One of the principal reasons, I think, to integrate an impact tranche into a larger Alternative infrastructure financing is to achieve some leverage on the social and environmental impact of the project. Each aspect of impact might be individually small (relative to project size), but the aggregate potential value, in comparison to an impact tranche that is also small (relative to project size) might achieve decent results.
I did some rough (and highly hypothetical) numbers on this, really to start to get a sense of scale. The Excel model is in the Download menu — sans goal-seek macros and not at all user-friendly anyway. But the on-open results in the impact tabs illustrate the ‘aggregate and leverage’ idea.