global wind power potential: physical and technological limits
TRANSCRIPT
Energy Policy 39 (2011) 6677–6682
Contents lists available at ScienceDirect
Energy Policy
0301-42
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Communication
Global wind power potential: Physical and technological limits
Carlos de Castro a,n, Margarita Mediavilla b, Luis Javier Miguel b, Fernando Frechoso c
a Applied Physics, Campus Miguel Delibes, University of Valladolid, 47011 Valladolid, Spainb Systems Engineering and Automatic Control, Paseo del Cauce s/n, University of Valladolid, 47011 Valladolid, Spainc Electric Engineering, Francisco Mendizabal s/n, University of Valladolid, Spain
a r t i c l e i n f o
Article history:
Received 5 November 2010
Accepted 13 June 2011Available online 29 June 2011
Keywords:
Renewable energy potential
Wind energy
Global energy assessment
15/$ - see front matter & 2011 Elsevier Ltd. A
016/j.enpol.2011.06.027
esponding author. Tel.: þ34 983423545; fax:
ail address: [email protected] (C. de Cast
a b s t r a c t
This paper is focused on a new methodology for the global assessment of wind power potential. Most of
the previous works on the global assessment of the technological potential of wind power have used
bottom-up methodologies (e.g. Archer and Jacobson, 2005; Capps and Zender, 2010; Lu et al., 2009).
Economic, ecological and other assessments have been developed, based on these technological
capacities. However, this paper tries to show that the reported regional and global technological
potential are flawed because they do not conserve the energetic balance on Earth, violating the first
principle of energy conservation (Gans et al., 2010). We propose a top–down approach, such as that in
Miller et al. (2010), to evaluate the physical–geographical potential and, for the first time, to evaluate
the global technological wind power potential, while acknowledging energy conservation. The results
give roughly 1 TW for the top limit of the future electrical potential of wind energy. This value is much
lower than previous estimates and even lower than economic and realizable potentials published for
the mid-century (e.g. DeVries et al., 2007; EEA, 2009; Zerta et al., 2008).
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The limited nature of fossil and nuclear fuels and the socio-ecological problems associated with them are important incen-tives for a global transition towards renewable energies, whichmust be accomplished in this century. The wind power and solarphotovoltaic technologies are gaining more support as candidatesfor this transition (Zerta et al., 2008; Greenpeace, 2010; Jacobson,2009; REN21, 2010; Schindler et al., 2007).
Most studies try to envision scenarios that avoid an abruptchange of the socio-economic global system and, therefore, searchfor ways to keep increasing energy consumption and relativeefficiency while, at the same time, solving such environmentalproblems as pollution and Climate Change (WEC, 1994; IPCC,2007; Jacobson, 2009; IEA, 2010).
The great majority of the studies that evaluate the long termtechnological potential of renewable energies conclude that theamount of renewable resources poses no limit from the techno-logical point of view, due to their extreme abundance. In theparticular case of wind energy, the usable power is said to beseveral times greater than the global amount of total energy usedtoday in the world; therefore, the technological limits are assumedto be unreachable for decades, and the concern is on the economic,
ll rights reserved.
þ34 983423358.
ro).
political or ecological limits imposed (REN21, 2010; Johanssonet al., 2004; Nakicenovic et al., 1998; Greenpeace, 2008)1.
Section 2 reviews the estimations of global wind powerresources present in the literature. Section 3 calculates the globaltechnical potential of wind energy using a top–down methodol-ogy. In Section 4, we demonstrate that the estimations of windpotential based on a bottom-up approach are not coherent withthe energy conservation principle and finally, in Section 5,conclusions are extracted.
2. Previous estimations of the technical potential of windenergy
Several authors have estimated the geographical, technical andeconomic power available in the Earth’s wind energy. Table 1summarizes the main results found in the literature.
If we take a look at the economic and sustainable potentialsthey establish, Schindler et al. (2007) and Zerta et al. (2008), forinstance, estimated 6.9 TW as the plausible development duringthis century. On the other hand, Graßl et al. (2003) established
1 ‘‘Renewable energy resources are immense and will not act as a constraint
on their development’’ (Johansson, 2004). ‘‘The world has tapped only a small
amount of the vast supply of renewable energy resources’’ (REN21, 2010). ‘‘They
can provide energy for any level of future energy demand’’ (Nakicenovic et al.,
1998). ‘‘In summary, wind power is a practically unlimited, clean and emissions
free power source’’ (Greenpeace, 2008).
Table 1Feasible wind power according to several authors. Most technical powers are primary, not electrical.
Authors Technical power (TW) Economic/sustainable power (TW)
Archer and Caldeira (2009) 1500 (jet stream, % feasible?)
Capps and Zender (2010) 39 (offshore)
DeVries et al. (2007) 4.5 (in 2050)
EEA (2009) 8.6 (Europe) 3.5 (Europe 2030)
Elliott et al. (2004)þMusial (2005) 1 (EEUU)
Greenblatt (2005) 70.4 (global)
Greenpeace (2008) Greenpeace 2010 1 (in 2050) 1.2 (in 2050)
Hoogwijk et al. (2004) 11 2,4-6 (economic)
Lu et al. (2009) 78 (onshore), 47 (offshore)
Miller et al., 2010 17–38 (onshore, geographical potential)
Schindler et al. (2007) 6.9 (sustainable)
WEC (1994) 55.2 (global)
Wijk and Coelingh (1993) 2.3 (onshore, OCDE)
Zerta et al. (2008) 6.9 (sustainable)
C. de Castro et al. / Energy Policy 39 (2011) 6677–66826678
4.5 TW as the economic and sustainable potential of wind energyduring this century, DeVries et al. (2007) gives 4.5 TW for 2050,while EEA (2009) envisions, only for Europe, a potential of 3.5 TWfor 2030. Greenpeace (2008) projects scenarios with 1 TW for2050 and cites the studies of several authors with sustainablepotentials of 15 TW. All of these economic or sustainable assess-ments are based on technical potentials while adding morerestrictions; therefore, technical global potentials are higher. Aswe shall see in Section 3, the technical potential we estimate inthis paper is one or two orders of magnitude lower than most ofthese technical potentials, in fact, it is lower than the estimatedeconomic and sustainable potentials cited in Table 1.
All of these studies that evaluate the technical power potential ofwind energy (see Table 1), except the one by Smil (2008) (with noexplicit methodology) and the one by Miller et al. (2010) thatcalculates a physical–geographical potential, use a bottom-upmethodology: they take the wind speed in many locations of theEarth’s surface, exclude the areas that are considered not suitable forwind farms, and calculate the energy that would be trapped in thoselocations with the technically available present or future windmills.
3. Estimation of technical potential of global wind energy
A top–down methodology would be based on the followingsteps: we must first take into account the global amount of poweravailable as kinetic energy from the wind in the atmosphere, P0,and in the lower atmosphere (altitude lower than 200 m), usableby windmills, P0(ho2 0 0). This power must be reduced by thegeographical constraints, which account for the percent of landsuitable for wind farms (excluding, for instance, remote andfrozen land areas and deep sea surfaces). We shall call thegeographical potential, the power available after the geographicalconstraints are considered, PG.
The technical potential, PT, is more restrictive than PG and takesinto account the energy that the windmills can extract, consider-ing current or plausible technological efficiencies. Economic powerPEc is even lower and considers the restrictions derived from thecosts of the technology, and finally, if the disturbances in theecosystems are taken into account, we end up with the estimationof the sustainable power Pe, which is the limit that we should notsurpass in order to preserve the health of the planet (see forinstance DeVries et al. (2007) for similar definitions).
In order to calculate the global potential of wind energy, weuse a top–down methodology based on six stages. The base data isthe kinetic energy contained in the atmosphere, and this amountis restricted by several constraints that subtract the energy that
cannot be transformed into electricity. These constraints are:
1.
The energy of the lowest layer of the atmosphere, f1,P0(ho2 0 0)¼ f1P02.
Reachable areas of the Earth (geographical constraint), f2.PG¼ f1f2P03.
Percentage of the wind that interacts with the blades of themills, f34.
Areas with reasonable wind potential, f45.
Percentage of energy of the wind speeds that are valid toproduce electricity (not too little or too much velocity), f56.
Efficiency of the conversion of kinetic energy into electricenergy, f6PT¼ f1 � f2 � f3 � f4 � f5 � f6 � P0
3.1. Global kinetic energy of the atmosphere
According to several authors, the kinetic energy that wind con-tains and is dissipated into other forms of energy varies between 340and more than 1200 TW (e.g. Gustavson (1979) 3600 TW; Lu et al.(2009) 340 TW; Lorenz (1967) 1270 TW; Wang and Prinn (2010)860 TW, Peixoto and Oort (1992) 768 TW, Skinner (1986) 350 TW,Sorensen (1979, 2004) 1200 TW, Keith et al. (2004) 522 TW). Theseestimations are obtained using observations or general circulationmodels of the atmosphere, and some consider the entire atmosphere,while others restrict it to altitudes lower than 1000 m.
In our study, we will take the one of Sorensen (2004),P0¼1200 TW, because it is for the entire atmosphere and alsogives turnover times of kinetic energy.
3.1.1. f1, the energy of the lowest layer of the atmosphere
Not all the wind of the atmosphere can be captured withwindmills, only that which is closest to the Earth’s surface. Thebiggest windmill today (as of October 2010) has a power of7.6 MW, the diameter of its blades is 126 m and it is 198 m high(Enercom, 2010). Although in future years the size of windmillsmight continue to grow, most of the mills installed today havediameters below 90 m (less than 2 MW), and those installed todate will continue in their locations at least till 2030. Therefore,we will consider that, at least during this century, the windmillswill have an average size of at most 200 m high and blades of100 m in diameter, and therefore, the layer of atmosphere whoseenergy can be extracted is that of the 200 m closest to the Earth’ssurface. Therefore, we are interested in an estimation of thepercent of the wind energy and power of the atmosphere that iscontained in those first 200 m. We use several methods to
C. de Castro et al. / Energy Policy 39 (2011) 6677–6682 6679
estimate and calculate this power: the first one based on theenergetic density of the atmosphere, the second based on theresidence time of winds and a third based on the AtmosphericBoundary Layer wind power.
The energy density of air increases with height, from 30 J/m3
at 100 m to more than 120 J/m3 at 5000 m, because the energy isdissipated at a higher rate where there is more friction, close tothe ground (Sorensen, 2004). In the layer below 200 m, thedissipation rate is around four times higher than above onekilometer (Sorensen, 2004). Taking 1200 TW as the kineticdissipation in 10,000 m of the atmosphere (troposphere), if theenergy were to be dissipated equally at any altitude, at 200 m (2%of the troposphere) only 24 TW will be dissipated. However, asthe dissipation rate in this layer below 200 m is around four timeshigher than that above one kilometer (Sorensen, 2004), then wecan conclude that the dissipation in the first 200 m is approxi-mately 96 TW (4�24 TW).
Another way to estimate the energy of the 200 m layer is usingthe residence time of the wind, which is the average time that itskinetic energy takes to dissipate into heat. We can calculate thekinetic energy of the lowest 200 m layer of air based on the mass andspeed of this layer. Based on the experimental data and the calcula-tions of Archer and Jacobson (2005), we can calculate the speed ofwind at any height as vh¼v10(h/10)a, v10 being the mean speed at10 m height (5.8 m/s according to Archer and Jacobson (2005)) and‘‘a’’ being a constant (Hellman’s exponent) taken as a¼1/7 (Archerand Jacobson, 2005). Using this formula, the mean speed of the windbetween 0 and 200 m is vmean 0–200 m¼7.9 m/s. The mass of this layerof air can be calculated by taking its density as 1.2 kg/m3 and thevolume as approximately 4pRT
2�200¼1.22�1017 m3. The mass of
the first 200 m of air ends up being matm 200 m¼1.47�1017 kg.Therefore, the kinetic energy of the layer of air closest to the surfacewould be Ec 200 m¼4.59�1018 J. The residence time of this air, tc 200 m,can be estimated as roughly half a day (lower than the meanresidence time in the atmosphere which is 7.4 days), since it ismainly determined by the night–day cycle and the direction andintensity of wind usually varies with that rhythm (Sorensen, 2004;Stull, 1988). According to these estimations, the power dissipatedin this layer is P0 200 m¼Ec 200 m/tc 200 m�106 TW, which is similarto the value of 96 TW calculated with the previous methodology.
A third method to estimate this power could be calculatedfrom the dissipated power of the Atmospheric Boundary Layer(ABL). Hermann (2006), gives 290 TW to this power. Generally,the boundary layer thickness is quite variable in time and space,ranging from a few hundred meters to a few kilometers. If we takethe mean thickness of this layer as 400–800 m (Stull, 1988;Hennemuth and Lammert, 2006), then the first 200 m arebetween a half and a fourth of this Boundary Layer with a smallerturnover time, but also with lower wind velocities; therefore,approximately between 1/2 and 1/4 of the wind power in the ABLwill be dissipated in the first 200 m: 72.5–145 TW, again roughly100 TW as the P0 (ho2 0 0) value.
Wind front
Fig. 1. In a wind park, the catching surface of each mill is S1. If the mills are all in a horiz
not brushed by the blades. On the other hand, even if the park has several lines of turb
Taking these three methodologies into account, we will takethe value of the power dissipated in the layer of air closest to thesurface as P0 (ho2 0 0)¼100 TW¼P0 f1, where f1¼0.083.
3.1.2. f2, geographical constraints, reachable areas of the Earth
Not the entire Earth surface is suitable for kinetic energyextraction: the deep sea areas (more than 200 m deep), areaspermanently covered by ice, high mountains, cities, protectedareas and natural parks, etc., could be excluded by geographicalconstraints; therefore, more than 3/4th of the Earth’s surface is notsuitable for wind farms (Archer and Jacobson, 2005; Capps andZender, 2010; Lu et al., 2009). On the other hand, the windiestcontinent is Antarctica, and wind has a lot more energy over thedeep seas than on the ground. We could, therefore, easilyestimate that less than 80% of the energy will be lost because ofthese geographical restrictions.
f2o0.2
3.1.3. f3, percentage of the wind that interacts with the blades
Current wind parks are designed leaving a space between millsto prevent efficiency losses due to turbine interference (‘‘Windpark effect’’). These distances are typically 4D in the directionperpendicular to the dominant wind and 7D in the paralleldirection, with D being the diameter of the blades (Archer andJacobson, 2005; Capps and Zender, 2010; Lu et al., 2009). Usingoptimal distributions of the mills such as this one, the efficiencyloss from the wind park effect is reduced to 2–5%.
A wind front 200 m in height going through a turbine of 100 min diameter, would not intersect any blade in 60% of the rectan-gular surface that it occupies (see S2 in Fig. 1). Although themomentum mixing from the surrounding air in the wake of thefirst line of the turbines of a wind park partially replenishes thewind power behind the turbines and could be used by the secondline of turbines, too many lines of mills cannot compensate thewake effect (see Section 4) and the necessary separation betweenmills (see S3 in Fig. 1). This means that a wind farm might try tocatch less than approximately 30% of the kinetic energy that goesthrough it, because the rest will simply never interact with theblades of the mill (see Fig. 1). Therefore: f3o0.3.
3.1.4. f4 areas with reasonable wind potential
Even in locations accessible for wind parks, it is reasonable tothink that not all of them will be occupied by wind parks, themills being restricted to those areas with the highest wind power.Today the areas are classified, according to wind power, intodifferent classes. Most parks today are in areas of class 5 or 6,since they are the most profitable technically and economically,and in order to produce the same energy in a park with wind ofclass 3 we need two times mills than in a park of class 6. Weconsider that the mills will be situated in areas of class 3 or higher(as in Archer and Jacobson, 2005). According to the study of
S1
S2
S3
ontal plane, each mill occupies a surface of the wind S1þS2, where S2 is the surface
ines, there will be a surface, S3, between mills where the flow of the wind is free.
C. de Castro et al. / Energy Policy 39 (2011) 6677–66826680
Archer and Jacobson (2005), approximately half of all the kineticenergies of the geographically accessible areas are in areas of class3 or higher (more than 75% of the areas are of class 1 and 2,although they carry a lot less energy per km2). Using thisestimation and considering that only locations of class 3 or higherwould be used we have: f4¼0.5.
Due to the wake effect (Christiansen and Hasager, 2005) andthe interactions of wind power removal with the flow, as is donein the study by Miller et al. (2010) (see Section 4), not the entirearea with reasonable wind potential could be fully replenishedwith wind farms. This means that there is a trade balancebetween f3 and f4 (the more lines in a wind farm the more f3
but also the more wake and removal flow effects, lowering f4).Therefore, f4o0.5.
3.1.5. f5, percentage of energy of the wind speeds that are valid to
produce electricity
Although wind turbines today are designed for a wide range ofspeeds, they have a limit on the highest and lowest speed they canuse (typically between 2.5 and 25 m/s) and they are designed to givethe maximum power in a particular spectrum of speeds. This meansthat during many hours of the year the turbine does not produceenergy that exists. Since the power of the wind grows with the cubeof its speed, if the turbine is designed to give the maximum energyat 12 m/s, for example, at 20 m/s it wastes more than 75% of thewind energy. Modern turbines can interact and produce electricitywith less than half of the energy that goes through them (Boccard,2009). We estimate that future designs will be able to improve thisratio and use three quarters of the energy that interacts with them,but not much more, therefore: f5¼0.75.
3.1.6. f6 efficiency of the conversion of kinetic energy into electric
energy
A windmill, at the blades, can transform into mechanical energyat most 59% of the kinetic energy that it effectively catches (Betzlaw). Modern turbines have a mechanical energy efficiency at theblades of less than 45% (Cetin et al., 2005). However, because thereare losses in the alternator and in the power converters, etc., present
Wind flow direction
S1
h
2h
Fig. 2. Hypothetical helicopter and tie turbines. If they are at an altitude h, the effectiv
interact with the tie or helicopter blades, S2 being roughly 2 h �10,000 (where 10,000
helicopters or ties of 2000 m2 and h¼1000 m, S2¼20�106 m2, therefore: f3¼S1/S2¼10
referred to the web version of this article).
turbines have an efficiency lower than 40% (Chang et al., 2003;Boccard, 2009). There are also losses due to maintenance, failures,etc. that the mill might have throughout its life time. Therefore, ifwe assume than in the future the losses relative to the Betz limit willonly be 15% (at present losses are 430%) then f6¼0.5.
Taking into account all these limitations and the global kineticenergy of the atmosphere calculated in this section, we canestimate that the final technical potential of the wind is
PToP0 � f1 � f2 � f3 � f4 � f5 � f6�0,0009 � P0
PT (ho200)�1 TWOne of the criticisms that we could pose concerning our own
methods comes from the fact that, although there are nocommercial turbines higher than 200 m, some scientific literatureis speculating with devices that would trap the energy at highaltitudes (4500 m) (Archer and Caldeira, 2009; Fagiano et al.,2009; Roberts et al., 2007).
These future technologies would be subject to many of therestrictions that we have calculated for low heights, and theprincipal restriction would be given by the factor f3, the energy ofthe wind that interacts with the blades. All of the designs of thesetechnologies are attached to the ground, and the ones withgreater perspectives are the helicopter and tie types (see Fig. 2).Since the wind fluctuates in direction and intensity, these mobiledesigns would need an area of operation and security a lot largerthan the fixed windmills.
As we can see in Fig. 2, f3 will not, for these designs, be greaterthan 10–4, and because we will also have the rest of the ‘f’ factors,then much less than 10–4
� P0 could be used from the kineticenergy in the entire atmosphere.
4. Criticisms of the bottom-up methodologies.
Bottom-up methodologies should take into account that eachtime a wind front goes through a turbine it reduces its speed andpower (wake effect, Christiansen and Hasager, 2005). Accordingto Christiansen and Hasager (2005), the speed of a wind front isreduced by 8–9% and the power by 25% when it goes through awind park and does not recover until the next 5–20 Km.
e area S1, in blue, is compared with S2, the surface of the wind front that does not
m is the thickness of the atmosphere). Even if S1 is made up of a total array of–4. (For interpretation of the references to color in this figure legend, the reader is
C. de Castro et al. / Energy Policy 39 (2011) 6677–6682 6681
In order to regenerate the energy loss by the wake effect on aglobal scale, only a huge kinetic energy transfer from above thewindmill atmospheric layer could partially justify a bottom-upapproach. But Wang and Prinn (2010) (see also Keith et al., 2004)use a general circulation climate model and show that the kineticenergy per unit mass in the atmospheric boundary layer isreduced by more than 10% in order to generate 5 TW of electricpower globally.
Climatic change and land use change (mainly deforestationand reforestation and buildings and urbanization) are causingchanges in the wind velocity distribution over the Earth. Wangand Huang (2004), give a 20% increase of the wind energy input tothe surface waves in the last half century and, on average,terrestrial near-surface winds have slowed down in recentdecades (McVicar and Roderick, 2010); human induced surfaceroughness changes are effectively reducing wind energy andchanging wind distribution, so windmills will also do this.
All these facts should be considered when using bottom-upmethodologies, since they would significantly reduce the effectivedensity of windmills in the suitable areas; but these considerationsare not found in the authors of Table 1 (except for Miller et al. 2010,who use a top–down methodology). These studies assume that theperturbation that a wind farm produces is negligible compared tothe global amount of resource; but those bottom-up methodologiesare not compatible with global data of kinetic energy present in theatmosphere and therefore violate the principle of energy conserva-tion as Gans et al. (2010) does explicitly on theoretical grounds,based on the wake effect, and Wang and Prinn (2010) doesimplicitly, using a 3D general circulation model.
In order to demonstrate that the bottom-up methodologiesthat many authors use to estimate the potential of wind energyare not valid, let us use the results of Table 1 and Section 2.
We are going to use the factors f1–f6, calculated in Section 2,and do the inverse calculation: using the technical potentialestimated by these authors, we shall find the amount of kineticenergy that they would require from the atmosphere. If theenergy required is greater than the global kinetic energy con-tained in the wind (1200 TW, as discussed in Section 3) theestimations are incoherent.
If we take into account the 72 TW that Archer and Jacobson(2005) consider feasible for just onshore wind potential, and addall the wind energy discarded because of impossible locations,480% (see Section 3), the energy discarded because of lowaverage speed (25%), the energy not used because of wrong speed(50%) and the interaction of the front with the blades (75%), wewould conclude that the global kinetic energy dissipated by theatmosphere in the 200 m closest to the surface should be higherthan 4000 TW, a lot higher than the 100 TW of energy that weestimated for the layer of 200 m closest to the surface, and evenlarger than the estimations of 1200 TW for all the energydissipated in the entire atmosphere.
In Capps and Zender (2010), they estimate a technical poten-tial of 39 TW of offshore energy by occupying locations that, theysay, carry only 2.73% of the kinetic energy of the sea winds andusing mills of 80m in diameter and a capacity factor of 0.5. Usingtheir own calculations, we would end up estimating that 7500 TWare carried by all the oceans in the 200 m layer of the atmosphereclose to the surface, six times greater than the estimation of thekinetic energy of the entire atmosphere.
5. Conclusions
The global assessment of the technological potential of windpower to produce electricity, based on the top–down approach,shows quite different results to those of previous works. The
technical assessment potential that has been obtained is one ortwo orders of magnitude lower than those estimated by authorsreferred to in Table 1, and is only comparable to the estimation bySmil (2008) (o10 TW), except for the physical–geographicalpotential estimated by Miller et al. (2010), also using atop–down methodology, which is 17–38 TW (our physical–geographical potential, applying only f1, f2 and f6, which iscomparable to Miller et al. (2010)’s methodology and calculations,would be o40 TW). This means that technological wind powerpotential imposes an important limit on the effective electricwind power that could be developed, against the commonthinking of no technological constraints by economic, ecologicalor other assessments.
According to the World Wind Energy Association (WWEA,2010), the electrical wind power produced today is �0.045 TWand this type of energy is growing at an annual rate of 425%. Ifthe present growth rate continues, we would reach the 1 TW weestimated in less than 15 years. Therefore, probably in thisdecade, we will see less growth than we saw in the previousdecade.
This limit poses important limitations to the expansion of thisenergy. Since the present exergy consumption of all energies is�17 TW, it implies that not more than 6% of today’s primaryenergy can be obtained from the wind.
On the other hand, transforming 1 TW of wind energy intoelectricity requires 1012 m2 (taking required land of 1 W/m2;Smil, 2010). This amount represents more than 5% of the landdedicated on the Earth to agricultural land (1.5�1013 m2), whichis a very significant impact on the landscape and the ecosystems,even if the space of the wind parks can be dedicated to otheruses too.
Furthermore, if the electric wind power of the world were toapproach 1 TW, we could generate a new class of ‘‘tragedy of thecommons’’ (Hardin, 1968) with the necessity of the internationalregulation of rights to winds. Without an effective regulation, in amedium-term future, we will see ‘‘wind park effect and wakeeffect’’ on a global scale, making new and old installed parks lessefficient.
Global assessment of potential energy based on bottom-upmethodologies has been used for renewable energies such astidal, wave or geothermal. Dubois et al. (2008) is an extreme case(in an undergraduate reviewed paper) that gives more than3000 TW as the resource of waves (158 TW as the technical–geographical potential), but the total energy transferred fromwind to sea waves is 60 TW (Wang and Huang, 2004), and thewave exergy dissipated on the sea coast is only 3 TW (Hermann,2006). A top–down methodology must start with this 3 TW as thegeographic potential, PG, and then apply their correspondingreduction factors, f3–f6.
A top–down review of the global assessment of potentialenergy from these renewable sources may be necessary in orderto obtain the best estimation for the top limit of primary energythat our society is able to use in a sustainable way.
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