What is the value of the winter power generation capacity of the south-facing photovoltaic exterior wall in northern regions?

“In the mid-to-high latitude regions of the Northern Hemisphere, the angle of solar incidence in winter is relatively small. Therefore, south-facing photovoltaic systems are expected to become an important supplement to rooftop photovoltaic systems in winter. Moreover, the surface of vertically installed facade photovoltaic components does not accumulate snow in winter, and can even utilize the reflection of ground snow to improve power generation.”

Similar descriptions have appeared more than once in previous articles of this issue. So, can the actual performance of south-facing photovoltaic systems in winter really exceed that of rooftop photovoltaics? Can photovoltaic buildings with south-facing photovoltaics achieve balanced photovoltaic power generation in both summer and winter? Data from a photovoltaic building power generation verification report from the Swiss Federal Office of Energy can answer these two questions.

In this report, the Swiss Federal Office of Energy statistically analyzed the power generation of five photovoltaic buildings in three cities between 2019 and 2021.

These five buildings are all multi-unit residential buildings, namely:

No. 158, Affolternstrasse and No. 67/69, Oberwiesenstrasse in Oerlikon, Zurich;

No. 186 and No. 188, Segantinistrasse in Zurich;

And No. 48, Oberwälserstrasse in Opfikon;

The first four buildings have photovoltaic installations on the roof and exterior walls and balcony railings facing east, west, south, and north. The fifth building has no rooftop photovoltaics, only exterior wall photovoltaics. The orientations of the above buildings are not exactly south or north, as shown in the building site plan above.

Since the photovoltaic arrays of different orientations are ultimately connected to different inverters or different MPPTs of the same inverter, the power generation of photovoltaic arrays of different orientations can be statistically analyzed independently. Taking No. 158, Affolternstrasse as an example, the figure below shows that between 2019 and 2021, the power generation per kWp of rooftop photovoltaics (dark orange line) in summer months was much higher than in winter months, while the power generation in spring and autumn was between winter and summer. East, west, and north-facing facade photovoltaic systems also have the above characteristics, and the power generation per kWp is much lower than that of rooftop photovoltaics in the same month. The monthly power generation per kWp of south-facing facade photovoltaics is basically the same in spring, summer, and autumn, about half of the summer power generation of rooftop photovoltaics; in winter, the monthly power generation per kWp of south-facing facade photovoltaics is close to that of rooftop photovoltaics, and may even be higher than the latter in snowy weather (e.g., January 2021).

However, in terms of total amount, even with the addition of facade photovoltaics, the total power generation of the building in winter is still much lower than that of rooftop photovoltaics in summer, and even lower than the summer power generation of the entire building's photovoltaic system. Therefore, (south) facade photovoltaics do not solve the seasonal imbalance in power generation of building photovoltaic systems in mid-to-high latitude regions (Zurich is at 47°N latitude). The monthly power generation of 1 kWp south-facing facade photovoltaics in winter is less than 1/8 of the monthly power generation of 1 kWp rooftop photovoltaics in summer. Considering that the cost per watt of facade photovoltaic systems is higher than that of rooftop photovoltaics, this further indicates that the persuasiveness of using the so-called “enhancing the winter power generation capacity of buildings” as the main reason for installing south-facing facade photovoltaics is insufficient. Unless the goal is to achieve energy self-sufficiency for a single building, in winter introducing external electricity may be a better solution for the entire system. The seasonal variations in power generation of the other four buildings are consistent with No. 158, Affolternstrasse.

The figure on the left below shows the daily power generation curve of No. 67/69, Oberwiesenstrasse in summer. It can be seen that the summer power generation peak of the east (south-leaning), north (east-leaning) facades appears in the morning, the summer power generation peak of the west (north-leaning) facade appears in the evening, and the power generation peak of the south (west-leaning) facade appears at 3 pm.

In this case, the peak power generation of the west facade coincides more with the evening peak electricity consumption, indicating that the west facade photovoltaics have high economic value in summer; unfortunately, the report does not disclose the time-of-use power generation curves of the other three seasons, so it is impossible to compare the annual performance of each facade.