Stated estimates for city-integrated wind and solar PV are too high

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 achieved if the entire city surface was PV – this does not violate physical laws, but is oblivious to economic trade-offs of cost vs efficiency. These physical and economic considerations explain why current PV generation rates in cities are typically lower than industrial sites.

Integrating renewables into cities is possible, and it might reconnect us to our environment with everyday reminders of our energy demands. A sound path begins by establishing achievable, near-term realities for renewables, and then incrementally leveraging these small successes.