Is there any upgrade available to increase the amount of energy we can accumulate, or perhaps an upgrade..

Not that I have seen as of yet, however, if you do the Daily Quests, you get a reward which then can be found in your bag under the treasure tab (10 points of stamina, please understand I disagree with how this items description reads, as I would take it to mean that it actually increases the amount of energy that we can store, however, it does NOT it simply refills some of the energy that you can accumulate, if you use it when your energy level is already full it will allow you to overfill it... ....i.e., 150/120.)