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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.
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
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
Conventional vs. vertical installation: Energy generation profile and spot market prices as of 26/05/2017; Source:/Next2Sun
Another significant difference is the change in the use of land. Compared to conventionally mounted panels, vertically mounted bifacial modules require significantly greater row spacing to avoid excessive self-hading. On the one hand, this leads to lower land utilisation rates and, on the other hand, it is precisely the larger row spacing that helps the land remain usable for agricultural purposes at least with certain restrictions. In the end, maintaining agricultural productivity means the possibility of reducing land lease costs in comparison to conventional power plants and thus the possibility of countering the increasing problem of land consumption based on an innovative cooperation model. Because project developers normally rather seek to achieve the maximum installed capacity to optimise margins when selling a power plant, this concept will be more interesting to energy suppliers. Thus, the popular doubts about land consumption can be dispelled. Here, interesting is the concept of collaborating with agricultural cooperatives which own vast areas of land, e.g. in Eastern Germany.
Another advantage of the vertical mounting is the minimisation of losses caused by snow depositing on thesurface of the modules. Moreover, according to initial studies, the losses caused by pollution are significantly lower (this is relevant in desert areas).6
The suitability of bifacial modules for use in certain types of power plants not only depends on the fact that both sides of the panel are used but also on the transparency of the glass-glass module. Thus, they are most suitable for installation in car ports, on noise barriers, as elements of buildings, in Floating PV 7, and also in the above-mentioned Agro PV.
For comparison purposes, the analysis of the cost and revenue structure takes into account the conventional design of optimally tilted, south-facing PV power plants.
The attentive reader will have already concluded at this point that the installation of bifacial modules requires fewer modules due to the higher specific yield. This reduces the installation, BoS and land lease costs.
By contrast, however, the prices of modules are higher, currently by around 10%, which could, however, also converge in the long term. The reason for this is that the production process is very similar to that of conventional modules3. Currently, the global production capacities are rather oriented at monofacial modules. Therefore, the price reduction effect arising from economies of scale has not yet had such a strong impact on the bifacial modules.
It should be, however, examined on a case-by-case basis what type of the module and what installation method to select for individual projects. If electricity is sold outside the EEG scheme, it is reasonable –in addition to determining LCOEs– to prepare a long-term electricity price forecast to determine the value of the produced electricity.
In the case of vertically mounted bifacial modules, specific investment costs are approx. 10 % - 20 % higher and the land lease costs are roughly twice as high due to the greater need for land. In the case of land lease costs, cost reductions are possible, depending on whether the piece of land is suitable for agricultural purposes and whether its continued use is possible. By contrast, bifacial modules offer approx. 5 % to 10 % higher yields and higher revenues from the sale of electricity at times of higher electricity prices. In addition, it is expected that the use of glass-glass modules will lead to longer servicelife.9
It is assumed that bifacial modules will increase in importance worldwide. The forecasts range from 20 % in202610 to 40 % in 2025.11 In addition to the price difference, difficulties also arise from the fact that the question of bifacial STC has not been solved yet. Other issues are the cleaning of the rear of the panels and the emergence of hot spots caused by the mounting structure. Positive is that the delivery times for bifacial modules have normalised and yield simulations render reliable values.
Bifacial PV modules generate more energy on the same module surface through a solar-active rear of the panel due to the reflectivity of the surrounding surface. With installation and BoS costs being at the same level, this leads to a higher yield that normally exceeds the higher cost of bifacial modules. In addition, bifacial modules are predestined for use in noise barriers, Floating PV power plants and vertically mountedPV systems. Because with vertically mounted power plants, land can continue to be used for agricultural purposes, this eliminates the controversial issue of land consumption12 and thus increases the chance ofobtaining the relevant permits. At the same time, the production profile of this type of power plants promises higher returns on the electricity exchange, which is particularly attractive in the case of power plants operated outside the EEG support scheme. Therefore, it is basically advisable in any case to consider bifacial PV modules and their vertical mounting in future projects.
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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 yield3 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.2013.08.040Yusufoglu, U. A., Pletzer, T. M., Koduvelikulathu, L. J., Comparotto, C., Kopecek, R. and Kurz, H. (2015a) ‘Analysis of the annual performance of bifacialmodules and optimization methods’, IEEE Journal of Photovoltaics, 5(1), pp. 320–328. doi: 10.1109/JPHOTOV.2014.2364406Shoukry, 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 PVSEC7 „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.pdf9 Next2Sun10 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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