Power density



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Wind and solar

Conversions of these two massive energy flows have become the two fastest growing components of new renewable electricity supply and there has been no shortage of ambitious national goals for their future expansion. Both of these flows differ fundamentally from biofuel harvests and conversions. On a positive side, they have substantially higher power densities, and wind turbines can share other productive (agricultural, pastoral, silvicultural) activities in the area required for their optimized spacing. But staggered wood or crop harvesting and phytomass storage (entire trees, wood chips, baled straw) can assure continuous supply while intermittent radiation and wind translate into, at best, moderately high and often fairly low capacity factors.



Wind Wind power has been the fastest growing category among the new ways of electricity generation and its expansion has been undoubtedly helped by its light footprint as the surfaces actually occupied by wind turbine foundations, access roads and transmission towers amount to a small fraction (<5%) of the area that contains properly spaced machines. In acknowledgment of this reality I have not used the power densities of wind farm spacing –- ranging mostly, as we have seen, between 2-4 Wi/m2 for name-plate capacities and around 1 (0.5-1.5) W/m2 for actual generation –- when quantifying the aggregate claims of energy systems but counted only actual footprints approximated by power density of 20 W/m2.

And yet adhering only to this apparently rational and easily defensible choice is misleading because power densities governing machine spacing are highly relevant in governing the expansion of the industry. The earliest stages of wind power development seek the locations that combine high wind speeds with persistent flows but the capacities of such sites are inevitably restricted, in exceptional circumstances to around 10, usually to less than 4 Wi/m2 and this fundamental power density limit (subject to only minor modification by better turbine design) eventually forces new capacities to sites with lower speed and lower wind frequencies. In the US, where high power density sites cover a large part of the Great Plains between northern Texas and North Dakota, such a shift is hardly a concern, but in Germany, where it has been underway for several years, it is already creating new complications.

Germany’s first phase of wind development, concentrated in coastal regions of windy northern states (Niedersachsen, Schleswig-Holstein) was to be followed by massive offshore projects but their costs –- and the costs and delays in connecting them to land by undersea cables and then transmitting electricity to the south –- refocused attention on the country’s much less windy central and southern regions. As result, there are plans to build 60,000 new wind turbines amidst orchards, vineyards, forests (requiring extensive tree felling), mountaintops (requiring new access roads for trucks and heavy cranes used to transport and assemble turbines) and even in protected areas in states ranging from Nordrhein-Westphalen to Bayern and from Baden-Würtemberg to Sachsen (Schulz 2013).

Such an expansion would greatly alter the appearance of German landscapes, particularly with the invasion of many forested areas, a particularly dubious quest given inherently low capacity factors in many of these central and southern locations. Still, in order to meet the country’s ambitious goal of 35% energy coming from renewables by 2020, many proponents of massive wind power development offer no relief. Winfried Kretschmann, Minister-President of the state of Baden-Württemberg and the first Green Party President of the German Bundesrat (in 2012-2013) insist that "es führt kein Weg daran vorbei, die Landschaft auf diese Weise zu verschandeln” (there is simply no other way but to disfigure the countryside like this).

How much this will be opposed remains to be seen, unlike in the UK where they have been (and not without justification) many vocal (and often admirably eloquent) opponents of defacement of the country’s remarkable landscapes for many years. Wind farms ‘’desecrate our national heritage’ (McMahon 2011, 18) “as if some malevolent creature from mythology shed its spawn over the land’’ (Etherington 2009, 10). Again, the insult would be more tolerable if much higher power densities of wind machines would allow to concentrate the generation into a much smaller number of locations. And disfiguration of landscapes is not the only consequence of limited power densities of wind power.

Necessity to install large numbers of machines tends to reduce the width of noise exclusion corridors, to increase the chances of large-scale bird fatalities and to affect many terrestrial species due to fragmentation of their habitat. In windy and sparsely inhabited Scotland the rule (as already noted) is for 2 km between wind farms and the edge of cities and villages, while in densely populated German states the minimum distances from houses are just 500 (in Bayern), even 300 m (in Sachsen). And, obviously, large wind farms with hundreds of smaller machines in forested and mountainous terrain will kill more birds –- particularly raptors who tend to use mountain slopes and ridge saddles between hills (Subramanian 2012) than single machines or a small grouping of large wind turbines located on flatlands.



PV generation Germany is also the prime example of forcing (through high subsidies) massive installations of PV panels in the country where the solar electricity generation has inherently low power densities not only due to relatively high latitudes (roughly 470-550 N) but also to very low capacity factors in climates governed by the prevailing frontal flows from the Atlantic: in 2012 the country had nearly 40% more PV capacity (32.4 GW) than sunny Spain, Italy and Greece combined (23.7 GW). But in 2013 Germany’s Siemens, one of the key benefactors of the country’s solar boom, published a study whose conclusion was that building and expanding Europe’s solar and wind installations in wrong locations is costing € 45 billion (about $60 billion) in unnecessary investment (Siemens 2013).

I read this with mute astonishment: one of the world’s largest engineering firms had apparently discovered the power of power densities! Details are telling: if Europe’s new PV capacities that are to be built by 2030 (about 140 GW) were located at the sunniest Mediterranean sites the EU could save 39 GW of installed capacity, even after accounting for the cost of additional south-to-north transmission. At the same time, Siemens and several other major German companies (since 2009 grouped in Dii GmbH consortium with other EU partners) was a member of consortium that is pushing for a solution that is the very opposite of PV expansion in Germany. DESERTEC Project (or EUMENA, EU and the Middle East and North Africa) is to make PV electricity generated in Sahara and the Middle East the principal supplier of European demand (DESERTEC 2014). That scheme would take full advantage of the highest possible power densities and it would rely on CSP, rather than on PV, to provide more evenly distributed supply.

Post-2010 upheavals in North Africa and Middle East have accentuated the project’s major weakness: costs and the necessity to construct HV links of unprecedented capacity aside, would it be wise, as the project envisages, to put large CSP facilities in Morocco, Algeria, Tunisia, Libya, Egypt, Syria, Saudi Arabia and Iraq (with most of these countries subject to sporadic or chronic violent conflicts) and then to transmit all electricity through a few chokepoints (Gibraltar, Sicily, Bosporus)? In this case the highest possible solar power densities are easily trumped by the inherent riskiness of the endeavor. In any case, Siemens (and Bosch) left the consortium in 2012, and there are no imminent prospects for DESERTEC becoming a major source of EU electricity. DESERTEC also highlights challenges of servicing very large areas of solar collectors or reflectors in arid environments full of airborne dust: these may not be necessarily higher labor costs (as mechanization and automation can replace, partially or fully, many functions that were previously performed by people) but higher material and energy costs.

Regular cleaning (at least two to three times a year) of these exposed surfaces with power washers or brushes would be obviously both water- and labor-intensive. The best solution appears to be the deployment of robots that glide along rows of PV panels, an innovation by Greenbotics of California that also brings large water savings (SunPower 2013). But this option requires higher inputs of energy-intensive materials to make the devices and more electricity to operate them, thus reducing the net generation of a solar facility. On the other hand, regular night-time cleaning can boost day-time generation. This might be a good solution for China with its extraordinarily high air pollution and chronic shortages of water, but more would be required for the DESERTEC projects or in Saudi Arabia: the country plans enormous solar capacities in the environment where many of them would be prone to be buried under sands in a matter of months.

Another complications whose true dimension cannot be fully answered is the longevity of PV panels mounted on rooftops. They make no new spatial claims as they are merely additions to built-up (impervious) areas but the length of their service may be rather short in many rapidly growing Asian cities where their optimal placement may be also fairly limited due to common shading and where their performance could be heavily degraded by extraordinarily high levels of air pollution (with airborne particulate matter in China’s largest cities reaching maxima an order of magnitude higher than the WHO air quality standards ).

Finally, a few clarifications regarding often-cited claims that imply extraordinarily high power densities of solar generation. Calculations that divide national electricity demand by peak solar power in sunny regions are misleading underestimates of actual land claims. For the US, 2010 electricity demand of 422 GW could be supplied by an area of some 2,800 km2 (square with sides of 53 km) in Arizona –- but only if that surface could generate year-round with the noon-time peak power density of 150 We/m2. NREL (2003, 1) offers somewhat less misleading number when it claims that “a 100-by-100 mile area of Nevada could supply the United States with all of its electricity.” In reality, that area (25,600 km2) could –- with average insolation of 220 W/m2 (Las Vegas mean), capacity factor of 25%, high average conversion efficiency of 15% and hence with average annual power density of 8.25 We/m2) –- generate about 211 GW, or half of the 2010 US demand.

Realistic theoretical estimates are highly dependent on assumptions used to construct a massive imaginary nationwide PV system. Denholm and Margolis (2008a) used long-term data on average irradiation for 48 contiguous states assumed a combination of 25% of rooftop and 75% ground-based modules (40% fixed arrays with 250 tilt, 25% single-axis and 10% two-axis tracking) coupled with appropriate long-term storage and ended up with total requirements of 181 m2/capita –- compared to per capita averages of 35 m2 for airports (or golf courses), 65 m2 for roofs, 162 m2 for major roads and nearly 840 m2 for urban areas. This means that in absolute terms entirely PV-based US electricity generation would require about 55,000 km2, slightly larger than the combined area of Massachusetts and Vermont and about 0.7% of the total area of the 48 states, and the average power density would be a very realistic rate of less than 8 We/m2.

In (largely) gloomy Germany the area needed by PV panels to supply all electricity generation (nearly 560 TWh in 2012) would be considerably larger. With average PV output of 100 kWh/m2 (recent annual mean for both roof and ground-based installations), require about 5,600 km2 covered with modules. That would be an equivalent of nearly 1.6% of Germany’s total area, 25% of the country’s built-up area or almost 15% of land claimed by settlements and transportation infrastructure; and it would be roughly 2.7 times the total area of all German roofs based on an estimate of roughly 25 m2 of roof area per person (Waffenschmidt 2008).



What it would take

If you are willing to engage in unbounded science (and engineering) fiction then this is, according to Jacobson and Delucchi (2011) what it would take to supply the world with 100% renewable energy in 2030 by using electricity (generated by wind, water and solar PV) and electrolytic hydrogen for all purposes: 3.8 million 5-MW wind turbines, 49,000 300-MW central solar plants, 40,000 300-MW solar PV plants, 1.7 billion 3-kW rooftop PV installations, 5350 100-MW geothermal plants, 270 new 1.3-GW hydrostations, 720,000 0.75-MW wave devices, and 490,000 1 MW tidal turbines. All of that would require only about 0.4% of world’s land for footprint and 0.6% for spacing, and we are assured that the “barriers to the plan are primarily social and political, not technological or economic” as the energy cost in a new wind-water-solar world “should be similar to that today.”

These assurances asides, the simplest reality check shows the fictional nature of these assumptions. In 2013 worldwide capacity in wind turbines reached about 330 GW while 13 TW (40 times as much) would be needed by 2030. Total rooftop and large plant PV capacity reached about 100 GW but 17.1 TW of these installations would be required (170 times as much); moreover, there was not a single 300-MW solar PV plant (five plants rated between 200 and 250 MW) but 40,000 would be needed by 2030. In 2013 there was only one central solar power facility rated at more than 300 MW, Ivanpah at 392 MW, but nearly 50,000 would be needed by 2030 (increase of four order of magnitude!). There were fewer than 50 geothermal stations rated at more than 100 MW but 5,350 would be needed (a 100-fold increase). Pelamis (2013) the world’s most advanced wave energy company, produced six 0.75-MW devices by the beginning of 2014 but 720,000 would have to operate by 2030 (increase of five orders of magnitude!). Finally, by 2013 there were fewer than 10 small tidal stations with aggregate installed power of much less than 1 GW while 490 GW would have to generate by 2030 (two orders of magnitude more).

Such a ramping-up of all kinds of capacities –- design, permitting, financing, engineering, construction, all going up by anywhere between one and five orders of magnitude in less than two decades –- is far, far beyond anything that has been witnessed in more than a century of developing modern energy systems. And that still leaves out two other key facts, namely that such a gargantuan renewable system would need enormous expansion of HV transmission and a creation of an entirely new, hydrogen-based, society. I am still not sure how we would fly with hydrogen (or electricity) or smelt pig iron, but apparently these are minor items, perhaps to be addressed by another grand design to be in operation by 2030. In any case, chances of a 100% water-wind-solar world to be ready by 2030 are essentially nil, but it is worthwhile to explore what it would (realistically) take to create a an increasingly non-fossil global energy system.



Making choices To begin with, even though the solar generation has the highest power densities (which may further improve with more efficient PV modules), it would be a very unwise engineering choice to aim at 100% PV generation even if moderately good storage options were available. Any sensibly designed all-renewable system would aim at combining different techniques: this would mean adding (as national conditions allow) substantial shares of wind generation and central solar power, and also (where possible) more geothermal electricity. These requirements would necessitate greatly expanded long-distance HV connections and this would result in additional large land claims in all countries with extensive territories, but because these claims are difficult to calculate without a great deal of specific assumptions I will leave their quantification aside.

And although electricity’s share of final energy use has been steadily rising (in the US it is now about 40%), fossil fuels dominate in all major economies where they have become principal sources for transportation, space heating and for an enormous variety of industrial processes. As a result, their substitution presents a greater transition challenge than displacing significant shares of thermal electricity by solar and wind generation. Setting an early emergence of a hydrogen economy aside, there are two basic paths toward their replacement, The first one is a complete substitution of fossil fuels by a range of phytomass fuels (wood, liquid biofuels, gasification of phytomass) resembling the current sources and able to fit the existing uses.

The second, a more desirable choice and less land-intensive choice, is to produce some phytomass fuels and substitute a large share of their current use by renewably generated electricity used directly to power electric cars, trains and many industrial processes, and to provide space heating, and indirectly to produce (with an inevitable reduction of efficiency) storable energies, an option that might include compressed air, hot water, ice, ammonia and, of course, hydrocarbons made with captured CO2 and some hydrogen. Consequently, the most realistic approach to delimit approximate land requirements of new renewables is to use relatively extreme but still plausible scenarios of future energy systems that might eventually displace all fossil fuels. Small countries would not have requisite flexibility of choice, but it would seem that large nations should have plenty of space to put eventually in place complex renewable energy systems. How much land such arrangements would claim is, of course, dependent on the composition of final energy use.

Higher degree of electrification reliant on generation in sunny and windy places would entail smaller land claims than large-scale substitution of gasoline, kerosene and diesel by biofuels. I have sketched two options for the eventual total displacement of fossil fuels for the US demand at 2012 level that entailed roughly 320 GW of fuel-generated electricity and 1.8 TW of coal, oil and gas, with 60% of this fuel total going for transportation. I have also assumed fairly high power densities: in all cases I have used better performances than today’s practices, and with wind-powered generation I am counting only the land actually occupied by pads and access roads, that is just 5% of area with optimally spaced turbines. The first option –- displacing all fossil fuel-based generation by solar and wind electricity, and substituting biofuels for all fossil fuels –- would claim about 470 Mha and the entire system would have low power density of 0.45 W/m2, above all due to enormous areas required to produce liquid biofuels.



Land required to displace 2012 US fossil fuel demand by new renewables

Electricity generated from fossil fuels: ~ 320 GW

50% displaced by PV (10 W/m2) = 16,000 km2

25% displaced by CSP (20 W/m2) = 4,000 km2

25% displaced by wind (20 W/m2)* = 4,000 km2

*counting only pads and roads

Liquid fuels*: ~ 1,100 GW

25% displaced by crop ethanol (0.4 W/m2) = ~ 688,000 km2

50% displaced by cellulosic ethanol (0.3 W/m2) = ~ 1,830,000 km2

25% displaced by biodiesel (0.2 W/m2) = 1,375,000 km2



*non-fuel uses are subtracted

Solid and gaseous fuels*: ~ 700 GW

50% displaced by wood combustion (1 W/m2) = 350,000 km2

50% displaced by phytomass gasification (0.8 W/m2) = ~438,000 km2



*coal and gas used for electricity generation were subtracted

Massive electrification -– with half of all fuels, or about 900 GW, replaced by electricity generated by the same mixture of solar and wind conversions –- would reduce the total to about 250 Mha, but as this total does not include losses involved in converting some of the generated electricity to storable fuels the eventual land claim would be significantly larger. In any case, these approximate calculations delimit plausible extremes: entirely renewable energy system would occupy roughly 25%-50% of the country’s territory (250-470 Mha), compared to about 0.5% (5 Mha) of land claimed by today’s fossil fuel/hydro/nuclear system.

Of course, there are many plausible adjustments of the grand total, but closer examinations reveal their limits. Perhaps most obviously, PV land claim could be cut by a third or by half by placing more panels on roofs rather than amassing them in solar farms in deserts or placing them on abandoned industrial properties or on strips of land along highways. An increasing share of renewable electricity generation can be moved offshore: large marine wind farms are already in existence (EWEA 2013) and several countries and many companies have been developing various means of ocean power extraction, ranging from classic ocean thermal energy conversion (OTEC) schemes (Faizal and Ahmed 2011) to various wave-power devices and turbines powered by tides or ocean currents and also including utility-scale undersea energy storage (Slocum et al. 2013).

All of these immersed or submersed conversion share two commonalities: they have to operate in demanding, corrosive environment, and (with the exception of strong offshore winds that can be harnessed with high capacity factors) thermal and kinetic energies they are converting have very low power densities. As a result, even OTEC, the oldest idea among ocean energy conversions, has not progressed from failed or abandoned trials to reliably operating prototypes. Similarly, wave devices are in the earliest stages of commercial development and except for Scotland there are no bold plans for their mass deployment. Demand for liquid fuels can be reduced by a third or more as more efficient vehicles get to dominate the market, and the same is true (in longer term, due to a slower turn-over of housing stock) about household energy use.

But even after cutting the lower estimate of future US renewable energy needs by a third we are left with nearly 170 Mha (about 17% of the country, and that is without additional transmission ROWs that would be neded to link the new renewable generation capacities), that is with more land that is in annually harvested crops. Large as it is, such a share could be, in extremis and costs aside, accommodated by the world’s third largest nation -– but that is not the case when analogical land claim calculations are done for smaller countries or for the largest island nations.

Even when assuming a very high average power density of 1 W/m2 (which would require higher rate of system’s electrification) the UK would need about 240,000 km2, or virtually its entire territory. Similarly, McKay (2013, 1) concluded that “in a decarbonized world that is renewable powered, the land area required to maintain today’s British energy consumption would have to be similar to the area of Britain.” Germany has gone further in installing wind and PV electricity-generating capacities than any other affluent economy, but setting up a completely renewable system based on the best available conversions would require, even with a high power density of 1 W/m2 and even with all roofs covered by PV panels, about 350,000 km2, again essentially the country’s entire area.

But it could be lot worse. Japan would complete decarbonize with nearly 600,000 km2 of land devoted to electricity generation and phytomass fuels, nearly 60% more than the area of the four main islands, and land requirements of fully renewable national energy systems would surpass entire territories for numerous high-energy countries ranging from such islands states as Singapore, Taiwan and Trinidad to such industrial powers as South Korea or the Netherlands. Again, assorted measures (from rooftop PVs and offshore wind to more efficient cars and lights) could cut these demands by a third or by half, but even then those fully renewable system would claim impractically large shares of national territories. Given these realities it is astonishing that Lovins (2011a, 40) would claim that “land footprint seems an odd criterion for choosing energy systems: the amounts of land at issue are not large” and that ‘’for civilian energy production, it’s merely an intriguing artifact.” Some artifact!

Power densities matter and this means that the transition from predominantly fossil fuel-based to purely renewable energy systems cannot take place –- even in affluent, populous countries with large territories and with excellent conditions for PV and wind electricity generation and for phytomass cultivation –- by simply following a variant of one of the just outlined replacement options. Large-scale international trade in renewables would help, as it does in modern fossil fuel system where trade accounts for almost 20% of all coal use, two-thirds of crude oil demand and nearly a third of natural gas supply (BP 2014; Cornot-Gandolphe 2013; BP 2013). But trading low energy-density phytomass fuels produced with low power densities in Amazonia would not be obviously the same as trading high energy density crude oil produced with the exceptionally high power densities in the Middle East.

Moreover, the biospheric realities mean that a truly massive trade in phytomass fuels that could be harvested on large scale and with high yields could be sourced from only few tropical countries, with Brazil dominant. Extensive new phytomass production in all other countries with large territories (Russia, Canada, USA, Australia) is greatly limited either by cold climate or by recurrent droughts while serious land scarcity eliminates cultivated phytomass as a major option for the world’s four most populous low-income nations, China, India, Indonesia and Pakistan. Marginal lands, barren hilly slopes and abandoned low-productivity farmland are claimed to have a large potential for producing biofuels but, given their aridity and poor soils, their low productivity (well below 1 W/m2) would come at a high environmental cost (soil erosion, nutrient loss, biodiversity reduction). Heavy reliance on Brazil would further imperil the always precarious state of Amazonian forests: after several years of declining rates the region’s deforestation rose by nearly 30% between August 2012 and July 2013 (INPE 2013).

Decentralized energy supply? Of course, purely renewable supply would be easier to realize if electricity generation were to be massively decentralized –- and decentralization of power and distributed generation have been the leading mantras of renewable energy advocates. The set-up entails electricity generation by small units that may or may not be connected to the grid but are always close to the point of final use, a solution that appeals to green sensibility as it conforms to small-is-beautiful ideal. Reality check is in order: how to square this with growing megacities whose densely-crowded high-rise blocks may average throughout the year more than 500 W/m2 and reach 1,000 W/m2 during the hours of their peak demand?

Since 2007 more than half of the world’s population has been living in cities, by 2050 that share will be above 70% and more than half will live in megacities with populations of more than 10 million, areas with the highest power density of final energy uses. Even if power densities of energy use in many megacities were to decline gradually in decades ahead, it would be impossible to supply them with decentralized PV. The world’s largest megacity, today’s metropolitan Tōkyō, offers a perfect example of these limits.



Decentralized PV generation in Tōkyō

Tōkyō metropolis 2,186.9 km2

Area of 23 special wards 621.3 km2

Metropolitan energy demand in 2010 723.5 PJ = 22.9 GW

Demand in 23 special wards ~ 600 PJ = 19 GW

Metropolitan power density ~ 10 W/m2

Power density in 23 wards ~ 30 W/m2

Average insolation 154 W/m2

PV efficiency 12%

Performance factor 0.85

Average power density of PV generation ~ 15 W/m2

Data sources: Tōkyō Metropolitan Government (2006 and 2012).


Even when the PV panels operating with 12% conversion efficiency were to cover the entirety of more than 600 km2 of 23 densely populated wards they could supply only half of the city’s energy requirements. And the entire metropolis, whose power density averages about 10 W/m2, could get all of its energy from PV panels only if they were to cover about 70% of its nearly 2,200 km2, an impossibility unless we resort to science-fiction visions of cities under plastic bubbles. Shares of annual demand that could be realistically delivered are only small fractions of the total. As previously noted, all potentially available roofs within Tōkyō’s most densely inhabited 23 specials wards are about 64 km2 (Stoll, Smith and Deinert 2013), that is just 10% of the total area; but practical availability will be a fraction of that, and roofs are a smaller share of a much less densely inhabited area outside the 23 wards.

Realistic assumptions can come up with slightly different shares but it is obvious that extensively used PV panels would supply less than 10% of annual demand of the entire metropolis, and less than 1% of the need in its core area where average power densities are an order of magnitude higher than the metropolitan mean (and could do so only if they were tied to adequate storage capacities). Not surprisingly, the Global Energy Assessment concluded that local renewables can therefore only supply urban energy in niche markets (e.g., low-density residential housing), but can provide less than 1% only of a megacity’s energy needs” (Global Energy Assessment 2012, 1347). And there may be other reasons to maximize urban PV generation: panel shading and air pollution resulting in low capacity factors and degraded performance and risk of a substantial capacity loss in earthquake-prone area must be set against the need to supply urban infrastructures that operate 24/7/365 and that need to cover demand that often peaks at night (with tropical household air conditioning).

Similar, or even larger, mismatch between power densities of production and final use will apply to many highly energy-intensive industrial processes, above all to smelting and casting of metals and to chemical syntheses raging from ammonia to plastics and composite materials. Obviously, the most efficacious way to supply megacities and energy-intensive industries would be by converting energies with even higher power density in their proximity, either by relying on domestic resources or by importing high energy density fuels -– while finding large nearby areas capable of supporting large wind or solar capacities might be impractical or impossible. As there is no such renewable option megacities would have to rely on HV links to distant concentrations of wind and solar generation capacities.

How disruptive that shift will eventually be to traditional centralized utilities remains uncertain. Edison Electric Institute concluded that despite the risks presented by a rapidly growing penetration of distributed energy penetration this shift (as long as its degree remains low, as it does in the US) is



not currently being discussed by the investment community and factored into the valuation calculus reflected in the capital markets. In fact, electric

utility valuations and access to capital today are as strong as we have seen in decades, reflecting the relative safety of utilities in this uncertain economic environment (Kind 2013, 2).

Another way to boost renewable generation would be to by having vastly expanded HV link and eventually a global grid. That has been an aspirational goal for decades (Fuller 1981; GENI 2014) but its early emergence (with links crossing the Bering Strait, and connecting North America with Europe via Iceland) is most unlikely, and future large –scale renewable electricity transmission will be limited for long time to regional interconnections (lines from Algerian Sahara to the EU, or from Arizona to New England, from Xinjiang to coastal China). Particularly advantageous links will be those that connect distant production and demand areas across several time zones in order to take advantage of different generation and demand peaks (three hour difference between CSP plants in the US Southwest and the populous Northeastern coast is perhaps the best example).

At least three other components must come together in order to make future renewable energy systems, dominated by electricity with an environmentally acceptable share of biofuels, possible: increased efficiency of all final energy uses; large-scale electricity storage to manage the stochasticity of renewable flows; and affordable means of using electricity to produce liquid fuels. Considerable improvements in efficiency of energy conversions would reduce the overall power demand and hence narrow the gap between power densities of renewable conversions and power densities of common energy uses, making solar and wind much more suitable for decentralized supply outside megacities.

Availability of mass-scale storage of electricity that could deliver on demand combined blocks of 107-1010 W; that would require not only very large numbers of distributed small-scale storages (on the order of 103-104 W, now commercially available and required in Germany and California) but also introduction of new forms of storage with the largest unit capacities of 107-109 W, something that can be done today only with large hydro stations and the largest pumped hydro storages can do. The third required component of an will be large-scale conversions of surplus wind and solar electricity generated during peak capacity hours to storable energies, preferably to high energy density fuels (best of all to synthetic hydrocarbons) or to hydrogen. Even if all road transport became electrified (an unlikely development anytime soon), such fuels would be required to power ocean shipping and flying, and hydrocarbons would be also needed for many synthetic processes.

This book has been preoccupied with quantification of fundamental physical qualities but the pace and extent of energy transitions will be obviously strongly determined by costs and real returns, and closer looks show that the new renewables are not exceptionally attractive. Perhaps most notably, standard calculations of the levelized costs of electricity (LCoE) have ignored the cost of integration measures at the system level but without such steps solar or wind cannot reach large shares of the overall supply. That is why Ueckerdt et al. (2012) came up with a new measure that quantifies the system LCoE and includes the integration costs of variable renewable energies (VRE).

Their key finding was that

at moderate wind shares (~20%) integration costs can be in the same range as generation costs of wind power and conventional plants. Integration costs further increase with growing wind shares. We conclude that integration costs can become an economic barrier to deploying VRE at high shares. This implies that an economic evaluation of VRE must not neglect integration costs (Ueckerdt et al. 2013, 1).

If wind’s share of electricity generation reached 20% its system-wide costs could be 50% higher, and if it reached 40% they might be double the traditional LCoE estimates due largely to stand-by power and recurrent overproduction costs. Similarly, an analysis by the Berkeley National Laboratory (Mills and Wiser 2012) found the adding the first 100 MW of solar PV to a grid can reduce dispatchable capacity by 40-70 MW in dispatchable capacity but as soon as PV’s share reaches just 10% these capacity credits shrink to 20-40 MW. These findings conforms to expectations and engineering realities but they have been inexplicably neglected by the proponents of rapid energy transition. On the other hand, proponents of PV and wind might argue that environmental benefits of high shares of renewables make them still the better choice.

As for the true (that is physical, not monetary) returns, Weissbach et al. (2013) used strict exergy concept and updated material databases to compare energy return on investment (commonly labeled EROI) of wind, PV, hydro, natural gas, coal and nuclear power plants on a uniform basis, an approach superior to previous studies. For the renewables they presented two values, unbuffered EROI and buffered return taking into consideration the cost of storage systems (primarily pumped hydro storage). All systems produce more energy than they consume (EROI>1) but two them are below the economic limit of 7: German solar PV has unbuffered EROI 3.9 and buffered return of 1.6, and corn for energy has EROI 3.5. European wind generation has unbuffered EROI of 16 but buffered rate (3.9) also falls below the economic threshold. In contrast, combined-cycle gas turbine generation has EROI 28, coal-fired plant 30 and nuclear (PWR reactors) 75.

All of this does not mean that a new global energy system based predominantly, if not solely, on conversions of renewable flows is impossible. But these realities make it clear that achieving it will be more challenging, and that it will take longer time, than most of its enthusiastic proponents would let us believe. Technical advances, gradual gains and fundamental innovations will keep making some of its components more affordable and more efficient, but there is no imminent prospect that they could eliminate the mismatch between inherently low power densities of stochastic renewable energy flows and relatively high power densities of final energy uses in modern urbanized societies.

Energy studies have accomplished a remarkable feat by largely ignoring space as a key organizing determinant of modern systems supplying fuels and electricity. But space matters: resources occur in specific locations and configurations and they are extracted, converted and use with largely circumscribed power densities. Fossil fuel-based civilization was always predestined to claim a relatively short time span of human evolution but it has evolved as a direct expression of high power densities of coals, hydrocarbons and thermal electricity. Without doubt, large-scale adoption of renewable energy conversions by societies that are dominated by megacities and concentrated industrial production will require a profound spatial restructuring of the existing energy system, a process that will have many major environmental and socioeconomic consequences.

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