thermodynamic analysis and particularly the entropy law to
economic theory [5] and has had a profound influence in the field
of ecological economics [6,7]. Also in the 1970s, ecologist Howard
T. Odum, working on energy flow analysis, stated that ‘the true
value of energy to society is the net energy, which is that after the costs
of getting and concentrating that energy are subtracted’ [8]. This
definition and approach had a great impact at the time even
influencing US policy with the publication of the Research and
Development Act in 1974, still part of US code today. It states that
‘the potential for the production of net energy by the proposed
technology at the stage of commercial application shall be analyzed
and considered in evaluating proposals’ [9]. Odum also had a major
influence in the field of ecological economics and amongst other
achievements is known for developing the concept of ‘emergy’ in
the 1990s, which is still being developed and used today in a wide
range of applications including the building environment [10,11].
Since the revival of the ‘net energy’ concept in the 1970s, net
energy analysis has been applied in many different fields, from the
fossil fuel and nuclear industries to renewable technologies [4,12–
16], being recognized as a valuable tool to consider life cycle
aspects of energy systems. The net energy analysis has been
defined as a ‘technique for evaluating which seeks to compare the
amount of energy delivered to society by a technology to the total
energy required to find, extract, process, deliver, and otherwise
upgrade that energy to a socially useful form’ [17]. There has been an
emerging debate about the usefulness of the ‘net energy’ concept in
decision-making particularly in the field of biofuels, with different
experts highlighting both the usefulness of the concept and its
potentially misleading application [18–20]. One of the reasons for
the disagreements about the usefulness of the concept of ‘net
energy’ is due to the difficulties of setting boundaries as happens
with any life cycle analysis methodology. Different methods and
degrees of accuracy have been used for ‘net energy’ analysis over
the years and results have being expressed in different terms such
as energy payback, energy return of investment, energy yield ratio,
net energy ratio, life cycle energy analysis (LCEA). A recent paper
by Mulder and Hagens [21] analyzed a range of different studies
and proposed a consistent framework of analysis for energy return
of investment (EROI) of energy production technologies. Richards
and Watt [22] also attempted to review the different types of ‘net
energy’ indicators that have been used over the years, focusing on
studies carried out on photovoltaics, and proposes the energy yield
ratio (EYR) as the most adequate indicator, having as its main
advantage the consideration of the lifetime of the product.