The University of Arizona

Developing and Expanding Renewable Energy and Fuel Options

By Joe Abraham | The University of Arizona | June 17, 2009

The burning of fossil fuels for energy production is a leading cause of increasing atmospheric greenhouse gas concentrations. Carbon-neutral and low-carbon energy sources will need to be developed in order to stabilize atmospheric concentrations at levels that avoid significant climate change.1 Southwest state climate action plans identify several renewable energy sources that can be developed within the region, including:

photo of dairy cows

Dairy cow manure can be managed to produce methane that, when combusted, can power the dairy and offset electricity that would otherwise be produced with fossil-fuels.
Credit: ©Eli and Ric, istockphoto.com

Discussion of these renewable energy and fuel options below is based, in part, on a review and analysis of state plans created between 2005 and 2007 in Arizona, New Mexico, Colorado, and Utah to help achieve statewide greenhouse gas emission reductions. Gubernatorial executive orders instigated the planning processes in Arizona, New Mexico, and Utah, while a coalition of stakeholders (including the state of Colorado) led by a regional non-profit organization initiated the Colorado plan.

Solar, wind, and geothermal energy

In the Southwest there is significant potential for additional generation of solar, wind, and geothermal energy. The U.S. Department of Energy and the Western Governor’s Association are working together to identify the best locations in Western U.S., Canada, and northern Mexico for developing large-scale solar, wind, and geothermal energy generation projects. Once developed, these projects will compliment large-scale renewable energy projects already delivering low-carbon energy to cities and other areas of high energy demand through the regional electrical grid.

Solar and wind energy can also be generated at a smaller scale at homes and businesses, effectively reducing demand for energy from the grid, which is predominantly fossil-fuel based. Federal, state, and local governments and utilities provide a variety of incentives for individuals and businesses to purchase and install such systems, including the practice of selling excess energy generated to the local electricity utility, commonly referred to as net metering.

Biogas from landfills and dairy farms

When organic matter decomposes in the absence of oxygen it produces methane, a potent greenhouse gas and biogas that can be combusted to generate electricity. Southwestern state climate plans identified landfills and dairy farms as relatively cost-effective sources for producing electricity from methane biogas with the potential to reduce several million metric tons of CO2-equivalents, or MMtCO2-e (Figure 1).

Graph of renewable energy recommendations

Figure 1. Southwest state climate action plan emission reduction options involving energy from biomass, biogas, and biofuels.
| Enlarge This Figure |
Credit: Joe Abraham and Rebecca Macaulay, CLIMAS, The University of Arizona

When we throw away trash, it is often taken to a landfill. Over time, material in landfills decomposes and produces methane that eventually reaches the surface of the landfill and escapes to the atmosphere. The largest landfills in the U.S. are required to control for methane and other landfill gases but they may just collect and burn the gas, which produces carbon dioxide, a less potent greenhouse gas.

Smaller landfills not required to control landfill gas emissions can install gas collection systems that can be used to power on-site electricity generators. Depending on the amount of methane produced, landfills can offset some or all of their energy needs, and in some cases sell energy to local utilities through the electrical grid. The Colorado plan estimates as much as 7.5 MMtCO2-e can be reduced from landfill methane-to-energy projects in the state from 2007 through 2020 at net cost over that time period. The estimated emissions reduced include not only the methane emissions captured and combusted but the emissions avoided from not using the equivalent amount of energy from fossil fuels as well.

Like landfills, cows and other farm animals can produce a significant amount of manure that produces methane when it decomposes. Dairy farms in particular were identified by southwestern climate plans as the most cost-effective sources of energy. The methane derived from dairy cow manure yields an estimated 1.8 to over 6 MMtCO2-e of emission reductions per state between 2007 and 2020 (Figure 1). The manure is collected in an anaerobic digester that generates methane produced by the decomposition of the manure. The methane is then concentrated and combusted to produce on-farm energy and possibly supply energy via net metering to the regional electrical grid. Thus, estimates of emission reductions include both the avoided methane emissions as well as the energy produced that would otherwise come from predominantly fossil fuel-based sources.

Biomass from agriculture and forest thinning activities

Electricity can also be generated by burning the residual vegetation from agricultural operations as well as wood and woody materials collected in forest thinning activities. In both cases energy production is relatively cost-effective when compared to conventional fossil fuel-based energy assuming the biomass is readily available and can be converted into energy with limited transport and energy-intensive processing.

Photo of thinned forest

Figure 2. Forest thinning helps produce biomass.
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Credit: Mike Wagner, Northern Arizona University forestry program

Arizona, New Mexico, and Colorado state climate action plans all recommend developing energy projects that derive biomass from forest thinning activities aimed at reducing wildfire risk in the region (Figure 1). State plans separated out biomass from forest thinning in residential areas and uninhabited areas like state and national forests. Forest thinning projects typically either burn the small trees and woody fuels close to where thinning takes place or leave thinned biomass in the forest to decompose. As an energy source it would be transported to centralized biomass-to-energy facilities like the one in Snowflake, Arizona, or cut and sold for home wood stoves and other space heating needs. Like the previous biogas examples, emissions reductions are based on the energy produced by the biomass that would otherwise be produced using natural gas or another fossil fuel-based energy source. In the case of biomass, however, methane emissions are not avoided.

Agriculture residues from crops like corn, wheat, sorghum, pecans, and orchard trimmings can similarly be used to generate electricity or heat energy. Arizona and New Mexico state plans estimated emission reductions based on agricultural biomass processed into pellets and used for home and business heating and industrial settings to produce heat and energy. Because the production of agricultural biomass is highly seasonal centralized biomass-to-energy facilities are not likely to rely solely on agricultural biomass. If processed into pellets, biomass could be stored and used year-round.

Biofuels

A number of crops and plants are being studied and used to produce biofuels like ethanol and biodiesel that can replace more carbon-intensive fossil-based fuels. Arizona, New Mexico, and Colorado state climate plans identify in-state ethanol production and use as a significant emission reduction option, with estimated reductions of 28, 7.5, and 15 MMtCO2-e, respectively (Figure 1). These emission reduction estimates, as well as ethanol’s estimated cost-effectiveness of $3 per ton of CO2 reduced, however, are based on the production of cellulosic ethanol, which as of June 2009 has not been produced commercially in the U.S. Currently, ethanol is produced using starch from corn kernels, while cellulosic ethanol would use a variety of in-state crop plant materials and other less valuable biomass sources, potentially diminishing controversy related to producing energy from food crops like corn.

Biodiesel is produced from a variety of sources including crops like soybeans and canola, waste vegetable oils, animal fats, and algae. Estimated emission reductions from increasing the production and use of biodiesel are significantly less than for ethanol (Figure 1), in part due to the assumption of cellulosic ethanol technology.

Curious about algae as a source of biofuels? Read Powered by pond scum by Michelli Murphy.

Production of ethanol and biodiesel in the Southwest is not without controversy. For example, in addition to displacing crops for food with crops for fuel, biodiesel agriculture may expand the amount of marginal farmland in production, potentially affecting efforts to build up the amount of carbon stored in soils. Water use is another concern. Depending on the production method, producing one gallon of ethanol requires anywhere from three to six gallons of water.2 In any case, state climate action plans emphasize the need for conducting lifecycle analyses to determine if a particular crop or biofuel production method will, in fact, reduce greenhouse gas emissions compared to conventional fossil fuel, or possibly result in negative environmental and social impacts.

Related Links

Pew Center on Global Climate Change report on increasing the use of wind and solar in the United States
| http://www.pewclimate.org/report/Wind-Solar-Electricity/June2009 |

U.S. Environmental Protection Agency Methane-to-Markets Landfill methane recovery fact sheet
| http://www.methanetomarkets.org/resources/factsheets/landfill_eng.pdf |

U.S. Environmental Protection Agency Methane-to-Markets animal waste recovery fact sheet
| http://www.methanetomarkets.org/resources/factsheets/agriculture_eng.pdf |

U.S. Forest Service Forest Products Laboratory report on wood energy sources and uses
| http://www.fpl.fs.fed.us/documnts/tmu/biomass_energy/sources-uses-of-wood-for-energy.pdf |

Fact sheet on biomass power plant in Snowflake, Arizona
| http://www.srpnet.com/about/stations/snowflakebiomass.aspx |

2007 U.S. National Renewable Energy Laboratory cellulosic ethanol research brochure
| http://www.nrel.gov/biomass/pdfs/40742.pdf |

2006 U.S. National Renewable Energy Laboratory biofuel research brochure
| http://www.nrel.gov/biomass/pdfs/39436.pdf |

References

  1. Hansen, J., et al. 2008. Target atmospheric CO2: where should humanity aim? The Open Atmospheric Science Journal, 2: 217-231.
  2. Aden, A. 2007. Water usage for current and future ethanol production, Southwest Hydrology, 6(5): 22-23.