Experts question new energy sources. Oct 31, 2002. AP.
(APOnline) — None of the known alternate energy sources are technically ready to take the place of fossil fuels experts say in a new study: Martin Hoffert, et al 2002 Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet, Science. Vol 298).
The study by 18 scientists and engineers in university, government and private labs evaluated technologies that would make energy without burning oil, coal or natural gas and found that no single system or combination of systems could replace these fossil fuels.
Hoffert said a combination of renewable energy sources — such as wind and solar power generation, or electrical power beamed from orbiting solar satellites, and nuclear fusion power plants — “are theoretically capable of keeping our civilization going into the future, but the problem is that we haven’t taken the challenge seriously enough to do research in it. We are putting practically nothing into really, seriously studying the problem.”
In 2002, the world’s power consumption was over 12 trillion watts, with 85% of it produced by burning fossil fuels [2008: 15 trillion watts, 81% from fossil fuels].
The study surveyed the entire field of alternate energy and found most systems have serious technical problems such as:
- Nuclear fission: It is not the final answer because of a shortage of uranium fuel. The proven reserves of uranium would last less than 30 years if nuclear fission was used to make 10 trillion watts of power, about a third of what will be needed by the end of the century.
- Solar power: To meet the current U.S. needs with solar power would require sun collectors covering some 1,000 square miles. To make the equivalent of 10 trillion watts of added power would require surface arrays covering almost 85,000 square miles, an area larger than the state of Kansas.
- Wind power: These systems must operate from remote areas and the current power grids could not manage the load.
- Solar power satellites: Orbiting solar arrays could make electricity, convert it to microwaves and then beam that energy to a ground antenna where it would be converted back to electricity. But to make 10 trillion watts of power would require about 660 space solar power arrays, each about the size of Manhattan, in orbit about 22,000 miles above the Earth.
- Hydrogen energy: Hydrogen does not exist in pure, natural reservoirs and has to be extracted from natural gas or water. The study found that more carbon dioxide and less energy is produced by the extraction of hydrogen than by burning natural gas directly. Extracting hydrogen from water using solar or wind power is not “cost effective,”.
- Nuclear fusion: After decades of study, science still has not learned how to extract power from the fusion of atoms.
Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet
Here is the discussion of renewable energy resources:
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 Wem−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 TWe) 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 kWem−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 We 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 GWeoutput. [10 TW equivalent (3.3 TWe) 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).
Capturing and controlling sun power in space. (A) The power relay satellite, solar power satellite (SPS), and lunar power system all exploit unique attributes of space (high solar flux, lines of sight, lunar materials, shallow gravitational potential well of the Moon). (B) An SPS in a low Earth orbit can be smaller and cheaper than one in geostationary orbit because it does not spread its beam as much; but it does not appear fixed in the sky and has a shorter duty cycle (the fraction of time power is received at a given surface site). (C) Space-based geoengineering. The Lagrange interior point L1 provides an opportunity for radiative forcing to oppose global warming. A 2000-km-diameter parasol near L1 could deflect 2% of incident sunlight, as could aerosols with engineered optical properties injected in the stratosphere.
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.
(A) The conventional LWR employs water as both coolant and working fluid (left). The helium-cooled, graphite-moderated, pebble-bed, modular nuclear fission reactor is theoretically immune to loss-of-coolant meltdowns like TMI and Chernobyl (right). (B) The most successful path to fusion has been confining a D-T plasma (in purple) with complex magnetic fields in a tokamak. Breakeven occurs when the plasma triple product (number density × confinement time × temperature) attains a critical value. Recent tokamak performance improvements were capped by near-breakeven [data sources in (68)]. Experimental work on advanced fusion fuel cycles and simpler magnetic confinement schemes like the levitated dipole experiment (LDX) shown are recommended.
The main problem with fission for climate stabilization is fuel. Sailoret 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, nonsuperconducting 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.
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.