Renewable Energy Sources II: Alternatives Part II

Renewable Energy Sources II: Alternatives Part II Lecture #11 PHYS 331/581 Spring 2019 Renewable Energy

Chapter Summary • Hydroelectric Power • Wind Power • Ocean Thermal Energy Conversion • Biomass as Energy • Geothermal Energy • Tidal Energy • Wave Energy • Today’s Focus • Biomass • Others

Recall Renewable Resources • Renewable means anything that won’t be destroyed by using it • sunlight (the sun will rise again tomorrow) • biomass (grows again) • hydrological cycle (will rain again) • wind (sunlight on earth makes more) • ocean currents (driven by sun) • tidal motion (moon keeps on producing it) • geothermal (heat sources inside earth not used up fast)

Renewable Energy Consumption

Another look at available energy flow • The flow of radiation (solar and thermal) was covered previously • earth is in an energy balance: energy in = energy out • 30% reflected, 70% thermally re-radiated • Some of the incident energy is absorbed, but what exactly does this do? • much goes into heating the air/land • much goes into driving weather (rain, wind) • some goes into ocean currents • some goes into photosynthesis

The Renewable Budget

Outstanding Points from Figure • Incident radiation is 1741015 W • this is 1370 W/m2 times area facing sun (R2) • 30% directly reflected back to space • off clouds, air, land • 47% goes into heating air, land, water • 23% goes into evaporating water, precipitation, etc. (part of weather) • Adds to 100%, so we’re done • but wait! there’s more…

Energy Flow, continued • 0.21% goes into wind, waves, convection, currents • note this is 100 times less than driving the water cycle • but this is the “other” aspect of weather • 0.023% is stored as chemical energy in plants via photosynthesis • total is 401012 W; half in ocean (plankton) • humans are 6 billion times 100 W = 0.61012 W • this is 1.5% of bio-energy; 0.00034% of incident power • All of this (bio-activity, wind, weather, etc.) ends up creating heat and re-radiating to space • except some small amount of storage in fossil fuels

iClicker Question • With respects to energy, hydroelectric power represents • A remnant electric power from storms • B remnant water energy from chemical bonds • C remnant energy of chemical bonding • D remnant gravitational potential energy of precipitation • E a form of fictitious energy

iClicker Question • What is true about hydroelectric power generation since 1950? • A It has always increased in MW produced • B It has always decreased in MW produced • C It has increased and decreased in total MW produced, but is now at a peak • D It has both increased and decreased in total MW produced • E The percentage of electric power produced by hydroelectric plants has generally increased over time

iClicker Question • What is about the maximum efficiency of energy generation using the wind? • A 20% • B 40% • C 60% • D 80% • E 100%

iClicker Question • Which state generates the most amount of electricity derived from wind power? • A Virginia • B Alaska • C Montana • D California • E Texas

Biomass • Biomass is any living organism, plant, animal, etc. • 401012 W out of the 174,0001012 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 generation of 131012 W, or to 0.61012 W of human biological activity • Fossil fuels represent stored biomass energy

Photosynthesis • Typical carbohydrate (sugar) has molecular structure like: [CH2O]x, where x is some integer • refer to this as “unit block”: C6H12O6 (glucose) has x=6 • Photosynthetic reaction: xCO2 + xH2O+ light [CH2O]x + xO2 1.47 g0.6 g16 kJ 1 g1.07 g • Carbohydrate reaction (food consumption) is photosynthesis run backwards • 16 kJ per gram is about 4 Calories per gram • Basically a “battery” for storing solar energy • usage just runs reaction backward (but energy instead of light)

Photosynthetic efficiency • Only 25% of the solar spectrum is useful to the photosynthetic 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 the useful wavelength bands) is stored as chemical energy • the rest is heat • akin to photovoltaic incomplete usage of photon energy • Net result is about 6%

Realistic photosynthetic efficiency

iClicker Question • The photosynthesis reaction • A takes in sugar and water and produces carbon dioxide and energy • B takes in sugar and sunlight and produces sugar and energy • C takes in sunlight and water to produce sugar and oxygen • D takes in sunlight, carbon dioxide and water to produce sugar and oxygen • E takes in sunlight, oxygen and water to produce sugar and energy

iClicker Question • Metabolic consumption of food is like photosynthesis in reverse in that • A you use oxygen and water to produce energy and carbon dioxide • B you use carbon dioxide and water to produce energy and oxygen • C you use sugar and oxygen to produce energy and carbon dioxide • D you use sugar and oxygen to produce carbon dioxide and water • E you use sugar and carbon dioxide to produce energy and oxygen

How much biomass is available? • Two estimates of plant production in book come up with comparable answers: • 1017 grams per year • 320 grams per m2 averaged over earth’s surface • consistent with 401012 W photosynthesis • U.S. annual harvested mass corresponds to 80 QBtu • comparable to 100 QBtu total consumption • U.S. actually has wood-fired plants: 6,650 MW-worth • in 2002, burned equivalent of 200,000 barrels of oil per day

Ethanol from Corn • One can make ethanol (C2H5OH: a common alcohol) from corn • chop; mix with water • cook to convert starches to sugars • ferment into alcohol • distill to separate alcohol from the rest

Does Ethanol as a Fuel Make Sense? • We put more energy into agriculture than we get out (in terms of Caloric content) by about a factor of two • at least in our modern, petrol-based mechano-farming • sure, we can do better by improving efficiencies • Estimates on energy return • controversial: some say you get out 0.7 times the energy out that you put in (a net loss); others claim it’s 1.4 times; often see numbers like 1.2 • 1.2 means a net gain, but 83% of your total budget goes into production; only 17% of crop is exported as energy

Ethanol, continued • Right now, using tons of fossil fuels to get ethanol • and not clear we’re operating at a net gain • Why on Earth are we trying? • corn has worked its way into much of our foods • high fructose corn syrup • cow feed • corn oil for cooking • powerful presence in the halls of Congress • the corn lobby is partially responsible for pervasiveness of corn in our diet (soft drinks)

iClicker Question • Fructose is bad for your health. • A True • B False • Sucrose is better for you than fructose. • A True • B False

Food For Thought • Differences between glucose, fructose, and sucrose • And then there is ethanol

Ethanol Issues, continued • Energy is a high-payoff business, especially when the government 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 is squeezing 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 CO2 by removing CO2-absorbing jungle

iClicker Question • The basic chemical formula for both glucose and fructose is C6H1206 • A True • B False

iClicker Question • Sucrose is a complex sugar made of glucose and fructose. • A True • B False

Quantitative Ethanol • Let’s calculate how much land we need to replace oil • an Iowa cornfield is 1.5% efficient at turning incident sunlight into stored chemical energy • the conversion to ethanol is 17% efficient • assuming 1.2:1 ratio, and using corn ethanol to power farm equipment and ethanol production itself • growing season is only part of year (say 50%) • net is 0.13% efficient (1.5%  17%  50%) • need 40% of 1020 J per year = 41019 J/yr to replace petroleum • this is 1.31012 W: thus need 1015 W input (at 0.13%) • at 200 W/m2 insolation, need 51012 m2, or (2,200 km)2 of land • that’s a square 2,200 km on a side

What does this amount of land look like? We don’t have this much arable land! And where do we grow our food?

Take Home Points • Hopefully this illustrates the power of quantitative analysis • 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

Other Renewable Resources • Consult text and other books for more on the other renewable resources • Note that there are few likely major players • Restricted by location and development costs • When considering most abundant renewable resources • consider the approximate value of QBtu available annually • compare to our consumption of 100 QBtu per year

Renewable Resources Review • Solar (photovoltaic, solar thermal) • get 100 QBtu/yr with < 2% coverage of U.S. land area • Wind • maybe 180 QBtu/yr worldwide, maybe 25 QBtu in U.S. • Biomass • if we divert 10% of the 40 TW global budget into energy, would net 4 TW, or 120 QBtu worldwide; maybe 7 QBtu in U.S., given about 6% of land area • Hydroelectric • 70 QBtu/yr feasible worldwide: twice current development • 5 QBtu/yr max potential in U.S.

Geothermal Energy • Geothermal: run heat engines off earth’s internal heat • could be as much as 1.5 QBtu/yr worldwide in 50 years • limited to a few rare sites Binary-cycleBinary-cycle power plants use moderate-temperature water (225 ºF–360 ºF, or 107 ºC–182 ºC) from the geothermal reservoir. In binary systems, hot geothermal fluids are passed through one side of a heat exchanger to heat a working fluid in a separate adjacent pipe. The working fluid, usually an organic compound with a low boiling point such as iso-butane or iso-pentane, is vaporized and passed through a turbine to generate electricity.

Geothermal Energy • Dry steam • Use very hot (>455 °F, or >235 °C) steam and little water from the geothermal reservoir. • Steam goes directly through a pipe to a turbine to spin a generator that produces electricity. • This type of geothermal power plant is the oldest, first being used at Lardarello, Italy, in 1904. • Flash steam • Flash steam power plants use hot water (>360 ºF, or >182 ºC) from the geothermal reservoir. • When the water is pumped to the generator, it is released from the pressure of the deep reservoir. • The sudden drop in pressure causes some of the water to vaporize to steam, which spins a turbine to generate electricity. • Both dry steam and flash steam power plants emit small amounts of carbon dioxide, nitric oxide, and sulfur • Generally 50 times less than traditional fossil-fuel power plants. • Hot water not flashed into steam is returned to the geothermal reservoir through injection wells.

Tidal Energy • Tidal: oscillating hydroelectric “dams” • a few rare sites are conducive to this (Bay of Fundy, for example) • can only generate when the tide is flowing in or out • only for about 10 hours each day • up to 1 QBtu/yr practical worldwide Tidal Energy System in France

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

iClicker Question • Are there any other alternative renewable energy resources? • A Yes • B No • Don’t forget that there is more to energy than meets the Earth

Renewable Energy Sources II: Alternatives Part II