours to warm the plants up and synchronise them so that the frequency of their output matches that of the electricity grid. As explained in detail in previous Module, coal plants in some regions have become much more flexible in recent years.

Figure 1 - Coal power plant

Gas generators work differently to coal and oil thermal generators and have two main types: open cycle gas turbines (OCGT) and closed cycle gas turbines (CCGT)

OCGTs consist only of a gas turbine connected to a generator, whereas CCGTs also have an additional process that harnesses the waste heat from the turbine. In the case of an open cycle gas turbine, air is drawn into a compressor, forcing it to a much higher pressure. The high-pressure air is then mixed with natural gas in a combustion chamber, where the gas / air mixture is ignited. The resulting hot gas drives a turbine as shown in Figure 2 below.

In a closed-cycle gas turbine, the heat that remains from the combustion of the gas and air is utilized.

After turning the turbine blades, the heat is piped to boil water in a steam cycle and generate further electricity from this method, as shown in Figure 3 below. CCGT plants therefore have a higher efficiency than OCGT plants, as the heat from combustion of natural gas is effectively used twice in different processes.

Figure 3: Combined Cycle Gas Turbine

OCGT plants are less efficient but have faster response times than CCGT

OCGT plants typically have an efficiency (which means the ratio of the thermal energy in the gas that is converted into generated electricity) of 35-42%. They are advantageous in that they can be started and ramped up extremely quickly to meet fluctuations in demand. In many power systems, they are used as peaking plants. CCGT plants have a higher efficiency, with modern state of the art plants exceeding efficiency levels of 60%. They typically have longer start up times however.

A nuclear power generator comprises many of the same parts as a fossil-fuel thermal power station as described above, but creates heat needed for the steam cycle differently.

The heat is created from a reaction called nuclear fission. This is the breaking up of atomic nuclei of certain elements which are heavy and relatively unstable, most commonly uranium, but in some cases plutonium and thorium. Fission is caused by firing neutrons at these elements, whose nuclei break-up (creating lighter elements) thereby releasing energy as heat. The break-up also releases new neutrons, which then cause further fission in what is known as a chain reaction. This is carefully controlled using special control rods, most commonly made of boron. The boron absorbs some of the neutrons, and the control rods are raised and lowered to manage the reaction rate at the desired level. This heat is then harnessed within a conventional steam cycle.

Due to the need to keep the reaction stable, nuclear power plants are relatively slow to ramp up or down to meet changes in demand.

They typically make up base-load and are combined with other more flexible generators to meet demand on the grid. Nuclear has a high capacity factor and would be running all the time aside from being shut-down periodically for maintenance and to replace the fuel rods, which typically last 12-24 months.

Figure 4: Nuclear power plant

Biomass - matter including wood and agricultural residues and animal and human waste - can be used to generate electricity in a variety of ways

The most straightforward and most commonly used method is through combustion of biomass to create heat, and then utilising this heat to turn a turbine in a steam cycle. Alternatively, biomass can be used to create gas. Gasification at low oxygen levels creates flammable gas that can be burned, allowing more energy to be extracted from the biomass. The gas, once cooled and cleaned, can be used in a gas engine as described above. Further to this, biomass such as agricultural and animal waste can create gas through a process called anaerobic digestion. Bacteria break down the waste in a zero-oxygen environment, releasing gas that can be burned to generate electricity.

In addition, biomass such as dry wood can be burned with coal

This process, known as co-firing, uses a conventional thermal power plant, but the mix of fuel results in lower emissions than if coal is burned alone. There are three ways in which this can be done. Direct co-firing is the simplest and most commonly deployed. This is where biomass and coal are burnt in the same furnace, at the same time but requiring different feed systems. In this method, biomass can provide up to around 15% of the required energy, but normally resulting in little or no loss in efficiency of the boiler. Indirect co-firing is a process that converts the biomass into gas, and then burns this gas alongside the coal. Finally, some coal plants may have a separate biomass boiler that enhances the steam production from the existing boiler.

Figure 6: Biomass power plant

Carbon capture and storage

Thermal plants using fossil and biomass fuels can be used with equipment that captures the CO2 that is produced from the combustion of these fuels, in a set of processes known as carbon capture and storage (CCS). The captured CO2 is transported to either depleted oil and gas fields, or to deep saline aquifer formations, where it can be injected and stored for very long periods (many centuries or more). Of the order 90% of the CO2 can be captured (using current technologies), but CCS does incur an efficiency penalty on the conversion of the thermal energy in the fuels to electrical energy, since the processes themselves require energy input. The efficiency reductions for CCS power plants are about 5-10 percentage points, relative to the efficiencies of the same types of plant without CCS. Although the different processes involved in CCS have been known and used for many decades, CCS power plants are still relatively rare, with only 2 commercial scale plants operational worldwide. In theory thermal power plants with CCS can be flexible, as per thermal plants without CCS, but much more research and demonstration is required to fully understand how CCS affects plant flexibility.

Figure 6: Carbon Capture and Storage power plant

Combined heat and power

In many countries it is common to utilise the waste heat from the combustion of fossil and other fuels which are used to generate electricity.

This co-generation of heat and electricity (commonly known as combined heat and power, or CHP) is a more efficient way of using the fuels, compared to generation of electricity and of heat in separate combustion processes. There are many different types – and sizes – of CHP plant, using a variety of methods to generate electricity, including (at larger scales) steam cycles, and (at smaller scales) engines similar to those found in cars. In both of these cases, the chemical energy in the fuel is converted to rotational kinetic energy to generate electricity.

Figure 7: Combined Heat and Power plant

Renewable electricity generation technologies

Hydropower

Hydroelectric power uses the kinetic energy of flowing water to turn turbines, thereby generating electricity

There are two main types of hydropower: dammed hydro (which in some cases can be used as pumped hydro storage) and run of river.

Dammed hydro uses the potential energy of water at a height

This water is often contained in lakes and reservoirs, which drives turbines as this water is released so that it can fall to a lower level, transforming its potential energy into kinetic energy. Hydro dams can provide a continuous generation of electricity, even in times of low rainfall. In times of severe drought however, reservoir levels may decrease to such an extent that electricity cannot be generated. Hydro dams can also be used as pumped storage, with a pump to drive water up to the reservoir during times of surplus or cheap electricity, before it is released during times of scarce or expensive electricity.

Figure8: Dammed hydro plant

Run of river hydro

Run of river hydro uses the natural flow of the water in a river to drive turbines

Run of river hydro is more variable than dammed hydro, as it is reliant on the flow of the river, that may vary with rainfall levels.

Figure 9: Run of river hydro plant

Hydro power is a renewable source of electricity, as the water cycle is constantly renewed. The construction of large scale hydroelectric dams can however cause changes to the way in which water flows and may contribute to water scarcity in some places.

Solar Photovoltaics (PV)

Solar PV converts energy in sunlight into electricity.

Key features of a solar PV array are:

?    A solar photovoltaic module is a solar cell encapsulated in weather-proof material.

?     The cell is made of semi-conductor material such as silicon. The photons within sunlight carry energy,   which  is  absorbed by  atoms or  molecules in  the   semiconductor material, freeing electrons which are then driven to an external electrical circuit, as direct current.

?     The DC power is then sent to devices called inverters that convert the DC electricity to AC, for use in the electricity grid.

The efficiency of commercially available solar cells (in terms of the ratio of the electrical power generated and the solar power hitting the module) is typically 15-22%.

Solar PV is dependent on sunlight and will therefore have a capacity factor that depends on the amount of sunlight/daylight in a given locality. Depending on the location and PV technology, solar PV typically has a capacity factor between 15 and 25%.

Solar PV coupled with energy storage (battery, hydro) can offer a more flexible solution.

Wind Power

Wind turbines convert kinetic energy in the wind to electrical energ

Turbines come in varying sizes and designs, with increasingly large turbines (which are more cost- effective) being made available by manufacturers. Key features are:

?     Blades of the turbine harness the kinetic energy in the wind to turn the turbine.

?     The movement of the turbine rotates an axel that connects to a gearbox.

?      A gearbox  increases the  relatively  slow speed of the  turbines to be fast  enough  to generate electricity.

?      The gearbox connects to a generator not dissimilar to that found within other power plants: magnets rotating around coils of wire generating alternating current. However, this current is not at the correct frequency for feeding into the electricity grid, so is first converted to direct current, and then fed into an inverter to generate the alternating current of the right frequency for grid connection.

?     This The turbine will generate the most power if it is facing towards the wind.

?      Modern turbines are able to rotate or ‘yaw’ towards the wind, maximising output. The blades can also be tilted to adjust output.

Wind speeds vary depending on different factors such as climate, geography and local topography

Winds speeds tend to be higher and more consistent at higher altitudes. Therefore, the higher the turbine hub, the greater the capacity factor is likely to be. However, as with solar PV, wind is weather-dependent as its output cannot be varied at will at any time.

Increasingly, wind turbines are being installed off-shore where wind speeds are higher and more consistent.

Offshore wind turbines can be larger as they are far from population centres, and can have significantly higher capacity factors than onshore wind turbines. But the additional infrastructure required to install them and connect them to the grid over long distances still makes them a more costly technology than onshore wind turbines, in terms of their LCOE.

Wind coupled with energy storage (battery, hydro) can offer a more flexible solution.

Figure 11: Wind turbine

Concentrated Solar Power

Concentrating solar power works by using an array of revolving mirrors, known as heliostats, to concentrate sunlight in one place to heat a fluid that can be used to generate electricity

At this point, normally the top of a tower at the centre of the plant, intense heat from the multiple reflections can be used to heat water, which is converted into steam which can be used in a conventional steam cycle to generate electricity.

Some concentrated solar power plants are instead used to heat molten salt

Molten salt can be stored for many hours and acts as a large thermal battery. It can be used in hours of darkness to generate electricity within a conventional steam cycle.

Concentrating solar thermal power plants have a typical efficiency between 15-25% and capacity factor between 29-44%

If there is no heat storage element, the capacity factor will depend on the hours of sunshine in a given location. Normally CSP plants are located in places with high insolation, such as in deserts.

Figure 12: Concentrating Solar Power

Tidal Energy

There are two main ways of harnessing tidal energy: using tidal range technology and tidal current/stream technologies

Hybrid applications can also exist where both technologies are deployed together.

Tidal range technology uses a barrage or a dam to harness power from the height difference created by the tide in a given location

As the tide changes, water will be trapped within the dam, and can be released in a controlled way to turn turbines. This kinetic energy is converted into AC electricity using a generator. This technology is most appropriately used in locations with a large tidal range (difference in water height at high and low tides).

Figure 13: Tidal barrage

Tidal current technology is instead deployed to harness power from the movement of water along a channel

Turbines are placed within a channel with an adequate tidal flow and turn with the movement of the tide in and out. The movement rotates an axel, generating AC power.

Tidal technology is not dispatchable, although owing to the regularity of tidal flows, it is highly predictable.

Geothermal

Geothermal electricity utilises heat from the earth’s core

Heat is continuously produced in the earth’s core and can be harnessed at varying depths. Since this heat is inexhaustible, geothermal energy is a renewable resource. There are a variety of ways in which the heat can be harnessed to generate electricity: through piping hot steam directly from underground wells, using hot water and rapidly expanding it to form steam, or using heat from hot water to evaporate another fluid. The resulting steam or evaporated fluid is used to turn a turbine and generate AC electricity.

Geothermal can be used as a continuous electricity source, owing to the heat’s constant presence, ramped up and down, by utilising the heat as required.

Figure 14: Geothermal plant

Waste-to-energy

The thermal treatment of residual waste is the final, indispensable stage of a modern waste management system according to the European waste hierarchy. This approach aspires to maximize resource-efficiency: When recycling is not an option, residual waste is required to be thermally treated. This allows for a safe and environmentally-sound waste disposal while generating highest possible energy yields and material recoveries.

The chemically inert bottom ash left over from the incineration contains recoverable recyclates, such as, ferrous and non-ferrous metals and is ideal for use for construction purposes. What’s more, the energy extracted from the waste is a valuable source for power to be exported to the national grid, as well as, for heat and steam to be used for both domestic and industrial purposes.

Waste-to-energy, reducing waste volumes by up to 90%, plays a decisive role in efforts to reduce the need for landfill sites which is a milestone to eliminate harmful methane emissions. Simultaneously, waste-to-energy plants produce minimal emissions that are significantly below the IED. Converting waste into valuable energy also helps to reduce reliance on fossil fuels, such as, coal, petroleum and gas. And because more than 60% of the waste-borne energy comes from renewable sources, it is largely CO2neutral.

The bottom line: Waste-to-energy is an important part of a sustainable waste management concept while contributing to an environmentally-sound energy generation.

The most common thermal treatment of waste worldwide is incineration, but there is no ‘one size fits all’ solution. The size and site of the plant will influence the type of technology chosen. Waste-to-energy plants consist of a number of differing components – fuel, plant and location being just a few. While the waste will differ from one bag to the next, our experience makes it easy for us to calculate important properties, such as, the energy available in the waste and the renewable content – details which are critical when selecting the best solution. The technology chosen needs to match the waste fuel in terms of both physical properties and environmental impact.

Despite the variants, however, all waste-to-energy plants incorporate the same basic stages:

Figure 14: Waste-to-Energy Plant