Humanity is currently facing one of the most critical challenges in its history: climate change. Climate change, the result of decades of human activity driven by the excessive burning of fossil fuels, has assumed alarming proportions. Its effects—observed in extreme weather events, ecosystem disruptions, ocean acidification, and the rapid loss of biodiversity, among others—serve as clear reminders of the urgent need to adopt more sustainable practices, requiring immediate global action (Weiskopf et al., 2020). Increasing greenhouse-gas emissions and dependence on conventional energy sources pose significant challenges for humanity (Paraschiv and Paraschiv, 2020). It is, therefore, necessary to rethink our relationship with energy and work toward a more sustainable energy matrix (Gil-García et al., 2024). In this context, the transition to renewable energy sources (RESs) is presented as a fundamental response, constituting an essential pillar in the fight against climate change and the effort to achieve a future where dependence on fossil fuels is gradually eliminated (Mufutau Opeyemi, 2021).

RESs not only offer a cleaner alternative to fossil fuels but are also characterized by their inexhaustible potential as they are constantly renewable and, therefore, do not disappear when their energy is used (Halkos and Gkampoura, 2020). RESs encompass a variety of technologies, including the following (Fernández-Guillamón et al., 2019a):

• Wind power, both onshore and offshore installations.

In 2022, RESs accounted for 30% of global electricity generation, with hydro, PV, and wind energy representing 90% of renewable electricity generation worldwide, as shown in Figure 1. Global electricity generation in 2022 was approximately 29,165 TWh, which means that renewable sources contributed approximately 8,750 TWh (International Energy Agency, 2023). Including this absolute value allows for a clearer understanding of the scale of renewable electricity generation. According to Bórawski et al. (2020), a total wind energy capacity of 323 GW is projected to be installed in the EU by 2030, with 253 GW being onshore and 70 GW being offshore. This capacity is expected to generate 2,681 TWh by 2030 compared to the 342 TWh produced by wind energy in 2010 (Ellabban et al., 2014). Meanwhile, wind power capacity in the United States is expected to reach 305 GW, while in China, it is projected to reach 230 GW by 2030 (Shahan, 2011). Additionally, the global solar capacity is forecasted to reach 192 GW by 2030, leading to an increase in solar energy production from 32 TWh in 2010 to 846 TWh in 2030 (Ellabban et al., 2014).

However, from the electrical grid point of view, both wind and solar plants have a common characteristic: their production depends on the instantaneous availability of primary energy (wind and solar irradiation, respectively), which makes it extremely difficult to control the energy they generate, and therefore, they cannot be used to follow demand fluctuations (Fernández-Guillamón et al., 2019b); a similar problem is faced by run-off-river hydroelectric power plants (Gerini et al., 2021). In this regard, run-of-river hydropower plants exhibit a similar behavior as they lack the capacity to regulate turbine flow and, consequently, power generation (Sarasua et al., 2007).

As a concrete example, with the rapid advancement of wind power technology, the penetration rate of wind energy in some regions of China has surpassed 20% (Zhu et al., 2011), leading to notable negative impacts on the dynamic stability of power grids and frequency and voltage regulation (Wu and Infield, 2014). Generally, these negative effects on the electrical grid are associated with a decline in the quality of the power supply, making it challenging to maintain frequency (Wang et al., 2016) and voltage (Georgilakis, 2008) within the limits set by the transmission system operators (TSOs). These issues are particularly problematic in isolated power systems, where load-shedding programs are often activated to compensate for fluctuations in power demand or generation from non-controllable renewable sources (Martínez de Lucas, 2018; Sarasúa et al., 2021). As a result, maintaining frequency within the required limits becomes significantly more difficult. Pre-planned load-shedding may be necessary (Sharma et al., 2011) as an emergency response when frequency falls below certain thresholds set by the TSO to prevent damage to consumer loads.

RESs are inherently intermittent, but the degree of intermittency varies among them. Wind, for instance, is a highly variable meteorological factor that fluctuates on an hourly, daily, weekly, monthly, and annual basis. In contrast, solar radiation is a more predictable energy source, with less variability, though it only operates during daylight hours. From a power quality perspective, solar energy provides more stable power and can be integrated with smaller energy-storage systems (Alam et al., 2014). This fluctuating nature of renewable energy sources presents challenges for consistent power production, making it difficult to ensure a continuous supply to end users and maintain grid stability.

In terms of unpredictability, wind speed forecasting has significantly improved in recent years, as highlighted by a review of wind power prediction models (Wang et al., 2011). Therefore, the primary challenge for effectively integrating these technologies is managing the intermittency and stochastic behavior that is inherent to most of them (Canales et al., 2015). Regarding demand independence, wind and solar resources do not align with consumption patterns, either in the short or long term. A potential benefit lies in situating future wind farms in locations where wind patterns better match the electricity demand rather than simply selecting sites with the highest average wind speed (Suomalainen et al., 2013).

As a consequence, hybrid renewable energy systems (HRESs), where different RESs are combined (with or without backup energy systems, such as energy storage systems, ESSs), have been developed as a solution to this problem (Lian et al., 2019). These HRESs ensure a constant supply of electricity even under varying conditions, thus offering higher reliability than individual power sources, especially for off-grid (isolated) systems. In addition, they make it possible to take advantage of the best of each technology, achieving greater efficiency, lower operating costs, and more flexible use of energy resources, and they can be adapted to various needs and contexts, from urban environments to remote areas (Sohail et al., 2022). Various methods are discussed in the literature to mitigate the negative impacts of high renewable energy penetration. In terms of ESSs, many researchers argue that storing energy in the short, medium, and long term is an effective way to support the growth of RESs and alleviate some of their limitations. For instance, Zhao et al. (2015), Aneke and Wang (2016), Parra et al. (2017), and Mitali et al. (2022) have conducted comprehensive reviews of available ESS technologies in recent years, highlighting the importance of technologies such as pumped-storage hydroelectric plants (PSHPs) due to the regulating capacity of hydropower plants, flywheels, batteries, and capacitors. Additionally, ESS capacity has grown significantly in recent years. According to Agency (2014), approximately 140 GW of large-scale ESSs were installed in global power grids in 2014. Of this capacity, 99% was contributed by PSHPs, with the remaining 1% coming from a combination of batteries, compressed air energy storage, flywheels, and hydrogen storage. However, these figures increased within just 4 years; by 2018, the global ESS capacity had risen to 170 GW, with PSHPs accounting for more than 96% of that (Gür, 2018). As noted by Barnhart and Benson (2013), ESSs are a valuable load-balancing technology, enhancing the reliability and flexibility of power grids—particularly in regions with strong climate-change policies—while also helping lower carbon emissions by reducing transmission loads and enabling power plants to operate at peak efficiency.

In a global context focused on the transition toward cleaner and more sustainable energy sources, HRESs are positioned as a promising alternative, providing flexible and efficient solutions to satisfy the growing demand for electricity in an environmentally friendly manner (Vivas et al., 2018). Today, there are numerous success stories of hybrid systems (Alternative Technology Association, 2017). The island of Ta’u (American Samoa) relied on diesel generators until 2016, when Tesla installed a hybrid electric system consisting of 1.4 MW of PV capacity and 6 MWh of batteries (Lin, 2017). Two Australian islands (King Island and Rottnest Island) use hybrid wind–PV systems. The first, with 2.45 MW and 470 kW of wind and PV power, respectively, is backed by a range of support and storage technologies (batteries, flywheels, diesel generators using biodiesel, etc.) (Tasmania, 2014). The second island combines 600 kW of wind and 600 kW of PV power with a diesel power plant (Rottnest Island Authority, 2024). The island of Eigg (Scotland) has a hybrid system consisting of three hydroelectric generators (100, 6, and 5 kW), four wind turbines (6 kW each), and a PV installation (170 kW), supplying 95% of the total electricity. The remaining 5% is generated by two 64 kW diesel generators, which serve as backup when RESs are in short supply or during maintenance (Isle of Eigg Heritage Trust, 2024). Two notable cases stand out in the Spanish electricity system. First, a hybrid wind and pumped-storage hydropower facility, known as Gorona del Viento, was commissioned in June 2014 within the El Hierro power system. This pumped-storage plant includes four Pelton units (4 × 2.83 MW), six fixed-speed pumps (6 × 0.5 MW), and two variable-speed pumps (2 × 1.5 MW). In recent years, the importance of this complex power plant has grown within the power system. During the summer of 2019, electricity demand was met continuously and solely by RESs for a total of 596 h (Sarasúa et al., 2021). Second, Termosolar Borges, located in Les Borges, is currently the only operational hybrid thermal solar–biomass power plant, boasting a capacity of 22.5 MWe. This hybrid approach allows the plant to supply electricity around the clock, achieving an efficiency rate of 90% (Mohaghegh et al., 2021).

The main objective of this work is to carry out a spatial analysis of the technical viability (from the resource point of view) of different technologies applicable to HRESs, including pairs of combinations between solar, wind, hydroelectric, biomass, and geothermal power plants. The study considers the entire national territory of Spain, encompassing both the mainland and insular regions, including the Balearic and Canary Islands, accounting for regional variations in the different RESs analyzed. To overcome this, an analysis of the main parameters and characteristics affecting each of the RES considered is carried out to determine the optimum location of such power plants. The research study offers a novel combination of geographic, climatic, and technical factors to identify the most suitable locations for HRESs, which can optimize resource utilization and address intermittency issues inherent to individual RESs. Even though this study provides a framework for HRESs in Spain, it offers insights that are broadly applicable to other regions striving to transition to a more sustainable and resilient electricity future. The remainder of this study is organized as follows: Section 2 describes the methodology used; the case study is presented in Section 3; Section 4 details the results obtained, which are discussed in Section 5; finally, the conclusions are presented in Section 6.

This work is based on a systematic literature review methodology, with a particular focus on the parameters affecting the efficiency and adaptability of several RES technologies. The RESs under analysis involve the following:

• Solar power plants (PV and thermal).

These technologies were selected due to their maturity, scalability, and commercial availability in the Spanish context. Moreover, several of them exhibit complementary generation profiles, such as diurnal (solar) versus nocturnal (wind) generation or controllable (geothermal, hydropower, and biomass) versus variable (solar and wind) output. Additionally, the chosen RESs can be scaled and adapted to a range of energy demands, which vary from small-scale systems to large utility-scale projects.

The research follows a three-stage process, as depicted in Figure 2:

1. Literature review and parameter identification: The first phase involves a comprehensive review of academic publications and institutional reports to identify the main factors influencing the efficiency and viability of the RES technologies. For each technology, technical and some economic parameters are identified to determine the suitable location of each RES, i.e., solar irradiation, wind speed, hydrological features, and biomass availability, among others.

The final HRES suitability map provides valuable insights for the following aspects:

• Resource optimization and planning: It allows for the identification of areas with the highest potential for RES generation, maximizing the use of available natural resources and identifying high-potential zones for renewable energy deployment.

In this paper, Spain is selected as the case study area, considering the mainland and island regions. For each RES under consideration, different databases are used to obtain the relevant information regarding them. Apart from that, the TSO responsible for the Spanish power grid, “Red Eléctrica de España” (REE), releases a report on the Spanish grid each year, offering a detailed analysis of key metrics and operational statistics. The most recent available report is for the year 2022 (REE, 2023). The key data related to the Spanish electricity system at the end of 2022 are outlined below (see Figure 3):

• The total installed capacity was 119,091 MW, with 59.2% sourced from RESs. From the RES sector, 95% is generated from hydropower, wind, and PV sources.

Spain’s high RES generation is largely due to a mix of policies, infrastructure investments, and favorable natural resources (Escribano Francés et al., 2013; Matti and Consoli, 2015; Alonso et al., 2016; González et al., 2020; Jorge-Vazquez et al., 2024):

• Policy support: Spain has implemented subsidies, feed-in tariffs, and favorable policies that incentivize investment in RES technologies, particularly in wind and solar, which are the major RES contributors (onshore wind and solar PV).

Based on the literature review of the main parameters affecting the performance of the RES technologies under analysis carried out in Section 2, the optimal locations to install such power plants are analyzed. Table 2 presents a summary of the different RESs considered, including the key variables, thresholds/criteria for each factor, and the data sources from which the information was obtained, which are explained and detailed later.

After that and following the HRES cases under consideration (Table 1), the locations where the plants of different HRESs could be installed are presented.

Once the maps for the optimum location of all individual RESs are obtained, suitability maps for different HRES combinations were generated by overlapping these individual layers. This spatial integration helps identify areas where multiple RESs coincide, allowing for the co-location and hybridization of technologies. Figures 9, 10 provide these composite maps, which are fundamental for prioritizing HRES deployment in Spain. It should be remarked that the selection of parameters across all RESs in this study balances technical importance, data availability, and spatial coverage, offering a comprehensive and realistic basis for regional HRESs planning in Spain. From them, it can be observed that cases 1, 2, 3, 5, 6, and 8 have an interesting potential along the territory of Spain. A spatial analysis of these maps reveals distinct regional patterns of suitability:

• Northern Spain shows high suitability for wind–hydro (case 5), wind–biomass (case 6), and hydro–biomass (case 8) combinations. This is attributed to high wind speeds and significant rainfall, combined with elevation gradients that favor hydropower. However, none of these are currently under deployment. Some northern regions can also be suitable for solar HRES, highlighting solar–wind (case 1) and solar–biomass (case 3), where the HRES system “Termosolar Borges” must be highlighted (refer to Section 1).

It should be noted that these maps show the possibility of diversification of RESs for HRESs, which is essential for a long-term strategy. The implementation of HRESs is also relevant when considering the reduction of carbon dioxide emissions and other pollutants produced by more traditional energy sources. Consequently, these findings suggest that Spain can strategically diversify HRES implementation based on regional strengths: for example, wind-heavy zones and solar-dominated regions can rely on hydro or biomass as dispatchable sources for regulation.

Although these suitability maps provide insights into where multiple RESs may geographically co-exist in Spain, it is important to note that the analysis does not account for physical or technical interactions between technologies nor does it consider economic, environmental, or regulatory aspects. As such, this study serves as a resource-focused screening, identifying high-potential zones for HRESs based on natural availability, which must be further evaluated with respect to technical, economic, and land-use compatibility in future analyses. However, these maps offer a foundation for policymakers and developers to explore site-specific HRES configurations that align with local environmental conditions and infrastructure. The limitations of the spatial analysis are as follows:

• Co-location feasibility: The maps assume ideal conditions for resource overlap but do not account for land-use conflicts (e.g., urban zones, Natura 2000 protected areas) or land availability for infrastructure.

The deployment of RES projects in Spain is shaped by a complex political and regulatory framework, which includes land-use restrictions, economic incentives, and national strategic planning. Land-use regulations, such as those related to environmental protection (e.g., Natura 2000 sites) and zoning constraints for agricultural or urban uses, can significantly limit the availability of suitable sites for new energy infrastructure (MIT, 2024a). Additionally, regulatory schemes introduce specific incentives and barriers that directly affect the feasibility of HRESs. For instance, support mechanisms such as feed-in tariffs and renewable energy auctions have historically played a key role in project viability, though recent changes have increased regulatory uncertainty (Sendstad et al., 2022). At the strategic level, the Integrated National Energy and Climate Plan (PNIEC) 2021–2030 sets ambitious targets for RES integration, storage capacity, and electrification while identifying priority areas for infrastructure development (MIT, 2024b). However, effective implementation of the PNIEC depends on interregional coordination and regulatory clarity regarding environmental permitting, grid access, and hybridization rules.

Consequently, the resource synergies must be weighed against practical constraints. To move from theoretical potential to actionable implementation, it is necessary to analyze several alternatives in each case to ensure that the optimal one is chosen from a multi-criteria point of view (technical, economic, environmental, regulatory, etc.), as proposed by Zhang et al. (2023). Mardani et al. (2015) presented a literature review on multi-criteria decision-making techniques, highlighting their ability to help stakeholders and decision-makers address uncertainties in environmental decision-making processes across various stages of sustainable and renewable energy system development, and several authors have focused on these techniques for different types of RES (Gil-García et al., 2023; García et al., 2024; Serrano-Gomez et al., 2024) and HRES (Elkadeem et al., 2021; Rezk et al., 2021; Ali et al., 2023) problems. Future research could build upon this spatial framework by integrating these other factors and metrics to provide a more comprehensive evaluation of HRES deployment potential.

Regarding the HRESs considered, even though not all of them are already deployed in Spain, there are several real systems around the world, which prove that they can be feasible from a multi-criteria point of view if all the conditions are satisfied:

• Case 1 (solar–wind): Due to the complementary nature of wind and solar energy, HRESs provide several advantages over standalone solar or wind technologies. Notably, the peak operating times for wind and solar systems occur at different periods throughout the day and year, enhancing overall energy production and reliability (Roy et al., 2022). Several PV –wind HRESs have been mentioned in Section 1, demonstrating their feasibility. However, all of them relied on backup technologies, whether through ESSs or conventional power sources. Even though some studies have focused on the complementarity of PV and wind power generation (Couto and Estanqueiro, 2020; 2021; Costoya et al., 2023), this HRES presents the main drawback of lacking inherent regulation capacity (Yashwant Sawle and Bohre, 2016); an additional successful case is Orgueirel (Portugal, 8.4 MW of PV and 11 MW of wind (EDP Global, 2023)). Considering thermal solar and wind HRESs, they have the regulation ability due to the thermodynamic cycle of the thermal solar power plant and its TES (Powell et al., 2017; Rahman et al., 2017). Moreover, some authors have improved their performance as an HRES by transforming the curtailed wind power into heat through an electric heater, which is later stored in the thermal storage sub-system of the solar thermal power plant (Ding et al., 2019). Although the combination of solar thermal and wind energy is not as common as other HRESs, it can be a growing area of interest for the future of RESs, especially in regions with high levels of wind and solar radiation. Reichling and Kulacki (2008) simulated a hybrid solar thermal–wind power plant, concluding that it produced better electric load matching characteristics than an energy-equivalent wind power plant. However, to the authors’ knowledge, there are no real installations of this type.

This study presents a comprehensive analysis of HRESs tailored to the Spanish energy context, focusing on combinations of solar, wind, hydroelectric, biomass, and geothermal power plants. A key contribution of this work is the development and application of a simple but effective geographic integration method, which enables precise identification of optimal locations for implementing HRESs by combining spatial resource availability and technical parameters.

The main conclusions of this study are as follows:

• Geographic integration enables targeted planning: This method produces simplified maps that identify the most suitable regions in Spain for each hybrid combination. This supports more effective site selection and resource optimization, providing a practical tool for energy planners and policymakers.

In summary, this study advances the academic understanding of HRESs through an extensive literature review of 10 HRESs providing practical insights into the spatial and technical feasibility of various HRES configurations in Spain. The findings demonstrate that strategically combining multiple RES technologies can create resilient, efficient, and scalable energy systems that support Spain’s transition toward a sustainable, low-carbon energy future.