Solar Versus Wind Power: Which Has The Most Stable Power Output?


Imagine yourself working for a major electric power utility or distributor, perhaps in the strategic planning department. Government is introducing mandates for renewable energy portfolios: a 5 to 20% required green power component is a certainty within the next few years. Both solar and wind power are technically viable choices within the service area. Which of the two technologies should the company first support or most invest in?

Restating the question, which source, wind v.s. solar, has the most even capacity, will be the least disruptive to customers, and require the least backup investment to achieve synergy with the existing business? In part, this question can be distilled down to "Capacity Factor." Two Carnegie Mellon Electricity Industry Center researchers sampled a few solar and wind power facilities over period of two years to document the capacity factor differences. Look at what they found:

"The power spectral density of the output of utility-scale wind farms and solar photovoltaic (PV) arrays is examined to provide information on the character of fluctuations in real power output; the power spectrum constrains the character of fill-in power. Both one second and one hour samples from several wind farms and ten second and one minute resolution data from four solar PV arrays are analyzed. The measured output power for wind follows a Kolmogorov spectrum over more than four orders of magnitude, from 30 seconds to 2.6 days. That for PV is significantly flatter; thus fluctuations at short time scales are larger relative to those at long time scales for PV than for wind. While wind's capacity factor varies from 32% at the sites examined to 40% at excellent sites, the capacity factor for a 4.6 MW PV array in Arizona is determined to be19% over two years."

A money quote from the full paper:

Due to the larger magnitude of solar PV power output fluctuations relative to those of wind at time scales shorter than approximately 31⁄2 hours, the costs of large scale solar PV integration are likely to be larger than those of wind.

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Citation & Image credit:Carnegie Mellon Electricity Industry Center Working Paper CEIC-08-04; The Spectrum of Power from Utility-Scale Wind Farms and Solar Photovoltaic Arrays, by Jay Apt and Aimee Curtright - "Fig. 3. Real power output (kW) sampled with one minute resolution for a 4.6 MW solar PV array in northeastern Arizona for one week."

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