In 2011, the capacity factor, based on average beginning and end of year capacity, was 29.8% for onshore wind (Table 6.5). The definitive reference for such information is the Digest of United Kingdom Energy Statistics (DUKES) 2012. The capacity factor tells us the ratio between actual power generated over a given time period and the theoretical maximum that could be generated if the turbine operated for 100% of the time. There is, therefore, a clear and significant net CO 2 reduction over a wind turbine’s lifetime. In the same report, a range of 2.5 – 8.6 is reported for CCGT. The average EROI for 60 operational wind turbines is 19.8, with a standard deviation of 13.7, which agrees with the range (EROI = 5.0 – 40.0) reported in the IPCC report mentioned above. The paper “ Meta-analysis of net energy return for wind power systems” (Renewable Energy, Volume 35 (2010) pages 218–225) reviews and synthesizes the literature on the net energy return for electric power generation by 119 operational and theoretical designs of wind turbines from 50 different analyses, ranging in publication date from 1977 to 2007. Equivalent data from the same Table for CCGT is 1.9 – 3.9 years.ĮROI or Energy Ratio is the ratio of energy generated to embodied energy. EPT is the operational time it would take the plant to recover its own embodied energy and for wind turbines, this varies between 0.1 and 1.5 years, according to Table 9.8 in the Special Report on Renewable Energy Sources and Climate Change Mitigation released by the Intergovernmental Panel on Climate Change in 2011”. the energy expended for its manufacture, operation (including fuel extraction, processing and transport) and decommissioning. Both terms involve the embodied energy of the generating plant, i.e. Related to carbon footprint are the Energy Return on Investment (EROI) and Energy Payback Time (EPT). In comparison, CCGT has a footprint ranging between 488 - 600 gCO 2eq/kWh. The data presented in the Postnote, based on 9 peer-reviewed international analyses of large (>500kW) onshore wind turbine performance, gives a range of 8 – 20 gCO 2eq/kWh over the most often quoted turbine lifetime of 25 years. For wind turbines, the footprint is dominated by indirect emissions, such as those produced during construction, and varies according to factors such as the local wind resource, because a higher electricity output means that total emissions are spread over a greater quantity of electricity. It is expressed in grams of CO 2 equivalent per kWh (gCO 2eq/kWh) of electricity generated. Carbon footprint aims to provide a complete picture of the emissions caused at all stages of a technology’s lifecycle, including construction and maintenance, the extraction, processing and transport of fuel (where applicable) and ultimate decommissioning and disposal. The so-called carbon footprint of different electricity generation technologies is addressed in Postnote Update No 383 (June 2011), produced by the Parliamentary Office of Science and Technology. In order to remedy this situation, the RPC has researched a number of respected, and in our view unbiased, publications which cover the topic and the data are presented here for the benefit of for readers. The editor attributed this to a lack of rigorous, evidence-based data surrounding net power and lifecycle performance of wind turbines. Members of the Renewable Power Committee (RPC) of the IMechE were disturbed by responses to the question “Do you accept that wind turbines produce a net COu2082 reduction over their entire lifecycle?” 24% of respondents answered “No” and 33% answered “Don’t know”. News that the atmospheric level of CO 2 observed at the Mauna Loa Earth System Research Laboratory in Hawaii will soon reach the 400ppm milestone calls to mind a survey on the subject of wind turbines conducted by Professional Engineering last December. Search our library and digital resources.
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