Yesterday I had a discussion about our current project. The discussion was actually not only limited to that topic, however a topic in general: power plant. When one has plan to build a power plant, he must know operational profile of the power plant. Is it used for base load, peak load, or follower load ? What does it mean?
In this picture I show you the difference between those three.
To understand better about the difference between the three load based on its capacity, please see graphic below.
Base load is minimum demand on an electrical supply system over a 24 hour period. The electricity is supplied by the grid’s most efficient plants operating continuously at their optimum capacity 24 hours per day. Actually, the demand is varies over time. To catch up the demand, electricity is supplied by smaller more flexible plants.
Characteristics of Base Load Power Plant are:
- Typically generated by very large and highly efficient thermal base-load generation plants
- Often called slow assets due to its slow response to changes in demand. If started from cold, these plants need more than two hours to come on stream before they are synchronized with the grid
- Plant ramp up rate is very slow. For example, coal fire power plants are only 1.5% to 3% of full rated capacity per minute. On the other hand, ramp up rate of gas turbine is 10-100 MW/minute, with an average 25 MW/min
- Tend to lead significant wear if it is used to provide variable load
Note: ramp rate is the rate at which a power plant can increase and decrease output.
Based load power plants include geothermal systems and nuclear fission. The generators driven by steam turbine is also possible with steam raised by burning fossil fuel (coal, oil, and gas). Electricity is also possible to be generated by hydro-electric with hydraulic turbine driving the generator.
Load following is characterised by power output which may change as often as every minutes in response to changing demand.
Characteristics of Load Following are:
- Because it is operated not at rated output/design output, its efficiency is reduced.
- Efficiency when operated at optimum load is not as high as base load
- Construction and capital cost is lower than base load
Figure below show several type of power plant and time required to reach its full power.
Peak supplies power plants are operated when there is exceptionally high demand of electricity. Typically peak supplies power plant is gas turbine. I think it is because ramp rate is high. It is the most flexible generator to adjust power level, yet the most expensive to operate.