# What's wrong with solar power on China's Tiangong space station?

There was recently an article on Golem.de which claimed, that the area of the solar panels in the picture are larger than half of a football field. Which is of course an ill defined unit of area (for national matches it has a range of 4050 m² to 10800 m², the standard seems to be 7140 m² (source)). So I was looking for better defined numbers.

German Wikipedia is quite helpful in that respect. It lists:

• Tianhe (core module): 2x67 m² so 134 m² for a total of 9 kW, or 10 kWp in best conditions (source)
• Mengtian: 2x110 m² so 220 m² for a total of 13.75 kW, or 14.6 kWp in best conditions (source)
• Wentian: 2x110 m² so 220 m² for a total of 13.75 kW, or 14.6 kWp in best conditions (source)

So we have a total of: 574 m² for 36.44 kW or 39.2 kWp

Then there is this page which claims that the ingenious solar cells have 30 % efficiency and the station has a total power of 27 kW. This made me wonder again, no way that is right.

The solar irradiance in space at earth orbit is 1361 W/m² (source) which leads to these:

Efficiency Tianhe: Irradiance: 182 kW -> 10 kWp / 182 kW = 5.48 %

Efficiency Mengtian / Wentian: Irradiance: 299 kW -> 14.6 kWp / 299 kW = 4.88 %

Which seems very low, but these are very flexible solar panels.

For comparison: The iROSA modules manage more than 20 kW from 109.2 m² which converts to 13.4 % efficiency. (source)

So I think there is some form of mistake in my research or my conclusions. Of course you can just say, that Chinese technology is lagging, but I have my doubts. I'm usually just skimming the news around space exploration, as I find it interesting but too far out of my scope to really get into it - so I guess my research is based on lousy references or something.

Edit: Another source mentions 800 m² large solar array. But it could be that it means the total structure and not just the active area, but that seems like a rather large overhead.

• I can't read the Chinese sources of Wikipedia - but could these power values be averaged over time? I.e half the time is spent in the dark. This would explain the strange "at least X kW and Y kW with good illumination". Dec 19, 2023 at 15:50
• @asdfex that's a good point, with the translation tools available to me, I couldn't get any more information from the sources though - one is an interview, the other a short news article, both only printing a single number. MS Edge built-in translation completely messes up the numbers (23.13 kW instead of 13.5), which is quite interesting, because the numbers are Arabic - and I guess there are no Chinese units for power involved which got translated correctly... Dec 20, 2023 at 11:53

This doesn't answer the whole question but should be helpful. Solar cells have come a long way in terms of efficiency. Space solar cells have usually (always?) been ahead of the terrestrial market.

The following graphic is research cells (i.e. usually better than either space or terrestrial):

Figure: Research cell efficiencies from https://en.wikipedia.org/wiki/Solar-cell_efficiency

In my understanding the western space cell manufacturers have been achieving somewhere in the range 27% - 30% for many years. It doesn't seem unreasonable that firms in China are in the same range.

Note, I haven't seen any folks mention (i.e. those who convey some credibility that they know what they are talking about) higher values than 30% for space production cells.

Lastly, note in the figure that those cells with the very highest efficiency are actually concentrator cells, i.e. using a secondary reflective surface to increase the local irradiance on the cell area.

NREL's research cells chart (see the earlier answer) is interesting and it suggests what might be in store in the future. But, there's a less well-known chart that provides more pragmatic information... NREL's Champion Photovoltaic Module Efficiency Chart

If you would like a deeper understanding of the efficiencies, I created an online utility that draws Sankey charts to illustrate energy losses. You can access it here. The "Aspirational Values" column in each table is editable. If you hover over the entries in the "Labels" column, popups will appear that provide you with more information and references. You should be able to plug your numbers into this utility and use the values it generates to check your own calculations. If you know how to debug Javascript code, you can even set breakpoints and watch variables in your browser.

NREL also has a utility called PVWatts which helps estimate the economic viability of a PV Solar project based on its location and several other factors.