Impact of Variability and Uncertainty of Wind Resources on the Grid

Fact: The amount of wind energy that is produced and delivered to the grid depends on the wind speed.

Fact: Although wind speeds can be forecasted (uncertainty in forecast can be reduced), the variability in wind speed cannot be controlled. Meaning wind has its own schedule, we may be able to predict it, but we cannot match its schedule of the demand schedule.

Fact: The goal of a Utility is to provide reliable power, whenever the customer demands (of course within reason).

Question: What happens when wind is strong (high supply) and there is low demand on the grid?

Question: What happens when there is no wind (low supply) and demand is high on the grid?

Before answering these questions let me present the background. For simplicity there are three types of generators in a grid: Base load generators, spinning reserve and non-spinning reserves.

Base load generators are the large thermal and nuclear power plants that supply electricity. These generators operate 24x7, and produce almost constant amount of electrical power running close to rated power capacity.

Spinning reserves are generators that are spinning (or on) all the time and react of changes in electrical energy demand. Examples are natural gas fired and hydro generators. The amount of fuel is regulated by the demand. These generators are typically running at low output but can react to changes in demand quickly by increasing or decreasing output without sharp drop in efficiency.

Non-spinning reserves are generators that are turned on in case of large spike in demand or large decrease in supply. The startup time is 10 to 30 minutes. In case of unexpected events, these reserves relieve spinning reserves running at full throttle.

These reserves are pooled across utilities and grid operators manage uncertainties in demand and supply with reserves they own and moving energy from other utilities.

When wind energy is added the grid, it introduces variability to the supply side. The same mechanisms of spinning and non-spinning reserves which are used to manage variability in the grid are also used to manage wind energy variability. Meaning when wind energy is on, spinning reserves reduce output. According to Brady and Gramlich[1], when there is high penetration of wind energy, there may be a need to increase reserves, but the increase is modest. Studies have shown that integration of 10 to 20 percent wind can cost $0.005/kWh.

[1] D. Brady, R. Gramlich, “Getting Smart About Wind and Demand Response,” Wind Systems, pp. 28-33, July 2009.

Article written by Dr. Pramod Jain

Email Pramod at pramod@frombeginningtowind.com

Visit Wind Energy Consulting and Contracting Inc.

IEC Classification of Turbines: Selecting the right turbine for the site based on wind data

The International Electrotechnical Commission (IEC) creates and publishes standards for wind turbines among other electrical and electronics equipments. The IEC 61400 deals with wind turbine generators (WTG). This blog entry will explain turbine classes. Turbine classes are determined by three parameters the average wind speed, extreme 50-year gust, and turbulence. The following table explains the classifications.

WTG Class	I	II	III	IV
Vave average wind speed at hub-height (m/s)	10.0	8.5	7.5	6.0
V50 extreme 50-year gust (m/s)	70	59.5	52.5	42.0
I15 characteristic turbulence Class A	18%
I15 characteristic turbulence Class B	16%
α wind shear exponent	0.20

For standards purposes, wind speeds are measured every 3 seconds, and every 10 minutes wind speed and standard deviation are recorded. For design load calculations purposes the wind speed over 10 minutes is assumed to be a Rayleigh distribution.

All wind speeds in the above table are at hub height. The extreme wind speed are based on the 3 second average wind speed. I₁₅ Turbulence is the standard deviation of wind speed measured at 15 m/s wind speed.

As an illustration consider GE 1.5sle, a Class IIA WTG and GE 1.5xle a Class IIIB WTG. The Class IIA WTG has a rotor diameter of 77m and hub heights of 65m and 80m. It is designed for average wind speed at hub height of 8.5 m/s with turbulence of 18%.

The Class IIIB WTG has a rotor diameter of 82.5m and hub height of 80m. Because the Class IIIB WTG is designed for lower wind speed (7.5 m/s at hub height) and lower turbulence (16%), the design loads are going to be smaller, therefore its blades are larger and hub height is taller. Bigger rotors of Class IIIB WTGs therefore capture more wind energy and yield higher capacity factors compared to Class I or II WTG.

In conclusion, a wind resource assessment that is based on onsite wind measurements can provide not only the annual average wind speed, but also provide turbulence and extreme wind conditions. This data is necessary to select the class of a turbine. Wind data that is typically used for prospecting like reanalysis data and 10m airport wind data do not provide information about turbulence.

Article written by Dr. Pramod Jain

Email Pramod at pramod@frombeginningtowind.com

Visit Wind Energy Consulting and Contracting Inc.