**1. Introduction**

On 12 December 2015, the 195 countries participating in 21st Conference of the Parties (Paris Climate Change Conference) [1], organised by the United Nations Framework Convention on Climate Change (UNFCCC) [2], signed the Paris Agreement [3]. This agreemen<sup>t</sup> aims to achieve, as soon as possible, a reduction on the carbon emissions to hold the increase in the global average temperature to well below 2 ◦C above pre-industrial levels. The generation and use of energy are the main contributors to climate change, with 60% of the total greenhouse gases (GHG) emissions. The reduction in energy sector emissions is mandatory to achieve the global warming objectives. Hence, the Paris Agreement determines by 2030 there will be a substantial increase in the use of renewable energy sources (RES) in the world energy mix.

This important agreemen<sup>t</sup> is one more step given in the fight against climate change, which has been developed by the international community in the last decades. For this purpose, governments and international organisations and institutions have designed scenarios, strategies and commitments focused on the mitigation and reduction of the present emission levels. In all of them, high RES penetration shares are mandatory, and, with this aim, ambitious plans have been determined.

Along these lines, the United States of America developed the SunShot Initiative [4], focused on the solar photovoltaic renewable source (PV), favouring its integration by means of being competitive with the traditional generation forms before 2020, and the Wind Program [5], designed to speed up the development and integration of wind energy. Likewise, the member countries of the European Union established the Roadmap 2050 [6] to set up the paths to achieve the European commitment to reach in 2050 GHG emissions below 80% of 1990 levels.

Nowadays, the RES technologies with higher integration level are wind and photovoltaic. Figure 1 shows the time evolution of wind and PV installed power worldwide. At the end of 2017, the installed power capacity was 384 GW in PV facilities and 494 GW in wind farms.

**Figure 1.** Wind and photovoltaic (PV) installed power worldwide. Source: [7,8].

Wind and PV electricity generation technologies presently offer technical and economic maturity levels. They allow high-efficiency generation almost everywhere at such a low cost compared with the traditional generation based on conventional thermal methods [9–15]. Moreover, among the renewable energy sources, wind and PV electricity generation technologies present high degrees of sustainability under multi-criteria analysis [16–18].

The International Energy Agency (IEA) remarks in its Energy Technology Perspectives 2017 (ETP2017) [19] that, the implementation of PV and onshore wind technologies are on-track to achieve their integration targets. Nevertheless, penetration shares for these technologies are still far from the targets fixed to contribute to the mitigation of GHG emissions. According to the IEA hi-Ren scenario (the high-renewables scenario—hi-Ren scenario—sees energy systems radically transformed to achieve the goal of limiting the global mean temperature increase to 2 ◦C target with a large share of renewables, which requires fast and strong deployment of photovoltaic and wind power and solar thermal electricity), the installed worldwide power capacity should reach 4674 GW by 2050 for PV and 2700 GW for onshore wind in the same period [20,21]. Innovative technical solutions and regulatory measurements are required to boost a massive RES integration to close the huge gap between the present status and the fixed targets in the next coming years.

The achievement of RES penetration targets is only feasible with actions addressed to facilitate their use in three fields with massive energy consumption: transport, buildings and industry. Among them, building integration shows the biggest potential to increase the share of RES in the energetic mix [22,23].

The widest field for RES building integration is found in the urban environment. Considerable research has been carried out to determine the PV [24–32] and wind potential [33–36] in urban areas and buildings.

PV presents a characteristic that favours its massive penetration in the urban environment: the dispersion degree. Solar radiation is received everywhere with such intensity levels that make possible the production of electricity. In addition, PV building integration offers environmental advantages as against its implementation on rural lands as the former gives a new value to the building roofs and facades.

In regard of wind energy, the installation of wind turbines in urban areas is not widely spread yet, but there are technical solutions to e fficiently take advantage of the urban wind stream with its special characteristics of turbulence and direction variability [33,37–41].

In relation to PV–wind hybrid plants (PV+W hybrid hereinafter), extensive research has been developed to quantify the synergies between solar and wind sources. A non-exhaustive list of references is shown in Table 1.


**Table 1.** Literature review reference list.

Cities are big electricity consumers. Therefore, RES integration in urban areas would also o ffer an important technical advantage because the generation would be placed near to the consumption point. This solution would improve the whole electric system e fficiency by reducing the transport and distribution of electricity losses. Moreover, it is a clear example of distributed generation with advantages associated with the control and managemen<sup>t</sup> of the electric network [51–53].

But the integration of a massive share of variable RES (VRES) in the electric power grid implies technical challenges and extra-costs. The electricity generated in PV and wind facilities have a non-manageable character; which means that it is not possible to control the supply instantaneously (except to reduce it) to match the demand. A high VRES 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 [54–56]. These measures present estimable costs for the system that could reach 25–35 €/MWh in high penetration scenarios [57,58].

Extensive research has been recently carried out showing that, with the use of adequate coordination control algorithms, large-scale systems made up of multiple individual subsystems can together contribute e fficiently in the achievement of global quantities of interest, even in the case that some of the sub-systems became adversarial or non-cooperative due to bad functioning [59]. This resilient performance is fully applicable to a massive integration of VRES based on the implementation of individual small facilities.

Due to the aforementioned, a deep knowledge of the performance of the facilities and their generation patterns becomes relevant. It is essential to understand how they could match the electricity demand, with the aim to o ffer better control and managemen<sup>t</sup> of the electricity fed into the grid and, consequently, collaborate to reduce the technical barriers and to decrease the integration cost.

With this target as the main objective of our work, we have carried out a study under the novel perspective to evaluate the supply–demand balance adaptation of PV+W hybrid plants integrated into an urban environment. To have results applicable on a global scale, we have considered hundreds of locations spread all over the world and multiple load profiles for the characterisation of demand. This article first analyses if PV+W hybrid facilities present generation patterns that adapt better to the demand profiles than if the facilities were installed individually, and second, determines a novel methodology to quantify the adaptation degree.

The novelty of our work is fundamentally based on three main grounds:


The main technical challenge arises from our requirement to obtain results applicable on a global scale. With that aim, we have considered only real weather data from hundreds of meteorological stations and multiple electricity load profiles for the characterisation of the demand in di fferent seasons and days. These requirements have obliged the authors to carry out extensive work to obtain and validate the input data and ge<sup>t</sup> it homogeneous.

Below in Section 2, we introduce the methodology developed to evaluate and quantify the level of adaptation of generation patterns to demand profiles. In Section 3, we present the results of applying this methodology to a wide number of locations worldwide and carry out a sensitivity analysis of the results. Finally, in Section 4, the conclusions of our study are discussed.
