Napel was part of a German double, as she took silver on Aquilina and compatriot Angelika Trabert won bronze on Ariva-Avanti. Great Britain's Natasha Baker won gold.
'It was a very emotional result for me. When I heard the score the tears were rolling down my face,' said Napel.
She came out of Beijing 2008 with a set of medals - gold, silver and bronze - but does not expect the same in London.
Napel said: 'No, this will not be possible because I believe that Great Britain will win the Team medal. I've already got silver here and I think Germany will get another silver in the Team competition.'
Trabert said: 'I'm super happy. The horse was more relaxed. I need horses with a strong character and a lot of self-confidence.
'I still have my first Paralympic horse - he's 23. Ariva is only my third horse. You can never sell them when they've done such a good service.'