# Opportunities for renewable energy sources in mountain areas and the Alps case

Nesrine Faraj, Valentina Maruzzo, Iacopo Benesperi, Antoine Bousquet, Anna Lushnikova, Marcello Baricco, Francesca Brunetti, Christophe Menezo, Christine Lartigau-Dagron, Nadia Barbero

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Nesrine Faraj, Valentina Maruzzo, Iacopo Benesperi, Antoine Bousquet, Anna Lushnikova, et al. Opportunities for renewable energy sources in mountain areas and the Alps case. Renewable and Sustainable Energy Reviews, 2025, 223, pp.115983. 10.1016/j.rser.2025.115983. hal-05156785

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Review article

# Opportunities for renewable energy sources in mountain areas and the Alps case

Nesrine Faraj<sup>a,b,c,1</sup>, Valentina Maruzzo<sup>a,c,1</sup>, Iacopo Benesperi<sup>a,1</sup>, Antoine Bousquet<sup>c,1</sup>, Anna Lushnikova<sup>b,1</sup>, Marcello Baricco<sup>a,1</sup>, Francesca Brunetti<sup>d,1</sup>, Christophe Ménézo<sup>b,1</sup>, Christine Lartigau-Dagron<sup>c,1</sup>, Nadia Barbero<sup>a,e,1,*</sup>

ARTICLE INFO

Keywords:

Alpine region

Climate change

Energy poverty

Mountain areas

Renewable energies

ABSTRACT

The transition to renewable energy sources is critical for addressing climate change and achieving sustainable development, especially in environmentally sensitive regions such as mountain areas. Mountains are extremely heterogeneous: they show unique features in terms of climate, topography and biodiversity, which makes them and their local populations more vulnerable to the negative effects of climate change and energy poverty. This review explores the specific challenges and opportunities associated with the implementation of renewable energies in mountain areas, with a particular focus on the Alps. It addresses the issue of energy poverty – a situation in which a household is not able to afford or is lacking access to essential energy services – and it provides a focus on the effects of climate change on mountain areas. This review also discusses the advantages and disadvantages in the potential use of various kinds of renewable energy sources in mountainous regions, including solar, wind, hydropower, and biomass; especially in the context of the Alps' unique environmental and socio-economic conditions. Despite the promising availability of these resources, some of them have not been extensively applied yet in this area, such as wind and solar power. Biomass and hydropower, on the other hand, have been largely exploited. Finally, an overview of the available energy storage systems is given, along with some practical examples of simulation or implementation of plants, highlighting the crucial role of storage and integration technologies in enhancing the reliability and efficiency of renewable energy systems in mountainous terrains.

Contents

<table><tr><td>1.</td><td>Introduction</td><td>2</td></tr><tr><td>2.</td><td>Energy poverty and the critical energy needs of mountain communities</td><td>3</td></tr><tr><td></td><td>2.1. Geographic challenges</td><td>3</td></tr><tr><td></td><td>2.2. Socio-economic challenges</td><td>5</td></tr><tr><td>3.</td><td>Climate change in mountains</td><td>6</td></tr><tr><td>4.</td><td>Renewable energies in mountain areas</td><td>7</td></tr><tr><td></td><td>4.1. Biomass</td><td>8</td></tr><tr><td></td><td>4.2. Hydropower</td><td>8</td></tr><tr><td></td><td>4.3. Wind</td><td>10</td></tr><tr><td></td><td>4.4. Solar photovoltaic</td><td>11</td></tr><tr><td></td><td>4.5. Solar thermal</td><td>13</td></tr></table>

<footnote><sup>a</sup> University of Torino, Department of Chemistry, NIS Interdepartmental Centre and INSTM Reference Centre, Via Quarello 15/A, 10135, Torino (TO), Italy</footnote>

<footnote><sup>b</sup> Université Savoie Mont-Blanc, LOCIE UMR CNRS 5271, bât. Hélios, 60 av. du lac Léman, Savoie Technolac, 73376, Le Bourget-du-Lac, France</footnote>

<footnote><sup>c</sup> Université de Pau et des Pays de l'Adour, E2S UPPA, CNRS, IPREM, Pau, France</footnote>

<footnote><sup>d</sup> CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, Via del Politecnico, 1, 00133, Roma (RM), Italy</footnote>

<footnote><sup>e</sup> Istituto di Scienza, Tecnologia e Sostenibilità per lo Sviluppo dei Materiali Ceramici (ISSMC-CNR), Via Granarolo, 64, 48018, Faenza (RA), Italy</footnote>

<footnote>* Correspondence to: c/o Dipartimento di Chimica - UniTOVia P. Giuria, 710125, Torino (TO), Italy.</footnote>

<footnote>E-mail address: nadia.barbero@unito.it (N. Barbero).</footnote>

<footnote>URL: http://www.moflab.unito.it (N. Barbero).</footnote>

<footnote n="1">¹ These authors contributed equally.</footnote>
<table><tr><td>5.</td><td>Energy management</td><td>14</td></tr><tr><td>5.1.</td><td>Storage</td><td>14</td></tr><tr><td>5.1.1.</td><td>Batteries</td><td>14</td></tr><tr><td>5.1.2.</td><td>Mechanical storage</td><td>15</td></tr><tr><td>5.1.3.</td><td>Hydrogen storage</td><td>16</td></tr><tr><td>5.2.</td><td>Integration</td><td>17</td></tr><tr><td>6.</td><td>Analysis and recommendations</td><td>18</td></tr><tr><td>7.</td><td>Conclusions</td><td>20</td></tr><tr><td></td><td>CRediT authorship contribution statement</td><td>20</td></tr><tr><td></td><td>Declaration of competing interest</td><td>20</td></tr><tr><td></td><td>Acknowledgments</td><td>20</td></tr><tr><td></td><td>Data availability</td><td>20</td></tr><tr><td></td><td>References</td><td>20</td></tr></table>

# 1. Introduction

Mountains have gained significant political and scientific interest over the past three decades and especially after United Nations' Conference on Environment and Development held in Rio de Janeiro in 1992: chapter 13 of the document "Agenda 21" edited during the conference is titled "Managing fragile ecosystems: sustainable mountain development" [1]. In this document, high mountain areas (e.g., the Alps, the Rockies, the Himalayas, etc.) are considered one of earth's hotspots as the impact of climate change is more pronounced in these areas [2,3]. Mountain areas have peculiarities in their morphology and landscape that influence installation processes and energy generation capabilities of renewable energy sources compared to other land surfaces. Therefore, it is useful to provide an organic view of these differences, to assist policy makers and energy companies in their decisions toward a correct transition to a post-carbon society. Before delving into this analysis, however, it is important to try to define what a mountain actually is, as it is not as straightforward a task as it could appear at first sight, and a global consensus has not been reached yet. It is not possible, in fact, to define a mountainous area based only on its altitude (e.g., there are elevated plateaus such as the short-grass Prairies in North America that are not classified as mountains) or its climate (as any cold climate category would include Arctic and Antarctic lowlands). Furthermore, mountains are usually very heterogeneous in several aspects, such as climate, topography, soil, land cover, and vegetation [4-6]. Initial attempts to systematically define a mountain were made in the 19th century, when criteria as elevation, volume, relief, and steepness were taken into account [7]. Later, elevation and ruggedness were selected in an attempt to reach an agreement on a global mountain convention [6,8,9]. In 2011, Körner et al. defined mountains as areas with more than 200 m difference in elevation within a 2.5 arcminute grid cell (corresponding to a 4.6 × 4.6 km² area near the equator) with 0.5 arcminute resolution irrespective of elevation [6,10].

Energy resource uses and development are closely related to the climate change that earth has been experiencing in the past few decades. Fossil fuels are to date the main source of energy used by humankind, despite the large emission of greenhouse gasses (mainly CO₂) associated with their combustion [12,13]. The reduction of CO₂ emissions, driven by the transition to renewable energy sources for energy production, is one of the goals included in United Nations' 2030 Agenda for Sustainable Development [14]. In the scope of this agenda, to achieve sustainable development policies must contain viable and environmentally friendly energy strategies as a core element [15-17]. In addition to being one of the main causes of climate change, to which mountains are particularly sensitive, energy production from fossil fuels is based on big plants that generate a large amount of energy, which then needs to be distributed to a large territory: distribution that is not always easy to achieve in mountain areas. The deployment of renewable energy sources scattered throughout the territory to address the local population needs would solve the energy transport issue, increase the

resilience of the electricity grid, and address Energy Poverty (EP) in these regions. EP is characterized by a situation in which a household either cannot afford essential energy services or lacks sufficient access to them. EP exerts a profound influence on both the quality of life and the health of individuals, impacting their overall standard of living within their country of residence. This review will describe how different renewable energy sources – with a focus on solar energy and photovoltaic electricity production – can adapt to and benefit from the morphological peculiarities of mountain areas, to provide clean and reliable energy to the local population. This review will also analyze what the effects of geographic and socio-economic characteristics of mountain communities related to EP are, and why mountain regions are particularly vulnerable to it.

Although – as previously stated – mountains are generally heterogeneous in their characteristics, when it comes to the deployment of renewable energies or to socio-economic peculiarities of the local population compared to lowlands inhabitants of their respective countries many similarities can be found across all mountain areas in the world. Therefore, this review will have a particular focus on the Alps as an example of a mountain range to analyze, with conclusions that can be extended to other mountain areas. The Alps are a range of mountains located in the center of the European continent and are considered the highest and largest mountain range system located entirely in Europe. The Alps span across an area of about 190,600 km² (Fig. 1) with a population of 13.6 million in eight Alpine countries: France, Monaco, Italy, Switzerland, Liechtenstein, Germany, Austria, and Slovenia [18]. The Alps also play an important role in providing water to the European continent and they have been defined as the "water tower of Europe" by Füreder et al. [19]. Moreover, Alpine regions are among the richest in Europe in terms of landscape diversity, and plant and animal species [20]. They are considered one of the most famous scenic mountain ranges in the world, and they are a vacation destination for about 120 million guests annually, who enjoy their stunning sceneries and practice many sports such as skiing and climbing [20,21]. However, the fragile Alpine ecosystem has historically reacted to even the tiniest of changes to environmental parameters such as average annual air temperature, precipitation conditions, and snow cover [22-24], which in turn lead to significant environmental and social changes such as life and habitat loss, water availability, etc. [25-28]. Several studies have shown that since the beginning of the 20th century the average annual air temperature of the Alps has risen by more than 1.5 °C [29-32]. It should be noted that this rise has been more accentuated during the second half of the 20th century, with an average increase of about 0.36 °C per decade between 1970 and 2019 [26,33]. The average annual air temperature is expected to further increase in the coming decades if actions are not taken to reduce the current high rates of greenhouse gas emissions [34]. Lastly, the worldwide temperature increase in the last decades has led to a sharp decrease in snow cover throughout all Alpine regions [35]. Throughout the following sections of this review there will be a particular focus on the analysis of EP in the Alpine regions and how renewable energy sources can adapt to this particular mountain range, in order to provide policy makers with a broad overview of the subject that can help them in their decisions for a correct energy planning for the whole region.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Typology of land use and national states of the Alps, as defined by the Alpine Convention.
*Source:* Reproduced with permission from Eurac Research from Ref. [11].</figcaption></figure>

## 2. Energy poverty and the critical energy needs of mountain communities

In addition to their geographical characteristics and their sensitivity to climate change, mountain areas are peculiar from a socio-economic perspective, too. Local populations, in fact, are known to suffer from EP, which is related to the inability of households to adequately meet their energy needs. EP has emerged as a vital problem worldwide in recent years, and it has received significant attention not only at academic level, but also within public policy and international organizations, such as the United Nations (UN), the International Atomic Energy Agency, the International Energy Agency, and the World Bank [36,37]. There is no specific definition of the concept of EP, and its estimation depends on the definition/indicators used. The problem of EP first gained public attention after Boardman's book entitled "Fuel Poverty: From Cold Homes to Affordable Warmth" in 1991, where the term fuel poverty was defined as the inability to obtain sufficient energy to maintain heat in the home [38]. This definition was later expanded to include all other domestic energy needs, such as insufficient access to lighting, energy to operate electrical appliances, and cooling at a reasonable price [39–41]. Due to the complexity of this phenomenon, different measurement methods have been proposed over time, which focus on different baseline parameters and conditions. One of the earliest approaches to the concept of EP is the 10% indicator method, for which a home is considered to be energy poor if it needs to spend more than 10% of its income on all domestic energy needs to meet the satisfactory standards of home warmth [42]. Later examples of quantitative indicators to measure and assess EP are the "Minimum Income Standard" [43], the "Low Income High Costs" [44], the "Degree of Coverage of Energy Needs" [45], and the "Low Income Low Energy Efficiency" methods [46]. It is our opinion that no indicator is better than the others. Rather, different indicators are better suited to picture the EP problem in a certain location depending on the political

priorities and the need of knowledge of the local government, and the ability to collect population data. For example, the latter three indicators are more complex compared to the 10% indicator method, and could therefore be considered more comprehensive depending on government priorities, but they come at the expense of requiring much more accurate data from the population to be calculated. EP is a specific form of poverty that has a range of negative consequences for human health and well-being and tackling it is an essential part of restoring social inequalities and injustice [47]. The severity of EP varies from region to region based on their characteristics.

In this section, we delve into the challenges that mountain areas face compared to their lowland counterparts that makes them more susceptible to EP. These challenges can be either geographic or socio-economic in nature, due to the terrain characteristics of these lands and to the kind of economy that can be developed in them. Understanding these challenges is crucial to devise effective energy policies to address the needs of the local populations while safeguarding the extraordinary biodiversity of these areas.

### 2.1. Geographic challenges

Altitude is one of the main characteristics of mountainous regions and the main contributor to the increase in energy demand in these areas. Temperatures are generally lower in mountain regions than in lowlands due to their higher altitude. Every 100 m increase in altitude the average annual temperature decreases by 0.558 K as the atmosphere density (and hence its thermal capacity) decreases [48, 49]. After altitude, latitude is the other important geographical factor that influences the other characteristics of these regions; especially their climate [50]. Areas with high altitude or high latitude show the same trend of temperature. They are both very cold in winter, their surface is largely covered by snow and ice, their air is quite dry, the feedback given by snow/ice albedo is comparable, changes
<caption>**Table 1**</caption>

Allocation of energy demand with altitude for a building located in Austria, Switzerland, Greece and north Italy (cooling is meaningless over 900 m for all cases. The percentage variation of energy needs at each level has been calculated in respect with the first level (the meteorological station with the lowest altitude)).

Source: Reproduced from Ref. [62] with permission from Springer International Publishing AG, copyright 2015.

<table><thead><tr><th>Country</th><th>Meteorological station</th><th>Altitude (m)</th><th>Annual HDD (°C d)</th><th>Percentual increase of heating needs (%)</th><th>Annual CDD (°C d)</th><th>Percentual decrease of cooling needs (%)</th><th>Percentual variation of total needs (%)</th><th>Ratio of heating/cooling needs (%)</th></tr></thead><tbody><tr><td rowspan="4">Austria</td><td>Linz-Stadt</td><td>263</td><td>2357</td><td></td><td>181</td><td></td><td></td><td>92.9</td></tr><tr><td>Flattach-Kleindorfl</td><td>735</td><td>2920</td><td>23.9</td><td>105</td><td>42.2</td><td>+19.2</td><td>96.5</td></tr><tr><td>Badgastein</td><td>1092</td><td>3247</td><td>37.8</td><td>20</td><td>89.0</td><td>+28.7</td><td>99.4</td></tr><tr><td>Lugano</td><td>273</td><td>1503</td><td></td><td>410</td><td></td><td></td><td>78.6</td></tr><tr><td rowspan="2">Switzerland</td><td>Chur</td><td>556</td><td>2101</td><td>39.8</td><td>184</td><td>55.2</td><td>+19.4</td><td>92.0</td></tr><tr><td>Davos</td><td>1594</td><td>3591</td><td>138.9</td><td>20</td><td>95.1</td><td>+88.8</td><td>99.4</td></tr><tr><td rowspan="3">North Italy</td><td>Bolzano</td><td>272</td><td>2236</td><td></td><td>260</td><td></td><td></td><td>89.6</td></tr><tr><td>Bressanone</td><td>569</td><td>2659</td><td>18.9</td><td>134</td><td>48.4</td><td>+11.9</td><td>95.2</td></tr><tr><td>Predazzo</td><td>1020</td><td>3233</td><td>44.6</td><td>20</td><td>92.3</td><td>+30.4</td><td>99.4</td></tr><tr><td rowspan="3">Greece</td><td>Lamia</td><td>143</td><td>1192</td><td></td><td>481</td><td></td><td></td><td>71.2</td></tr><tr><td>Konitsa</td><td>542</td><td>1543</td><td>29.4</td><td>249</td><td>48.2</td><td>+7.1</td><td>86.1</td></tr><tr><td>Karpenisi</td><td>980</td><td>2074</td><td>74.0</td><td>20</td><td>95.8</td><td>+25.1</td><td>99.0</td></tr></tbody></table>

in atmospheric circulations are present in both types of area, and the amount of solar radiation received is higher compared to other lands [51–55]. When Arctic and Alpine regions are compared, lower maximum summer temperatures are typically found in Alpine regions, as they are windier [56]. Mountain climate is characterized by cold winters and short summers, which in turn affect the energy needs of these regions. Nevertheless, the relationship between temperature and altitude is complex: firstly because local factors can deeply affect regional scales, but also because climate variability is higher at high altitudes [29,57–61]. As a result, valleys close to each other can show very different climate patterns.

A few studies have focused on evaluating the thermal energy needs of mountainous regions [62–64]. B. P. Bhatt et al. explored fuelwood consumption across various altitudes in the Himalayan region (Garhwal Himalaya, Uttarakhand, India), revealing a significant correlation between altitude and energy demand [63]. They found that at higher altitudes there is an increase in fuelwood consumption, especially during the winter months for heating purposes. Katsoulakos et al. noticed a direct relationship between altitude and temperature, observing a notable temperature decrease with increasing altitude, which in turn results in a significant rise in thermal energy demand in mountainous regions [64]. Papada and Kaliampakos used the methodological tool of degree days (DD) – which is one of the simplest methods and a well-known index of energy demand – to quantify the energy demand of high-altitude buildings in three different Alpine regions (Austria, Switzerland, and Northern Italy) and in Greek mountainous regions thanks to the availability of long-term temperature data [62]. A DD describes the intensity and duration for which the external air temperature is above or below a specified threshold value, known as the base temperature ($T_b$) [65,66]. $T_b$ is a crucial parameter for the calculation of DD; it is not equal to the household desired temperature and it depends on the properties of the examined building, which means that its value varies for different buildings and regions. In Papada's and Kaliampakos's study two quantitative indicators – heating degree days (HDD) and cooling degree days (CDD) – were used to indicate the energy demand for heating and cooling of buildings. These two indicators are proportional to climatic conditions as they are based on air temperature records and on the characteristics of the building's shell. In this study, the $T_b$ used for Switzerland and Austria was 14 °C, while that for Northern Italy and Greece was 16 °C (note: the final choice of $T_b$ was based on the assessment of individual researchers by taking into account the special climatic conditions and temperatures of the examined areas). As shown in Table 1, heating needs rose gradually with altitude, whereas cooling needs decreased. Although the demand for cooling decreased with altitude, the total energy needs increased in all cases. Therefore, the majority of energy needs in mountainous regions consisted of thermal loads. For instance, in the case of Austria,

the heating needs at an altitude of 1092 m were 1.4 times higher than those at an altitude of 263 m, while cooling needs were 9.1 times lower. The total energy needs (heating and cooling) were 1.3 times higher. In Switzerland, the increase in energy demand at higher elevations was more pronounced, as the heating needs of a building at an altitude of 1594 m were 2.4 times higher than those at an altitude of 273 m, while the cooling needs were 20.5 times lower. The total energy needs were 1.9 times higher. In summary, through the studied cases it was proven that thermal energy demand was strongly influenced by altitude. Although latitude also had an effect on air temperature, altitude proved to be the main factor affecting energy demand in all the cases studied. In addition, they addressed the challenge of limited availability of long-term, high-accuracy meteorological data in mountainous regions by introducing a model based on the decrease of surface temperature with altitude, known as the “lapse rate value” [67]. Through regression analysis of temperature-altitude relationships, they determined region-specific lapse rate constants for Austria, Switzerland, Greece, and north Italy. These lapse rate constants, such as 4.9 °C km⁻¹ for Austria, 5.2 °C km⁻¹ for Switzerland, 5.5 °C km⁻¹ for Greece, and 6.0 °C km⁻¹ for North Italy, enabled accurate estimation of air temperatures at different altitudes in mountainous terrains, enhancing the precision of energy demand calculations for buildings in high-altitude areas. This innovative approach not only provided a solution to the scarcity of meteorological data but also underscored the importance of considering local climatic conditions in energy planning for mountainous regions.

Given the morphology of mountainous regions, remoteness and isolation are two characteristics of these areas, which contribute to the exacerbation of the EP problem. Their combination creates obstacles to the supply of energy, since it is not profitable to expand power grids towards remote areas with low energy demand. As a result, these areas are usually the last to be electrified when energy companies expand their grid and sometimes they are not electrified at all [68, 69]. Moreover, fuel prices are higher in mountainous regions (5%–7% higher in the case of the Greek mountains [64,70]) compared to lower regions due to the higher cost of fuel transportation, given the large distance between mountainous areas and oil refineries, and the poor condition of the road network. For example, the annual energy costs of a Greek house at an altitude of 1000 m are 80% higher compared to equivalent costs at sea level [64]. All these factors lead to the over-exploitation of energy sources that are locally available, such as biomass, and decrease the use of other commercially available sources, which are usually cleaner and more efficient. A survey taken by Li et al. in 2016 studied the difference in energy consumption among the plains, semi-mountainous and mountainous areas of the Loess hilly region of China [71]. In the mountainous region the consumption of commercial energy decreased when farmers were able to self-produce their energy (i.e. straw and grass, biogas). The choice among the available
commercial sources (e.g. different types of fuel) was mainly dictated by their availability and price. Another survey taken by Cai and Jiang in 2007 led to a similar conclusion: when looking at Laoxiancheng, a mountainous village, biomass (fuelwood) was mainly exploited as energy source due to the difficulty of liquefied petroleum gas (LPG) and coal transportation, together with economic difficulties [72]. The shift to more efficient commercial sources is observed in more urbanized areas. In India, firewood consumption increases with altitude, going from 1.07 kg per capita per day at 500 m up to 2.80 kg per capita per day above 2000 m. Kerosene, LPG and cow dung are used almost only at low altitudes, accounting for 23% of the energy requirement up to 1000 m and less than 2% above 1500 m [63]. The over-exploitation of the available resources can exert a negative effect on the environment. Biomass for example does not normally undergo complete combustion, producing carbon black, which can accelerate the melting of the Hi-malayan glaciers and decrease crop production as it forms black clouds that reduce the amount of solar energy available for the land [73].

In other mountainous areas of the world, access to electricity can be higher. For instance, the Tajik Pamirs, in Tajikistan, is a mountainous region in which the largest part of the population (94%) has access to electricity, although its availability is strongly limited [74]. This area was under Soviet Union influence until 1991: until then, 96% of the energy supply was imported as fossil fuels [75]. With the acquisition of independence, this supply source was drastically reduced and thus the local population had to shift their way to produce energy [74]. Nowadays, 98% of the energy is obtained through biomass sources in the West part of the Pamir mountains, leading to high losses (up to 80%) of forest cover. Three types of villages were defined based on the survey carried out by Hoeck et al. in 2007: some of them have access to electricity supply thanks to both grid connection and the presence of mini hydropower stations (MHPSS) nearby; furthermore, they are rich in biomass resources. Then, there are villages receiving electricity from the MHPSS with low biomass availability, and finally there are villages in which there is no supply of energy and biomass resources are scarce [74]. Generally, however, energy availability in mountain areas is low. For instance, between 2011–2012, electricity represented the main energy source for only 12.3% of the buildings present in Phakding, Pangboche, Thamo, and Thame, four villages in the region of Khumbu (Nepal) located at different altitudes [76]. Indeed, electricity is accessible by only 3% of the rural population of Nepal [77]. In Ngari Prefecture, Tibet, the electricity is accessible by only 10% of the population, and the most used energy source is biomass, which due to the rarefied atmosphere shows a low efficiency upon burning [78]. An extensive use of biomass is also present in Zhatong city, China, where 96.6% of the area is mountainous. In the valley, coal is used to cover 66.8% of the households’ energy needs, whereas up in the high mountain firewood is covering 72.6% of them. Here, too, the excessive use of the forest’s wood is causing environmental deterioration of the area [79].

The Alps' case is different from the mountain ranges described above, as this mountain range is composed of first-world countries. To the best of our knowledge, there is no data on the lack of electrification in the Alps: according to data from the World Bank all countries included in the Alpine region have reached 100% electrification, although micro-settlements up in the mountains may be too little to influence this percentage. For what concerns energy generation, Eurac Research has published a report in 2017 concerning energy production and consumption in the EUSALP region; report that was updated in 2019 [80]. The latest revision of this report was compiled in 2023 based on 2021 data, this time in the form of a website at the URL www.alpine-energy-data.eu. The EUSALP is a region larger than the Alpine one, encompassing lowland areas of Italy, France, Germany, Austria and Slovenia. Looking at the data for purely mountainous regions of Italy and Austria, hydropower has the lion's share of energy production – with percentages above 85% and often above 90% – followed by solar photovoltaic and biomass with percentages between

5% and 8%; little energy is produced using fossil fuels. The big outlier is Switzerland, here hydropower is responsible for 55% of the country's energy production, followed by nuclear with 29% and solar with 4.4%. Even in the Swiss case, fossil fuels only amount to 2% of the total energy production.

## 2.2. Socio-economic challenges

A building's age is one of the most important physical determinants with regard to energy consumption and it is directly related to heating and cooling energy needs. This is due to the fact that old buildings consume more energy, as they do not have good insulation and efficient energy management systems [82–85]. Buildings in mountainous areas are usually old and so they have high energy needs, which in turn makes them more vulnerable to EP [86–89]. Furthermore, their heating is responsible for the emission of CO₂, which can be decreased with a more efficient thermal insulation of the building [90]. Karani et al. studied the phenomenon of EP in Agrafa, a mountainous municipality located in Greece [91]. This region was electrified only around 1980 [64], and it is still lacking a gas interconnection. According to the survey, 62.4% of the families in this region are classified as energy poor. In this area, 17.2% of the buildings were constructed before 1960, and 47.1% between the 60's and 80's. All of them do not have a thermal insulation system, and in 50% of the cases no action has been taken to increase the energy efficiency of the buildings. They are up to 22% smaller than newer houses (built after 2000), and the annual income of the households is lower when compared to that of people living in the newer buildings. Another study conducted by Katsoulakos in the mountainous area of Metsovo, Greece, highlighted how variable the energy performance is when comparing buildings made in different years: when built before 1985, 75% of them are not insulated and have outdated and not efficient window frames. In general, 114 of the 750 households present in Metsovo spend 24% of their total income for their energy needs, a value decreasing to 10.7% when referred to the overall area, which is still higher than the EP limit. Furthermore, the protection of the vernacular architecture set by the law is hindering the exploitation of solar energy in the area [92].

In addition to problematic geographical peculiarities, mountainous areas are also characterized by low industrialization and lack of investments, which in turn negatively affects household income (Fig. 2) [64,81]. Energy price is one of the major factors contributing to EP, as energy costs take a significant percentage of a household's income [93,94]. Moreover, mountain dwellers tend to use unsafe fuels and heating systems (e.g., combustion of biomass) to heat their homes, due to their financial problems and high fuel prices. Biomass combustion can achieve high performance and safety when coupled with modern wood boilers or enclosed and vented fireplaces, however in the large majority of mountainous areas it is used with low performance burning tools such as ancient wood boilers inside the house or in open fireplaces. This leads to environmental problems due to an increased emission of greenhouse gasses, and it poses a serious threat to human health that can lead to fatal accidents. For example, a case study in Northern Greece showed that a high percentage of households (78%) tend to improperly heat their dwellings due to financial problems, and many fatal accidents due to carbon monoxide poisoning have been recorded in poor households [70]. Another study conducted by Huboyo et al. in 2014 measured the PM2.5 and CO levels in the kitchens of coastal and mountainous dwellings of Indonesia, where biomass is used as cooking fuel. While CO levels were comparable between the two locations, PM2.5 levels were double in the mountainous area, revealing a higher risk of health problems [95]. Moreover, insufficient heating and living in cold houses has a physiological and psychological effect on vulnerable people suffering from EP, and may pose a significant risk to their physical and mental health (such as respiratory diseases, depression, etc.) [96,97].
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Energy vulnerability index qualitative indicators in Metsovo, Greece.</figcaption></figure>

*Source:* Reproduced from Ref. [81] with permission from Springer International Publishing AG, copyright 2021.

The problem of poverty can be exacerbated by humanitarian crises as well. In 2015, Nepal faced two heavy crises: an earthquake with a magnitude of 7.8 damaged 30% of the country's electricity infrastructure, and a shortage in the amount of fuel provided by India that affected the people's way of living. Since many of the other gas suppliers increased their selling price, a transition process started to increase the exploitation of the low price energy sources present in the region, such as biomass [98]. In Nepal, the consumption of energy in mountainous areas is mainly confined to the household sector: the heating and cooking processes generate pollutants, the exposure to which is higher in rural regions, where families often rely on low quality equipment to cook and buildings are less efficiently ventilated. The incomplete combustion of biomass together with the accumulation of smoke bring a high threat to health [99]. Residential wood burning, with its associated health issues, is common in the Alps as well. Concentrations of carbon black and particulate matter in air are especially high in areas where air tends to stagnate in the same place with little dispersion, where pollutant concentrations can reach levels typical of much more urbanized areas [100,101].

### 3. Climate change in mountains

Beginning with 1970, a sharp increase of the global warming rate has been noticed, owing to the rising concentrations of greenhouse gasses caused by anthropogenic emissions [103]. Climate change, along with its related consequences, is becoming more and more perceivable in our everyday lives. In the past few years, extreme meteorological phenomena are increasing in their frequency and intensity, and they are damaging the environment and the economy of the areas where they take place. A 2 °C global rise of average temperature is expected, but it will not be homogeneous throughout the earth's surface, as local and global factors, with their feedback effects, vary [104–107]. Mountains are going to be heavily affected by global warming, as they are particularly sensitive to it [55,108]. Temperatures in these areas are currently rising at a rate double than the average: strongest warming is registered in summer and spring (due to the reduction of snow cover and snow-related albedo), and daily minimum temperatures increase at

a faster rate compared to daily maximum temperatures (with an extent of increase that differs from mountain to mountain) [35,109–111]. The precipitation cycle will change during the 21st century [23]. An increase of precipitation in the North of Europe and a decrease in the South are predicted, with the Alpine region located at a transition zone of this shifting pattern [23,112]. Summer precipitation will decrease, especially in Southern regions, while winter precipitation will increase. Rainfalls will partially replace snowfalls, leading to a snow cover and depth reduction (which will modify the annual global radiation and relative humidity cycles, too). Extreme precipitation phenomena will become more frequent, with intensities that will gain up to +30% [113]. This is a key point for mountains, as when the quantity of water falling as precipitation is greater than 50 mm per day, the probability of floods and slopes instabilities increases up to 25% [114, 115]. All geomorphological processes, such as erosion, discharge, and sedimentation rates will be accelerated. The subsequent deposition of sediments on agricultural lands will cause a decrease of production. The melting of permafrost will increase the probability of rockfalls and landslides, which will release greenhouse gasses [116,117].

The rapid warming in mountain regions will lead to a reduction of snow mass up to 25% over the next 10 to 30 years, and the decline of the cryosphere is a phenomenon that has already been happening for the past few decades (Fig. 3) [102]. Glaciers are fragile changing elements [118]: their length and surface are continuously reducing, leading to the turning of glaciers into smaller ice masses, named glacierets [119–122]. Areas in which glaciers (or snow in general) are not present any more are recolonized by plants, leading to the so-called “greening phenomena” [123–125]. On a long-time scale, the glaciers retreat and disappearance will bring to a reduction in biodiversity [126, 127], with an impact to the economy, too [128]. Snow mass reduction is not yet visible from satellite, but “greening phenomena” are already observable in the Alps and in the mountains of central Asia [129–132]. Droughts will become more frequent, and snow decrease will bring to a reduction of thermal insulation, usually provided by the layer formed during winter, as well as of meltwater availability during the growing season [28,133]. Rumpf et al. investigated how snow cover and vegetation productivity has varied during the past 40 years in
<figure><img src="image_5.png" /></figure>

<caption>**Fig. 3.** Synthesis of observed physical changes and impacts on ecosystems and human systems and ecosystems services in eleven high mountain regions over past decades that can at least partly be attributed to changes in the cryosphere. Only observations documented in the scientific literature are shown, but impacts may also be experienced elsewhere.
*Source:* Reproduced from Ref. [102] with permission from Cambridge University Press, copyright 2022.</caption>

the European Alps (1984–2021) [28]. Their results showed that the decrease of the annual amount of snow was more pronounced than that in the summer period alone (9% against 4%). Snow depth at meteorological stations was decreasing, and greening occurred in 77% of the European Alps above the tree line, with a trend that was stronger than that related to snow cover loss. A feedback effect implies that continued greening will cause earlier snowmelt, resulting in greater snow cover losses [134,135].

Phenomena such as the 2003 heat wave will become more frequent: extreme summer temperatures and their associated droughts will have severe impacts on human health, agriculture, ecosystems, and hydrology [136,137]. If warming continues at the currently predicted rates, even more pronounced changes will be expected in all mountain areas. As life there strongly relies on temperature, the study of their ecosystems is of primary importance to promptly detect the potential consequences of climate change on them [2,138,139]. Aquatic ecosystems are also extremely sensitive to climate changes, as the temperature affects water chemistry (for instance, pH varies with temperature for several reasons: for example biological processes of production and decomposition are temperature-related, and dissociation of water into H⁺ and OH⁻ is higher at higher temperature) [48,140,141]. Long-term analysis of climate measurements is essential to improve the understanding of climate and validate the climate change scenario [142–144]. Warming is going to affect biodiversity (e.g., species extinctions, changes in the composition of assemblages), water resources (e.g., a decrease of the extent of glaciated areas and snow packs), and natural hazards (e.g., floods), but an accurate estimation of the effects of climate change in mountain systems is difficult due to uncertainties associated with the different climate scenarios (see below) and the existence of non-linear feedbacks between the various impacts [145].

Climate change can trigger several kinds of feedback effects on the ecosystem. Edaphic factors (physical, chemical, and biological properties of the soil that result from biologic and geologic phenomena or anthropogenic activities) can at least partially compensate for the changes: for instance, through the soil a positive or negative feedback loop can be induced [146,147]. Furthermore, the high topographic complexity of mountains modulates the impact of climate change on vegetation [148,149]. Complex topography may, for example, provide sheltered (e.g., cooler and more moist) sites where species can persist even though the general climate becomes unfavorable for them. Greening exerts a feedback effect, too, as more plants can sequebrate more atmospheric CO₂. This positive effect, however, will be overshadowed by the many negative ones [150]. For instance, greening phenomena result not only in an increase of productivity of indigenous plant species,

but also in compositional and functional changes of the vegetation and its associated biota [151]. This change could lead to large-scale structural changes across the Alps, impacting on water provision, economy, recreational activities, and landscape aesthetic value [152]. Climate change profoundly alters the composition and size structure of forests, however the topographic complexity and heterogeneity of mountains can mitigate these effects: qualitative and quantitative responses of mountain forests to climate warming are buffered and delayed, as they are for the biota living inside them [153–155].

Many studies and theoretical models attempt to predict climate change scenarios and their effects by relying on mid-term and long-term collected data [156–160]. Furthermore, several initiatives are working on providing useful information about climate scenarios in layman's terms based on data from regional climate services. Among them are Swiss CH2011 and CH2018 scenarios in the Western Alps and Styrian STMK12 scenarios in the Eastern Alps [107,161,162]. European Alps have one of the widest collection of data, as many mountain observatories have been working since the nineteenth century. The main problem, however, is that regional and synoptical air patterns influence the climate of high altitude basins in a not straightforward and intuitive way, therefore data extrapolation should be conducted carefully [144]. Sometimes, asynchronism is present among mountain and continental territories, which suggests that climate trends in the two regions may not be synchronized. For instance, maximum advances of mountain and continental glaciers are often asynchronous [163]. Furthermore, in certain cases (as in the Rocky Mountains between 1951–1996) warming at lower elevations may result in increased orographic precipitation at higher elevations (due to the increased percentage of water vapor in there), and hence greater snowfall, increase of albedo, and lower net radiation (due to the precipitation increase) [144]. The higher snow cover will result in the increase of cloud cover in summer months due to the higher level of humidity, thus reducing the direct shortwave radiation. In the short term, then, this positive feedback loop may occur, with a temperature decrease at high altitudes.

## 4. Renewable energies in mountain areas

As it has been described above, mountain areas face EP issues. This is not only due to economic issues, but also to geographical issues, such as remoteness, isolation, and scarce population density, which makes building large, centralized energy infrastructure non-profitable. Given these factors, building small, localized electricity networks powered by in loco renewable energy sources could solve the aforementioned

<footnote n="1">¹ includes Hindu Kush, Karakoram, Hengduan Shan, and Tien Shan; ² tropical Andes, Mexico, eastern Africa, and Indonesia;</footnote>

<footnote n="3">³ includes Finland, Norway, and Sweden; ⁴ includes adjacent areas in Yukon Territory and British Columbia, Canada;</footnote>

<footnote n="5">⁵ Migration refers to an increase or decrease in net migration, not to beneficial/adverse value.</footnote>
<caption>**Table 2**
Annual amount of energy available from RE sources in the Alpine region and its current production level.</caption>

*Source:* Reproduced from Ref. [164], released with CC-BY 4.0 license by Hastik et al. 2016.

<table><thead><tr><td>Energy source</td><td>Potential (TWh)</td><td>Current production (TWh)</td></tr></thead><tbody><tr><td>Forest biomass</td><td>74.7</td><td>70 (94%)</td></tr><tr><td>Hydropower</td><td>145</td><td>100 (69%)</td></tr><tr><td>Wind</td><td>56.4</td><td>4 (7%)</td></tr><tr><td>Solar photovoltaic</td><td>64.5</td><td>6.5 (10%)</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Forest cover changes in different Alpine regions. Black bars are relative increments of the latest survey compared to the first one (gray bars, normalized to the same length for all regions).</figcaption></figure>

*Source:* Reproduced from Ref. [168] with permission from Elsevier, copyright 2017.

geographical issues, while providing affordable energy prices to mountain users. Table 2 shows the example of REs use in the Alpine region, describing the energy potential for each kind of RE source and their currently exploited percentage. In the following sections, we will analyze in depth the deployment of solar photovoltaic panels in these regions, and we will provide an overview of the potential of other renewable energy sources.

## 4.1. Biomass

Biomass is a term that refers to all organic materials that originate from plants (including algae, trees and crops), which are used as fuel for heating or electricity production. Biomass is considered a form of renewable energy despite its CO₂ emissions because all the greenhouse gasses emitted were previously sequestered from the atmosphere during photosynthesis to produce plant matter. The CO₂ absorption/emission process is cyclical, and as such it is carbon neutral (i.e. it does not contribute to the rise of CO₂ concentration in the atmosphere) when taking into account the whole life cycle of the plant that is burned [165-167]. Mountain forests are a vital source of wood fuel, which can be exploited without excessive impacts. The 19th century has seen an increase in forest area and biomass in the Alps (see Fig. 4 and Table 3), which has increased the continuity and volume of biomass fuel in the region [168].

In addition to being burned directly, biomass can also be used to produce biogas and biohydrogen, which are energy carriers of growing interest in agricultural mountain areas. Indeed, biogas and biohydrogen are considered as important fuels due to their renewable, sustainable and stockable characteristics. Biogas is produced through the anaerobic digestion of organic materials such as agricultural waste and animal manure. Biohydrogen can be generated through the microbial fermentation of organic compounds [169]. Both processes rely on renewable feedstocks, making biogas and biohydrogen inherently sustainable and

contributing to the reduction of dependence on finite fossil fuel resources. In addition, coupling hydrogen and biogas with solar energy is highly complementary in terms of generation support or assistance in meeting heat, electricity, and low-carbon mobility needs.

There are two management strategies for the extraction of forest biomass from the Alps, since large parts of the territory are protected: removal of dead residues and harvesting of forest residues (such as branches and tops) after selective cutting [170,171]. The annual theoretical potential of biomass energy in the Alps is estimated to be about 60 TWh. However, almost 40% of the Alpine region is protected, imposing restrictions on biomass usage. These restrictions vary in different protected areas, with highly protected zones prohibiting extraction entirely, while sustainable management criteria allow for significant extraction in low protection areas [172,173]. Furthermore, forest biomass potentials across the Alps exhibit significant variation, influenced by factors including soil fertility, climatic conditions, and the complex composition of tree species [174]. Despite this variability, biomass energy is now one of the most utilized renewable energy sources within the Alpine region [164,175]. Biomass energy usage in Alpine regions has grown significantly due to the increasing need for renewable energy and the region's commitment to reducing greenhouse gas emissions. For example, in Switzerland wood fuels accounted for 4.9% (41 PJ) of the total final energy consumption in 2019, which was 836 PJ overall. This share increased to 5.3% in 2020 as total energy consumption dropped due to the pandemic [176]. In the Italian Alps, biomass energy production is expanding, particularly in regions like Lombardy and Piedmont. In Lombardy, biomass contributes to 871.7 MW of installed power, with 55 thermoelectric plants generating electricity from biomass [177]. In the French Alps, biomass is increasingly being considered a valuable resource, with forest chips emerging as a key fuel source. The region has over 1.7 million hectares of forest and significant potential for biomass energy, though its accessibility is variable. The French Alps are facing rising demand for fuelwood, with a focus on forest chips from thinning and residues. Despite the challenge of difficult terrain, efforts are underway to increase biomass extraction while ensuring sustainable forest management [178].

For biomass to reach its full potential as an energy source, there is a critical need for implementing sustainable practices. This would not only enhance the efficiency and sustainability of biomass energy production but also ensure the preservation of the Alpine ecosystems [179]. Extraction of forest biomass, in fact, may have a negative impact on habitat quality, wildlife and biodiversity even when limited to the collection of wood residues and deadwood removal. Deadwood and wood residues (e.g., standing snags, twigs, fallen logs, broken off tops and limbs, and stumps), in fact, play an important role on several levels: (i) logging residues attract animal species that lay eggs in the piles, (ii) soil disturbance affects algae and species that reproduce in vegetation, (iii) deadwood extraction leads to habitat fragmentation for dependent species, (iv) deadwood and wood residues provide food and cover resources for many species of wildlife [180-184]. On the other hand, one positive impact has been identified, which is the removal of stumps leads to increased seedlings of deciduous species [185]. Lastly, poor road conditions may limit access to woody biomass [186].

## 4.2. Hydropower

Rainfall and snowfall are very important climatic variables, from both environmental and economical perspectives. The Alps played an important role in the regional distribution of precipitation for the period 1971-1990, as precipitation volume was higher in low-lying regions adjacent to the Alps compared to other regions of the same height above sea level [187]. A complex orography influences the distribution of precipitation in the alpine system [188]. The inner parts of the Alps are drier compared to those located in the rims, and the driest conditions are found in valleys that are shadowed in the direction of moisture-bearing winds, in which annual precipitation values are
<caption>**Table 3**

Change of wood volume (live and dead) and species composition during the last decades.</caption>

*Source:* Reproduced form Ref. [168] with permission from Elsevier, copyright 2017.

<table><thead><tr><th rowspan="2">Region</th><th colspan="2">Live volume</th><th colspan="2">Dead volume</th><th colspan="3">Change species share per decade (%)</th></tr><tr><th>(m³ ha<sup>-1</sup>)<sup>a</sup></th><th>Change per decade (%)</th><th>(m³ ha<sup>-1</sup>)<sup>a</sup></th><th>Change per decade (%)</th><th>Spruce</th><th>Fir</th><th>Beech</th></tr></thead><tbody><tr><td>Vorarlberg (Austria)</td><td>411</td><td>6.1</td><td>11.1</td><td>50.7</td><td>1.9</td><td>-2.6</td><td>-0.3</td></tr><tr><td>Tyrol (Austria)</td><td>328</td><td>9.3</td><td>10.8</td><td>60.7</td><td>-0.6</td><td>0.2</td><td>0.1</td></tr><tr><td>Salzburg (Austria)</td><td>346</td><td>7.8</td><td>10.6</td><td>58.7</td><td>-1.1</td><td>0.0</td><td>0.0</td></tr><tr><td>Carinthia (Austria)</td><td>352</td><td>13.8</td><td>7.7</td><td>49.8</td><td>-0.1</td><td>-0.2</td><td>0.4</td></tr><tr><td>Styria (Austria)</td><td>352</td><td>9.3</td><td>8.6</td><td>47.5</td><td>-0.1</td><td>0.1</td><td>0.7</td></tr><tr><td>Liechtenstein</td><td>409</td><td>2.4</td><td>30</td><td>NA</td><td>-5.8</td><td>-3.8</td><td>2.5</td></tr><tr><td>Northern Swiss Alps</td><td>456</td><td>0.4</td><td>36.5</td><td>105.8</td><td>-2.1</td><td>0.6</td><td>0.5</td></tr><tr><td>Central Swiss Alps</td><td>347</td><td>7.7</td><td>32.3</td><td>39.3</td><td>-1.1</td><td>0.7</td><td>0.3</td></tr><tr><td>Southern Swiss Alps</td><td>278</td><td>17.1</td><td>22.5</td><td>59.1</td><td>-0.9</td><td>0.0</td><td>0.9</td></tr><tr><td>Northern French Alps</td><td>274</td><td>12.0</td><td>NA</td><td>NA</td><td>5.1</td><td>11.0</td><td>20.0</td></tr><tr><td>West. Slovenian Alps</td><td>313</td><td>9.9</td><td>13.9</td><td>NA</td><td>-0.8</td><td>-1.0</td><td>1.3</td></tr><tr><td>East. Slovenian Alps</td><td>352</td><td>16.9</td><td>36.1</td><td>NA</td><td>1.5</td><td>-2.6</td><td>0.6</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Large hydropower stations with capacities of more than 10 MW in the Alps.</figcaption></figure>

*Source:* Reproduced with permission from the Permanent Secretariat of the Alpine Convention from Ref. [191].

comparable to those of lowlands nearby [189]. Precipitation decreases in the Western Alps towards the South-West and in the central Eastern Alps towards the East. The overall amount of precipitation is highly variable; for example, in the period from 1971 to 2008 average annual values ranged from 400 up to 3000 mm per year [189]. Precipitation maxima are observed during summer in the inner parts of the Alps, while in the prealpine belt maxima are found in spring and autumn. Annual precipitation is about 500–600 mm in valleys parallel to the alpine range and about 1500–2000 mm in transverse-oriented valleys [190]. Climate change is going to affect precipitation patterns. Although it is not easy to predict the weather in the long term, prediction models show that the annual average precipitation throughout Europe is not going to change, but that summers will become dryer and extreme events will be more and more frequent [115].

Hydropower, also known as hydroelectric power, is the energy generated using natural water flowing downstream to drive turbines that in turn power generators to generate electricity. This type of RE has a high potential in mountains and it has been exploited in the Alps since the beginning of the 20th century, when many artificial reservoirs and channels were built and used to divert and regulate the flow of water streams in the area [192]. This mountain range is also the origin of four major rivers (Danube, Po, Rhine, Rhone) that can be used for hydropower generation [193]. The amount of energy that can be generated with hydropower is a function of water density, volume flow rate of the water stream, net hydraulic head and of the efficiency of the electric components of the power plant [194]. Volume flow rate and hydraulic head in particular are greatly dependent on

the geographical position in which the plant is installed. The volume flow rate mainly depends on precipitation, which is abundant and frequent in the Alps due to the high-altitude climate [195,196]. The hydraulic head is proportional to the height difference between a water reservoir and the electricity-generating turbine: given the presence of long slopes, mountains can generate large hydraulic heads. Therefore, mountainous regions are ideal for the development of both small and large hydroelectric power plants useful for local needs, due to the abundance of water and the presence of slopes, which facilitate the increase of hydraulic head and volume flow rate.

Hydropower is also the most exploitable renewable energy in the Himalayan region [197]: Bangladesh, India, Nepal and Pakistan have a hydroelectric potential of 334 GW [198], but only one third of it is currently exploited [199–201]. This is mostly due to the fact that the development of large hydropower projects requires high manufacturing costs due the remoteness of the region and its geological features; together with several economic, technical, and social problems that can be encountered (poor plant maintenance, lack of trained staff, damage to nearby households due to blasting activities, decrease of crop production in downstream areas due to the limited availability and quality of water, and vegetation disruption) [198]. Despite its low utilization with respect to its potential, hydropower is still the most used energy source in the region [202]. Among the eight Alpine countries, Germany, Austria, Italy, France, and Slovenia stand out for their substantial contributions to Europe’s hydropower production [203]. Germany takes the lead in installing hydropower plants within the Alpine region, closely followed by Austria, Italy, France, and Slovenia (Table 4) [204]. The Alps house approximately 550 large hydropower stations with outputs exceeding 10 MW, alongside numerous smaller stations [191] (Fig. 5).

As mentioned above, the Alps have experienced an increase in temperature that resulted in rapid snowmelt, and further warming is expected in the future. These high temperatures lead to the degradation of permafrost and thus to a significant change in the hydrological regime of water courses and flow rates. In particular, flow rates will increase during the melting of glaciers and then decrease dramatically once they disappear at high altitudes. As a result, the entire hydrological cycle will be severely affected, with consequences for evaporation and precipitation that will have significant impacts on the seasonality of flows, which may cause problems for hydropower plants [205–209]. In addition, there are strong signs that the availability of water resources in the Alps will decrease significantly in the future [210,211]. In this context, efforts to mitigate the effects of climate change on the mountain water cycle are seeing the development of initiatives to create water reservoirs, the use of which can generate conflicts of use: as a stockpile for making artificial snow, as a stockpile for feeding humans and animals, for cultivation, etc. To this multitude of uses should be added the possibility of using these storage volumes as micro-steps to

<footnote><sup>a</sup> Assessed at the last available inventory.</footnote>
<caption>**Table 4**
Hydropower production in Alpine areas.</caption>

*Source:* Reproduced from Ref. [204] with permission from Elsevier, copyright 2019.

<table><thead><tr><th>Country</th><th>Plants in 2011</th><th>Plants in 2007</th><th>Capacity in 2011 (MW)</th><th>Production (GWh a<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Germany</td><td>7512</td><td>7503</td><td>1723</td><td>8352</td></tr><tr><td>Austria</td><td>2993</td><td>2354</td><td>1284</td><td>5778</td></tr><tr><td>Italy</td><td>2601</td><td>1835</td><td>2896</td><td>10958</td></tr><tr><td>France</td><td>1935</td><td>1825</td><td>2110</td><td>6820</td></tr><tr><td>Slovenia</td><td>471</td><td>456</td><td>118</td><td>587</td></tr></tbody></table>

store energy in the form of potential energy. To avoid wasting water for this purpose, the combination of solar photovoltaic (floating or not) and small-scale pumped-hydroelectric storage units makes perfect sense [212]. Apart from possible future issues due to climate change, there are side effects that may result from the use of hydroelectric power: the power plants and the use of dams can increase flood risk due to the alteration of water flow in the rivers, lead to modification of water quality, and cause a loss of biodiversity and introduce barriers for fish migration [213–215].

### 4.3. Wind

All mountain regions are characterized by local valley and mountain breezes, which alternate daily in day and night cycles (Fig. 6). At dawn, mountain slopes are quickly heated up by the sun, while valleys remain cooler for longer. This temperature gradient generates a convection current, leading to upslope breezes from the valley to the mountain. At night, conversely, slopes get colder faster due to higher thermal radiation and the resulting higher-density air flows toward the valley, generating a downslope breeze [86,216]. During persistent periods of high pressure, however, cold air accumulates at the bottom of valleys, as the amount of energy lost by thermal radiation becomes important. The colder air stagnates at the bottom due to the constraining effects of valley boundaries, and temperatures are higher at greater altitudes. A temperature inversion is generated, which usually does not exceed 1000 m of height [59]. Mountain and valley breezes are of key importance: they influence the air quality in industrialized valleys, helping to disperse pollutants. Given their relative constancy and strength, they are also an excellent source of wind energy, especially along mountain ridges.

Wind turbines convert wind's kinetic energy into mechanical (and subsequently electrical) power. The energy converter, named propeller, which converts kinetic energy into mechanical work, is the most essential part of a wind turbine [217]. Overall, wind turbines are comprised of three parts: a hub with sizable blades, a gearbox, and a tower with a rotor yaw mechanism. They can be classified according to multiple factors, such as turbine technology (onshore or offshore), application type (standalone or grid-connected), rotation speed (fixed or variable), orientation of the turbine (horizontal or vertical axis), output power, and type of generator (synchronous, asynchronous, or DC generator) [218]. Depending on the rotor axis, horizontal or vertical axis wind turbines can be found. Both can operate at different wind speeds; the former type is that most widely used as it is characterized by higher performances, while the advantages of the latter are that it can operate regardless of wind direction, it can better handle turbulence, and it allows to reach a higher density of installations per unit surface [219]. According to Betz's law, which describes the principle of extracting mechanical work from airflow, the power that wind can exert on a wind propeller depends linearly on air density, quadratically on the propeller's blades length, and cubically on wind speed [220]. Air density depends on many factors and it is subject to large local variations. Despite this, it is a function of both air temperature and pressure, and therefore it decreases with increasing altitude [221]. The decrease in air density is counterbalanced by an increase in wind speed [222], which is mainly due by the reduced friction between the earth's surface and the movement of air in the free atmosphere [216]. The net result is that the

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** The flow of air in mountain and valley breezes.</figcaption></figure>

maximum extractable power from wind increases more or less linearly with altitude [223,224]. Winds do not flow equally throughout the Alps, and the Northern part of the mountain range has more potential than the Southern part [223]. In the Austrian Alps the best location for wind turbines is along mountain ridges, which are swept by winds of 7 m s⁻¹ average velocity, for a potential of 1600–2900 kWh m⁻² a⁻¹ of energy production [223]. Turbine siting was also found to be important in the Swiss Alps, with energy generation capabilities that can vary between 6 and 13.3 TWh a⁻¹ depending on where turbines are placed [225].

There are shortcomings regarding the installation of wind turbines in mountain areas. From a technical point of view, running propellers in cold conditions and ice formation on propeller's blades is a serious
concern in terms of both turbine performance and safety [226,227]. Barber et al. compared the annual energy production of wind turbines at two different sites: the Gütsch Alpine Test Site at an altitude of 2330 m (“non-extreme” icing), and the Bern Jura Test Site at a lower altitude of 800–1500 m (“extreme” icing) [228]. Compared to predictions from theoretical models, icing can cause a reduction of up to 2% in annual energy production at the Gütsch Alpine Test Site and of up to 17% at the Bern Jura Test Site. From an ecological perspective, wind farms cause (albeit small) habitat loss (e.g., birds, bats, etc.) [229–231]. Finally, for what concerns social aspects, wind turbines can affect tourism and people’s willingness to accept such projects, which can alter the natural beauty of landscapes or cause noise disturbance [232–234].

## 4.4. Solar photovoltaic

As seen in Table 2 for the Alps case, solar energy is still quite underdeveloped in mountain areas. Given the fact that the sun is the largest renewable energy source on the earth surface, photovoltaics (PV) could provide a significant contribution to the energy requirements of local populations, especially as solar panels can be more performing at higher altitudes than in lowlands [235–238]. Apart from the quality of the PV cells it is composed of, in fact, the efficiency and energy output of a solar panel are also dependent on geographical factors: latitude, altitude, climate, and topography. Before analyzing how these geographical factors influence energy generation, it is important to review the different ways in which sunlight can reach a solar panel: it can be direct (in a sunny day), diffuse (e.g. when passing through clouds), or *via* albedo (reflected from the ground), as shown in Fig. 7 [239]. The current main technology of solar panels – silicon – is most effective when receiving direct light orthogonally with respect to its surface [240–242]. Latitude determines the amount of sunlight a certain area can receive throughout the year, and the average incident angle of sun rays [243]. Altitude also affects the amount of direct sun rays that can reach a panel, as at higher altitudes air is thinner and usually cleaner, leading to a lower amount of light scattering derived by aerosols and particle suspended in the atmosphere [244–248]. Higher locations are also unconcerned with low-lying clouds (thus reducing the amount of cloudy weather) and colder, which improves the efficiency of solar panels [235,249,250]. Climate affects air temperature and the amount of clouds, which both affect the energy output of the panel. Finally, topography is important for the albedo: this is especially true in mountain areas, as their slopes are covered in snow in winter times, significantly increasing the amount of sunlight reflected by the ground [245,247,251,252].

The benefits of high altitudes for PV panels have been known for a long time: almost 30 years ago Bergauer-Culver and Jäger were simulating the operation of PV plants in locations relatively close to each other but with very different altitudes within the Austrian Alps [235]. Their work showed how plants higher up in the mountains were more productive especially in winter months compared to their lower counterparts, and how overall the energy yield increased almost linearly with altitude. These results were confirmed experimentally in 2019 by Karpić et al. although with a very small, custom system [236]. Despite a general consensus that PV energy generation is higher at higher altitudes compared to lowlands, a study from Ferry et al. showed how the situation in the South of France may be different, as lowland villages seem to have a higher potential than their mountainous counterparts [253]. Nevertheless, one aspect of major importance should be taken into account: due to all the aforementioned peculiarities of mountain topography, PV panels installed at high altitudes are especially efficient in producing energy during winter months, especially when compared to lowland territories. This higher winter production can help to mitigate the energy gap during these months, when energy demand is highest [254].

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Different ways in which sunlight can reach a solar panel.
*Source:* Reproduced from Ref. [239] with permission from Springer International Publishing AG, copyright 2019.</figcaption></figure>

Thanks to the flexibility of the installation of solar panels – which can be mounted on buildings, on the ground, or on water surfaces in both small and big plants – this technology is particularly suited for rough and impervious areas such as mountains, as it can be tailored to fit the space available. For this reason, there are many studies that simulate and predict the performance of PV systems at high altitude, as well as reports on the actual performance of existing installations, in both developed and emerging countries. As one of the largest mountain ranges in the world, at a latitude with high solar radiation levels, and with a population of 53 million people [255], the Himalayas are a prominent location for the deployment of photovoltaic plants. Ramachandra et al. have investigated the solar radiation potential of the Indian region of Himachal Pradesh using satellite data, calculating an average global horizontal insolation value of 4.5 kWh m⁻² per day [256]. Sinha and Chandel have done a similar study for combined PV-wind power systems [257]. Chaudhary and coworkers estimated the performance of different technologies of solar panels in the various climate conditions of India, showing that the Himalayan region of India has the best PV potential in the country [258]. Wang et al. in 2009 have analyzed the state of the art of solar exploitation in Tibet [78]. Solar energy in the region was mainly used for heating purposes, although 9 MW of PV plants existed. Given the local population’s sparseness, the authors suggest that small PV installations can provide reliable electricity in the area. They also conclude that the main obstacles to PV penetration in the region are the high hydropower potential, and the lack of local talents for the deployment of PV panels. Not only Tibet, but the Himalayan region in general is composed of developing countries with a high level of electricity-poor population (and general poverty) and a lack of a well-developed national grid. From a technical point of view, then, micro-grids disconnected from the national network and stand-alone buildings are mostly being built here [259]. From an economic point of view, the presence of subsidies and a careful intervention of the central government is paramount for the successful penetration of solar energy in these countries. A prime example of this is the collaborative effort between Grameen Shakti and Infrastructure Development Company Limited to deploy Solar Home Systems (SHS) in Bangladesh *via* micro-financing. For more than a decade this program has helped the rural, off-grid population in Bangladesh reduce their dependency from kerosene by installing PV system in their houses, with the benefits of increased children study time, women’s empowerment, and costs savings, with good users’ satisfaction [260]. Studies were also made to introduce a similar system in nearby countries [261]. In later years, however, poor decisions from both the funding agency and the national government have led to an almost complete collapse of this previously very successful program [262]. On a smaller scale, at the cross between actual usefulness and promotion of the PV technology,
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** View from above of Rubí, the largest PV plant in Peru.
*Source:* Reproduced from Ref. [270], released with CC-BY 4.0
license by Cacciuttolo et al. 2024.</figcaption></figure>

Singh and coworkers have designed a solar tree for the mountainous regions of India [263].

The Andean region is facing issues similar as those of Himalaya, with several communities that are isolated, energetically poor, and economically poor [264–267]. A large portion of the Andes, however, is located in a part of the world where the Global Horizontal Irradiance is very high, and several studies have been conducted to predict the exploitation of the solar resource in Chile [268,269], Peru [270], Ecuador [271], Argentina [272], Bolivia [273], and Colombia [274]. A distinction should be made between large, grid-connected power plants [270,275] (Fig. 8), and off-grid systems for local communities. In the latter case, oftentimes the difference for the local population is between having or not having access to electricity. Despite this, not all deployment projects are successful. On this regard, of particular interest is the analysis of Feron et al. who have studied successful and failed off-grid projects in Peru, Ecuador, and Chile [265]. In their article they show how technological aspects are tightly interwoven with social aspects for a successful implementation of a community-wide off-grid project. This means that it is not sufficient to select good materials for the project deployment and provide maintenance for it, but that the involvement and empowerment of final users, together with stable and sane government decisions, is crucial for the success of off-grid rural electrification.

The involvement and empowerment of the local population is not important only in poor communities of developing countries, but in more educated, rich and developed countries as well. Although the deployment of REs and solar in particular is paramount to meet countries' climate goals, in fact, they have a significant impact on the land and the landscape where they are deployed, sparking frictions with the local population if not properly explained [276]. Despite these frictions, a study conducted in Switzerland by Vuichard and co-workers shows how the local population is overall in favor of the installation of PV plants (also of large sizes) high up in the mountain [277]. Their results show that the population living close to where a PV plant would be seated is even more in favor of it than the national average, however they also reinforce the fact that the involvement of the local population is paramount for a successful project, as well as the implementation of mitigation elements that would make the PV plant less noticeable (e.g., the use of green-colored panels). There is also high social acceptance for placement of photovoltaic panels in situations where there

is already an alteration of the landscape, such as replacements for avalanche barriers [278]. For this last particular application scenario, Díaz and van Vliet have conducted an in-depth study about public acceptance and society's concerns [279]. The importance of installing PV panels on the ground and at high altitude instead of limiting oneself to rooftops is highlighted by Ratnaweera et al. who show that these areas have a much higher potential compared to residential rooftops both is absolute value and in energy generation capacity per square meter [280]. Moser and co-workers in 2013 were already trying to assess the PV potential of the South Tyrol region in Italy, including that of non-conventional surfaces such as transport infrastructure and artificial lakes, to reduce the amount of ground-deployed PV [281]. Despite having remarked the importance of the social aspect so far, Dujardin et al. took a purely economic view in their assessment of the feasibility of PV installation in the Swiss mountains [282]. Their calculations, using two different models, predict that revenues for Alpine PV plants can be 20% higher compared to urban installations, percentage that increases to 33% for the first GW of installed capacity in Switzerland. Walch and Rüdisüli on the other hand focus on regional equality, working with only rooftop installations and trying to either minimize the amount of panels needed to reach Swiss' goal of 35 Twh PV expansion target by 2050 for a fast integration, or to maximize regional equity by making as many districts self-sufficient from an energy generation point of view [283]. They conclude that in the first scenario only the best 46% of the country's rooftops are needed to achieve the goal. However, this scenario also generates a large surplus of energy supply in certain regions of the countries during part of the year, which could pose a strain on the local grid, while allowing only 40% of the districts to reach self-sufficiency. On the other hand, the maximization of equality could greatly reduce these local surpluses while allowing more than 70% of the districts to reach self-sufficiency, at the cost of installing PV systems on 86% of the country's roofs.

Most of the discussion on the application of PV in the Alps so far has been based on calculations, models, and predictions; based on irradiance data and some other reasonable assumptions. Fortunately, the Alps have been hosting PV installations for many years, so it is possible to analyze real data of PV systems working through the years. At the research station of Jungfraujoch, in the Swiss Alps, there is a small grid-connected PV plant of 1.13 kWp in operation since fall 1993 at a height of 3454 m. In 2004 Prof. Häberlin reported on the first 10 year of operation of the power plant: he reported that there were no interruptions of electricity supply throughout all the period, and that only one panel showed signs of degradation (delamination) and it was replaced while still perfectly working [284]. Also for what concerns the other electrical equipment of the plant, there were only minor issues due to faults. The plant produced more energy with much smaller summer/winter variations compared to two other Swiss plants located at a lower altitude. In 2014 the plant was more than doubled in size with new panels [285]. Although there are no other reports of the performance of this power plant, a report from 2021 indicates that it is still in operation and producing data [286]. Overall, the PV laboratory at the Bern University of Applied Sciences monitors several PV plants in Switzerland, although not much data is available for them [287]. Carigiet et al. installed and monitored for two years several mono- and bifacial panels at a location at 2500 m of altitude in the Swiss Alps with different inclinations of 30°, 70° and 90°, and compared them to a reference installation in the urban region of Zurich with a 30° inclination [238]. The energy yields of the panels at high altitude were higher compared to the urban ones in all cases, in the order 70° > 30° > 90°. The 90° panels in particular had an energy generation similar to those in the urban region, but with a higher percentage in the winter months. Losses due to snow cover were minimal even for the panels with 30° inclination. The bifacial modules were 13% to 55% better performing compared to their monofacial counterparts depending on the time of the year. Egger et al. also reported that a PV test site in Pitztaler Gletscher, Austria, at 3000 m, had an average yearly energy
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Examples of integrated PV installations on isolated buildings in the Alps. From top left to bottom right: Doragno Castle (reproduced with permission from Progetto deltaZERO, photo Luciano Carugo), eV-Chalet (reproduced with permission from GruppoSTG Fabbrica Srl), Cable car station of Naturno (reproduced with permission from Eurac Research), Chalet La Pedevilla (reproduced with permission from Pedevilla Architects, photo Alessandra Ianniello), Monte Rosa Hut (reproduced with permission from Earth & Deplazes Architekten AG: Valentin Bearth, Andrea Deplazes, Daniel Ladner; photo Tonatiuh Ambrosetti), and Útia da Ju (reproduced with permission from the owner). Photos taken from the Integrated PV website of Eurac Research [291].</figcaption></figure>

generation production about 25% higher than a PV test site in Inzig, Austria, at 625 m [288]. Frischholz et al. compared a PV test site (the one previously mentioned from Carigiet et al.) to two utility-scale plants – one at a higher altitude and one in the Swiss Midlands [289]. The Alpine plants outperformed the Midlands plant both in terms of annual energy production and especially in terms of energy production during winter times. The authors also report that the Alpine utility-scale plant is consistently less efficient (of about 8%) compared to the test site, and they attribute this to a suboptimal orientation of the panels due to the building constraints imposed by the dam wall. There are other PV test sites at high and very high altitude both in the Alps and in the rest of the world; the PV Resources website has an overview of them in their “PV Power Plants in High Altitudes” webpage [290].

We have seen how solar photovoltaic deployed high up in the mountains could provide a significant contribution to the energy needs of villages, cities, and countries located near these regions. In addition to this, there is a specific scenario in which PV (with adequate storage) outperforms all other REs, and it is the electrification of isolated buildings at high altitude, such as mountain refuges, huts, and ski resorts. In this case, in fact, the flexibility in the installation of PV panels makes this technology preferable to hydropower, which is not available anywhere, and of wind, which does not work well in micro plants. The difficulty in carrying biomass and fossil fuels to these remote sites makes PV a better alternative also compared to burning fuels. Forget and Ayroles comment on a number of these projects for self-sufficiency, some with good and others with bad results [292]. In general, the failed projects involved the presence of multiple people or multiple entities in the decision-making process, which led to disagreements on how to proceed. Eurac Research, a research center based in Bolzano, Italy, on its Integrated PV website hosts a number of examples of isolated buildings now powered by solar photovoltaics; namely: Doragno Castle [293], eV-Chalet [294], Cable car station of Naturno [295], Chalet La Pedevilla [296], Monte Rosa Hut [297], and Útia da Ju [298] (Fig. 9 [291]).

As with all the other RE technologies, there are downsides in the deployment of PV in mountain regions as well, although a distinction should be made between roof- and ground-mounted PV plants. Contrary to all other technologies, in fact, there are no downsides to the installation of solar panels on existing building roofs, at least from an environmental perspective [180,299]. When the panels are ground-mounted, however, there is competition for land use, with all its associated problems. We have already mentioned above the social

issues that arise, mostly related to aesthetic concerns. From a purely natural point of view, possible concerns are the alteration of habitats and microclimatic changes due to shading effects and reflections, which can change the plant community. However, the presence of mounted panel does not appear to be a barrier to the movement of wildlife [180].

### 4.5. Solar thermal

Photovoltaic cells are increasingly being installed in mountain areas, in refuges for instance, but there is also a demand for low-temperature thermal energy, which could be met by solar thermal systems. Solar thermal systems provide a clean and cost-effective alternative to conventional heating methods and refer to harnessing the sun's energy to generate thermal energy, which can be used for various applications [300–302]. Unlike PV systems that convert sunlight directly into electricity, solar thermal systems utilize sunlight to heat water, oil, air, or other fluids. Solar thermal systems find application in a wide range of sectors, including residential, commercial, and industrial settings. Common applications include water heating, space heating, air conditioning, and industrial processes such as drying and desalination [303]. The system can be characterized by natural or forced circulation and solar thermal energy can be effectively utilized in mountainous regions, where the availability of sunlight can be harnessed despite the challenging terrain and climate conditions.

In practice, conventional thermal systems are rarely used in the mountains due to several reasons: the availability and reliability of electrical energy for pumps and controls are often not guaranteed, extremely low temperatures can negatively impact pump seals, and control devices are highly susceptible to damage during storms. Solar thermal systems can provide hot water for remote mountain cabins and lodges where flat plate or tube collectors can be installed on rooftops or nearby sunny slopes. Spontaneous heat transport systems have been installed for snow melting and for sanitary hot water production [304]. The first example was installed in a refuge at an altitude of 1570 m above sea level and after this first trial in another refuge at an altitude of 3650 m. The system, as demonstrated in some years of operation, can satisfy any low-temperature heat requirement. Up to now, it has been used for snow melting, for the production of hot sanitary water, and for heating a sewage treatment plant. During out-of-season periods the system can also be used to keep frost out of the winter-room present in mountain refuges. The advantage of maintenance-free operation of this system makes it ideal for mountain applications.
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Ragone plot of several storage technologies.
*Source:* Reproduced from Ref. [306], released with CC-BY 4.0 license by Ben Elghali et al. 2018.</figcaption></figure>

# 5. Energy management

As already mentioned above, mountainous regions are not always connected to the grid: diesel generators have been a widespread solution to provide energy. With the transition to more sustainable, renewable energy sources, the flexibility of power on demand of the generators is lost, as energy production is tied to external, natural factors. For this reason, it is paramount to couple RE sources to energy storage systems, in order to supply electricity in a stable manner even when production is lower than demand. Even for areas connected to the main electricity grid, integration of diffused renewable energy plants needs to be implemented carefully, to avoid grid issues. Finally, renewable energies allow the formation of microgrids, where the energy produced can be shared across a local community rather than just a single dwelling, but without connection to the main grid.

## 5.1. Storage

There are several technologies employed in energy storage; there is not one technology better than the others, but different technologies are more appropriate for different applications. From a performance point of view, the parameters to take into account are the specific energy/energy density, and the specific power/power density. The first one measures the amount of energy that can be stored, while the second one the amount of power that can be delivered, both per unit weight (specific) or volume (density). The various storage technologies can be placed together in a single plot that shows their performance in terms of these two parameters: the Ragone plot [305] (Fig. 10 [306]). The Ragone plot is very useful to evaluate technologies where energy and power are coupled, while it is more difficult to place in the plot technologies for which these two parameters are decoupled, such as redox flow batteries or pumped hydro. Energy and power densities are especially relevant when size and weight are important (e.g., for mobility applications), but for static and/or large energy storage needs more important are the price of the unit of energy stored, the use of materials that are available in large quantities on earth, the flexibility of installation, the ability to store energy for a long period of time, and the lifetime of the storage device.

Depending on the power operation range of the plant and its geographical location, one storage method can be better suited than others. On the large scale in the 100 MWh–100 GWh range, pumped hydro or compressed air are preferred for their ability to deliver high power, even if they are characterized by a low density of energy storage both per volume and mass [307,308]. In the case of 1 MWh, flywheels [309], supercapacitors, superconducting coils, or batteries represent a better solution [310,311]. While flywheels and supercapacitors are preferred when high power and low energy density are required, batteries are used in high energy and medium power applications. Lastly, hydrogen

storage is gaining more and more interest to stock energy for its cleanliness and its cost efficiency, especially when seasonal storage is required [312–314]. Table 5 provides a summary of the various storage technologies, with their main advantages and disadvantages. All of them have been implemented in micro-grid systems, as a single storage method or combined with each other. Reviews on their applications have been published [315,316], and several cost analyses (levelized cost of storage) were carried out [317–319].

Remote Area Power Supply systems rapidly spread on the market for small-scale systems in the kW range. They are traditionally based on photovoltaic or wind input and employ diesel generator backup and/or battery storage to fill the gaps left by the RE sources [320]. However, as the supply fluctuations of renewable energies can be in a very long time scale (up to months), these technologies are not economically valuable for off-grid applications, as the use of fuel-based generators is increasing the overall cost of the system, especially when located in remote areas. On the other hand, it can be really expensive to extend the electricity grid, even in isolated but not rural areas: for instance, in the North of Australia more than 100 communities are not connected to the grid, and the cost of its extension was estimated to be around ~0.6 million Euro per kilometer in 2011 [321]. There is a lot of research conducted on stand-alone power supply systems exploiting different storage technologies, as they stand as a more convenient alternative than grid extension in many remote areas. The use of lithium batteries for short-term applications along with hydrogen storage to address seasonal variations in energy supply represents a solution that is more expensive at the stage of initial investment, but cheaper in the long term and suitable for off-grid applications. This is especially true when two components of the system, namely the electrolyzer and the fuel cell, which will be described later, are coupled in a single unit in the unitized regenerative fuel cell, a solution meant to decrease the cost associated to the hydrogen storage [322,323]. These storage-guaranteed systems are suitable for small installations, farms and communities not connected to the grid, as well as for military and humanitarian purposes.

### 5.1.1. Batteries

Although they have historically been the first kind of energy storage systems and their price is projected to decrease, batteries rely on materials whose availability is relatively scarce, and it is projected that their use on a large scale will not be possible for more than 50 years [324]. Furthermore, they can store electrical energy only for a relatively short period (perhaps a few days) without the need to be topped-up, and they require replacement after a few years. Among the many types of existing batteries, the Li-ion cell technology is nowadays the most used thanks to its high energy density and long-life cycles [325–327]. However, these batteries also have several drawbacks. For example, the exposure to high temperatures can lead to swelling, increase of impedance, and increased speed of degradation [328]. Short-circuits can greatly increase the battery's surface temperature, leading to a remote possibility of thermal runaway [329], while over-discharge and overcharge can decrease their performances [326,330]. From a commercialization perspective, they are subjected to regulations on their transport (especially air transport). Li-ion storage has been used coupled with photovoltaic panels to provide electricity to the highest lodge in Taiwan for more than seven years, allowing a deep understanding of the main problems associated with its use on a long time scale [331]. The study revealed that in mountainous areas the use of this type of batteries is more economical and environmentally friendly than the use of lead batteries, which are suffering from leaks and thus represent a potential source of contamination for the environment. This latter kind of batteries has been used as energy storage for a long time as well: they have a lower energy density compared to Li-ion batteries, but higher safety, lower cost and higher sustainability [332]. Recycling of the lead acid batteries has been efficiently implemented, whereas lithium batteries still suffer from a low recycling rate [333].
<table><caption>**Table 5**
Comparison of different energy storage technologies.</caption><thead><tr><th>Technology</th><th>Scale of application</th><th>Advantages</th><th>Disadvantages</th></tr></thead><tbody><tr><td>Pumped hydro</td><td>100 MWh–100 GWh</td><td>Large scale<br>High power<br>Long lifetime<br>Long storage period</td><td>Low energy density<br>High manufacturing cost<br>Favorable geography required</td></tr><tr><td>Compressed air</td><td>100 MWh–100 GWh</td><td>Large scale<br>High power<br>Long lifetime<br>High energy efficiency</td><td>Low energy density<br>High manufacturing cost<br>Favorable geography required</td></tr><tr><td>Flywheel</td><td>1 MWh</td><td>High power<br>Long lifetime<br>High energy efficiency<br>Low maintenance</td><td>Low energy density</td></tr><tr><td>Superconducting coil</td><td>1 MWh</td><td>High power<br>High energy efficiency</td><td>Low energy density<br>High cost</td></tr><tr><td>Supercapacitor</td><td>1 MWh–1 kWh</td><td>High power<br>Fast charging<br>High cycling efficiency</td><td>Low energy density</td></tr><tr><td>Battery</td><td>1 MWh–1 kWh</td><td>High energy density<br>High energy efficiency</td><td>Medium power<br>Availability of raw materials<br>Toxicity</td></tr><tr><td>Hydrogen storage</td><td>Variable</td><td>Long term/seasonal storage<br>Clean (if green hydrogen)</td><td>High cost and complexity<br>Low round trip efficiency</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Scheme of a redox flow battery.
Source: Reproduced from Ref. [334] with permission from the Royal Society of Chemistry, copyright 2021.</figcaption></figure>

Redox-flow batteries represent a promising technology for energy storage [334]. The key advantage of this battery technology is the decoupling of power and energy. Power is provided by the reduction and oxidation reactions of the electrolyte that are taking place in two half cells, which are separated by an ion exchange membrane. The generated electrons flow through an external circuit, generating electricity: the bigger the half cell area, the higher the power. Two storage tanks of liquid electrolytes in either reduced or oxidized state, which represent the amount of energy a battery can store, are connected to the two half cells: the larger the tanks, the higher the energy (Fig. 11). Several electrolyte couples and cell designs have been studied [335–337]. Independent power and energy sizing, an efficient control of the cell’s temperature allowing it to operate in the best conditions, and absence of self-discharge are some of the advantages related to their use, while the relatively high dimensions and decrease of efficiencies due to shunt currents are some of the drawbacks [338].

### 5.1.2. Mechanical storage

Both pumped hydro and compressed air technologies require specific geological and geographical features, which limit the suitable sites in which these technologies are cost effective. Pumped hydro storage is well known and widely adopted [339], with a long system lifetime, which in turn makes it convenient despite the high cost associated with its construction [317]. This system is classically composed of a pump and a turbine, along with water reservoirs at higher and lower altitudes (Fig. 12). This technology is well suited and convenient for large power capacities (up to 2000 MW). The storage period can be of years, and between 71% to 85% of the energy used to pump the water in the elevated reservoir can be recovered during the stage of energy production [315]. Its use is profitable in many complementary services [340]. Several approaches have been developed to minimize the drawbacks of this storage system: e.g., the use of off-stream systems or waste water to minimize the impact on biodiversity [341], or the use of the naturally pressurized waters of oceans or lake depths [342,343]. Similar techniques have been developed, which exploit the movement of solid masses. For instance, the Underground Pumped Hydroelectric Storage, in which a mass of soil is lifted thanks to the water filling of a reservoir beneath it. This system can give comparable efficiencies to the classical one, and can be used when two water reservoirs at different heights are not available [344,345]. In gravity storage, a solid mass can be lifted during the charging stage of the battery, and subsequently released to allow the generation of electricity when need. Several systems have been developed to do so, which differ in their architecture. They have been described and compared by Tong et al. in 2022 [346]. On some of them, simulations were carried out to optimize the key parameters of the system and thus its efficiency [347].

The compressed air storage system consists of a compressor, turbines, a container for the compressed air (e.g., rock caverns), a motor that can engage with either the compressor or the turbine, and other auxiliary components. During a low electricity demand period air is compressed (4–8 MPa) using the excess of energy, then when energy is needed the air previously compressed is heated and expanded through a high pressure turbine, and subsequently mixed with fuel and burnt through a low pressure turbine. These turbines are connected to the grid and produce energy. The efficiency of this system is about 80% over one year due to its low losses [315]. The first systems that have been built are located in Huntorf, Germany, which has been in operation since 1978, with a storage capacity of around 310,000 m³,
<figure><img src="image_14.png" /><figcaption>Fig. 12. Conceptual scheme of a pumped hydro energy storage system connected to a photovoltaic power plant.</figcaption></figure>

Source: Reproduced from Ref. [307] with permission from Elsevier, copyright 2015.

and in McIntosh, Alabama, USA, built in 1991, with a storage capacity of 500,000 m³ [315]. Pumped hydro and compressed air systems can be coupled together, even if an increase of the system complexity has to be taken into account and evaluated [348].

Flywheels are a clean technology characterized by high power density, long lifetime, high efficiency and no special need for maintenance. The energy is stored through the angular momentum of a spinning mass. During the charging stage a motor is rotating the flywheel, whereas during the discharging stage the motor acts as a generator of energy from the rotational energy of the flywheel. The efficiency of flywheels is about 90%–95%, and are normally used when high power for a short time is required [309,315]

### 5.1.3. *Hydrogen storage*

When used directly as a burning fuel, hydrogen represents an alternative to fossil fuels: its conversion into energy is more efficient than that of other fossil fuels and the only dangerous species originated by its combustion is $NO_x$ [349]. However, hydrogen is usually not burned directly, but its energy is exploited electrochemically inside fuel cells [350]. It can be produced during off peak hours from renewable energy plants, through carbon free resources, satisfying the increasing energy needs along with the necessity to move to “carbon neutral” technologies to decrease the $CO_2$ emissions linked to the extensive use of the fossil fuels [351]. Once produced, it can be used in loco or it can be shipped where needed, to provide a cleaner energy source through pipelines (Fig. 13), while remaining convenient from the energetic profile point of view (and in some cases even more convenient than transport of electricity over a long distance) [352]. In the 90’s, a pilot project in which $H_2$ was produced in Quebec from renewable energies and then shipped to Europe was started and evaluated [353]. Another study demonstrated the feasibility of the exploitation by Italy and Spain of the high hydropower potential of Zaire, which was producing more energy than that consumed in the country, through the long-distance

shipping of $H_2$ produced in excess [354]. In the case of off-grid systems, $H_2$ generated from the surplus of renewable energy is used in loco. In this case, direct coupling (i.e. the entire production of renewable energy is used to feed an electrolyzer) is avoided, and only the surplus of energy is used for $H_2$ production instead. Afterwards, a fuel cell is able to directly convert $H_2$ into electricity [355], or the gas can be used directly for heating and cooking, or as fuel in $H_2$-powered combustion engine vehicles. $H_2$ is stored directly as a pressurized gas (in the order of MPa), or in a metal hydride hydrogen storage. When $H_2$ is stored as a gas, its production with a high pressure electrolyzer allows it to reach higher efficiencies compared to production at atmospheric pressure [356] (although subsequent compression is still required). The main problem related to direct $H_2$ storage is its low energy density. Thus, metal hydride systems have been developed to provide a compact alternative, which are particularly suited in remote areas. These tanks are composed of materials able to exothermically absorb $H_2$ and endothermically desorb it. Thus, they must show good heat transfer ability to avoid overheating during charging and overcooling during discharging. When using metal hydride $H_2$ storage, the electrolyzer works under pressure, to allow $H_2$ absorption by the metal hydride. In addition, the $H_2$ desorption pressure in the storage tank must be high enough to enable the process at ambient pressure without the necessity of external heating. Several metal hydride containers were investigated [357–361], and it was demonstrated that this type of storage can be more compact compared to alternative systems having batteries as storage method [321].

Several articles have been published describing storage systems in mountainous areas. Degiorgis et al. in 2007 designed a plant in Valle d’Aosta to exploit hydroelectric and solar RE sources [362]. With the use of an electrolyzer $H_2$ was produced and used to supply the entire needs of the village: heat, electricity and transportation. PV was employed to supply energy for vehicles, whereas the hydropower station was used to generate $H_2$ to supply the heating needs, which
<figure><img src="image_15.png" /><figcaption>Fig. 13. Schematic of an H₂ storage system powered by renewable energy sources.</figcaption></figure>

accounted for 99% of the total electricity consumption. Alberizzi et al. in 2020 modeled a system to fulfill the energy requirements of a mountain hut located at 2200 m [363]. In the region, in fact, there are several ski resorts and huts, which are mainly electrified with diesel generators. A hybrid system exploiting solar and wind energy with battery storage was optimized taking into account the real absorbed loadings of the lodge. Casero and coworkers in 2021 modeled a stand-alone micro-grid system hut in Bachimana mountain, located in the Spanish Pyrenees, in Panticosa, at an altitude of 2200 m [364]. Different configurations were studied comparing the use of H₂ and batteries as energy storage system using the HOMER software, and a lifetime cycle assessment analysis was carried out. The study revealed that even if the starting cost of the H₂ system was higher, the impact on the environment was lower. Mamhud et al. in 2022 studied four possible configurations of a solar/wind/biomass/fuel mixed system to supply the remote community of Balnasari Qani village in Afghanistan [365]. Young et al. in 2023 studied the feasibility of the use of hydropower with H₂ storage for two small communities close to Bhutan, in which up to 80% of the people do not have access to electricity despite the high availability of renewable energies of the Himalayan region [366]. In particular, the paper focuses on the effect of H₂ storage using either hydropower or a mixed PV/wind system. When micro-hydroelectric generators were not coupled with H₂ production, it was estimated that only 22% of the generated electricity was used, whereas the rest was not efficiently exploited. Moreover, the already existing PV/batteries systems experienced a large number of failures due to batteries, lack of protocols, and unauthorized modifications of the systems [367]. The potential for the use of other sources was demonstrated as well. For instance, the installation of wind turbines coupled with solar panels in the Khumbu Valley, Nepal, was simulated by Castellani and coworkers in 2017 to reduce the dependence of this region from commercial fuels and firewood [368]. Climate changes must be taken into consideration when designing and deploying new RE plants. For instance, Farinotti et al. in 2018 studied the variation of hydropower potential supply and

<figure><img src="image_16.png" /><figcaption>Fig. 14. Energy flows of the stand-alone energy systems modeled for Valle dell'Eugio-Locana. The flows E1, E2, E3, E4, E5, E6, E7 are electricity flows; the flows E8 and E9 are the energy associated to the hydrogen flows.</figcaption></figure>

Source: Reproduced from Ref. [370] with permission from Elsevier, copyright 2004.

water storage in all areas in which glaciers are present: by the end of the 2050, 68% of potential water reservoirs will become ice-free, using the RCP 2.6 scenario [369]. The retreat of the glaciers will have a great impact on the energy supply of several countries.

## 5.2. Integration

The integration of renewable energy sources into existing power systems is a crucial step towards achieving a sustainable energy future, but it presents significant challenges, primarily due to the variability and intermittent nature of these energy sources. Traditionally, electricity grids have operated on a centralized model, with power generated from large-scale, fossil fuel-based power plants distributed over long distances to consumers. However, the decentralized nature of renewable energy generation requires a shift towards a more distributed grid model [371]. Solar and wind energy production, for example, depends on factors such as weather patterns, time of day, and wind speeds, leading to fluctuations in energy output. To manage this variability, grid operators need to implement smart grid technologies and
advanced energy management systems that can dynamically adjust to changes in energy supply and demand [372,373]. A microgrid is an integrated energy system comprising a localized collection of distributed energy resources, storage solutions, and electrical loads, capable of operating in both isolated and grid-connected mode [374, 375]. Additionally, the integration of renewable energy into the grid enables the bidirectional flow of electricity, allowing consumers to become “prosumers” who both consume and generate electricity. This shift towards a more decentralized energy system empowers individuals and communities to take control of their energy production and consumption, reducing reliance on centralized utilities and fossil fuels [376,377]. Advancements in smart grid technologies, demand response systems, and community-based microgrids further enhance the resilience and flexibility of power systems, paving the way for a cleaner, more sustainable energy landscape [378,379].

ALPGRIDS addressed energy challenges in rural Alpine regions by promoting local, sustainable energy production through microgrids [380]. This project not only enhanced low-carbon communities and stabilized billing but also prevented blackouts by providing reliable local energy. Additionally, the project established the ALPGRIDS network, facilitating the development and replication of smart grids across five European countries, namely Austria, Germany, France, Italy and Slovenia. Complementing ALPGRIDS, the AMETHyST project aims to deploy green H₂ ecosystems for energy storage and mobility purposes [381]. Sawhney et al. conducted a study in Savona, Italy, focusing on hybrid microgrid integration within a Positive Energy District [382]. The AC hybrid microgrid incorporated various resources such as a CHP unit, rooftop PV plants, micro wind turbines, storage battery systems, and EV charging stations, with potential consideration for V2G chargers. Using HOMER Grid software, the study optimized system design, achieving cost-effective solutions and reducing greenhouse gas emissions. The outcomes highlighted the economic viability and environmental benefits of the microgrids, demonstrating its potential for sustainable energy solutions. Ski resorts in the Alps face significant challenges in powering their operations, including voltage drops caused by the low capacity and long line lengths of power grids, the need for efficient integration of renewable energy sources, and the requirement to support electric mobility infrastructure for future electric vehicles. To address these challenges, the implementation of Alpine microgrids could offer a transformative solution that enhances electricity infrastructure and boosts tourism in the region [383–385]. According to a study by Sulligoi and co-workers [384], microgrids can effectively mitigate voltage drops, integrate renewable energy sources like small/micro hydropower, and support electric mobility, thereby improving the overall reliability and sustainability of the electricity supply.

As mentioned above, microgrids and small energy communities can also decide to forgo main grid integration and remain off-grid. In this case, diversification of RE sources to have a relatively stable energy production in different weather conditions and deployment of large enough energy storage systems must be implemented, to avoid power cuts. At the first stage of the development of off-grid, stand-alone systems exploiting renewable energy, several simulations and small scale experiments have been carried out to investigate their feasibility from an energetic point of view and to try to optimize their efficiency through the proper choice of each component. Solar power was coupled with H₂ storage [387], and some studies showed the feasibility of the use of these systems in mountainous locations [388]. Wind power was also coupled with batteries and H₂ [389], as well as hydro power and H₂ [390]. Wind and solar power were coupled to increase the reliability of the system [391–393], however these systems are more difficult to design and optimize [394]. Once again, it is important to underline that location has a profound influence on the efficiency of the system, making one kind of energy source preferable than another. For instance, stand-alone energy systems were modeled for Valle dell'Eugio-Locana, in the North-West of Italy [370] (Fig. 14). This system was targeted to

provide 3 MWh per year for a residential building, and it compared three RE sources (solar, wind and hydro) and two types of storage (batteries and H₂ storage) in order to obtain the most suited plant design. Several PV and H₂ mixed plants (eventually involving storage with batteries) were built in universities, cities, or more remote areas of several countries, and deeply studied to understand their features [395–402]. Plants in regions with extreme conditions were also investigated, using wind/H₂ systems [386] (Fig. 15, PV/H₂ systems [403], or mixed systems [404]). Bielman et al. in 2011 studied the efficiency of different systems comprised of PV and batteries with or without H₂ as energy storage method placed in different location of Switzerland [405]. This study highlighted the ability of H₂ to provide a complete energetic independence, especially as seasonal fluctuations of the energy sources required an excess of additional battery capacity in systems with only battery storage. Despite the extensive research carried out, there are still some challenges to be addressed when implementing these systems in microgrids: (i) technical, as a careful design of the plant and all related procedures is required; (ii) economical, related to the high investment costs; (iii) market-related, as the definition of the energy prices will be of key importance; (iv) regulatory, as the legislation for these systems has not been fully developed yet [406].

## 6. Analysis and recommendations

Current degree of electrification and EP of settlements and villages in mountain areas vary greatly around the world and it is therefore impossible to draw unified conclusions or give simple recommendations that can be universally valid. As diverse as the different mountain ranges are around the world, so are the populations living in them. In addition to this, very little openly accessible data exist on energy poverty of the population of local territories, as most reports only address the matter at a national level of scale. The same applies to the energy needs of local populations and how these needs have changed over time, and to the availability of renewable resources in a territory and how these have evolved with time: the little data that exists is often scattered in a myriad of little local reports that are hard to find and even harder to collate together to get a bigger picture. The availability of this currently lacking data is paramount to compile studies that can highlight which RE technology, in which size, and on which site to install to support the growth and the well-being of the local population without over-using the land for energy production. Therefore, in this section, rather than giving recommendations based on hard facts, we will only provide relatively informed opinions that stem from the literature reviewed here and our personal knowledge and experience, which can still be useful to guide policymakers and energy companies in the selection of the most suitable technologies and locations for their deployment.

In areas with little to no electrification, the deployment of renewable energy can radically change the quality of life of the inhabitants, providing access to better heating and food cooking, and light and electricity to work, study, and socialize after the sun sets. This translates to better health and a higher level of empowerment of the population. These areas are mostly found in poorer countries with no resources available to extend the national grid to these remote locations. For this reason, the creation of a microgrid based on at least one but better on two complementary sources, such as solar and wind or solar and hydropower, coupled with enough storage to provide a reliable service, can be both an environmentally friendly and a cost-effective solution to satisfy the needs of the local population and raise their living conditions to an acceptable level. Non only this, but the communal caring of the energy system and the longer hours of social activity that it provides can foster social relations within the community and create a common purpose, thus strengthening social bonds and increasing the sense of security and safety. In these countries, the deployment of such microgrids should be financed and handled by the local or central
<figure><img src="image_17.png" /><figcaption>Fig. 15. System schematic of the wind/hydrogen demonstration plant installed at Utsira.</figcaption></figure>

Source: Reproduced from Ref. [386] with permission from Elsevier, copyright 2010.

government, after talking to the local populations to understand their needs, in order to desing the most cost-effective solution.

In richer countries, even rural and remote areas are usually con-
nected to the national grid and therefore properly electrified, with the
possible exception of very isolated lone buildings, for which individual
solutions exist as exemplified in Fig. 9. Nevertheless, we have seen
how even in these countries EP is relatively higher in mountain areas
compared to lower lands; a gap that needs to be bridged. Apart from
EP considerations, transition to renewable energy is paramount to
combat climate change and preserve the natural environment, and
rich countries should be at the forefront of a fast transition, with all
territories within a country giving their contribution. From an ethical
and sociological point of view, energy generation close to the place of
energy consumption makes the local population responsible for their
energy habits and raises their awareness of this topic, which can lead to
energy consumption reductions and better energy efficiency practices.
From a technical point of view, generation close to consumption leads
to fewer losses along power lines, which reduces the amount of genera-
tion needed, and to the reduction in number of long power lines, which
cuts distribution costs and reduces their impact on the land. In these
countries, in addition to fully public efforts towards the renewable
energy transition, the expansion of subsidies and financial incentives
for RE projects can promote private or semi-private investments as well.
These incentives and subsidies are of particular importance when they
target low-income households.

For what concerns which type of renewable energy to implement,
as seen in Table 2 sources such as forest biomass and hydropower
are already widely utilized in mountain areas and especially in the
Alps. Therefore, there is a need to further exploit other renewable
energy sources, particularly wind and solar energy. According to the
International Energy Agency, solar energy has become the cheapest
energy source in the world, presenting a significant opportunity to
address energy poverty and climate change issues more effectively. Not
only that, but hydropower will or at least may change the shape of
the territory irreversibly, while – not taking into account all the infras-
tructure work surrounding the plant – solar panels leave the ground
they were mounted on intact once disassembled, and the footprint of
wind turbines is also relatively little compared to their total size. Unless
forests are cut to make space for such installation, the impact of solar
and wind energy on the mountain environment is much smaller com-
pared to that of hydropower. Furthermore, with the deterioration of
weather conditions due to climate change, we have experienced more

extreme weather – be it droughts or floods – throughout the world in
recent years that affect water supply. This, coupled with the reduction
in mass of perennial mountain glaciers due to temperature increase,
will make hydropower energy less reliable in the future especially in
mountain areas. Biomass energy, while renewable, is only sustainable
if used with moderation, otherwise it leads to deforestation; in the Alps
especially, its limit has almost been reached.

Although the need to deploy renewable energy should be unques-
tioned by now, it is true that compared to coal- and gas-fueled power
stations renewable energy plants are more impactful from a land use
point of view, and their intrinsic diffused and delocalized nature bring
them closer to the eye of a larger number of people compared to
the centralized system of fossil fuel plants. This is especially true for
non-fossil fuel-producing areas, whose populations are not exposed to
the land devastation derived from their extraction. Therefore, the con-
struction of PV or wind plants also prompts considerations regarding
their social acceptance and the consequent necessity to educate and
engage the people in the area, to make them part of the decision
process and raise awareness of its importance. As an example for PV in
particular, in the context of Switzerland there appears to be a notable
degree of social acceptance (64%) observed [277]. Factors such as the
environmental impact, ownership models, and the visual design of the
PV panels can significantly influence how these projects are perceived
and accepted by local communities. It is essential to address these
aspects to ensure that the implementation of solar and wind energy
infrastructure aligns with both environmental priorities and community
preferences. This is especially true in richer countries, where people
already live comfortably for the most part and are more concerned
about aesthetic matters, especially in locations whose economy rely
heavily on tourism. Fewer resistances are expected in poorer countries,
where people will be more interested in reaching adequate standards
of living.

Finally, to efficiently exploit REs, storage methods need to be im-
plemented to guarantee a continuous supply of energy. Batteries are
normally used in the short term, whereas H₂ can be an efficient long-
term storage system. In mountains especially, what potential is left for
the implementation of hydropower may be used for pumped hydro
storage systems to complement solar and wind energy, rather than a
primary source of renewable energy.
## 7. Conclusions

The exploitation of renewable energy in mountain areas is both a necessity and an opportunity. Addressing energy poverty, which refers to limited access for households to affordable and reliable energy services, is crucial, as it significantly affects the quality of life and health of individuals, thereby influencing the overall living standards of a country. This review highlighted how the geographic and socio-economic challenges unique to mountain regions exacerbate the difficulties in energy access. Renewable energy offers a dual solution: it has the potential to enhance energy accessibility, and to mitigate the impacts of climate change, making it an essential strategy for sustainable development in these areas.

There is a large variety of renewable energy sources, with hydropower, biomass, solar and wind being the most relevant in mountain regions. Each of them has its on strengths and drawbacks, and there is no one source that is generally better than the other: the area in which it needs to be deployed determines the best specific choice. However, in the past hydropower and biomass have been the renewable sources of choice and their potential is already highly exploited in mountain areas. On the other hand, wind and solar have seen little use so far, so they have the greatest potential for expansion in the future.

For renewable energy to be reliable, especially when derived from more intermittent sources such as wind and solar, it is important to design power plants that combine both energy generation and storage, to provide continuous services. On this regard, several short-term and long-term storage solutions exist, with different levels of readiness. These plants, implemented in microgrids, can be connected to the main grid or remain off-grid. In all cases, a transition from a central unit providing energy to a decentralized system based on REs can provide a more sustainable and resilient electricity system. Several examples of integrations demonstrated the feasibility and the profitability of the use of REs in systems that are strongly location-dependent and hence need careful design and optimization. The international ALPGRIDS project for example allowed to form a network and to install smart grids in several locations throughout the Alps.

The exploitation of renewable energy can significantly boost the local economy in mountain areas. The development, installation, and maintenance of renewable energy systems create a wide range of job opportunities and stimulate economic growth in mountain communities. Additionally, renewable energy projects can attract investment and support local businesses, thereby fostering economic activity and improving infrastructure and services, which enhance the overall quality of life. Furthermore, the integration of renewable energy can complement and enhance the tourism industry, which is vital for many mountain regions. For example, a reliable energy supply can improve amenities and enhance the visitor experience at ski resorts and other tourist attractions. Overall, the transition to renewable energy can foster a more sustainable and diversified economy, ensuring long-term socio-economic benefits for mountain areas.

Given the visual impact that renewable energy plants often have on the territory, engaging local populations, making them an active part of the decision process, and raising knowledge and awareness around the importance and benefits of renewable energy deployments is essential for policymakers aiming to facilitate sustainable renewable energy development in mountain areas. By ensuring equitable energy access and ownership models, protecting the environment with sensible plant designs in appropriate areas, and garnering community support, policymakers can advance the region's long-term socio-economic and environmental sustainability.

### CRediT authorship contribution statement

**Nesrine Faraj:** Writing – original draft & review, Supervision.
**Valentina Maruzzo:** Writing – original draft & review, Supervision.

**Iacopo Benesperi:** Writing – original draft, review & editing, Supervision.
**Antoine Bousquet:** Writing – review.
**Anna Lushnikova:** Writing – review.
**Marcello Baricco:** Writing – review, Funding acquisition.
**Francesca Brunetti:** Writing – review.
**Christophe Ménézo:** Writing – review, Funding acquisition, Supervision.
**Christine Lartigau-Dagron:** Writing – review, Funding acquisition, Supervision.
**Nadia Barbero:** Conceptualization, Writing – original draft, review & editing, Project administration, Funding acquisition, Supervision.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors thank Elena Merlino for the drawing of Fig. 6. The authors thank the UNITA alliance, UPPA, and UniTO for the funding of NF and VM, and UniTO for the Grant for Internationalization (GFI), Programmazione Triennale 21–23, “Green technologies: renewable energies and the green transition”. NF, VM, IB, MB, and NB also acknowledge support from Project CH4.0 under the MUR program “Dipartimenti di Eccellenza 2023–2027” (CUP: D13C22003520001). This publication is part of the project NODES, which has received funding from the MUR and M4C2 1.5 of PNRR with grant agreement no. ECS00000036. VM acknowledges the Vinci project 2023 – Chapitre 2 funded by the Université Franco Italienne (project number: C2-168). NF, VM, MB, CM, CLD, and NB would like to thank European Alliance UNITA (101004082 – UNITA – EAC-A02-2019 / EAC-A02-2019-1 and 101124853 – UNITA – ERASMUS-EDU-2023-EUR-UNIV) for its support. IB acknowledges the PNA4Energy project, funded under the MUR program “PNNR M4C2 Initiative 1.2: Young Researcher – Seal of Excellence” (CUP: D18H22001950007).

### Data availability

No data was used for the research described in the article.

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