Wednesday, August 24, 2011

Different 'fuels' at different times

An article entitled ‘We can all do little things for the environment’ appeared in The Kitchener-Waterloo Record on 23 August 2011.  Though the theme was broadly environmental, there was a paragraph’s focus upon electricity issues. Notwithstanding the issue I would take with the comment that ‘On summer weekdays, hydro costs almost double between 7 a.m. and 7p.m.’ (see my earlier posting), the phrase that I will come back on is the following:  ‘In Ontario, if we all run our air conditioning units during the peak time it means Ontario has to start up high polluting coal-fired plants, or buy electricity from American coal plants. Very dirty!’  I thought it would be worthwhile to dig into this.
To do this, I went to the site of a small company called Sygration – harvesting data from the Independent Electricity System Operator in Ontario, these are usefully put into interesting form there.  I pulled out ‘yesterday’ (23 August 2011), as well as 21 July 2011 – a day on which the University of Waterloo weather station says that the temperature reached 37.6°C!  (I was fortunate enough to be in a lake on that day! :>)  Not surprisingly, peak output by Ontario’s power plants was 19,135 MW (hour ending 5pm EDT) on 23 August, but 25,692 MW (same hour) on 21 July – some 34% higher!  In terms of energy, generation on that hot day – across all 24 hours – was 33% higher.  So, across all hours of the day, power plants across the province had to be ‘geared up’ that much more.
That invites a discussion about ‘dispatching’.  Electricity cannot be easily (cheaply) stored.  As such, it needs to be generated when it is needed.  Choices are thus made by system operators as to which plants to ‘call’.  Cheap ones are obviously preferred first, but consideration must also be given to those that are easily (or not easily) ‘ramped up or down’ --  that is, responsive to changes in dispatch instructions from the system operators.  Of the major contributors in Ontario, nuclear is usually run ‘all of the time’; hydropower is generally cheap and ‘rampable’, and thus used often and/or strategically.  The fossil fuels (coal, gas and gas/oil) are not only dirtier, but also often more expensive; they can be, however, responsive to changes in system-wide demand.
So that is meant to help explain the chart below – where the relative contributions of Ontario generators on these two days are listed.  On the ‘lower demand day’ (23 August), nuclear and hydro predominate, and fossil fuels contribute less than 15%.  On the ‘higher demand day’ (21 July), fossil fuels – including the need to operate the Lennox ‘oil/gas plant’ – contribute almost one-third. 

percentage share to total daily energy on 21 July 2011
Percentage share to total daily energy on 23 August 2011
Gas and gas/oil

It might also be useful to think about how the fuels are used during different time-periods – namely, Ontario’s three (summertime) ‘bands’.  I present this in a couple of visuals below – plotting MW generation against time (EST), and then shading the off- (green), mid- (yellow) and on-(red) peak periods.  On 21 July, gas/oil, in particular, plays a relatively predominant role during the on-peak period; the fossil fuels certainly ‘ramp up’ in the morning, but take a while to ‘tail off’ in the evening.  In 23 August, all of those fuels are ‘near the bottom’, given how much nuclear meets all of the needs.
The bottom line? – carbon intensity varies with time, and it is, for the most part, higher during the high-demand/‘on-peak’ periods.

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