Simultaneous input and output flows to/from storage can happen if all of the following conditions are true:
1) The storage has no efficiency losses (no STG_EFF specified, or it is 1)
2) There are no costs specified on the input or output flow
3) The storage capacity represents the nominal max. amount that can be stored (i.e. no NCAP_AFC is specified)
4) The input and output flows are at the same timeslice
Under these conditions the storage would thus be passing the commodity flow directly through the storage, without any storage operation (the input and output are at the same timeslice). And, therefore no capacity is needed for the stored amount, because nothing gets stored. However, usually this condition does not cause any problem as such. The storage effectively does nothing at all, although you see an input and output flow.
But if the issue you are seeing does not fit with this explanation, I would be happy to investigate your case, if you can provide the model input files (*.DD and *.RUN).
1) The storage has no efficiency losses (no STG_EFF specified, or it is 1)
2) There are no costs specified on the input or output flow
3) The storage capacity represents the nominal max. amount that can be stored (i.e. no NCAP_AFC is specified)
4) The input and output flows are at the same timeslice
Under these conditions the storage would thus be passing the commodity flow directly through the storage, without any storage operation (the input and output are at the same timeslice). And, therefore no capacity is needed for the stored amount, because nothing gets stored. However, usually this condition does not cause any problem as such. The storage effectively does nothing at all, although you see an input and output flow.
But if the issue you are seeing does not fit with this explanation, I would be happy to investigate your case, if you can provide the model input files (*.DD and *.RUN).