Review

City-integrated renewable energy for urban sustainability

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Science  20 May 2016:
Vol. 352, Issue 6288, pp. 922-928
DOI: 10.1126/science.aad9302

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  • Confirming Practical Estimates for City-integrated Photovoltaic and Wind Power Densities
    • Deborah A. Sunter, Postdoctoral Researcher, University of California, Berkeley
    • Other Contributors:
      • John O. Dabiri, Professor, Stanford University
      • Daniel M. Kammen, Professor, University of California, Berkeley

    While we are happy to engage in scientific debate, Miller et al. must recognize that the Kammen and Sunter article primarily is a review article. If the scientific concerns are with the content of the cited peer-reviewed publications, we suggest that Miller et al. pursue discussions with the authors and editors of those publications. Through these discussions if any of the cited publications are retracted, then please do let us know so the review can be updated accordingly.

    The upper limit presented for the power density of solar photovoltaics is clearly stated in the Kammen and Sunter article as being the “potential performance, based on optimal conditions and technologies currently available in the laboratory.” The Solar I/II power plant was included in the last rebuttal to show how even a slightly newer construction (started in 2013) had significant increases in power density, 26% increase when compared to the average of the three plants cited by Miller et al. Photovoltaic efficiencies have improved greatly. Industrial-scale solar farms constructed over 5 years ago are clearly not using “technologies currently available in the laboratory”. The Kammen and Sunter article is forward looking at where the potential for the photovoltaic industry could be in upcoming years as these laboratory technologies are brought into the market, not backward-looking at where the industry was 5 years ago.

    Miller et al. state that “the math here is obvious, but incorporating s...

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    Competing Interests: None declared.
  • Establishing practical estimates for city-integrated solar PV and wind
    • Lee Miller, Postdoctoral Researcher, Harvard University
    • Other Contributors:
      • Vaclav Smil, Professor, University of Manitoba
      • Gernot Wagner, Research Associate, Harvard University
      • David Keith, Professor, Harvard University

    We challenged Kammen & Sunter [1] because their city-scale estimates were too high. Industrial-scale solar farms currently generate less than 20 We/m^2 (Kammen & Sunter noted 10-120 We/m^2) and city-scale wind farms could generate no more than about 1 We/m^2 (Kammen & Sunter noted 2.5-30 We/m^2). When compared to the ~28 W/m^2 average consumption rate of the 10 most populous cities, the 29 W/m^2 of Berkeley (CA), or the 47 W/m^2 of Cambridge (MA), hopefully our reason for forcing this debate is immediately clear.

    We are trying to establish practical estimates for city-integrated solar PV and wind. Reflecting on our debate thus far, we are converging on solar. The opposite is true for wind, where their revised estimate of 35 We/m^2 is higher and therefore more wrong than before.

    Figure 1a shows the average consumption rates of the 10 most populous cities in the US, as well as the chronology of solar PV and wind estimates raised in this debate so far.

    Kammen & Sunter’s original manuscript [1] stated a solar PV range of 10-120 We/m^2. We responded with observed solar farm data that was perhaps overly precise at 8.7-13.2 We/m^2, as Sunter, Dabiri, and Kammen then responded with another observation that was just above our range. Solar farms on flat ground could hit 20 We/m^2 in the very near-future, but the estimate is only part of the story here. We would be quite cautious in using any operational solar farm data as applicable to c...

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    Competing Interests: None declared.
    Attachments
  • Kammen, Daniel M., and Sunter, Deborah A. (2016) “City-integrated renewable energy for urban sustainability,” Science, 352, 922 – 928. DOI 10.1126/science.aad9302
    • Daniel Kammen, Professor, University of California, Berkeley
    • Other Contributors:
      • Deborah Sunter, Postdoctoral Researcher, University of California, Berkeley
      • John Dabiri, Professor, Stanford University

    Kammen, Daniel M., and Sunter, Deborah A. (2016) “City-integrated renewable energy for urban sustainability,” Science, 352, 922 – 928. DOI 10.1126/science.aad9302

    By Deborah Sunter, John Dabiri, and Daniel M. Kammen

    The advances in renewable energy power densities have been immense. While such great achievements may be jarring, all the power densities brought into question have been peer-reviewed and are part of a new frontier for science.

    In response to our paper Miller et al. states that the power density range for wind is “wrong” based on the global estimates of a limit to wind energy generation. However, these global estimates are based on the assumption of a fully developed atmospheric boundary layer over the wind farm, sometimes referred to as the wind turbine atmospheric boundary layer (WTABL). While even a first-principles analysis of the fully developed WTABL regime predicts that an order of magnitude improvement over current HAWT farm power densities is feasible (1), this flow regime does not most accurately represent wind behavior in urban areas. This condition of a fully developed WTABL is only achieved if the surface roughness is constant in the downwind direction over very long distances, i.e. if there are is a consistent topography over that length scale. In the letter to the editor, the authors even state a required distance downwind of “10km.” For a wind farm in the plains, that condition could be achieved, e.g. if all of the turb...

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    Competing Interests: None declared.
  • Stated estimates for city-integrated wind and solar PV are too high
    • Lee Miller, Postdoctoral Researcher, Harvard University
    • Other Contributors:
      • Vaclav Smil, Professor, University of Manitoba
      • Gernot Wagner, Research Associate, Harvard University
      • David Keith, Professor, Harvard University

    Figure 1 of Kammen & Sunter places the energy demand of present-day cities (~20 W m^-2) between “typical [and]... potential [renewable energy] performance, based on optimal conditions and technologies currently available in the laboratory.” Yet the Wind range of 2.5-30 W m^-2 is wrong, the Solar PV range of 10-120 W m^-2 is misleading, and cold weather downtowns often require up to 500 W m^-2.

    After a wind farm extends downwind for more than ~10 km, including the consideration that wind turbines and the atmosphere will be directly interacting limits the electricity generation rate to about 0.3-0.5 W m^-2 globally, and about 1-1.5 W m^-2 in windy locations [1-6] atypical of cities. First recognized by [7], this limit relates to the atmosphere’s ability to continually generate motion (~2 W m^-2 globally), rather than the specifics of the wind technology.

    Solar PV, based on observed 2015 generation rates and footprints from 3 newly constructed solar farms, generates about 11 W m^-2 (8.7 W m^-2 over 2400 acres by Agua Caliente Solar Project; 10.5 W m^-2 over 3500 acres at Topaz Solar Farm; 13.2 W m^-2 over 1475 acres at Central Valley Solar Ranch, [8-13] consistent with [14,15]). 10 W m^-2 is, therefore, not “conservative” (p.922), but rather representative of state-of the art industrial facilities presently operating in regions of flat terrain with excellent insolation, and receiving ongoing maintenance. A doubling of PV generation to ~20 W m^-2 could be...

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    Competing Interests: None declared.