How can theoretical modelling be converted into viable product? Many a times modelling is just for the sake of modelling like exploring some properties of materials which does not help in converting the theoretical modelling into a viable product that can serve humanity. 

How one can become expert enough or which areas one should focus to model such problems and materials which can eventually result in some useful product. 

Like we have solar cells but every other day we see a research article being published just by tweaking the parameters which does not result in better or greater product. 

I want to see modelling and theory becoming useful sooner than we normally see.
I'm assuming that you're really asking, "how do I do something useful with modelling and theory."

The easiest answer is to work in a modelling and simulation field that makes useful products. Computational Electromagnetics is used a lot in RF, Finite Element Analysis is used in mechanical product design.

The broken part of your reasoning is that "more expertise = more practically useful." This is wrong. Usefulness is an instrinsic property of a task. A task is either useful or not useful. Expertise is only important in as much it allows you to perform a task. Therefore, you need to:


Pick a useful task
Have enough expertise to perform that task.


Sounds obvious, but perhaps it isn't.