In 1824 Jean Baptiste Joseph Fourier in effort to explain why the Earth was warming faster than expected theorized that elements in the atmosphere were trapping solar radiation and reflecting it back to Earth. Later Svante Arrhenius expanded upon this explanation and coined the term ‘greenhouse effect’. When global warming was first proposed as a threat to the Earth, most rejected or ignored it, despite the fact that it was backed by solid science and some intriguing initial data, largely in favor of theories attributing the warming to changes in solar or orbit cycles, theories that have no valid evidence of support now. Other individuals elected to not out-and-out reject global warming and while gathering further data hypothesized that perhaps there was a much shorter lifespan for carbon intensive energy producing methods, like burning coal and natural gas due to their association with a future unmanageable temperature increase. In response to the potential loss of fossil fuels, trace/zero emission energy alternatives had to be developed and deployed to fill the eventual energy gap. From the trace/zero alternatives that have become available, wind power has seen significant growth in the last decade easily outpacing growth of all other trace/zero alternatives (solar, biomass, geothermal, fusion, hydroelectric) combined. However, there may be a problem with this massive wind deployment that harkens back to carbon intensive energy, in that wind may not actually be self-sustainable at the level that would be required to meet future energy demands in the United States. Is wind just another type of coal?
At the turn of the century wind power only accounted for 5.6 million MW-h of utilized electricity (approximately 0.29% of the total U.S. demand at the time).1 That abysmal statistic changed over the last decade largely due to the application of a government tax subsidy. The Federal Renewable Energy Production Tax Credit is designed to aid in the development and deployment of renewable energy technologies. Specifically for wind power it provides a 2.1 cent tax credit for every kw-h of energy produced from an operational wind turbine and utilized by a different source.2 Under most conditions the tax credit is only applicable to a given project for the first ten years of operation. A new portion of legislation that passed in early 2009 also allows renewable operators to instead of taking the production tax credit to opt for a federal business energy investment credit which typically covers approximately 30% of initial capital costs.2 However, unlike most of the major subsidies for the fossil fuel industry, which have permanent status unless explicitly eliminated, the Federal Renewable Energy Production Tax Credit has a typical lifespan ranging from 1-3 years and if not directly renewed it naturally expires. The current wind power subsidy was renewed earlier this year and is slated for expiration Dec 31, 2012.2
A vast majority of the success in wind power development and deployment can be attributed to this subsidy because first, despite the so-called risk-reward attitude that is supposed to drive capitalism, private companies have become more and more squeamish when taking investment risks unless the government footing a significant portion of the bill. Second, and more obvious, the years in which the subsidy had expired deployment of wind power fell substantially as shown in the figure below.3
The universal nature, albeit not universal value, of the renewable subsidies does demonstrate that the principal reason for selecting wind power over another trace/zero emission source like geothermal is a lower capital and/or operation cost. However, these economical considerations only focus on the short-term present value to both the producer and the country as a whole, but unfortunately such thinking is common. Also it is important to understand that all major domestic energy providers receive subsidies with those given to fossil fuel derived industries dwarfing those given to alternative energy sources, thus alternative energy sources are not receiving any type of governmental advantage over fossil fuels. In fact they are still actually at a significant disadvantage.
While wind power growth has been impressive over the last decade, an important question is how much of the future electricity demand can be reasonably provided by wind power with current technology? To calculate the required perimeter landmass a reference average of acres per MW will use Horse Hollow Wind Energy Center in Taylor and Nolan Counties in Texas, which can produce 735 MW of peak power from 421 turbines that cover a landmass of 47,000 acres equaling approximately 64 acres/MW.4 The fact that wind speeds in West Texas are above the national average and the peak power production, not the average power production, is being used makes the use of the above value as a reference value more than valid. Realistically the land value derived from this example more than likely will be an underestimation.
Next assume an average nameplate potential (average operational capacity) of 2200 hours per year (which is a little higher than current existing wind capacity)1, which would generate an average of 0.029 acres/MW-h or 34.4 MW-h per acre. Then assume that over the next decade total overall electricity demand drops by 22.5% from the demand of approximately 4,000,000,000 MW-h in 20071 due to incorporation of new energy efficiency technologies into the residential and commercial sectors and any new structures. Taking all of those assumptions into account, how much land would be required for wind power to replace 50% of the electricity provided by only coal-based sources in 2007 within a decade? Recall that replacing coal with a trace/zero energy source (not just natural gas) will be essential to warding off catastrophic climate change. With current technology wind power would require 45,795.2 square miles of land to generate the approximate 1.008 trillion kW-h (458 GW) produced by part of the coal industry in 2007, which would account for 32.5% of the anticipated electricity demand in this scenario.
That land value seems excessively high doesn't it? The reason that wind farms like Horse Hollow need to be so large is that if wind turbines are built within close proximity of each other overall energy output drops significantly because of elements like wake turbulence, which can interfere with the pressure variations that produce wind. Basically the proximity of turbines is related to the overall average wind speed in that the closer together the turbines the higher the probability that the average wind speeds those turbines experience will decrease.
Wind proponents argue that the total landmass devoted to wind power can be misleading due to the distribution pattern and chief vertical nature of wind turbines. Basically a large portion of the landmass attributed to a wind farm can be regarded as void space, which can be used for other tasks. The primary use of this void space is as farmland because any large vertical structures, even trees, disrupt wind speed as previously discussed. However, farmland conversion can be sub-optimal because wind farms are largely situated on land where high wind speeds are the primary concern and quality soil, water availability and distribution infrastructure are all distant seconds. Fortunately for wind proponents, some of the best pieces of land for wind farms are in the Midwest which already handle a large agricultural sector. Overall the true amount of ‘land use’ for wind power lies somewhere in between the distribution perimeter and the total amount of space occupied by the turbines themselves, but any wind farm will put significant constraints on the land used within its perimeter. Finally computer modeling is used before actually building a wind farm in order to predict the number of turbines and their placement within a given perimeter to maximize overall energy generation.
Note that EIA information from 2007 is being utilized in this analysis because electricity generation and demand from 2008 and 2009 are heavily skewed because of the economic recession. It is rational to believe that electricity demands will increase once again in 2010-2011. It is unknown whether or not future demand will rival that seen in 2007, unlikely but still unknown, but to suggest that 2007 demand will be significantly higher than any future demand seems naïve and unreasonable.
In even the most optimistic scenario of wind growth generating approximately 458 GW of wind power in a reasonable time period (the next two decades) seems unrealistic. If a lower goal were established, under the above scenario parameters wind power would still require approximately 282 GW to provide only 20% of the hypothesized demand. Unfortunately although the total costs and land use appear daunting, they may not be the biggest problem. There is reason to believe that wind power may not have the staying power once thought. In wind advocate circles the United States has been referenced as “the Saudi Arabia of Wind” in effort to describe its vast wind resources and the fact that it could easily be the world leader in wind power generation if current levels of investment continue. However, wind already suffers from the concern of intermittence, the reason behind its low capacity, but is this concern smaller than it should be?
A potentially groundbreaking study documented wind patterns in the United States from the mid 70s to the present and came to the conclusion that there has been an overwhelming dominance of decline in wind speeds at both the 50th and 90th percentile (basically average and top-end wind speeds).5,6 There is little information to evaluate if low-end wind speeds are changing because uncertainty values encapsulate a significant amount of those speeds. Placing a quantitative value on the above research, across portions of the Midwest and regions east of the Mississippi an approximated 10% drop in average wind speed has occurred just in last decade. It is not surprising that this drop in wind speed comes within the same time period that the Earth has experienced its most rapid pace of warming. Clearly this finding, if valid, is extremely important for the future growth and sustainability of the wind industry, but how could this decrease in wind speed be happening and exactly what influence will it have on wind power in the future?
Recall that wind is primarily the result of variance is air pressure brought on by two factors: differential heating between the equator and the poles, which influences the jet stream, mid-latitude westerlies, trade winds and polar easterlies, and planetary rotation, which creates circular motion of air (Coriolis Effect) and leads to monsoons. Unfortunately the rate of accelerated global warming is higher at the North and South poles than other regions of the Earth, which is closing the temperature difference between regions, in turn altering wind patterns and generally increasing the probability of slower wind speeds, for the greater the temperature variance the greater the maximum wind speed.
Although there is too little information to completely confirm whether or not this drop in wind speed is an aberration or a trend, most of the rationalities that have been suggested to explain these wind speed drops without acknowledging the legitimacy of the decrease are rather weak. Some argue that measurement technologies have evolved in the last 35 years and the different nuances in each device creates a level of error that could lead to these conclusions. Really, so the entire drop in wind speeds can be primarily attributed to instrumentation? Others claim that measurement devices, that were once in open space, have recently become obstructed by buildings and new trees which misrepresent wind speeds in that localized region. Such a rationality seems improbable based on the number of different regions that seem to be experiencing significant drops in average wind speed. Finally others claim computer models do not necessarily predict a decline in wind speed. When did the results of computer models take precedence over direct empirical measurements? Also computer models have predicted wind speed reductions in Europe, which creates some level of contradiction because there are similarities between the stock sources that drive wind patterns between the U.S. and Europe.7 Overall what all skeptics of this information seem to be forgetting is the physics behind wind generation as described above. The fact that the temperature variance between regions is shrinking demonstrates increased viability for the conclusion that wind speeds throughout the world should decrease in kind.
So how does a decrease in wind speed affect the power generation of a wind turbine? A wind turbine functions under the premise of transforming the kinetic energy of the wind into either mechanical or electrical energy (depending on the type of system used). The original mechanism of wind energy was the mechanical wind pumper which converted the kinetic energy into mechanical energy as a means to pump water. However, wind turbines and their associated wind power now more commonly convert kinetic energy into electrical energy. There are two types of wind turbines for electrical generation: horizontal-axis or vertical-axis. Horizontal-axis turbines are much more popular in industry most likely because they are more cost effective in maximizing blade diameter which is tied directly to the amount of energy that a wind turbine can produce.
The overall efficiency of a generic wind turbine is actually rather skewed in that the maximum efficiency is attained at approximately 18 mph, but the turbine reaches maximum power output at between 56 to 57 mph. The reason maximum power is not attained at maximum efficiency is because recall the equation governing kinetic energy (E = ½*m*v^2) as wind speed increases the amount of energy relative to only the velocity is squared. In addition the mass of air moving through the turbine also increases proportionally to its velocity, thus the change in the amount of energy available for collection is proportional to the cube of the wind speed. Note that the above explanation does not directly deal with calculating the power generated from a wind turbine, but addresses how the energy available in the wind changes with wind speed. Specifically calculating the power derived involves incorporating momentum flow rates, continuity principles and power equations, a level of applied physics that goes beyond the scope of this post.
Under normal circumstances efficiency of a given system is extremely important, but in the case of wind power due to the fact that wind is not a controllable finite resource, i.e. changes in wind are heavily outside of the control of humans, efficiency is much less important vs. maximum power output. Therefore, when discussing wind turbines, efficiency is an issue that is not imperative. With relation to the ability of a turbine to absorb energy from passing wind, the Betz limit (59% transfer) is the maximum level. The graph below illustrates the change in power generation in relation with wind speed. Note that the graph is not a mathematically perfect representation of the generic 2.1 MW wind turbine, but it is accurate enough that one can get a general idea of the relationship and make fairly accurate calculations.
Suppose that the conclusion that average wind speed is dropping is correct, what does that mean for the wind power industry? Well, if it is assumed that an 8% drop in wind speed can be universally applied, a significant drop-off in power production can be expected. For example if the average wind speed in a given generic wind farm containing 300 2.1 MW turbines falls from 15 mph to 13.8 mph the annual total power loss, when using the performance information from the above figure, is approximately 310,143.32 MW-h or about 27.76%.
The two primary reasons for the significant drop-off with such a small reduction in wind speed is the cubing property associated with the kinetic energy transfer and the low capacity of wind power. Regarding the second reason, suppose a wind turbine was constructed in an environment that had an average wind speed of 40 mph, the 8% reduction would have no effect on wind power (an average speed of 36.8 mph still attains a maximum level of power generation). Unfortunately the second reason is a problem because wind turbine technology is not cost-effective enough to construct turbines that exist at a high enough altitude to capture wind gusts with such a high average speed.
Another problem with the potential reduction of wind speed is early results localize it primarily in the Midwest which is the region that has the highest average wind speeds and is the principal construction venue for wind farms due to both the high average wind speed (12-17 mph) and the relatively flat and open spaces which lend themselves well to cultivating, if not already, wind farm void space. Therefore, if the wind reduction trend is accurate, there will be a significant reduction in power generation within the optimal region of power generation.
Understand that this analysis is not to say that wind power should be abandoned because such a stance would be silly and irresponsible. Instead it is important to remember to diversify the mix of energy generation for the United States in the future. If no new trace/zero emission energy sources become scientifically and economically available, most of which seem unlikely (anti-matter, greater than break-even hot fusion, cold fusion, space-based solar power, etc.), the total available trace/zero emission options are limited. In fact it stands to reason that significant future growth in trace/zero emission energy can be confined to four unique options: wind, solar, geothermal and biomass. A vast majority of the potential that exists for hydroelectric has already been realized as secondary tidal generation systems do not look promising in pushing hydroelectric power far beyond that which is already generated as well as account for any losses accrued from further drought conditions in the West, thus hydroelectric does not appear to be a valid candidate for significant further growth.
Unfortunately growth in geothermal and biomass has barely budged in the last decade and despite vast improvement in manufacturing, solar power is still heavily dependent on photovoltaic costs. There has been a lot of talk surrounding thermal solar plants, especially in the Mojave Desert, but very little has come from this so-called vast potential. In fact solar energy incorporation into the grid could grow at 30% annually over the next two decades and still be a relative non-factor in electricity production (255,533,901 MW-h in 2030; approximately 8.24% of the total electricity demand in the previously hypothesized demand scenario). The sad state of trace/zero emission energy source growth is demonstrated in the table below.1,8
* delineates annual growth rate from last year to current year;
Looking at the above table is rather disconcerting as wind power is generating double-digit growth rates from 1999 to the present in all years in which a government-based wind subsidy was available. However, the growth rates or absolute energy generation for the other three major alternative sources are completely and utterly pathetic. Although solar has an impressive growth rate over the last two years, the recession has limited the availability of capital for the solar industry, so unless something like the ACES is signed into law, it is rational to expect those growth rates to decrease significantly. Also the absolute energy generation of solar is extremely small comparative to the other alternatives, thus high growth rates are easier to achieve (note that the absolute gains in solar vs. geothermal from 2007 to 2008 were nearly identical, but solar grew at 37.75% and geothermal grew at 1.5%). Regardless, as previously mentioned, it is unlikely barring a technological miracle, that solar will become a major contributor to the grid in the next two decades. This reality is unfortunate because the next two decades will be the most critical time in both avoiding an energy crisis and serious detrimental climate change. Geothermal and biomass have significant capacities, but need a huge shot in the arm from a funding, infrastructure and willingness to actually undertake new projects standpoint, all which do not appear to be forthcoming.
If the trend of wind speed reduction continues, at the moment there is no reason to dismiss such a conclusion, then with the current rate of fossil fuel energy/electricity substitution relying so heavily on wind power, the United States will either almost definitely face an energy crisis of significant proportion in the future or be guaranteed significant detrimental climate change. To avoid such a crisis, energy diversity is essential, instead of giving huge subsidies to coal, oil and natural gas, a significant percentage of those subsidies need to be siphoned off and directed towards geothermal, biomass and solar-based sources. Unless specific quotas are assigned, the simple administration of a renewable energy standard will not drive this diversity because most producers will simply direct efforts to wind power to fill the requisite percentage. The current and future energy market is just like the stock market the key to long-term growth and stability in the overall portfolio is diversity, such an ideal is not achieved by throwing all the alternative energy eggs into the wind power basket.
1. “Electric Power Annual 2007: Table ES1. Summary Statistics for the United States, 1996 through 2007.” Energy Information Administration. January 2009.
3. Original Source: American Wind Energy Association [Secondary Source - Climate Progress: “Energy and Global Warming News for September 1: Big Money returns to Wind Power.” http://climateprogress.org/2009/09/01/energy-and-global-warming-news-for-september-1-big-money-returns-to-wind-power/]
4. Mims, Christopher. “The World's 10 Largest Renewable Energy Projects.” Scientific American Magazine. June 4, 2009.
5. Pryor, S, et, Al. “Wind speed trends over the contiguous United States.” J. Geophys. Res. 2009. 114: D14105 - doi:10.1029/2008JD011416.
6. Pryor, S, Barthelmie, R, Takle. “Wind speed trends over the contiguous USA.” IOP Conf. Series: Earth and Environmental Science. 2009. 6: doi:10.1088/1755-1307/6/9/092023.
7. Hennemuth, B, Hollweg, H-D, Schubert, M. “Change of Wind Speed in Europe in
Regional Climate Model Scenario Projections.” Service Group Adaption – SGA. Model & Data / MPI-M, Hamburg, Germany. 2008.
8. Preliminary Energy Data for 2008. Energy Information Administration. 2009. http://www.eia.doe.gov/cneaf/alternate/page/renew_energy_consump/rea_prereport.html