Saturday, 17 December 2011

Does wind displace carbon dioxide emissions?

Does wind displace CO2 in the electrical power system?  Simple question.

On the surface, here's the simple answer:

Since electricity can't easily be stored, whenever wind blows (or the sun shines on solar panels or snow-melt cascades through a run-of-river hydro generator) then some other generating source must be throttled back.  In Ontario, it's the job of the Independent Electricity System Operator (IESO) to issue commands to various generators to maintain that balance.  It's called dispatch.

Normally, the IESO will throttle back the generator with the highest variable price (called the marginal generator).  It's critical to understand that the IESO uses the variable price, not the full price that would include capital costs.  For renewable energy, that price is based on the generator's variable cost which is essentially zero in the short run.  For fossil fuel generators, that price is based on fuel value.  

Therefore, when a renewable energy generator ramps up, a fossil fuel generator ramps down.  So, when wind goes up, coal or gas use goes down.  After that, it's simple math to figure out the CO2 impact.  And that's where you see claims that a KWh of wind displaces x tonnes of CO2.  That's also why x is different for every jurisdiction.  Each jurisdiction has a different coal/gas mix.

Now it gets slightly more complicated:

What is being displaced, coal or gas?  If the IESO is optimizing the system purely on price to the consumer, the fuel most frequently displaced (at least historically) is gas because its fuel cost has been higher than coal.  It's a bit of a toss-up right now.  However, in Ontario, coal versus gas is less relevant because the government has intervened in a couple of ways.

1. In 2009, it placed a coal price adder on power generated from coal.  The government owns OPG, so this was simple to do.  The coal price adder was simply a form of carbon tax.  The concept worked for awhile until the recession knocked demand down and the adder became an inadequate deterrent for OPG to not burn coal.
2. It then placed a CO2 tonnage limit on OPG with the limit dropping every year.  At some point that declining tonnage limit was simply translated into a generator unit shutdown schedule.

So, clearly, coal was being taken out of the system.  The number of TWh (a TWh is 1,000 GWh or 1,000,000 MWh) of energy from coal went from 23.2 TWh in 2008 to 12.6 TWh in 2010. [source: IESO]. It was replaced by new sources of renewable energy (predominantly wind at 2.8 Twh).  Plus natural gas and imported hydro for the times when the wind doesn't blow.  OPA contracted for a few 1000 MW's of natural gas generation and Hydro One built a 1100 MW tie-line to Hydro Quebec to accomplish this.

Simple math would suggest that the reduction of energy supplied from coal would exactly match the increase in energy derived from wind plus natural gas plus imported hydro.  Not quite.

Now it gets complicated:
If you want to skip the details, just go to the end of this section to see what the experts say.  Otherwise, here we go.
Claims have been made by many anti-wind or pro-fossil fuel advocates that all the theoretical displacement of CO2 is lost by the need to keep backup fossil fuel generators running.  The argument typically goes like this:

1. Assume a 100 MW wind farm.
2. That wind farm will need a 100 MW gas turbine to back it up.
3. That gas turbine will cycle up and down in an inverse relationship with wind.
4. In doing so, it is typically running at a fraction of full capacity, where its CO2 output per kWh goes through the roof.
5. In some cases, at low wind speeds, wind actually causes a net increase in CO2 output.

Let's take this apart, line by line.

1. You don't have a 100 MW wind farm.  You have a network of wind farms, rated in total at closer to 1500 MW, connected to an electricity system.

2. The 1500 MW wind network will need backup for the summer peak when wind is at its seasonal low.  Solar will be at its seasonal high, though, so as solar expands, less backup is required.  And if tie-lines to Quebec are strengthened, then even less backup is required.  However, some gas capacity will likely be needed, especially since Pickering nuclear capacity starts coming off line in about 5 years.

3. The electricity system already has enough spinning reserve to cover the largest contingency in the system.  In Ontario, that's the loss of a Darlington unit - roughly 900MW.  Depending on the time of day and week, that reserve could be Ontario hydro-electric (Ontario actually has quite a bit of stored water), Ontario gas, Quebec hydro-electric via the tie-line; or gas turbines.  Any variations in wind plant output will be handled by this and other reserves.  The IESO has done this sort of balancing for years and they're very good at it.  Renewable energy increases the variability somewhat, but it's manageable.  After all, today, the variability in the output of wind is less than the variability in demand that occurs hour-to-hour during the day.

4. The only way that this concept comes off the rails is if the backup is unresponsive (this is called a low ramp rate).  Hydro-electric is very responsive but there may be times when it's not available and thermal generation is required.  Historically, only peaking gas turbines had that kind of ramp rate (and they were inefficient) but modern combined cycle gas turbines provide the necessary CO2 efficiency and high ramp rates.  Tom Adams, the energy watchdog, used to worry about gas turbine ramp rates but he's revised his thinking (he's still against wind turbines, I believe, but he's changed the basis of his argument).

5. I think that I've proven that item 5 is impossible.  If one wants further proof, though, just look at the hourly generation factors for gas turbines - at the unit level.  You have to get beyond the IESO data and look at individual turbines.  For example, Greenfield GS has a rating of 1005.0 MW, but it is actually composed of 3 gas turbines plus a steam turbine (the complex is called a combined cycle gas turbine, or CCGT).  If you look at the output per turbine over the course of the day, you'll see that they typically come on at over 70% of each turbine rating, sometimes in sequence and sometimes all together.  This is because the operators don't want to run at inefficient levels and because the IESO is able to dispatch it's turbine fleet as discrete units.  In other words, they don't say "OK, everyone run at 30% output".  Instead they say, "OK, you three run full out, you seven standby".

So, what's the answer?

If this explanation isn't  sufficient, maybe the Institute of Electrical and Electronic Engineers (IEEE) can help. The IEEE has collected numerous studies by utilities around the world and they all point to the same conclusion.  In an article  in the Power Engineering Society Journal, they demonstrate that only 4% of CO2 displacement is lost by a need for backup. 

If you want to listen to more experts from Ontario's power system, go to Glen Estill's wonderful blog.

1 comment:

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