Page 420 - A Comprehensive Guide to Solar Energy Systems

430 A COmPREhEnSIVE GuIdE TO SOLAR EnERGy SySTEmS

the longer term (in decisional/consequential LCAs) grid mix where the PV plant is
being installed.
2. PV as replacement of the nonrenewable energy resources used in the power grid mix.
This approach calculates the EPBT by using the nonrenewable PE only; renewable
PE is not accounted for, neither on the demand side, nor during the operation phase.
This approach calculates the time needed to compensate for the nonrenewable
energy required during the life cycle of a PV system. The annual electricity generation
(E agen ) is likewise converted to PE equivalent considering the nonrenewable PE to
electricity conversion efficiency of the average (in attributional LCAs) or the long-term
marginal (in decisional/consequential LCAs) grid mix where the PV plant is being
installed. The result of using this approach must be identified as nonrenewable energy
payback time (nREPBT) to clearly distinguish it from the EPBT derived from the 1st
approach. The formula of nREPBT is identical to that of EPBT described above except
replacing “PE” with “nonrenewable PE.” Accordingly, grid efficiency, ηG, accounts for
only nonrenewable PE.

Both EPBT and nREPBT depend on the grid mix; however, excluding the renewable
PE makes nREPBT more sensitive to local or regional (e.g., product-specific use of hydro-
power) conditions, which may not be extrapolated to large global scales. In contrast, EPBT
metric with an average large-scale (e.g., Eu, uS, or World) grid conversion efficiency may
not capture the conditions of local or regional climates. The calculated EPBT and nREPBT
do not differ significantly in case the power plant mix of a country or region is dominated
by nonrenewable power generation. however, because an increasing share of renewable
energies is expected in future power grid mixes as well as within the PV supply chain, the
two opposing effects of a reduction in the CEd of PV and an increase in grid efficiency will
require careful consideration, and the numerical values of EPBT or nREPBT may come to
vary considerably according to the chosen approach.
Therefore it is important to choose the approach that most accurately describes the
system parameters and satisfies the goal of the LCA study.
EROI is defined as the dimensionless ratio of the energy generated over the course of
its operating life, over the energy it consumed (i.e., the CEd of the system). The electricity
generated by PV needs to be converted to PE so that it can be directly compared with CEd.
Thus EROI is calculated as:
= /EPBT
G
EROImJPE-eq/mJPE-eq=TEagen/ηG−EO&m/CEd=T/EPBT EROI [MJ PE-eq /MJ PE-eq = ] (T ( )[ E agen / η ) − E O&M ]/CEDT
where T is the period of the system operation; both T and EPBT are expressed in years.
EROI and EPBT provide complementary information. EROI looks at the overall energy
performance of the PV system over its entire lifetime, whereas, EPBT measures the point
in time (t) after which the system is able to provide a net energy return. Further discussion
of the EPBT and EROI methodology can be found in the IEA PVPS Task 12 LCA Guidelines,
and a discussion of its misrepresentation in a few publications is discussed by Raugei
et al. [3–5]