I am often asked, what is the "standard" or "safe-level" for penetration of wind energy into the grid. This blog will attempt to provide answers and explains the issues related to variability of wind power.
Fact: Wind energy (or solar energy) is variable; it is produced only when wind is blowing (or sun is shining), and the amount of energy produced depends on the wind speed (or amount of radiation).
Fact: Electricity demand is variable; consumers turn on appliances at will. And in most grids, electricity is provided by the grid reliably to meet this variable demand.
Fact: In general, there is no correlation between demand and renewable energy supply; if you are lucky, when the consumer loads are high, renewable energy production is also high.
Fact: At any given point of time, the amount of electrical energy supply must be equal to the amount of energy consumption, if the grid has no electrical storage.
Fact: Modern wind plants are grid friendly. It provides both real and reactive power, and have low voltage ride-through (LVRT) capability. Electrical energy is provided at desired voltage and frequency. Forecasts of wind plant output is reasonably accurate for when the wind is hour-ahead to 6 hours ahead. Accuracy of day-ahead forecasts is being improved continuously.
So the question is, how is this real-time balance achieved? The answer is, flexible generation units; there are different names like peaking generation units or reserve units (spinning and non-spinning). Spinning reserves are online and spinning, these units sense the changes in load and produce appropriate amount of energy to deliver to the grid--examples of such units are gas-fired generators and hydro-electric plants. Non-spinning reserves are generators that are turned on and start producing energy in a few minutes; for example dispatch center may turn on a diesel generator at 9AM every weekday because the demand starts rising at 9AM. Both spinning and non-spinning reserves provide the ability to balance the energy supply and demand. Then there are the base-load generation units that provide almost constant electrical energy through out the day. Examples are coal-fire or nuclear power plants that are run at 90%+ of rated capacity.
Consider an illustrative example: Grid has demand with the following profile (over a period of one year), minimum demand of 900MW, maximum demand of 2,200MW, and average demand of 1,600MW. In this situation, it would make sense (with out going into lot of detailed demand analysis) to have a base load generation of 900MW and reserves of at least 1,300MW. The reason for saying at least is because generator failures need to be accounted; reserves not only provide energy during peak demand times, but also serve as backup then base-load and/or other reserve generators fail. So, reserves determine reliability of a grid to meet demand.
Base-load generation plants have some flexibility (depending on specs)--they may be run between say 80% and 100% capacity. So these plants also follow the load within limits. Below the minimum level of rated capacity, the plant become too inefficient or unworkable.
As a rule, base-load generation is the cheapest, non-spinning reserve is next, and spinning reserve is the most expensive. It turns out this is not always the case: Non-spinning reserves like Diesel or Heavy Oil plants produce electricity at a higher cost than gas-based plants. So the grid operator dispatches plants (turns them on or off) based on cost, contracts, response rate, operating range, and few other factors.
In this back drop, wind energy generators provides variable source of energy into the grid. For illustration, lets consider the following simplistic cases (real life is much more complicated):
- Case 1: Penetration level of wind is small (less than 5% in annual energy terms):
- When grid is at minimum demand and supply is from base-load generation plants, then the grid is able to absorb wind energy by adjusting output of base-load plants--remember base-load plants have a range in which output may be adjusted. Note if wind plants are providing 5% energy annually and the wind plants have a capacity factor of 33%, then at time of peak wind, the wind plants will provide 15%-plus of energy into the grid when demand is low
- When grid is at maximum demand, and supply is from base-load and reserves (spinning and non-spinning), then wind energy displaces reserves (grid operator decides how to distribute the reduction among the reserves)
- When grid demand is at some intermediate level, then again grid operator decides how to distribute the reduction
- Case 2: Penetration level of wind is higher (between 5% to 20%):
- When grid is at minimum demand and supply is from base-load generation plants, then the grid is able to absorb wind energy by adjusting output of base-load plants--remember base-load plants have a range in which output may be adjusted. Note if wind plants are providing 20% energy annually and the wind plants have a capacity factor of 33%, then at time of peak wind, the wind plants will provide 60%-plus of energy into the grid when demand is low
- When grid is at maximum demand, and supply is from base-load and reserves (spinning and non-spinning), then wind energy displaces reserves (grid operator decides how to distribute the reduction among the reserves)
- When grid demand is at some intermediate level, then again grid operator decides how to distribute the reduction
It is clear that the problem is at times of minimum demand and when the base-load generation cannot be reduced below the minimum acceptable level. In these cases, there are unfortunately no options other than curtail wind energy production.
To be continued ...