Page 406 - A Comprehensive Guide to Solar Energy Systems

Chapter 21 • (EROI) and (EPBT) for PVs 415

To properly compare the two EROIs the crude oil must first also be converted into electric­
ity, or the electricity must be expressed as primary energy equivalent. Another lesson from
the exchanges is that it is important to define energy quality of energy input data in EpBT
calculations. Energy invested is the readily available energy diverted from other possible
societal uses [19] and as such it is provided as energy carriers, however, they can be mea­
sured in terms of thermal energy and electricity. Therefore, a quality correction must be
made to provide comparability. As previously discussed, in some cases the electricity value
is multiplied by 3 to provide this correction. The CED, however, describes the total primary
energy that must be harvested from the varying environmental sources to produce a given
amount of usable energy carrier throughout the lifecycle [35], and thus arguably already
provides the energy mix in comparable units. Discounting the quality of energy when
accounting can lead to incomparable analyses. Therefore, explicitly stating objectives of
evaluating economical and effective use of available energy carriers versus efficient use
of primary energy resources and using consistent methods in measuring these differing
flows is paramount to comparable EpBT and EROI studies for pv and other fuels.

21.2.4 Pathways to PV Net Energy Analysis Using CED

As can be inferred from the methodological issues surrounding the EROI of pv systems,
the more precise is the question being asked, the more precise will be the answer the EROI
analysis can provide. This section is adapted from Raugei et al. [22] and summarizes dif­
ferent pathways for pv NEA and EROI analysis using two families of energy metrics. The
following definitions apply to the equations in this section and are reported per square
meter of the pv system over the system lifetime:

Irr total solar irradiation over system lifetime (MJ m )
−2
PV module energy harvesting efficiency (MJ MJ )
−1
η PV
−2
PE primary energy directly harvested over system lifetime (MJ m )
(Irr)(-η PV )
Inv energy investment to build, operate and dismantle PV system, in terms of its primary
−2
energy demand (M m )
−1
PR performance ratio (MJ MJ )
Out can be accounted for in direct energy units such as electricity (Out el ), or in terms of its
equivalent primary energy (Out PE-eq )

21.2.4.1 EROI el : Energy Output Expressed in Terms of Direct Energy
When expressing energy output in terms of direct energy, which in the case of pv is always
electricity, the EROI of pv electricity may be calculated as:
EROI el = Out/Inv (21.4) EROIel=Outel/Inv
el