By courtesy of Sci. Am. 2006, Sep.
A plan to keep carbon in check
R H Socolow and S W Pacala
At the present rate of growth, emissions of CO2 will double by 2056, reaching 14 billion tons of carbon a year in 2056. The atmospheric concentration of CO2 will be headed above 560 ppm, double the preindustrial value. The 560 ppm level would mean about 1200 billion tons of atmospheric carbon, up from the current 800 billion tons. The difference of 400 billion tons actually allows for roughly 800 billion tons of emissions, because half the CO2 emitted into the atmosphere enters the planet’s oceans and forests.
There are 15 ways, each preventing the release of 25 billion tons of carbon over 50 years. Chosen seven of them (avoiding doulbe-counting) can collectively contribute to a fixed annual CO2 emission as 2006 till 2056. Then the atmospheric carbon will reach 1000 billion tons. From 2056 to 2106, future technology might help further decrease the CO2 emission, then hopefully the atmospheric carbon concentration would stop increasing.
Here are 15 ways to make a wedge:
a. end-user efficiency and conservation: 1. increase fuel economy of 2 billion cars from 30 to 60 mpg; 2. drive 2 billion cars not 10000 but 5000 miles a year (at 30 mpg); 3. cut electricity use in homes, offices and stores by 25%.
b. power generation: 4. raise efficiency at 1600 large coal-fired plants from 40 to 60%; 5. replace 1400 large coal-fired plants with gas-fired plants.
c. carbon capture and storage (CCS): 6. install CCS at 800 large coal-fired plants; 7. install CCS at coal plants that produce hydrogen for 1.5 billion vehicles; 8. install CCS at coal-to-syngas plants plants.
d. alternative energy sources: 9. add twice today’s nuclear output to displace coal; 10. increase wind power 40-fold to displace coal; 11. increase solar power 700-fold to displace coal; 12. increase wind power 80-fold to make hydrogen for cars; 13. drive 2 billion cars on ethanol, using one sixth of world cropland.
e. agriculture and forestry: 14. stop all deforestation; 15. expand conservation tillage to 100% cropland.
Oil accounted for 43% of global carbon emissions from fossil fuels in 2002, while coal accounted for 37%; natural gas made up the remainder.
39% US share of global carbon emissions in 1952; 23% US share in 2002
Fueling our transportation future
J B Heywood
Transportation accounts for 25% of worldwide greenhouse gas emissions.
The US light-duty vehicle fleet currently consumes 150 billion gallons (550 billion liters) of gasoline a year, or 1.3 gallons of gasoline per person a day. If other nations burned gasoline at ethe same rate, world consumption would rise by a factor of almost 10.
Today’s gasoline spark-ignition engine is about 20% efficient in urban driving and 35% efficient at its best operating point. Real-world driving phenomena reduce the engine’s average efficiency so that only about 10% of the chmical energy stored in the fuel tank actually drives the wheels. With a 10% efficient vehicle and with the driver, a passenger and luggage-a payload of some 300 pounds, about 10% of the vehicle weight-only 1% of the fuel’s energy in the vehicle tank actually moves the payload.
In US roughly 20% of the corn crop is being converted to ethanol. Much of this is blended with gasoline at the 10% level in so-called reformulated gasolines. US plans to double ethanol production from the current 2% of transportation fuel by 2012.
An efficient solution
E K Jochem
The annual global primary energy demand is 447000 petajoules (a petajoule is roughly 300 gigawatt-hours), 80% of which comes from caron-emitting fossil fuels such as coal, oil and gas. After conversion these primary energy sources deliver roughly 300000 petajoules of so-called final energy to customers in the form of electricity, gasoline, heating oil, jet fuel, and so on. Then the next step, conversion of electricity, gasoline, and the like to useful energy in engines, boilers and lightbulbs, causes further energy losses of 154000 petajoules. Therefore, two thirds of the primary energy are lost during the two stages of energy conversion.
Almost 35% of greenhouse gas emissions come from buildings.
The nuclear option
J M Deutch and E J Moniz
Global electricity consumption is projected to increase 160% by 2050.
More than 20000 megawatts of nuclear capacity have come online globally since 2000, mostly in Far East.
MIT study, “the future of nuclear power”, dexcribed a scenario whereby worldwide nuclear power generation could triple to one million megawatts by the year 2050, saving the glove from emissions of between 0.8 billion and 1.8 billion tons of carbon a year, depending on whether gas- or coal-powered plants were displaced. At this scale, nucler power would significantly contribute to the stabilization of greenhouse gas emissions, which requires about seven billion tons of carbon to be averted annually by 2005.
The rise of renewable energy
D M Kammen
Solar power: 5000 megawatts is the global generating capacity of solar power; 37% is the top efficiency of experimental solar cells; 20-25 cents is the cost per kilowatt-hour of solar power.
Wind power: 60000 megawatts is global generating capacity of wind power, expected to triple by 2014; 0.5% is the fraction of US electricity produced by wind turbines. 1.9 cents is the tax credit for wind power, per kilowatt-hour of electricity.
Ethanol: 16.2 billion liters of ethanol produced in US in 2005; 2.8% is ethanol’s share of all automobile fuel by volume; $2 billion is the annual subsidy for corn-based ethanol.
High hopes for hydrogen
J Ogden
The number of vehicles worldwide, now 750 million, is expected to triple by 2050. And 97% of transportation fuel currently comes from crude oil.
Current automotive PEM fuel cells last only about 2000 hours, less than half the 5000 hour lifetime needed for commercial vehicles.
Today’s fuel-cell cars are handmade pecialty items that cost about $1 million apiece. If fuel-cell cars were mass-produced, the cost of their propulsion systems would most likely drop to a more manageable $6000 to $10000. That price is equivalent to $125 per kilowatt of engine power, which is about four times as high as the $30-per-kilowatt cost of a comparable internal-combustion engine.
Hydrogen production now consumes 2% of global energy, and its share is growing rapidly. If all this hydrogen were devoted to fuel-cell cars, it would power about 150 million vehicles.
If hydrogen were produced from natural gas, the most common method today, and used in an efficient fuel-cell car, the total greenhouse gas emissions would work out to be about 110 grams per kilometer driven. This amount is somewhat less than the total emissions from a gasoline hybrid vehicle (150 grams per kilometer) and significantly less than thsoe from today’s conventional gasoline cars (195 grams per kilometer).
Although electrolysis and biomass gasification face no major technical hurdles, the current costs for producing hydrogen using these methods are high: $6 to $10 per kilogram. A kilogram of hydrogen has about the same energy content as a gallon of gasoline, but it will propel a car several times as far because fuel cells are more efficient tan conventioanal gasoline engines. Future technologies and large-scale production and distribution could lower the price of hydrogen at the pump to $2 to $4 per kilogram.
US currently has only about 100 small refueling stations for hydrogen, set up for demonstration purposes. In contrast, the country has 170000 gasoline stations. GM has estimated that prociding national coverage for the first million hydrogen vehicles in US would require some 12000 hydrogen stations in cities and along interstates, each costing about $1 million. Building a full-scale hydrogen system serving 100 million cars in US might cost several hundred billion dollars, spent overdecades. But the infrastructure costs of maintaining and expanding the North American gasoline economy over the next 30 years will total $1.3 trillion, more than half of which will be spent in oil-producing countries in the developing world. To enable fuel-cell vehicles to enter mass markets in 10 to 15 years, hydrogen fuel must be widely available at a competitive price by then.