In general, there are two major categories of solar energy technologies; photovoltaics (PV) and concentrating solar power (CSP). PV cells convert solar radiation directly into electricity, whereas mirrors are used in CSP to reflect and focus sunlight to generate heat and convert it into power. Solar energy installations can be distributed or large scale depends on the size and location. Distributed solar energy systems (typically ˂1 MW) are independent of the grid and they are often building integrated. Large scale solar installations are geographically centralized and sometimes built far away from where the power is needed, e.g. in deserts where the solar irradiation is high and the population density is low [7].

Solar energy is a great alternative to carbon-intensive energy sources and has enormous potential to minimize polluting emissions and thus contribute to climate change mitigation [8]. Zhai et al. [9] found that the total amount of avoided CO₂ emissions can range from 6.5% to 18.8% if 10% of the power in 10 different states in the US were generated from PV.

Solar PV power generation plays a significant and growing role in many countries. The annual global market for solar PV increased by 100 GW in 2018 to reach the total installed capacity of 505 GW [10]. Figure 1 shows that the majority of global solar development is in the PV sector. In 2018, the CSP counted only for less than 1.1% (5.5 GW) of the total global installed capacity. By the end of 2018, 640 TWh of PV electricity was generated worldwide, which is 2.4% of the annual global electricity generation [10]. In developing countries, off-grid solar PV systems continue to contribute to the electrification of rural areas. It is believed that 5% of the people in Africa are powered by off-grid PV systems [10].

Figure 1

The global installed PV and CSP capacity (GW)

Despite the advantages of solar energy, it hurts the environment. The environmental impact of solar energy started in the seventies [11] [12]. Neff [13] conducted one of the earliest studies about the environmental impacts of PV plants where he studied the adverse impact on land, climate, and emissions. Later, life cycle assessment techniques were used to analyze the environmental impact of PV [14] [15] [16] [17] [18] [19] [20] [21] [22]. These assessments take into consideration the type of the plant (ground based, BIPV, PV or CSP), geographical location, production, operation, maintenance, and decommissioning of the entire plant. For example, Lovins [12] touched upon the benefits and harmful effects of energy sources from the source to end-use form. Some studies focused on regional aspects of specific geographical locations (e.g., Serbia; [23]) or the impact on wildlife [24].

In the year 2018, the global wind power market grew by 51 GW to reach the total installed capacity of 591 GW (Figure 2) [10]. Most of the wind turbines in the world (96%) are onshore wind turbines. China is the world leader in wind power with approximately 210 GW installed capacity. The USA is the second-largest then Germany and India (Figure 3) [10].

The power capacity and size of modern wind turbines are becoming larger with time. Large wind turbines have a larger swept area, they are powerful, and can reduce the cost of the generated renewable power. The modern wind turbines are rated between 0.5 and 2 MW with a rotor diameter ranges from 40 to 90 m [25].

Figure 2

The global wind energy (installed GW)

Rapidly falling costs per kilowatt-hour has made both solar and wind energy an attractive option for new power generating capacity in many markets around the world. China is a world leader in PV installations (Figure 3). 37% of the worlds' wind installations and 35% of the world's solar installations are in China [10].

Figure 3

Wind and solar energies - the top six countries

Despite that wind power is environmentally benign, it still has some impact on human life [26]. There are a significant number of life cycle analyses of the environmental impacts of wind turbines, with considerable inconsistencies due to site selection or system size. For example, Raadal et al. [27] [28] showed that onshore wind turbines with a capacity between 30 kW and 3MW can generate total emissions of 4.6–55.6g CO₂ /kWh. In another study conducted by Lenzen and Munksgaard [29], higher values of greenhouse gas emissions (7.9 to 123.7g CO₂ /kWh) are reported for onshore wind turbines with a capacity between 0.3 kW and 3 MW.

All power production facilities affect the environment and the ecosystems on Earth. Solar and wind systems are raising in various regions in the world and are no longer specific anymore to coastal, mountainous, desert or arid regions. Thus the importance of studying the environmental impact essential for the sustainability of these systems. This impact on the environment varies according to the system type and its geographical location on the planet.

The disagreement of the results of the life cycle analyses in the literature is caused mainly by the inconsistencies of the used parameters, such as solar radiation or wind conditions, site selection, and installation sizes. Some modeling approaches use different pathways when evaluating the same environmental impact which may affect the result of life cycle analysis [30]. Overall, such an inconsistency in life cycle analysis results limits the decision-making process, since it prevents explicit determination of the environmental impact of certain wind or solar installations. This study provides a detailed description of the solar and wind installations' environmental impact on flora and fauna. This description may help in conducting a detailed life cycle analysis and a sophisticated approach, which may lead to practical methods and successful implementation of sustainable solar and wind power production.

Both wind and solar systems have shown significant growth in the last years, they also have a significant impact on the environment, such as noise, biodiversity impact, visual and climatic impact. Although the above-mentioned impacts seem insignificant when compared with traditional fuels, their impact on the whole ecosystem shouldn’t be ignored. It is important to investigate these negative impacts, especially the potential of long-term impact in order to come up with intelligent solutions to maintain the sustainability of solar and wind energy.

This review was conducted by searching several scientific publishers' websites, research databases and the internet using the following search-terms: solar and environmental impact, solar and land, solar and water, solar and biodiversity, solar and visual, solar and noise, solar and health, solar and microclimate, wind and environmental impact, wind and land, wind and water, wind and biodiversity, wind and visual, wind and noise, wind and health, wind and microclimate.

The integration of small-scale distributed installations or large scale solar plants into the built environment will likely have a negligible direct impact on biodiversity [31]. In this section, the impact of both CSP and PV plants on undisturbed habitats will be the focus of the discussion.

Many studies quantifying the direct impact of large scale solar projects on biodiversity [24] [32] [33] [34]35]. During construction, installation, and operation of both PV and CSP, the soil is graded and in some cases, herbicides are used to eradicate the vegetation. The graded soil and new roads can further increase organisms' mortality rates and the extinction of native species [36] [37].

In arid or semiarid regions, the desert vegetation controls the transport of sand, dust, and sediments. Removal of vegetation, grading the land and the construction of roads by solar energy infrastructure causes soil loss by storms and floods [38] [39].

Large scale solar energy infrastructures may block the movement as well as seasonal migration of wildlife species which may cause a gene migration disruption between different species [40] [41]. This habitat loss and fragmentation could threaten biodiversity and disrupt the eco-system balance [42] [43].

To date, most solar thermal plants are constructed in arid or semiarid areas, which typically have a unique and fragile soil and habitat. An Environmental assessment statement of solar projects in desert regions southwest of the USA [44] reported that the solar plant would significantly impact and threaten desert tortoise, as well as special-status animals and plant species in the local area. The statement has affirmed the necessity of extensive efforts to mitigate impacts on wildlife and habitat.

Concentrated solar systems could pose a marginal threat to insects, and birds flying nearby the focus of the reflectors. McCrary et al. [45] study the quantitative impact of concentrated solar plants on wildlife, where they calculated the death of birds, bats, and insects. They found that the impact was low compared to other anthropogenic activities.

In the case of PV plants, the occupation of land has a major impact on wildlife and habitat. An environmental impact report prepared for a utility-scale PV project (550 MW_p) in central California found that tens of protected species in the region are affected [46]. Also, the shade of PV arrays changes the microclimate, reshaping the vegetation biodiversity.

Besides the direct impact, there are also indirect and regional impacts on biodiversity. The elevated levels of greenhouse gases and nitrogen oxides resulting from the increased vehicle activity or gas burners in concentrated solar plants may promote the growth of invasive grasses that increase fire occurrence frequencies [47] [48]. Additionally, chemicals such as antifreeze agents, herbicides, dust suppressants, rust inhibitors that are used during the maintenance of solar plants may have long term harmful effects of the local and regional biodiversity [24] [49].

To protect solar energy facilities from floods, water channels are established in the vicinity of support structures to facilitate the quick divert of surface water and have it flow away from the facility. This diversion of flow can dramatically affect water availability and desert habitat quantity and quality [50].

Dust. Large scale solar facilities substantially impact the landscape through site preparation, grading, vegetation removal, and construction of roads. All of these activities result in dust and particle emissions, especially in the desert and arid regions [51]. Dust emissions can alter plant fertility and water-retention potential of the soil. Physiologically, dust emissions can adversely impact photosynthetic processes, and water consumption of desert plants [52]. Mobilized dust particles can cause root exposure, the burial of plants, and scratch plants leave and stems. The plant damage caused by dust can indirectly affect nutrients and habitat quantity and quality in an adverse manner.

During plant operation, natural dust emissions reduce heliostat, parabolic trough, and panel efficiency in converting sunlight into power or thermal heat. To overcome this challenge, solar plants use various dust suppressants on mirrors and panels. These suppressants can drastically impact the soil, hydrology of the region, and the surface water runoff dynamics [53] [54] [55].

Water is a crucial component of power production systems. [61]. Solar power installations vary in their water use. PV systems need a small amount of water (0.02 m³/MWh), for panel cleaning and dust suppressions in deserts or places rich with dust [62]. While in thermal CSP systems, the majority of the water (85-95%) is used in the cooling cycle and only a small amount of water is used in producing the steam.

Arid and semiarid regions are blessed with high solar irradiance and are suitable for large scale solar projects [63]. However, they are water-constrained areas and water use by solar plants may cause conflict and create competition on the water for agricultural and domestic needs [64] [65].

The water consumption rates in CSP depend on the cooling techniques; wet, dry, or a hybrid cooling (a combination of both) [65].

Concentrating solar thermal power plants with wet cooling system has a considerably large water footprint and consumes large quantities of water (~3 m³/MWh), which is nearly equivalent to the quantity used in traditional nuclear or coal thermal plants [64] [65] [66] [67]. The solar power plants with wet cooling systems are more profitable and efficient [68] compared to plants that use dry cooling units. It is worthy to mention that the majority of the water used in the cooling cycle is reused, but with lower quality and that may cause pollution to the aquifer.

Water is also used in cleaning heliostats and parabolic troughs to reduce reflectivity losses [69]. The back of the mirrors used for concentrated power systems is coated with silver, the solar radiation passes the reflector twice and thus why CSP systems are more affected by dust compared to PV. The volume of water used for cleaning depends on solar technology and region. Usually, plants installed in desert regions (rich with dust storms) consume more water for cleaning. Although other dust removal technologies (e.g., electrostatic, robot cleaning chemical sprays) exist, most are new, and their environmental impact is not completely understood [64] [70].

Land-use efficiency is a key parameter that determines the effectiveness of land use for energy production. It also defines the plant′s generated power relative to the used area of land (W/m²) by the construction throughout the life cycle of the installation [63] [71]. The energy conversion chain includes raw materials exploration, extraction, processing, manufacture, plant construction, power generation, operation, maintenance, decommissioning, and disposal. Thus, energy production processes highly impact the land. In some cases, the damage caused to the land is irreversible and the land cannot be restored to its original state [71] [72] [73].

Utility-scale renewable energy generation facilities can have substantial footprints in terms of land and water use [22]. Many studies in the literature that investigate land use characteristics.

The type and capacity of large scale solar systems are the main factors impacting land-use efficiency. Hernandez et al. [63] found that the land-use efficiency of large scale solar systems in California, to be 4.6 W/m² for PV and 11.2 W/m² for CSP. Fthenakis and Kim [71] found out that the direct land use of PV ranges between 402 and 463 m²/GWh.

The impact of large scale solar systems on land-use and land-cover change is relatively smaller than other energy systems [74]. PV systems require the largest land-use when compared to wind, hydroelectric, and biomass [63] [71] [75].

Zanon and Verones [76] discussed the risk of PV plants on the use of fertile areas or the impact of the construction and operation of the facility on the landscape. The competition between land use for agriculture and energy always attracted the attention of researchers, some studies compared the suitability of land (arable, marginal) for agriculture and energy [77] [78] [79] [80]. Tsoutsos et al. [22] study the impact of large scale solar plants on land use, they point out that PV plants can impact the potentially cultivable land and the power output is therefore considered as competitive with food production, similar to the energy-food competition on land in the case of energy crops production [81].

The amount of solar radiation is the major factor impacting the land needed for solar plants. The higher solar radiation leads to less land use for the same capacity of solar plants [82]. CSP plants require land that should be relatively flat. The land used by CSP includes solar arrays, heliostats or parabolic troughs, substations, thermal storage facilities, access roads, service buildings, and other infrastructure.

The visual impact of PV systems is highly dependent on the nature surrounding the system. In the case of installations near areas with natural beauty, the visual impact can be significantly high. In the case of building integrated systems, it may have a positive aesthetic and educational impact. It can contribute to controlling the shading, lighting, and ventilation [22]. It’s also attractive when used in areas where the public can see it e.g. Parks, where it may serve as a tool for increasing public awareness of these technologies [22].

Usually, CSP plants are installed far away from the public residential areas and have a very minimum visual impact. The visual impact of the solar towers is high compared to parabolic concentrators, Stirling dish engines, and Fresnel plants, due to the height of the tower which may reach up to 240 meters [83].

Solar water heaters both flat panels and evacuated tubes are usually designed to fit closely to the existing roof design. Furthermore, architects use solar collector elements to enhance the aesthetic appeal of a building.

There is almost no noise generated from small and large scale PV power plants. CSP plants are less noisy compared to other renewable and non-renewable power plants, such as wind farms or coal and gas-fired plants. In CSP thermal plants, the steam or gas turbine is located in the middle of the plant and is surrounded by heliostats or parabolic troughs. Thus the turbine noise is unlikely to disturb the neighboring public.

The reflected radiation from the surface of the PV modules, mirrors, or from the receiver in a solar tower can cause a glare that may lead to a temporary reduction or loss of vision.

In hot seasons and due to the infrared component of the solar insolation, the temperature of the photovoltaic panels can reach 70 °C or higher [84]. This increase in PV module temperature reduces the electrical output of the module, heats the air surrounding the system, and influence the microclimate of the site. Also, such changes in temperature may cause a problem in the outdoor built environment and impact the outdoor thermal comfort conditions and generate a heat island effect. The overheating of the PV arrays can be avoided by passive cooling [84].

In the case of concentrating systems, thermal facilities can increase the albedo in deserts by 30%-56%, which could impact local temperatures and precipitation through wind pattern changes and evapotranspiration. Also, large concentrating power plants, specifically the dry-cooled facilities, may generate a significant amount of waste heat that can be transferred by wind creating drought conditions. The heat generated by solar tower facilities, especially at the focal point, can burn birds and insects [22] [32] [85] [86].

In CSP plants, collectors create shading and this may increase or decrease the soil temperature depending on the season and the location [87].

Many chemicals are used in manufacturing PV panels. For example, in thin-film solar cells, carcinogens such as cadmium are used. Similarly, many other toxic chemicals are used to maintain solar panels clean. European Union initially banned the use of cadmium and other heavy metals. However, in 2010, the EU excluded the PV panel from this ban in order to attain the set targets for renewable power generation [88]. The inappropriate handling in the recycling process of PV modules can expose the public to toxic materials such as cadmium, arsenic, and silica dust [89] [90] [91].

In CSP systems, the heat transfer oil is carried by long pipelines that connect the long rows of mirrors, dishes, or parabolic troughs. Some solar thermal facilities have thermal storage units that use molten salts as a heat storage medium which may leak into the surrounding environment [22] [24].

Large scale concentrated solar facilities, especially wet-cooled solar facilities, use toxic chemicals in their cooling and recirculating systems, such as antifreeze agents, rust inhibitors, acids, alkali, heavy metals, and herbicides [22] [49].

During the construction of large scale solar projects, hazards like soil-borne pathogens [92] and different sizes of particulate matter [93] [94] are released. These hazards can lower the local air quality and impact public health [95].

Assessment of the potential effects of any proposed energy project on the local flora and fauna is essential for any environmental assessment. Bird disturbance is known as a major issue for wind energy development, particularly for remote sites with rare habitats. The direct impact of wind energy is the death caused by the collisions of birds with wind energy plants. Birds are considered the largest victim groups in mortality caused by collision with wind turbines around the world [98]. However, deforestation and urban expansion can cause much higher bird fatalities [24]. Sovacool [99] argues that wind turbines kill about 95% fewer birds compared to fossil fuels. For example, for a 1GW wind power plant, the annual birds killed, is only 20, while 2000 were killed by vehicles and high voltage transmission lines, and 1500 were killed caused by hunting [100]. Some studies indicate that birds can quickly adapt to newly installed wind turbines [101].

The indirect wind energy impacts are habitat disruption, avoidance, and displacement [102]. For example, sea creatures are impacted by offshore wind turbines. Thomsen et al. [103] reported that dab and salmon's behaviors can be influenced by the perceived pile-driving pulses caused by wind turbines construction and operation.

Factors affecting birds' collision with wind turbines. There are several factors that lead to a collision of the birds with wind turbines [104]

Flashlights. The light emitted from wind towers attracts birds especially during foggy nights and poor weather conditions. Thus, birds fly toward the wind turbines and this increases collision possibilities with wind towers and blades [105].

Weather conditions. Gregory et al. [106] point out that 3 out of 48 bird mortalities happen when the weather is not a factor. Migrating birds tend to fly at lower altitudes during bad weather such as storms, low clouds, and rain. This increases birds' collision potential with wind turbines.

Tower design. Old wind turbines have a small capacity, low hub heights, and shorter blades. Old wind farms usually have a large number of small turbines. The low height of the turbines attracts birds' nesting and the blades' high rotation speed increase the bird mortalities [102]. Josimovic [107] shows that high wind generators' (145m) do not present a risk for the migrating birds since they fly at much higher altitude.

Noise pollution is considered a critical environmental impact of wind farms. The wind turbine noise can be a major annoyance to people [108] [109]. The value of properties nearby the wind farms may be negatively impacted by the noise greeted by wind turbines.

There are two types of noise emitted by wind turbines; mechanical and aerodynamic noise. The noise caused by the moving components such as bearings, gear, and power generator is called mechanical noise. The level of this noise is affected by turbine size, wear and tear, lack of maintenance, and poor component designs [110]. The mechanical noise can be lowered by using proper acoustic insulation of the turbine house and by the use of anti-vibration support footings [111]. The second type of noise is the aerodynamic noise which is caused by the dynamics of how the airflow and passes the blades of the turbine. The interaction of blades with turbulent wind causes the "whooshing" sound. In general, the aerodynamic noise increases by the increase of the blade tip speed [112]. The reduction of the aerodynamic noise can be achieved by proper blades design [111]. The noise of the blade is less with slow blade tip speeds and with large wind turbines [113].

Some researchers have done research and surveys on the impact of aerodynamic noise's low frequency on people and have found that it may disturb people who live close to wind turbines. Miller and Keith [113] point out that sound pressure levels may cause anxiety indications such as headaches to those who live close to the wind turbines. They also mention that wind turbines' low-frequency aerodynamic noise can cause sleep disorders and hearing problems. However, Leung and Yang [25] point out that having obstacles in the surrounding lines of the wind farm may decrease the noise pollution.

Pedersen [114] has studied wind turbines’ sound pressure impact on peoples’ well-being, showing that those who were disturbed significantly by wind turbines suffer from stress symptoms and headaches. Punch et al. [115] have reviewed and summarized the literature in this field and concluded that wind turbines' low-frequency aerodynamic noise may cause sleeping and hearing problems, and may damage the vestibular system. However, there are suggested possible solutions to solve this issue. For example, some suggested house structure changes to reduce or even eliminate the noise or to build the wind farm at 2 km distance from where they live [109]. Also, optimizations made on blade design [116] can reduce the level of noise by 0.5 to 3.2 dB.

The visual impact of wind turbines is another critical factor that influences the public perception of this technology. However, among all environmental impacts of wind turbines, measuring and estimating the visual impact is a challenging task [101]. People are subjective when they evaluate the visual impact; some individuals enjoy looking at wind farms, while some people have contrasting views. Visual impact surveys conducted in the UK have shown that the majority (70%) of people doesn’t oppose wind turbines [117]. While some surveys point out that people from the tourism industry believe that wind farms may negatively impact local tourism [25].

The wind turbines’ visual impact is affected by several factors, e.g. the proximity of the observer to the wind farm, size, type, color or contrast, number of wind turbines, wholly or partially seen, the number and rotational speed of blades, the arrangement of the turbines across the site, the local lighting conditions (shadow flickering) operation time and local turbine history [118] [100].

Carefully planned wind farms can fit well into and be part of the landscape. For example, Josimovic [107], argues that visual impact was not a major problem for Bavaniste wind farm in Serbia, and they argue that the farm will act as a catalyst in developing the local tourism, which contradicts the observation of Leung and Yang [25].

As the wind turbines become larger in size and number, there are some speculations that they could affect the local climate. In the latest studies of Chapman [119] and Miller and Keith [113] they argue that wind power can impact the local climate by changing the macro-climate at the local level, but its impact on the global average surface temperature would be insignificant.

The land use of wind power depends on the turbines’ spacing and their configuration. The land use includes land preparation, construction, access roads, and transmission lines. Usually, wind turbines use less than 10% of the wind farm areas [120]. Compared to other energy systems, wind energy has a minimal land footprint. However, the visual impact on the landscape may affect the value and uses of neighboring lands.