Bifacial modules


Bifacial PV modules are recently delivered by several manufacturers. Because there is either no or low price difference in the price of such bifacial and monofacial modules it is reasonable to use bifacial modules which could produce 5-20% more energy (in comparison with monofacial PV module with the same nominal output power).
Polar axis solar tracker and/or tracking concentrator is always mounted on high support structures (to avoid contact of the rotating PV array with the ground). It improves back side energy collection in comparison with fixed PV arrays or even roof integrated PV modules. The energy gain can be in the range 10-15% (for typical albedo 0,3).
Additionally solar trackers/concentrators are usually oriented to the west, before backtracking, in the morning. The bifacial modules enable to collect direct back side solar radiation before backtracking. According to local climate the resulting energy gain could be 2-5%.
It is also very advantageous that bifacial PV modules (fixed), transparent for infrared radiation, has lower operating temperature against monofacial ones (about 5-9oC). It is especially advantageous by solar trackers and tracking soft (C=1,6) concentrators where PV modules are exposed to higher solar radiation than on fixed racks. As indicated above solar trackers/concentrators are always mounted on high support structures which improve cooling of PV modules by air flow. Measured temperature of bifacial c-Si PV modules on the tracker was usually lower (by 5-8oC) than that of roof integrated monofacial c-Si modules.
Even in soft concentrators (C=1,6) with bifacial PV modules there is very low temperature difference against roof integrated monofacial modules (lower than 5oC). The reduced temperatre of bifacial modules can also increase the energy gain by 2-5%.
The reduced temperature is also very important for lifetime of PV modules in soft (C=1,6) concentrators. It should help to avoid degradation of polymer encapsulants of modules cased by higher temperatures (of monofacial modules).
The new bifacial PV modules with reduced temperature sensitivity (HIT) can further increase the system energy gain by up 10%. A synergic combination of all above effects can boost energy gain by 15-25% in comparison with the same tracker/concentrator with monofacial modules.
The polar axis solar tracker with c-Si bifacial PV modules will therefore deliver by about 50% more energy than fixed c-Si monofacial PV array with the same rated output power. The tracking bifacial soft concentrator will even double the energy gain against fixed monofacial PV array (Fig.1). Concerning the PV pumping systems there is 100% and 150% pumping capacity surplus for tracks and tracking concentrators respectively.


Fig.1 Idealised power/time dependence

The customer e.g. in Spain will therefore get the support 0,4 EUR per kWh of the 5 kWh systems. The 5 kW tracking bifacial system will deliver energy equivalent to 7,5 kWh fixed system, but for 7,5 kW system the support would be only 0,2 EUR per kWh. There is 5 kW limit for the 0,4 EUR/kWh support in Spain, for bigger systems the support is 0,2 EUR/kWh.
The same facts are valid also for tracking ridge concentrators where the energy gain is about 100% (i.e. 5 kWh tracking bifacial concentrator gives the same energy output like the 10kWh fixed monofacial system).
Another calculation shows that 5 kW tracking concentrator is by about 2700 EUR more expensive than 5 kW fixed mono stand but the double energy gain will pay back the price within less than 6 months (5 kW x 0,4 EUR x 10hrs. x 180 days = 3600 EUR) in sunny arid climate.

Operating temperature measurement of bifacial modules

Bifacial as well as monofacial reference PV modules were delivered by the same manufacturer. The design of both modules has been very similar. The only difference is back surface grid of bifacial cells and back surface glass encapsulant of the bifacial module. Cell temperatures were calculated using the temperature dependence of the open circuit voltage.
Our measurements of both bifacial glass/c-Si/glass and monofacial glass/c-Si/foil flat plate PV modules indicate that the operating temperature Top = 41oC of bifacial modules was by 12oC lower than that of monofacial ones Top = 53oC at AM 1,5 solar radiation, wind speed below 1 m.s-1 and ambient temperature Tamb = 22oC.
Another measurement indicate that bifacial PV modules at soft concentrators (Cgeometrical = 1,5, Coptical = 1,35) are still by about 3oC less hot Top = 50oC than monofacial modules at one sun radiation.

The study shows that main reasons are as follows:

1) Bifacial Si PV cells are transparent for the infrared part of the solar radiation. As the infrared radiation above 1,100 nm represents more than 20% of the solar energy a proportionally lower energy is absorbed in the bifacial module in comparison with non transparent monofacial one.

2) Opaque back surface protection foil also contributes to the higher temperature of monofacial modules. Typical packing density of c-Si PV modules is about 0,85. It means that 15% of the module area, not covered by PV cells, can absorb substantial quantity (about 50%) of the incident solar radiation while there is negligible absorption in the bifacial modules.

The experimental results are in good agreement with our calculation as well as with soft concentrator (C=1,6 and 2,2) measurements [1] of PV modules (both glass/Si/glass and glass/Si/foil) presented by Stacey and Mc Cormick (with monofacial cells only).
In conclusion using soft concentrators (Cgeometrical = 1,5-1,7 , Coptical = 1,35-1,45) with bifacial PV modules the over heating of modules is eliminated.
Additionally module mounted on high pedestal solar trackers/concentrators can be well cooled by air while fixed modules are frequently roof integrated so the natural air flow is substantially restricted.