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Load Ranges of Power Plants

Power Demand_PSU_052021A
[Power Demand - Pennsylvania State University]

- Overview

Power plants use three load ranges:
  • Base load: The minimum level of electricity demand required over a period of 24 hours. This load is needed to provide power to components that keep running at all times.
  • Intermediate load: The load from mid-morning until the evening.
  • Peak load: The time of high demand, often for only shorter durations. Peaking power plants, commonly known as peakers, operate during times of high demand.

Power plants are used in these ranges according to their operational and economic properties. A high load factor means that the total capacity of the plant is utilized for the maximum period, which results in lower cost of the electricity being generated. 

Plant load factor (PLF) is the ratio between the actual energy generated by the plant to the maximum possible energy that can be generated.


- Base Load Power

Baseload power is the minimum amount of electricity that needs to be supplied to the grid at any given time. Power plants need to meet day-to-day trends in electricity usage, but it is not optimal for a power plant to always produce the maximum required power. Therefore, there are baseload power plants, such as coal-fired power plants that provide the minimum required power, and peak power plants that meet fluctuating demand. Power demand fluctuates widely throughout the day, so baseload power may not be sufficient. The grid needs to use peak shaving power, which is electricity that is provided to meet changing power demands. 

Baseload electricity must be provided by a stable and reliable source of electricity. They are also sometimes schedulable to cover unreliable intermittent power sources. Power plants that provide baseload electricity typically run year-round - so have a high capacity factor - and use non-renewable fuels. Some baseload power plants include coal-fired and nuclear power plants.


- Fuels for The Base Load Power Plants

Baseload power supplies are plants that operate continuously to meet 24/7 minimum power demand levels. Baseload plants are often large and are key components of an efficient grid. Baseload plants generate electricity at a constant rate and are not designed to respond to peak demand or emergencies. Baseload power generation can rely on renewable or non-renewable sources. 

Non-renewable resources (fossil fuels) include: coal, nuclear fuel. Renewable resources include: hydropower, geothermal, biomass, biogas, and solar thermal resources with associated energy storage.


- Fuels for The Intermediate and Peak Load Power Plants

Baseload power plants are usually coal-fired or nuclear because they produce low-cost fuel and steady-state electricity. Hydro and geothermal energy can also be used for baseload generation if these resources are available within the region.

Renewable energy systems such as solar and wind power are best suited for medium-load power plants. These are intermittent energy sources whose output and capacity factors depend on weather conditions, daily and seasonal variations. Therefore, unless there is an efficient energy storage system in place, they cannot be relied on to meet continuous demand for electricity supply, nor can they be used immediately to respond to peak demand. However, as intermediate energy sources, solar and wind energy systems can be efficient and help reduce reliance on fossil fuels. 

Peak power generation is often attributed to systems that can be easily stopped and started. Possibilities are gas and oil plants, hydroelectric facilities. 

As things stand, we can see that the baseload electricity niche is currently dominated by non-renewable energy systems and is therefore unsustainable. But here's a tentative question: Can baseload power be supplied entirely by renewables? Or can't we avoid coal entirely? Clearly, the difficulty with renewables lies in their intermittency in time and place bias. Comparing typical capacity factors for various energy systems reflects this difficulty.

Jackson Hole Mountain_Wyoming_111421A
[Jackson Hole Mountain, Wyoming]

- The Time-of-Day Power Demand 

Typically, electricity demand changes periodically every day, reaching a maximum value during the working hours during the day, dropping to a minimum value late at night and early in the morning, but never falling below a certain base value. 

This base load is typically 30-40% of maximum load, so the amount of load allocated to base load equipment is adjusted to that level. Electricity demand above the benchmark (above the benchmark) is handled by intermediate and peak power plants, which are also included in the grid. 

The main advantages of baseload power plants are cost efficiency and reliability at optimum power levels. The main drawbacks are slow response times, lack of fuel flexibility, and inefficiency when operating below full load.


- Nominal Capacity Rating

Baseload plants (and other energy conversion facilities) are characterized by their rated capacity. For example, if a power plant is rated at 1,000 megawatts, that means it can produce 1,000 megawatt-hours of electricity per hour at full load. Actual power generation may be less, depending on demand or operating conditions, and can be characterized by a capacity factor (CF):

CF = [actual generated output] / [maximum possible output]

For example, let us calculate the capacity factor for a 1,000 MW base load power plant that generated 512,000 MWh of electricity over the month of January.

In this case, the maximum energy that can be generated by the plant at full capacity over this month can be determined as follows:

E(max) = 1,000 MW x 31 days x 24 hour/day = 744,000 MWh


CF = E(real) / E(max) = 512,000 / 744,000 = 0.69 (69%)

There are many reasons why a factory's capacity factor is below 100%. Some of them are:

  • reduce the demand for electricity for a certain period of time;
  • insufficient capacity due to maintenance;
  • equipment failure/interruption;
  • resource/fuel shortage;
  • equipment upgrades (resulting in high nominal capacity).


[More to come ...]

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