Base load is the output achieved at the thermal limit of the machine. More often than not the first limit reached is the exhaust temperature limit which is inherent in the design of the engine.
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If we learn from our mistakes I'm getting a great education!
For a power plant base load is usually described as 85% of maximum possible out put. Take the plant out put in MW hr for the year and divide by the plant rating times the number of hours in a year.
Usually plants that are used for peaking or high demand only will have PLF less than 35%.
Base load refers to operation and is a value (x% of maximum continuous rating or MCR of the machine or unit) agreed upon between the supplier and grid operator. Under base load operation electricity is generated at a constant rate but the unit may load follow within a certain designated bandwith.
For a Gas Turbine, "Base Load" is defined as that load (at rated firing temperature) consistent with maximum anticipated life of hot gas path parts. "Peak Load" is defined as operation at higher than rated firing temperature. Operation at "Peak Load" will result in a reduction of life for hot gas path parts.
The actual load achieved at "Base Load" depends on the application (fuel, ambient conditions, site elevation, inlet system losses, exhaust system losses, etc.).
Adding to the above, base load connotes continuous operation, even though it may be load following as metengr mentions, while peaking (not "peak load" because even a base loaded unit may be brought to "peak load" from time to time as it follows load) connotes a unit that is only brought on line when the load demand peaks. Peakers may or may not run at "peak load" but what is generally recognized about a peaker is that they rarely run.
I once heard a peaker plant manager tell a supplier that he only ran ~500 hrs per year and never wanted to exceed 800 hrs/yr because that would throw him into the requirement to install CEMS (Continuous Emissions Monitoring) equipment which also required monthly reporting and he didn't want to do that so he was happy to keep his load factor at below 500 hrs/yr. Now that is a peaker.
Typically speaking, a combustion turbine isn't used in a base load application unless it is used as part of a combined cycle or a co-gen. On a simple cycle basis their heat rate just doesn't permit it. Nuclear plants and large coal units are run as base load units and CT's are brought on and off line to manage the peak loads.
Adding to rmw's comments, some combustion turbine plants exist for the primary purpose of serving as spinning reserve. Their fast start and fast loading capability allows them to reach full load fast enough to satisfy their spinning reserve rating while being at rest. This permits other units, typically nuclear or coal, to operate at higher, more efficient loadings while the GT plant consumes no fuel. Each start for these plants is brutal, but since relatively few starts are involved, these plants can be very effective both economically and environmentally. These plants almost always use light fuel oil stored on site for the sake of having an assured fuel supply. They are very costly when they run, but they make lots of money or save lots of fuel when they are not running. Their economics can be astoundingly strong. The base load for these plants may seem relatively high, but this is based on a relatively short operating life.
In other cases, the exhaust from a combustion turbine may be used in a process plant. In these cases, the operating temperatures may be unusually low and the operating life can be amazingly long. The seemingly low generating efficiency is more than compensated by the use of the exhaust heat.
As others have indicated, the true meaning of base load is very much application specific.
Scotty is right, but in some cases a calculated turbine inlet temperature limit is the first limit to be the determining factor. Depends on the manufacturer.
I.e if your temperature limit is 1000C then once you hit that with your guide vanes wide open, then you are at base load
I must come from a different school of thought. Aside from the comments about base load vs Peak load which would pertain to any piece of generating equipment, steam, gas, hydro, diesel, etc, with respect to a CT, I always considered base load being the name plate load at which point the CT was rarely operating at a limiting parameter such as temperature control, be it EGT or turbine inlet temperature. When the unit was then pushed to the parameter that limited it, and you couldn't add any more fuel or load, then it had hit its peak and that was what was referred to as peak load.
But then again, that was back when we often had to read the instruction manuals by candle light because there wasn't electricity everywhere.
Nameplate load values are not relevant to base load. The actual base load MW will be what the GT is at when it hits the temperature limits
Peak load is when the GT base load limiting factor has been increased, normally for a limited time, to allow x% more MW. In both cases the GT has hit the limiting parameter and the fuel valve(s) will modulate to keep the EGT or inlet temp to that value. (Assuming the MW setpoint is set abouve actual MW at that point)
