Renewable Energy II: Biomass and Photosynthesis, Slides of Energy and Environment

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Renewable Energy II

Biomass

Other Renewables

Biomass

•^

Biomass is any living organism, plant, animal, etc.

•^

12

W out of the 174,

(^12)

W incident on

the earth from the sun goes into photosynthesis– 0.023%– this is the fuel for virtually all biological activity– half occurs in oceans

•^

Compare this to global human power generationof 12

12

W, or to 0.

12

W of human

biological activity

•^

Fossil fuels represent

stored

biomass energy

Photosynthetic efficiency

•^

Only 25% of the solar spectrum is useful to thephotosynthetic process– uses both red and blue light (reflects green), doesn’t use

IR or UV

•^

70% of this light is actually absorbed by leaf

•^

Only 35% of the absorbed light energy (in theuseful wavelength bands) is stored as chemicalenergy– the rest is heat– incomplete usage of photon energy just like in solar PV

•^

Net result is about 6%

Realistic photosynthetic efficiency

Location

Plant Production

(g/m

2

per day)

Solar Energy

Conversion Efficiency

Potential Maximum

Polluted stream (?!)

Iowa cornfield

Pine Forest

Wyoming Prairie

Nevada Desert

Q

Ethanol from Corn

•^

One can make ethanol (C

H 2

OH: a common 5

alcohol) from corn– chop; mix with water– cook to convert starches to sugars– ferment into alcohol– distill to separate alcohol from the rest

Why are we even talking about Ethanol?!

•^

We put more energy into agriculture than we getout (in terms of Caloric content) by about a factorof 2–10– at least in our modern, petrol-based mechano-farming– sure, we can do better by improving efficiencies

•^

Estimates on energy return from corn ethanol– controversial: some say you get out 0.7 times the

energy out that you put in (a net loss); others claim it’s1.4 times; often see numbers like 1.

– 1.2 means a net gain, but 83% of your total budget goes

into production; only 17% of crop is exported as energy

Q

Ethanol problems, continued

•^

Energy is a high-payoff business, especially when thegovernment helps out with subsidies–

thus the attraction for corn ethanol (which

does

get subsidies)

•^

Can supplant actual food production, driving up price of food–

there have been tortilla shortages in Mexico because corn ethanol issqueezing the market

-^

after all, we only have a finite agricultural capacity

-^

both land, and water are limited, especially water

•^

Ethanol from sugar cane can be 8:1 favorable–

Brazil doing very well this way: but corn is the wrong answer!

-^

but lookout rain forests: can actually increase CO

2

by removing CO

absorbing jungle

Quantitative Ethanol

•^

Let’s calculate how much land we need to replace oil–

an Iowa cornfield is 1.5% efficient at turning incident sunlight intostored chemical energy

-^

the conversion to ethanol is 17% efficient

-^

assuming 1.2:1 ratio, and using corn ethanol to power farm equipmentand ethanol production itself

–^

growing season is only part of year (say 50%)

-^

net is 0.13% efficient (1.5%

–^

need 40% of 10

20

J per year = 4

^10

19

J/yr to replace petroleum

–^

this is 1.

^10

12

W: thus need 10

15

W input (at 0.13%)

–^

at 200 W/m

2

insolation, need 5

^10

12

m

2 , or (2,200 km)

2 of land

–^

that’s a square 2,200 km on a side

The lesson here

•^

Hopefully this illustrates the power of quantitativeanalysis– lots of ideas are floated/touted, but many don’t pass the

quantitative test

– a plan has to do a heck of a lot more than sound good!!!– by being quantitative in this course, I am hoping to

instill some of this discriminatory capability in you

Q

Other renewables

•^

We won’t spend time talking about everyconceivable option for renewable energy (consulttext and other books for more on these)

•^

Lots of imagination, few likely major players

•^

As a way of listing renewable alternatives, we willproceed by most abundant– for each, I’ll put the approximate value of QBtu

available annually

– compare to our consumption of 100 QBtu per year

Renewables, continued

•^

Geothermal: run heat engines off earth’s internalheat– could be as much as 1.5 QBtu/yr

worldwide

in 50 years

– limited to a few rare sites

•^

Tidal: oscillating hydroelectric “dams”– a few rare sites are conducive to this (Bay of Fundy, for

example)

– up to 1 QBtu/yr practical

worldwide

•^

Ocean Thermal Energy Conversion (OTEC)– use thermal gradient to drive heat engine– complex, at sea, small power outputs