Useful for what? The paper mentions off-grid sensors, which seems like a useful application. Looks like the numbers are 0.5W/m² which means a square meter of th

Useful for what? The paper mentions off-grid sensors, which seems like a useful application. Looks like the numbers are 0.5W/m² which means a square meter of these could power e.g. an ESP8266. It's obviously not going to power your home with those numbers, but it's a little unimaginative to say that it has no useful application.