Solar and Renewable Energies

Transcription

1 Physics 162: Solar and Renewable Energies March 9, 2009 Prof. Raghuveer Parthasarathy Winter 2010

2 Lecture 18: Announcements No homework this week Today: Finish solar; biofuels; hydrogen Thursday: Finish + Concluding remarks; Review Study questions for the last few lectures, posted

3 Lecture 18: Announcements Final Exam: Friday. March 19, 8 10am Cumulative, but emphasis on post midterm material Same format as midterm (closed book, no calculators,...) RP out of town (at least) Tu Th, March Makeup office hours next week: Monday 2:00 3:30 GTF review session next week Tues. 2:00 3:30pm, Willamette 175. As before, bring questions! Note course evaluations (online)!

4 Last time: Solar power Average insolation roughly 200 W/m 2. What can we do with all this power? Generate electricity w/ photovoltaic cells Make things warm Store energy as chemical energy (fuels or food)

5 Solar Heating Solar heating: using solar power for heating Passive solar heating (no pumps, fans; just the design of the building itself) Active solar heating (pumps, fans)

6 Building orientation: Southfacing windows! High R insulation retain solar generated heat Distribute energy through the building e.g. floors with high thermal conductivity Store thermal energy, for when the sun isn t shining materials with high heat capacity Passive Solar Heating what material properties desired functions (a good exam question in here...)

7 Solar Power Electricity Solar heating is great, but we also love electricity Can we use solar power to generate electricity? Yes. Two ways: direct conversion of electromagnetic radiation to electrical energy (photovoltaics) thermal energy + heat engine

8 Solar Thermal Power We can focus sunlight (with a lens or mirror). Concentrated sunlight high temperature; use this to run a heat engine + generator electricity. This is referred to as solar thermal power Is concentrated to this area Light from this area

9 Solar Thermal Power Heliostats: First significant implementation: Solar One (Calif.): ; 1800 heliostats; 10 MW power. Presently none in U.S. Spain: PS10 (shown) m 2 mirrors focus light to tower. 11 MW, completed Part of a series; expected 300 MW by

10 Heliostats: Solar Thermal Power Solar Two (CA). Solar Three is in Spain; I don t know why. A bit of info at Heliostats: Typical power generated per overall area*: about 5 30 W/m 2 Pretty good! * DJC McKay, Sustainable Energy without the hot air (2009), p. 184

11 Solar Thermal Power Concentrated sunlight; thermal energy; run a heat engine. How? Either... Heat fluids, pipe them to the heat engine. Place the heat engine directly at the focus. (Some engine types, e.g. the Stirling Engine we ve seen that runs using external temperature differences, are wellsuited to this.)

12 Solar Thermal Power Another way to harness solar thermal power: Solar updraft tower (Solar chimney) Transparent roof; tall hollow center Sunlight hot air. Hot air rises up chimney Moving air turns turbines electricity Q: Is this a heat engine? A. yes B. no Thermal energy converted to kinetic energy, electricity

13 Solar Thermal Power Another way to harness solar thermal power: Solar updraft tower (Solar chimney) Transparent roof; tall hollow center Sunlight hot air. Hot air rises up chimney Moving air turns turbines electricity Disadvantage: T H isn t very hot (low efficiency) Advantage: Few moving parts; low maintenance High construction costs; low operation costs; presently prototypes. Future?

14 Solar Thermal Power That s solar thermal power: sunlight heat something (thermal energy) generate electricity using a heat engine, generator.

15 Land Use Given the amount of solar power, The nation s entire energy needs could be met by this square: Nathan Lewis, CalTech

16 Land Use Oregon s electricity could be supplied by solar cells with 10% efficiency spanning this 10 mile 10 mile square:

17 Issues with photovoltaics So what s limiting the use of solar cells? Cost Energy storage requirements [start demo]

18 Energy storage Energy storage is huge challenge for solar power. (And wind power) You collect solar power in the daytime. How do you use it at night? How do you run a car with it? We ll look at a few issues of energy storage batteries biofuels (unrelated to photovoltaics) hydrogen Brief, but as you ll see many old issues reappear: combustion; energy density; etc.

19 Batteries Batteries:Chemical reactions separate opposite charges (positively and negatively charged molecules). The electrical potential energy is harnessed when we hook up a battery. What s the energy density of a battery? [Demo] This is something we can figure out.

20 Batteries What s the energy density of a battery? Consider a D battery. How much energy does it contain? I don t know but I know it could, roughly, power a small flashlight for a few hours. An estimate: Things you and I know: Power 4W (like a 4W nightlight bulb) Time 5 hours (it d probably run for 5 hours) Mass? Probably a tenth of a pound; about 0.2 kg. How can you determine the energy density (J/kg) from this?

21 Batteries What s the energy density of a battery? Power 4W (like a 4W nightlight bulb) Time 5 hours Mass? 0.2 kg. Energy = power time = 4 W 18,000 s = 72,000 J So: Energy density = Energy / mass = 72,000 J / 0.2 kg = 360,000 J/kg, i.e. roughly 0.4 MJ/ kg. Note: a very rough estimate, but requiring only things we already know

22 Batteries What s the energy density of a battery? Another (more exact) approach: Look at a D battery: V = 1.5 Volts, and it says 12,000 mah What is this?? milli Amp hours. (?) We ve figured out that power P = IV. (Units: Watts = Amps volts). So Energy = IVt, where t = time So 12,000 mah 1.5 V = 12, A h 1.5 V = 18 Amp Volt hours = 18 Watt hours = 65,000 Watt seconds = 65,000 Joules = Energy. (Those strange mah tell how much energy is in the battery.) Again, this energy / mass = 0.4 MJ/kg

23 Batteries What s the energy density of a battery? Another approach: measure current, voltage... [demo]

24 Batteries Batteries: Energy density < 1 MJ/kg Compare to fossil fuels: 50 MJ/kg A problem! Need lots of batteries to store electrical energy, e.g. from solar cells, wind Furthermore, batteries are expensive toxic There s been a lot of improvement in battery performance. Still: it s a huge challenge.

25 Solar power Average insolation roughly 200 W/m 2. What can we do with all this power? Generate electricity w/ photovoltaic cells Make things warm Store energy as chemical energy (fuels or food)

26 Biofuels Another approach to solar power: Our goal is to store the energy received in sunlight. Plants do this (photosynthesis) let s use them. Biofuels. Plants: solar energy chemical energy. Burn biofuels, release chemical energy. istockphoto.com

27 Biofuels What is the energy density of biofuels? Typical values: MJ/kg Ethanol: 30 MJ/kg Methanol: 20 MJ/kg Most sugars: 20 MJ/kg Biodiesel (vegetable oil): 40 MJ/kg Quite high! Not as high as gasoline (46 MJ/kg), but not bad; far better than batteries. Portable (e.g. for cars) But don t get too excited (wait a few slides)...

28 Biofuels Plants: solar energy chemical energy. Burn biofuels, release chemical energy. (Like fossil fuels: Combustion thermal energy; heat engine work) Burning biofuels: carbon neutral? Burning biofuels combustion: Isn t this what we do with fossil fuels? Why doesn t this release CO 2? Answer: it does, but the plant s carbon came (recently) from the atmosphere in the first place. istockphoto.com

29 Biofuels Photosynthesis: water + CO 2 + energy (sun) sugars (molecules of C + H + O) + O 2 (oxygen) Burn this (e.g. wood), or use fermentation or other processes to these things into liquid fuels like ethanol, and burn this Combustion (as we ve seen): fuel + O 2 CO 2 + water + energy released. (Fossil fuels: the CO 2 was taken from the atmosphere hundreds of millions of years ago, and is rapidly being dumped back now.)

30 Biofuels Biofuels sound appealing, but there are several serious problems. As seen above, biofuels are carbon neutral, but only if (1) no fossil fuels are used in their production. (2) no biomass (e.g. jungles) is destroyed to create crop land this sends the CO 2 stored in these plants into the atmosphere.

31 Biofuels At present, this is very much not the case. Modern farming is very energy intensive. U.S. Corn based ethanol: Energy out = only 1.1 Energy input! Greenhouse gas emissions only 13% less than fossil fuels. Brazilian sugarcane ethanol: Greenhouse gas emissions 90% less than fossil fuels. But: concerns about deforestation. We might be able to lower the energy needs of farming & bio ethanol production, etc., but there are deeper problems...

32 Biofuels An even deeper biofuel problem... Silicon solar cells. Efficiency* is roughly: A % B % C. 1 10% D % If efficiency = 10%, how many W/m 2 do we get? A. 200 B. 20 C. 2 D. 0.2 =10% of 200 W/m 2 * Input solar energy to desired output form (electrical energy)

33 Using biofuels Plants [1] Plants: electromagnetic energy chemical energy (typically photosynthesis, energy stored in sugars) [2] After this: thermal energy (burning, e.g. wood fires, or biofuels in engines) OR chemical energy (eating) Note that step [1] sets the max. energy available (i.e. how much of the solar energy the plant gets.) After this step, any steps involving tractors, animals, power plants,... can only lower the amount of useful energy available.

34 Biofuels So how efficient is the first step? Typical plants. Efficiency* is roughly (guess): A % B % C. 1 10% D %! If efficiency = 0.1%, how many W/m 2 do we get? A. 200 B. 20 C. 2 D. 0.2 =0.1% of 200 W/m 2 * Input solar energy to desired output form (chemical energy)

35 Biofuels A fundamental biofuel problem: Plants aren t very efficient at harnessing solar energy. from DJC McKay, Sustainable Energy without the hot air (2009) * w/ optimal irrigation, GMO crops

36 Biofuels A fundamental biofuel problem: Plants aren t very efficient at harnessing solar energy. Most plants: efficiency < 0.1 % Even the best crops: efficiency roughly < 0.5% (If plants wanted to be more efficient, they d be black! Don t be upset with them they have other jobs, too, like metabolism, reproduction... They didn t evolve to please you!) istockphoto.com

37 Biofuels A fundamental biofuel problem: Plants aren t very efficient at harnessing solar energy. Also: farming, processing, transportation all require energy, so energy output is even lower. Note: ed article about energy in / out of food from DJC McKay, Sustainable Energy without the hot air (2009)

38 Biofuels For biofuels to supply all the world s energy, 30% of the total land on Earth would have to be covered with energy farms devoted solely to biofuels. Think of our solar cell example: 200miles 200 miles of 10% efficiency cells could power the U.S. So we d need 2000 miles 2000 miles of 0.1% efficiency cells Also: using crops for energy can drive food costs up we re barely feeding 7 billion as it is!

39 Biofuels Of course, we don t need to ask for all the world s energy. There is a role for biofuels since they can create convenient liquid fuels, e.g. ethanol for transportation and... if we can invent cheap new ways to easily generate ethanol from waste biomass (e.g. corn leaves, stalks; grasses; cellulosic ethanol )... if we can harness unusual biofuels (e.g. algae fed by CO 2 emissions from power plants, creating ethanol about 4 W/m 2!) But: biofuels can t solve our present energy problems.

40 Biofuels All of a sudden, you know, we may be in the energy business by being able to grow grass on the ranch! And have it harvested and converted into energy. That s what s close to happening. George W. Bush, February 2006 Why are people so fond of biofuels? High energy density ( 20 MJ/kg), portable Wishful thinking Politics! ( Farmers are good people..., Hemp powered cars, man! ) hempcar.org

41 Biofuels Why are people so fond of biofuels? Wishful thinking Politics! ( Farmers are good people... ) High energy density ( 20 MJ/kg), portable Biofuels: high energy density, low efficiency does that make them good or bad, or is this a meaningless question? You (having taken this course) can realize that energy density and overall efficiency are two totally separate things!

42 A calculation: Biofuels Suppose you burn 1 kg/day of a biofuel (20 MJ/kg) that comes from a plant with 0.1 W/m 2 efficiency. Ignoring extra inefficiencies of energy conversion (Carnot, etc.!), how much area of plant coverage do you require? Could think of this as a unit conversion: 1 kg/day? m 2. Try setting this up... (Solution Thursday)

43 Energy storage A few issues of energy storage batteries biofuels (unrelated to photovoltaics) hydrogen

44 Hydrogen Hydrogen(H) is not an energy source. It s an energy storage medium. Often misundersood. Water: H 2 O. Stable configuration of hydrogen and oxygen. It takes energy to break the bonds: water + energy input hydrogen (H 2, gas) + oxygen (O 2, gas) + energy stored in H H and O O bonds Combining hydrogen and oxygen releases energy: hydrogen + oxygen H 2 O (water) + released energy (There are other possible reactions involving hydrogen, but this is the most important.)

45 Hydrogen There isn t any free hydrogen gas on Earth. It has to be formed by using energy to break up water (or other chemicals) E.g. electrolysis using electrical energy to break up water E.g. photolysis using light + semiconductors to break up water. (Is there anything semiconductors can t do?...) Lots of present day research on this

46 Hydrogen Combining hydrogen and oxygen releases energy: hydrogen + oxygen H 2 O (water) + released energy Note: no CO 2 emissions; water is the only product. Very clean......if the hydrogen gas was generated in the first place using a carbon free energy source So hydrogen isn t good or bad in itself it is simply a storage medium for the energy that went into its creation.

47 Hydrogen Hydrogen s appeal is mainly for transportation: a replacement for fossil fuels No emissions High energy density (next graph), so portable Lots of infrastructure issues: hydrogen gas stations? We don t have a good way of creating lots of cheap hydrogen yet. If/ when we do, what do we with it? Fuel cells that directly combine hydrogen, oxygen, to create water and release energy

48 Hydrogen The hydrogen doesn t have to be stored as a gas; various chemical hydrides bind hydrogen in ways that can allow its release. Cost and energy density (graph) remain issues with considerable room for improvement... Energy density in MJ/liter (scaled to incorporate engine efficiency; text fig. 11 4).

49 Summary: Solar Power & related A lot of solar power reaches Earth 10,000 our power consumption. It s the only alternative energy source that can possibly totally sustain us. Solar power for heating Solar thermal energy electricity Photovoltaics: direct conversion of light to electrical energy how they work how efficient they are

50 Summary: Solar Power & related Energy storage: A key issue for solar energy batteries: chemical charge separation. Low energy density. biofuels:photosynthesis provides a natural way to store solar energy at high energy density; combustion releases it. Several serious problems mainly due to low efficiency of initial energy conversion. hydrogen: a non fossil fuel, high energy density storage medium. The future?...