Sometimes, if you deal with or work on a topic for a long time, things become so obvious that you cannot believe that others do not understand it as well. One such thing is that electricity production from PV in the last years has become so cost effective that it will be hard to beat by any other technology in future. This is not yet understood, particularly by many politicians – or it is but there is a strong lobby behind or other interests so they don’t get this point. Another obvious fact is that in order to minimise a PV system’s LCOE (levelised costs of electricity), low cost but powerful and long-lasting modules have to be used in order to minimise the costs for the balance of system (space, material, installation, maintenance). This is not yet understood, for example by many venture capital providers, who still invest in low cost but low-efficiency (and often nondurable) PV module technologies.
Finally, and the topic of this blog post: if you want to maximise the kilowatt-hour output of a ground-mounted PV system or of one on a flat roof, bifaciality offers a very easy possibility to achieve this. Many PV scientists are still claiming that there is hardly any albedo out there and that bifacial modules are way too expensive, so therefore there is no real room for bifaciality. However, we are very confident that bifaciality will become an extremely important technology, for the following reasons:
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- High-efficiency solar cell technologies are getting bifacial anyhow (passivated rear-side of advanced concepts make this possible);
- Many module producers are introducing or even completely switching to glass-glass modules (thin glass becomes cost effective and glass-glass leads to longer lifetime of a module);
- Balance of system (BOS) costs are approaching a level that is hard to be further reduced and consequently high power modules (smaller area of system lead to lower costs) are the only way to achieve a further reduction of the total system cost.
The benefits of bifacial
When rooftops are the main market in the country, such as in the Netherlands, bifaciality is of course not that interesting. However, on flat roofs and in ground-mounted installations of large systems (for example in desert areas) bifaciality is extremely helpful in order to drive down the LCOE (€0.06-0.07/kWh compared to €0.08-0.09/kWh in southern Germany according to a Fraunhofer IPA/ISE study) and also to increase the electricity generation time. The largest enemy of bifaciality is the old-fashioned “Wp thinking” of customers instead of a modern “kWh mentality”.
There are still several technological challenges to bifaciality which are not trivial – how to design and where to place the junction boxes, how to build shadow-less and stable mounting systems and – most importantly – how to standardise bifacial measurements and how to simulate the bifacial benefit of individual installations.
However, several companies and institutes individually have been working on tackling these challenges. In addition, bifacial workshops and working groups created through those are tackling these challenges (www.bifipv-workshop.com). These workshops are also spreading to other future PV countries such as Egypt and Chile, demonstrating the growing appeal of the technology within the international PV community.
Figure 1 shows an example of a bifacial solar cell from ISC Konstanz, BiSoN, which will be produced in MegaCell in Italy [http://www.megacell.it/press-release/]. More details on different bifacial solar cell technologies, modules, history and measurements of bifacial devices, can be found in the latest issue of Photovoltaics International. On the right hand side of Figure 1 you see how important bifaciality is getting in terms of filed patents within the last years.
Figure 2 shows why these patents have been filed. The table on the left shows the values of the albedos for different surfaces – fresh snow and highly reflective roof coatings being the best and showing albedos of up to 95%. The bifacial gain observed during a long measuring time (months to years) for many different installations is depicted on the right of Figure 2. For sand, with an albedo around 35%, it can increase the system performance between 15-20% which is an increase that is very interesting from the economic (LCOE) point of view.
In order to reach the same energy yield (kWh/kWp) as a bifacial PV system based on modules with 20% efficient bifacial solar cells, operating with a bifacial gain of 15%, a standard monofacial PV system requires the use of expensive 23% efficient solar cells.
Most of the c-Si solar cells at the moment are based on mc-Si technology as can be seen from Figure 3 and the trend does not seem to change.
However what is changing visibly is that also in the case of the mc-Si technology the wafer quality is improving a lot, making advanced solar cell technologies feasible also here. Many solar cell producers, such as REC, Hanwha Q CELLS, Sunrise, Trina Solar and others have introduced mc-Si PERC solar cells into mass production, reaching efficiencies between 18% and 19%. Lu’an Solar, MegaCell supported by RCT, Schmid and others are even about to introduce bifacial mc-Si solar cells with boron back surface field in pilot production.
So generally speaking: advanced c-Si solar cell technologies with boron back surface field or emitter offer the possibility of making and using these devices in bifacial mode. In other words, future technologies (based on p- and n-type Si) exceeding 20% efficiency will be or can be easily made bifacial.
N-type mono c-Si solar cells are the best example of that. Figure 4 depicts a selection of n-type technologies in mass production until 2014 and the new-comers in red. ECN and European OEMs have successfully transferred their n-type technology to cell and module manufacturers outside Europe (Yingli, Mission Solar Energy). Currently, ISC Konstanz is transferring the BiSoN process to MegaCell in Italy and will continue in 2015 in Egypt. Mission Solar and MegaCell are investing in the installation of additional solar cell capacity because of bifaciality exclusively. Figure 5 summarises all front B-emitter screen-printed solar cell technologies in research and production.
In 2009, ECN was one of the leaders in this technology with their PASHA technology achieving an efficiency of 18.5% of. Since then, many actors (academics, equipments suppliers or cell producers) have reported efficiencies between 20% and 21%. Experts in the field are confident that performances over 21% should be reached in 2015.
Using such cells in glass-glass modules PVG Solutions has built the word’s largest bifacial PV power plant up to now in Japan of around 1MWp, which is depicted in Figure 6.
Bifacial outlook
To summarise the advantages explained above:
- There is a small technological step from advanced cell concepts (e.g. PERT) to bifacial technology;
- Glass-glass modules offer a long lifetime;
- 20% bifacial cells can be produced at much lower cost than for example 23% high-efficiency monofacial cells;
- Many installation sites provide sufficient albedo without the need for additional investment.
For these reasons we are sure that in the coming years, bifacial PV will play an important role on flat roofs and in ground-mounted systems, and thus, we will see many and larger bifacial systems like the one shown in Figure 6. MegaGroup already has contracts for large bifacial PV-systems in places such as Europe, Egypt and Chile.
