Coal And Gas Plants
Coal can be used to create electricity when it is pulverized and burned to boil water into steam. The steam is used to spin a turbine and the turbine shaft spins a generator to generate electricity. Steam generators are also known as ‘thermal plants’. Natural gas and nuclear plants can also make steam to create electricity. However, in many natural gas plants the combustion gases are utilized directly to spin a turbine and generator. This is referred to as a simple cycle or ‘combustion’ gas turbine. If after passing through the combustion turbine the exhaust gases are used in a second stage to produce steam, this is called a combined cycle. The average efficiency of a steam generator (thermal plant) is about 33%, the average efficiency of a natural gas combustion turbine (simple cycle) is about 30%, and the average efficiency of a natural gas combined cycle plant is about 45%.
Roles Of Power Plants
Plant efficiency alone is not widely valued by utility managers. Generation must ‘follow’ the load (or demand): as load increases or decreases across the grid, generation for that grid must be increased or decreased to match load at any given time (as well as to provide the appropriate amount of reserves). Power plant units can be characterized by how they are used to meet this ever-changing demand.
A ‘baseload’ generating unit is run fairly steadily throughout a day and throughout the year. They are dispatchable, which means that they can be controlled to run at specified levels over specified time periods. Together, baseload units can provide a significant portion of overall generating resources on a grid at a given time. They are often steam generators that cannot ramp up or down quickly and that operate most efficiently when run near capacity. Coal, nuclear, geothermal, natural gas, and hydro are all potential baseload resources.
‘Cycling’ or load following units are dispatchable but are ramped up or down throughout a day to meet changing demand. Hydropower and combined cycle gas plants are commonly utilized as load following resources. ‘Peaking’ units are a type of load following unit that are utilized primarily to meet peak demand on a given day. Although they are less efficient than thermal plants, gas combustion turbines can be started or stopped relatively quickly, can be ramped between load levels quickly, and can operate at a wider range of power outputs than thermal plants. Combustion turbines are therefore most often used as peaking units. To some degree, solar can be used a summer peaking units when the peak is due to increased demand for air conditioning on sunny days.
Renewable Energy Integration
Because solar and wind are not dispatchable–they cannot be controlled to provide a specified output at any given time – and because their fuel is free, these resources are used to generate electricity whenever they are available. Load following and sometimes even baseload resources must therefore not only be managed to be responsive to changing demand, but also to changing supplies of wind and solar.
In addition to the responsiveness of different types of power plants to changing supply and demand, cost is also a foremost consideration for those operating the electric grid. Plants that are least expensive to run are typically operated first to meet the demand for electricity at any given time, followed in order by plants that are more expensive to operate. Historically, this has been the reason behind widespread use of coal, but decreasing natural gas prices have resulted in greater use of gas and decreased use of coal to meet demand.Last updated: October 18, 2017 at 11:31 am