That trend persisted for 200 years, but reached an apogee in the mid 1980s. Since then, fossil-based EROIs have tended to decrease as the rich fossil reserves have become more scarce–forcing a move to poorer grade or more di¢ cult to reach resources; and to longer shipping distances as local resources became depleted or uneconomic. Additionally, societal focus on reducing the environmental impact of energy supply has led to energy-consuming additions to O&M operations which have generally reduced EROI.

Technologies for wind, solar, and nuclear infrastructures are relatively immature on the time scales of energy infrastructure evolution, and as such they entail signi...cant remaining opportunities for innovation and improvement. Wind and the photovoltaics in particular have experienced extensive technological progress during the past decade. Thus the parameters we employed for the illustrative calculations of Table1 are based on data that can be updated. Table 2 presents estimates based as far as possible on current assumptions and data for these technologies.

Let us consider ...rst the case of wind. Kubiszewski et al (2010) present a meta-analysis of EROI for 119 wind turbines from 50 studies between 1977 and 2007. Their analysis notes that there is a positive relationship between EROI and the power rating of the new generation of windmills which they attribute to: (i) utilization of more advanced technologies (e.g. improved aerodynamic designs) which permit the newer, larger windmills to achieve higher levels of energy conversion e¢ ciency; (ii) greater hub height and rotor diameters on new windmills–wind speed generally increases with height, and the power delivered by the turbine is proportional to the cube of wind-speed and the square of its diameter; and (iii) economies of scale. As a result, the signi...cant increase in the power rating of wind turbines in recent years has been accompanied by a substantial increase in their EROI.

The average EROI for all the studies (operational and conceptual) surveyed by Kubiszewski et al is 25.2 (n = 114; std:dev = 22:3) while the corresponding average for just the operational studies is 19.8 (n = 60; std:dev = 13:7). If we use the average EROI from the operational studies, then from (2) and (4)– assuming T = 30 and a constant capitalization-to-variable expenditure ratio–the implied value for the energy payback period �1 is approximately .98 years. From (11) and (12), the corresponding growth parameter � and doubling time �2 of wind infrastructure are estimated to be .15 and 4.80 years respectively. The large variance in the published EROI estimates raises doubts about the meaning of representative (e.g. average) values. Moreover, some existing studies do not properly account for the backup capacity that is needed to compensate for wind ‡uctuations and thus some reported EROI ratios are overestimated. As wind penetration rises above certain threshold levels, the required investment in backup capacity and the supporting grid could potentially increase nonlinearly. While the surplus energy that can be realistically delivered by wind needs to be studied very carefully, the trend is unmistakable. The performance metrics of wind power, especially those related to sustainability, have experienced substantial improvements in recent years and now compare favorably to those of baseload electricity mainstays coal and nuclear ...ssion.

The ...gures of merit for PV systems have been a matter of considerable controversy during the past decade. Some studies have asserted that PV systems will never achieve 15