Biofuels, Solar and Wind as Renewable Energy Systems_Benefits and Risks Episode 1 Part 2 pps

However, other investigations are not as optimistic and, in fact, sug-gest that geothermal energy systems are not renewable because the sources tend todecline over 40–100 years Bradley 1

The efficiency of solar ponds in converting solar radiation into heat is estimated

to be approximately 1:4, assuming a 30-year life for the solar pond (Table 1.2) A

100 ha (1 km2) solar pond can produce electricity at a rate of approximately $0.30per kWh (Australian Government 2007)

Some hazards are associated with solar ponds, but most can be avoided withcareful management It is essential to use plastic liners to make the ponds leakproofand prevent contamination of the adjacent soil and groundwater with salt

1.5.2 Parabolic Troughs

Another solar thermal technology that concentrates solar radiation for large-scaleenergy production is the parabolic trough A parabolic trough, shaped like the bot-tom half of a large drainpipe, reflects sunlight to a central receiver tube that runsabove it Pressurized water and other fluids are heated in the pipe and used to gen-erate steam that drives turbogenerators for electricity production or provides heatenergy for industry

Parabolic troughs that have entered the commercial market have the potential forefficient electricity production because they can achieve high turbine inlet tempera-ture Assuming peak efficiency and favorable sunlight conditions, the land require-ments for the central receiver technology are approximately 1,100 ha per1 billionkWh per year (Table 1.2) The energy input:output ratio is calculated to be 1:5(Table 1.2) Solar thermal receivers are estimated to produce electricity at approxi-mately $0.07–$0.09 per kWh (DOE/EREN 2001)

The potential environmental impacts of solar thermal receivers include the cidental or emergency release of toxic chemicals used in the heat transfer system.Water availability can also be a problem in arid regions

ac-1.6 Photovoltaic Systems

Photovoltaic cells have the potential to provide a significant portion of future U.S.and world electrical energy (Energy Economics 2007) Photovoltaic cells produceelectricity when sunlight excites electrons in the cells The most promising photo-voltaic cells in terms of cost, mass production, and relatively high efficiency arethose manufactured using silicon Because the size of the unit is flexible and adapt-able, photovoltaic cells can be used in homes, industries, and utilities

However, photovoltaic cells need improvements to make them economicallycompetitive before their use can become widespread Test cells have reached ef-ficiencies of about 25% (American Energy 2007), but the durability of photovoltaiccells must be lengthened and current production costs reduced several times to maketheir use economically feasible

Production of electricity from photovoltaic cells currently costs about $0.25per kWh (DOE 2000) Using mass-produced photovoltaic cells with about 18%

efficiency, 1 billion kWh per year of electricity could be produced on approximately2,800 ha of land, and this is sufficient electrical energy to supply 100,000 people(Table 1.2, DOE 2001) Locating the photovoltaic cells on the roofs of homes,industries, and other buildings would reduce the need for additional land by anestimated 20% and reduce transmission costs However, because storage systemssuch as batteries cannot store energy for extended periods, photovoltaics requireconventional backup systems.

The energy input for making the structural materials of a photovoltaic systemcapable of delivering 1 billion kWh during a life of 30 years is calculated to beapproximately 143 million kWh Thus, the energy input per output ratio for themodules is about 1:7 (Table 1.2, Knapp and Jester 2000)

The major environmental problem associated with photovoltaic systems is theuse of toxic chemicals, such as cadmium sulfide and gallium arsenide, in their man-ufacture Because these chemicals are highly toxic and persist in the environment forcenturies, disposal and recycling of the materials in inoperative cells could become

a major problem

1.7 Geothermal Systems

Geothermal energy uses natural heat present in Earth’s interior Examples aregeysers and hot springs, like those at Yellowstone National Park in the UnitedStates Geothermal energy sources are divided into three categories: hydrothermal,geopressured-geothermal, and hot dry rock The hydrothermal system is the simplestand most commonly used for electricity generation The boiling liquid underground

is produced using wells, high internal pressure drives, or pumps In the UnitedStates, nearly 3,000 MW of installed electric generation comes from hydrothermalresources, and this is projected to increase by 4,500 MW

Most of the geothermal sites for electrical generation are located in California,Nevada, and Utah Electrical generation costs for geothermal plants in the Westrange from $0.06 to $0.30/kWh (Gawlik and Kutscher 2000), suggesting that thistechnology offers potential to produce electricity economically The US Department

of Energy and the Energy Information Administration (DOE/EIA 2001) projectthat geothermal electric generation may grow three- to fourfold during the next20–40 years However, other investigations are not as optimistic and, in fact, sug-gest that geothermal energy systems are not renewable because the sources tend todecline over 40–100 years (Bradley 1997, Youngquist 1997, Cassedy 2000) Exist-ing drilling opportunities for geothermal resources are limited to a few sites in theUnited States and world (Youngquist 1997)

Potential environmental problems of geothermal energy include water shortages,air pollution, waste effluent disposal, subsidence, and noise The wastes produced

in the sludge include toxic metals such as arsenic, boron, lead, mercury, radon, andvanadium Water shortages are an important limitation in some regions Geothermalsystems produce hydrogen sulfide, a potential air pollutant; however, this could be

processed and removed for use in industry Overall, these environmental costs ofgeothermal energy appear to be minimal relative to those of fossil fuel systems.

1.8 Biogas

Wet biomass materials can be converted effectively into usable energy using obic microbes In the United States, livestock dung is normally gravity fed or in-termittently pumped through a plug-flow digester, which is a long, lined, insulatedpit in the earth Bacteria break down volatile solids in the manure and convert theminto methane gas (65%) and CO2(35%) (Pimentel 2001) A flexible liner stretchesover the pit and collects the biogas, inflating like a balloon The biogas may be used

anaer-to heat the digester, anaer-to heat farm buildings, or anaer-to produce electricity A large facilitycapable of processing the dung from 500 cows costs nearly $300,000 (EPA 2000).The Environmental Protection Agency (EPA 2000) estimates that more than 2000digesters could be economically installed in the United States

The amount of biogas produced is determined by the temperature of the tem, the microbes present, the volatile solids content of the feedstock, and theretention time A plug-flow digester with an average manure retention time ofabout 16 days under winter conditions (17.4◦C) produced 452,000 kcal/day and used262,000 kcal/day to heat the digester to 35◦C (Jewell et al 1980) Using the samedigester during summer conditions (25◦C) but reducing the retention time to 10.4days, the yield in biogas was 524,000 kcal/day, and it used 157,000 kcal/day forheating the digester (Jewell et al 1980) The energy input per output ratios for thesewinter and summer conditions for the digester were 1:1.7 and 1:3.3, respectively.The energy output of biogas digesters is similar today (Hartman et al 2000)

sys-In developing countries such as sys-India, biogas digesters typically treat the dungfrom 15 to 30 cattle from a single family or a small village The resulting energyproduced for cooking saves forests and preserves the nutrients in the dung Thecapital cost for an Indian biogas unit ranges from $500 to $900 (Kishore 1993) Theprice value of a kWh biogas in India is about $0.06 (Dutta et al 1997) The total cost

of producing about 10 million kcal of biogas is estimated to be $321, assuming thecost of labor to be $7/h; hence, the biogas has a value of $356 Manure processedfor biogas has fewer odors and retains its fertilizer value (Pimentel 2001)

1.9 Ethanol and Energy Inputs

The average costs in terms of energy and dollars for a large modern corn ethanolplant are listed in Table 1.4 In the fermentation/distillation process, the corn is finelyground and approximately 15 L of water are added per 2.69 kg of ground corn Afterfermentation, to obtain a liter of 95% pure ethanol from the 8% ethanol and 92%water mixture, the 1 L of ethanol must be extracted from the approximately 13 L

of the ethanol/water mixture To be mixed with gasoline, the 95% ethanol must be

Table 1.4 Inputs per 1,000 L of 99.5% ethanol produced from corna

communi-m 20 kg of BOD per 1,000 L of ethanol produced (Kuby et al 1984).

n 4 kWh of energy required to process 1 kg of BOD (Blais et al 1995).

o DOE (2002).

further processed and more water removed, requiring additional fossil energy inputs

to achieve 99.5% pure ethanol (Table 1.4) Thus, a total of about 12 L of wastewatermust be removed per liter of ethanol produced, and this relatively large amount ofsewage effluent has to be disposed of at an energy, economic, and environmentalcost

To produce a liter of 99.5% ethanol uses 43% more fossil energy than the energyproduced as ethanol and costs 44c/ per L ($1.66 per gallon or $2.76 per gallon in-cluding the subsidy) (Table 1.4) The corn feedstock requires more than 33% of thetotal energy input In this analysis the total cost, including the energy inputs for thefermentation/distillation process and the apportioned energy costs of the stainlesssteel tanks and other industrial materials, is $436.92 per 1,000 L of ethanol produced(Table 1.4)

The largest energy inputs in corn-ethanol production are for producing the cornfeedstock, plus the steam energy, and electricity used in the fermentation/distillationprocess The total energy input to produce a liter of ethanol is approximately7,570 kcal (Table 1.4) However, a liter of ethanol has an energy value of only5,130 kcal Based on a net energy loss of 2,440 kcal of ethanol produced, 43% morefossil energy is expended than is produced as ethanol

1.10 Grasslands and Celulosic Ethanol

Tilman’s research (Tillman et al 2006) has merit in the explanation of field iments with various combinations of species of natural vegetation, and the produc-tivity of diverse experimental systems The outstanding, 30-year effort by the LandInstitute in Kansas (Jackson 1980) to develop multi-species perennial ecosystems

exper-that deliver high productivity for long periods has been de facto endorsed by Tillman

et al., albeit without acknowledgement

However, there are concerns about two items First, the statement by Tillman

et al that crop residues, like corn stover, can be harvested and utilized as a fuelsource This would be a disaster for agricultural ecosystems Without the protec-tion of crop residues, soil loss may increase as much as 100-fold (Fryrear andBilbro 1994) Already the U.S crop system is losing soil 10 times faster than sus-tainability (NAS 2003) Soil formation rates are extremely slow or less than 1 t/ha/yr(NAS 2003, Troeh et al 2004) Increased erosion will facilitate soil-C oxidation andcontribute to the greenhouse problem (Lal 2003)

Tillman et al assume about 1,032 L of ethanol can be produced through the version of the 4 t/ha/yr of grasses harvested However, Pimentel and Patzek (2007)reported a negative 50% return in switchgrass conversion Based on the optimisticdata of Tillman et al., and converting all 235 million ha of U.S grassland intoethanol, only 12% of U.S petroleum would be provided (USDA 2004, USCB2004–2005)

con-In addition, to achieve the production of this much ethanol would mean ing about 100 million cattle, 7 million sheep, and 4 million horses now grazing

displac-on 324 millidisplac-on ha of U.S grassland and rangeland (USDA 2004, Mitchell 2000).Already overgrazing is a problem on U.S grasslands and a similar problem existsworldwide (Brown 2001) Thus, the assessment of the quantity of ethanol that can

be produced on U.S and world grasslands by Tillman et al appears to be undulyoptimistic

1.11 Methanol and Vegetable Oils

Methanol can be produced from a gasifier-pyrolysis reactor using biomass as afeedstock (Hos and Groenveld 1987, Jenkins 1999) The yield from 1 ton of drywood is about 370 L of methanol (Ellington et al 1993, Osburn and Osburn 2001).For a plant with economies of scale to operate efficiently, more than 1.5 million

ha of sustainable forest would be required to supply it (Pimentel 2001) Biomass isgenerally not available in such enormous quantities, even from extensive forests, atacceptable prices Most methanol today is produced from natural gas

Processed vegetable oils from soybean, sunflower, rapeseed, and other oil plantscan be used as fuel in diesel engines Unfortunately, producing vegetable oils foruse in diesel engines is costly in terms of economics and energy (Pimentel andPatzek 2005) A slight net return on energy from soybean oil is possible, if thesoybeans are grown without commercial nitrogen fertilizer The soybean under

favorable conditions will produce its own nitrogen Even assuming a slight net ergy return with soy, the total United States would have to be planted to soybeansjust to provide soy oil for U.S trucks!

en-1.12 Transition to Renewable Energy

Despite its environmental and economic benefits, the transition to large-scale use ofrenewable energy presents several difficulties Renewable energy technologies, all

of which require land for collection and production, will compete with agriculture,forestry, and urbanization for land in the United States and world The United States

is at maximum use of its prime cropland for food production per capita today, but theworld has less than half the cropland per capita that it needs for a diverse diet (0.5 ha)and adequate supply of essential nutrients (USDA 2004) In fact, more than 3.7billion people are already malnourished in the world (UN/SCN 2004, Bagla 2003).With the world and US populations expected to double in the next 58 and 70 years,respectively, all the available cropland and forestland will be required to providevital food and forest products (PRB 2006)

As the growing U.S and world populations demand increased electricity andliquid fuels, constraints like land availability and high investment costs will restrictthe potential development of renewable energy technologies Energy use based oncurrent growth is projected to increase from the current U.S consumption of 102quads to approximately 145 quads by 2050 Land availability is also a problem, withthe US population adding about 3.3 million people each year (USCB 2007) Eachperson added requires about 0.4 ha (1 acre) of land for urbanization and highwaysand about 0.5 ha of cropland (Vesterby and Krupa 2001)

Renewable energy systems require more labor than fossil energy systems Forexample, wood-fired steam plants require several times more workers than coal-firedplants (Giampietro et al 1998)

An additional complication in the transition to renewable energies is the tionship between the location of ideal production sites and large population cen-ters Ideal locations for renewable energy technologies are often remote, such asdeserts of the American Southwest or wind farms located kilometers offshore Al-though these sites provide the most efficient generation of energy, delivering thisenergy to consumers presents a logistical problem For instance, networks of dis-tribution cables must be installed, costing about $179,000 per km 115-kV lines(DOE/EIA 2002) A percentage of the power delivered is lost as a function ofelectrical resistance in the distribution cable There are complex alternating cur-rent electrical networks in North America, and 3 of these are tied together by DClines (Nordel 2001) Based on these networks, it is estimated that electricity can betransmitted up to 1500 km

rela-A sixfold increase in installed technologies would provide the United States withapproximately 46 quads (thermal) of energy, less than half of current US consump-tion (Table 1.1) This level of energy production would require about 159 million ha

of land (17% of US land area) This percentage is an estimate, and could increase

or decrease depending on how the technologies evolve and energy conservation isencouraged

Worldwide, approximately 473 quads of all types of energy are used by thepopulation of more than 6.5 billion people (Table 1.1) Using available renewableenergy technologies, an estimated 200 quads of renewable energy could be pro-duced worldwide on about 20% of the world land area A self-sustaining renewableenergy system producing 200 quads of energy per year for about 2 billion people(Ferguson 2001) would provide each person with about 5,000 L of oil equivalentsper year, approximately half of America’s current consumption per year, but anincrease for most people of the world (Pimentel et al 1999)

The first priority of the US energy program should be for individuals, ties, and industries to conserve fossil fuel resources and reduce consumption Otherdeveloped countries have proved that high productivity and a high standard of livingcan be achieved with the use of half the energy expenditure of the United States(Pimentel et al 1999) In the United States, fossil energy subsidies of approximately

communi-$40 billion per year should be withdrawn and the savings invested in renewableenergy research and education to encourage the development and implementation

of renewable technologies If the United States became a leader in the development

of renewable energy technologies, then it would likely capture the world market forthis industry (Shute 2001)

The current subsidies for ethanol production total $6 billion per year (Koplow2006) This means that the subsidies per gallon of ethanol are 60 times greater thanthe subsidies per gallon of gasoline!

1.13 Conclusion

This assessment of renewable energy technologies confirms that these techniqueshave the potential to provide the nation with alternatives to meet nearly half offuture U.S energy needs To develop this potential, the United States would have tocommit to the development and implementation of non-fossil fuel technologies andenergy conservation People in the U.S would have to reduce their current energyconsumption by more than 50% and this is entirely possible Eventually we will

be forced to reduce energy consumption The implementation of renewable energytechnologies now would reduce many of the current environmental problems asso-ciated with fossil fuel production and use

The United States’ immediate priority should be to speed the transition from thereliance on nonrenewable fossil energy resources to reliance on renewable energytechnologies Various combinations of renewable technologies should be developedconsistent with the characteristics of the different geographic regions in the UnitedStates A combination of the renewable technologies listed in Table 1.3 should pro-vide the United States with an estimated 46 quads of renewable energy by 2050

These technologies should be able to provide this much energy without interferingwith required food and forest production.

If the United States does not commit itself to the transition from fossil to newable energy during the next decade or two, the economy and national securitywill suffer It is of critical importance that U.S residents work together to conserveenergy, land, water, and biological resources To ensure a reasonable standard ofliving in the future, there must be a fair balance between human population densityand use of energy, land, water, and biological resources

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Can the Earth Deliver the Biomass-for-Fuel

we Demand?

Tad W Patzek

Abstract In this work I outline the rational, science-based arguments that question

current wisdom of replacing fossil plant fuels (coal, oil and natural gas) with fresh plant agrofuels This 1:1 replacement is absolutely impossible for more than a few

years, because of the ways the planet Earth works and maintains life After these fewyears, the denuded Earth will be a different planet, hostile to human life I argue that

with the current set of objective constraints a continuous stable solution to human life cannot exist in the near-future, unless we all rapidly implement much more

limited ways of using the Earth’s resources, while reducing the global populations

of cars, trucks, livestock and, eventually, also humans

Keywords Agriculture · agrofuel · biomass · biorefinery · boundary · crop ·ecology · energy · ethanol · fuel production · model · mass balance · net energyvalue· plantation · population · sustainability · thermodynamics · tropics · yield

2.1 Introduction

The purpose of this work is to:

1 Show that the current and proposed “cellulosic” ethanol (a “second generation”agrofuel) refineries are inefficient, low energy-density concentrators of solarlight

2 Prove that even if these refineries were marvels of efficiency, they still would

be able to make but a dent in our runaway consumption of transportation fuels,because the Earth simply has little or no biomass to spare in the long run.The fundamental energy unit I use in this work is

1 exajoule (EJ) or 1018joules