Readers letters
News that the atmospheric level of CO2 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. 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”. The editor attributed this to a lack of rigorous, evidence-based data surrounding net power and lifecycle performance of wind turbines.
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 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. 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. It is expressed in grams of CO2 equivalent per kWh (gCO2eq/kWh) of electricity generated. 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.
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 gCO2eq/kWh over the most often quoted turbine lifetime of 25 years. In comparison, CCGT has a footprint ranging between 488 - 600 gCO2eq/kWh.
Related to carbon footprint are the Energy Return on Investment (EROI) and Energy Payback Time (EPT). Both terms involve the embodied energy of the generating plant, i.e. the energy expended for its manufacture, operation (including fuel extraction, processing and transport) and decommissioning. 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”. Equivalent data from the same Table for CCGT is 1.9 – 3.9 years.
EROI or Energy Ratio is the ratio of energy generated to embodied energy. 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. 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. In the same report, a range of 2.5 – 8.6 is reported for CCGT.
There is, therefore, a clear and significant net CO2 reduction over a wind turbine’s lifetime.
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. The definitive reference for such information is the Digest of United Kingdom Energy Statistics (DUKES) 2012. In 2011, the capacity factor, based on average beginning and end of year capacity, was 29.8% for onshore wind (Table 6.5). The benefits of the higher wind resource at offshore locations are highlighted in the same table by an increased capacity factor of 36.8%.
Clearly a wind turbine cannot generate electricity when there is no wind at all – however the fact remains that in the UK wind turbines generate electricity between 70-85% of the time.
According to Table 6.4 of DUKES 2012, 10.4 TWh of electricity was generated by 4.6GW installed capacity of onshore wind in 2011.
For readers who are interested in other aspects of wind turbine operation, such as health risks, we recommend the Centre for Sustainable Energy report “Addressing Public Concerns about Wind Power”, which debates some of the misconceptions surrounding wind energy.
The RPC recognises the challenges involved in efficiently integrating onshore (and offshore) wind into the UK’s electricity infrastructure. But we are confident that these challenges can be overcome through the modernisation and effective management of the UK’s aging grid infrastructure and electricity balancing system, together with the ingenuity of our creative engineers. Local and national electricity storage solutions will also support greater utilisation of wind power.
Ian Burdon, Chairman, Renewable Power Committee
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