So it is actually the minimum energy required as it assumed a 100% efficient vacuum. The difference of 100 pascals (sing the linear calculation) has a difference in the final answer of around 1% but the final answer would still be that it would take around 500 GJ.
An upper limit would be imposed by assuming a low efficiency pump of an exponential difficulty increase with non-linear efficiency (meaning the efficiency will decrease as the volume of gas left in the tube decreases) and that will end up being a much higher energy requirement total.
Yeah ist gives the minimum (ideal) energy for the assumed state (p = absolute zero pressure), but the state itself is unrealistic and poses the maximal necessary energy required to achieve that state (compared to realistic ones with some mbar).
More Work has to be done if you pump out 100% rather than <100%.
The statement on the pumping efficiency is correct.
Well the energy to pump down to 1 mbar (for the tube that size) is around 0.5 TJ when using a linear formula
The energy required to pump to 0 mbar (again when using that formula) is considered 0.5 TJ, however realistically it would require an infinite amount of energy.
I do understand where you are coming from but for the vast majority of the population, I would like to keep it somewhat simplistic. I am not trying to argue with you, just stating why I stated it was a lower limit (as an approximation)