Renewable energy
Renewable energy is energy harnessed from naturally replenishing sources on a human timescale, such as solar radiation, wind, flowing water, geothermal reservoirs, biomass, and ocean tides, which generate power without depleting finite stocks like fossil fuels.[1] In 2023, these sources accounted for 30% of global electricity generation, with hydropower as the dominant contributor at 14%, while wind and solar rapidly expanded to comprise a growing share through unprecedented capacity additions exceeding 500 gigawatts annually, driven by cost reductions of over 80% for solar photovoltaics and 70% for onshore wind since 2010 via technological learning and scale.[2][3][4] However, renewables met only about 15% of total primary energy demand that year, constrained by the intermittency of solar and wind—necessitating backup from dispatchable sources or costly storage to maintain grid reliability—and their lower energy densities, which demand extensive land areas for equivalent output compared to denser fuels, sparking debates over environmental trade-offs including habitat fragmentation and resource extraction for manufacturing.[5][6][7]Fundamentals
Definition and Classification
Renewable energy refers to energy derived from natural sources or processes that are replenished at a rate comparable to or faster than their consumption on human timescales, such as solar radiation, wind, flowing water, geothermal heat, and biomass growth.[1][8] This contrasts with non-renewable energy sources, which rely on finite stocks of fuel accumulated over geological timescales, including fossil fuels like coal, oil, and natural gas, as well as nuclear fuels like uranium, whose reserves deplete with extraction and use without natural replenishment at usable rates.[9][10] The classification of renewable energy primarily follows the underlying natural resource or process harnessed, with technologies grouped accordingly into major categories: solar energy, which captures sunlight via photovoltaic cells or thermal systems; wind energy, generated by turbines converting kinetic energy from atmospheric motion; hydropower, derived from the gravitational potential of water in rivers or reservoirs; geothermal energy, tapping heat from Earth's subsurface; biomass and bioenergy, produced from organic materials like wood, crops, or waste through combustion, gasification, or biofuels; and marine energy, including tidal barrages and wave converters exploiting ocean movements.[11][1] These categories exclude nuclear fission, despite its low-carbon attributes, as the fissile material is not naturally replenished.[9] Within classifications, distinctions arise based on scalability, intermittency, and sustainability constraints; for instance, biomass is considered renewable only if harvesting rates do not exceed regrowth, avoiding net deforestation or soil depletion, while large-scale hydropower can alter ecosystems through damming despite water's renewability.[12] Emerging subcategories, such as ocean thermal energy conversion, remain minor but fit under marine renewables. Overall, renewables accounted for approximately 29% of global electricity generation in 2022, underscoring their diversity but also variability in output reliability compared to dispatchable non-renewables.[13]Physical Principles
Renewable energy technologies convert forms of energy from ongoing geophysical and solar-driven processes into usable mechanical or electrical power, subject to fundamental thermodynamic and conservation laws that impose efficiency limits. These sources exploit kinetic, potential, radiant, thermal, or chemical energy, often with inherent variability due to atmospheric and orbital dynamics. Solar energy capture relies on the photovoltaic effect, where incident photons with energy exceeding the semiconductor's band gap generate electron-hole pairs, establishing a voltage across a p-n junction that drives current when connected to a load. Photons below the band gap pass through unused, while excess energy above the band gap dissipates as heat via thermalization, leading to the Shockley-Queisser limit of approximately 33.7% efficiency for single-junction cells under AM1.5 spectrum at optimal band gap of 1.34 eV.[14] Concentrated solar power systems instead use thermal principles, absorbing sunlight to heat a fluid that drives a heat engine, bounded by Carnot efficiency \eta = 1 - T_c / T_h, where T_h and T_c are hot and cold reservoir temperatures in Kelvin; practical systems achieve 20-40% due to low \Delta T relative to fossil alternatives.[15] Wind power harnesses the kinetic energy of atmospheric motion, primarily induced by solar heating gradients creating pressure differences and Coriolis effects. Turbine blades, acting as airfoils, decelerate airflow to extract momentum, converting it to rotational torque via lift and drag forces, with power P = \frac{1}{2} \rho A v^3 C_p, where \rho is air density, A swept area, v wind speed, and C_p the power coefficient. Conservation of mass and momentum dictates the Betz limit, capping C_p at 16/27 or 59.3%, as full extraction would halt downstream flow, violating continuity.[16] Hydropower transforms gravitational potential energy of elevated water masses into kinetic energy through controlled release, typically via penstocks, where E_p = m g h (mass m, gravity g, head h) converts to flow velocity v = \sqrt{2 g h} by Bernoulli's principle, spinning turbines connected to generators. Efficiency approaches 90% in large installations, limited mainly by hydraulic losses rather than fundamental thermodynamics, though output varies with precipitation and evaporation cycles.[17] Geothermal energy draws on conductive and convective heat transfer from Earth's interior, sourced from radiogenic decay and residual accretion heat, with flux averaging 0.087 W/m² globally. Systems circulate fluids to extract thermal energy, employing heat engines or direct use; binary cycle plants use organic Rankine cycles between reservoir temperatures (often 100-200°C) and ambient, yielding Carnot-limited efficiencies of 10-20% due to modest \Delta T, as \eta scales with temperature ratio.[18] Biomass derives chemical energy stored through photosynthesis, where solar photons drive endothermic reactions fixing CO₂ and H₂O into carbohydrates via chlorophyll-absorbed light (primarily 400-700 nm wavelengths), with quantum yield limited by energy mismatches and electron transport chains, achieving 1-3% overall solar-to-biomass conversion. Combustion or gasification releases this as heat, convertible to work via heat engines again bounded by Carnot efficiency, though direct biochemical pathways like anaerobic digestion yield lower-grade methane.[19]Renewability vs. Finite Alternatives
Renewable energy sources are defined as those derived from natural processes that replenish continuously or cyclically on timescales comparable to human use, rendering them inexhaustible in aggregate flow despite flow-limited availability at any instant.[9] In contrast, finite alternatives such as fossil fuels and nuclear energy depend on depletable geological stocks accumulated over geological epochs, which diminish with extraction and do not regenerate within practical human horizons.[20] This fundamental dichotomy underpins the classification: solar, wind, and hydropower, for instance, tap into perpetual fluxes driven by solar input and gravitational forces, whereas coal, oil, and uranium represent fixed inventories subject to eventual exhaustion.[21] The scale of renewable inflows vastly exceeds global demand, illustrating their theoretical abundance. The average solar irradiance on Earth equates to approximately 342 watts per square meter, yielding a total incident power of about 174 petawatts—over 10,000 times the world's primary energy consumption of roughly 18 terawatts as of recent estimates.[22] Fossil and nuclear reserves, by comparison, constitute less than 10% of the annual solar resource in energy-equivalent terms.[23] Proven global reserves of oil, natural gas, and coal correspond to roughly 47, 52, and 132 years of supply at 2021 consumption rates, respectively, though these ratios extend with new discoveries, enhanced recovery techniques, and shifts in demand; nonetheless, they remain bounded by planetary endowments.[24] Nuclear fuel presents a nuanced case within finite alternatives: identified recoverable uranium resources, totaling around 6 million tonnes at costs below $130 per kilogram, suffice for current reactor fleets and projected expansions through 2050, potentially extending further via breeder reactors or thorium cycles that recycle fuel more efficiently.[21][25] However, without such advancements, uranium's finite abundance—primarily from ancient supernova remnants—limits long-term scalability akin to fossil fuels, distinguishing it from renewables where the primary driver (e.g., solar fusion) persists for billions of years.[21] Empirical assessments confirm renewables' edge in raw renewability, though practical deployment hinges on technological capture rates rather than source depletion.[1]Primary Technologies
Solar Energy
Solar energy captures sunlight to produce electricity or heat via photovoltaic (PV) effect or thermal concentration. In PV systems, semiconductor materials such as crystalline silicon absorb photons, exciting electrons to generate direct current electricity convertible to alternating current via inverters.[26] Concentrated solar power (CSP) systems employ mirrors or lenses to focus sunlight onto a central receiver, heating a fluid to produce steam that drives turbines for electricity generation.[27] PV dominates global deployment due to modularity and scalability, while CSP offers potential for thermal storage but remains limited by higher costs and site requirements.[28] Global cumulative PV capacity surpassed 2.2 terawatts by the end of 2024, with over 600 gigawatts added that year alone, driven by manufacturing scale-up primarily in China.[28] Annual additions reached record levels, accounting for nearly three-quarters of new renewable capacity installations.[29] Commercial PV module efficiencies typically range from 20% to 24%, with laboratory records exceeding 30% for tandem perovskite-silicon cells as of mid-2024.[30] [31] CSP plants, by contrast, achieve system efficiencies around 15-20% but enable dispatchable power through molten salt storage.[32] The levelized cost of energy (LCOE) for utility-scale solar PV averaged $0.043 per kilowatt-hour globally in 2024, reflecting an 89% decline since 2010 due to falling module prices and improved manufacturing.[33] [34] However, solar's intermittency limits capacity factors to 10-25% globally, averaging about 23% in high-insolation U.S. regions, necessitating grid-scale storage or fossil backups for reliability.[35] [36] Environmental impacts include habitat disruption and water use from mining silicon, silver, copper, and other materials, with polysilicon production being energy-intensive and often reliant on coal-powered facilities.[37] [27] End-of-life panel recycling remains underdeveloped, posing waste management challenges.[38] Despite cost advantages in sunny locales, full-system integration costs, including transmission and storage, often exceed simple LCOE figures.[39]Wind Power
Wind power generates electricity by converting the kinetic energy of wind into mechanical power through rotating blades on turbines, which drive generators. Modern horizontal-axis wind turbines typically feature three blades mounted on a nacelle atop a tower, with rotor diameters exceeding 150 meters for utility-scale units. The power output follows the cubic relationship to wind speed, rendering generation highly sensitive to velocity variations; turbines operate efficiently between cut-in speeds of 3-4 m/s and rated speeds around 12-15 m/s, shutting down above 25 m/s to prevent damage.[40] The utilization of wind for mechanical tasks dates to antiquity, with Persian windmills for grinding grain around 500-900 AD and Chinese water pumps by 200 BC. The first electricity-generating wind turbine appeared in Scotland in 1887, built by James Blyth, followed by Charles Brush's American version in 1888. Post-1970s oil crises catalyzed modern development, with Denmark pioneering large-scale deployment via Vestas and Siemens turbines. By the 1990s, subsidies and technological advances enabled rapid scaling, culminating in multi-megawatt offshore prototypes by the 2000s.[41] Onshore wind dominates installations, comprising 93% of global capacity as of 2023, due to lower construction costs and easier access, though constrained by land-use conflicts and variable terrain winds yielding capacity factors of 30-40%. Offshore wind, situated in marine environments, benefits from steadier, stronger winds (often >8 m/s), achieving capacity factors up to 50-60%, but incurs higher expenses from foundations, cabling, and maintenance logistics. Offshore turbines are larger, with hub heights over 100 meters and capacities surpassing 10 MW per unit, versus onshore averages of 2-3 MW.[40][42] Global installed wind capacity reached 1015 GW by the end of 2023, with China accounting for over half of additions, followed by the United States, Germany, and India. Electricity generation from wind grew by 216 TWh in 2023, representing approximately 7-8% of worldwide electricity, though actual output varies regionally—Denmark exceeds 50% wind penetration, while global intermittency limits higher shares without backups. Capacity expansions slowed slightly in 2024 to 1133 GW total, per IRENA estimates, amid supply chain bottlenecks for components like steel and composites.[40][29] Unsubsidized levelized cost of energy (LCOE) for onshore wind averaged $24-96/MWh in 2023 analyses, with medians around $40-50/MWh, influenced by site-specific factors like wind resources and financing; offshore LCOE ranged higher at $50-140/MWh due to installation premiums, though declining with scale. These figures exclude integration costs such as grid upgrades or firming capacity, which can add 20-50% to system-level expenses, as wind's variability necessitates dispatchable reserves like natural gas. Lazard's assessments highlight wind's competitiveness against new fossil builds in optimal conditions but underscore rising capital costs from inflation and permitting delays.[43][44] Environmental impacts include habitat fragmentation from turbine footprints and access roads, covering about 0.1-1% of farm area effectively, alongside noise and visual alterations prompting local opposition. Bird and bat collisions cause mortality rates of 0.2-0.4 birds per GWh globally, lower than fossil fuels' pollution-driven deaths but cumulative for raptors and migrants; mitigation via radar curtailment reduces strikes by 50-70%. Material demands feature rare earth elements like neodymium in permanent magnet generators, sourcing from mining-intensive processes in China, which generate toxic waste and supply vulnerabilities. Lifecycle emissions are low at 11 gCO2/kWh, comparable to nuclear but above unsubstantiated green claims ignoring end-of-life recycling challenges for composites.[45][46] Key challenges stem from intermittency, with output fluctuating hourly and seasonally, eroding capacity credits to 10-20% in many grids and demanding overbuilds or storage for reliability. Grid integration requires enhanced forecasting, reactive power support, and transmission expansions, as uncoordinated penetration risks frequency instability and curtailments—evident in Europe's 2021 wind droughts necessitating fossil ramps. Supply chains for blades and towers face steel volatility, while offshore faces hurricane vulnerabilities and marine ecosystem disruptions from noise during construction.[47][46]Hydropower
Hydropower, or hydroelectric power, generates electricity by converting the kinetic energy of flowing or falling water into mechanical energy via turbines, which then drives generators.[48] This process typically involves impounding rivers behind dams to create reservoirs, releasing water through penstocks to spin turbines, though run-of-river systems operate without large storage. Globally, hydropower held an installed capacity of approximately 1,412 gigawatts (GW) in 2023, accounting for the largest share of renewable electricity generation at around 4,185 terawatt-hours (TWh), or roughly 15% of total worldwide electricity production.[49][50] Capacity additions slowed to 13 GW in 2023, below the five-year average, amid challenges like droughts and permitting delays.[48] China leads in hydropower with the world's largest facility, the Three Gorges Dam on the Yangtze River, boasting 22.5 GW of capacity and annual output exceeding 100 TWh.[51] Other top producers include Brazil, the United States, Canada, and Russia, where hydropower constitutes over 50% of electricity in countries like Brazil.[52] Pumped storage hydropower (PSH), which stores energy by pumping water uphill during low demand and releasing it for generation, added 8.4 GW globally in 2023, reaching 189 GW total and enhancing grid flexibility.[53] Unlike intermittent sources such as solar and wind, hydropower offers dispatchable, reliable baseload power with high capacity factors often exceeding 50%, enabling rapid ramping to meet peak demand or balance grid fluctuations.[52][48] It produces no direct emissions during operation, with lifecycle greenhouse gas emissions typically under 20 grams CO2-equivalent per kilowatt-hour, far below fossil fuels.[48] Facilities last 50-100 years with low operational costs, providing economic stability once built.[52] Despite these benefits, hydropower entails significant environmental and social costs. Dams fragment rivers, blocking fish migration and altering aquatic habitats, while trapping sediments reduces downstream fertility and reservoir silting shortens lifespan.[54] Tropical reservoirs can emit substantial methane from submerged vegetation decay, with some studies equating emissions to coal in certain cases.[55] Construction often displaces communities—such as over a million people for Three Gorges—and floods ecosystems, though proponents argue benefits outweigh impacts when managed sustainably.[54] Vulnerability to droughts, as seen in 2023's 5% generation drop from low precipitation, underscores hydrological dependence.[50] High upfront costs and long development timelines further limit expansion in suitable geographies, which are increasingly scarce.[48]Geothermal Energy
Geothermal energy harnesses thermal energy from the Earth's subsurface, primarily from hot water and steam reservoirs formed by natural heat flows driven by radioactive decay in the mantle and core, as well as residual heat from planetary formation. This heat is extracted via wells drilled into geothermal reservoirs, typically located in tectonically active regions such as volcanic arcs or rift zones, where temperatures exceed 150°C at depths of 1-3 km.[18] The process involves pumping geothermal fluids to the surface, using their heat to generate electricity through turbines or directly for heating applications, with reinjection of cooled fluids to maintain reservoir pressure and sustainability.[56] Three primary types of geothermal power plants convert this heat to electricity. Dry steam plants, the oldest and simplest, pipe high-temperature steam (above 200°C) directly from reservoirs to turbines, as exemplified by The Geysers field in California, which has operated since 1960.[57] Flash steam plants, comprising about 70% of global capacity, handle hotter fluids (above 180°C) by flashing pressurized hot water into steam in low-pressure separators, then directing it to turbines; double-flash variants enhance efficiency by a second flashing stage.[58] Binary cycle plants, increasingly common for lower-temperature resources (100-180°C), transfer heat from geothermal fluids to a secondary working fluid with a lower boiling point, such as isobutane, which vaporizes and drives a turbine without direct steam use, allowing closed-loop operation and broader resource applicability.[56] As of the end of 2023, global installed geothermal electricity capacity stood at approximately 16.3 GW across over 30 countries, generating over 97 TWh annually, with a high average capacity factor exceeding 75%—far surpassing variable renewables like wind (under 30%) or solar PV (under 15%).[59] The United States leads with the largest capacity, primarily in California and Nevada, followed by Indonesia, Turkey, New Zealand, and Iceland, where geothermal supplies up to 25% of electricity in some nations.[60] Resource potential remains vast; the U.S. Geological Survey estimates identified U.S. hydrothermal resources could support 9 GW, with undiscovered additions of 30 GW, while enhanced geothermal systems (EGS) accessing hot dry rock could unlock over 500 GW domestically through hydraulic fracturing to create artificial reservoirs.[61] Globally, the International Energy Agency assesses technical potential exceeding 42 TW for EGS at depths under 5 km.[62] Geothermal systems provide dispatchable baseload power with minimal intermittency, emitting about 38 g CO₂/kWh—roughly 10 times less than coal—and 97-99% fewer sulfur compounds and particulates than equivalent fossil plants, though trace gases like H₂S require mitigation via abatement technologies.[63] [64] Plants occupy small land footprints (typically 1-8 acres per MW) compared to solar or wind, minimizing habitat disruption.[65] However, deployment is geographically constrained to areas with sufficient heat flow, such as 10% of global land surface, necessitating high upfront exploration and drilling costs (often $5-10 million per well) that can exceed $100/kW installed.[66] Environmental risks include induced seismicity from fluid injection, potential land subsidence, and water contamination from extracted minerals like arsenic or silica if reinjection fails, though modern practices emphasize closed-loop systems to limit these.[67] Resource depletion over decades is possible without proper management, but long-term sustainability is achievable with monitoring, as demonstrated by stable output at mature fields like Larderello, Italy, operational since 1913.[68]Biomass and Bioenergy
Biomass encompasses organic materials derived from recently living organisms, including wood and wood residues, agricultural crops and residues, animal wastes, municipal solid waste, and dedicated energy crops such as switchgrass or miscanthus.[69] Bioenergy refers to the energy produced from these feedstocks through thermochemical processes like direct combustion, gasification, and pyrolysis, or biochemical processes such as anaerobic digestion and fermentation.[70] Direct combustion burns biomass to generate heat or steam for electricity, typically achieving efficiencies of 20-40% in power plants.[71] Gasification converts biomass into syngas for fuels or power, while fermentation produces liquid biofuels like ethanol from sugars or starches.[72] In 2023, global bioenergy electricity generation totaled 697 terawatt-hours (TWh), accounting for 2.4% of worldwide electricity production, with growth of 3.1% from 2022.[73] Installed bioenergy power capacity reached 151 gigawatts (GW) by 2024, representing 4.4% of total renewable capacity, following additions of 4.6 GW in 2024 primarily from solid biomass plants.[74] [75] Bioenergy also supplies significant heat—over 10% of global heating in some regions—and transportation fuels, with biofuels comprising ethanol from corn or sugarcane and biodiesel from vegetable oils like soy or rapeseed.[76] Investments in bioenergy are projected to reach $16 billion in 2025, a 13% increase, driven by policy support despite slower capacity growth in advanced economies.[77] Lifecycle greenhouse gas (GHG) emissions from bioenergy vary widely but are not inherently zero or lower than fossil fuels without strict sustainability measures.[78] When sourced from waste residues, emissions can be 50-90% below those of coal or natural gas equivalents, assuming rapid biomass regrowth absorbs released CO2.[79] However, harvesting whole trees or mature forests for pellets or chips often results in net emissions exceeding fossil fuels for decades due to carbon debt from slow forest regrowth—up to 40-50 years in some cases—and supply chain inefficiencies like overseas transport.[80] [81] Biofuel production from crops can drive indirect land-use change, including deforestation, increasing emissions by 20-100% compared to gasoline in lifecycle analyses.[82] Sustainability challenges include competition for arable land with food production, leading to higher food prices and expansion into forests or grasslands, which releases stored carbon and reduces biodiversity.[83] In regions like the southeastern U.S. and Indonesia, biomass demand has been linked to primary forest loss, with certifications such as the Sustainable Biomass Program criticized for inadequate verification and enabling greenwashing.[84] [85] High dependence on biomass in developing countries exacerbates deforestation and soil degradation, contributing to 15% of global anthropogenic GHG emissions from land-use changes.[86] Despite these issues, bioenergy's dispatchable nature provides grid flexibility, though its scalability is limited by feedstock availability and lower energy density compared to fossil fuels.[87]Emerging Technologies
Marine and Tidal Energy
Marine energy encompasses technologies that harness kinetic, potential, and thermal energy from oceans, including tidal streams, waves, currents, and salinity gradients, distinguishing it from more established renewables due to its nascent commercial scale. Tidal energy specifically exploits the gravitational pull of the moon and sun on seawater, creating predictable twice-daily cycles in suitable coastal locations, while wave energy captures irregular surface motions driven by wind. These resources offer high energy density—up to 800 times that of wind at equivalent speeds—but global installed capacity remains under 550 MW as of 2024, dominated by barrage systems rather than modern stream turbines.[88][89][90] Key tidal installations include the 240 MW La Rance barrage in France, operational since 1966 and undergoing €100 million renovations through 2025 to sustain output, and South Korea's 254 MW Sihwa Lake plant, the largest by capacity, which generates over 500 GWh annually despite initial environmental setbacks from reduced oxygen levels. Tidal stream projects, using underwater turbines akin to submerged wind rotors, have progressed slowly; Scotland's MeyGen site hosts the world's largest array at 6 MW operational capacity, with consents for 86 MW and potential expansion to 398 MW, though deployment lags due to grid connection delays. Wave energy converters (WECs), such as point absorbers or oscillating bodies, face greater variability; prototypes like those tested by Ocean Energy Europe total under 1 MW in active operation, with 12.6 MW decommissioned post-demonstration by 2024, highlighting reliability issues in harsh conditions.[91][92][93] Theoretical global potential exceeds 2,000 TWh/year for tidal and wave combined, sufficient to meet significant fractions of regional demand where geography aligns—such as narrow straits or high-wave coasts—but extractable energy is constrained by site-specific flow speeds above 2 m/s for viability and limited suitable estuaries. In the U.S., marine resources could theoretically supply up to 57% of electricity needs, yet practical yields are curtailed by high upfront costs and permitting hurdles. Levelized cost of energy (LCOE) for tidal ranges from 110-480 €/MWh and 160-750 €/MWh for wave, far exceeding unsubsidized solar or wind at under 50 €/MWh, with recent farm estimates at 0.12-0.20 USD/kWh reflecting capital-intensive mooring and cabling in corrosive saltwater.[94][95][96] Deployment faces technical barriers including biofouling, extreme storm survivability, and fatigue from cyclic loading, alongside environmental effects like altered sediment transport, marine mammal collisions with rotating blades, underwater noise disrupting migration, and electromagnetic fields from cabling affecting sensitive species. Barrages can impound estuaries, reducing tidal flushing and impacting fish nurseries, as observed early at Sihwa before mitigation via sluice adjustments. While emissions-free during operation, lifecycle impacts from manufacturing rare-earth magnets in turbines parallel offshore wind concerns, underscoring that scalability hinges on cost reductions via modular designs rather than overhyping predictability as a panacea for intermittency elsewhere in renewables.[97][98][99][100]Enhanced Geothermal Systems
Enhanced geothermal systems (EGS) engineer artificial heat reservoirs in hot, dry rock formations where natural permeability and fluid saturation are insufficient for conventional geothermal extraction.[101] This approach accesses geothermal resources beyond traditional hydrothermal sites, targeting depths of 3 to 10 kilometers where temperatures exceed 150°C but rock impermeability limits fluid flow.[102] By creating engineered permeability, EGS enables closed-loop or open-loop circulation of water or other fluids to transfer heat to the surface for electricity generation via steam turbines, offering a dispatchable, low-emission baseload power source independent of weather or time of day.[103] The core process entails drilling vertical or directional wells using advanced techniques adapted from oil and gas, such as polycrystalline diamond compact bits and managed pressure drilling, followed by hydraulic stimulation to fracture the rock and enhance connectivity between injection and production wells.[104] Fluid is then injected under pressure, heated by conduction from the surrounding rock, and extracted through production wells, with heat exchangers converting thermal energy to electricity at efficiencies of 10-20% depending on reservoir temperature and flow rates.[105] Unlike conventional geothermal, which relies on permeable aquifers, EGS requires precise control of fracture networks to minimize short-circuiting and sustain long-term productivity, often incorporating tracers and seismic monitoring for optimization.[106] Development traces to U.S. Department of Energy (DOE) pilots in the 1970s, such as the Fenton Hill project in New Mexico, which demonstrated feasibility but highlighted stimulation challenges.[107] Recent advancements leverage horizontal drilling and fracking from shale gas, with the DOE's FORGE (Frontier Observatory for Research in Geothermal Energy) in Utah achieving breakthroughs in reservoir creation since 2018.[108] In 2024, Fervo Energy reported successful EGS demonstration in Nevada, producing 3.5 MW with plans to scale to 400 MW by 2028, while DOE-funded projects aim for commercial viability through reduced drilling costs from 20-30% of total capital expenses.[109] International efforts, including in Australia and Europe, focus on superhot rock EGS (>400°C) for higher efficiency, though deployment remains limited to pilots as of 2025.[110] EGS holds potential to supply 20% of U.S. electricity by 2050, tapping an estimated 500,000 exajoules of accessible heat in the continental crust, equivalent to thousands of years of national energy demand at current rates.[104] [103] The DOE's Enhanced Geothermal Shot targets capital costs of $3,700 per kW and levelized cost of electricity (LCOE) below $45/MWh by 2035, down from current estimates of $70-150/MWh driven by drilling and stimulation expenses.[111] Projections indicate LCOE could reach grid parity by 2027 in favorable sites with capacity factors over 90%, outperforming intermittent renewables in reliability.[112] [113] Deployment faces technical hurdles including maintaining fracture permeability over decades, as mineral precipitation and thermal contraction can reduce flow rates by 50% or more post-stimulation.[114] Induced seismicity from fluid injection poses risks, necessitating real-time monitoring and mitigation protocols observed in projects like Basel, Switzerland, where a 2006 pilot triggered a 3.4 magnitude event leading to suspension.[115] [116] High water consumption—up to 10 million liters per MW-year—and upfront costs exceeding $10 million per well limit scalability without subsidies or technological leaps in materials and automation.[117] [118] Despite these, empirical data from recent pilots affirm EGS's causal potential for firm, zero-carbon power if economic barriers subside through iterative field testing.[119]Advanced Storage Innovations
Flow batteries represent a key electrochemical innovation for long-duration energy storage, decoupling power and energy capacity to enable scalable grid applications with cycle lives often exceeding 20,000 full equivalents. Vanadium redox flow batteries (VRFBs), the most mature variant, utilize vanadium ions in differing oxidation states for reversible reactions, achieving round-trip efficiencies of 75-85% and operational lifespans beyond 25 years without significant capacity fade.[120] Recent advancements include non-vanadium alternatives like iron-based flow batteries, which leverage abundant, low-cost materials such as iron salts to potentially halve electrolyte expenses compared to VRFBs while maintaining comparable voltage and safety profiles.[121] [122] These systems address lithium-ion limitations in material scarcity and degradation, with pilot deployments demonstrating multi-hour discharge for renewable smoothing.[123] Mechanical gravity-based storage emerges as a degradation-resistant option for durations of 4-24 hours or more, converting electrical surplus into gravitational potential by lifting composite blocks or pistons in water-filled shafts. Energy Vault's G-VAULT systems, for instance, employ crane-lifted 35-ton blocks stacked in towers, yielding round-trip efficiencies of 80-85% and lifespans over 30 years with no chemical degradation.[124] Gravitricity's underground variants repurpose disused mine shafts, suspending weights up to 2,000 tons to generate power via winch descent, with response times under a second for grid stabilization.[125] These innovations bypass rare-earth dependencies, though site-specific geography limits widespread adoption compared to electrochemical rivals.[126] Thermal energy storage advancements, particularly in molten salt configurations, facilitate dispatchable output from concentrated solar power by storing heat at 565°C for 10+ hours. Innovations like single-tank thermocline designs with particulate fillers reduce material use by 30-50% versus two-tank systems, enhancing cost-effectiveness for hybrid renewable plants.[127] [128] Efficiencies exceed 95% for sensible heat retention, though corrosion and freezing risks necessitate alloyed salts or additives.[129] Chemical hydrogen storage targets seasonal needs by electrolyzing surplus renewable electricity into H2, compressed or converted to ammonia for volumes up to gigawatt-hours, but round-trip efficiencies languish at 30-50% due to electrolysis and reconversion losses.[130] [131] The U.S. Department of Energy identifies it as viable for multi-day buffering where density trumps efficiency, yet empirical pilots reveal 2-3 times higher energy input requirements versus batteries, constraining economic viability absent subsidies.[132] Solid-state batteries, employing ceramic or polymer electrolytes, offer grid-potential enhancements in density (up to 500 Wh/kg) and thermal stability over liquid lithium-ion, mitigating fire risks for stationary use.[133] However, manufacturing scalability and interface dendrite formation persist as barriers, with commercial grid deployments projected post-2030 despite lab efficiencies nearing 90%.[134] These technologies collectively aim to lower levelized cost of storage below $100/kWh by 2030, contingent on material innovations and policy support.[132]Speculative Concepts
Space-based solar power (SBSP) proposes collecting solar energy via large orbital arrays and transmitting it to Earth as microwaves or lasers for conversion to electricity, potentially providing continuous baseload power unaffected by weather or night cycles.[135] Proponents argue it could deliver terawatts of clean energy, with NASA's 2024 study outlining a phased development path starting with small prototypes by 2030, though it estimates launch costs at $1-10 per watt versus under $1 per watt for terrestrial solar.[135] Technical challenges include efficient wireless transmission efficiency (projected at 10-50% end-to-end), orbital assembly requiring in-space manufacturing, and safety concerns over beam alignment to avoid atmospheric or biological disruption, rendering large-scale deployment uneconomic without drastic reductions in space access costs.[135] A 2024 analysis concludes SBSP remains speculative, as ground-based alternatives continue to scale more rapidly and cheaply.[136] Airborne wind energy systems aim to harvest stronger, more consistent winds at altitudes of 200-1,000 meters using tethered kites, drones, or balloons equipped with turbines or generators, potentially yielding 2-10 times the power density of ground-level turbines.[137] China's 2025 S1500 prototype, a 1-megawatt airborne turbine, demonstrates feasibility for off-grid applications by dynamically adjusting height to optimize wind capture, with tests showing reduced material use compared to tower-based designs.[138] However, durability in harsh upper-air conditions, tether management to prevent entanglement, and regulatory hurdles for airspace integration pose significant barriers, with commercialization projected beyond 2030 absent proven long-term reliability.[137] These concepts, while theoretically superior in resource access, hinge on overcoming engineering and economic obstacles that have delayed prototypes for decades.[137]Technical Challenges
Intermittency and Variability
Intermittency in renewable energy refers to the non-dispatchable nature of sources like solar photovoltaic (PV) and wind, where output fluctuates unpredictably due to weather dependencies, contrasting with controllable fossil fuel or nuclear generation.[139] This variability occurs across timescales, from seconds (turbulence in wind) to intra-hour changes (clouds passing over solar arrays), diurnal cycles, daily weather shifts, and seasonal patterns, necessitating additional system flexibility to maintain grid balance.[140] Empirical data from grid operations indicate that high penetrations of these variable renewable energy (VRE) sources increase reserve requirements and risk of supply-demand imbalances, as seen in analyses of major power markets where wind intermittency correlates with elevated curtailment or backup activation.[141] [142] Solar PV generation displays pronounced diurnal variability, with output ceasing entirely at night and peaking around solar noon, achieving typical capacity factors of 21-34% depending on location and insolation class, far below the 24/7 potential of baseload plants.[143] Cloud-induced ramps can exceed 1% of capacity per minute for individual plants, though aggregation across large areas mitigates this to under 13% per 5 minutes in distributed systems; however, such events still strain grid response capabilities without sufficient fast-ramping reserves.[144] Seasonally, solar output peaks in summer months in mid-latitude regions, with inter-annual variability influenced by atmospheric patterns, but remains absent during extended cloudy periods regardless of forecasting accuracy.[145] Wind power exhibits variability driven by wind speed distributions, with onshore capacity factors averaging 36% fleet-wide in the U.S. as of 2022, though subject to rapid changes from gusts or fronts yielding ramp rates that challenge conventional plant flexibility.[146] Diurnally, wind often strengthens at night in many locales, partially offsetting solar's absence, but seasonal patterns differ regionally—for instance, U.S. wind generation peaks in spring and dips in summer, with persistent cycles independent of annual totals over multi-decadal records.[147] [148] Offshore wind shows higher factors (often >40%) but retains intermittency, as correlated system needs amplify effective capacity credits below nameplate ratings.[149] Combining solar and wind provides partial diurnal and seasonal complementarity—solar filling wind's summer lulls and vice versa in winter-dominant wind areas—but imperfect correlations result in residual variability, with studies showing up to 30% swings in combined potential across seasons.[150] [151] NERC assessments highlight that elevated VRE shares, as projected to exceed 20-30% in some regions by 2033, erode reliability margins without enhanced dispatchable capacity or storage, evidenced by increased outage risks in high-renewable scenarios from 2007-2023 data. Grid operators thus require overbuilding VRE capacity (often 2-3 times peak load needs) or flexible backups, as intermittency reduces effective capacity credits to 10-20% for solar in peak summer systems.[152] This dynamic underscores causal limits on VRE scalability without parallel investments in mitigation, per analyses from NREL and IEA modeling.[153][139]Energy Storage Limitations
Renewable energy sources such as solar and wind exhibit significant intermittency, generating power only when sunlight or wind is available, which often mismatches demand patterns and leads to periods of over- or under-supply on the grid.[154] Effective energy storage is essential to shift excess generation to times of scarcity, but current technologies face profound limitations in scale, duration, and cost that prevent reliable, high-penetration renewable systems without fossil fuel backups.[155] As of 2022, global grid-scale battery storage capacity totaled approximately 28 GW, predominantly added in the prior six years, representing a fraction of the terawatt-scale renewable generation capacity worldwide.[155] Lithium-ion batteries dominate grid-scale storage due to their deployability, yet they are optimized for short-duration applications, typically 2-4 hours of discharge at full power, which addresses daily peaks but fails to cover multi-day or seasonal lulls in renewable output.[156] [157] For instance, net load peaks in high-renewable scenarios can extend beyond 8 hours, necessitating long-duration energy storage (LDES) technologies capable of 10-100+ hours, but such systems remain underdeveloped, with lithium-ion economics discouraging durations beyond 4 hours due to diminishing returns on additional capacity.[158] Round-trip efficiencies for lithium-ion systems hover around 85-90%, incurring losses that compound over extended cycles, while battery degradation reduces usable capacity over 10-15 years of operation, limiting long-term viability.[156] Economic barriers exacerbate these technical constraints; although installed costs for battery projects fell 93% from $2,571/kWh in 2010 to $192/kWh in 2024, grid-scale deployment remains capital-intensive, with levelized costs for storage-integrated renewables exceeding those of dispatchable alternatives in many analyses.[159] NREL projections for 2025 indicate utility-scale lithium-ion systems at around $300-400/kWh for 4-hour variants, but scaling to LDES could double or triple expenses due to material demands and unproven engineering.[160] Supply chain vulnerabilities, including reliance on lithium, cobalt, and nickel mining concentrated in geopolitically sensitive regions, further hinder rapid expansion, as global production struggles to meet projected demands for net-zero pathways requiring 35-fold growth in battery capacity by 2050.[161] [155] Alternative storage methods like pumped hydroelectric (which accounts for over 90% of existing capacity) offer longer durations but are geographically constrained, requiring specific topography and water resources unavailable at scale globally.[155] Emerging options such as compressed air, flow batteries, and thermal storage promise 8-24+ hour capabilities, yet face efficiency losses below 70%, high upfront costs, and commercialization delays, with few deployments exceeding pilot stages as of 2025.[162] These limitations collectively underscore that energy storage cannot yet enable renewables to supplant baseload power without overbuilding generation capacity by factors of 2-3 times or retaining fossil/nuclear flexibility, as evidenced by real-world grids like California's, where storage shortfalls contributed to reliability risks during extended low-renewable periods.[163] [164]Grid Integration Requirements
Grid integration of renewable energy sources, particularly variable ones like wind and solar, necessitates adaptations to power system operations traditionally designed around dispatchable synchronous generators. These adaptations address the inherent intermittency and non-synchronous nature of inverter-based resources (IBRs), which contribute minimal rotational inertia compared to conventional fossil fuel or nuclear plants.[165] Low system inertia accelerates frequency deviations following disturbances, requiring enhanced frequency response capabilities to maintain stability within limits such as 59.5–60.5 Hz in North American grids.[166] Grid codes in regions like Europe and Australia now mandate IBRs to emulate inertia through synthetic controls in inverters, providing virtual inertia via rapid power adjustments.[167] Ancillary services form a core requirement, including primary frequency regulation, which IBRs must deliver within seconds using fast-ramping capabilities absent in traditional setups.[168] For instance, NREL analyses indicate that at 50–100% renewable penetration, grids demand augmented reactive power support and voltage ride-through to mitigate fault-induced instability, as IBRs can disconnect en masse without proper controls.[169] Ramping requirements escalate due to intra-hour variability; solar output can fluctuate by 30–50% in minutes from cloud cover, necessitating flexible reserves equivalent to 10–20% of peak load in high-penetration scenarios.[170] Transmission expansions, such as high-voltage direct current (HVDC) lines, are often required to aggregate distant renewable resources and reduce curtailment, with the IEA estimating global needs for 80 million km of new lines by 2040 to accommodate renewables.[171] Operational protocols must evolve to incorporate forecasting accuracy, with errors below 5% for day-ahead wind/solar predictions enabling better reserve scheduling.[172] Demand-side management and energy storage provide balancing, but peer-reviewed studies highlight that without grid-forming inverters—capable of establishing voltage and frequency autonomously—high IBR shares risk cascading failures, as observed in events like Australia's 2016 blackout.[173] Upgrading IBR performance standards, per NERC guidelines, includes mandatory overcurrent injection during faults and seamless black-start capabilities for restoration.[168] These requirements underscore that while technical solutions exist, scaling to 80%+ renewables demands coordinated investments exceeding $500 billion annually globally, per IEA projections, to avoid reliability gaps.[171]Economic Analysis
Cost Structures and LCOE Critiques
The cost structures of renewable energy sources such as solar photovoltaic (PV) and wind power are dominated by capital expenditures (CAPEX), which account for 70-90% of lifetime costs due to the absence of fuel expenses, with operational expenditures (OPEX) limited primarily to maintenance and minor replacements.[174] These CAPEX costs have declined significantly; for instance, solar PV module prices fell by over 80% from 2010 to 2023, driven by economies of scale and manufacturing efficiencies in Asia.[175] Wind turbine costs similarly decreased by approximately 70% over the same period, though offshore installations remain higher at $2,000-4,000 per kW installed capacity as of 2024.[176] However, these structures do not inherently include expenditures for balancing intermittency, such as backup capacity or storage, which can add 20-50% to total system investments in grids with over 30% renewable penetration.[177] Levelized Cost of Energy (LCOE) serves as a common metric to compare generation costs, calculated as the net present value of total lifetime costs (CAPEX, OPEX, financing) divided by the present value of expected energy output, typically expressed in dollars per megawatt-hour ($/MWh).[178] In Lazard's unsubsidized 2025 analysis, utility-scale solar PV LCOE ranges from $29 to $92/MWh, onshore wind from $27 to $73/MWh, and offshore wind from $72 to $140/MWh, reflecting variations in capacity factors (20-30% for solar, 35-50% for onshore wind) and regional factors like solar irradiance or wind speeds.[178] These figures position renewables competitively against new coal ($68-166/MWh) or gas combined-cycle ($39-101/MWh) under baseline assumptions, but LCOE relies on simplified projections of output and discount rates, often assuming 5-7% weighted average cost of capital.[174] [179] Critiques of LCOE emphasize its failure to incorporate intermittency and system-level integration costs, treating variable renewables as equivalent to dispatchable sources despite their low capacity factors and unpredictable output.[180] Standard LCOE omits expenses for firming capacity—such as gas peaker plants or batteries required for reliability—which can elevate effective costs by $25/MWh for wind and $43/MWh for solar when added as a "cost of intermittency" adjustment.[181] For example, in regions like California or Germany with high renewable shares, grid balancing and curtailment costs have driven wholesale prices volatility, with backup needs increasing total system expenses by 50-100% beyond isolated generator LCOE.[182] [183] This metric also neglects transmission upgrades, estimated at $10-30/MWh for remote wind/solar farms, and the opportunity costs of overbuilding capacity to achieve firm power equivalence.[184] Economists like Paul Joskow have argued since 2011 that LCOE distorts comparisons by ignoring output-value correlations, where renewables often generate during low-demand periods, reducing their marginal economic value.[180]| Technology | Base LCOE ($/MWh, 2025 unsubsidized) | Estimated Firming Add-On ($/MWh) | Effective System Cost Range ($/MWh) |
|---|---|---|---|
| Utility-Scale Solar PV | 29-92 | 43 | 72-135 |
| Onshore Wind | 27-73 | 25 | 52-98 |
| Offshore Wind | 72-140 | 25-40 | 97-180 |
Subsidies and Market Distortions
Renewable energy technologies, particularly wind and solar, have received substantial government subsidies worldwide, often in the form of tax credits, feed-in tariffs, and direct payments, which alter competitive dynamics in energy markets. In the United States, federal subsidies for renewables totaled $15.6 billion in fiscal year 2022, more than doubling from $7.4 billion in 2016, with the Production Tax Credit (PTC) offering up to 2.75 cents per kilowatt-hour for qualifying electricity generation and the Investment Tax Credit (ITC) providing a 30% credit on capital costs under the Inflation Reduction Act extensions. These mechanisms reduce the effective cost to developers, encouraging deployment but shifting expenses to taxpayers and obscuring true economic viability when intermittency requires backup capacity. Globally, support for clean energy investments reached $1.7 trillion in 2023, though explicit subsidies form a subset driven by policy mandates rather than market demand alone.[187][188][189] Such subsidies introduce market distortions by incentivizing overproduction during favorable conditions, leading to negative wholesale electricity prices in high-penetration grids. In regions with priority dispatch for subsidized renewables, generators continue outputting power even when market prices fall below zero to capture fixed payments, suppressing signals for efficient resource allocation and stranding investments in flexible generation. For instance, the PTC has amplified negative pricing incidents by rewarding production volume irrespective of market value, distorting flexibility markets and favoring inefficient curtailment over storage or demand response. In Germany, the Renewable Energy Sources Act (EEG) surcharge, which funded feed-in premiums, contributed up to one-fourth of household electricity prices at its peak of 6.88 cents per kWh in 2017, elevating retail costs to sustain subsidized expansion amid rising system integration expenses.[190][191][192] These interventions crowd out unsubsidized alternatives and inflate overall system costs, as subsidies fail to internalize the need for dispatchable backups or grid upgrades, resulting in higher consumer bills despite apparent wholesale price reductions from the merit-order effect. Empirical analyses indicate that while renewables depress spot prices during peak output, the fixed subsidy costs—passed through levies or taxes—exacerbate price volatility and deter investment in baseload capacity, as seen in Europe's increasing reliance on imported fossil fuels during low-renewable periods. In the UK and Germany, policies akin to feed-in tariffs have correlated with sustained high retail prices, with EEG reforms in 2022 reducing the surcharge to 3.72 cents per kWh but not reversing cumulative burdens estimated in tens of billions of euros. Critics, including analyses from energy economists, argue this favors intermittent sources over nuclear or advanced fossils, leading to suboptimal energy mixes where total societal costs exceed unsubsidized benchmarks.[193][194][195]Investment Trends and Projections
Global investment in renewable energy reached record levels in 2024, with clean energy transitions—including renewables—totaling $2.1 trillion, an 11% increase from the prior year, surpassing fossil fuel investments by a factor of approximately 2:1.[196] [197] Solar photovoltaic projects dominated, accounting for the largest share due to continued cost declines, while wind and energy storage saw notable but slower growth amid supply chain constraints and higher upfront costs.[197] These trends reflect policy-driven incentives, such as tax credits and mandates, which have channeled capital despite underlying challenges like intermittency requiring complementary grid and storage investments that lag behind generation capacity additions.[198] In the first half of 2025, investment in new renewable energy projects hit $386 billion globally, up 10% from the same period in 2024, though regional disparities emerged, with U.S. commitments falling to under $40 billion from $57 billion in the second half of 2024 due to policy uncertainties and investor risk reassessments.[199] [200] China continued to lead, capturing over half of announced solar and wind capacity financing, while emerging markets outside China received disproportionately less, exacerbating energy access gaps despite high potential returns in unsubsidized contexts.[201] Fossil fuel investments, meanwhile, remained focused on existing asset maintenance rather than expansion, totaling around $1 trillion annually, as capital shifted toward renewables but highlighted dependencies on government support for the latter's scalability.[202] Projections indicate renewable capacity investments will sustain momentum through 2030, with the International Energy Agency forecasting a near-60% rise in renewable energy consumption across power, heat, and transport sectors under current policies, driven by solar and wind additions exceeding 1,000 GW annually by decade's end.[201] BloombergNEF anticipates energy storage deployments to exceed 92 GW/247 GWh globally in 2025, a 23% increase, underscoring efforts to mitigate variability, though grid infrastructure underinvestment—projected at only $400 billion yearly versus needed $600 billion—poses risks to reliability and cost-effectiveness.[203] These forecasts assume stable subsidies and supply chains, but empirical data from delayed projects and rising material costs suggest potential shortfalls if economic realities, such as levelized costs exceeding dispatchable alternatives without incentives, deter private capital.[197]Comparative Economics
The levelized cost of electricity (LCOE) for new utility-scale solar photovoltaic (PV) installations ranges from $29 to $92 per megawatt-hour (MWh), while onshore wind LCOE ranges from $27 to $73/MWh, according to unsubsidized estimates that exclude tax credits or subsidies.[204] In comparison, combined-cycle natural gas plants have an LCOE of $45 to $108/MWh, new coal plants $69 to $168/MWh, and advanced nuclear $142 to $222/MWh.[204] These figures, derived from U.S.-focused analyses by investment firm Lazard, suggest renewables hold a cost advantage over fossil fuels and nuclear for marginal generation capacity, driven by declining capital costs for solar panels and wind turbines—solar module prices fell over 80% since 2010.[204] However, LCOE calculations often assume average capacity factors and omit intermittency-related expenses, such as the need for overbuilding capacity or pairing with storage; for instance, solar PV plus four-hour battery storage elevates LCOE to $60 to $210/MWh.[204] Capacity factors further underscore economic disparities, as they measure actual output relative to maximum potential. Globally, nuclear plants averaged 81.5% in 2023, enabling near-constant dispatchable power, while coal and natural gas combined-cycle plants typically achieve 40-60%.[205] In contrast, solar PV operates at 20-25% and onshore wind at 30-35% on average, necessitating 3-5 times more installed capacity than dispatchable sources to deliver equivalent annual energy.[206] U.S. Energy Information Administration (EIA) projections for 2030 reflect this in adjusted LCOE: utility solar PV at $26-38/MWh (capacity-weighted), onshore wind at $19-32/MWh, versus natural gas combined-cycle at $46/MWh and advanced nuclear at $67-81/MWh, though these incorporate partial subsidies under the Inflation Reduction Act.[174] Low capacity factors inflate total system capital requirements for renewables, eroding their apparent per-MWh advantage when firm, on-demand power is required. At high penetration levels, integration costs—encompassing backup generation, storage, and grid reinforcements—significantly elevate effective expenses for variable renewables. Estimates indicate $28-32/MWh added for solar and wind at substantial shares (e.g., 30-50% of grid supply), due to balancing supply-demand mismatches and curtailment during oversupply periods.[207] Real-world outcomes bear this out: Germany's Energiewende policy, achieving 59% renewable electricity in 2024, correlates with household prices averaging 0.30-0.40 EUR/kWh, far exceeding France's nuclear-dominant system at around 0.20 EUR/kWh despite similar economic conditions.[208] [209] U.S. averages remain lower at 0.12-0.15 USD/kWh (equivalent to 0.11-0.14 EUR/kWh), supported by diverse dispatchable sources.[208] These differentials arise not merely from generation costs but from renewables' reliance on flexible backups like gas peakers, whose underutilization during renewable peaks imposes hidden inefficiencies absent in baseload nuclear or coal.[210]| Energy Source | Typical Capacity Factor (%) | Unsubsidized LCOE Range ($/MWh, New Build) |
|---|---|---|
| Nuclear | 80-90 | 142-222 |
| Natural Gas CC | 40-60 | 45-108 |
| Coal | 40-50 | 69-168 |
| Onshore Wind | 30-35 | 27-73 |
| Solar PV | 20-25 | 29-92 |