Energy cheaper than from coal
Published on July 11th, 2010
When economic well-being measured by the gross domestic product exceeds a threshold, birthrate drops sharply.
Global warming now threatens irreversible climate damage, ending glacial water flows needed to sustain food production for hundreds of millions of people, and shrinking the polar cold water regions of the ocean where algae start the ocean food chain. Atmospheric CO2 dissolving into the ocean acidifies it, killing corals and stressing ocean life. Demand for biofuels increases destruction of CO2 absorbing forests and jungles.
Burning coal for power is the largest source of atmospheric CO2, which drives global warming. Airborne coal soot causes 24,000 annual deaths in the US and 400,000 in China. We seek alternatives such as burying CO2, or substituting wind, solar, and nuclear power.
The world population growing from 6.7 to 9 billion will increase resource competition, exacerbating environment stress. Yet the OECD nations, with adequate energy supplies, have birthrates lower than needed for population replacement.
Nations with GDP per capita over $7,500 have sustainable birthrates. Electricity for water, sanitation, lighting, cooking, refrigeration, communications, health care, and industry contributes to economic development. Those nations with per capita electricity of 2,000 kWh/year (1/6 US use and an average power of 230 W) do achieve GDP of $7,500 per capita, which leads to sustainable birthrates.
Taxing carbon seeks to encourage energy sources that do not emit CO2, yet this has not been effective in Europe. Developing countries will not agree to carbon taxes and forgo an advantage they perceive led to prosperity in OECD nations. Alternatively, a source of energy cheaper than from coal would dissuade all nations from burning coal, without imposing tariffs or taxes that reduce economic productivity. Affordable electric power can also help developing nations reach modest levels of prosperity and lifestyles that include sustainable birthrates.
The objective for energy cheaper than from coal is $0.03/kWh and a capital cost of $2/watt of generating capacity. How can the liquid fluoride thorium reactor produce energy cheaper than from coal?
Fuel costs. Thorium fuel is plentiful and inexpensive; one ton worth $300,000 can power a 1,000 megawatt LFTR for a year – enough power for a city. Just 500 tons would supply all US electric energy for a year. The US government has 3,752 tons stored in the desert. US Geological Survey estimates reserves of 300,000 tons, and Thorium Energy claims 1.8 million tons of ore on 1,400 acres of Lemhi Pass, Idaho. Fuel costs for thorium would be $0.00004/kWh, compared to coal at $0.03/kWh.
Capital costs. The 2009 update of MIT’s Future of Nuclear Power shows new coal plants cost $2.30/watt and PWR nuclear plants cost of $4.00/watt. The median of five cost studies of molten salt reactors from 1962 to 2002 is $1.98/watt, in 2009 dollars. The following are fundamental reasons that LFTR plants will be less costly than coal or PWR plants.
Pressure. The LFTR operates at atmospheric pressure, without a massive reactor vessel pressurized to 160 atmospheres, and without a large containment dome needed to contain any accidentally released radioactive materials propelled by pressurized steam. One concept for the smaller LFTR containment structure is a concrete building below grade, with a concrete cap at grade level to resist aircraft impact.
Safety. PWRs are safe because of defense in depth – multiple, independent, redundant systems engineered to control faults. LFTR’s intrinsic safety keeps such costs low. A molten salt reactor can’t melt down because the core is already molten — its normal operating state. The salts are solid at room temperature, so if a reactor vessel, pump, or pipe ruptured the salts would spill out and solidify. There is no explosion potential because the pressure in the reactor is atmospheric. If the temperature of the salt rises too high, a solid plug of salt in a drain pipe melts and the fuel drains to a dump tank; the Oak Ridge researchers turned the reactor off this way on weekends.
Heat. The LFTR safely operates at high temperatures. Salt remains liquid below 1400°C; internal graphite core structures maintain integrity even above this. Molten salt heat capacity exceeds that of the water in PWRs or liquid sodium in LMFBRs, allowing more compact heat transfer loops. The molten salt heat exchange loop components of high-nickel metals such as Hastelloy-N are qualified up to 750°C.
Helium gas (green) is successively heated by 700°C molten salt (red) from a LFTR heat exchanger as it passes through high, medium, and low pressure turbines (T). The gas cycles back through three successive compressors (C), cooled by fluid (blue) that transfers rejected heat externally. The recuperator (R) transfers some energy from the compression cycle back to the expansion cycle. The generators (G) produce electricity. (Diagram courtesy of Per Peterson of UC Berkeley.)
Brayton Cycle. The triple reheat closed cycle Brayton turbine achieves a 45% efficiency of conversion from thermal to electric power, compared to 33% typical of existing nuclear and coal power plants using traditional Rankine steam cycles. The Brayton rejected heat to power ratio is thus 1.2 (55/45) rather than Rankine’s 2.0 (67/33) so the cooling requirements are nearly halved, reducing cooling tower costs and making air cooled LFTRs practical in arid regions where water is scarce. This compact Brayton turbine machinery is a quarter the mass, suggesting a similar cost reduction.
Boeing, producing one $200 million airplane per day, is a model for LFTR production.
Mass production. Commercialization of technology leads to lower costs as the number of units increase. Experience benefits arise from work specialization, new processes, product standardization, new technologies, and product redesign. Business economists observe that doubling the number of units produced reduces cost by a percentage termed the learning ratio, seen in the early aircraft industry to be 20%. Today Moore’s law in the computer industry illustrates a learning ratio of 50%. In The Economic Future of Nuclear Power University of Chicago economists estimate the learning ratio is 10% for nuclear power reactors.
Boeing, producing one $200 million airplane per day, is a model for LFTR production. Reactors of 100 MW size costing $200 million can be factory produced. Manufacturing more, smaller reactors traverses the learning curve more rapidly. Producing one per day for 3 years creates 1095 production experiences, reducing costs 65%
Research. Cost reductions are presaged by current engineering research. Compact, thin-plate heat exchangers may reduce fluid inventories, size, and cost. Possible new materials include silicon impregnated carbon fiber with chemical vapor infiltrated carbon surfaces and higher temperature nickel alloys. Operating at 950°C can increase thermal/electrical conversion efficiency beyond 50%, and also improve water dissociation to create hydrogen for manufacture of synthetic fuels such as methanol or dimethyl ether that can substitute for gasoline or diesel oil, another use for LFTR technology.
In summary, LFTR capital cost targets of $2/watt are supported by simple fluid fuel handling, high thermal capacity heat exchange fluids, smaller components, low pressure core, high temperature Brayton gas turbine power conversion, simple intrinsic safety, factory production, the learning curve, and new technologies already under development. A levelized $2/watt capital cost contributes $0.02/kWh to the power cost. With plentiful, inexpensive thorium fuel, LFTR can generate electricity at <$0.03/kWh, underselling power generated by burning coal.
Producing one LFTR of 100 MW size per day could phase out all coal burning power plants worldwide in 38 years, ending 10 billion tons of CO2 emissions from coal plants now supplying 1,400 GW of electric power. Low LFTR costs are vital to this coal replacement strategy, achievable if this goal is maintained during every design choice. Inexpensive electric power can also assist developing economies to improve prosperity, encouraging lifestyles with sustainable birthrates.
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