My guess would be that it's the same gel coming out as going in, it just doesn't have a "charge" left in it.
I imagine the "tank" that the spent "fuel" would get emptied into would run this spent "fuel" through an attached system that would effectively re-charge the gel for the next consumer.
Think of, in the Matrix, the goo that Neo and all of the other humans were suspended in so the machines could harness their body heat.
