Several of the studies suggested the need for substantial transmission expansion in order to facilitate high-renewables-penetration scenarios. While some transmission expansion is certainly warranted, large-scale expansion of the bulk transmission network is costly and will face substantial siting barriers. Thus there is, in our opinion, a need for creative thinking about how to most effectively use existing transmission resources, with perhaps a minimum amount of network expansion, to facilitate high-renewables scenarios. As an example of the type of analysis that is needed, Hoppock and Patino-Echeverri (2010) compared the levelized cost of energy from distant, high-capacity-factor wind sites to energy from near, lower-capacity-factor sites. They found that transmission investment costs could make the lower-quality sites less expensive. The existing technology available for studying optimal transmission expansion is a significant barrier to further progress in this area. Only in the most recent study (NREL, 2012) did analysts attempt to optimize their transmission expansion plans, and in this case a transportation model of the grid was used rather than a power-flow model. Research on optimization methods for transmission switching (Barrows & Blumsack, 2012; Fisher et al., 2008) and transmission expansion (Alguacil, Motto, & Conejo, 2003)
suggest approaches to this problem, but substantial research is needed before these algorithms can be applied to large-scale problems.
As methods for optimal system expansion planning improve, future large-scale studies should examine creative combinations of offshore wind, strategically located onshore wind, solar (which is often more easily located near load centers and is increasingly cost competitive), storage, controllable AC or DC transmission lines, and demand response resources. There may be synergies among these technologies that enable higher-penetration scenarios while minimizing curtailment, ancillary service costs, and reliability effects.