**5.2 Challenges**

There are several challenges which hefts to be considered so as to accomplish sustainable energy efficient facility design actually, these are highlighted to lure attention from the researchers within the field;

#### *5.2.1 Retrofit-ability as design criterion*

Inclusion of retrofit-ability as a design criterion ends up in a design which will be continuously valid for whole life span of a facility and at the same time reminiscent

to a new energy efficient design of the upcoming facility. Retrofit-ability insertion as a design criterion at process level up to facility level is kind of challenging because the facility designs has to be developed for all energy-capital trade-offs furthermore as other scenarios. The developed designs must be compiled together to urge a design which can be subsequently modified without hindering the old design specified as the primary design being most inexpensive and least efficient (in terms of energy consumption not energy-capital trade-off) and shall be improved with minimal capital investment throughout its life-cycle. All designs must be further developed to form provision to incorporate all identified alterations within the facility from start till end. Developing a mathematical framework to capture all elements of the sustainable energy efficient design is difficult and lack of insights; preferably it can cause to integrate processes/stream which are never studied from the unification perspective and data/information is unavailable for unifying them. The foundation of integration starts with process, which approach ascends up to facility level, leading to unifying the entire facilities process with options of direct, indirect, hybrid integration options among the processes. Consequently, springing up with sustainable energy efficient facility design is extremely difficult, even the plant level design is difficult to realize. As shown even one sustainable process design generates lots of values to the facility by improving its performances, hence, it's advisable that in developing sustainable design evolutionary-approach may be adapted versus the more time-consuming revolutionary-approach.
