Tag Archives: Global warming

It’s just the future of the world. No big deal.

Never would I have thought that something as distant and mundane as the climate could upset me so.

But I’ve been pondering climate change more and more with every passing week, and more and more it’s becoming clear that we (humans) have two options if we want to avoid the icky consequences of climate change:

  1. Stop burning fossil fuels for energy.
  2. Find a new planet to live on.

Tongue-in-cheekiness aside, I truly do believe in Option 2—space exploration—as a vital long-term goal for humanity. Unfortunately, the time for climate action is limited, and new worlds aren’t built in a day. So today I thought it would be a good idea to link climate change with my own interests and background in energy, to summarize and share some of the unadulterated facts. I hope to inspire you to take a moment to think, to personalize and internalize the numbers, to dedicate your own prodigious intellectual resources to finding a solution to our shared little problem.

I draw numbers and inspiration (and a few direct quotes) from a 2009 review paper by Stanford’s Mark Jacobson, whose work I highly recommend to anyone interested in energy and the environment.


Climate Change and Air Pollution

Key point: Climate change and air pollution have a host of deleterious effects on human and environmental health.

Climate change (or global warming) is caused by an increase in the greenhouse effect induced by the following human inputs into the atmosphere (from most to least significant):
  • CO2
  • Fossil and biofuel soot
  • Methane
  • Halocarbons
  • Tropospheric ozone
  • Nitrous oxide

So what? Well, here’s my own little climate-change mnemonic:

The effects of climate change are FAST.

  • Food and Freshwater
    • Shifts viable locations for agriculture
    • Shifts the timing and magnitude of freshwater availability
  • Animals and plants, Acidity
    • Alters natural habitats substantially, causing extinction of plant and animal species
    • Increases the acidity of the ocean (dissolved CO2 forms carbonic acid, H2CO3) – The pH of the world’s oceans has dropped by ~0.1 since pre-industrial levels
  • Storms, Sea levels, and Sickness
    • Makes tropical storms more severe
    • Shrinks glaciers, snow pack, and ice levels worldwide, causing sea levels to rise
    • Increases the prevalence or geographic spread of various diseases (e.g., West Nile)
  • Temperature
    • Increases global temperatures and accompanying heat-related health risks

Air pollution is the 6th leading cause of death worldwide; it has been implicated by the WHO in 2.4 million deaths worldwide each year. Indoor and outdoor pollutants can increase ER visits, reduce worker productivity, and induce all of the following health conditions: asthma, respiratory illness, cancer, and cardiovascular disease.

Slightly troubling note: Human-created aerosol particles in the atmosphere—sulfates, nitrates, chlorides, ammonium, potassium, carbon, H2O—mask ~50% of the current global warming potential; if we reduce air pollution, much of that warming potential will be unveiled.


Energy Production and Use

Key point: Both climate change and air pollution are fundamentally linked to energy production and use; both arise from the burning of solid, liquid, and gaseous fuels. Solving these problems will thus require fundamental changes to the energy sector.

A few important numbers on energy and emissions:

  • Global energy production = 133 PWh/yr = 15.2 TW (2005)
  • Global electricity production = 18.2 PWh/yr = 2.1 TW (2005)
  • 25% of total US CO2 emissions are directly exhausted from vehicles on the road; another 8% are due to production and transport of fuel. Operation of fossil-fuel-burning vehicles thus accounts for over 33% of total US CO2 emissions, which we can eliminate by powering vehicles with non-emitting or lower-emitting technologies.
  • In the US, each person uses an average of ~14,000 kWh of electricity per year.
When evaluating alternative energy technologies, we need to think about several key metrics and consequences:
  • Resource availability: How much raw energy is available in a given energy resource? A renewable technology that’s extremely efficient and always available may have very little impact on climate change or pollution if the available potential energy is not enough to satisfy a major fraction of total energy demand.
  • Total CO2 emissions: Emissions can be measured in grams of CO2 equivalent per kWh (CO2e/kWh): In the context of emissions (see clarification here: http://www.skepticalscience.com/Carbon-dioxide-equivalents.html), this unit refers to the amount of CO2 required to produce a global warming effect equal to whatever is emitted by a particular technology, for each unit of energy produced.
    • Lifecycle emissions: How much CO2 (equivalent) is emitted during the production, operation, and disposal of a given technology for each unit of energy generated? Higher lifecycle emissions make a technology less attractive for mitigating climate change and air pollution.
      • The energy payback time is the amount of time for which a power plant must operate to produce the same amount of energy that was required to build it.
      • A longer technology lifetime amortizes the fixed CO2 emissions over a longer period of energy generation, reducing the emission per unit of energy produced.
    • Opportunity-cost emissions: If a renewable generation source takes a long time to plan and deploy, significant emissions may occur during that period; those emissions could have been eliminated if a more quickly deployable renewable source were used instead.
      • The planning-to-operation time encompasses the time to site, finance, permit, insure, construct, license, and connect the technology to the grid.
      • Such delays allow existing, higher-emitting power generation plants to operate longer, and limit our ability to respond to changing energy needs and the need for immediate action against climate change.
      • To minimize opportunity-cost emissions, we want generation sources with a long lifetime and fast planning and installation.
  • Land use and effects on wildlife: Renewable energy sources tend to require more land area than conventional power plants, due to the low energy density of renewable resources. Appropriation of land for energy generation may encroach on or damage wildlife habitats, as well as reducing the total amount of land available for alternative human use.
  • Vulnerability to disruption: Disruptions in the energy supply affect all human activities that are tied to the electrical grid. Centralized energy sources (e.g., nuclear plants) increase the risk of disruption compared to distributed sources (e.g., wind turbines, solar cells), since a single localized catastrophe can wipe out a large portion of local generation capacity.
  • Intermittency: Many renewable resources are by nature dependent on the caprices of complex and unpredictable environmental systems. Wind turbines produce energy only when and where the wind blows. Solar cells and solar thermal converters operate efficiently only when the sun shines on them directly, with no clouds, haze, or human obstructions in between. An energy technology that suffers from high intermittency may not be able to reach high penetration—i.e., to satisfy a large proportion of electricity needs—because it would be unable to fulfill electricity demand consistently. Other technologies must then compensate for the shortfall.
    • Intermittency makes grid-planning more difficult and increases the stress (from ramping) on other generation technologies.
    • The capacity factor is the average fraction of the peak rated power output of an energy source that is sustained continuously over the source lifetime.
      • The capacity factor can be calculated by dividing the total energy actually produced in a given time period by the total energy produced if the source produced 100% of its peak output for that entire period: If a 100Wp solar cell produces 50W for 6 hours a day and 0W for the remaining 18 hours, the numerator would be 50W*6h = 300Wh, the denominator would be 100W*24h = 2400Wh, and the capacity factor would be 300/2400 = 0.125.
      • High intermittency reduces the capacity factor of a given technology.
    • Several strategies may reduce the impact of intermittency:
      • Average out variations by combining generation sources from wide geographical areas. This requires a significant investment in grid infrastructure for long-distance electrical transmission.
      • Smooth out total generation by combining multiple high-intermittency renewable sources (e.g., solar, wind, wave) and using low-intermittency sources (e.g., hydro, geothermal) to provide for excess demand.
      • Use smart meters to charge and discharge electric vehicles at appropriate times to match electricity demand to supply.
      • Deploy an efficient utility-scale energy storage technology: Charge a battery with a solar panel when the sun is shining, and discharge the battery at night to charge an electric vehicle or iPhone. Such a technology does not exist today.
      • Forecast the availability of renewable sources more precisely (e.g., when the sun will shine and when the wind will blow) and ramp up other power plants to offset any excess demand.

Alternative Energy Technologies

Note on energy units: Although terawatts (TW = 1012 watts (W)) are units of power (energy per unit time), I will use it here to describe the total energy available for each renewable technology. To facilitate comparison with our global time-averaged energy production and demand (~15 TW total, of which ~2 TW is in the form of electricity), I’ve converted the total available energy (given by Jacobson in petawatt-hours (PWh) = 1015 watt-hours (Wh)) to the continuous available power (in W) by dividing by the number of hours in a year (8766 ~= 9000 ~= 104 for order-of-magnitude estimates). I think the lack of consistent and personally relevant units cripples clear discussion and understanding of energy issues (what the hell is a tonne of oil equivalent (toe) anyway?)—I like to stick with two units: watts (or kW, MW, GW, and TW) for power, and watt-hours (primarily kWh) for energy.

Solar energy: Concentrated solar power (CSP) and solar photovoltaics (PVs)
  • Essence of CSP: Concentrate sunlight to heat up a thermal transfer medium and generate steam to run a turbine.
    • Leading CSP technologies: Linear parabolic trough, power tower, parabolic dish (no thermal storage)
    • One primary benefit of CSP is the built-in storage capacity: By heating up and storing a material with sufficient heat capacity (e.g., molten nitrate salts (NaNO3, KNO3), steam, synthetic oil, graphite), we can counteract the intermittency of solar energy.
    • Total available energy: ~1000 TW = ~2/3 that of solar PVs (the area required per MW is ~1/3 larger for CSP than for OVs)
    • Capacity factor: 0.13-0.25 (without storage)
    • Lifetime: ~40 years
    • Energy payback time: 5-7 months
    • Planning-to-operation time: 2-5 years
    • Total emissions: 9-11 g CO2e/kWh
  • Essence of PVs: Use an energetically asymmetric semiconductor device to absorb light and generate electricity.
    • Leading PV technologies: Si (amorphous, polycrystalline, microcrystalline), Thin-fim (CdTe, CIGS, CZTS)
    • Solar cells can be deployed in large farms (usually 10-60 MWp, proposed up to 150 MWp) or on rooftops (90% of installed PV capacity is currently on rooftops; a future estimate for that number is 30%). Even the largest solar farms are smaller (in peak capacity) than average fossil-fuel (~600 MWp) or nuclear plants (~1 GWp), hence reducing the risk for disruption of the energy supply caused by centralization.
    • Total available energy: ~1700 TW (~20% of this is realistically harvestable, given the low insolation at high latitudes and competing land uses)
    • Capacity factor: 0.1-0.2 (depending on location, cloudiness, tilt, efficiency)
    • Lifetime: ~30 years
    • Energy payback time: 1-3 years
    • Planning-to-operation time: 2-5 years
    • Total emissions: 19-59 g CO2e/kWh (depends on insolation and amount of energy used in production)
Wind energy
  • Essence of wind: Convert the kinetic energy of moving air (wind) into rotational energy in a turbine to generate electricity.
    • Wind physics
      • The instantaneous power density of wind (usually 100-1000 W/m2) is proportional to the cube of the instantaneous wind speed. Finding locations with high wind speeds is thus crucial for successful wind power generation.
      • Wind speeds at a given height follow a Weibull distribution, which for wind can be approximated quite accurately by the Rayleigh distribution (a special case of the Weibull with shape parameter k=2).
      • Due to shear forces (the friction of air flowing across rough land), wind speeds increase roughly logarithmically with height: Taller wind turbines are thus more efficient. Modern turbines typically have a tower height of ~80 m (the hub or axis of rotation is at that height).
    • In 2007, 94 GW of peak wind generation capacity was installed globally; actual energy production satisfied ~1% of the ~2 TW of global electricity demand.
    • Wind energy requires the smallest footprint of all renewable technologies, although the total land area required is larger due to the array spacing between turbines needed for efficient operation (i.e., each turbine decreases the wind speed behind it, reducing the power density available to nearby turbines).
      • The area required for each turbine is approximately A = 4D x 7D, where D is the rotor diameter.
    • Total available energy: ~72 TW (~6 TW in US)
      • Average wind speeds of over 7 m/s are required for wind turbines to be economically competitive with other energy technologies. Such speeds exist over ~13% of global land area.
    • Capacity factor: 0.33-0.35
    • Lifetime: 20-30 years
    • Energy payback time: 2-4 months
    • Planning-to-operation time: 2-5 years
    • Total emissions: 3-7 g CO2e/kWh (lowest of all technologies surveyed)
Hydroelectric energy
  • Essence of hydro: Release the gravitational potential energy of water in an elevated reservoir, allowing it to flow through a turbine.
    • Hydroelectric plants are built on rivers, either by damming (to flood large areas of land and create a reservoir with water storage) or by introducing turbines that do not significantly alter the course of water flow.
    • Pumped hydro (pumping water uphill, storing it, and releasing it through a hydroelectric plant when electricity is needed) is the only energy storage technology that has been successfully demonstrated at the utility scale (~100 GW, compared to hundreds of MW for all other storage technologies), with storage efficiencies of 70-90%.
    • Hydropower is the largest installed renewable energy source globally, accounting for 17.4% (~36.5 GW) of total electricity production in 2005. Hydroelectric plants are often used as peaking plants since they can start producing power quickly (15-30s when in spinning-reserve mode) to match demand variations and smooth the intermittency of other generation sources—hydropower thus complements other renewables like solar and wind.
    • Total available energy: ~2 TW (5% has been tapped)
    • Capacity factor: 0.42
    • Lifetime: 50-100 years
    • Energy payback time: ~1 year
    • Planning-to-operation time: 8-16 years
    • Total emissions: 16-61 g CO2e/kWh
Ocean energy: Wave and tidal
  • Essence of wave energy: Convert the kinetic energy of wind-driven waves on the surface of the ocean to mechanical motion in a floating generator.
    • The power of a wave is proportional to the density of water (more dense => larger mass (per unit volume) => more kinetic energy), the group velocity of the wave (which is proportional to the wave period), and the height of the wave squared (the intensity or power of any wave is proportional to the amplitude squared).
    • Total available energy: 480 GW
      • Given that 2% of the world’s ~8×105km of coastline has wave power density greater than 30 kW/m
    • Capacity factor: 0.21-0.25
    • Energy payback time: 1 year
    • Planning-to-operation time: 2-5 years
    • Total emissions: 40-60 g CO2e/kWh
  • Essence of tidal energy: Use undersea turbines to harness the energy of oscillatory undersea currents caused by the gravitational attraction of the moon and the sun.
    • The ocean is continuously transitioning between high tide and low tide, with 4 such transitions (2 in each direction) every day. This predictability (6 hours in each direction) makes tidal energy a potential baseload generation source.
    • Tidal currents must have a speed greater than ~2 m/s for tidal power to be economical—lower than the 7 m/s required for wind energy, since water is more dense than air.
    • Tidal turbines are usually mounted on the sea floor, with the rotors either directly exposed or preceded with a narrowing duct to direct water toward them.
    • Total available energy: ~800 GW (~20 GW practical)
    • Capacity factor: 02.-0.35
    • Energy payback time: 3-5 months
    • Planning-to-operation time: 2-5 years
    • Total emissions: 34-55 g CO2e/kWh
Geothermal energy
  • Essence of geothermal: Drive water deep into the ground, allowing the thermal energy in the Earth’s crust to heat it up, and use the hot water to drive a steam turbine.
    • Geothermal energy arises from radioactive decay and planetary formation.
    • Leading geothermal technologies: Dry steam, flash steam, binary
      • Dry and flash steam systems are used when geothermal reservoir temperatures are 180-370ºC or higher; both require the drilling of two boreholes, one for steam (dry) and/or liquid (flash) flowing upward and one for condensed water flowing downward.
        • The dry steam approach uses (vapor-phase) steam pressure to drive a turbine directly.
        • The flash steam approach (most common today) converts hot liquid water into steam in a low-pressure flash tank, which then drives a turbine.
        • Neither system (currently) condenses greenhouse gases (CO2, NO, SO2, H2S) released from the underground reservoir during extraction, instead releasing them to the atmosphere and exacerbating climate change.
      • Binary cycle systems (~15% of current systems) are used when reservoir temperatures are 120-180°C.
        • Hot water rising from a borehole is used to heat and evaporate a low-boiling point “binary” or “working” organic fluid (e.g., isobutene, isopentane); the resulting vapor is used to drive a turbine.
        • Since they are closed-loop systems, binary cycle plants do not emit any greenhouse gases into the environment.
        • Because they can be operated at intermediate temperatures, binary systems may be the most promising and flexible of the geothermal approaches.
    • Total available energy: ~170 TW (but only ~100 GW at reasonable cost)
    • Capacity factor: 0.73 (good)
    • Lifetime: ~30 years
    • Energy payback time: 10+ years
    • Planning-to-operation time: 3-6 years
    • Total emissions: 16-60 g CO2e/kWh

Nuclear energy

  • Essence of nuclear (fission): Use slow neutrons to split the nuclei of heavy elements (e.g., U-235, Pu-239) into high-energy products (e.g., Kr-92, Ba-141, neutrons, gamma rays), which collide with and boil water to drive a steam turbine.
    • The most common nuclear fuels are uranium-235 and plutonium-239.
      • Uranium is typically stored as small ceramic pellets in metal fuel rods, which are used in a reactor for 3-6 years before being replaced.
      • The radioactive waste products resulting from nuclear fission must be stored in proper containment to isolate it from the biosphere for many thousands to millions of years (i.e., until the radioactive elements have decayed sufficiently).
    • The US has more nuclear power plants than any other country (~25% of the world’s total), followed by France, Japan, and Russia.
    • Total available energy: 0.4-14 TW (lower number for once-through thermal reactors, higher number for light-water and fast-neutron reactors)
    • Capacity factor: 0.81 (really good)
    • Lifetime: ~40 years
    • Energy payback time: 1-2.5 years
    • Planning-to-operation time: 10-19 years
    • Total emissions: 68-180 g CO2e/kWh
Fossil energy with carbon capture and storage (CCS)
  • Essence of CCS: Burn fossil fuels (coal in particular, since it’s abundant) but capture and store the resulting carbon emissions to reduce the impact on climate and air quality.
    • Emitted CO2 can be captured and injected deep underground in geological formations (e.g., saline aquifers, depleted fossil fuel fields, unminable coal seams) or into the deep ocean.
      • Worldwide, geological formations have the potential to store up to ~2000 Gt CO2, while we emit ~30 Gt CO2 every year. Clearly CCS is not a permanent solution.
      • Deep ocean sequestration of CO2 makes the ocean more acidic due to the formation of carbonic acid (H2CO3), and the sequestered CO2 eventually equilibrates with surface levels and is re-emitted into the atmosphere.
        • This option has been dismissed by most credible sources.
      • An alternative CCS approach is to combine CO2 with common metal oxides (e.g., quicklime (CaO), MgO, Na2O) to form solid carbonate minerals (e.g., CaCO3, MgCO3, Na2CO3) that can be easily sequestered. This approach requires a lot of raw oxide material and large energy inputs to speed up the reaction (via high temperatures and pressures).
    • Carbon capture technologies can reduce CO2 emissions by up to 85-90%.
      • But the energy required to capture and store CO2 will significantly reduce the power output (by 10-40%) of a power plant equipped with CCS technology.
      • And non-CO2 pollutants are still emitted, at even higher rates than before (more fuel must be burned to generate the same power output).
    • No major power plant today has successfully implemented carbon capture and storage.
    • Total available energy: ~1 TW for 200 years (~60% of annual electricity production)
      • Based on proven coal reserves
    • Capacity factor: 0.65-0.85
    • Lifetime: 30 years
    • Energy payback time: ??
    • Planning-to-operation time: 6-11 years
    • Total emissions: 310-570 gCO2e/kWh
Bio energy: Corn and cellulosic ethanol (E85)
  • Essence of biofuels: Let plants (or animals) convert solar energy into chemical energy, then burn the plants (or animal dung) or decompose them into liquid ethanol for transportation fuels.
    • Biofuels can be solids, liquids, or gases derived from organic matter: wood, grass, straw, manure, corn, sugarcane, wheat, sugarbeet, or molasses. These materials can be burned directly (e.g., for heating, cooking, and transportation) or converted into liquid ethanol for transportation.
      • The sugars and starches in crops are used fermented by microorganisms to create ethanol. Cellulose from switchgrass, wood, or miscanthus (a fast-growing grass) can also be converted into ethanol through fermentation, although the process is far slower than for the crops mentioned previously.
      • E85 fuel consists of 85% ethanol and 15% gasoline.
    • The amount of land required for bioethanol production is massive: We would need to triple the US land area dedicated to corn farming (to ~10% of the 50 states) just to power all US vehicles with corn ethanol.
      • The demands on freshwater for irrigation would be similarly immense.

The conclusion?

Wind and solar energy, coupled with baseload hydro and a bit of geothermal, can turn the climate change equation upside down, slashing emissions while improving the long-term sustainability of our energy supply. Biofuels and CCS do more harm than good. Existing technologies can make a big difference already; technology advances will make renewable technologies more and more attractive by both economic and climate-change metrics.

So where do we go from here?

Start reading. Start learning. Start protesting.

Thanks for reading!

And thanks to my ONE Labmates—in particular Patrick and Geoffrey—for many frightful discussions on energy and climate change.

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