solar and renewable energies - university of oregon...

Physics 162:

Solar and Renewable Energies

Prof. Raghuveer Parthasarathy

[email protected]

Winter 2010

Physics 162: Solar and Renewable Energies R. Parthasarathy Winter 2010

February 23, 2010

Lecture 14: Announcements

• Reading: Wolfson 8.1‐8.3, Chapter 9

• Homework: Problem Set 6. Due Thursday Feb. 25

• RP’s Office Hours today (out of town)– GTF Billy Scannell, 1:30‐2:30pm


Last time...

• Thermal (blackbody) spectrum: EM radiation emitted by all objects

• Higher T (More thermal energy) →– More electromagnetic radiation AND

– Electromagnetic radiation with shorter wavelengths


Thermal Radiation• Thermal radiation : not just one wavelength, but a wide range. (Important, we’ll see, to solar cells.)

• Note:

• Peak wavelengthdepends on temperature

• Total power depends on temperature


Power (in

com

plicated

units)

Sun: T ≈ 5300 K

Thermal radiation

• Total emitted power depends strongly on temperature...

• σ is a constant• A is the surface area of the object• T is the absolute temperature (Kelvin)• ε is “emissivity.” Describes how well EM radiation is absorbed or emitted (same thing)

For a perfectly “black” object, ε =...

4P ATεσ=


A. 1B. 0C. Depends on T

For a perfectly “shiny” object, ε = 0

emissivity• Emissivity can vary with wavelength.

• I.e. an object can have high ε at some wavelengths, and low ε at others.• In energy‐efficient windows, coatings that give low‐εin the infrared range.


Thermos• Mechanisms of heat flow, illustrated by...

• A thermos: minimize heat flow– low conduction: vacuum

– low convection: vacuum

– low radiation: reflective

istockphoto.com


Heat Capacity• Topics related to thermal energy and heat

• Heat transfer: how much thermal energy flowsbetween objects at different temperatures?

• A topic like the “opposite” of the last one:

• Heat capacity: If we supply some amount of energy to an object, how much does its temperature change?


Heat Capacity & Specific Heat• The relation between Q and ΔT depends on

– how much material there is: Mass, M

– material property: “specific heat,” c

• Q = M c ΔT• c is the specific heat

– Table. Water has a very high c.

• “M c” is the heat capacity


Heat Capacity• Topics related to thermal energy and heat

• Heat engines: Fundamental limit for converting thermal energy to low‐entropy forms; depends on TH, TC.

• Heat transfer: how much thermal energy flowsbetween objects at different temperatures?

• One more (like the “opposite” of the last one):

• Heat capacity: If we supply some amount of energy to an object, how much does its temperature change?


Heat Capacity & Specific Heat• Suppose you supply an amount of thermal energy (Q), how much does T change?

• If an object cools, how much energy (Q) does it release?

• The relation between Q, and the change in temperature (ΔT) depends on– how much material there is: Mass, M

– an intrinsic property of the material: “specific heat,” c

text’s symbol, commonly used.





• The equation (needn’t memorize): Q = M c ΔT• As noted, c is the specific heat

• “M c” is the heat capacity – combines the intrinsic property with the amount (mass) of the material





• The equation (needn’t memorize): Q = M c ΔT• What (SI) units should c have?

A. kg × K × JB. W / K / kg

C. J / kg / K

Note: both kg and K in denominator

Same as


J = kg × (J/kg/K) × K

Jkg K

Heat Capacity & Specific Heat• Q = M c ΔT• Specific heat: units of J/kg/K (or J/kg/°C)

• c shows the energy needed to raise T of 1 kg of the material by 1 °C

• Note: Water has a very high specific heat!


Heat Capacity• Q = M c ΔT• An iron thumbtack and a big iron bolt are removed from a hot oven. They are red hot and have the same temperature. When dropped into identical containers of water of equal temperature, which one raises the water temperature more?

A. thumbtack

B. bolt


Same ΔT, same c. Bolt: higherM→More heat (Q) transferred as it cools →Larger ΔT for the water.

Specific Heat Example• Q = M c ΔT• For example... consider a 5m ×10m × 2 m swimming pool (volume 100 m3, density 1000 kg/m3, so M=100,000 kg). What energy is required to heat it from 20 °C to 30 °C?

• Answer: Q = M c ΔT = 105 kg × (4×103 J/kg/K) × 10 K = 4 × 109 J.

• How much power would it take to heat the swimming pool in 10 hours (≈40,000 s)?

• Answer: Power = Energy / time ≈ 4 × 109 J / 4 × 104 s = 105 W = 100,000 W = 100 kW. A lot!


Heat Capacity & Specific Heat• In our swimming pool example, ... Q = 4 × 109 J. This is the extra thermal energy we put into our 30°C pool.

• Suppose we heated our pool with an electrical heater (converting electrical energy to thermal energy). How much electrical energy did we need?


A. 4 ×109 JB. Some larger number, which we would calculate using the Carnot relation

C. Some smaller number, which we would calculate using the Carnot relation

D. Impossible to determine

There’s no fundamental limit on converting low‐entropy forms to thermal energy!

Heat Capacity & Specific Heat• In our swimming pool example, ... Q = 4 × 109 J. This is the extra thermal energy we put into our 30°C pool.

• Suppose we used the pool to run a heat engine. How much energy could we convert to electricity?


A. 4 ×109 JB. Some larger number ... Carnot

C. Some smaller number ... Carnot

D. Impossible to determine

Plugging in TH =30 °C, TC = 20 °C, in Kelvin, ... emax = 0.03!

So we’d get 0.03× (4 × 109) = 0.1 × 109 J.

Heat Capacity of Water• Water has a large heat capacity (≈ 4‐5 ×“typical” substances)

• Important! It takes a lot of energy to change the temperature of water. Explains:– hot water bottles (stay warm a long time)

– water‐cooled car radiators (& other machines)

– on‐ and off‐shore breezes: a given amount of sunlight raises the land & water to different temperatures; this leads to convection → breezes. (Very important to climate.)


Thermal Energy and Heat

• That’s thermal energy and heat.

• Next– Geothermal Energy

– Solar: Solar Thermal Energy

– Electricity & Semiconductors

– Solar: Photovoltaics (Solar Cells)


The Geothermal Resource

• Geothermal energy flow, from the Earth’s interior: about 0.025% of the total energy reaching the Earth’s surface. (The rest: nearly all from the sun)

• Origin: Decay of radioactive materials in the Earth; “primordial” energy left over from the Earth’s formation (grav. potl. energy → thermal energy)

• It’s hotter deeper in the Earth. How much?...


The Geothermal Resource• It’s hotter deeper in the Earth. How much?...

• The geothermal gradient: About 25 °C per km of depth.

• What energy flow does this correspond to?

• We need to know:


A. the thermal conductivity of the Earth

B. the specific heat of the Earth

C. the heat capacity of the Earth

Energy flow (heat) –depends on thermal conductivity (& temperature, etc.)


• It’s hotter deeper in the Earth. How much?...

• The geothermal gradient: About 25 °C per km of depth.

• What energy flow does this correspond to? Rock (granite) thermal conductivity k = 3.4 W / m / K.

What does our “gradient” mean? It’s (TH‐TC)/d. (Think)

What’s “A”? Whatever. We’ll calculate H/A, i.e. the energy flow per unit area.


( )H CHk A T

dT

=−

( )H CH kT

A dT−

=

The Geothermal Resource• What energy flow does this correspond to? Numbers (note d→meters)... H/A = 0.085 W/m2. Less than 0.1 W per square meter.

• What’s the total energy flow for the planet? Above # × surface area of Earth → 40 × 1012 W.– This is about 3× humankind’s current energy consumption rate.

• We could have just “looked up” this number, but it’s important to see where it comes from. Unless one changes the geothermal gradient (!), we can’t possibly get more than this.



• What’s the total energy flow over the planet? 40 ×1012 W. About 3× humankind’s current energy consumption rate.

• What energy flow does this correspond to? H/A = 0.085 W/m2. Less than 0.1 W per square meter.

• There is a lot of geothermal power, but it’s very diffuse. Not uniformly so: some regions have large geothermal gradients due to magma near surface; often associated with volcanic activity, tectonic plate boundaries...


Regions with some of the greatest geothermal energy potential are located along geologic plate boundaries (gray lines) and close to active volcanoes (dots).


• Categories of geothermal resources:

• Natural steam reservoirs. Naturally occurring steam, drives turbines.– e.g. The Geysers power plant, north of San Francisco, 2000 MWe in 1989.

– Very desirable. Very rare.

• Hot water reservoirs. Naturally heated underground water. Fine for heating; poor for electricity (why?...)



• Categories of geothermal resources:

• Geopressured reservoirs. Hot water trapped under pressure. Could extract thermal and mechanical energy, but so far impractical. (Also contains lots of dissolved methane.)

• Normal gradient in dry rock. Drill hole in dry rock; find high temperatures. How to harness? Unknown; technologically unlikely.

• Molten magma. Very hot. No technology yet exists to exploit this.


Geothermal Energy Technology

• We’ve been using geothermal power for thousands of years (spas, bathing, etc.)


“Bathers enjoy geothermallyheated waters in Iceland’s Blue Lagoon.”


• Geothermal heating: Directly use geothermallyheated water. Many cities, e.g. Reykjavik, Copenhagen; Boise (largest in U.S.)


“Geothermal hot water carried through tubing installed in city sidewalks melts snow from walkways in Klamath Falls, Oregon.”


• Geothermal electricity generation:– Hot steam: Drive turbines. Rare.

– Can use hot water, but difficult.

– Problem: low efficiency. Why? Low TH ; Carnot! “High” TH = 200 °C (e.g. steam), emax ≈ 0.4. TH = 100°C → emax = 0.2.

– Presently: 0.4% of the U.S. electrical energy. (California produces one‐third of the world’s geothermal electricity.)


Geothermal Issues and Impacts

• Environmental Issues

• Renewable? It depends...– Yes if harnessing a steady energy flow (heat conduction)

– No if removing hot fluid (liquid or steam). Often the case. E.g. The Geysers, CA (see text p. 232 if curious)




• Renewable? It depends...

• Emissions: Pollutants and CO2? Some, because of dissolved gases in geothermal fluids. (Not a lot)– CO2: about 13% as much CO2 as coal plants with the same electricity production

– Sulfur dioxide: about 1% of that of a coal plant




• Renewable? It depends...

• Emissions: Pollutants and CO2? Some, because of dissolved gases in geothermal fluids. (Not a lot)

• Aesthetics (scenic geothermal areas)

• Land subsidence (“slumping” of ground level) if water is not reinjected; seismic fault lubrication if it is. Important?

• Overall: Pretty “green,” though not perfect.


Geothermal energy

• Good for direct heating and small amounts of electrical power generation in particularly “geothermal rich” regions

• It’s not going to solve our energy problems.


Solar Energy

• Our discussion of “warm things” leads us to the most important alternative energy source: solar energy.

• Most important: why?

• There’s lots of it.– How much?

– How do we harness it?

– Issues?


Solar Energy• What is solar energy?

• In the sun: Nuclear Fusion (mass energy →thermal energy)– Core temperature about 15 million K

– Surface temperature (determines thermal radiation) about 5500 K

• Thermal energy → electromagnetic radiation (light), traveling through space. Some reaches Earth.

• Very steady (≈ 0.1% variation for various astrophysical reasons)


Solar Constant• How much?

• The Solar Constant (S) is the power per unit area,where the area is oriented perpendicular to the sunlight


Area A, perpendicular to sunlight

Power P = S×A

S = 1400 W/m2

Solar Constant• If the area is not perpendicular, the power is less than S × A ...


Area A, notperpendicular to sunlight

Solar Constant• If the area is not perpendicular, the power is less than S × A ...


Consider this smaller “projected” area

Solar Constant• If the area is not perpendicular, the power is less...


... which is why it’s cooler at larger latitudes (farther from the equator)

Sunlight• Illustration: the sun


Is it (in the N. hemisphere):

A. Winter

B. SummerThe N. hemisphere is tilted toward the sun

Total Solar Power• The Earth intersects the same amount of solar energy as would a flat disk with an area equal to Earth’s cross‐sectional area (πRE2).


• How much power is this?• S × πRE2= 1.74 ×1017 W

Total Solar Power

• S × πRE2= 1.74 × 1017 W.• This is the total amount of solar power hitting the Earth.

• Humankind’s energy usage rate: 1.5 × 1013 W. • The amount of solar power is about 10,000 times greater than humanity’s power consumption – so even if we harness 0.0001× of it, we’re all set!


Insolation

• The power incident on a square meter of ground in Oregon is less than the power incident on a square meter in Arizona. Why?


A. The ground in Oregon is, on average, more “tilted” with respect to the sunlight

B. It’s warmer in Arizona

C. Arizona is closer to the sun.

Be sure to see why this isn’t actually an explanation. It’s true because of (A).Radius of Earth: 6 ×106 m

Earth‐sun distance: 2 × 1011 m

Insolation

• The power incident on a square meter of ground in Oregon is less than the power incident on a square meter in Arizona.

• We characterize this by the insolation ‐‐ the actual incident power per square meter of the Earth’s surface.– Greatest in the tropics, lowest at high latitudes

– Greatest at midday, lowest at sunrise, sunset


Insolation• Insolation ‐‐ the actual incident power per square meter of the Earth’s surface.– Greatest in the tropics, lowest at high latitudes

– Greatest at midday, lowest at sunrise, sunset


“Insolation measured on a mid‐October day in Middlebury, Vermont, at 44° north latitude. The dashed curve represents an ideal cloudless day.”

Average solar power• The actual solar power per unit area reaching the Earth’s surface is less than S=1400 W/m2 due to – “Tilt” (average ×½ ‐‐ see text)

– Nighttime (×½)– Reflection of light by clouds, etc. (lose about 30%)

• Avg. reaching the Earth’s surface: ≈ 200 W/m2.– Higher at lower latitudes, midday, less cloudy places

– A rough average; a round number to remember.

– 2,000× higher than humankind’s power consumption!

• Another rough number: midday on a clear day, the sunlight carries: ≈ 1000 W/m2.


Remember

Solar Energy: house• Some rough calculations (like text’s 9.1):

• The roof area of a house is around 150 m2. What is the average solar power incident on a typical house roof? Compare this to the average U.S. household energy consumption rate of 3.5 kW.

• 200 W/m2 × 150 m2 = 30,000 W = 30 kW. Much more than necessary. It’s not unrealistic to imagine fully solar powered homes.


Solar Energy: car• Some rough calculations (like text’s 9.1):

• The roof area of a car is around 7 m2. What is the average solar power incident on a typical car roof? Is this enough to power a 300 hp engine?

• 200 W/m2 × 7 m2 = 1,400 W.

• 300 hp × (750 W/hp) = 225,000 W. Much more than the incident solar power.

• So we don’t imagine (directly) solar powered cars...


Solar Energy: car• So we don’t imagine (directly) solar powered cars...

• ... unless we are content with less powerful cars

• or we drive things like this:


“Stanford University engineering students built this solar‐powered car they named Solstice. It won the 2005 North American Solar Challenge.”

Cars powered indirectly by solar power, generated elsewhere, are feasible

Distribution of Solar Energy• Bright, sunny Arizona desert: Average insolation ≈300 W/m2.

• A more northern, cloudier place like Eugene (or Portland)? [Guess]


A. 150 W/m2

B. 60 W/m2

C. 30 W/m2

Not as low as you might think!

Distribution of Solar Energy• Various cities


Not so different as you might think!

Distribution of Solar Energy• The U.S.


Distribution of Solar Energy• Even when cloudy, insolation isn’t zero: some light passes through clouds, and “diffuse insolation” is scattered throughout the sky. (That’s why the sky isn’t black)


“... a mid‐October day in Middlebury, Vermont.”

Distribution of Solar Energy• In N. hemisphere, longer summer days increase average insolation; shorter winter days decrease it.


Theoretical maximum horizontal‐surface insolation (solid curves) versus latitude for January and June. Also shown are actual data points for selected world cities in January (open circles) and June (solid circles).

Solar Spectrum• Don’t forget the solar spectrum: Power is spread over a large range of wavelength.

• Is this important? It depends...


http://en.wikipedia.org/wiki/Image:Solar_Spectrum.png

For solar heating: no. The amount of power is all that matters.

For electricity generation (solar cells): yes.

Solar power• So: Midday on a clear day, sunlight carries: ≈1000 W/m2.

• Average insolation is smaller, depends on latitude, cloud cover, etc. Roughly ≈ 200 W/m2. Higher in some places, but not tremendously lower (even in Oregon!).

• What can we do with all this power?– Make things warm

– Generate electricity

• Stay tuned...


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