Hoffert, Martin I., et al. November 1, 2002. Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet, Science. Vol 298,:981-987.

Renewables

Renewable energy technologies include biomass, solar thermal and photovoltaic, wind, hydropower, ocean thermal, geothermal, and tidal (36). With the exception of firewood and hydroelectricity (close to saturation), these are collectively <1% of global power.

All renewables suffer from low areal power densities.

Biomass plantations can produce carbon-neutral fuels for power plants or transportation, but photosynthesis has too low a power density (∼0.6 W m⁻²) for biofuels to contribute significantly to climate stabilization (14, 37). (10 TW from biomass requires >10% of Earth’s land surface, comparable to all of human agriculture.)

PV and wind energy (∼15 W_em⁻²) need less land, but other materials can be limiting [Pacca].

For solar energy, U.S. energy consumption may require a PV array covering 10,000 square miles (a square ∼160 km on each side (26,000 km²) (38). The electrical equivalent of 10 TW (3.3 TW_e) requires a surface array ∼470 km on a side (220,000 km²). However, all the PV cells shipped from 1982 to 1998 would only cover ∼3 km²(39). A massive (but not insurmountable) scale-up is required to get 10 to 30 TW equivalent.

More cost-effective PV panels and wind turbines are expected as mass production drives economies of scale. But renewables are intermittent dispersed sources unsuited to baseload without transmission, storage, and power conditioning. Wind power is often available only from remote or offshore locations. Meeting local demand with PV arrays today requires pumped-storage or battery-electric backup systems of comparable or greater capacity (40). “Balance-of-system” infrastructures could evolve from natural gas fuel cells if reformer H₂ is replaced by H₂from PV or wind electrolysis (Fig. 2A). Reversible electrolyzer and fuel cells offer higher current (and power) per electrode area than batteries, ∼20 kW_em⁻² for proton exchange membrane (PEM) cells (21). PEM cells need platinum catalysts, >5 × 10⁻³ kg Pt m⁻² (41) (a 10-TW hydrogen flow rate could require 30 times as much as today’s annual world platinum production). Advanced electrical grids would also foster renewables. Even if PV and wind turbine manufacturing rates increased as required, existing grids could not manage the loads. Present hub-and-spoke networks were designed for central power plants, ones that are close to users. Such networks need to be reengineered. Spanning the world electrically evokes Buckminster Fuller’s global grid (Fig. 2B). Even before the discovery of high-temperature superconductivity (42), Fuller envisioned electricity wheeled between day and night hemispheres and pole-to-pole (43). Worldwide deregulation and the free trade of electricity could have buyers and sellers establishing a supply-demand equilibrium to yield a worldwide market price for grid-provided electricity.

Space solar power (SSP) (Fig. 3, A and B) exploits the unique attributes of space to power Earth (44,45). Solar flux is ∼8 times higher in space than the long-term surface average on spinning, cloudy Earth. If theoretical microwave transmission efficiencies (50 to 60%) can be realized, 75 to 100 W_e could be available at Earth’s surface per m² of PV array in space, ≤1/4 the area of surface PV arrays of comparable power. In the 1970s, the National Aeronautics and Space Administration (NASA) and the U.S. Department of Energy (DOE) studied an SSP design with a PV array the size of Manhattan in geostationary orbit [(GEO) 35,800 km above the equator] that beamed power to a 10-km by 13-km surface rectenna with 5 GW_eoutput. [10 TW equivalent (3.3 TW_e) requires 660 SSP units.] Other architectures, smaller satellites, and newer technologies were explored in the NASA “Fresh Look Study” (46). Alternative locations are 200- to 10,000-km altitude satellite constellations (47), the Moon (48, 49), and the Earth-Sun L₂Lagrange exterior point [one of five libration points corotating with the Earth-Sun system (Fig. 3C)] (50). Potentially important for CO₂ emission reduction is a demonstration proposed by Japan’s Institute of Space and Aeronautical Science to beam solar energy to developing nations a few degrees from the equator from a satellite in low equatorial orbit (51). Papua New Guinea, Indonesia, Ecuador, and Colombia on the Pacific Rim, and Malaysia, Brazil, Tanzania, and the Maldives have agreed to participate in such experiments (52). A major challenge is reducing or externalizing high launch costs. With adequate research investments, SSP could perhaps be demonstrated in 15 to 20 years and deliver electricity to global markets by the latter half of the century (53, 54).

Fission and Fusion

Nuclear electricity today is fueled by ²³⁵U. Bombarding natural U with neutrons of a few eV splits the nucleus, releasing a few hundred million eV (²³⁵U + n → fission products + 2.43n + 202 MeV) (55). The²³⁵U isotope, 0.72% of natural U, is often enriched to 2 to 3% to make reactor fuel rods. The existing ∼500 nuclear power plants are variants of ²³⁵U thermal reactors (56, 57): the light water reactor [(LWR) in both pressurized and boiling versions]; heavy water (CANDU) reactor; graphite-moderated, water-cooled (RBMK) reactors, like Chernobyl; and gas-cooled graphite reactors. LWRs (85% of today’s reactors) are based largely on Hyman Rickover’s water-cooled submarine reactor (58). Loss-of-coolant accidents [Three Mile Island (TMI) and Chernobyl] may be avoidable in the future with “passively safe” reactors (Fig. 4A). Available reactor technology can provide CO₂ emission–free electric power, though it poses well-known problems of waste disposal and weapons proliferation.

The main problem with fission for climate stabilization is fuel. Sailor et al. (58) propose a scenario with²³⁵U reactors producing ∼10 TW by 2050. How long before such reactors run out of fuel? Current estimates of U in proven reserves and (ultimately recoverable) resources are 3.4 and 17 million metric tons, respectively (22) [Ores with 500 to 2000 parts per million by weight (ppmw) U are considered recoverable (59)]. This represents 60 to 300 TW-year of primary energy (60). At 10 TW, this would only last 6 to 30 years—hardly a basis for energy policy. Recoverable U may be underestimated. Still, with 30- to 40-year reactor lifetimes, it would be imprudent (at best) to initiate fission scale-up without knowing whether there is enough fuel.

What about U from the seas? Japanese researchers have harvested dissolved U with organic adsorbents from flowing seawater (61). Oceans have 3.2 × 10⁻⁶ kg dissolved U m⁻³ (62)— a ²³⁵U energy density of 1.8 MJ m⁻³. Multiplying by the oceans’ huge volume (1.37 × 10¹⁸ m³) gives 4.4 billion metric tons U and 80,000 TW-year in ²³⁵U. Runoff and outflow to the sea from all the world’s rivers is 1.2 × 10⁶m³ s⁻¹ (63). Even with 100%²³⁵U extraction, the flow rate needed to make reactor fuel at the 10 TW rate is five times as much as this outflow (64). Getting 10 TW primary power from²³⁵U in crustal ores or seawater extraction may not be impossible, but it would be a big stretch.

Despite enormous hurdles, the most promising long-term nuclear power source is still fusion (65). Steady progress has been made toward “breakeven” with tokamak (a toroidal near-vacuum chamber) magnetic confinement [Q ≡ (neutron or charged particle energy out)/(energy input to heat plasma) = 1] (Fig. 4B). The focus has been on the deuterium-tritium (D-T) reaction (→ ⁴He + n + 17.7 MeV). Breakeven requires that the “plasma triple product” satisfy the Lawson criteria: n × τ ×kT ≈ 1 × 10²¹ m⁻³ s keV for the D-T reaction, where n is number density; τ, confinement time; T, temperature; and k, Boltzmann’s constant (66, 67). Best results from Princeton (Tokamak Fusion Test Reactor) and Europe (Joint European Torus) are within a factor of two (68). Higher Qs are needed for power reactors: Neutrons penetrating the “first wall” would be absorbed by molten lithium, and excess heat would be transferred to turbogenerators. Tritium (12.3-year half-life) would also be bred in the lithium blanket (n + ⁶Li → ⁴He + T + 4.8 MeV). D in the sea is virtually unlimited whether utilized in the D-T reaction or the harder-to-ignite D-D reactions (→ ³He + n + 3.2 MeV and → T + p + 4.0 MeV). If D-T reactors were operational, lithium bred to T could generate 16,000 TW-year (69), twice the thermal energy in fossil fuels. The D-³He reaction (→ ⁴He + p + 18.3 MeV) is of interest because it yields charged particles directly convertible to electricity (70). Studies of D-³He and D-D burning in inertial confinement fusion targets suggest that central D-T ignitors can spark these reactions. Ignition of D-T–fueled inertial targets and associated energy gains of Q ≥ 10 may be realized in the National Ignition Facility within the next decade. Experiments are under way to test dipole confinement by a superconducting magnet levitated in a vacuum chamber (71), a possible D-³He reactor prototype. Rare on Earth, ³He may someday be cost-effective to mine from the Moon (72). It is even more abundant in gas-giant planetary atmospheres (73). Seawater D and outer planet³He could power civilization longer than any source other than the Sun.

How close, really, are we to using fusion? Devices with a larger size or a larger magnetic field strength are required for net power generation. Until recently, the fusion community was promoting the International Thermonuclear Experimental Reactor (ITER) to test engineering feasibility. Enthusiasm for ITER waned because of the uncertainty in raising the nearly $10 billion needed for construction. The U.S. halted ITER sponsorship in 1998, but there is renewed interest among U.S. fusion scientists to build a smaller-sized, higher-field, non-superconducting experiment or to rejoin participation in a half-sized, redesigned ITER physics experiment. A “burning plasma experiment” could produce net fusion power at an affordable scale and could allow detailed observation of confined plasma during self-heating by hot alpha particles. The Fusion Energy Sciences Act of 2001 calls on DOE to “develop a plan for United States construction of a magnetic fusion burning plasma experiment for the purpose of accelerating scientific understanding of fusion plasmas (74).” This experiment is a critical step to the realization of practical fusion energy. Demonstrating net electric power production from a self-sustaining fusion reactor would be a breakthrough of overwhelming importance but cannot be relied on to aid CO₂ stabilization by mid-century.

The conclusion from our ²³⁵U fuel analysis is that breeder reactors are needed for fission to significantly displace CO₂ emissions by 2050. Innovative breeder technologies include fusion-fission and accelerator-fission hybrids. Fissionable²³⁹Pu and/or ²³³U can be made from²³⁸U and ²³²Th (75). Commercial breeding is illegal today in the United States because of concerns over waste and proliferation (France, Germany, and Japan have also abandoned their breeding programs). Breeding could be more acceptable with safer fuel cycles and transmutation of high-level wastes to benign products (76). Th is the more desirable feedstock: It is three times more abundant than U and ²³³U is harder to separate and divert to weapons than plutonium. One idea to speed up breeding of ²³³U is to use tokamak-derived fusion-fission hybrids (68, 77). D-T fusion yields a 3.4-MeV alpha particle and a 14-MeV neutron. The neutron would be used to breed ²³³U from Th in the fusion blanket. Each fusion neutron would breed about one ²³³U and one T. Like²³⁵U, ²³³U generates about 200 MeV when it fissions. Fission is energy rich and neutron poor, whereas fusion is energy poor and neutron rich. A single fusion breeder could support perhaps 10 satellite burners, whereas a fission breeder supports perhaps one. A related concept is the particle accelerator-fission hybrid breeder (56): Thirty 3-MeV neutrons result from each 1000-MeV proton accelerated into molten lead; upon injection to a subcritical reactor, these could increase reactivity enough to breed ²³³U from Th, provide electricity, and power the accelerator efficiently (∼10% of the output). The radiotoxicity of hybrid breeder reactors over time is expected to be substantially below LWRs.

These ideas appear important enough to pursue experimentally, but both fission and fusion are unlikely to play significant roles in climate stabilization without aggressive research and, in the case of fission, without the resolution of outstanding issues of high-level waste disposal and weapons proliferation.

Concluding Remarks

Even as evidence for global warming accumulates, the dependence of civilization on the oxidation of coal, oil, and gas for energy makes an appropriate response difficult. The disparity between what is needed and what can be done without great compromise may become more acute as the global economy grows and as larger reductions in CO₂-emitting energy relative to growing total energy demand are required. Energy is critical to global prosperity.

References

Cembalest M (2021) 2021 Annual Energy Paper. JP Morgan Asset & Wealth Management.