Page 419 - A Comprehensive Guide to Solar Energy Systems

Chapter 22 • Life Cycle Analysis of Photovoltaics: Strategic Technology Assessment 429

EPBT [ ] =yr CED / ) (22.2)
G
(( / E agen η ) − E O&M EPBTyr=CEd/Eagen/ηG−EO&m
−1
where E mat , E manuf , E trans , E inst , and E EOL are defined as above; additionally, E agen [mJ el yr ]:
annual electricity generation, E O&m [mJ PE-eq ]: annual PE demand for operation and main-
−1
tenance, and η G [mJ el (mJ PE-eq ) ]: grid efficiency, i.e., the average life-cycle PE to electricity
conversion efficiency at the demand side.
For systems in operation for which records exist, the annual electricity generation
(E agen ) is taken from the actual records. Otherwise, it would be estimated with the simple
equation below (note the units into parentheses]:

[

−1
−2
[kWh yr −1 ] = irradiationkWh myr −1 ] × moduleefficiency kW (kWp ) m −2  ×
E agen  
performanceratio [dimensionless ] Eagen kW h yr−1=irradiationkW
h m−2yr−1×module efficiency
where, irradiation is the global irradiation on the plane of the PV, module efficiency is the kWkWp−1 m−2×performance
reported by the manufacturer-rated efficiency measured under 1 kW m irradiance, and ratiodimensionless
−2
the performance ratio (PR) (also called derate factor) describes the difference between
the modules’ (dC) rated performance (the product of irradiation and module efficiency)
and the actual (AC) electricity generation. It mainly depends upon the kind of installation.
mean annual PR data collected from many installations show PR values of 0.75 from resi-
dential and 0.80–0.90 from utility systems.
In general, the PR increases with (1) declining temperature and (2) monitoring the
PV systems to detect and rectify defects early. Shading, if any, would have an adverse
effect on PR. This is why well-designed, well-ventilated, and large-scale systems have a
higher PR.
E agen is then converted into its equivalent PE, based on the efficiency of electricity con-
version at the demand side, using the grid mix where the PV plant is being installed. note
that E agen is measured (and calculated) in units of kilowatt hours (kW h), and we first have
to convert it to mJ (1 kW h=3.6 mJ) and then use ηG to convert mJ of electricity to mJ of
PE (mJ PE-eq ). Thus, calculating the primary-energy equivalent of the annual electricity gen-
eration (E agen /η G ) requires knowing the life-cycle energy conversion efficiency (η G ) of the
country-specific energy-mixture used to generate electricity and produce materials. The
average η G values for the united States of America and Europe are, respectively, approxi-
mately 0.30 and 0.31.
Based on the above definition, there are two conceptual approaches to calculate the
EPBT of PV power systems:

1. PV as replacement of the energy resources used in the power grid mix. This approach
calculates the time needed to compensate for the total (renewable and nonrenewable)
PE required during the life cycle of a PV system. The annual electricity generation
(E agen ) is converted into its equivalent PE, based on the efficiency of electricity
conversion at the demand side, using the current average (in attributional LCAs) or