Solar PV systems technology

Photovoltaics (or commonly known as PV) is the static and direct conversion of sunlight into electricity (DGS, 2008). A classic PV system is made primarily of a combination of photovoltaic modules (Figure 12 left, mounted onto a residence’s rooftop as an example (NSR, 2013c)) connected in series forming strings. These strings are then hooked up in parallel to solar inverters (Figure 12 right), which convert the produced DC (direct current) power from the modules into AC (alternate current) power, then fed into the grid of the building.

Figure 12: A residential 8.6 kWp PV system in Singapore with thin-film

modules and two inverters (NSR, 2013c).

Smaller PV installations such as the 8.6 kWp thin-film system shown in Figure 12 are usually connected to low-voltage, single-phase grids (230 VAC, 50 Hz for the case of Singapore). Bigger systems, such as the 300.4 kWp polycrystalline setup at the Renewable Energy Corporation (REC) solar wafer, cell and module plant in Singapore (Figure 13, (NSR, 2013c)), might be connected to the medium-voltage grid of the facilities in which they are hosted.

Similar systems are flourishing in other tropical locations of the world, such as Brazil, as it could be seen in Figure 3 for the case of a medium-voltage connected system and Figure 14 for the first residential PV system connected to the electricity grid in the state of Minas Gerais, located in the city of Belo Horizonte, a 3.6 kWp monocrystalline setup (SPB, 2012).

Figure 13: An industrial 300.4 kWp PV system at the REC facility in Tuas,

Singapore with polycrystalline modules (NSR, 2013c).

Figure 14: A 3.6 kWp monocrystalline PV system in a residence in Belo

Horizonte, Brazil (SPB, 2012).

Crystalline silicon wafer-based technologies have lead most of the development of solar photovoltaics. Its presence can be linked to 80-90% of the PV systems found in the world (SOLARBUZZ, 2013b). Thin-film amorphous silicon and other variants like microcrystalline, had a spurt of growth around the year 2005, increasing their market share from 10% to close to 20%. That was especially the case since a raw material bottleneck

occurred (2006-2008), favoring the rise of thin-film systems, which utilize less silicon for their production. However, due to ongoing consolidation in the PV manufacturing sector and continuous preference towards crystalline-based technology systems, thin-film products have been further diluted back to ~10% and below of the total market volume. Monocrystalline silicon cell efficiency record is achieved with the deposition of a heterojunction intrinsic layer (“HIT cells” as trademarked by Sanyo, later acquired by Panasonic). The most recent record as of 2015 stands at 25.6%. For multicrystalline silicon, the record efficiency is 20.8% (GREEN et al., 2015). In terms of other PV technologies, cadmium telluride (CdTe) modules and systems have been heavily deployed by American company First Solar. During tough economic market conditions (2011-2013), the company performed well and deployed MW- size solar parks primarily in the United States (see the 550 MWac solar farm in Figure 15 (FIRST_SOLAR, 2013)). First Solar announced in 2015 a breakthrough record efficiency of 21.5% for its CdTe cells (PV_MAGAZINE, 2015).

Figure 15: Desert Sunlight Solar Farm (550 MWac PV plant), built in

California (FIRST_SOLAR, 2013).

Apart from efficiency improvements in solar cell technologies, a current trend in crystalline wafer-based modules is the launch of bifacial products, glass-glass laminates which allow light to go through the panel, which then bounces off the ground/roof underneath, returning partially to the panel, boosting its power output.

Another recent frontier in photovoltaic system deployment has been the launch of “floating PV systems”. These systems have been aimed

at minimizing land resource utilization, especially for area-stricken countries such as Japan or Singapore (NCCS, 2011). Other mentioned benefits of floating PV are the extra cooling provided by the water to the panels, thus allowing for a higher energy harvesting, as well as ability of the system to prevent algae growth (TRAPANI and REDÓN SANTAFÉ, 2015). Figure 16 shows two MW-level floating PV systems deployed in Japan in fresh water canals and reservoirs (NIKKEI, 2013; TECHXPLORE, 2015).

Figure 16: 1.2 MWp floating PV system at the Okegawa (left) and 1.7 MWp

Hyogo (right) prefectures in Japan (NIKKEI, 2013; TECHXPLORE, 2015). In terms of how different PV technologies react to the sun’s electromagnetic irradiation, Figure 17 shows the relative spectral responses of different PV modules (multicrystalline Si (silicon), amorphous Si, CdTe and CIGS). The single-junction a-Si module has a peak response at around 600 nm and is able to convert light into electricity from ~320-800 nm. For the CdTe module technology, the corresponding range is 300-900 nm. The spectral response range for the CIGS module is around 350-1150 nm, similar to that of a crystalline silicon wafer-based module (LIU et al., 2014b).

Modules are the driving force of the system, with the remaining “balance of systems” (commonly abbreviated as BOS) composed by inverters, mounting systems (for the interface between modules and ground/rooftop/floats), DC and AC cabling & switches/isolators, surge protections, etc. Modules have historically represented the majority of the cost of a PV system (in the order of 70% of the total cost), but this ratio has drastically changed, falling to around 50% in 2015 (see section 2.1.2).

Figure 17: Relative spectral response of various PV technologies (multicrystalline Si, amorphous Si, CdTe and CIGS), measured under STC temperature (25°C), with the AM1.5G spectrum (grey) also shown as reference (LIU et al., 2014b).

As much as solar PV can and have caused enthusiasm in governments and population for a faster rollout, a few aspects historically prevent even faster growth rates. They can be named as follows:

 Cost-competitiveness of photovoltaics (discussed in 2.1.2);  Grid integration challenges (discussed in 2.1.3);

 Variability aspect of the energy generation profile from PV (discussed in 2.1.4);

Apart from the various challenges discussed next, solar PV has a bright future, as covered in subsection 2.1.5 with a market outlook.