Power Density

by Vaclav Smil

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

  The availability of mass-scale storage of electricity that could deliver on demand combined blocks of 10'-1010 W 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 the introduction of new forms of storage, with the largest units having capacities of 10'-109 W, something that can be done today only with large hydro stations and that the largest pumped hydro storages can do. The third required component of a renewable energy system is the large-scale conversion 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), fuels would still be needed to power ocean shipping and flying, and hydrocarbons would also be needed for many synthetic processes.

  This book has been preoccupied with quantifying fundamental physical qualities, but the pace and extent of energy transitions will be strongly determined by costs and real returns, and a closer look shows that the new renewable energy sources are not exceptionally attractive. This may be a surprising statement given the fact that unsubsidized levelized costs of electricity (LCoe) generated by new renewables is becoming increasingly competitive with the dominant conventional conversions (Lazard 2013; USDOE 2013a). But standard calculations of LCoE do not account for different dispatch characteristics, for potential stranded costs of distributed generation and for social costs affecting those unable to afford distributed generation.

  Most notably, published levelized costs 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 and co-workers (2013) 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 systemwide costs could be 50% higher, and if it reached 40% they might be double the traditional LCoE estimates owing largely to standby power and recurrent overproduction costs. Similarly, an analysis by the Berkeley National Laboratory (Mills and Wiser 2012) found that adding the first 100 MW of solar PV-based electricity to a grid can provide capacity credit equal to 40-70 MW, but as soon as PV's share reaches just 10%, these capacity credits shrink to 20-40 MW. These realities have been inexplicably neglected by the proponents of a rapid energy transition. On the other hand, proponents of PV-based solar and wind energy might argue that the environmental benefits of high shares of renewables make them still the better choice, and, undoubtedly, in some already well-interconnected systems integration costs might be relatively modest

  As for the true (that is physical, not monetary) returns, Prieto and Hall (2013) called the attention to the low energy return on investment (EROI) for Spain's solar energy, putting it at no more than 2.45 in terms of thermal equivalents, far below 12-14 needed to maintain the modern civilization. Their findings were strengthened by Weissbach and co-workers (2013) who used a strict exergy concept and updated material databases to compare the EROI of wind, PV, hydro, natural gas, coal, and nuclear power plants on a uniform basis, an approach superior to any used in previous studies.

  For the renewables they present both an unbuffered EROI and a buffered return that takes into consideration the cost of storage systems. All systems produce more energy than they consume (EROI greater than 1), but two them are below the economic limit of 7: German solar PV has an unbuffered EROI of 3.9 and a buffered return of 1.6, and corn for energy has an EROI of 3.5. European wind generation has an unbuffered EROI of 16, but its buffered rate (3.9) also falls below the economic threshold. In contrast, combined-cycle gas turbine electricity generation has an EROI of 28, a coalfired plant has an EROI of 30, and pressurized water reactors have an EROI of 75 (fig. 8.3).

  All of this does not mean that a new global energy system based predominantly, if not solely, on conversions of renewable energy flows is impossible. But these realities make it clear that achieving it will be more challenging and will take longer than most of its enthusiastic proponents would have 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 the low power densities of stochastic renewablesource energy flows and the 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 their harnessing, conversion, and use proceed with power densities whose values are fundamentally limited by environmental constants and circumscribed by advancing technical capabilities. Modern civilization has evolved as a direct expression of the high power densities of fossil fuel extraction, but that extraction is predestined to claim only a short time span of human evolution. New energy arrangements are both inevitable and desirable, but without any doubt, if they are to be based on large-scale conversions of renewable energy sources, then the societies dominated by megacities and concentrated industrial production will require a profound spatial restructuring of the existing energy system, a process with many major environmental and socioeconomic consequences.

  Figure 8.3

  EROI of energy production. Modified from Weissbach and co-workers (2013). Carl De Torres Graphic Design.

  Basic Systeme International d'Unites (SI) Units

  Energy, Power, and Associated Units

  Other Units Used in Calculations and Text

  Multiples Used in the Systeme International d'Unites

  Submultiples Used in the Systeme International d'Unites

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