Actually you are not exporting, it's PowerBI that's importing ;-)
Anyway, PowerBI uses the BC web services, to me more precise the ODATA web services, at this moment.
So you will need permissions in BC to access the web services and also to the underlying pages/queries/tables that these web services are built on.
That means you need to investigate exactly which objects (tables/pages/queries) that you want to make available via web services and you could then create a permissionset for that.
On the other hand, you could also create a dataflow, or a power bi report that only fetches the data. For the refresh of that dataflow/report you use a user that has those permissions. Then in dataflows or PowerBI you can share these datasets with other users, and manage security in PowerBI.
(If you want every PowerBI user to be able to do refreshes, then they all needs to be valid users in BC, using dataflows/shared datasets you only need 1 refresh-BC-user.)
