Mitigation options for greenhouse gas induced global warming

Introduction

Total energy consumption today is about 12 TW (12 trillion watts), 85% of which is generated by fossil fuels.

The growing atmospheric Carbon dioxide concentration will reach 550 ppm by the end of the century if left unchecked (Fig. 1).

To stabilize Carbon dioxide at 550, 450 or 350ppm by the end of the century will require 15, 25, or >30 TW of emission free power by mid-century.

Will conservation help and if so by how much? Is conservation a long-term solution?

Certainly a country such as the United States can reduce carbon dioxide emissions substantially through conservation. Western European countries in general emit about half the carbon dioxide per capita as the United States (see table I). [Look at the per-capita emissions of Great Britain, France, Germany, Italy, Holland and Belgium. Also check out Japan and other countries.]

Can we conserve? Yes we can and we have to. During the Arab oil embargo, of the mid 1070's the United States actually reduced its consumption of fossil fuels through conservation measures (fig 2). At the same time oil companies, both large and small, greatly intensified exploration in the lower 48 states. Formerly uneconomical wells were brought back on line because of substantially higher oil prices. Nevertheless the annual production of oil, in the lower 48 states, continued the decline that had started some years before.

Methods for reducing emissions that also save money are many (table II) but there is some reluctance to impliment these for they often involve upfront costs.

The Kyoto agreement calls for reductions of Carbon dioxide emissions by industrial nations to 5% below 1990 levels by 2008 to 2012. Growth of emissions from developing nations growing at >5% will probably swamp savings from developed nations. Carbon dioxide emissions growth rates match population growth plus increases in standards of living (Fig. 3, fig. 4).

Conservation measures will help and will hopefully buy time but we must also develop ways to generate 100% to 300% of toady's world power consumption with zero emissions.

The most effective way to reduce carbon dioxide emissions and sustain economic growth is to develop revolutionary changes in energy production. Present U. S. policy emphasizes domestic oil production not energy technology research (see Cheney report www.whitehouse.gov/energy/National-Energy-Policy.pdf).

Sources of energy and energy efficiency

Primary energy is found in metastable chemical and nuclear bonds and in natural fluxes. Examples of the former include fossil fuels, fission fuels, and fusion fuels, among the latter are flows of; solar photons, wind, water and heat.

Energy conversions always involve dissipative losses. By energy efficiency we mean the ratio of usable energy output to total energy input.

The efficiencies of mature technologies are well known, the most efficient being electric generators (98% to 99%) and electric motors 90% to 97%) efficient. Others include:

Rotating heat engines

Gas and Steam turbines (35-50%)
Diesel engines (30-35%)
Gasoline engines (15-25%)

Fuel cells (50 to 55% now and potentially 70%)

Renewable energy converter examples

Photovoltaic cells (15- 20 with a potential of 24%)
Wind turbines (30-40% theoretically 59%)
Photosynthesis (1-2%)

Electric lighting

High pressure sodium vapor (15-20%)
Fluorescent (10-12%)
Incandescent (normal electric bulbs) (2-5%)

How much can efficiencies be improved?

Old technologies are near their limits such as primary energy conversion into electricity (30-36%) which yields the nominal 3 kW (thermal) to 1 kWh (electrical). Hybrid cars (30-35%) can increase the automobile efficiency over internal combustion engines (15-25%). Cogeneration of power can be 39-50% efficient.

Life styles can of course effect emissions. Ultra fuel efficient cars (gas/electric hybrids) can deliver 68 miles per gallon however demand for SUV's has lowered the fuel economy of the U.S. fleet of cars and light trucks to a 21 year low of 20 mpg.

Remedies?

Decarbonization of carbon dioxide

Hydrogen can be extracted from fossil fuels and the hydrogen burned. A mixture of steam and methane for instance can produce the following reaction.

2H2O + CH4 -- > 4 H2 + CO2

However the CO2 released from the fossil fuels used to produce the steam and the CO2 released from this reaction can exceed the emissions of carbon dioxide released by burning the methane directly.

However if the CO2 can be sequestered and not returned to the atmosphere then the process is emission free.

Sequestration of carbon dioxide

If carbon dioxide released from the burning of fossil fuels can be put in reservoirs other than the atmosphere the rate of carbon dioxide increase in the atmosphere can be slowed.

Potential reservoirs (Fig. 5)

The biosphere

Ocean life
Land life
Soils

The simplest air capture of carbon dioxide is through forests, however, the capacity of this reservour is limited because the whole terrestrial biosphere reservoir is about the size of the atmospheric reservoir. Furthermore the residence time of carbon in trees is about 7 years and this is also the residence time of organic carbon in the shallow ocean (upper 1000 meters). Consequently the biosphere although it may take up part of the carbon dioxide we are generating it is not a long term solution.

The deep ocean (Fig. 6)

Under the pressure of the deep ocean carbon dioxide becomes denser than water and would stay at depth. Whoever it would react with sea water over time causing it to become more acidic. This is also a negative effect of increasing atmospheric carbon dioxide and should be avoided in order to maintain the health of the ocean.

Placing the carbon dioxide in old oil and gas wells or coal seams or deep saline aquifers.
If the carbon dioxide remains in the gaseous phase there is always the danger that it might leak to the surface and reach the atmosphere.

Artificial carbonate rocks

Perhaps the most promising ways to sequester carbon dioxide emissions is a technique being developed by a
Columbia University scientist, Klaus Lackner, which combines carbon dioxide with magnesium from rocks. There are a class of rocks that are very rich in iron and magnesium called serpentines. These rocks are found along the traces of ancient subduction zones. The reaction

Mg++ + CO3= --> MgCO3

The reaction is exothermic so it does not require energy to make it happen and the end product, magnesium carbonate, is stable and can be put back in the earth. The concept call for building power plants near outcrops of serpentine and shipping in fossil fuels. The carbon dioxide would be removed from the flu gasses and combined with magnesium for the serpentine and the magnesium carbonate put back in the serpentine mine. Since there are ample supplies of serpentine this is a promising technique for sequestering large amounts of carbon dioxide.

Renewables (flows of solar photons, wind, water and heat) (Fig. 7, Fig. 8)

  1. Biomass burning
  2. Solar thermal
  3. Photovoltaic
  4. Wind
  5. Hydropower
  6. Ocean thermal
  7. Geothermal
  8. Tidal

Biomass burning and hydroelectric resources are nearly fully utilized today, the rest account for< 1% of global power and are generally not competetive with fossil fuels. Because of the inefficiency of photosynthesis it would require an area equal to all the agricultural land on earth to produce 10 TW of biopower.

Photovoltaic and wind require less area but other factors can be limiting. To supply all of present US energy consumption would require an array of PV cells 160km on a side or 26,000 km2. All pv cells shipped between 1982 and 1998 would cover only 3 km2, Furthermore storage facilities are needed to supply electricity at night. This is also true for wind power for wind is less strong at night than in the daytime. A global electrical grid is another possible solution but it would have to be superconducting to reduce transmission losses. Such a grid could take continual advantage of the daylight side of the earth.

Perhaps an even better would be to put the photovoltaic cells in space. The solar flux is eight times greater in space than on a spinning clouded earth. The collected energy could be transmitted to earth via microwave radiation that is not impeded by clouds. If 50 – 60% efficiency can be achieved in the microwave energy transfer 75 to 100 We (electricity) could be available at earth's surface for every square meter of PV panels in space. This is less than a quarter of the area needed for earth surface PV panels to achieve the same output. NASA and DOE have estimated that a PV array the size of Mahattan could produce 5 GW ( billion watts). To produce 3.3 Twe would require 660 such units. With adequate funding space solar panels could be launched in ten to fifteen years.and be generating electricity by the latter half of this century.

Fission

Available reactor technology can produce emission free power but disposal of waste and the possibility of weapons proliferation has cooled interest in this source of power. Further in the long run it is not clear that enough fuel can be mined to supply large amounts, say 10 TW by 2050, for the long run. Estimates that fuel to generate this much power would only last a generation (35 years).

Fusion

This is a most promising source of electricity for the long run for there is unlimited fuel available and emissions are not dangerous. However to date the experements that have been done have not yet reached 0% efficiency.


Jim Hays, Spring 2004.