an EROI greater than 1–i.e., their cumulative electricity generated will be less than their embodied primary energy. The energy costs of re...ning and purifying crystalline silicon and the complex process of turning silicon wafers into PV cells are mainly responsible for the high fraction of gross production expended for the PVs’initial emplacement. Such studies have raised doubts about the sustainability of PV systems (Odum, 1996). More recent studies, however, re‡ecting the signi...cant technological change of the last few years, claim that PV systems are able to reimburse their initial energy requirements within a few years of operation (Bankier and Gale, 2006). Thus, there is a huge variance across studies in their estimates of EROI and the payback time �1 for PV systems–re‡ecting both methodological di¤erences (choice of system boundaries) as well as di¤erences in cell manufacturing technology (silicon puri...cation and crystallization process), insolation, type of encapsulation, frame and array of support, and the type of system application (autonomous vs grid-based).

A recent report based on data from 26 OECD countries presented the following estimates: (i) for roof-mounted PV systems (taking into account not only the panels but also the wires and electronic devices) the payback time �1 is in the range of 19 to 40 months and for PVfacades the corresponding range is 32 to 56 months; (ii) EROI is between 8 to 18 for roofmounted systems and between 5.4 to 10 for facades (IEA, 2006). Based on these ranges, the implied values for the growth parameter � are between .03 and .08 for roof-mounted systems and between .01 and .04 for facades; the doubling time �2 is: between 8.3 and 27.5 years for roof-mounted systems and between 17.8 and 75.7 years for facades. A study by Fthenakis and Alsema (2006), based on more recent Life Cycle Inventory data from 12 European and US photovoltaic companies, estimated the payback times for ribbon, multi-, and mono-Si technologies to be 1.7, 2.2, and 2.7 years respectively. The corresponding doubling times then are estimated to be 9.11, 13.07, and 18.20 years.

The EROI estimates for nuclear power have been equally disparate. As with the other generating technologies, one of the main issues has been de...ning the appropriate boundaries of analysis. For nuclear lifecycles employing the gaseous di¤usion technology, fuel enrichment is the key energy input–more than half the lifetime total. Centrifuge technology reduces enrichment energy consumption to 1/12 of its di¤usion-based value and correspondingly E reduces h, the O&M component of EROI; down to a value closer to<sup>theinitial</sup> Pnp'T<sup>;</sup> emplacement component. During the past two decades, nuclear power plants have also achieved increasingly higher capacity factors. In the United States, for example, the average capacity factor ' for plants in operation in 1980 was 56.3 percent. By 1990, it had risen to 66 percent, and in 2007 to 91.8 percent. Similar developments occurred in other countries. In Russia, capacity factors increased from 56% in 1998 to 75% in 2007 (NEI, 2008). Most of today’s nuclear plants were originally designed for 30 or 40-year operating lives. However, improvements in system components and upgrading of structures have extended e¤ective plant lifetimes. Re‡ecting these technical improvements, many of the currently operating reactors in the United States are expected to be granted licence extensions raising T from 40 to 60 years (Nuclear News, May 2009).

The following set of calculations is based on the most recent data reported in WNA (2009) on the total energy required to build and run a nuclear power plant and the lifecycle 16