Bifacial PV … and who is Albedo?

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In addition to the potential for achieving decreased levelised cost of energy (LCOE), bifacial PV modules offer new installation and marketing opportunities. This, in turn, comes with related advantages such as additional yields and higher power densities. In addition, due to their special energy generation profile, vertically mounted bifacial modules offer electricity price advantages in times when electricity has to be marketed without the EEG support and can help unlock new use for otherwise inaccessible areas.


Fundamental Information

In principle, bifacial photovoltaic (PV) modules are simply PV modules whose full rear contact has been replaced with a fingerprint contact. This means that also the rear of the panel can be exposed to solar radiation and be used to produce power. How much solar radiation will be received by the rear of the panel depends, most of all, on the reflectivity of the surrounding surface and on the manner of mounting of the modules. The so-called albedo value is the measure of how well a surface reflects solar radiation. For example, the albedo value is approx. 10 % for high sun above deep water, 10 % - 30 % for meadows, and up to 75 % - 95 % for freshly fallen snow.1 The albedo value of a surrounding surface is a significant factor for the yield gain (see illustration2).

 

Another factor is the so-called “bifaciality factor” that is a measure of how much energy the rear of the panel will produce compared to the front of the panel. At present, in bifacial modules, bifaciality factors normally range between 70 % and 95 %. For example, a bifaciality factor of 95 % means that, in the same conditions, the rear will produce only 95 % of energy produced by the front. But, in the end, it is the yield gain that matters, and here, if a facility is sited favourably in ideal installation conditions (high albedo value e.g. snow or a bright roof surface, perfect orientation, no shading), the possible values of the yield gain can range between 30 % and 50 % p.a.3 In Germany, however, it should be rather expected that the yield gain will vary between 5 % and 15 % p.a., depending on the system design and the surface underneath the module.

 

 Basic considerations about influence of albedo on yield gain

Basic considerations about influence of albedo on yield gain

 

 

Innovative System Designs

What is special about bifacial modules is that, besides the yield gains generated from the rear side, this type of the module also offers other installation options. Certainly, the most radical change compared to the conventional panels is the vertical installation. Most common is the east/west orientation of the panels. This entails two main differences compared to the conventional layout.

 

First of all, the vertical layout involves a changed production profile. Instead of a single midday maximum, two peaks are now produced: one in the morning and one in the afternoon, although production past midday remains comparatively high. In addition to the fact that the few comparable power plants generate yield gains of between 5 % and 10 %, the real advantage is that more electricity is now generated amid generally increasing spot market prices. In Germany and all over the world, it can be seen that more and more power plants are being planned or built without the EEG support.4 In the case of these power plants, the future electricity price quoted on the electricity exchange will be important for the considerations relating to profitability. The ability of avoiding the so-called „cannibalisation effect“ (i.e. the fall in the midday peak price even to the level of negative prices) of PV power plants can become a decisive factor. The analysis of the Spanish electricity market clearly illustrates the controversial nature of the problem of non-subsidised electricity prices.5

Standard vs. vertikale Installation: Erzeugungsprofil und Spotmarktpreise am 26.5.2017; Quelle: Next2Sun 

 Conventional vs. vertical installation: Energy generation profile and spot market prices as of 26/05/2017;  Source:/Next2Sun

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1 Dobos, E. (1996) ‘Albedo’, Engineering (London), 237(7), p. 21. doi: 10.1081/E-ESS

2 Dupeyrat et al., 2014; Investigations on albedo dependency of bifacial PV yield
3 Guo, S., Walsh, T. M. and Peters, M. (2013) ‘Vertically mounted bifacial photovoltaic modules: A global analysis’, Energy, 61, pp. 447–454. doi:10.1016/j.energy.2013.08.040
Yusufoglu, U. A., Pletzer, T. M., Koduvelikulathu, L. J., Comparotto, C., Kopecek, R. and Kurz, H. (2015a) ‘Analysis of the annual performance of bifacial
modules and optimization methods’, IEEE Journal of Photovoltaics, 5(1), pp. 320–328. doi: 10.1109/JPHOTOV.2014.2364406
Shoukry, I. (2015) Bifacial Modules – Simulation and Experiment. University of Stuttgart

4 https://www.pv-magazine.de/2019/03/04/enbw-plant-photovoltaik-anlagen-mit-400-megawatt-ohne-foerderung-in-deutschland/
5 https://www.pv-magazine.de/2019/03/01/photovoltaik-in-spanien-kurzer-ueberblick-ueber-einspeisetarife-sowie-projektentwicklung-und-finanzierung-von-freiflaechenanlagen/

6 Rabanal-Arabach, J., Mrcarica, M., Schneider, A., Kopecek, R. and Heckmann, M. (2016) ‘The Need of Frameless Mounting Structures for Vertical Mounting of Bifacial PV Modules’, in 32nd EU PVSEC
7 „Floating PV– Schwimmende Photovoltaikanlagen als neuer Trend“ (EnEws: Ausgabe Februar 2019)
8 Based on PV Magazin Webinar am 25.10.2018; Slides abrufbar unter: https://16iwyl195vvfgoqu3136p2ly-wpengine.netdna-ssl.com/wp-content/uploads/2018/10/2018-10-25-Bifacial-Webinar-SOLTEC.pdf
9 Next2Sun
10 International Technology Roadmap for Photovoltaic (ITRPV), March 2019 [Daten für “true bifacial”]
11 https://www.bloomberg.com/news/articles/2018-05-17/watch-out-for-double-sided-solar-panels-taking-off-in-china

12 https://www.pv-magazine.de/2019/03/04/fdp-politiker-photovoltaik-anlagen-in-unseren-regionen-sind-verbrechen-an-der-menschheit/

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