4.1. Fixed-Tilt Ground-Mounted Photovoltaic Systems
shows the CED of the analysed PV systems, while Figure 4
, Figure 5
and Figure 6
illustrate the respective LCA impact indicators, namely global warming potential (GWP), acidification potential (AP), and ozone depletion potential (ODP), all expressed per kWp
—the stacked bars show the individual contributions of the main life cycle stages. Each PV technology is also shown separately according to the country or region in which it was manufactured. The average efficiency for each technology is assumed in accordance with the latest report by the Fraunhofer Institute for Solar Energy Systems [40
], specifically: 17% for sc-Si PV, 16% for mc-Si, 15.6% for CdTe PV, and 14% for CIGS PV.
The results clearly show that the most impacting step for c-Si technologies is from SoG-Si supply to finished PV cells, which includes ingot/crystal growth and wafer and cell production, and especially so in the case of sc-Si PV systems (because of the energy intensive CZ crystal growth process).
highlights that, per kWp
, c-Si PV systems are overall twice as energy-demanding to produce as CdTe PV systems. Figure 4
illustrates the resulting GWP indicator per kWp
: c-Si PV technologies generally have higher values in comparison with thin film PV panels, and in particular, the lowest GWP values are for CdTe PV, especially when production takes place in Malaysia. A similar trend is shown in Figure 5
, in which the lower values of AP per kWp
are those for CdTe PV, and secondly for CIGS PV; conversely, sc-Si PV shows the highest AP values, followed by mc-Si PV. Also in terms of ODP results (Figure 6
), CdTe PV is still the best performer, followed by CIGS PV, and then mc-Si and sc-Si PV.
These new results show a remarkable improvement for current production CdTe PV modules when compared to similar modules produced in 2005 (the most recent production year for which CdTe PV inventory data are directly available in the Ecoinvent V3.1 Database). Over one decade, the CED per kWp
for the CdTe PV modules manufactured in the US has been reduced by approximately 62%, while the GWP, ODP, and AP results are also down by respectively 63%, 65%, and 71%. The current CdTe PV systems also show improvements when compared to previously published results [46
] referring to more recent (2010–2011) production data; in this case the CED is down by approximately 30%, and the GWP is down by 37%.
It is noted, however, that the CED of complete ground-mounted CdTe PV systems are not much lower than previously reported values, because the new inventory data for the ground-mounted BOS provided by First Solar led to a higher energy demand (831 MJ/m2
) than the previously used data from the c-Si PV BOS (542 MJ/m2
, First Solar) [47
]. The same also applies to the calculated EPBT
values (Table 1
From a geographical perspective, it is also clear from the results that the considered impact indicators (GWP, AP, ODP) are generally lower when the manufacturing takes place in Europe in comparison with the USA and China, and in particular the Chinese production chain consistently shows the highest indicator values. This is despite the fact that the CED associated to the Chinese c-Si PV production is actually slightly lower than that for the European and USA manufacturing chains—this seeming incongruence depends on the large reliance of the Chinese electric grid on coal [43
]. The input grid mix composition is also responsible for a significant share of the impacts in the case of CIGS PV produced in Japan (a country where, after the 2011 nuclear incident in Fukushima, over 90% of the energy resources used for electricity generation are fossil fuels [42
The BOS contribution is generally fairly low, with the partial exception of the AP results, which are negatively affected by the comparatively large amounts of copper and aluminium required.
, Figure 8
, Figure 9
and Figure 10
then illustrate the same results (CED, GWP, AP and ODP) expressed per kWhel
. These results are computed assuming a performance ratio of 0.8 and a lifetime of 30 years [18
]. Also, in order to provide results applicable to different contexts, three different irradiation levels are used, which are respectively representative of irradiation on a south-facing, latitude-tilted plane in Central-Northern Europe (1000 kWh/(m2
·yr)), Central-Southern Europe (1700 kWh/(m2
·yr)), and the Southwestern United States (2300 kWh/(m2
·yr)). In the figures, different symbol sizes (small, medium, and large, respectively) are used to refer to these three specific irradiation levels.
Unsurprisingly, the best energy and environmental performance as measured by all considered metrics is that of CdTe PV systems installed in the Southwestern US, with CIGS PV as a close second. At the other end of the scale, the highest impact in terms of GWP and AP are those for the Chinese produced sc-Si PV, mainly due to this technology’s higher demand for input electricity, coupled with the prominence of coal in the Chinese electricity grid mix.
As illustrated in Table 1
, the energy pay-back times of the analysed PV technologies were found to range from 6 months (for CdTe PV installed in the US South-West) to approximately 2–3 years (for c-Si PV installed in Central-Northern Europe).
illustrates the positioning of the analysed PV systems along the curve defined by the non-linear relation of EROIPE-eq
(often referred to as the “net energy cliff” [48
]). This figure makes it abundantly clear that, while the individual EROIPE-eq
values for the different PV systems over the three considered irradiation levels span a comparatively large range—from ~10 for sc-Si PV at 1000 kWh/(m2
·yr) to ~60 for CdTe PV at 2300 kWh/(m2
·yr)—in fact, all data points sit on what may be considered the “safe”, quasi-horizontal portion of the “cliff”. In other words, all PV systems afford the benefit of over 90% of their gross energy output being available as net usable energy to the end user (NTG
4.2. A Comparison to 1-Axis Tracking Installations
Generally, tracking PV systems provide the benefit of boosting the energy yield in comparison with fixed-tilt installations because the panels are mounted on a structure that follows the movement of the sun. In particular, one-axis trackers have one degree of freedom (the movement occurs along a single axis of rotation). The results shown below correspond to a horizontal rotational axis in the North-South (N-S) direction with the panels facing East in the morning and facing West in the late afternoon. Tracking could be further optimized with the horizontal rotational axis tilted south if the topography allows, which would give the benefit of a flatter profile throughout the day.
On one hand, the invested energy (and associated environmental impacts) for building the tracking BOS are higher than for conventional fixed-tilt PV systems, since tracking installations require larger amounts of structural steel and copper cabling; also, they use electricity during the usage phase for tracking actuators. On the other hand, the key advantage of tracking systems is the ability to harvest more direct beam irradiance, thereby requiring fewer PV modules per kWh produced in comparison with fixed-tilt installations.
The energy and environmental performance of tracking systems are highly influenced by site latitude and diffused light conditions; in particular, sites with lower (<40%) diffused light benefit more from tracking systems. Also, the gain in PV yield is reported to range from +10% to +24% over tropical and subtropical latitudes (0°–40°) [49
shows the maximum achievable variations in LCA impact assessment results (GWP, AP, OPD) and EPBTs for a range of one-axis tracking PV systems, expressed as relative to the corresponding values for fixed-tilt PV installations, assuming a best-case scenario of 2300 kWh/(m2
·yr) irradiation, and +24% enhanced capture efficiency with respect to latitude tilt fixed installations.
In general terms, the c-Si PV systems were found to benefit the most from tracking installations (over −10% impact). Instead, the advantage from tracking for CdTe PV (and also to a lesser extent for CIGS PV) appear to be much smaller, due to the very good performance of these thin film technologies in the first place, and hence the comparatively larger share of their overall impacts are due to the BOS itself.