Wind Integration: Incremental Emissions from Back-Up Generation Cycling (Part IV – Further Reflections)
Three previous posts have examined the emissions problem related to intermittent industrial windpower that is firmed up with fossil-fuel generation.
- Part I presented a framework of the necessary considerations and an interim assessment of the effects on fossil fuel consumption and CO2 emissions until sufficiently comprehensive studies can be performed in the areas indicated. This analysis shows approximately the same gas burn and an increase in related emissions, including CO2, compared to the no-wind case.
- Part II reviewed the simplistic, incomplete approach that is usually claimed by wind proponents and policy makers. Introducing necessary considerations shows the dramatic, negative impacts presented in Part I.
- Part III critically reviewed an article by Milligan et al, introduced in a post on Knowledge Problem in response to Part I. The Milligan article claims negligible reductions from the theoretical maximum and contains questionable material.
This post deals with issues raised in comments and other feedback received to date. Further comments and debate on new issues will continue this series.
Reciprocating Engine Gas Plants as Wind Shadowing/Back-up
It has been suggested by Donald Hertzmark and Robert Peltier of MasterResource that reciprocating engine gas plants as wind shadowing/back-up be recognized as a partial solution to the wind emissions problem. It is also mentioned by Milligan et al.
Specifically, Midwest Energy (MWE) in Kansas has implemented a natural gas-fired plant consisting of nine 8.4 MW reciprocating gas engines to help support MWE’s 325 MW total system demand and back-up power supply in the event of a transmission outage. The MWE system will also be accommodating 49 MW of industrial wind power by the end of 2009, representing 16 per cent of the peak load in capacity terms.
An additional advantage of the small multi-engine configuration is its ability to provide back-up power for the wind component. The reciprocating engines are fast-starting and represent a spinning reserve capability, which suits them for this task, especially as individual engines can be added or removed from production as needed, as opposed to the ramping up and down of a larger unit, such as a gas turbine. It is important to note that the capacity ranges for gas turbine plants start at the top end of those for the reciprocating engine plants. The question is: is this a better solution than gas turbine plants for wind shadowing/back-up?
In addressing this, some considerations are:
- What is the heat rate penalty for this configuration in a wind back-up/shadowing role? Although the heat rate for these engines is about 10 per cent greater than CCGT, it is less than that for OCGT. There are indications that the heat rate penalty is less than that of both types of gas turbine plants.
- What are the CO2 emissions per unit of electricity produced and how does this vary with frequent ramping across the full range of the complete plant? Having multiple small engines would appear to help in this respect.
- The plant has catalytic converters in the exhaust system that creates CO2 emissions through converting CO to CO2. How much does this add to CO2 emissions?
- The effect of frequent ramping and start/stop conditions on plant life, operations and maintenance compared to normal load following/peaking and infrequent back-up requirements.
- Even if the result is less emissions than the OCGT/CCGT/wind combination, what is the overall effect relative to the CCGT/No-wind case as a starting point. This should be qualified by any requirement for peaking plants not otherwise provided for in the no-wind case.
- How well can this configuration scale to larger power systems in terms of gas supply and coordination of units?
Here are some numbers that put the relative size of gas turbine and reciprocating engines in perspective. For a larger scale wind capacity of 3,200 MW, as used in the previous calculator sample runs:
- The number of engine-generator sets would involve 5-8 gas turbines and about 380 reciprocating engines.
- The number of plants would be 1-4 for gas turbines and 63 for average-sized reciprocating engine plants of 6 engines each. The total acreage required for these plants is not clear, possibly because of storage considerations, but from the information available appears to be about the same in total.
The consensus amongst those asked to review the idea was that this approach would probably not scale well to larger wind implementations in the many-thousands-of-megawatt range. One consideration is the need to deliver gas to the sites. Although there may be some application for such gas plants in small, more localized installations, the question remains: why bother with introducing wind into the mix? Further detailed studies might provide answers.
There is another issue related to this. The calculator does not take into account the consideration that multiple gas turbine engine-generator sets provide the ability to allow some gas turbines to run more efficiently than the calculator results might show. Countering this are other considerations, such as, the grid topology may not allow this type of co-operation of plants across an electricity system.
Campbell Paper Considerations
This is to further address the considerations raised by the Campbell paper and more completely answer a question raised in the comments to Part I.
Campbell addresses issues surrounding the substitution away from baseload generation to peaking and mid-merit, and intermittent sources, as the result of increasing intermittent production. During peak hours the substitution is to peaking and mid-merit, and, during off-peak, is assumed to be to intermittent sources. He finds that if peaking and intermediate technologies are more carbon intensive than non-renewable base load technologies, this substitution can more than offset the emission benefits derived from the output of the renewable technology.
In the case of off-peak periods, a closer look at the base load generation is necessary. Inasmuch as base load generation plants are incapable of shadowing wind output at higher wind penetration levels, peaking or mid-merit plants might have to be employed and base load production curtailed. Further, if base load generation is hydro and run-of-river, as at Niagara Falls, there is no reduction in CO2 emissions. If hydro is impounded, then closer examination is required into the correspondence of wind production with the need to conserve water supply in the reservoir to assess if CO2 emissions are saved as a result.
The calculator does not address the considerations raised by Campbell, which tend to be more electricity system specific. Within the context of the Campbell evaluation, the calculator looks at the interaction between wind and peaking and mid-merit gas turbine plants in connection with wind’s random and highly volatile output, whether or not base load generation is displaced. The calculator shows the effect on fossil fuel and CO2 emissions as a result of this interaction, which is not “frictionless”. In most cases the effects shown by Campbell would be additive to that of the calculator, and in a few cases perhaps somewhat offsetting.
In summary, Campbell does not take into account the interaction between wind and wind shadowing/back-up production, which is the subject of the calculator. The effects of the two approaches are most likely additive and, combined, may produce results even more disadvantageous to the introduction of wind plants than either alone.
Even if some savings are managed to be squeezed out by the presence of wind, it is important to remember that the effect of such for an electricity system will be very small to negligible. This will be true regardless of how much wind is installed. The costs to realize these small gains, again if somehow they can be realized, are large in many terms, including the price of electricity and other forms of taxpayer support, the misdirection of industrial/economic activity into non-productive channels, the impact on local environments and economies, despoilment of natural settings, health and safety issues, distraction from better approaches to meet societal goals, and the divisiveness created within communities and even families.
In addition to the entries listed in Parts I–III:
Peltier, Robert, Top Plants: Goodman Energy Center, Hays, Kansas, POWER, September 2009.