No question access to cheap capital (and lots of it) is vital to support growth in the deployment of renewable energy assets. And REITs could be a good vehicle for doing this. There are, however, a couple of important hurdles that won't easily or clearly be overcome.
1. Treating solar as real property for internal revenue code purposes means it is no longer property that qualifies for the Investment Tax Credit (which excludes real property) - it's a nuanced issue, and while a solution is possible it isn't easy (and it's not 100% it can be done w/o a legislative change - nothing good happens when Congress gets involved).
2. The general push around REITs is to tighten the application of REIT rules as there is some view that the conversion to REITs by non-core real estate businesses is an expansion beyond the intent of the REIT rules (I don't necessarily agree with this, but it is the current landscape). The upshot is that it’s more likely that REITs get narrower, rather than broader in scope.
Securitization and yieldcos (which have their own challenges), and I think at some point soon MLPs, are actually more likely to be the new capital vehicles that drive growth for solar and other renewables over the coming years - but agree it would be great to add REITs to the list as well.