Wind turbines produce electricity which is delivered to the
grid. Variations in wind velocity cause yield variations. Conventional power
stations are forced to compensate these variations by adjusting their output.
This has a negative effect on the efficiency of the latter stations. Using data
provided by CBS, the Dutch Institute for Statistics, an estimate is made of the
so called “turning point”. This is the point where the efficiency reduction of
conventional power stations balances out the fuel saving of the wind turbines,
and where the CO2 emission reduction turns negative as well. In the
Netherlands the data for the year 2007 show this to be the case at an
efficiency reduction of all power stations of about 2 %. The Dutch government uses an incorrect
formula for calculating the fuel and emission saving from wind energy. On this
subject parliament has been incorrectly advised by government.
In a previous article1 we
calculated the negative effect on the fossil fuel saving obtained when using
wind generated electricity. The effect is caused by the stochastic nature of
wind availability, which requires frequent and rapid output adjustments of the
fossil fuel powered plants. In our sums we used German data, because wind
turbines have been erected there in large quantities. We also used
somewhat too high estimates of the
efficiency of fossil fuel power stations.
We now turn our attention to the Dutch
situation, where thanks to our CBS, the National Bureau of Statistics, actual
data on the fuel input and electricity output of various kinds of power
stations are available.
The Dutch government2 and public conclude that the fossil fuel saved ΔF can be related to the efficiency and the total fossil fuel use of the system as follows:
The amount of fossil fuel used in a situation where wind is providing electricity as well thus becomes:
When combining equations (1) and (4) for ΔF = 0 we find:
Generally the following equation holds for the efficiency loss at the turning point:
an efficiency loss of this magnitude
actually occur? We were unable to find data on this effect. In our earlier
article we called this “ a well-kept secret”. We did receive promises from researchers
that they would try and send us
the relevant information. So far, we have not received data. Electricity
producers, apart from EPZ(Zeeland), have not provided us with the information.
Again, we have received promises that this will be subject of study.
The Netherlands uses various power sources for electricity generation. Most use fossil fuel. The share of each power type is presented in table 1.
Some electricity providers in the Netherlands
also supply (waste) heat for space heating and the process industry. We have
assumed that this heat supply is driven by the demand for electricity, thus not
requiring additional capacity variations in the electrical power generation.
This does not apply to decentralized combined heat and power generation. Those
systems are in most cases driven by the heat demand. Without further specific
data one cannot decide whether this generating capacity can be varied in
response to rapid variations in electricity demand (or supply from other
sources). The table also shows data on import and export of electricity. We do
not know the degree to which this electricity import into and export from the
Netherlands is controlled or
controllable by rapid demand fluctuations inside the country, just by price
differentials or by any other factor.
We will calculate the efficiency effect for various assumptions. We want to determine the efficiency loss caused by the wind supply fluctuations at the point where the fuel saved by the wind is equal to the fuel lost by the efficiency reduction of the back up system. In other words, the point where ΔF = 0. At this point the fuel consumption F from formulae (1) to (5) above is obviously equal to the fuel figures given by CBS. However, for the generated electricity E we need to add to Ei the electricity generated by the wind (0,39 Gwyr, from table 1), unless the figure already includes the wind contribution. In the first case we get:
In the first row, the Total Supply
includes the nett result of import and export. Because the Netherlands was a
nett importer this resulted in electricity without fossil fuel expenditure and
thus a higher overall fuel efficiency. The import only costs money, but that is
not under consideration here.
These calculations indicate how small the overall efficiency decrease of conventional electricity production needs to be to reach the point where no fuel is saved or CO2 emission reduced in the case when these conventional power stations are required to compensate wind electricity variability. Obviously, wind variability can be larger than that required to reach the turning point and in such a case wind energy leads to increased fossil fuel use and CO2 emission.
Let us also bear in mind that the building, erection and maintenance of wind turbines requires fossil fuel as well. The turbine steel and other material and the (concrete) foundation require energy for their manufacture. A wind farm requires a conventional power station with a power equal to the maximum power of the wind farm. A wind farm with a 25% duty factor requires a back up station with four times the power of that belonging to the actual electricity produced by the wind farm. A critical analysis of the pay back time of the wind farm fully accounting for all fossil fuel expenditure in its manufacture, operation, maintenance and back up requires another study.
Our preliminary estimate, based on data from J. van Oorschot13 shows the energy pay-back time to be minimally 1,5 years. This period is a very optimistic minimum. In this estimate the energy costs for the manufacture of the back up units, the extra high tension grid, transformers and regulating systems etc. The Dutch Gasunie, managing the natural gas distribution grid, announced in December 2009 to plan extra gas pipelines, amongst others because wind energy requires so much extra natural gas. In our estimate we have used the overly optimistic output formula used by the Ministry of Economic Affairs which we have shown here to be wrong. In addition one has to account for the fossil energy costs of maintenance of the turbines, which especially in the case of offshore placement will be significant.
It is quite conceivable that the efficiency reduction of the conventional power systems is not noticed by the operators. Wind electricity has as yet only a tiny contribution in Holland: only some 3,3% of the national electricity production and therefore less than 0,3% of the national energy use. (Do bear in mind that the amount of conventional back up required is about 4 times as large). The wind contribution has grown rather smoothly, and thus the resulting efficiency decrease is also slowly growing. In the period of wind turbine capacity addition conventional installations have probably been replaced by more efficient ones. This may well have masked the negative effect of the wind turbine addition. As an example, dual cycle gas turbines run at almost 60% efficiency, although it takes about one hour before this level is reached. As long as the steam cycle is not operational (i.e. during this hour) these machines run like open cycle gas turbines at 25 to 30% efficiency.
In figure 1 the development of the efficiency of fossil fuel driven power stations is shown for the period 1998 to 2008. The CBS figures for the years 2007 and 2008 are provisional. (There is also a difference between tables, but within one table there is consistency.) It is clear that the efficiency is not a stable quantity. It can only be calculated 'after the fact' and without detailed additional data from the producers the cause of the variation remains unknown.
Efficiency of fossil fueled plants &
amount of wind produced electrical energy.
Drawn line represents decreasing efficiency trend,
neglecting the 'not yet corrected' years 2007 & 2008.
In an earlier paper1 we proposed an algorithm to calculate the effect on the overall efficiency of all conventional power stations while only part of the powerstations effectively are required to back up the wind turbines. We start from the equation:
In the Netherlands in 2007 a = 4,63%, and we assume b to be 25% .(This is more favourable than in Germany, where the nation-wide value for b was on average over several years about 17%, with variations over the years from 14% to 21 %.) The turning point in Holland is therefore always some 75% of the average 'normal' efficiency factor. The result of the calculations is shown in table 3, where we show the overall efficiency reduction ΔR versus the efficiency of the actual units doing the back up.
The efficiency of the units which compensate wind energy fluctuations as a function of the
total efficiency reduction of the conventional units (based on CBS-data for the
Netherlands, 2007). We thank Kent Hawkins12 for his contribution to this table.
Table 3 demonstrates that at a hardly noticeable efficiency
decrease for the overall system (Small compared to the yearly fluctuations in
this overall efficiency number.) the stations effectively providing the back up
are operating at the indicated reduced efficiencies. In case all back up for wind farms in
Holland would be provided by open cycle gas turbines, having a 25% to 30%
efficiency only (Electricity producer NUON is doing this for its offshore wind
farm), in 2007 we
would have been beyond the turning point. Under the same conditions as in 2007 (same b, same
total of electricity supply, but with twice
the amount of wind electricity and consequently less conventional contribution), the
Rbu threshold would remain ~ 34% and again there would be no saving.
More fuel would be burned
and more CO2 emitted than in the case where no wind was being used.
Pay-back of the energy spent during construction and being spent to run the
extra kit is obviously out of the question.
We have also calculated, using formula (4) how much fuel is saved for the 2007 wind energy production of 0,39 GWyr when using different realistic back up configurations. Figure 2 presents the results, being the fuel saving ΔF as a function of the efficiency decrease ΔR. It is shown that in all configurations the fuel saving turns zero when roughly 2% efficiency is lost due to forced output variations to compensate for wind variability.
Fossil fuel savings as a function of
decreasing efficiency for different
use of wind energy for electricity generation in combination with the
requirement for fossil fuel powered stations to compensate for wind fluctuations
can easily lead to loss of the expected saving in fuel use and CO2
emission. In addition, the conventional
stations will be subject to accelerated wear and tear.
1 ) K. de Groot & C. le Pair: De brandstofkosten van windenergie; een goed bewaard geheim (The fuel costs of wind energy: a well-kept secret.); SPIL 263 – 264 (2009) p.15 ff.; also on the WWW .
2 ) Senternovem, the R&D agency of the Ministry of Economic Affairs responsible for a.o. Innovation and energy supply, confirmed as follows: (E-mail S. te Buck to S. Zwerver 2010 01 04) “…In this calculation we start from the primary energy requirement which is avoided and therefore we divide by the efficiency of the electricity generation…”. In our symbols: ΔF = Ew / R, as in (2) with ΔR = 0. However, because ΔR ≠ 0, the remarkable consequence of this official calculation method is that it would become possible to eliminate fossil fuel use worldwide by causing the wind turbines in the Netherlands to sufficiently lower the efficiency of conventional power stations. ( R → 0 , ΔF → ∞ ) ☺ .
3) F.Udo: Besparen windmolens CO2? (Do wind turbines save CO2?), Dec. 2009: http://www.groenerekenkamer.com/node/946
4) B. Chr. Ummels: Power system operation with large-scale wind power in liberalised environments; Diss. TU Delft 26 feb 2009.
5) J. Soens: Impact of Wind Energy in a Future Power Grid; Diss. Leuven, 2005 12 05.
6) G. Dijkema, Z. Lukszo, A. Verkooijen, L. de Vries & M. Weijnen: De regelbaarheid van elektriciteitscentrales, een 'quickscan' in opdracht van het Ministerie van Economische Zaken; TU Delft, Fac. Techniek Bestuur & Management i.s.m. DNC, Delft 2009.
7 ) K. Hawkins: Wind Integration: Incremental Emissions from Back-Up Generation Cycling (Part I: A Framework and Calculator). http://www.masterresource.org/2009/11/wind-integration-incremental-emissions-from-back-up-generation-cycling-part-i-a-framework-and-calculator/#comments . Later Hawkins updated his calculator without substantial difference in the outcomes. It can be found via this webpage.
8 ) Katzenstein, W & Jay Apt: Air Emissions due to Wind and Solar Power; Environ.Sci.Technol. 43 (2009) 253-258. They notice among else: "...studies have not accounted for the change in emissions from power sources that must be paired with... such as wind..."
uses different units for electrical and fossil energy. In this paper we use a single unit: GWyr.
1 GWyr = 8,76 x 109 kWh = 31,536 1015 Joule
(Ws). 1015Joule is also called petaJoule, PJ. In de
graph, figure 1, we use GWh, with 1GWyr = 8760 GWh.
11) J.C.L. van Cappelle, priv.
comm.: The efficiency of (the Dutch) nuclear plant at full
capacity ( = 515 MWe ) is 37,7%. At 10% capacity reduction this becomes 37,3%. Below
35% capacity the efficiency falls so
rapidly, that power generation is no longer realistic. NB. This is the heat
efficiency, there being virtually no fossil fuel efficiency.
12) Kent Hawkins, Priv.comm. (see note 7)
notified us of an error in table 3 of the previous version of this paper. We
gratefully acknowledge his contribution to the present version of table 3, in
which a third column is now added. We also changed the explanation in the text.
13) J. van Oorschot,
previously dir. R&D and Bus. Dev. Volker Wessels Stevin, priv.
comm.: VWS is involved in the construction and erection of wind farms. His
R&D-department made the following sums: a 3 MW wind turbine requires ~ 400 ton steel
for its frame and ~ 300 ton for the turbine.
Steel costs 8,2 MWh energy per ton. Thus in total 5740 MWh energy. De foundation needs 3000
(7800 ton). This contains 300 ton cement. Cement manufacture requires 0,5 MWh/ton
energy thus total 150 MWh. Together with steel 5890 MWh. In these
energiesums no acount is taken of
the energy needed for the sand and gravel (dredging), the transportation costs
and the installation costs. VWS estimated total energy requirements to be some
10000 MWh. If the turbine has a ‘duty factor’ of 25% , it will produce 6570 MWh
in one year . Using the simple (wrong) formula this would mean a payback time of 1,5
year. Part of the energy requirements of the manufacture of ‘back-up’
conventional generators, that of the extra grid (in Germany over 2700 km high
tension lines), transformers, here the subsea connectors to England and
Norway etc. have to be added. One also has to subtract the energy requirements of the
maintenance, which will be substantial in
case of off shore operations. We(ClP & KdG) show in this paper that the simple
formula for pay back is much too optimistic. We put the nett electricity yield closer to 0 than to
6570 MWh in one year. Assuming it would
be 2000 MWh in one year, then we are looking at almost 5 year pay back of the
wind turbines themselves.
11) J.C.L. van Cappelle, priv. comm.: The efficiency of (the Dutch) nuclear plant at full capacity ( = 515 MWe ) is 37,7%. At 10% capacity reduction this becomes 37,3%. Below 35% capacity the efficiency falls so rapidly, that power generation is no longer realistic. NB. This is the heat efficiency, there being virtually no fossil fuel efficiency.
12) Kent Hawkins, Priv.comm. (see note 7) notified us of an error in table 3 of the previous version of this paper. We gratefully acknowledge his contribution to the present version of table 3, in which a third column is now added. We also changed the explanation in the text.
13) J. van Oorschot, previously dir. R&D and Bus. Dev. Volker Wessels Stevin, priv. comm.: VWS is involved in the construction and erection of wind farms. His R&D-department made the following sums: a 3 MW wind turbine requires ~ 400 ton steel for its frame and ~ 300 ton for the turbine. Steel costs 8,2 MWh energy per ton. Thus in total 5740 MWh energy. De foundation needs 3000 m3 concrete (7800 ton). This contains 300 ton cement. Cement manufacture requires 0,5 MWh/ton energy thus total 150 MWh. Together with steel 5890 MWh. In these energiesums no acount is taken of the energy needed for the sand and gravel (dredging), the transportation costs and the installation costs. VWS estimated total energy requirements to be some 10000 MWh. If the turbine has a ‘duty factor’ of 25% , it will produce 6570 MWh in one year . Using the simple (wrong) formula this would mean a payback time of 1,5 year. Part of the energy requirements of the manufacture of ‘back-up’ conventional generators, that of the extra grid (in Germany over 2700 km high tension lines), transformers, here the subsea connectors to England and Norway etc. have to be added. One also has to subtract the energy requirements of the maintenance, which will be substantial in case of off shore operations. We(ClP & KdG) show in this paper that the simple formula for pay back is much too optimistic. We put the nett electricity yield closer to 0 than to 6570 MWh in one year. Assuming it would be 2000 MWh in one year, then we are looking at almost 5 year pay back of the wind turbines themselves.
14) The Minister obtained her results by using graphs in the Delft report (6). Non-linearity was not included in the calculations. Neither was the need incorporated to use low efficiency generators because of the rapid change of wind power. Finally, F. Udo, priv. comm., found a calculation error even in the (too) simple, straight forward, departmental computation.