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6.4.1: Problems with Wind Power

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    85109
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    Wind power offers great advantages indeed: no \(\mathrm{CO}_{2}\) emission, not toxic emissions of any other kind, and the wind gives away it's energy completely for free. So, why don't we shut down all the polluting power plants and install instead enough wind turbines to generate as much power as those polluting plant do?

    Well, it surely looks as a very good scenario for everyone who is concerned about the environment, the dwindling resources of mined fuels, and the devastating effects of mining. Yet, there are certain problems with wind power that make such a "grand plan" not fully realistic.

    First of all, we should take into consideration something that is called the Capacity Factor. It is defined as follows. Consider a wind farm with a number of turbines. Each turbine has a "nameplate power", i.e., the maximum electric power it can generate (for instance, for the Vestas V8o turbine discussed in the preceding section it is \(2 \mathrm{MW}\) ). Now, add up the nameplate powers of all individual turbines comprising the farm. It will be the total maximum power output of the entire farm. Then, think of a prolonged time period: a month, a quarter, or a full year - call it \(\Delta t\). Multiply the maximum power output by this time period, and you will get the maximum energy output from the farm over \(\Delta t\).

    Then, find out what is the actual amount of energy that the farm has generated over the the time period considered - and calculate the capacity factor, defined as:

    \[ \text { capacity factor }=\frac{\text { actual energy output over } \Delta t}{\text { maximum energy output over } \Delta t} \]

    The bad news is that the capacity factors for wind power facilities are not very high. An unquestionable leader in wind power technology is Denmark – it’s a very windy country, and most of its wind power comes from off-shore wind farms. The best of those attain the capacity factor as high as 50%, and for all installations it reaches about 40%. But Mother Nature is not as generous for the United States – here, the average for all states, according to the Energy Information Administration, is 27%.

    Suppose that one day we decide to eliminate all fossil fuel power plants and to replace them with wind turbines. In 2016, according to the U.S. EIA, 33.8% of electric power generated in the US came from natural gas power plants, and 30.4% from coal-burning plants. And the total installed nameplate power was 1177 GW. So, 64.2% out of 1177 GW, = 726.7 GW was the nameplate capacity of all fossil fuel facilities. And the nameplate capacity of all windpower facilities was 82 GW. But due to the capacity factor, these facilities generated only 27% of 82 GW = 22.1 GW. What’s the conclusion? Well, it is that if we wanted to eliminate all 726.7 GW obtained from fuel burning and replace by 726.7 GW generated by wind, we would need to increase the existing potential of windfarms 726.7/22.1 = 33 times! It means that next to each existing wind farm we would need to build 32 wind farms of the same size!

    It would be a grandiose project – unfortunately, it still won’t be enough. The problem is the natural intermittency of wind – especially, over lands, there may be pronounced changes in the the wind velocity – sometimes ir- regular variations in the wind speed and direction occurring over relatively short periods of time, so they should be termed as “fluctuations”. In Fig \(\PageIndex{1}\) it is shown how such fluctuations look like in Poland, a country located in central Europe1. The total consumption of electric power in Poland may reach about 25 GW in the peak hours, so that the utility sector needs to have at least 5 GW of reserve power in “peakers” ready to be launched on a short notice in order to compensate for a possible sudden hushing of the wind.

    Output fluctuates over the month from 1000 MW to 4000 MW
    Figure \(\PageIndex{1}\): Typical pattern of windpower electric generation in Poland. The data are from December 2019, recorded every hour over 31 times 24 = 744 hours. Source: hourly data published by the operator of the national grid in Poland ”Polskie Sieci Elektroenergetyczne (PSE)” and systematically monitored on Mrs. Jozefa Sokolowska’s blog – chart based on the data collected in her blog, with her permission.

    And now, a crucial question – how much power should be generated? From any worker of an utility company at any point of the globe you will get the same answer: As much as the customers demands! It cannot be less, because the grid which is not able to satisfy the needs of all customers may react with “black-outing” some regions of the area it services. And a surplus is not better, a grid into which too much power is “pumpeded” may react irresponsibly. Utility companies know how to handle such situations

    – normally, in the system they administer there are several different types of power plants, some of them able to quickly change their output power to respond to the fluctuations in the windpower generation. For instance, hydroelectric facilities may increase or decrease their output power in a few minutes. And if there are pumped storage hydroelectric plants in system, they may be quite happy if there is a sudden surge in the windpower. Another “backup power sources” widely used are natural gas turbines, whose start-up times from zero to full power is typically about 30 minutes.

    And if it’s not enough, the utilities may still rely on import from or ex- port to other regions. In Europe, there are special transmission lines between several countries that can be activated in such situations – one example is the 2 GW line connecting Denmark with its “sister county” Norway. Denmark relays heavily on windpower – and in Norway, power is generated almost ex- clusively in hydroelectric plants. If in Denmark there is a surge in windpower generation, the surplus is sent to Norway, which then curtails the output from its hydro plants – and if there is a “dip” in windpower in Denmark, Norway sends more water to its turbines and sends the surplus power to Denmark. In the US individual states can help each other in the same way, or simi- lar export-import may be arranged between American states and Canadian provinces.

    There may be, however, even more serious situations which the utility workers call “ramp events”. Normally, the utility systems in which a sig- nificant percent of power comes from wind farms observe the forecasts with attention. If the wind are expected to weaken, they make plans for many hours ahead how to use their backup sources. A ramp event is a situation when the dip in windpower appears to be much more serious than that fore- casted. An event of such kind that received much publicity had occurred in Texas on February 26, 2008. In short, on that day in the afternoon the wind died out completely in a western part of the state where there were several farms with large turbines. Over three hours their contribution to the state’s grid fell by 1500 MW. It coincided with a spike in demand. The grid opera- tors started heavily importing power from other regions – but so much power had to be sent that the transmission lines started overheating. The system was nearly collapsing – but fortunately, the operators had a “last resource” they decided to use. Namely, being aware that such situation may happen, some time before they had struck an agreement with several large industrial customers – allowing to cut off power for them in emergency situations after a 10-minute warning period. 1200 MW was taken off from the grid, and the collapse was avoided. With the exception of the aforementioned industrial plans, other power consumers were totally unaware that the situation really dramatic and if drastic measures were not taken at the last moment, the collapse of the grid was imminent.

    Windpower dropoff on February 26 2008 from a steady level of > 2000 MW to a few hundred over ~6 hours
    Figure \(\PageIndex{1}\): Total Texas windpower profile on February 26, 2008 (from the National Renewable Energy Laboratory Technical Report NREL/TP-5500- 49218).

    The Texas 02/26/2008 ramp event was described and analyzed in several professional reports and even in periodics of wider circulations2. What was learned from it certainly helped grid operators to be better prepared for such possibly-catastrophic events in the future. It should be noted that the collapse of the grid in one region may trigger an avalanche effect causing that eventually a whole macro regions are blacked out. Such “cascading” was responsible for two major power outages in the Northeast in 1965 and in 2003. In each tens of millions of people were left without electricity for many hours.

    In conclusion – there is no doubt that winds as very friendly and clean source of electric power – and may also be used for other purposes, e.g., wind may again start helping commercial vessels to cross the oceans. The available wind power is some 40 times larger than all electric power currently consumed by humanity. And no matter how much windpower we use, there is no fear that the wind resources will be ever depleted – our Sun is taking care of that.

    On the other hand, the facts that have been presented above should be taken as a warning against “overenthusiasm”. The capacity factors of 30% or 40% may look good if one looks only at them – in such narrow perspective one may conclude that the wind sector alone may be able to satisfy the needs of a country as big as the U.S.. Well, the capacity factor becomes highly misleading if one “forgets” that it is an average based on data collected over a long period, usually over the entire year, or over a season. The factor’s value alone doesn’t tell anything about the sudden windpower surges, die-outs and ramp events. Here a positive example may be the Danish-Norwegian partnership. Denmark is a leading nation as far as the windpower is concerned, with the capacity factor as high as 42%. It deserves praise, but it also means that the Danish utility grid may be knocked down in the case of a sudden wind hush. In the Danish-Norwegian partnership a recipe has been worked out that fixes such problems: namely, Norway, who is able to considerably increase the output of it’s hydroelectric facilities “on call”, simply acts a gigantic “back-up generator” for the entire Danish national utility system (which relies, as noted, in 42% on windpower – in professional terminology, one would say: the windpower penetration in Danish electric power generation system is 42%.).

    Unfortunately, there is no “another Norway” – or, rather a “Super-Norway” next to the U.S. borders which would be able to come to rescue of the U.S. wind sector in case of a huge developing ramp event. Such a backup system should exist inside the U.S.. Is it possible to get one? Yes, definitely. One way towards that is diversity : the entire system should consist of not only of one major component, but of several renewable energy “branches”, each of which should have some “reserve capacity”. Also, the success in develop- ing a comprehensive renewable energy sector, capable of eliminating power sources using fossil fuels, depends very much on developing in parallel an en- ergy storage system mighty enough to balance out sudden substantial losses of power due to an unexpected ramp event – let us note that such events are not limited to windpower. Solar power may also exhibit ramping effects. In general, solar and wind power are not easy to manage due to their variable and stochastic nature – especially, when penetration of one of those sources or both attains a high level. This discussion will be continued in the chapters devoted to solar power and to the storage of energy

    1. Poland is often referred to as an “Eastern European” country – which is perhaps correct from the viewpoint of political sciences. But geographically, Poland is located in the very center of the continent of Europe. No matter how this center is defined, either as the center of gravity of a figure obtained by clipping Europe out of the World map, or as the center of a circle circumscribed about such figure.

    2. See, e.g., Renewables Test IQ of the Grid by Dan Charles, in SCIENCE, 10 Apr. 2009, vol. 324, www.sciencemag.org)


    6.4.1: Problems with Wind Power is shared under a CC BY 1.3 license and was authored, remixed, and/or curated by Tom Giebultowicz.

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