**5. Conclusions**

An important technical challenge for a massive RES integration is the lack of manageability of the generation to match the demand. A high RES penetration requires the application of measures focused on planning, operation and flexibility of the whole system to respond to the uncertainty and variability in the supply–demand balance in short timescales. These measures present tangible costs to the system.

The results of this study lead us to state that the implementation of PV+W hybrid plants in urban areas would widen the RES integration limits and reduce the cost of high RES penetration because of the improvement of the manageability derived of a better adaptation to the demand profiles.

Additionally, our work gives valuable and quantifiable support to decision-makers to favour RES penetration into the urban environment, which constitutes a perfect example of distributed generation, with the advantages that this type of generation presents for the electric system.

As a result, a massive installation of PV+W hybrid plants would bring benefits for the whole electric system. Therefore, the author's workgroup of the Department of Electrical, Electronic and Control Engineering [87] propose and recommend the implementation of PV+W hybrid plants.

A massive integration into the urban environment presents financial, technical and regulatory barriers. The adequation of existing buildings could require the evaluation of subsidies to avoid financial constraints that could slow down the integration. Moreover, the facilities integrated into the urban environment will be likely owned by consumers (i.e., particulars or small/medium size companies) that could use part of the generation for self-consumption. This situation will require specific legislation to regulate the energy trading and implement technical requirements to avoid negative impact on the distribution networks.

**Author Contributions:** Conceptualisation: S.C.-R.; methodology: S.C.-R.; software: S.C.-R.; validation: S.C.-R.; formal analysis: S.C.-R.; investigation, S.C.-R.; resources: S.C.-R.; writing—original draft preparation: S.C.-R.; writing—review and editing: A.L.-R., S.C.-R., R.G.-O. and A.C.-S.; supervision: A.C.-S.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank Laj Cheenikkaparambil Abdullah for his contribution in making this article more readable.

**Conflicts of Interest:** The authors declare no conflict of interest.
