Bulk electrical power extraction through tunnel transmission technology from renewable energy concentrated zones

Domain: neuroscience
Published: 2026-04-02
Editorial outcome: major_revision

Abstract
Mountainous regions, such as the Alps, Appalachians, Andes, and Qingzang Plateau, offer
vast renewable energy potential, but geographical and ecological challenges limit conventional
overhead transmission lines. Underground alternatives, including gas-insulated lines and cables,
provide viable solutions but require comprehensive techno-economic evaluation for sustainable
deployment. This study integrates multi-physics analysis, combining electromagnetic, thermal,
and fluid dynamics, with lifecycle cost assessment per unit transmission task to compare
overhead and underground tunnel transmission systems. Optimized conductor designs and
ventilation strategies enhance cable capacity and improvRe economic performance, particularly in
regions with high external costs. Here, we sh ow that underground tunnel based power
transmission systems, especially gas-insulIated lines, deliver cost-effective, high-capacity
transmission solutions for mountainous renewable energy concentrated regions, outperform
overhead power transmission infrastructure when external costs, including ecological, landscape,
and infrastructure constraRints, are significant, thus advancing the goals of sustainable renewable
energy integration and net-zero.

Introduction
Facing the global energy crisis and increasing urgency of transitioning towards a low-
carbon future1,2, effective utilization of renewable energy has become crucial. By 2027, the global
renewable energy generation capacity is projected to increase by approximately 2,400 GW3. The
capacity and type of renewable energy construction expected to be completed by 2027 in
countries/regions around the world is provided in Supplementary Figure 1. A critical challenge
in realizing this growth lies in transmitting power from remote, resource-rich regions to demand
centers, ensuring feasibility, stability, and alignment with environmental policies and public
expectations4.
Untapped renewable energy resources are particularly concentrated in mountainous and
highland regions5,6, where elevated terrain enhances solar irradiance7, wind speeds8, and
hydropower potential. Other areas, such as offshore wind sites, also represent globally significant
resources, but the present study focuses on mountainous and plateau regions where corridor
scarcity and ecological sensitivity are especially pronounced. However, these areas frequently
overlap with ecologically sensitive or scenic regions9,10. For example, the Appalachian Mountains
in eastern North America hold an estimated 1,859 GW of wind power generation capacity
potential11, while the Andes in South America possess about 243 GW of combined wind and
geothermal generation capacity potential12. The NEuropean Alps offer 19.9 GW of wind generation
capacity potential, 45 GW of solar generat ion capacity potential, and 8.56 GW of hydropower
generation capacity potential8. In Chin L a, the Qingzang Plateau is targeted for the development of
227 GW of hydropower generation capacity and 96 GW of photovoltaic generation capacity13–15.
Indonesia’s Central Range (New Guinea Highlands) contains approximately 26 GW of renewable
generation capacity potential, most of which is hydropower, leveraging the area’s rugged terrain16.
These figures, which are shown in Table 1, are based on regional studies that report only partial,
technically proven reserves rather than the full exploitable potential of each mountain range.
Although the methodologies differ—ranging from geographic information system-based spatial
analysis to hydrological modeling—they consistently highlight the vast untapped renewable
potential of these regions17,18.
However, transmitting renewable energy from these areas faces significant challenges:
Geographical and Corridor Constraints: Mountainous regions require long-distance exports to
lowland load centers, yet feasible overhead corridors are sharply limited by rugged terrain and
regulatory siting restrictions. On the Qingzang Plateau, for instance, four planned transmission
pathways (northern, southern, central, and central-branch) intersect with multiple nature reserves,
leaving only ~16 feasible HV circuits compared to ~37 required for the planned renewable
buildout (≈40% realizable via OHL)19,20. Comparable potential corridor scarcity exists in the
Alps, where Natura 2000 and UNESCO sites restrict routing options, and in the Appalachians,
where national forests and the Appalachian Trail limit siting flexibility. Ecological and Tourism
Protection: Beyond technical siting limits, these mountain regions also carry exceptional
ecological and scenic value21. Overhead lines fragment habitats and impair landscapes, triggering
strong public opposition. In the Alps, willingness-to-pay (WTP) studies indicate more than
$900,000 per kilometer per year to undergroun d lines, reflecting the premium placed on
preserving biodiversity and visual integrity 22,23.
For decades, traditional overLhead transmission lines (OHL) have dominated power
delivery, yet they inadequately address these challenges in renewable-rich mountainous and
plateau scenarios. This Rhas spurred exploration of alternatives, with underground tunnel
transmission emerging as a viable solution for long-distance, high-capacity power transport.
Unlike OHL, tunnels bypass surface constraints and ecological impacts, enabling delivery
through challenging terrains while preserving landscapes24,25. The undergrounding alternatives
include direct-buried and tunnel installations; while direct burial is generally cheaper, in
mountainous, corridor-scarce, and environmentally sensitive segments, continuous trenching is
often infeasible and operators therefore typically adopt accessible cable tunnels for bulk
transmission25.
However, applying underground transmission tunnel technologies—such as cross-linked
polyethylene (XLPE) AC or DC cables or higher-voltage gas-insulated lines (GIL)—to
renewable-rich regions presents multiple unresolved challenges26. First, economic costs are a
primary concern. Although, studies have quantified the additional ecological and landscape costs
of overhead transmission lines (OHLs) and the economic benefits of undergrounding, clear
criteria and guidelines for determining which regions should prioritize underground solutions
versus OHLs, based on regional characteristics, remain absent. Second, the design of current-
carrying capacity for lines within tunnels has been evaluated by finite element analysis,
accounting for electromagnetic and thermal coupling27–30. However, few studies have
investigated how the spatial arrangement of lines within tunnels impacts their current-carrying
capacity31. Third, ventilation is critical for maintaining eRnvironmental stability in long, confined
tunnel spaces. Previous research has proposed vent ilation strategies for cable tunnels using finite
element analysis32, and a recent study oIptimized ventilation by constraining conductor
temperatures and factoring in energy loss costs33,34. However, this work was limited to a single-
tunnel scenario and focused solely on reducing ventilation energy losses, without exploring the
coupled relationship between ventilation and current-carrying capacity in long-distance tunnels.
Furthermore, current research has not sufficiently elucidated the interplay between multi-physics
fields—encompassing electromagnetic, thermal, and fluid dynamics—and economic benefits in
critical tunnel design aspects, such as line layout and ventilation. This gap hinders the
development of comprehensive design guidelines and economic evaluation frameworks for long-
distance tunnel transmission in mountainous regions.
In this work, we present a comprehensive framework for evaluating underground tunnel
transmission in renewable-rich mountainous and plateau scenarios, delivering three key
contributions: (1) A multi-physics coupling methodology integrating electromagnetic-thermal-
fluid finite element analysis with annualized lifecycle cost (LCC) per unit transmission task
(MW-km) evaluation, enabling rigorous comparison of GIL, UGC AC, and UGC DC under
realistic constraints. (2) A techno-economic model that accounts for ecological and landscape
costs, demonstrating that underground solutions, particularly GIL, achieve lower annualized LCC
per unit transmission task (MW-km) than overhead lines when external costs exceed
$550,000/km/year, redefining economic feasibility for renewable energy transmission in
mountainous regions worldwide. (3) Quantitative insights into spatial and ventilation
optimizations, demonstrating up to 57% capacity enhancements (e.g., AC cables from 1,273 A to
2,003 A) and 20% LCC savings in annualized LCC per unit transmission task (MW-km) via
parameterized simulations, providing actionable design gRuidelines for sustainable energy delivery.
This framework equips policymakers and engineers with a decision-support matrix, accelerating
deployment of underground tunnels to uInlock mountainous renewables, foster net-zero
transitions, and balance economic, environmental, and social imperatives.

Results
Comparative Techno-Economic Evaluation of Transmission Technologies for Renewable
Energy Concentrated Zones
To evaluate the economic viability of various high-capacity power transmission methods,
we analyzed alternating current (AC) and direct current (DC) overhead lines (OHL), underground
tunnel AC cables (UGC AC), and gas-insulated lines (GIL).
We selected 500 kV AC and ±600 kV DC overhead lines as benchmarks, as these are
widely adopted and technologically mature transmission systems. Cost data for these lines were
sourced from the 2024 MISO handbook, which provides U.S. power infrastructure construction
costs, with the tower type specified as an angled dead-end structure, suitable for mountainous
and high-altitude regions rich in renewable energy35. While China has implemented mature 1,000
kV AC and ±1,000 kV DC overhead lines with estimated costs of approximately $3,802,580/km
and $1,801,444/km, respectively, these data stem from specific projects and the technologies are
less commonly applied globally36. Thus, they are excluded from the main analysis but provided
in Supplementary Table 1 for reference.
For underground tunnel AC transmission, construction costs were derived from the 2025
IET report on underground cable systems37. Because published cost data for long tunnel GIL
systems remain scarce, we explicitly note that the Su-Tong GIL corridor is a short river-crossing
tunnel (Yangtze crossing) and use it as a conservative reference , while Supplementary Tables 1–
2 compile additional project data (including an Chinese GRIL tunnel) to broaden the evidence base.
Operation and maintenance costs were estimated based on industry averages and multiple
research reports. The Su-Tong GIL project haIs a rated current of 6,300 A. However, based on a
State Grid Corporation of China report and our subsequent studies38, dual-circuit lines operate at
a reduced current of 3,150 A under normal conditions to maintain stable tunnel temperatures.
Therefore, the capacity calculations are based on 3,150 A. Energy loss data for AC and DC
overhead lines were obtained from EIA report39. For UGC AC and GIL, where empirical loss
data are scarce, we conducted coupled electromagnetic-thermal-fluidic finite element analysis,
estimating energy losses of 11.91%/1,000 km for UGC AC and 6.71%/1,000 km for GIL
(Calculation details are shown in the Methods Section). These numbers align closely with
industry expectations and also with the data we collected from State Grid Corporation of China,
which is 15.2%/1,000 km and 5.96%/1,000 km for UGC AC and GIL, respectively. The data are
given in Supplementary Table 2.
Transmission lines in renewable-rich mountainous regions often traverse ecologically
sensitive or scenic areas, where OHLs significantly impact ecosystems and aesthetic value40. To
quantify these costs, we adopted the willingness-to-pay (WTP) metric, using the Alps as a case.
Two studies reported residents’ WTP for undergrounding overhead lines in the Alps at
$984,000/year/km and $892,043/year/km, respectively. We used their average, $938,022/year/km,
for cost estimation22,41.
Although we also investigated underground DC transmission, limited case studies and
data availability constrained the cost analysis. However, we posit that its costs are comparable to
underground AC cables, as both primarily incur additional expenses from tunnel construction and
equipment installation compared to overhead lines.
Table 2 summarizes the economic cost data for thRese transmission methods. Although the
data are drawn from diverse countries, projects, and studies, it still serves a reliable reference for
comparative analysis.
To determine the most suitable transmission method for renewable energy-rich regions,
we evaluated lifecycle unit capacity costs, analyzing total and per-unit-length costs as a function
of transmission distance, which is shown in Fig. 1.
Ecological and landscape costs constitute a significant portion of overhead line expenses,
particularly in sensitive regions like the Alps. These costs vary by region, so we plotted cost
curves with and without ecological considerations. When including ecological costs, overhead
lines exhibit the highest lifecycle unit costs: approximately $430/year/MW/km (DC) and
$400/year/MW/km (AC) at a 500 km distance. Underground AC cables follow at
$321/year/MW/km, and GIL at $263/year/MW/km.
Excluding ecological costs, overhead lines are significantly cheaper, with costs of
$33/year/MW/km (DC) and $28/year/MW/km (AC). Thus, without ecological costs, GIL costs
are roughly 8 times higher than overhead lines, and costs for underground AC cables are
approximately ten times higher. When ecological costs are excluded, overhead AC lines are more
cost-effective for distances below 800 km, while DC lines become preferable beyond 800 km.
This aligns with trends in prior studies42.
Acknowledging that infrastructure costs—particularly tunnel excavation—are highly
project-specific and dependent on local geological conditions, we conducted a comprehensive
sensitivity analysis to validate the robustness of our economic model. We evaluated the impact
of cost variations across three key dimensions: tunnel civil engineering costs (ranging from −50%
to +100% to cover geological complexities), electrical equipment costs, and operation and
maintenance expenses43. The detailed parameter settinRgs and the resulting sensitivity analysis
(presented as a tornado diagram) are provided in Supplementary Table 3 and Supplementary
Figure 2. The results demonstrate that the Ieconomic advantage of GIL is resilient to cost
uncertainties; even under scenarios of significantly higher tunnel construction costs (e.g., in
complex terrain), the GIL system remains a cost-competitive alternative to overhead lines when
high ecological externalities are factored in.
The choice between overhead and underground solutions hinges on ecological, landscape,
and other external costs, such as additional expenses for constructing new lines in fully occupied
corridors, forest clearing, ecological fees, and steep terrain construction. Further analysis reveals
that when these external costs reach $550,000/km/year, overhead lines’ unit costs approach those
of GIL; at $720,000/km/year, they align with underground AC cables. Therefore, we conclude
that underground GIL systems become more cost-effective when external costs exceed
$550,000/km/year in target regions, offering a viable alternative for sustainable, high-capacity
power transmission.
Electromagnetic Safety Analysis of Transmission Technologies in Underground Tunnels
Electromagnetic safety is a critical consideration for underground tunnels housing high-
voltage equipment. The confined geometry and proximity of conductors, maintenance personnel,
and surrounding infrastructure necessitate rigorous evaluation of electric and magnetic field
distributions to ensure operational safety and compliance with regulations44.
Following IEEE standards, occupational exposure limits for 50 Hz AC magnetic fields
are 2,710 μT and electric fields are 5 kV/m; for DC, magnetic fields are limited to 353,000 μT
and electric fields are limited to 5 kV/m (based on stricter head-exposure criteria)45,46. ICNIRP
standards limit electric fields to 10 kV/m and magnetic fields to 1,000 μT for AC 50Hz47. For this
analysis, we adopt the stricter standards: AC (50 Hz): 5 kV/m, 1,000 μT; DC: 5 kV/m, 353,000
μT.
For electric fields, both XLPE cablIes (AC and DC) and gas-insulated lines (GIL)
incorporate grounded shielding layers or enclosures, resulting in external electric fields near zero
during normal operation. However, it is crucial to distinguish normal operation from fault
conditions. While the aRforementioned shielding is effective under steady-state scenarios,
insulation breakdowns—though relatively infrequent with typical failure rates of approximately
0.1 to 0.5 failures per 100 km·year for modern high-voltage systems —pose a different set of
risks. In the event of an insulation fault, short-circuit currents (often reaching tens of kA) will
flow through the cable sheath. Without proper mitigation, this massive current induces significant
longitudinal voltages along the sheath, creating life-threatening touch potentials at the cable
surface. To mitigate this risk, we recommend the installation of Sheath Voltage Limiters (SVLs)
within the system—typically at cross-bonding points—to clamp potential overvoltages to safe
levels. Additionally, implementing an equipotential grounding grid throughout the tunnel is
advised. These measures ensure that the tunnel environment remains safe for maintenance
personnel access.
A more common scenario involves partially grounded shielding due to distant grounding
points. Simulations indicate that with a shielding layer voltage of approximately 1 V, the electric
field at 0.1 m from the conductor is about 3.76 V/m, well below safety thresholds. These
simulation results are detailed in Supplementary Figure 3. For these simulations, excitation
voltages were 500 kV (AC XLPE), 500 kV (DC XLPE), and 1,000 kV (GIL), with currents of
2,000 A (AC), 2,000 A (DC), and 6,000 A (GIL). Thus, electric field intensities and touch
potentials in tunnels are controllable and safe through the combination of conductor shielding
and fault protection measures.
Magnetic field analysis is complex due to the influence of multi-phase conductor
arrangements and shielding interconnection mIethods. We conducted magnetic field simulations
in tunnel environments, evaluating AC and DC cables in circular and square tunnels with stacked
and wall-mounted configurations (see Supplementary Figure 4 for schematics and dimensions).
For GIL, we analyzed the tunnel configuration of the Su-Tong GIL corridor 38 (Supplementary
Figure 5).
Regarding shielding grounding methods, DC cable shielding does not produce induced
voltage due to the absence of time-varying currents, allowing solid bonding48. GIL enclosures,
designed as continuous conductors to carry the return current, significantly reduce external
magnetic fields and transmission losses, typically employing solid bonding or multiple-point
bonding rather than cross-bonding. In contrast, long-distance AC XLPE cables, due to high
shielding losses, generally use cross-bonding. In our tunnel model simulations, DC cable
shielding and GIL enclosures use solid bonding, while AC cable shielding employs cross-bonding.
Magnetic field intensity distributions in tunnel environments are shown in Fig. 2.
In tunnels, GIL exhibits the lowest external magnetic field intensity, posing no health risks
unless directly contacting the conductor surface. DC cables produce higher magnetic fields,
approximately 500 μT within 0.6 m, but these remain well below the DC safety limit of 353,000
μT. AC cables show significantly higher magnetic fields, exceeding 1,000 μT within a 0.3 m
radius for both stacked and wall-mounted configurations. Simulation results suggest that wall-
mounted configurations allow maintenance personnel to avoid high-intensity magnetic fields in
the tunnel’s central walkway. For stacked configurations, placing conductors entirely along
tunnel walls is preferable to minimize exposure.
Spatial Configuration Effects on Power Loss and Thermal Performance in Underground
Tunnels
The operation of transmission syste ms in tunnel environments differs significantly from
overhead lines due to the confined, elongated space with limited heat dissipation49. A primary
challenge is balancing increased current-carrying capacity—enhancing economic value—with
the pronounced thermal effects it induces. Excessive heat, causing conductor temperatures to
exceed 90°C for XLPE cables (or 105°C for GIL)38, reduces conductor lifespan. Variations in
conductor spatial arrangements influence power loss and temperature through magnetic field
coupling and altered heat dissipation structures. To investigate these effects, we conducted finite
element analysis on the impact of conductor spatial arrangements on temperature and power loss
for three tunnel configurations.
We defined spatial parameters based on installation methods: for stacked cable
configurations, L represents the distance between the centers of two circuits; for GIL, L
denotes the horizontal distance between circuits. In square wall-mounted and GIL scenarios, H
indicates the vertical distance between adjacent conductors within the same circuit. In circular
wall-mounted scenarios,  represents the angle between the line connecting the topmost
conductor to the tunnel center and the horizontal axis. These parameters are illustrated in
Supplementary Figure 4. The resulting conductor temperature variations and tunnel thermal
losses are shown in Fig. 3.
For AC cables in stacked configurations with fixed intra-phase distances, increasing the
inter-circuit distance (L) has minimal impact on power loss but reduces conductor temperature.
In circular stacked scenarios, total losses increase slightly from 345 W/m (L = 0.4 m) to 347
W/m ( L = 1.4 m), while temperature decreases from 131.02°C to 126.42°C. In square stacked
scenarios, losses rise from 338 W/m ( L = 0.4 m) to 344 W/m ( L = 1.5 m), with temperature
dropping from 125.41°C to 121.86°C. This indicates that increasing the distance between heat
sources slightly increases losses (primarily shielding layer losses, with conductor losses
unchanged) but enhances heat dissipation, lowering conductor temperature and slightly
increasing current-carrying capacity from 1,273 A to 1,304 A (circular) and 1,310 A to 1,335 A
(square). In wall-mounted configurations, increasing intra-circuit conductor distance
significantly reduces shielding layer losses, from 64 W/m to 2 W/m (circular) and 88 W/m to 3
W/m (square), boosting current-carrying capacity by over 200 A in both scenarios. Further
analysis attributes this to the cross-bonding shielding method used, where larger intra-phase
distances allow balanced three-phase induced currents to neutralize shielding layer currents,
reducing losses. For AC cables, I²R losses dominate across all configurations. In stacked
configurations, shielding layer losses are significant (64–88 W/m, ~18–25% of total losses of
338–347 W/m), despite mitigation by cross-bonding. In wall-mounted configurations, shielding
layer losses drop to 2–3 W/m, making I²R losses even more dominant. AC cables exhibit higher
I²R losses than DC cables due to two factors: (1) shielding layer losses increase conductor
temperature, raising resistance, and (2) skin effect increases effective conductor resistance.
Simulations using a uniform multi-turn conductor model (single turn) estimate I²R losses without
skin effect at 168.37 W/m, compared to 235.63 W/m with skin effect, indicating a skin effect
contribution of ~67.26 W/m (~19% of stacked losses, ~32% of minimum wall-mounted losses).
Note that skin effect is a component of I²R losses, manifesting as increased resistance.
For DC cables, the static magnetic field eliminates shielding layer induced currents and
associated thermal losses, as well as AC-specific skin effects, resulting in lower losses and
temperatures compared to AC cables, unaffected by spatial arrangement. However, increasing
heat source distance improves dissipation, slightly raisiRng current-carrying capacity from 2,605
A, 2,658 A, 2,655 A, and 2,668 A to 2,663 A, 2,6 84 A, 2,714 A, and 2,710 A across the four
scenarios, with increases ranging from 26 A tIo 59 A. For DC cables, I²R losses are the sole loss
mechanism, as shielding layer losses and skin effect are absent.
For GIL, prior magnetic environment analysis shows low external magnetic fields and
minimal inter-phase coupling due to larger line distances, resulting in negligible power loss
changes with spatial arrangement. Capacity increases, driven by improved heat dissipation from
increased L and H , are modest, from 4,411 A to 4,466 A. For GIL, I²R losses dominate (976–
991 W/m, ~70–72% of total losses), with enclosure losses (386–394 W/m, ~28%) also significant,
aligning with the data from a Sutong GIL related reference50. Skin effect contributes ~11% of
total losses, as estimated using the same multi-turn conductor model.
Overall, increasing inter-conductor distance and adopting wall-mounted configurations
significantly enhance AC cable current-carrying capacity, reducing power loss and allowing
increases from 1,273 A to 2,003 A under temperature constraints. For DC cables, the absence of
induced currents limits the impact of spatial changes, and for GIL, low external fields and large
distances minimize loss variations, with capacity gains primarily from improved heat dissipation.
These findings provide a clear guidance for tunnel based power transmission design for
renewable energy dispatch from energy concentrated zones.
Simulations assumed cross-bonded grounding and balanced three-phase currents. With
unbalanced currents, AC cable shielding losses increase. In circular stacked and wall-mounted
scenarios, with phase angles of 0°, ±90°, and 0°, ±60°, shielding losses in stacked configurations
rise from 109 W/m to 144 W/m (0°, ±90°) and 248 W/m (0°, ±60°), and in wall-mounted
configurations from 22 W/m to 37 W/m (0°, ±90°) and 149 W/m (0°, ±60°), which is shown in
Supplementary Figure 6. This significantly reduces cRurrent-carrying capacity, particularly in
stacked scenarios.
Cost-Optimized Ventilation Design for Enhanced Transmission
Beyond reducing power losses, active ventilation is a critical method for lowering
conductor temperatures and maintaining tunnel thermal stability. However, in long-distance
tunnels, both ventilation shaft construction and air velocity control directly impact the economic
costs of transmission51,52. This study employs finite element analysis to optimize ventilation shaft
spacing L and tunnel air velocity
v
venti
w in d
under a minimum-cost objective for AC, DC, and GIL
tunnels.
To address the computational complexity of three-dimensional (3D) simulations, we
developed a two-dimensional (2D) model using a negative heat source to approximate the
complex electro-magnetic-thermal-fluid coupling in 3D simulations. This approach maintains
consistency in key results, such as air heat flux (represented as negative heat source power in 2D),
while reducing simulation time from 5,283 seconds to 47 seconds per scenario (Fig. 4a).
Specifically, during the validation process for converting the 3D model to a 2D one, a 3D model
with a length of 10 meters along the tunnel direction was used; and the air temperature at the inlet
was set to a constant 20°C during the simulation. Notably, the simplified 2D model is capable of
simulating scenarios with any ventilation distance. Details are provided in the Methods section.
Building on the techno-economic analysis and reliable industry cost references, we
established key cost factors, including baseline tunnel construction, ventilation shaft construction,
and ventilation operation costs, to support the cost-optimization analysis (see Methods for cost
assumptions). The results, based on these assumptions and parametric finite element analysis, are
presented in Fig. 4.
Previous analyses constrained conductor core temRperatures to 90°C for cables and 105°C
for GIL to estimate losses and temperature var iations. However, to account for practical
ventilation requirements, this section furtheIr limits tunnel air temperature to 40°C and GIL
enclosure temperature to 70°C50. Consequently, unventilated scenarios yield lower current-
carrying capacities than previously estimated. For round wall-mounted AC tunnels, without
ventilation, the capacity is limited to 1,575 A to maintain air temperature below 40°C. With shaft
spacing of L
ve n ti
= 6 km and air velocity of v
wind
= 0.5 m/s, the capacity increases to 2,002 A; at
L
ve n ti
= 1 km and v
wind
= 3 m/s, it reaches 2,081 A. While higher air velocities could further
increase capacity, our analysis, based on cost assumptions, shows that ventilation energy costs
scale with the cube of air velocity, significantly increasing overall costs. Nonetheless, tunnels can
be designed with high-velocity ventilation potential to accommodate peak transmission demands.
At the optimal point, the annualized LCC per unit transmission task is $405/year/MW/km, higher
than the $321/year/MW/km for UGC AC in the economic analysis, due to a lower simulated
capacity (3,464 MVA) compared to the reference case. Cost assumptions are detailed in Methods,
with optimal cost surfaces shown in Supplementary Figure 7.
For DC tunnels, assuming costs comparable to AC tunnels, the optimal configuration
occurs at
L
ve n ti
= 7 km and v = 0.75 m/s, yielding a capacity of 2,754 A and a cost of
wind
$251/year/MW/km, significantly lower than AC tunnels. This is attributed to the absence of
shielding layer losses and skin effect, resulting in lower total losses and higher capacity under the
same temperature constraints, enhancing economic efficiency. Thus, DC cable tunnels are
recommended for superior cost-effectiveness under equivalent conditions.
For GIL tunnels, the optimal configuration is at L
ve n ti
= 3 km and v
wind
= 1 m/s, with a
capacity of 5,902 A and a cost of $211/year/MW/km, notably lower than the $263/year/MW/km
from the economic analysis. Unlike AC and DC simulations, GIL parameters align with the Su-
Tong GIL project. However, the Su-Tong tunnel, crossing a riverbed, cannot accommodate mid-
tunnel ventilation shafts, restricting shaft spacing to 5.5 km (full length). This limits operating
current below the rated 6,300 A due to thermal constraints. Our simulations demonstrate that
optimizing shaft spacing and air velocity can reduce effective costs by approximately 20%
compared to the Su-Tong project.

Methods
Lifecycle cost analysis
To assess the economic viability of powerP transmission systems, we calculated the
construction costs of transmission lines and substations separately. For new lines, we assumed
the need for substations or converter stations at both ends to match the line’s capacity.
For 500 kV AC and ±600 kV DC overhead lines, construction and substation/converter
station costs, including operation and maintenance, were sourced from the 2024 MISO report35.
For AC substations, the report requires additional parameters such as land area and circuit breaker
counts. We estimated these costs using typical parameters for substations of this voltage class
(see Supplementary Table 4 and Supplementary Table 5 for billing items, unit costs, and
assumptions).
Construction and substation costs for UGC AC and their corresponding capacities were
derived from the 2025 IET report37. For gas-insulated lines (GIL), we referenced the construction
parameters of China’s Su-Tong 1000 kV GIL corridor, with a total cost of 4.763 billion CNY. As
specific cost breakdowns were unavailable, we estimated substation/terminal station costs at 16–
26% of the total, based on similar projects. This portion covers terminal station construction,
auxiliary systems, and project management. Terminal stations ensure seamless integration
between GIL and ultra-high-voltage overhead lines. We used a mid-range estimate of 1.013
billion CNY for these costs.
Lifecycle costs were evaluated over the full operational lifespan of each system. AC and
DC overhead lines were assigned a conservative lifecycle of 30 years, reflecting the minimum
expected duration in challenging mountainous regions61. Underground AC cables have a 30-year
lifecycle, while the tunnels themselves have a minimum lifespan of 50 years. Thus, cable and
tunnel costs were calculated separately, with cables and tunnels accounting for 42.5% and 57.5%
of total costs, respectively, as per the IET report37. For GIL, both the lines and tunnels have a 50-
year lifecycle. Although lifetimes differ by component (OHL: 30 years; UGC cable: 30 years;
tunnels and GIL: 50 years), all costs are annualized consistently over their respective service lives
within the LCC framework.
Operation and maintenance costs for AC overhead lines were estimated at 5% of
construction costs annually62. For DC overhead lines, this was set at 1% per year43. Underground
tunnel maintenance costs are lower relative to construction costs, at approximately 0.5%
annually62. Another study suggests AC overhead and UGC maintenance costs are 1.5% and 0.15%
of construction costs, respectively63, confirming UGC costs rate are roughly one-tenth of AC
overhead lines. Due to limited data on GIL maintenance, we assumed the same 0.5% of
construction costs as UGC AC.
Energy loss data for AC and DC overhead lines were sourced from EIA report39. For UGC
AC and GIL, where data are scarce, we relied on finite element simulations coupling
electromagnetic, thermal, and fluidic effects. For UGC AC (500 kV, dual-circuit, six lines), when
v is 0.5 m/s and
wind
L
ve n ti
is 6 km, the annualized LCC per unit transmission task is optimal with
the total loss was approximately 371.3 W/m at a current of 2000 A. Assuming a power factor of
0.9, the total power is 3117 MW, yielding an energy loss of 11.91%/1000 km. For GIL, when
v
wind
is 1 m/s and L
ve n ti
is 3 km, the loss was 556.8 W/m at a current of 5902 A, with a dual-
circuit power of 18,400 MW (power factor 0.9), resulting in an energy loss of 6.71%/1000 km.
Ecological and aesthetic costs are critical for transmission projects in renewable-rich
mountainous regions, encompassing ecosystem restoration, scenic value degradation,
biodiversity loss, and land reclamation. Quantifying these costs lacks a universal standard, so we
employed the willingness-to-pay (WTP) and hedonic pricing methods to estimate societal costs
of avoiding visual and ecological impacts from overhead lines64. A discrete choice experiment in
Italy’s Alps found that the social marginal benefit of undergrounding per kilometer far exceeds
construction costs, with residents’ WTP at $984,000/year/km22. Another study reported WTP
values ranging from €229,796/km/year to €1,555,223/km/year across different regions, averaging
€771,464/km/year ($892,043/year/km)41. Given the consistency between these studies, we used
their average, $938,022/year/km, for estimation.
Underground tunnels may impact groundwater systems or permafrost due to cable heat,
generating ecological costs. However, these effects are complex, poorly quantified, and less
studied. They are likely far lower than overhead lines’ ecological and aesthetic costs. For
groundwater, open-pit mine dewatering systems cost $10,000–$20,000 per well annually 65. For
permafrost, mitigation costs along the Alaska Highway are approximately $11,000/km/year66.
Adding a conservative $3,000/km/year for drainage system inspection and maintenance, we
estimated total ecological costs for underground cable tunnels at $14,000/km/year and GIL
tunnels at $17,000/km/year. These costs are modest and likely subsumed within the high
construction and maintenance expenses of tunnels, but we include them as a conservative
estimate for a holistic evaluation.
To evaluate the economic feasibility of different transmission technologies, we calculated
the annualized lifecycle cost (LCC) per unit effective power capacity and per unit effective
transmission task, which are shown in Fig. 1. The annualized LCC per effective MW C is
MW
given by:
A +A +O +O +E
C = L S L S C (1)
MW P
eff
The annualized LCC per effective MW per km
C
MW -k m
is derived by:
C
MW -k m
=
C
Md W (2)
A
L
represents the annual amortized line capital cost ($/year), the yearly amortized cost
of the transmission line capital investment. For UGC AC:
A
L
=
C
L ,c
L

c
d
+
C
L ,t
L

t
d
(3)
For other technologies:
C d
A = L (4)
L L
where C
L
is capital cost per kilometer of the transmission line , C
L ,c
and C
L ,t
represent
capital cost per kilometer for the cable component and tunnel component of UGC AC,
respectively. L is the lifecycle (years), L
c
and L
t
are the lifespans of UGC AC cable and tunnel,
which are 30 and 50 years, respectively. d is the transmission distance (km).
A represents the yearly amortized cost of the substation or converter station capital
S
investment, calculated as:
A
S
=
C
L
S (5)
where C is the total capital cost for constructing both end substations or converter
S
stations.
O
L
and O
S
are the yearly cost for maintaining the transmission line, given by:
 O
L
O
=
S
O
=
L
O
,p
S

,p
C

L
C

S
d
(6)
where O
L ,p
is the line operation and maintenance cost proportion of the capital cost per
year, and O
S ,p
is the station operation and maintenance cost proportion of the construction cost
per year.
E
C
is the yearly cost associated with environmental and landscape impacts ($/year),
computed as:
E
C
= E  d (7)
where E is the annual ecological and landscape cost per kilometer ($/year/km).
P
e ff
is the actual power delivered after accounting for energy losses, defined as:
 L d 
P =P  1− loss  (8)
eff  1000 
where P is the rated power (MW), and L is the Energy loss rate (%/1000km).
loss
Tunnel Ventilation Cost Configurations and Assumptions
To evaluate the impact of tunnel ventilation on transmission capacity and optimize
economic costs per unit transmission task (MW·km), we established cost assumptions for
ventilation shaft construction, ventilation equipment, and energy costs associated with air velocity.
Below, we outline the key cost assumptions and their sources.
For AC and DC underground cable tunnels, the annualized lifecycle baseline cost was
derived from the IET report for a 420 kV, 7,482 MVA project, with a total cost of $72,164,405/km,
split into 42.5% for electrical cables and 57.5% for tunnel construction, with lifecycles of 30 and
50 years, respectively37. This yields an annualized cost of $1,852,220/km/year. However, our
model considers a 500 kV AC double-circuit scenario with a rated current of ~2,000 A and power
of 3,464 MVA, significantly lower than the reference case. Thus, we referenced IET costs for 75
km projects at 2,494 MW ($1,226,900/km/year) and 4,988 MW ($1,693,500/km/year),
interpolating linearly to estimate a baseline cost of $1,408,407/km/year for 3,464 MVA. For DC
tunnels, as the tunnel dimensions match those of AC tunnels in our simulations, we assumed
identical baseline construction costs, as the primary expense is the tunnel itself, with minimal
cost differences between AC and DC cables. Additional cost parameters, drawn from the IET
report, Indianapolis water tunnel, HS2 tunnel projects, and related case studies67, include:
ventilation shaft construction at $7,000,000/shaft and maintenance at $46,000/shaft/year;
ventilation energy cost at $28,000/km/year for an air velocity of 1 m/s, scaling cubically with
velocity. These costs vary significantly with geological conditions and tunnel depth, and our
analysis adopts typical values for methodological clarity. Baseline tunnel costs include ventilation
system construction, maintenance, and shaft costs at 5 km intervals. In simulations, we assumed
air enters at 20°C at each shaft, carries heat to the next shaft’s exit, and re-enters at 20°C in the
subsequent segment, allowing the axial tunnel length to be set as the shaft spacing.
For GIL tunnels, the baseline cost was calculated directly from the Su-Tong GIL project,
with a 50-year lifecycle, yielding an annualized cost of $2,016,243/km/year. Due to the larger
tunnel cross-sectional area, ventilation energy costs increase. Fan shaft power (P) scales as
P v
wind
3
4
D

 , where D is tunnel diameter, indicating power is proportional to diameter. Scaling
by tunnel diameter relative to AC/DC tunnels, we estimated GIL ventilation energy cost at
$108,889/km/year for 1 m/s. This aligns with Su-Tong GIL data, where a 160 m³/s airflow
(equivalent to ~1.85 m/s) requires two 210 kW axial flow fans, consuming ~3.68 GWh/year38. At
a global average industrial electricity price of $0.161/kWh68,69, this equates to $592,480/year, or
~$93,575/km/year at 1 m/s, accounting for airflow losses, closely matching our estimate of
$108,889/km/year. For GIL ventilation shafts, based on HS2 project costs for rural tunnels with
similar dimensions and shaft spacing, we assumed construction costs of $16,200,000/shaft and
maintenance costs of $106,000/shaft/year67. The tunnel ventilation cost configurations and
assumptions are also shown in Supplementary Table 6.
Integrated Techno-Economic Multi-Physics Analysis Framework for Underground
Transmission Tunnels
This paper presents a comprehensive, finite element-based framework that integrates
electromagnetic, thermal, and fluid dynamics analyses with economic cost optimization to design
cost-effective underground transmission tunnels, which is shown in Fig. 5.
This integrated approach models gas-insulated lines (GIL), XLPE AC, and DC cables
under varying spatial arrangements and ventilation conditions, providing a unified methodology
for sustainable energy infrastructure. The analysis process comprises four interconnected steps,
detailed below:
1. Line and Tunnel Foundation Modeling:
Finite element models were constructed based on transmission line types, tunnel
dimensions, and material parameters, with a 20°C boundary condition applied 25 m from the
tunnel center to simulate ambient conditions. For GIL, the model includes four layers: inner SF
6
gas, conductor, outer SF gas, and shell, defined by their geometric and material properties
(shown in Supplementary Figure 8). For XLPE cables (model YJLW03-Z-290/500 kV-1, 2,500
mm²), the complex internal structure (e.g., copper conductor, XLPE insulation, semiconductive
buffer, aluminum sheath, HDPE jacket) was simplified by grouping non-critical layers into
composite layers (shown in Supplementary Figure 9 and Supplementary Table 7), reducing
computational complexity while preserving key metallic structures for accurate loss and cost
calculations. The stranded copper conductor was modeled as a single circular cross-section, with
a correction factor
N
con
applied:
N
con
=
A
S
co
co
n
n
(9)
where A
con
is the area of conductor cross-section which is 2500 mm2 in this case, and S
con
is the
cross-sectional area of the conductor in the modeling, which can be calculated as:
S
con
D
c2 o n
2
2 9 4 1 .6 6 m m 2  =
 
 (10)
where D
con
is the diameter of the conductor.
This factor N
con
adjusts thermal and electrical conductivities to ensure precise heat and
loss estimates, critical for economic assessments.
2. Electromagnetic Field Safety Analysis
Electromagnetic field safety was assessed to ensure compliance with tunnel safety
standards, minimizing operational risks. For electric field analysis, a voltage source was applied
to the conductor, with the shielding layer grounded, typically yielding zero external electric field.
High-impedance grounding faults were simulated using a circuit module connecting the shielding
layer to a non-zero resistor before grounding. For magnetic field analysis, a current source was
applied within the tunnel environment, evaluating spatial magnetic field distributions against
safety standards. GIL exhibited low stray magnetic fields, DC cables had higher but safe fields,
while AC cables exceeded limits over a larger range, favoring wall-mounted configurations to
reduce central tunnel impacts and safety-related costs.
3. Line Spatial Position and Heat Loss
Spatial layout parameters (e.g., L, H,  for stacked, wall-mounted, or GIL setups, as in
Results) were defined to optimize capacity and reduce losses. Using a 2D model, parametric
simulations varied current (100 A steps) and layout parameters, with current sources applied to
conductors. Cross-bonding, common in long-distance transmission, significantly reduced AC
cable shielding layer losses. This was modeled by replicating the magnetic field module for three
segments (each one-third of the total length), connecting shielding layers via a circuit module for
cross-bonding, and grounding through a zero or small resistor. Conductor temperature, conductor
losses, and shielding layer losses were analyzed to inform cost calculations. Optimal layouts
minimized temperature or total losses for a fixed current. Under temperature constraints (90°C
for XLPE cables, 105°C for GIL), maximum current-carrying capacity was estimated via
interpolation, guiding layout selection to maximize economic value while ensuring
electromagnetic safety.
4. 3D-to-2D Model Simplification for Ventilation Optimization
Finite element simulations of air, treated as a fluid, and tunnel ventilation typically require
three-dimensional (3D) models to capture axial airflow dynamics. However, 3D simulations are
computationally intensive, with long run times and complex meshing issues, particularly for long
tunnels where transmission line multi-layer structures (e.g., conductor, insulation, sheath) are
small relative to tunnel length. Simplifying these structures alters thermodynamic behavior. To
address this, we developed a two-dimensional (2D) model that uses a negative heat source to
approximate the heat removed by axial airflow, significantly reducing computational complexity
while maintaining accuracy.
The negative heat source q (W) is expressed as:
D2
 ( )v c
air 4 wind p,air
q=− (T −T )L (11)
L mref rep
venti
where
air
 = 1.2kg/ m³ is the air density, D is the tunnel diameter, c
p ,a ir
= 1005 J/(kg·K) is the
specific heat capacity of air, T is the temperature of air, T
m re f
= 20 °C is the reference inlet air
temperature, L
rep
= 1 m representative unit length for calculation. This negative heat source
simulates the cooling effect of forced ventilation. The expression is derived as follows:
1) Air Mass Flow Rate Calculation: The mass flow rate m& of air through the tunnel cross-
section is determined by the air density, cross-sectional area A D 2 4  = , and axial air velocity:
m
a ir
A v
wind a ir
( D 2 4 ) v
wind
   & =   =   (12)
2) Heat Removal over a Ventilation Segment: Over a segment L
ve n ti
, the air absorbs heat,
causing a temperature rise  T = T
o u t
− T
m re f
. The total heat removal rate Q (in W) for the
segmment is given by the connective energy balance:
Q = m &  c
p ,a ir
  T (13)
Here,  T is approximated as (T −T ) , where
mref
T represents the local average
temperature in the air domain, and T
m re f
is the inlet reference temperature, providing a linear
approximation for heat transfer.
3) Heat Removal per Unit Length: To distribute the heat removal along the tunnel, the
rate per unit length q
l
(in W/m) is:
Q m&c (T −T )
q = = p,air mref (14)
l L L
venti venti
For the representative unit length L
rep
(set to 1 m for simplicity in the 2D cross-section),
the adjusted negative heat source q=−q L .
l rep
This negative heat source, derived from air mass flow rate, heat removal over a ventilation
segment, and distribution per unit length, reduced simulation time from 5,283 s to 47 s (Fig. 4a)
while capturing cooling effects. Parametric simulations varied shaft spacing
L
ve n ti
, air velocity
v
wind
, and current, with conductor (90°C for cables, 105°C for GIL) and air (40°C) temperature
constraints. Capacity, losses, and LCC per unit transmission task were calculated using cost
assumptions from Methods, yielding optimal cost surfaces (Supplementary Figure 7). The cost-
optimal L
ve n ti
and v
wind
balanced capacity gains with ventilation energy costs, optimizing
economic performance.
In summary, this integrated techno-economic framework unifies electromagnetic, thermal,
and fluid dynamics analyses with economic cost optimization to design efficient underground
transmission systems for renewable energy. Through baseline modeling, electromagnetic safety
analysis, spatial layout thermal assessment, and 2D ventilation optimization, it provides a
comprehensive methodology for simulating and optimizing tunnel systems. By incorporating
multi-physics simulations and cost evaluations, the framework guides the planning, design, and
operation of sustainable energy infrastructure, maximizing capacity, ensuring safety, and
minimizing LCC per unit transmission task.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper analyzes and anticipates underground tunnel power transmission technology as a promising candidate to the problem of transporting energy from mountainous regions. The performances of overhead line, underground cable, as well as GIL are discussed, such as costs, electromagnetic safety, et al. However, overall, the conclusions in this paper still need to be further verified, and some analysis methods used in this paper are lack of credibility.\n1.In Table I, it is recognized that the ecological cost, public opinion cost and climatic disaster loss of UGC and GIL are all zero. Obviously, it is too arbitrary and idealized. For example, the underground cable is also affected by the natural disaster, such as floods, earthquakes, mudslides.\n2.Correspondingly, the conclusion that “with a typical typical transmission line life cycle of 40 years, underground transmission tunnels will even be 72.4% cheaper than overhead transmission lines.” looks beyond normal cognition. If this conclusion is right, all of the overhead line should be replaced by the underground tunnels. But actually, most of the high voltage transmission uses the overhead line.\n3.Similarly, it is claimed that “the 1,000 kV GIL can carry roughly six times the power capacity of a 500 kV XLPE cable, commercially available cable solution under state-of-art technology level. Under conditions demanding high-density, large-capacity transmission, GIL technology therefore may offer superior overall cost-effectiveness.” This conclusion is very arbitrary and lack of theoretical support. The authors hope to obtain the conclusion underground cable of GIL is better than overhead line for large capacity and long distance transmission. This is contrary to objective facts.\n4.What is the main innovation of this paper? This manuscript looks like an analysis report, while not a scientific paper.\n5.In addition, the dc transmission technology should also be discussed. Because compared with ac transmission, HVDC system is more suitable for long-distance transmission.", "reviewer_2": "This paper presents an analysis of underground tunnel power transmission—specifically Gas Insulated Lines (GIL) and cables—as a robust solution for renewable energy transport, highlighting their advantages over overhead lines in terms of environmental resilience. Through finite element simulations, the authors evaluated key performance factors and offered some recommendations for optimizing conductor layout, technology selection, and thermal management.\n1. Introduction, sentence 2: please review for grammar and coherency.\n2. Introduction: “the Papua Mountains” – please name the mountains using a common geographical name to avoid any ambiguity.\n3. Figure 1: The bottom part of the figure does not help in understanding the constraints in relation to particular technologies. It is suggested to replace it with a tabled description.\n4. Introduction: “3. Extreme Weather Hazards: These regions often have highly variable terrain and extreme climates 21, leading to challenges such as repeated icing, wildfires, and microclimatic difficulties22.” – The meaning of the phrase “microclimatic difficulties” is not clear. The authors are asked to explain its relation to the extreme weather hazards.\n5. Introduction: “This configuration greatly reduces the footprint, environmental impact, and susceptibility to external weather conditions, while maintaining stable performance under a wide range of operational scenarios.” – It is suggested to support this statement with a relevant reference.\n6. Introduction: “(1) Developing a triad evaluation methodology integrating coupled electromagnetic-thermal-fluidic finite element modeling with life-cycle cost analysis, enabling holistic comparison of GIL and cable systems under real-world constraints;” - please review for grammar and coherency.\n7. Results, Subsection “Cost, Environment, and Public Perception: Evaluating Transmission Economics”, paragraph 3: It is said that the nominal capital cost of a GIL solution is about ten times that of a tunnel-based cable system, but it is actually 11.5 times. Please provide a correct number or rephrase (e.g., to say it is an order higher of that…).\n8. The thickness of the cable and the GIL should be mentioned before Figure 2.\n9. Figure 3: visibility must be improved. Particularly, the details in subfigure c cannot be seen well.\n10. Figure 5b: please explain the analysis of the conductor temperature variation with spatial positions between the GIL circuits. How was it obtained and how can the results be interpreted?\n11. Figure 6: different traces (of angles and distances) are only visible in subfigure d, but are overlapping and in other subfigures. Figure 6 must be improved or made in a different fashion.\n12. Figure 8: visibility is low. The scales on the right of the subfigures cannot be seen at all.\n13. Results, last paragraph: “Specifically, the current simulation considers only a 10 m tunnel segment with an inlet air temperature fixed at 20 °C.” – it is recommended to explain it earlier, but not at the end of the section.\n14. Methods, Thermal effect finite element modeling: It would be good to cover more scenarios and consider different phase angles (e.g., 30°, 60°, 90°, 120°, 150°) rather than just 0° and normal (presumably 120°). Is it possible to identify which loss mechanisms (e.g., I²R loss, skin effect, shielding layer loss) dominate in which configurations?\n15. Methods, Simplification of conductor and shield layer losses in the 3D model: “As a result, it is not feasible to simplify this portion of the heat sources a fixed one.” - please check for grammar and coherency.\n16. Methods, last paragraph: “The simplified curves for various spatial positions are plotted in Supplementary Figure 7a, and the theoretical calculations almost perfectly match the simulation results, with all five curves exhibiting a correlation coefficient ( ) of 1.0000.” It is recommended to demonstrate the comparison results between the analytical method and the simulation in a table form. What is the accuracy of the simulation?", "reviewer_3": "* I don't see a clear focus in the paper. What exactly is it trying to show? The content is far too broad (ranging from flow calculations to cost estimates) without really delivering anything new or going into enough detail to be able to make general statements.\n* One example is the tunnel construction measures together with geological challenges, which are certainly the main influencing factors for projects of this kind and are completely neglected in the paper. In addition, many of the figures, especially the cost estimates, are incomprehensible or, in my opinion, simply incorrect. In any case, much more precise and comprehensible source references would be necessary here.\n* In terms of methodology, individual aspects are picked out, but there is no overall view. Take line 423 as an example: why does the tunnel cost so much more for a GIL? Furthermore, it makes no sense to compare lines with different transport capacities in monetary terms. From an electrical engineering perspective, there are also a number of aspects that need to be addressed (e.g., connection to the existing grid) that have been completely neglected.\n* With more precise information for a specific example, the content of the paper could certainly provide an interesting comparison as a case study , but it is far from a general assessment from which meaningful conclusions can be drawn.", "reviewer_4": "Reviewed the article “Bulk electrical power extraction through tunnel transmission technology from renewable energy concentrated zones” and suggested the following comments;\n1.0 Abstract should be improved with key findings, real-world relevance, and specific challenges to engage readers more effectively.\n2.0 The introduction section is weak and needs to be enhanced with a comparative analysis of recent works and an emphasis on the novelty of your work.\n3.0 Cite a suitable reference at line number 97 to demonstrate the significance of your work.\n4.0 Some sentences are lengthy (e.g., the first sentence). Breaking them into shorter, clearer statements would improve readability.\n5.0 Does the 28.4% improvement assume ideal conditions? Discuss real-world limits (e.g., tunnel geometry, maintenance access).\n6.0 This article would benefit from clearer methodological details, validation, and contextualization.\nThe paper can be accepted after this major revision", "reviewer_5": "General: Has outage time due to failures been estimated for cables and for GIL? Both risk of failure and time-to-repair are of importance and need to be quantified in economic terms.\nGeneral: The availability of the transmission (in terms of hours per annum) is of utmost importance and needs to be addressed and need to be quantified in economic terms.\nGeneral: The National Grid Operator in Sweden has published information regarding relative merits of cable versus overhead lines, giving very strong arguments in favour of overhead lines. It is unfortunately in Swedish, but could probably be useful to browse using a translation app. See https://www.svk.se/om-kraftsystemet/om-transmissionsnatet/teknik/\nDetailed comments:\nLine 42: 2400 GW would represent ”installed generation capacity” and not energy which is measured in TWh.\nLine 47-48: the sentence “significant amount of untapped renewable energy is generally concentrated in plateaus worldwide” seems to indicate that other areas do not have such resources. The fact is that several other areas are of great interest internationally, for example sea-based wind power. Pls clarify sentence\nLine 47-48: Pls provide arguments why only high-altitude sites are discussed\nLine 52-57: energy is confused with power. Be sure to call GW resources “power resources”\nLine 52-57: Stating that the Andes mountain range has only 0.3% of exploitable wind power compared to the Appalachian mountain range is very surprising considering the vast area of the Andes. Pls clarify text.\nLine 52-57: the reviewer strongly doubts the accuracy of cited renewable power reserves. Are the figures taken out of different contexts?\nLine 71: The phrase “of the planned construction capacity” is slightly unclear. The word “construction” usually refers to building activities. A better formulation could be : “of the installed generation capacity”\nLine 84-85: Explain how an overhead transmission line can be “restricted by surface channels”?\nLine 89: 500 kV XLPE AC cables are not suitable for longer distances due to the reactive power of such cables. DC cables are typically used for long distance interties – e.g. from wind turbine fields in the North Sea and north-south intertie in Germany. (however never in tunnels!). Explain why discussion is limited to AC cables\nLine 92: The phrase “pressurized insulating gas (typically SF6)” may be in some contradiction to current restrictions on use of SF6 and is opposed by the Kyoto protocol regarding reduction of global warming by reducing greenhouse gases. The matter needs further elaboration.\nLine 97: The example of the Su-Tong is an example of a short connection between overhead lines (crossing Yang Tse river). How is that showing promise for GIL in long distance interties?\nLine 108: “In addition, critical technological questions remain largely unexplored.” Subsequent paragraphs do not discuss why AC cables would benefit from being in a tunnel versus buried in the ground.\nLine 108 continued: Why are DC cables not discussed? Choice of tunnel vs buried cable.\nLine 117: term “transformative contributions” has unclear meaning, pls clarify\nLine 118: does “triad evaluation” mean “three-pronged”?\nLine 118: “electromagnetic-thermal-fluidic” is not understood by the reviewer. Please elaborate\nLine 123: Three-digit accuracy of this estimate (72.4%) is not credible. More reasonable would be to state that “normalized levelized costs compared to overhead lines across mountainous terrain scenarios could be cut to one third”\nLine 123 contnd. What have the costs been normalized against?\nLine 123 contnd. What are “levelized costs”?\nLine 129: three-digit precision of this estimate is not credible. Write “c:a 28%”\nLine 143-145: Weather induced failures of overhead transmission are cast as a major advantage for cables and GIL. However, failures occur for both cables and GIL (possibly due to other causes) and in such cases time-to repair is substantially longer than for overhead lines. Failure rates and time-to-repair of cables and of GIL need to be discussed.\nLine 148 - 150: Impression is given that cables and GIL have no failures. This is not true. Better discussion needed.\nLine 155, Table 1: Ecological cost is exemplified for “scenic spot or ecological protection zone” Why is this cost applied for the whole high voltage intertie? Most of the intertie’s length would presumably not be subject to this cost:\nLine 159: the estimated time to reach equivalence in cost for OHL and UGC is strongly questioned in view of comment to Line 155.\nLine 161: Overhead line life expectancy could reasonably be estimated to be twice that of cables\nLine 170 ff: While it is not surprising that electric fields are negligible in the vicinity of both cable and GIL, it seems that the very substantial risk of very high touch potentials that can occur if the cable or GIL experiences an insulation fault has been neglected. The possible touch potential is life threatening.\nLine 182, Fig 2 e and f: It seems that the x-axis shows distribution along the length of the cable/GIL. Radial distribution would be expected. Pls clarify\nLine 182, Fig 2 e and f: Assuming that the position axis depicts radial distribution and has radius 0 located at the center of the conductor: Red curve in f) shows an electric field of 0.6 V/m at position 0. This is the inside the conductor core where field should be zero. Pls rectify or comment.\nLine 182, Fig 2 e and f: Assuming that the position axis depicts radial distribution and has radius 0 located at the center of the conductor: Red curve in f) shows an electric field of ~0.55 V/m at position 0.1 m. This is at the surface of the conductor core where field should be on the order of 2 MV and not 0.55 V. Pls rectify or comment.\nLine 194: Reference is made to recommended public exposure to magnetic fields. The public is not expected to have access to the tunnel. Reference should be made to occupational exposure of professionals to magnetic fields which is 1000µT at 50 Hz (see: ICNIRPLFgdl.pdf table 3)\nLine 202: ICNIRP guidelines accept external electric field strength up to 10 kV/m at 50 Hz (see: ICNIRPLFgdl.pdf table 3) Why do you cite only 10 V/m?"}, "author_response": "Response to Reviewers' Comments:\nReviewer 1\nComments to Author\nComment 1:\nIn Table I"}]

Decision letter