Renewable Energy Integration and Wind Challenges

March 2012

• According to Ontario's IESO, there are 4,700 MW of intermittent generation - primarily wind - projects underway and expected to be in commercial operation by the end of 2012.
• The addition of intermittent resources, and in particular wind, will present operating challenges for the IESO because of the variable nature of the output (called ramping) and the quantity of supply being added.
• Adding wind capacity will contribute to surplus baseload generation and increase ramping requirements just as the Ontario grid's most flexible ramping resource - coal - is being phased out.
• The IESO is putting considerable effort into integrating renewable energy and addressing the operating challenges associated with intermittent generation.

Ontario's Long Term Energy Plan calls for 10,700 MW of non-hydro renewable energy to be installed by 2018. Much of this will be variable or intermittent generation, and much of it will come from wind. According to Ontario's Independent Electricity System Operator (IESO), there are 4,700 MW of variable generation (primarily wind) projects underway and expected to be in commercial operation by the end of 2012.

The addition of these resources will present significant operating challenges for the IESO, arising from the variable nature of the output and the quantity of supply being added. There are problems associated with integrating wind into electrical grids.

Wind Characteristics

While wind output varies, it is considered a baseload resource. The hourly output of a wind fleet can vary greatly from hour to hour. Having too much wind can be just as big a problem as having too little.

In April 2011, Aegent discussed some of the results from its aggregated analysis of Ontario's wind fleet output for January 2009 through December 2010. Output values were normalized based on the output capacity available at any given time, to account for the growth of the wind fleet over that period. The average wind output of the Ontario wind fleet over the analysis period was 27.8%. This value is consistent with the approximate 30% average output usually attributed to onshore wind.

Hourly Output

A frequency distribution provides more details and insights on how the wind fleet operates.

In addition to the average output noted above, there are at least two key observations. The "mode" or most common output level is just 8.5% (of total fleet output). The median output - where the output falls below/above 50% of the time - is 21.5%. The low mode in particular points to why it's not uncommon for system operators to count not more than 10% of installed wind capacity towards firm supply.

Hourly Changes

Wind output can also change relatively quickly, that is, in less than an hour - going from a high to a low or vice versa. This is the characteristic that presents the greatest challenge for electricity system operators. These are sometimes referred to as ramping characteristics.

The following frequency distribution provides more details and insights on how the wind fleet output changes from one hour to the next.

As expected, the distribution is symmetrical. Also, 80% (between the 10th and 90th percentiles) of occurrences fall within the range of -/+ 4.6%.

Operational Challenges

The addition of significant intermittent resources, and wind in particular, will add to a number of IESO operating challenges. Surplus Baseload Generation

Surplus baseload generation (SBG) occurs when there is more baseload generation supply (nuclear, hydro and wind) online than there is load. Low load hours are most common in shoulder months and typically occur during the early morning hours. Exports can help temper the problem but at times, inter-tie limits and load conditions in adjoining markets limit this relief.

In dealing with SBG, the IESO first relies on hydro and nuclear "maneuvers". Baseload hydro maneuvering is generally preferable and takes place by varying water flows over a very narrow range or by spilling water. The length of time a hydro unit is impacted can be very short. Nuclear maneuvering involves reducing output by reducing reactor output or using a steam bypass process to reject heat energy from a unit's conventional thermal loop. Nuclear maneuvering can affect unit output for a number of hours.

When these maneuvers cannot deal with the quantity of SBG present, the last resort is to shut down a nuclear unit. This can take the unit offline for two to four days. A more detailed discussion of SBG and the IESO's daily SBG Forecast reports can be found at: http://www.ieso.ca/imoweb/marketdata/sbg.asp

Local Oversupply

Conditions of local oversupply are similar to a global oversupply. Within a specific area, supply exceeds demand and transmission is not sufficient to carry away the excess power. This leads to the area being somewhat "landlocked" operationally, meaning there can be a reduced range of resources available to deal with the oversupply situation.

Operational Needs

As mentioned, intermittent resources and wind in particular have varying output levels. These changes are also referred to as ramping. Load also has ramping characteristics, generally increasing in the morning and tapering off at night. When the ramping directions of wind and load are positively correlated, this is a good thing, as it reduces the need for other ramping generation. A problem arises when the directions of change of these two factors oppose each other - wind output is falling while load is rising, and vice versa. This increases the need for other forms of ramping generation. Ideally, the IESO can manage these swings by using resources with a more flexible output range, such as coal-fired or natural gas-fired generation.

Outlook

Adding any type of baseload generation will exacerbate Ontario's SBG problem. Two Bruce A nuclear units with a combined nominal output of 1,500 MW will come online in 2012 and wind will also contribute significantly to the problem. The IESO has stated that to manage this it will be relying on coal generation as one of the options to providing flexibility.

Impact of Coal Phase-out

Growing wind capacity will contribute to SBG and greatly increase ramping requirements, just as the Ontario grid's most flexible ramping resource - coal - is being phased out. This means that in terms of accommodating rapid wind fleet growth, the phase-out of coal-fired generation could not be happening at a worse time. Coal-fired units are particularly valued for their output range flexibility, commonly being able to operate between 10% and 100% of their rated output. A theoretical 100 MW coal-fired unit can then run between 10 MW and 100 MW and is said to have 90 MW of "ramp depth".

Natural gas-fired units are replacing coal. They typically have much less flexibility and the amount is a function of a number of factors: simple or combined cycle design, gas turbine type, steam turbine (if applicable) and heat recovery steam generator type and configuration. The vast majority of Ontario's newer gas fleet procured through the Ontario Power Authority's Clean Energy processes are combined cycle plants. The IESO views them as having a minimum output of about 60%, meaning a theoretical 100 MW unit would run between 60 MW and 100 MW and so have 40 MW of ramp depth.

To achieve 1,000 MW of ramp depth, about 1,100 MW of coal-fired generation is required while 2,500 MW of natural gas-fired generation is needed. Put another way, the approximately 3,500 MW of coal-fired generation still online in Ontario in early 2012 has about 3,150 MW of ramp depth - the same depth provided by 7,900 MW of natural gas-fired generation.

IESO Activities

The IESO has and will put much effort into integrating renewables. Past activities occurred under the IESO's stakeholder engagement SE-57 / Embedded and Renewable Generation (http://www.ieso.ca/imoweb/consult/consult_se57.asp). That initiative was superseded by SE-91 / Renewable Integration (http://www.ieso.ca/imoweb/consult/consult_se91.asp).

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